INTRODUCTION

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Mitosis and differentiation are two cellular processes that are generally viewed as mutually exclusive events (43). Cell cycle progression is controlled by the activity of cyclin-dependent kinases (CDKs), which are finely regulated by accumulation of cyclins and association with the newly discovered CDK inhibitory proteins (CKIs). G₁-phase CKIs, including INK4 (inhibitor of CDK4), p21^CIP1, p27^KIP1, and p57^KIP2, bind CDK-cyclin complexes and thus antagonize CDK activity. The CKIs maintain the retinoblastoma (Rb) tumor suppressor protein in an active, hypophosphorylated form that effectively binds and inhibits the E2F transcription factors (60). Since cellular differentiation occurs in the G₀ phase of the cell cycle, emerging evidence suggests that CKIs not only restrain cellular growth but also promote differentiation (38). Indeed, expression of p21^CIP1 and maintenance of the active state of the Rb protein is correlated with cell cycle arrest of muscle cells, and this is associated with their terminal differentiation (21, 54, 62). Recently, Liu et al. (41) demonstrated that overexpression of p27^KIP1 or p21^CIP1 in the absence of differentiation agents can lead to terminal differentiation of promonocytic cells.

The finding that overexpression of cell cycle inhibitors is associated with cellular maturation does not necessarily indicate that cell growth and differentiation cannot occur simultaneously. For example, the increase in cellular proliferation caused by stem cell factor occurs concomitantly with enhanced megakaryocytic differentiation (61). Furthermore, germ line disruption of the three major CDK inhibitors, INK4 (59), p21^CIP1 (10, 14), and p27^KIP1 (17, 34, 51), has now been reported, but none of these strains of knockout mice has global defects in differentiated tissues and organs. Similarly, although the E2F transcription factor is well characterized as a cell cycle progression factor, germ line disruption of E2F was recently shown to lead to the opposite phenotype of hyperplasia rather than the expected state of hypoproliferation (18, 71). Although p27^KIP1 is expressed in differentiating promyeloid cells treated with vitamin D₃ (23), this event occurs several days after the cells begin to differentiate (72). This finding has recently been confirmed in human primary precursor CD34⁺ cells undergoing myeloid differentiation (63). More importantly, new evidence demonstrates a differentiation-inhibiting function of p21^CIP1, and this inhibitory role can be separated from the suppressive effect of this inhibitor on cell cycle progression (15). Since differentiation antigens are required to induce the expression of p21^CIP1 (21), these more recent data might indicate that expression of cell cycle inhibitors is more important for maintenance of differentiated cells in G₀ (20) than for directly promoting the initial stages of differentiation.

Insulin-like growth factor I (IGF-I) is well known as a G₁ progression factor (2) that maintains the expression of the DNA polymerase  cofactor/proliferating-cell nuclear antigen (PCNA) (47) and increases the growth of lymphoid and myeloid cells (35, 69). Human HL-60 myeloid precursor cells undergo differentiation toward the macrophage lineage in the presence of vitamin D₃ (13). We recently demonstrated that these cells express an endogenous type I IGF receptor and that inhibition of this intrinsic tyrosine kinase receptor blocks the growth and vitamin D₃-induced maturation of these cells (40). These data suggested that IGF-I promotes both the proliferation and differentiation of myeloid precursor cells. Here we establish that IGF-I is required for optimal induction of terminal macrophage differentiation induced by vitamin D₃, including CD11b, CD14, and -naphthyl acetate esterase (NAE). Western blot analysis revealed that this IGF-I-promoted macrophage differentiation does not lead to early induction of p27^KIP1 but, rather, causes an increase in expression of cyclin E and hyperphosphorylation of Rb. By using a novel double-labeling flow-cytometric technique that couples the expression of both differentiation (CD11b) and proliferation (PCNA) antigens, we unambiguously demonstrate that 75% of the early maturing cells simultaneously express both markers. Furthermore, early in the differentiation program (24 h), enriched populations of CD11b-positive cells incorporate [³H]thymidine. It therefore appears that early induction of the p27^KIP1 CDK inhibitor and cessation of mitosis are not necessarily required for the early events that lead to the terminal differentiation of myeloid cells.

	MATERIALS AND METHODS

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Antibodies and reagents. Powdered RPMI 1640 tissue culture medium (MediaTech, Herndon, Va.) was supplemented with 2 g of sodium bicarbonate per liter, 100 U of penicillin per ml, and 100 µg of streptomycin per ml (all from Sigma Chemical Co., St. Louis, Mo.). Fetal bovine serum (FBS) (HyClone Laboratories Inc., Logan, Utah [containing <25 pg of endotoxin/ml by the Limulus amoebocyte lysate assay; Associates of Cape Cod, Inc., Woods Hole, Mass.]) was heat inactivated at 56°C for 30 min. The human promyeloid cell line HL-60 was purchased from the American Type Culture Collection (Rockville, Md.). These cells have a cell cycle length of 23 h, and the mean duration of G₁, S, G₂ and M are 11, 9, 2, and 1 h, respectively (29). Recombinant human IGF-I was purchased from Intergen (Purchase, N.Y.), and the 1,25-(OH)₂ vitamin D₃ (vitamin D₃) was kindly provided by Milan Uskokovic, Hoffmann-La Roche. Rat anti-human CD11b (Mac-1, immunoglobulin G2b kappa chain [IgG2b ]) monoclonal antibody (MAb) was purchased from Boehringer Mannheim (Indianapolis, Ind.), and the irrelevant isotype-matched rat IgG2b was obtained from Sigma Chemical Co. The purified mouse anti-human cyclin E MAb (IgG1 ) and mouse anti-p27^KIP1 MAb (IgG1 ) were obtained from Oncogene Science, Inc. (Uniondale, Calif.) and Transduction Laboratories (Lexington, Ky.), respectively. Mouse anti-human CD14 (gp55, IgG2a ), the irrelevant isotype-matched mouse IgG2a Ab, and the mouse anti-human Rb tumor suppressor protein (pRb, IgG1 ), which recognizes both phosphorylated and dephosphorylated forms of Rb, were purchased from Pharmingen (San Diego, Calif.). Mouse anti-proliferating cell nuclear antigen (DNA polymerase  cofactor/PCNA) MAb (IgG1 ) and mouse antibromodeoxyuridine (anti-BrdU) MAb (IgG1 ) were purchased from MBL International Co. (Watertown, Mass.). The F(ab')₂ fragment of fluorescein isothiocyanate (FITC)-conjugated goat anti-rat IgG and anti-mouse IgG Ab was obtained from Cappel (Durham, N.C.).

Differentiation of HL-60 cells with vitamin D₃. Human promyeloid HL-60 cells undergo differentiation toward a macrophage phenotype following addition of vitamin D₃ in serum-containing medium (13). The cells were maintained at 37°C in an atmosphere of 95% air and 7% CO₂. They were passaged twice weekly in 10% FBS and studied between passages 20 and 40. The cells were washed three times (400 × g at 4°C) and then incubated in serum-free medium (RPMI 1640 supplemented with 5 µg of human transferrin [Sigma Chemical Co.] per ml and 30 nM sodium selenite [Sigma Chemical Co.]) for 24 h prior to each assay. Vitamin D₃ (final concentration, 1 µM; stock solution diluted in ethanol) was added, and the cells were cultured for the indicated times in the presence or absence of IGF-I (100 ng/ml). A similar amount of ethanol (0.1%) was added to the control, non-vitamin D₃-containing RPMI 1640.

Cytochemical detection of intracellular esterase activity. The enzyme NAE is an intracellular nonspecific esterase that is induced in human mature macrophages but is absent in immature myeloid blast cells (5). To determine the expression of this enzyme, HL-60 cells were seeded at 4 × 10⁵ cells per well in 24-well culture dishes. The cells were treated with vitamin D₃ (1 µM) in the presence or absence of IGF-I (100 ng/ml) for 2 days. At the end of the culture period, NAE activity was evaluated by using a commercially available diagnostic kit (Sigma Chemical Co.). Briefly, 2 × 10⁴ cells (in 0.5 ml) were cytocentrifuged and fixed with citrate-acetone-formaldehyde fixative onto microscope slides. After being rinsed, the cells were incubated with NAE substrate for 30 min and counterstained for 2 min in hematoxylin. The percentage of cells on each slide that contained cytoplasmic black formazan deposits, characteristic of NAE activity, was determined by counting at least 500 cells.

Western blotting of p27^KIP1, cyclin E, and p110^Rb. Cells were washed once with cold phosphate-buffered saline (PBS) (1.5 M NaCl, 19 mM Na₂HPO₄ · H₂O, 8.4 mM KH₂PO₄) and lysed on ice in cell lysis buffer containing 50 mM sodium HEPES, 150 mM NaCl, 50 mM NaF, 25 mM -glycerophosphate, 10 mM sodium pyrophosphate, 20 mM p-nitrophenyl, 1% Nonidet P-40, 10 µg of leupeptin per ml, 10 µg of pepstatin A per ml, 1 mM sodium vanadate, 1 mM EDTA, 1 mM benzamide, and 1 mM phenylmethylsulfonyl fluoride (all from Sigma Chemical Co.). Insoluble material was removed by centrifugation at 12,000 × g for 10 min, and the protein concentration was determined by the Bradford dye method with a protein kit (Bio-Rad Laboratories, Richmond, Calif.) with bovine serum albumin (Sigma Chemical Co.) as the standard. Equal amounts of cell extract (50 µg) were subjected to electrophoresis in sodium dodecyl sulfate (SDS)-10% polyacrylamide gels and transferred to polyvinylidene difluoride nitrocellulose sheets (Bio-Rad Laboratories) on a Bio-Rad Western transfer unit. The blotted nitrocellulose was washed twice for 15 min each with distilled water and was subsequently blocked for 20 min at room temperature with freshly prepared PBS containing 3% nonfat dry milk. The membrane was then incubated with 1 µg of mouse anti-cyclin E MAb, mouse anti-p27^KIP1 MAb, or mouse anti-p110^Rb MAb per ml at 4°C for 24 h. After two 15-min washes with distilled water, the polyvinylidene difluoride membrane was incubated for 1 h at room temperature with a goat anti-mouse IgG1 secondary Ab linked to horseradish peroxidase (Amersham Corp.). Following additional washes with PBS-0.05% Tween (Sigma Chemical Co.), Western blot analysis of the specific protein was performed with a standard enhanced chemiluminescence kit (Amersham Corp.). The specificity of the primary Ab was confirmed by the absence of detectable proteins as assessed by blotting an identical sample with an isotype-matched control Ab followed by the appropriate alkaline phosphatase-conjugated secondary Ab. The intensity of hypophosphorylated and hyperphosphorylated Rb as detected on autoradiographs was determined by laser densitometry with a Molecular Dynamics (Sunnyvale, Calif.) personal densitometer equipped with ImageQuant 3.3 software as previously described (45).

Flow cytometry to detect cell surface CD11b and CD14. Macrophage development was determined by using flow cytometry to assess the increase in the percentage of cells binding to MAbs specific for the mature macrophage surface antigens, CD11b and CD14, as previously described by others (5, 9, 67). Vitamin D₃-treated cells (10⁶) were washed once in PBS supplemented with 0.5% FBS and 0.25% bovine serum albumin (BSA) (wash buffer). The cells were then incubated for 30 min at 4°C in PBS with either rat anti-human CD11b MAb (0.2 µg) and its isotype-matched control (IgG2b) or mouse anti-human CD14 MAb (1 µg) and its isotype-matched control (IgG2a). After two washes, the cells were incubated in PBS with the secondary FITC-conjugated goat anti-rat or anti-mouse F(ab')₂ fragment for additional 30 min at 4°C. Subsequently, the cells were washed twice and fixed in PBS containing 1% formaldehyde until analyzed by flow cytometry (EPICS V; Coulter Instruments, Miami, Fla.). For each sample, the immunofluorescence intensity of cells stained with the isotype-matched control did not exceed that of 5% of the cell population compared to cells incubated with only the secondary F(ab')₂ fragment. The isotype-matched control Ab coupled with the secondary F(ab')₂ fragment was used to establish a bitmap of at least 5,000 cells of uniform size.

Cell cycle analysis in conjunction with cell surface immunofluorescence. Cell cycle analysis was performed by washing 10⁵ cells three times with RPMI 1640 and then culturing the cells in serum-free defined medium for 24 h. Serum-starved cells were then incubated with medium alone (control) or with vitamin D₃ (1 µM), IGF-I (100 ng/ml), or both for 5 days. The cells were collected at the indicated times and enumerated with a cell counter (Coulter Instruments). Subsequently, the cells were washed once with PBS and then fixed with 80% ethanol at 4°C for 24 h. After three washes, the fixed cells were incubated for 1 h with a propidium iodide (PI; 20 µg/ml [Sigma Chemical Co.]) solution containing 0.1 mg of RNase A (Sigma Chemical Co.) per ml. The cells were then subjected to cell cycle analysis on an EPICS V flow cytometer.

Flow cytometry to simultaneously detect both surface CD11b and intranuclear PCNA or BrdU incorporation. Cells (2 × 10⁶) were first subjected to the indirect-labeling method described above, to define the surface leukocyte differentiation marker CD11b, without 1% paraformaldehyde fixation. They were subsequently fixed in 90% methanol at 20°C for 30 min. After two washes with ice-cold PBS, the cells were blocked with normal sheep serum (NSS [Sigma Chemical Co.], 50% in PBS) at room temperature for 10 min. A mouse anti-PCNA MAb (50 µg/ml) or the same amount of irrelevant isotype-matched mouse IgG1 Ab was then added, and the reaction mixture was incubated at 4°C for 10 min and washed twice more with NSS. After centrifugation, the cells were dissolved in PBS containing phycoerythrin (PE)-conjugated secondary sheep anti-mouse IgG1 Ab (Sigma Chemical Co.) (1:100). The reaction mixture was incubated at room temperature for 30 min. The cells were analyzed by flow cytometry after two washes with PBS buffer containing 2% FBS. In each experiment, cells labeled with the relevant primary Ab, but with only either FITC or PE secondary Ab, were used to establish background gating.

Sorting of CD11b-positive and -negative cells and [³H]thymidine incorporation. Cells (8 × 10⁶) were differentiated for 24 h with vitamin D₃ and IGF-I and then labeled with an anti-CD11b antibody (without fixation), which included five washes in PBS supplemented with 0.5% FBS and 0.25% BSA. CD11b-positive and -negative cells were separated by flow cytometry (>98% enriched compared to the isotype-control antibody) with a modified EPICS 753 cell sorter (Coulter Instruments) by using serum-free medium as sheath fluid and the settings described above. Triplicate samples (10⁵ cells) were sorted into wells of a 96-well plate (200-µl total volume) and incubated in serum-free medium with or without IGF-I (100 ng/ml), both in the absence of vitamin D₃. After 18 h, 1 µCi of [³H]thymidine (ICN Biomedical Inc., Irvine, Calif.) was added to each well, and the plate was incubated for an additional 6 h. The cells were harvested, and incorporation of [³H]thymidine was determined with an LS 6000IC scintillation counter (Beckman, Irvine, Calif.) as previously described (44).

Statistical analysis. Data were analyzed by using the Statistical Analysis System (58), with Student's t test being used to detect differences between treatments. Differences of at least P < 0.05 were considered to be significant.

	RESULTS

Top Abstract Introduction Materials and Methods Results Discussion References

Kinetics of IGF-I-enhanced CD11b expression in differentiating promyeloid cells. Although generally recognized as a progression factor that acts early in the cell cycle (2), IGF-I has been reported to promote the differentiation of both primary B lymphocytes and granulocytes (12, 19, 28, 32, 33, 37). The major inducer of hepatic IGF-I synthesis, growth hormone, is synthesized by leukocytes (8, 68) and promotes the development of several lineages of hematopoietic cells (48-50, 69). Since human (65) and fetal bovine (56) serum contain an average of 150 ng of IGF-I per ml, as well as various amounts of growth hormone, we developed a defined serum-free system for differentiating HL-60 promyeloid cells into macrophages. Terminal macrophage maturation is accompanied by the sequential expression of differentiation markers (67), and induction of the -subunit of the ₂-integrin heterodimer (CD11b/CD18; CR3) (16) is an early event in this process (9, 67). We therefore measured cell surface expression of CD11b over a 48-h time span, during which IGF-I enhanced macrophage differentiation in a time-dependent fashion (Fig. 1). Expression of CD11b was increased at all time points in cells treated with both IGF-I and vitamin D₃ compared to those cells treated with either of these two agents separately. IGF-I acted early in the differentiation process induced by vitamin D₃, as indicated by a significant increase in the percentage of CD11b-positive cells as early as 12 h (20% ± 2%; P < 0.05) compared to cells in serum-free medium (4% ± 1%) or treated with vitamin D₃ alone (10% ± 2%). This enhancement in CD11b expression caused by IGF-I in vitamin D₃-treated cells persisted throughout the 48-h time span. Consistent with our previous results, IGF-I did not affect macrophage development in the absence of vitamin D₃ (42). In separate experiments, we demonstrated that as little as 10 ng of IGF-I per ml plus vitamin D₃ (1 µM) caused a threefold increase in the proportion of CD11b-positive cells at 48 h (25% ± 3% versus 74% ± 4%; P < 0.01). Although receptors for both growth hormone and the closely related protein prolactin are members of the hematopoietic cytokine receptor superfamily (27, 64), neither of these hormones, at concentrations ranging from 1 to 1,000 ng/ml, increased the expression of CD11b (data not shown). These data establish that the IGF-I-induced increase in CD11b expression occurs with as little as 10 ng of IGF-I, can be detected as early as 12 h following the initiation of macrophage differentiation, and is not mimicked by either growth hormone or prolactin.

View larger version (21K): [in this window] [in a new window]   FIG. 1.   Enhancement of CD11b expression by IGF-I occurs as early as 12 h following addition of vitamin D3. Cells were cultured in serum-free medium alone (Med) or with IGF-I (100 ng/ml), vitamin D3 (VD3; 1 µM), or vitamin D3 plus IGF-I (VD3+IGF-I). During the 48-h incubation, the proportion of cells expressing CD11b was determined by flow cytometry. As early as 12 h, IGF-I increased (P < 0.05) the expression of CD11b in vitamin D3-treated cells compared to those incubated with vitamin D3 alone. Vitamin D3, in the absence of IGF-I, moderately increased (P < 0.05) CD11b expression at 24 h and later. IGF-I alone failed to increase CD11b expression above control levels. Plus signs indicate differences (P < 0.05) between medium or IGF-I alone and vitamin D3 alone, and asterisks indicate differences (P < 0.05) between vitamin D3 alone and vitamin D3 plus IGF-I. Values are expressed as means and standard errors of the mean (n = 3).

IGF-I enhances the expression of CD11b, CD14, and intracellular macrophage esterase in differentiating promyeloid cells. To determine whether IGF-I increases the expression of differentiation markers other than CD11b, which is an early event in terminal macrophage differentiation (9), we measured the expression of two additional proteins, NAE and CD14, at 24 h (Fig. 2A) and 48 h (Fig. 2B). Cells were treated with vitamin D₃ (1 µM) in the absence or presence of IGF-I (100 ng/ml), and intracellular NAE activity was assessed by enumerating cells with intracellular black formazan deposits. Surface expression of CD14 was assessed by flow cytometry. HL-60 cells cultured in serum-free medium for either 24 or 48 h express very little CD11b, CD14, or NAE (<6% positive cells). However, these cells can be induced as early as 24 h to express all of these markers equally by treatment with a combination of vitamin D₃ and IGF-I (42% ± 5%, 42% ± 4%, and 41% ± 3% positive cells, respectively; P < 0.01). In the absence of IGF-I, however, vitamin D₃ was significantly less effective in promoting the development of CD11b, CD14, and NAE (15% ± 2%, 9% ± 2%, and 12% ± 2%, respectively). IGF-I alone did not promote the expression of any macrophage differentiation marker (P > 0.10). The proportion of differentiated cells at 48 h was roughly double that detected at 24 h for all three differentiation markers, and identical trends in the expression of these macrophage differentiation markers were measured at this later time point (Fig. 2B). In serum-free medium at 48 h, addition of vitamin D₃ increased (P < 0.05) the proportion of cells expressing CD11b, CD14, and NAE (25% ± 4%, 23% ± 2%, and 22% ± 2%, respectively). Addition of IGF-I to vitamin D₃-treated cells increased by approximately threefold the proportion of cells expressing these markers (76% ± 5%, 78% ± 3%, and 79% ± 3%, respectively; P < 0.01). These results extend the findings in Fig. 1 by establishing that IGF-I also significantly enhances the expression of CD14 and NAE, both of which are expressed only by mature macrophages, as early as 24 h following induction of differentiation.

View larger version (25K): [in this window] [in a new window]   FIG. 2.   IGF-I promotes vitamin D3-induced expression of CD11b, CD14, and NAE in vitamin D3-treated promyeloid cells in a time-dependent manner. HL-60 cells were incubated in serum-free medium alone (Med) or with IGF-I (100 ng/ml), vitamin D3 (VD3; 1 µM), or vitamin D3 plus IGF-I (VD3+IGF-I). After 24 h (A) or 48 h (B), the proportion of cells expressing CD11b or CD14 surface antigens was determined by flow cytometry and NAE expression was determined by intracellular staining. IGF-I alone did not affect the expression of any of the differentiation markers, whereas vitamin D3 alone elicited a moderate increase in the expression of CD11b, CD14, and NAE at both 24 and 48 h. The combination of both IGF-I and vitamin D3 significantly (P < 0.01) increased the proportion of cells expressing CD11b, CD14, and NAE over that of cells incubated with only vitamin D3. Asterisks indicate that vitamin D3 increased the proportion of macrophage marker-expressing cells (P < 0.05) over that of cells cultured in medium or indicate that vitamin D3 plus IGF-I increased (P < 0.01) marker-expressing cells compared to vitamin D3 alone. Values are means and standard errors of the mean (n = 3).

Cell growth is enhanced by IGF-I in vitamin D₃-treated HL-60 cells. IGF-I, acting as a classical progression factor, promotes cells to pass through the G₁/S phase checkpoint and increases DNA synthesis in a number of cell types (2). We therefore tested whether this growth factor is also able to enhance the growth of cells cultured with the differentiating agent vitamin D₃. HL-60 cells were incubated in medium, vitamin D₃ (1 µM), IGF-I (100 ng/ml), or both vitamin D₃ and IGF-I for 5 days. The cells were harvested at the indicated times and enumerated with a cell counter (Coulter Instruments). As shown in a typical example (Fig. 3A), vitamin D₃ did not affect the growth rate compared to that of control cells in medium only whereas IGF-I potently increased cell proliferation regardless of the presence of vitamin D₃. A summary of three independent experiments showed that addition of IGF-I to vitamin D₃-treated cells increased the cell number by 1.8 ± 0.2- and 2.2 ± 0.2-fold at 2 and 3 days, respectively (P < 0.05). At these times, the growth of cells treated with IGF-I together with vitamin D₃ was similar to that of cells treated with IGF-I alone (2.0 ± 0.3- and 2.5 ± 0.2-fold, respectively). After 3 days, however, cells treated with both IGF-I and vitamin D₃ lost their growth potential (2.3 ± 0.2-fold at 5 days) whereas cells cultured with only IGF-I continued to expand (3.1 ± 0.2-fold; P < 0.05; n = 3).

View larger version (24K): [in this window] [in a new window]   FIG. 3.   Cell cycle analysis during vitamin D3-induced macrophage differentiation. Cells (5 × 105) were washed three times with RPMI 1640 and cultured in serum-free defined medium for 24 h. Serum-starved cells were then incubated in medium alone (Med) or with vitamin D3 (VD3; 1 µM), IGF-I (100 ng/ml), or both (VD3+IGF-I) for 5 days. Cell samples were collected at the indicated times and enumerated with a Coulter cell counter. Cell cycle analysis was performed by flow cytometry with PI. (A) Accumulation of HL-60 cells. While cells cultured in medium alone or vitamin D3 failed to increase in cell number, IGF-I increased cell growth, even in the presence of vitamin D3, on days 2 and 3. On days 4 and 5, cells treated with IGF-I continued to accumulate whereas cell numbers plateaued in cells treated with both IGF-I and vitamin D3. (B and C) Time courses of S (B) and G0/G1 (C) phase distribution. IGF-I increased the percentage of cells in S phase, regardless of the presence of vitamin D3, compared to that of cells cultured in medium or vitamin D3 alone on days 1 and 2. Accordingly, the percentage of cells in G0/G1 phase was reduced when cells were treated with IGF-I. On day 3 and thereafter, cells treated with both IGF-I and vitamin D3 withdrew from the cell cycle and accumulated in the G0/G1 phase whereas cells stimulated with IGF-I alone continued to replicate. The standard deviation of triplicate samples was less than 10% of the mean at each time point. Data are representative of three independent experiments (see the text for a summary of the results).

IGF-I increases cyclin E and suppresses p27^KIP1 expression. The recently discovered CDK inhibitors, including the G₁-phase p27^KIP1 and p21^CIP1, potently inhibit cellular proliferation and have been suggested to be required for differentiation in skeletal muscle cells (38). However, the role of these cell cycle inhibitors in differentiating hematopoietic cells has only recently begun to be elucidated (24, 41). p27^KIP1 can associate with and inhibit a broad range of CDK-cyclin complexes of the G₁/S transition phase (60). Indeed, the amount of this protein associated with cyclin E-CDK2 greatly increased in U937 human leukemic cells after treatment with phorbol myristate acetate for 24 to 72 h (3). We therefore tested the possibility that IGF-I promotes cell cycling by differentially regulating the expression of p27^KIP1 and cyclin E, and we investigated whether this occurs early in the differentiation program of promyeloid cells. Cells were incubated in medium, vitamin D₃ (1 µM), IGF-I (100 ng/ml), or both vitamin D₃ and IGF-I for 1, 2, or 3 days. As shown in Fig. 4A, p27^KIP1 expression was detected in cells incubated in either serum-free medium alone or treated with vitamin D₃ in serum-free medium, corresponding well to the finding that cells in both these treatments were arrested in the G₀/G₁ phase of the cell cycle on day 3 (Fig. 3). Addition of IGF-I almost completely suppressed the expression of p27^KIP1, even in the presence of vitamin D₃, at least through 72 h. However, as in the experiments by Liu et al. (41) with more mature U937 cells and by both Hengst and Reed (23) and Wang et al. (66) with promyeloid HL-60 cells, expression of p27^KIP1 is significantly increased at later time points in the differentiation program. In contrast, expression of cyclin E was greatly increased by treatment with IGF-I (Fig. 4B). On day 1, cyclin E expression was increased by approximately eightfold in IGF-I-treated promyeloid cells compared to that in control cells in serum-free medium, and this enhancement was maintained throughout the 3-day experiment. Although the increase in cyclin E expression caused by IGF-I was clearly reduced by vitamin D₃, cyclin E expression remained enhanced compared to the level in cells treated with vitamin D₃ alone, amounting to roughly a 3-, 3-, and 20-fold increase on days 1, 2, and 3, respectively. These results establish that the IGF-I-promoted progression through the cell cycle in the initial stages of terminal macrophage differentiation occurs concomitantly with an inhibition of the G₁-phase CKI p27^KIP1 protein and an increase in cyclin E expression.

View larger version (31K): [in this window] [in a new window]   FIG. 4.   IGF-I inhibits p27KIP1 and enhances cyclin E expression in the early stages of macrophage development. Cells were cultured in serum-free medium for 24 h and then incubated in serum-free defined medium alone (Medium) or with vitamin D3 (VD3; 1 µM), IGF-I (100 ng/ml), or vitamin D3 plus IGF-I (VD3+IGF-I) for 1, 2, or 3 days. Equal amounts of protein (50 µg) in whole-cell lysates were resolved by electrophoresis on SDS-10% polyacrylamide gels and electrophoretically transferred onto a nitrocellulose membrane. The membrane was then probed with a MAb against p27KIP1 (A) or against the 45-kDa cyclin E protein (B). The 35-kDa protein bound to the cyclin E Ab has been previously reported and is likely to be a degraded product of cyclin E (23). Bands were visualized by enhanced chemiluminescence, and the molecular masses of protein standards are indicated on the left. A representative gel from one of three experiments is shown.

IGF-I induces hyperphosphorylation of Rb tumor suppressor protein. Rb tumor suppressor protein is a putative target of the G₁-phase cyclin-CDK complexes, including cyclin E-CDK2 and cyclin D1-CDK4 (4). The hypophosphorylated form of Rb binds to E2F transcription factors and inhibits their activity (4), whereas phosphorylation by CDKs dissociates Rb from E2F factors, which in turn activates the cell cycle machinery (36). More importantly, suppression of Rb expression in U937 promonocytic cells resulted in a diminished ability of these cells to differentiate response to vitamin D₃ (7). Since IGF-I suppresses p27^KIP1 and augments cyclin E expression, we wondered whether the phosphorylation of Rb tumor suppressor protein is also regulated by IGF-I in developing myeloid cells. As shown in Fig. 5, cells maintained in serum-free medium or treated with vitamin D₃ alone did not express Rb protein on day 1, supporting previous findings that the Rb gene is downregulated at the onset of cell cycle arrest in HL-60 cells (72, 73). Although the Rb protein appeared on days 2 and 3, these cells expressed Rb protein in a form that was approximately 90% hypophosphorylated (p^Rb) compared to the hyperphosphorylated form (pp^Rb). This finding is consistent with the concept that most of these cells are in the G₀/G₁ phase of cell cycle. However, IGF-I induced the expression of the Rb protein as well as its phosphorylation, with more than 90% of Rb in the hyperphosphorylated state on days 1 and 2, regardless of the presence of vitamin D₃. Even on day 3, approximately 80% of the Rb protein was in the hyperphosphorylated form following treatment with IGF-I, which was similar to that in cells treated with both vitamin D₃ and IGF-I (66% hyperphosphorylated). These data show that the IGF-I-promoted enhancement of vitamin D₃-induced macrophage differentiation and promotion through the cell cycle is associated with phosphorylation of the Rb tumor suppressor protein.

View larger version (22K): [in this window] [in a new window]   FIG. 5.   Phosphorylation of Rb protein during IGF-I-enhanced cell differentiation. Cells were washed three times and cultured in defined medium overnight. Subsequently, these serum-deprived cells were incubated in defined medium alone (Medium) or with vitamin D3 (VD3; 1 µM), IGF-I (100 ng/ml), or vitamin D3 plus IGF-I (VD3+IGF-I) for 1, 2, or 3 days. Equal amounts of protein (50 µg) in whole-cell lysates were resolved by electrophoresis on SDS-10% polyacrylamide gels and transferred onto nitrocellulose. The blot was probed with a MAb against Rb tumor suppressor protein and visualized by enhanced chemiluminescence. Hypophosphorylated (pRb) and hyperphosphorylated (ppRb) forms of the 110-kDa Rb protein are indicated on the right, and the molecular mass of the Rb protein is shown on the left. Similar results were observed in three independent experiments.

CD11b is expressed in differentiating macrophages at all cell cycle phases. Since IGF-I enhanced differentiation in vitamin D₃-treated promyeloid cells as early as 12 to 24 h (Fig. 1), a time point when these cells are progressing through the cell cycle (Fig. 3), these data suggested that differentiating cells might continue to divide early in their differentiation program. These data are in accord with the results of Liu et al. (41), who showed that differentiation of U937 cells, as assessed by expression of mRNA for CD14, occurs long before their exit from the cell cycle. Similar results with HL-60 cells have been found at the protein level for both CD14 and nonspecific esterase by Wang et al. (66). We therefore determined whether CD11b protein is expressed throughout the cell cycle instead of only on cells that are arrested in the G₀/G₁ phase. As shown in a representative example in Fig. 6, cells incubated in medium alone and remaining in the G₀/G₁ phase were mostly negative for CD11b expression (70% of cells), with another 5% of the G₀/G₁-phase cells expressing this leukocyte surface antigen. Averaged over three independent experiments, 76% ± 2% of control cells were in G₀/G₁ and only 5% ± 2% expressed CD11b. Treatment with vitamin D₃ alone induced CD11b expression in a small proportion of both G₀/G₁ (7%) and S-plus-G₂/M (7%) cells. When combined over three separate experiments, the proportion of CD11b-positive cells averaged 17% ± 4%, which was greater (P < 0.05) than the 5% CD11b-positive cells cultured in medium alone. Addition of IGF-I to vitamin D₃-treated cells greatly enhanced macrophage differentiation in both G₀/G₁ (36%) and S-plus-G₂/M (30%) cells, for a total of 66% CD11b-positive cells (Fig. 6). This increase in CD11b-positive cells averaged 69% ± 3% in three independent experiments, which was greater (P < 0.05) than in cells treated with vitamin D₃ alone. More importantly, CD11b-positive cells appeared throughout each phase of cell cycle. For example, 35% ± 2% (n = 3) of cells in G₀/G₁ expressed CD11b whereas 26% ± 2% (n = 3) of G₀/G₁-phase cells did not. In the S-plus-G₂/M phases, 32% ± 2% (n = 3) of the cells expressed CD11b compared to only 7% ± 1% that did not (n = 3; P < 0.05). These data demonstrate that at the onset of terminal macrophage differentiation, all cells are not arrested in G₀/G₁ and that early-differentiated promyeloid cells are capable of progressing through cell cycle.

View larger version (43K): [in this window] [in a new window]   FIG. 6.   Cell cycle analysis of cells expressing the CD11b differentiation marker. Human HL-60 cells were washed three times and cultured in serum-free medium for 24 h. The cells (106) were then cultured in medium alone or with vitamin D3 (VD3; 1 µM) or vitamin D3 plus IGF-I (VD3+IGF-I; 100 ng/ml) for 48 h. Cell samples were then subjected to simultaneous DNA content (bottom graph) and cell surface immunofluorescence (top graph) analysis by flow cytometry. Very few cells in medium alone expressed the CD11b marker (5% in G0/G1 and 2% in S+G2/M). Vitamin D3 alone induced a small increase in CD11b expression (7% in G0/G1 and 7% in S+G2/M). With the addition of IGF-I to vitamin D3-treated cells, a majority of cells (66%) expressed CD11b, with a significant number of cells (30%) being found in S+G2/M. These data are representative of three independent experiments (see the text for a summary of the results).

Expression of CD11b occurs concomitantly with expression of proliferation antigens. Since CD11b-expressing cells are able to progress through the cell cycle, we wondered whether IGF-I simultaneously promotes both cell growth and differentiation at the initial stages of terminal macrophage differentiation. We tested this possibility at the single-cell level by developing a novel flow-cytometric technique to simultaneously detect a cell surface differentiation marker and one of two definitive nuclear proliferation antigens, PCNA (DNA polymerase  cofactor) and the thymidine analog BrdU. As shown in Fig. 7A, cells incubated in medium alone that were PCNA positive (48%) remained mostly CD11b negative (46%). Averaged over three independent experiments, PCNA-positive, CD11b-negative cells amounted to 45% ± 3% of the total population. Vitamin D₃ alone induced a small proportion of both PCNA-positive (7%) and -negative (6%) cells to undergo macrophage differentiation by 24 h. Averaged over three independent experiments, 12% ± 2% of the cells incubated with vitamin D₃ expressed CD11b. Interestingly, half of these differentiating cells (6% ± 1% of the entire cell population [n = 3]) also expressed the PCNA marker. More importantly, addition of IGF-I to the vitamin D₃-treated cells substantially enhanced macrophage differentiation, as shown by the proportion of CD11b-positive cells increasing from 12% ± 2% to 35% ± 2% (n = 3; P < 0.05). Moreover, 70% of these CD11b-positive cells (24% ± 2% of the entire cell population) coexpressed PCNA. Similar results were observed when the thymidine analog BrdU, which is incorporated into nuclear DNA during replication, was used as an indicator of cell proliferation (Fig. 7B). Indeed, 21% of the vitamin D₃-treated cells, composed of 5% expressing CD11b and 16% remaining negative for CD11b, incorporated BrdU. Vitamin D₃ alone induced CD11b expression in 9% of the cells (10% ± 2% when averaged over three experiments). Addition of IGF-I to vitamin D₃-treated promyeloid cells doubled the proportion of the BrdU-labeled cells from 21 to 40% (38% ± 2% in three experiments [P < 0.05]) and increased the CD11b-positive cells from 9 to 36% (37% ± 3% in three experiments [P < 0.05]). Among the CD11b-expressing cells, 73% (27% ± 2% of the entire cell population) were also positive for BrdU, indicating active DNA synthesis in these early-differentiated cells. This result corresponds closely with the 70% CD11b-positive cells that express PCNA. Collectively, these data provide definitive evidence that individual cells undergoing IGF-I-enhanced macrophage differentiation simultaneously express the CD11b differentiation marker concomitantly with both PCNA and a marker of DNA replication, BrdU.

View larger version (36K): [in this window] [in a new window]   FIG. 7.   Simultaneous expression of CD11b and nuclear proliferation antigens after treatment with vitamin D3 and IGF-I in HL-60 cells. Cells were cultured in medium alone or with vitamin D3 (VD3; 1 µM) or vitamin D3 plus IGF-I (VD3+IGF-I; 100 ng/ml) for 24 h, followed by double staining for flow cytometry. (A) Labeling of surface CD11b differentiation antigen and intracellular PCNA. Only 5% of the cells expressed CD11b in medium alone (3% were PCNA negative, and 2% were PCNA positive). Vitamin D3 induced CD11b expression in 13% of the cells (6% were PCNA negative, and 7% were PCNA positive), while the addition of IGF-I further increased the expression of CD11b by nearly threefold (34%). More importantly, 74% of the CD11b-positive cells (25% of the entire cell population) also expressed PCNA. Results summarized over three independent experiments are given in the text. (B) Double staining of CD11b and the nuclear BrdU antigen. Only 5% of the control cells expressed CD11b (3% were BrdU negative, and 2% were BrdU positive). A small proportion of the cells (9%) underwent differentiation after treatment with vitamin D3 (4% were BrdU negative, and 5% were BrdU positive). However, the addition of IGF-I to vitamin D3-treated cells increased the proportion of CD11b-positive cells to 36%. Among the CD11b-expressing cells, 72% (26% of the entire cell population) were in a proliferative state, as indicated by BrdU incorporation. These data are representative of three independent experiments (see the text for a summary of the results).

CD11b-positive cells incorporate [³H]thymidine early in the terminal differentiation program. Flow cytometry was used to separate HL-60 cells that had been incubated for 24 h with vitamin D₃ and IGF-I into populations that expressed CD11b and those that did not. As expected from our earlier results (40), incubation of CD11b-negative cells with IGF-I in serum-free medium for an additional 24 h significantly (P < 0.01) increased their incorporation of [³H]thymidine by approximately fivefold (Fig. 8). CD11b-positive cells cultured in medium alone incorporated an amount of [³H]thymidine that was not significantly different from the amount incorporated by CD11b-negative cells cultured in medium alone. More importantly, even in CD11b-positive cells, IGF-I increased the incorporation of [³H]thymidine by 4-fold (P < 0.05) (Fig. 8). These data are consistent with the idea that very early during development of macrophages, cell differentiation and cell proliferation can occur simultaneously.

View larger version (35K): [in this window] [in a new window]   FIG. 8.   Enriched populations of CD11b-positive and -negative cells incorporate [3H]thymidine. Following a 24-h treatment with vitamin D3 (1 µM) and IGF-I (100 ng/ml), the cells were sorted by flow cytometry on the basis of expression of CD11b, and both CD11b-negative (>98% negative) and -positive (>98% positive) cells were cultured for another 24 h in serum-free medium alone (Med) or in medium plus IGF-I. Incorporation of [3H]thymidine was increased by IGF-I in both CD11b-negative (P < 0.01; n = 4) and CD11b-positive (P < 0.05; n = 5) cells. Asterisks indicate statistical significance, and values are means and standard errors of the mean.

	DISCUSSION

Top Abstract Introduction Materials and Methods Results Discussion References

The recently discovered CKIs, including the G₁-phase p27^KIP1 and p21^CIP1, not only inhibit cell cycle progression but have been suggested to be required for the differentiation of a variety of cells (43). Unlike skeletal muscle cells, where differentiation programs are inversely related to cell cycle progression (21, 62), we establish here that proliferation and the early events associated with terminal differentiation of promyeloid cells into macrophages are not mutually exclusive processes. Indeed, IGF-I clearly promotes cell differentiation toward the macrophage lineage in vitamin D₃-treated promyeloid cells, as indicated by increasing the expression of the -integrin adhesion molecule CD11b (Fig. 1), the CD14 lipopolysaccharide receptor (Fig. 2), and the mature macrophage-specific esterase enzyme, NAE (Fig. 2). This increase in expression of differentiation markers, which occurs within 24 h (Fig. 2A), is accompanied by an increase in the percentage of cells in the S phase of the cell cycle, a reduction in the percentage of cells in G₀/G₁ (Fig. 3B and C), and a subsequent doubling of the number of cells (Fig. 3A). More importantly, expression of the CKI p27^KIP1 is not induced in the presence of IGF-I as promyeloid cells begin to terminally differentiate towards the macrophage lineage, as determined by acquisition of the differentiation antigen CD11b (Fig. 1 and 4A). Accordingly, IGF-I increases cyclin E expression (Fig. 4B) and maintains the Rb tumor suppressor protein in a hyperphosphorylated state in differentiating macrophages (Fig. 5). Simultaneous analysis of DNA content and cell surface CD11b established that IGF-I enhances macrophage differentiation in all phases of the cell cycle, instead of only those arrested in G₀/G₁ (Fig. 6). A novel double-labeling flow-cytometric analysis at the single-cell level revealed simultaneous expression of CD11b and two different nuclear proliferation markers, PCNA and BrdU, in nearly 75% of myeloid precursor cells as they initiated terminal macrophage differentiation (Fig. 7). Finally, the finding that sorted populations of CD11b-positive cells incorporate significant amounts of [³H]thymidine is consistent with the idea that these cells synthesize DNA (Fig. 8). These data provide clear evidence that complete cell cycle arrest is not required at the onset of myeloid-cell differentiation and establish that the initial stages of terminal macrophage development are characterized by (i) expression of three different markers of macrophage differentiation, (ii) proliferation, (iii) expression of low levels of p27^KIP1, and (iv) maintenance of Rb in a state of hyperphosphorylation. Collectively, the results indicate that p27^KIP1 and hypophosphorylated Rb are not necessary to initiate macrophage differentiation.

These data are consistent with recent results (23) that established a negative relationship between the expression of p27^KIP1 and cyclin E protein during promyeloid proliferation, but they do not support a role for this CKI in the initial events leading from promyeloid cells to macrophages. For example, IL-2 promotes the G₁/S transition by inhibiting p27^KIP1 in T lymphocytes (52). Similarly, induction of p27^KIP1 leads to G₁ cell cycle arrest in cyclic AMP-treated macrophages cultured with CSF-1 (30). Although these data suggest an important role for p27^KIP1 in inhibiting cell cycle entry, they do not directly address the possibility that p27^KIP1 is involved in early maturation events of macrophages. New findings that ectopic overexpression of p27^KIP1 leads to monocyte differentiation in U937 cells in the absence of a priming differentiation signal (41) and that p27^KIP1 is expressed in differentiating HL-60 cells (23) also support a role for p27^KIP1 in monocyte differentiation. The fact that the expression of p27^KIP1 is increased only late in the differentiation program of U937 cells (>48 h) strongly supports a role for this protein in the final stages of macrophage differentiation (41, 66). These data showing a direct role of p27^KIP1 in leading to cell cycle arrest in both T cells and macrophages is consistent with the notion that this CKI is somehow involved in promoting the differentiation of hematopoietic cells. In contrast, our experiments examined the potential contribution of p27^KIP1 at much earlier stages of myeloid-cell differentiation in a progenitor myeloid cell rather than the more differentiated monocytic U937 cells. Our rationale for this objective was that although vitamin D₃ has been reported to induce the expression of p27^KIP1, this protein is not detectable at significant levels until 4 days after initiation of cellular differentiation in HL-60 cells (23, 66). Recent studies on expression of the CKI p21 in normal myeloid-cell differentiation found a similar result for human primary CD34⁺ progenitors. Indeed, expression of p21 is not apparent until 9 to 12 days after initiation of differentiation process whereas expression of a characteristic leukocyte maturation marker, the granulocyte colony-stimulating factor receptor, starts on days 3 to 6 (63). Our data extend these results by showing that in the presence of IGF-I, p27^KIP1 is not induced during the first 48 h of macrophage differentiation (Fig. 4). However, during this initial 24- to 48-h period, the cells had clearly initiated their terminal differentiation program, as assessed by expression of CD11b, CD14, and NAE (Fig. 1 and 2). However, in more differentiated human U937 cells, expression of p21^CIP1 at both the mRNA and protein levels is increased by 4 h following addition of vitamin D3 (41). It is therefore possible that this CDK inhibitor, rather than p27^KIP1, regulates the development of more mature macrophages.

Recent evidence suggests that induction of CKIs such as p21^CIP1 and p27^KIP1 may be more important for maintenance of terminally differentiated cells in the G₀ phase of the cell cycle than for the initiation of differentiation, and our results are not inconsistent with this hypothesis. Expression of the muscle-specific protein MyoD transactivates the p21^CIP1 promoter and induces the expression of its transcripts (21, 46). Moreover, induction of p21^CIP1 is correlated with terminal cell cycle arrest in several fully differentiated cell lineages, including muscle, skin, cartilage, and nasal epithelial cells, in a p53-independent fashion (54). Accordingly, induction of p21^CIP1 occurs as a consequence of a terminal differentiation event in normal epithelial cells, and one major consequence of this CDK inhibition is an increase in the growth-inhibitory activity of hypophosphorylated Rb protein. However, new data have now demonstrated an inhibitory role of p21^CIP1 in the differentiation of primary mouse keratinocytes, suggesting a distinct function for this CKI in cell development that can be separated from its effects on cell cycle control (15). Indeed, cell cycle regulators have been shown to expand their role in cell differentiation. For example, Rb protein appears to direct the differentiation program in myoblasts (53), adipocytes (11), and hematopoietic cells (7) because these cells fail to differentiate in the absence of Rb protein. Thus, it is possible that the CKI p27^KIP1, which is induced long after the majority of promyeloid cells have initiated their differentiation program (Fig. 1 and 4A) (23), plays a role in maintaining the postmitotic state of differentiated cells by preventing them from reentering the cell cycle. Differentiation antigens expressed later in the development of hematopoietic cells may function to induce the expression of p27^KIP1 and therefore maintain mature, terminally differentiated macrophages in the G₀/G₁ phase of the cell cycle.

In contrast to previous reports (5, 9), we used a defined culture system, which excluded FBS, to differentiate vitamin D₃-treated HL-60 promyeloid cells into macrophages. Although IGF-I alone failed to induce terminal macrophage differentiation, addition of IGF-I to vitamin D₃-treated cells potently increased the expression of three different mature macrophage antigens, including CD11b. This differentiation antigen is induced by a novel nuclear transcription factor, MS-2, during monocyte differentiation (16). Similarly, vitamin D₃ induces macrophage differentiation and, after several days, causes cell cycle arrest. This late exit from the cell cycle was recently shown to be associated with a reduction in the expression of cyclin E (23), a decline in the expression of DNA polymerase  cofactor/PCNA (26) and hypophosphorylation of the Rb tumor suppressor protein (73). In contrast, we investigated some of the earliest events associated with macrophage differentiation and separated the effects of vitamin D₃ from those of FBS, which contains abundant amounts of IGF-I. This 70-amino-acid peptide is well known to be a mitogenic proliferation factor for many types of hematopoietic cells (35, 69). IGF-I is well known to exert its proliferative actions as a G₁ progression factor very early in the cell cycle near the G₀-G₁ interface (2). Since by 24 h most of the CD11b-positive cells had already passed completely through their S phase, it is likely that at least some of the CD11b-positive cells reentered the cell cycle and incorporated [³H]thymidine in their second round of DNA synthesis. Our results also confirm that IGF-I increases PCNA expression in HL-60 cells (55). Indeed, an intact IGF-I receptor is required for simian virus 40 T-antigen-induced transformation in 3T3-like cells (6) and for the expression of PCNA in platelet-derived growth factor-induced fibroblasts (47). Although IGF-I is likely to act largely by maintaining the hyperphosphorylated state of Rb tumor suppressor protein (57) and by increasing cyclin E expression (70), the regulation of Rb phosphorylation and cyclin E by IGF-I in hematopoietic cells has remained unknown. Here we clearly establish that IGF-I increases the expression of cyclin E and the phosphorylation of Rb tumor suppressor protein in promyeloid cells. More importantly, these events occur early (<24 h) and at the same time as these cells are differentiating from promyeloid cells into the macrophage lineage.

The newly identified c-Mpl ligand thrombopoietin promotes megakaryocyte progenitor proliferation at the same time as expression of the differentiation marker gpIb, a component of the platelet von Willebrand factor receptor (31). Simultaneous expression of proliferation and differentiation markers in these cells suggests a critical role of c-Mpl ligand in both events and supports our results that hematopoietic cells can simultaneously differentiate and continue to divide. Similarly, c-kit ligand has now been reported to stimulate the proliferation of human megakaryocytes by increasing the expression of cyclin A and the ratio of hyperphosphorylated to hypophosphorylated Rb protein (61). Incubation with c-kit ligand simultaneously leads to increased expression of IIb/IIIa platelet-related glycoprotein (gpIIb, IIIa), indicating enhanced megakaryocytic differentiation. Likewise, cytokine-regulated proliferation of human hematopoietic stem cells occurs concomitantly with induction of the ₁-integrin's very late antigens 4 and 5 (VLA-4 and VLA-5 [39]), which suggests that maturation of these pluripotent blood stem cells is functionally linked to cell growth. In addition, during lymphocyte differentiation, T-cell-dependent B-cell activation induces isotype switching, and this event has recently been shown to be positively related to the division cycle number (24). The correlation between the percentage of IgG1-positive B cells and division number was independent of time after stimulation, arguing for a requirement of cell division during B-cell maturation. Interestingly, rapamycin, which abrogates the activation of p34^cdc2 and p33^CDK2 necessary for entry into S phase, also inhibits the differentiation of normal B lymphocytes into Ig-secreting plasma cells (1). Taken together, these reports support our findings with myeloid progenitors that cell growth and early differentiation events occur simultaneously.

Collectively, these data clearly show that IGF-I promotes macrophage differentiation and that this process occurs concomitantly with progression through the cell cycle. The finding that a single cell can express both differentiation and proliferation antigens early in the development of myeloid cells unequivocally argues that these two cellular processes are not exclusive events. Indeed, the IGF-I-induced elevation of cyclin E expression, hyperphosphorylation of Rb, and inhibition of CKI p27^KIP1 show that permanent withdrawal from the cell cycle does not necessarily precede cell differentiation. Indeed, nearly 75% of early-differentiated promyeloid cells express PCNA and incorporate BrdU, and purified populations of CD11b-positive cells incorporate substantial amounts of [³H]thymidine. Thus, both p27^KIP1 and hypophosphorylation of Rb are not induced at the initiation of the terminal differentiation program in promyeloid cells. Since the cells acquire p27^KIP1 and Rb hypophosphorylation later in their differentiation program, the expression of these proteins is likely to be of clinical relevance because disruption of either Rb (25) or CKI (22) function results in terminally differentiated cells reentering the cell cycle during carcinogenesis.