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Previous Article | Next Article 
Molecular and Cellular Biology, July 2004, p. 6231-6240, Vol. 24, No. 14
0270-7306/04/$08.00+0 DOI: 10.1128/MCB.24.14.6231-6240.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Beth Israel Deaconess Medical Center,1 Department of Cell Biology, Harvard Medical School,4 Department of Cardiology, Boston Children's Hospital, Boston, Massachusetts,2 Friedrich Miescher Institute for Biomedical Research, Basel, Switzerland3
Received 12 April 2004/ Returned for modification 23 April 2004/ Accepted 28 April 2004
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ABSTRACT |
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INTRODUCTION |
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S6Ks have been implicated as important regulators of body and organ size. Deletion of the dS6K gene in the insect Drosophila melanogaster resulted in a high incidence of embryonic lethality, and surviving adults displayed a severe reduction in body size (27). Deletion of S6K1 in mice was not lethal, but mice were approximately 20% smaller at birth and this was maintained throughout adulthood (39). Furthermore, all organs examined were proportionately smaller. The authors suggested that the phenotype was more dramatic in Drosophila than in mice because Drosophila only expresses one form of S6K. By contrast, mice also express S6K2, and this could possibly compensate, in part, for the loss of S6K1. More recently, the characterization of S6K1–/– S6K2–/– mice was reported (30). Absence of both S6K1 and S6K2 impaired animal viability, and mice were similar in size to that described for S6K1–/–.
In vitro and in vivo models of cardiac hypertrophy have suggested that S6Ks play a key role in the stimulation of protein synthesis in the heart. In isolated cardiac myocyte models of hypertrophy (induced by angiotensin II, phenylephrine, or insulin) rapamycin, which inactivates S6Ks via mTOR, inhibited protein synthesis (2, 35, 48). In mice, our investigators previously showed that aortic banding, exercise training, or transgenic expression of insulin-like growth factor 1 receptor (IGF1R) or constitutively active phosphoinositide 3-kinase (caPI3K) induced cardiac hypertrophy (25, 40, 42). In each model, S6K1 activity and/or S6 phosphorylation was elevated in the heart. By contrast, mice expressing a dominant-negative PI3K (dnPI3K) mutant in the heart had significantly smaller hearts, and S6K1 activity and S6 phosphorylation were depressed (40). Furthermore, rapamycin can attenuate and regress pressure overload-induced cardiac hypertrophy (24, 42). Together, these studies suggest that hypertrophic stimuli regulate heart size, at least in part, by the activation of S6Ks.
Despite reasonable evidence to suggest that S6Ks play a key role in determining heart size, it was not clear whether S6Ks alone are critical regulators for the induction of cardiac hypertrophy. The aim of the present study was to determine whether S6Ks regulate heart size in vivo and whether S6Ks are critical effectors for the development of physiological or pathological cardiac hypertrophy. For this purpose we (i) generated and characterized cardiac-specific S6K1 and S6K2 transgenic mice, (ii) subjected S6K1–/–, S6K2–/–, and S6K1–/– S6K2–/– mice to a pathological stress (aortic banding) or a physiological stress (exercise training), and (iii) genetically crossed transgenic and knockout mice with IGF1R and PI3K transgenic mice.
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MATERIALS AND METHODS |
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The HA-S6K2 eukaryotic expression plasmids encoding WT and KD alleles of human S6K2 (54-kDa isoform) have also been described previously (22, 23). The cDNA inserts for WTS6K1, KDS6K1, RRS6K1, WTS6K2, and KDS6K2 were cloned into a SalI-digested 
-myosin heavy chain promoter construct (clone 26; a gift from J. Robbins) (13), and cardiac-specific transgenic mice were generated as previously described (40). Animal care and experimentation were approved by the Institutional Animal Care and Use Committee of the Beth Israel Deaconess Medical Center.
Generation of S6K1–/–, S6K2–/–, and S6K1–/– S6K2–/– mice. S6K1–/–, S6K2–/–, and S6K1–/– S6K2–/– mice were previously generated and characterized (30, 39). During postnatal life, S6K1–/– and S6K1–/– S6K2–/– mice are approximately 20% smaller by weight than wild-type mice (S6K+/+), whereas S6K2–/– mice are not significantly different from S6K+/+ mice (30, 39). Organ weights are proportionately smaller in S6K1–/– and S6K1–/– S6K2–/– mice.
Injection of IGF1. Intravenous injection of IGF1 to mice results in the activation of S6K1 and S6 phosphorylation in the heart. To examine whether loss of S6K1, S6K2, or both altered the activation of S6K1 and S6 phosphorylation, mice were injected with IGF1. S6K+/+, S6K1–/–, S6K2–/–, and S6K1–/– S6K2–/– mice were anesthetized with intraperitoneal injections of 2,2,2-tribromoethanol (Avertin; Aldrich). IGF1 (0.5 mg/kg of body weight; rhIGF1; gift from Genentech Inc.) or the same volume of phosphate-buffered saline (PBS) was intravenously injected via a jugular vein for 15 min as described elsewhere (41). On completion, hearts were harvested.
Ascending aortic constriction. Ascending aortic constriction was performed in male mice at 11 to 13 weeks of age, as described previously (42, 46).
Swimming training. Groups of 14 to 16 mice at 8 to 12 weeks of age were subjected to chronic swimming training for 4 weeks as described elsewhere (26).
Administration of rapamycin. Rapamycin is a macrocylic triene antibiotic which has effects on growth by forming a gain-of-function inhibitory complex with FKBP12 (FK506-binding protein) (4, 5, 47). This complex binds to mTOR and inhibits its function. Rapamycin is a potent inhibitor of S6K1, preventing activation of S6K1 by all known agonists at subnanomolar concentrations (4).
To confirm that any changes in heart size induced by the S6K transgenes were specifically due to changes in S6K activity, rapamycin (4 mg/kg/day; gift from Wyeth-Ayerst) was administered to S6K transgenics as previously described (41). The solvent for rapamycin was 0.1% sodium carboxymethylcellulose-0.125% polysorbate 80 in water. Rapamycin or solvent was intraperitoneally administered to nontransgenic (NTg) or S6K transgenic mice from 3 to 4 weeks of age. Mice were sacrificed at 4 weeks, heart weight was measured, and S6K1 activity and S6 phosphorylation were measured in heart lysates.
Echocardiography. Echocardiography was performed as described elsewhere (26). For aortic banding studies, to evaluate the degree of stenosis, the pressure gradient across the constriction was assessed using Doppler echocardiography. A nonimaging Doppler pencil transducer (continuous wave) was placed at the apex and orientated towards the proximal ascending aorta. The peak velocity (in meters per second) was measured, and the maximum instantaneous gradient (millimeters of Hg) was calculated using the following Bernoulli equation: pressure gradient = 4 x (velocity)2.
Histological analysis. Heart sections were cut and stained with Masson trichrome as described elsewhere (40).
Biochemical analysis. Preparation of heart lysates and immunoblotting were performed as previously described (40). S6K1 activity and phosphorylation of S6 (235/236) were examined as described elsewhere (26, 40, 41). S6K2 transgene expression was confirmed with an anti-N-terminal S6K2 antibody (30). S6K2 activity was examined in the same way as S6K1 except that immunoprecipitation was performed with an antihemagglutinin (anti-HA) antibody (F-7; Santa-Cruz Biotechnology, Inc.).
Northern blot analysis.
Northern blot analysis was performed as previously described (40). Total RNA (10.0 µg) was electrophoresed in 1.3% denaturing formaldehyde agarose gels and blotted onto Hybond N membranes (Amersham). Membranes were probed with atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), -skeletal actin, and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) radiolabeled probes. The probes for ANP (40), BNP (42), -skeletal actin (26), and GAPDH (17) have previously been described.
Cross-breeding of S6K1 transgenics and S6K knockout mice with IGF1R and PI3K transgenic mice. Our investigators have previously generated and characterized cardiac-specific transgenic mice expressing IGF1R (25), caPI3K, or dnPI3K (40). To genetically determine the relationship of S6Ks with the IGF1R-PI3K pathway, we performed two independent analyses: (i) WTS6K1 and RRS6K1 mice were crossed with dnPI3K mice, and (ii) S6K1–/– and S6K1–/– S6K2–/– knockout mice were crossed with IGF1R and caPI3K transgenic mice.
Statistical analysis. Results are expressed as means ± the standard errors of the means (SEM). When comparing groups, statistical significance was determined using one-way analysis of variance. If the analysis of variance showed significance (P < 0.05), it was followed by the Fisher's projected least significant difference post hoc test. The significance level was P < 0.05.
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RESULTS |
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-myosin heavy chain promoter construct). Six lines of transgenic mice per gene construct were initially produced. For confirmation of transgene product in each of the transgenic lines, cardiac tissue lysates from transgenic mice were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and probed with an anti-HA antibody and an anti-S6K1 or anti-S6K2 antibody (Fig. 1). Transgenic lines with significant transgene expression were subsequently analyzed. The heart weight/body weight (HW/BW) ratios of WTS6K1 and RRS6K1 transgenic mice were approximately 10 and 15% greater, respectively, compared to NTg mice at 3 months of age (Table 1). There was no evidence of cardiomyopathic changes, such as necrosis, fibrosis, or myocyte disarray, in hearts from mice at 3 months of age (data not shown), cardiac function as assessed by echocardiography was normal (Table 2), and life span appeared normal.
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S6K activity and S6 phosphorylation in S6K1 and S6K2 transgenic mice. To confirm the activity of the transgene products in S6K1 transgenic mice, heart tissue lysates were immunoprecipitated with a C-terminal anti-S6K1 antibody and subjected to an in vitro kinase assay using glutathione S-transferase-S6 as a substrate. S6K1 activity in heart lysates from WTS6K1 and RRS6K1 transgenics was significantly elevated compared to activity in NTg and KDS6K1 animals (Fig. 2A) (WTS6K1, 6.6-fold ± 1.2-fold, n = 7 from four separate experiments; RRS6K1, 11.4-fold ± 2.6-fold, n = 7 from three separate experiments). At the time of these experiments, S6K1 was thought to be primarily responsible for the phosphorylation of S6. Unexpectedly, phosphorylation of S6 was not significantly elevated in hearts from WTS6K1 and RRS6K1 mice (Fig. 2B).
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, AMPK, JNK11, and GSK3ß) at 1 µM, a concentration 10- to 20-fold higher than that required to inhibit mTOR in cell-based assays (7). The HW/BW ratios for WTS6K1 and RRS6K1 transgenics were significantly elevated compared to that in NTg littermates at 4 weeks of age (Fig. 4A). The HW/BW ratio for rapamycin-treated WTS6K1 mice was significantly smaller than that of vehicle-treated WTS6K1 mice and was not different from that of NTg mice (Fig. 4A). Rapamycin reduced S6K1 activity and S6 phosphorylation in the hearts of WTS6K1 mice to a similar degree to rapamycin-treated NTg (Fig. 4B). These results suggest that the cardiac hypertrophy observed in WTS6K1 transgenics was due to overexpression of WTS6K1. In RRSK1 transgenics, rapamycin caused some reduction in the HW/BW ratio and S6K1 activity (Fig. 4). However, the HW/BW ratio and S6K1 activity of RRS6K1 transgenics were still significantly greater than those of NTg mice. Unexpectedly, despite the modest effect of rapamycin on S6K1 activity in hearts of RRS6K1 transgenics, S6 phosphorylation was completely inhibited (Fig. 4B). Because rapamycin also inhibits S6K2 activity (18), these data suggest that S6K2 may have a primary role in mediating S6 phosphorylation in vivo rather than S6K1.
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-skeletal actin was elevated in hearts from RRS6K1 transgenic mice (Fig. 6A). Consistent with these findings, the expression levels of ANP and -skeletal actin were depressed in S6K1–/– mice, and there was a trend of lower BNP (Fig. 6B). ANP but not BNP or -skeletal actin was elevated in WTS6K1 transgenics. The reason for this finding is not clear.
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45%; RRS6K1, 41%; Fig. 7A), but RRS6K1 transgenics displayed a decreased response to aortic banding (NTg, 50% increase; RRS6K1, 20% increase; Fig. 7B). The degree of aortic stenosis, measured by the aortic pressure gradients (AoPG) across the bands of NTg and RRS6K1 transgenic mice, was similar (31.6 ± 2.9 and 26.6 ± 1.6 mm Hg, respectively). The reason for the blunted hypertrophic response in RRS6K1 transgenics subjected to aortic banding is not clear. We recently reported a similar phenomenon in IGF1R transgenics (25). This may be related, in part, to some signaling molecules playing distinct roles for the induction of pathological and physiological cardiac hypertrophy (26).
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(i) Cross-breeding WTS6K1 and RRS6K1 with dnPI3K transgenic mice. Because S6K1 is considered to be downstream of PI3K, we hypothesized that the S6K1 transgenes would rescue the fall in S6K1 activity in hearts of dnPI3K mice and would be sufficient to rescue the small heart phenotype of dnPI3K transgenics. We previously showed that expression of a constitutively active Akt mutant (Akt is a downstream effector of PI3K) was able to rescue the small heart phenotype of dnPI3K transgenics (41). In the present study, S6K1 activity was lower in dnPI3K transgenics than in NTg (Fig. 9A), as previously shown (40). S6K1 activity was elevated in double transgenic mice expressing both the dnPI3K mutant and RRS6K1 transgene and not different from that observed in RRS6K1 transgenics (Fig. 9A). A similar finding was apparent in double transgenic mice expressing the dnPI3K mutant and WTS6K1 transgene (data not shown). Rescuing S6K1 activity had no impact on heart size. The HW/BW ratio of double transgenics was not different from that of dnPI3K transgenics alone (Fig. 9B).
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DISCUSSION |
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Transgenic expression of WTS6K1 induced a modest increase in heart size, which could be attenuated by treatment with rapamycin. By contrast, hypertrophy induced by overexpression of a rapamycin-resistant mutant (RRS6K1) was not significantly affected by rapamycin treatment, suggesting that the increase in heart size in S6K1 transgenics was due to overexpression of S6K1. Transgenic expression of S6K2 had no significant effect on heart size. This is consistent with previous reports that demonstrated that deletion of S6K1 reduced body size, whereas deletion of S6K2 had no effect (30, 39). When we began these studies, reports suggested that S6K1 regulated growth via the phosphorylation of S6. S6 phosphorylation was inhibited in embryonic stem cells with S6K1 deleted (45). Thus, it was somewhat surprising that S6 phosphorylation was not elevated in hearts of S6K1 transgenics. However, very recent work characterizing S6K2–/– and S6K1–/– S6K2–/– mice suggests that S6K2 rather than S6K1 is largely responsible for the phosphorylation of S6 (30). This is consistent with our findings in the heart. S6 phosphorylation was not elevated in hearts of S6K1 transgenics, and S6 phosphorylation increased normally in S6K1–/– mice in response to IGF1. By contrast S6 phosphorylation under basal conditions, as well as in response to IGF1 stimulation, was markedly depressed in S6K2–/– mice. Although, based on these data, we expected S6 phosphorylation to be elevated in hearts of S6K2 transgenics. The reason why S6 phosphorylation was not elevated in S6K2 transgenics is not clear. Though, it could be due to the relatively modest increase in S6K2 activity (approximately twofold) in our transgenic mice. Alternatively, overexpression of S6K2 alone may not be sufficient to increase S6 phosphorylation. As eluded to by Pende et al. (30), body size is reduced in S6K1–/– mice with little effect on S6 phosphorylation, whereas body size of S6K2–/– mice is normal but S6 phosphorylation is reduced. The authors suggested that S6K1 and S6K2 may exert specific functions through distinct substrates.
Cardiac hypertrophy can be induced by pathological stimuli (e.g., pressure or volume overload) or physiological stimuli (e.g., developmental growth, exercise training). Both forms of hypertrophy have distinct characteristics and appear to be mediated by distinct signaling pathways (1, 6, 8, 12, 15, 16, 19, 26, 28, 29, 36, 40, 44, 49). A number of pieces of evidence led us to hypothesize that deletion of S6Ks would modulate, at least in part, the hypertrophic response to aortic banding, exercise training, or IGF1R- or caPI3K-induced hypertrophy. Our investigators previously reported that (i) transgenic expression of IGF1R or a caPI3K mutant resulted in cardiac hypertrophy which was associated with increased S6K1 activity and S6 phosphorylation (25, 40), (ii) PI3K was critical for physiological cardiac hypertrophy induced by chronic exercise training (26), and (iii) rapamycin significantly inhibited S6K1 activity and attenuated and regressed cardiac hypertrophy induced by aortic banding (24, 42). However, in contradiction to this evidence, deletion of S6Ks had no significant effect on hypertrophy induced by aortic banding, swimming training, or the IGF1R-PI3K pathway. Some studies in Drosophila have suggested that PI3K and S6K are on distinct and parallel signaling pathways (32, 33). DS6K activation was independent of PI3K activity in cells from Drosophila. Furthermore, enlarged eyes and ommatidia size induced in transgenic flies expressing PI3K were not affected in flies lacking dS6K (33). Our data, demonstrating that S6K1 transgene expression was unable to rescue the small heart phenotype of dnPI3K transgenics, suggest that S6K1 is not responsible for the heart size defect in dnPI3K.
It is noteworthy that functional redundancy of related genes or compensatory upregulation of other genes may mask the effect of a knockout, even if the gene has an important function in the normal animal. Pende et al. reported that deletion of S6K1 and S6K2 in mice uncovered a functional redundant mitogen-activated protein kinase (MAPK)-dependent pathway in mouse embryo fibroblasts and hepatocytes which was able to phosphorylate S6 (30). However, in the present study, an intravenous injection of IGF1 (known to activate the PI3K and MAPK pathway in cardiac myocytes [21, 34]) did not phosphorylate S6 in hearts from S6K1–/– S6K2–/– mice. Thus, at least in the heart, it appears that a MAPK pathway phosphorylating S6 is not present.
While we cannot rule out the possibility that a compensatory pathway in S6K knockout mice may mask the role of S6Ks under normal conditions, data obtained from S6K1 transgenic mice also suggest that S6Ks alone are not the critical regulators of heart size. In dnPI3K transgenic mice with reduced S6K1 activity, rescuing S6K1 activity with the S6K1 transgenes was unable to rescue the small heart phenotype observed in dnPI3K mice. The heart size of double transgenic mice expressing the dnPI3K and S6K1 transgenes was not different from the heart size of mice expressing the dnPI3K mutant alone. Consistent with these data, Stolovich et al. (43) reported that transduction of growth signals into translational activation of terminal oligopyrimidine tract mRNAs was fully reliant on the PI3K pathway but independent of S6K1 and S6 phosphorylation. Finally, we recently reported that an mTOR-independent pathway is able to regulate heart size under certain conditions induced by pressure overload (24).
In summary, we have shown that overexpression of S6K1 modestly increased heart size; that deletion of S6Ks had no significant effect on exercise-, pressure overload-, IGF1R-, or caPI3K-induced cardiac hypertrophy; and that rescuing S6K1 activity in dnPI3K transgenic mice was unable to rescue the small heart phenotype. Together, these studies suggest that S6Ks alone are not critical for the induction of cardiac hypertrophy.
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ACKNOWLEDGMENTS |
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This work was supported by National Institutes of Health grant RO1 HL65742 to S.I.
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FOOTNOTES |
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REFERENCES |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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2. Boluyt, M. O., J. S. Zheng, A. Younes, X. Long, L. O'Neill, H. Silverman, E. G. Lakatta, and M. T. Crow. 1997. Rapamycin inhibits alpha 1-adrenergic receptor-stimulated cardiac myocyte hypertrophy but not activation of hypertrophy-associated genes. Evidence for involvement of p70 S6 kinase. Circ. Res. 81:176-186.
3. Brown, E. J., and S. L. Schreiber. 1996. A signaling pathway to translational control. Cell 86:517-520.[CrossRef][Medline]
4. Chou, M. M., and J. Blenis. 1995. The 70 kDa S6 kinase: regulation of a kinase with multiple roles in mitogenic signalling. Curr. Opin. Cell Biol. 7:806-814.[CrossRef][Medline]
5. Chung, J., C. J. Kuo, G. R. Crabtree, and J. Blenis. 1992. Rapamycin-FKBP specifically blocks growth-dependent activation of and signaling by the 70 kd S6 protein kinases. Cell 69:1227-1236.[CrossRef][Medline]
6. D'Angelo, D. D., Y. Sakata, J. N. Lorenz, G. P. Boivin, R. A. Walsh, S. B. Liggett, and G. W. Dorn II. 1997. Transgenic Gq overexpression induces cardiac contractile failure in mice. Proc. Natl. Acad. Sci. USA 94:8121-8126.
7. Davies, S. P., H. Reddy, M. Caivano, and P. Cohen. 2000. Specificity and mechanism of action of some commonly used protein kinase inhibitors. Biochem. J. 351:95-105.[CrossRef][Medline]
8. Duerr, R. L., S. Huang, H. R. Miraliakbar, R. Clark, K. R. Chien, and J. Ross, Jr. 1995. Insulin-like growth factor-1 enhances ventricular hypertrophy and function during the onset of experimental cardiac failure. J. Clin. Investig. 95:619-627.
9. Dufner, A., and G. Thomas. 1999. Ribosomal S6 kinase signaling and the control of translation. Exp. Cell Res. 253:100-109.[CrossRef][Medline]
10. Fingar, D. C., S. Salama, C. Tsou, E. Harlow, and J. Blenis. 2002. Mammalian cell size is controlled by mTOR and its downstream targets S6K1 and 4EBP1/eIF4E. Genes Dev. 16:1472-1487.
11. Fumagalli, S., and G. Thomas. 2000. S6 phosphorylation and signal transduction, p. 695-717. In N. Sonenberg, J. W. B. Hershey, and M. B. Mathews (ed.), Translational control of gene expression. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.
12. Geenen, D. L., A. Malhotra, and P. M. Buttrick. 1996. Angiotensin receptor 1 blockade does not prevent physiological cardiac hypertrophy in the adult rat. J. Appl. Physiol. 81:816-821.
13. Gulick, J., A. Subramaniam, J. Neumann, and J. Robbins. 1991. Isolation and characterization of the mouse cardiac myosin heavy chain genes. J. Biol. Chem. 266:9180-9185.
14. Hernandez, J. M., M. J. Fedele, and P. A. Farrell. 2000. Time course evaluation of protein synthesis and glucose uptake after acute resistance exercise in rats. J. Appl. Physiol. 88:1142-1149.
15. Iemitsu, M., T. Miyauchi, S. Maeda, S. Sakai, T. Kobayashi, N. Fujii, H. Miyazaki, M. Matsuda, and I. Yamaguchi. 2001. Physiological and pathological cardiac hypertrophy induce different molecular phenotypes in the rat. Am. J. Physiol. Regul. Integr. Comp. Physiol. 281:R2029-R2036.
16. Izumo, S., B. Nadal-Ginard, and V. Mahdavi. 1988. Protooncogene induction and reprogramming of cardiac gene expression produced by pressure overload. Proc. Natl. Acad. Sci. USA 85:339-343.
17. Kasahara, H., S. Bartunkova, M. Schinke, M. Tanaka, and S. Izumo. 1998. Cardiac and extracardiac expression of Csx/Nkx2.5 homeodomain protein. Circ. Res. 82:936-946.
18. Lee-Fruman, K. K., C. J. Kuo, J. Lippincott, N. Terada, and J. Blenis. 1999. Characterization of S6K2, a novel kinase homologous to S6K1. Oncogene 18:5108-5114.[CrossRef][Medline]
19. Lembo, G., H. A. Rockman, J. J. Hunter, H. Steinmetz, W. J. Koch, L. Ma, M. P. Prinz, J. Ross, Jr., K. R. Chien, and L. Powell-Braxton. 1996. Elevated blood pressure and enhanced myocardial contractility in mice with severe IGF-1 deficiency. J. Clin. Investig. 98:2648-2655.[Medline]
20. Levy, D., R. J. Garrison, D. D. Savage, W. B. Kannel, and W. P. Castelli. 1990. Prognostic implications of echocardiographically determined left ventricular mass in the Framingham Heart Study. N. Engl. J. Med. 322:1561-1566.[Abstract]
21. Liu, T., H. Lai, W. Wu, S. Chinn, and P. H. Wang. 2001. Developing a strategy to define the effects of insulin-like growth factor-1 on gene expression profile in cardiomyocytes. Circ. Res. 88:1231-1238.
22. Martin, K. A., S. S. Schalm, C. Richardson, A. Romanelli, K. L. Keon, and J. Blenis. 2001. Regulation of ribosomal S6 kinase 2 by effectors of the phosphoinositide 3-kinase pathway. J. Biol. Chem. 276:7884-7891.
23. Martin, K. A., S. S. Schalm, A. Romanelli, K. L. Keon, and J. Blenis. 2001. Ribosomal S6 kinase 2 inhibition by a potent C-terminal repressor domain is relieved by mitogen-activated protein-extracellular signal-regulated kinase kinase-regulated phosphorylation. J. Biol. Chem. 276:7892-7898.
24. McMullen, J. R., M. C. Sherwood, O. Tarnavski, L. Zhang, A. L. Dorfman, T. Shioi, and S. Izumo. 2004. Inhibition of mTOR signaling with rapamycin regresses established cardiac hypertrophy induced by pressure overload. Circulation 109:3050-3055.
25. McMullen, J. R., T. Shioi, W. Y. Huang, L. Zhang, O. Tarnavski, E. Bisping, M. Schinke, S. Kong, M. C. Sherwood, J. Brown, L. Riggi, P. M. Kang, and S. Izumo. 2004. The insulin-like growth factor 1 receptor induces physiological heart growth via the phosphoinositide 3-kinase(p110) pathway. J. Biol. Chem. 279:4782-4793.
26. McMullen, J. R., T. Shioi, L. Zhang, O. Tarnavski, M. C. Sherwood, P. M. Kang, and S. Izumo. 2003. Phosphoinositide 3-kinase(p110) plays a critical role for the induction of physiological, but not pathological, cardiac hypertrophy. Proc. Natl. Acad. Sci. USA 100:12355-12360.
27. Montagne, J., M. J. Stewart, H. Stocker, E. Hafen, S. C. Kozma, and G. Thomas. 1999. Drosophila S6 kinase: a regulator of cell size. Science 285:2126-2129.
28. Neri Serneri, G. G., M. Boddi, P. A. Modesti, I. Cecioni, M. Coppo, L. Padeletti, A. Michelucci, A. Colella, and G. Galanti. 2001. Increased cardiac sympathetic activity and insulin-like growth factor-I formation are associated with physiological hypertrophy in athletes. Circ. Res. 89:977-982.
29. Neri Serneri, G. G., M. Boddi, L. Poggesi, I. Simonetti, M. Coppo, M. L. Papa, G. F. Lisi, M. Maccherini, R. Becherini, A. Boncompagni, T. Toscano, and P. A. Modesti. 2001. Activation of cardiac renin-angiotensin system in unstable angina. J. Am. Coll. Cardiol. 38:49-55.
30. Pende, M., S. H. Um, V. Mieulet, M. Sticker, V. L. Goss, J. Mestan, M. Mueller, S. Fumagalli, S. C. Kozma, and G. Thomas. 2004. S6K1–/–/S6K2–/– mice exhibit perinatal lethality and rapamycin-sensitive 5'-terminal oligopyrimidine mRNA translation and reveal a mitogen-activated protein kinase-dependent S6 kinase pathway. Mol. Cell. Biol. 24:3112-3124.
31. Proud, C. G. 2004. mTOR-mediated regulation of translation factors by amino acids. Biochem. Biophys. Res. Commun. 313:429-436.[CrossRef][Medline]
32. Radimerski, T., J. Montagne, M. Hemmings-Mieszczak, and G. Thomas. 2002. Lethality of Drosophila lacking TSC tumor suppressor function rescued by reducing dS6K signaling. Genes Dev. 16:2627-2632.
33. Radimerski, T., J. Montagne, F. Rintelen, H. Stocker, J. van der Kaay, C. P. Downes, E. Hafen, and G. Thomas. 2002. dS6K-regulated cell growth is dPKB/dPI(3)K-independent, but requires dPDK1. Nat. Cell Biol. 4:251-255.[CrossRef][Medline]
34. Ren, J., W. K. Samson, and J. R. Sowers. 1999. Insulin-like growth factor I as a cardiac hormone: physiological and pathophysiological implications in heart disease. J Mol. Cell. Cardiol. 31:2049-2061.[CrossRef][Medline]
35. Sadoshima, J., and S. Izumo. 1995. Rapamycin selectively inhibits angiotensin II-induced increase in protein synthesis in cardiac myocytes in vitro. Potential role of 70-kD S6 kinase in angiotensin II-induced cardiac hypertrophy. Circ. Res. 77:1040-1052.
36. Scheuer, J., and P. Buttrick. 1987. The cardiac hypertrophic responses to pathologic and physiologic loads. Circulation 75:I63-I68.
37. Schluter, K. D., and H. M. Piper. 1999. Regulation of growth in the adult cardiomyocytes. FASEB J. 13(Suppl.):S17-S22.
38. Shah, O. J., J. C. Anthony, S. R. Kimball, and L. S. Jefferson. 2000. 4E-BP1 and S6K1: translational integration sites for nutritional and hormonal information in muscle. Am. J. Physiol. Endocrinol. Metab. 279:E715-E729.
39. Shima, H., M. Pende, Y. Chen, S. Fumagalli, G. Thomas, and S. C. Kozma. 1998. Disruption of the p70(s6k)/p85(s6k) gene reveals a small mouse phenotype and a new functional S6 kinase. EMBO J. 17:6649-6659.[CrossRef][Medline]
40. Shioi, T., P. M. Kang, P. S. Douglas, J. Hampe, C. M. Yballe, J. Lawitts, L. C. Cantley, and S. Izumo. 2000. The conserved phosphoinositide 3-kinase pathway determines heart size in mice. EMBO J. 19:2537-2548.[CrossRef][Medline]
41. Shioi, T., J. R. McMullen, P. M. Kang, P. S. Douglas, T. Obata, T. F. Franke, L. C. Cantley, and S. Izumo. 2002. Akt/protein kinase B promotes organ growth in transgenic mice. Mol. Cell. Biol. 22:2799-2809.
42. Shioi, T., J. R. McMullen, O. Tarnavski, K. Converso, M. C. Sherwood, W. J. Manning, and S. Izumo. 2003. Rapamycin attenuates load-induced cardiac hypertrophy in mice. Circulation 107:1664-1670.
43. Stolovich, M., H. Tang, E. Hornstein, G. Levy, R. Cohen, S. S. Bae, M. J. Birnbaum, and O. Meyuhas. 2002. Transduction of growth or mitogenic signals into translational activation of TOP mRNAs is fully reliant on the phosphatidylinositol 3-kinase-mediated pathway but requires neither S6K1 nor rpS6 phosphorylation. Mol. Cell. Biol. 22:8101-8113.
44. Tanaka, N., T. Ryoke, M. Hongo, L. Mao, H. A. Rockman, R. G. Clark, and J. Ross, Jr. 1998. Effects of growth hormone and IGF-I on cardiac hypertrophy and gene expression in mice. Am. J. Physiol. 275:H393-H399.
45. Tang, H., E. Hornstein, M. Stolovich, G. Levy, M. Livingstone, D. Templeton, J. Avruch, and O. Meyuhas. 2001. Amino acid-induced translation of TOP mRNAs is fully dependent on phosphatidylinositol 3-kinase-mediated signaling, is partially inhibited by rapamycin, and is independent of S6K1 and rpS6 phosphorylation. Mol. Cell. Biol. 21:8671-8683.
46. Tarnavski, O., J. R. McMullen, M. Schinke, Q. Nie, S. Kong, and S. Izumo. 2004. Mouse cardiac surgery: comprehensive techniques for the generation of mouse models of human diseases and their application for genomic studies. Physiol. Genomics 16:349-360.
47. Thomas, G., and M. N. Hall. 1997. TOR signalling and control of cell growth. Curr. Opin. Cell Biol. 9:782-787.[CrossRef][Medline]
48. Wang, L., X. Wang, and C. G. Proud. 2000. Activation of mRNA translation in rat cardiac myocytes by insulin involves multiple rapamycin-sensitive steps. Am. J. Physiol. Heart Circ. Physiol. 278:H1056-H1068.
49. Wettschureck, N., H. Rutten, A. Zywietz, D. Gehring, T. M. Wilkie, J. Chen, K. R. Chien, and S. Offermanns. 2001. Absence of pressure overload induced myocardial hypertrophy after conditional inactivation of Gq/G11 in cardiomyocytes. Nat. Med. 7:1236-1240.[CrossRef][Medline]
50. Zhang, H., J. P. Stallock, J. C. Ng, C. Reinhard, and T. P. Neufeld. 2000. Regulation of cellular growth by the Drosophila target of rapamycin dTOR. Genes Dev. 14:2712-2724.
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5 in each group.