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Mol. Cell. Biol., 08 1996, 4128-4136, Vol 16, No. 8
Z Wu, NL Bucher and SR Farmer
The differentiation of 3T3 preadipocytes into adipocytes is accompanied by
a transient induction of C/EBPbeta and C/EBPdelta expression in response to
treatment of the cells with methylisobutylxanthine (MIX) and dexamethasone
(DEX), respectively. In this report, we demonstrate that peroxisome
proliferator-activated receptor gamma (PPARgamma) expression in 3T3-L1
preadipocytes is induced by MIX and DEX, suggesting that C/EBPbeta and
C/EBPdelta may be involved in this process. Using a tetracycline-responsive
expression system, we have recently shown that the conditional ectopic
expression of C/EBPbeta in NIH 3T3 fibroblasts (beta2 cells) in the
presence of DEX activates the synthesis of peroxisome PPARgamma mRNA.
Subsequent exposure of these cells to PPAR activators stimulates their
conversion into adipocytes; however, neither the expression of C/EBPbeta
nor exposure to DEX alone is capable of inducing PPARgamma expression in
the beta2 cell line. We find that unlike the case for 3T3 preadipocytes,
C/EBPdelta is not induced by DEX in these 3T3 fibroblasts and therefore is
not relaying the effect of this glucocorticoid to the PPARgamma gene. To
define the role of glucocorticoids in regulating PPARgamma expression and
the possible involvement of C/EBPdelta, we have established an additional
set of NIH 3T3 cell lines expressing either C/EBPdelta alone (delta23
cells) or C/EBPdelta and C/EBPbeta together (beta/delta39 cells), using the
tetracycline-responsive system. Culture of these cells in
tetracycline-deficient medium containing DEX, MIX, insulin, and fetal
bovine serum shows that the beta/delta39 cells express PPARgamma and aP2
mRNAs at levels that are almost equivalent to those observed in fully
differentiated 3T3-L1 adipocytes. These levels are approximately threefold
higher than their levels of expression in the beta2 cells. Despite the fact
that these beta/delta39 cells produce abundant amounts of C/EBPbeta and
C/EBPdelta (in the absence of tetracycline), they still require
glucocorticoids to attain maximum expression of PPARgamma mRNA.
Furthermore, the induction of PPARgamma mRNA by exposure of these cells to
DEX occurs in the absence of ongoing protein synthesis. The delta23 cells,
on the other hand, are not capable of activating PPARgamma gene expression
when exposed to the same adipogenic inducers. Finally, attenuation of
ectopic C/EBPbeta production at various stages during the differentiation
process results in a concomitant inhibition of PPARgamma and the adipogenic
program. These data strongly suggest that the induction of PPARgamma gene
expression in multipotential mesenchymal stem cells (NIH 3T3 fibroblasts)
is dependent on elevated levels of C/EBPbeta throughout the differentiation
process, as well as an initial exposure to glucocorticoids. C/EBPdelta may
function by synergizing with C/EBPbeta to enhance the level of PPARgamma
expression.
Copyright © 1996, American Society for Microbiology
Induction of peroxisome proliferator-activated receptor gamma during the conversion of 3T3 fibroblasts into adipocytes is mediated by C/EBPbeta, C/EBPdelta, and glucocorticoids
Department of Biochemistry, Boston University School of Medicine, Massachusetts 02118, USA.
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