A role for C/EBP beta in regulating peroxisome proliferator-activated receptor gamma activity during adipogenesis in 3T3-L1 preadipocytes

The differentiation of 3T3-L1 preadipocytes is regulated in part by a casca de of transcriptional events involving activation of the CCAAT/enhancer-bin ding proteins (C/EBPs) and peroxisome proliferator-activated receptor gamma (PPAR gamma) by dexamethasone (DEX), 3-isobutyl-1-methylxanthine (MIX), an d insulin. In this study, we demonstrate that exposure of 3T3-L1 preadipocy tes to DEX and insulin fails to induce adipogenesis as indicated by a lack of C/EBP alpha, PPAR gamma2, and adipose protein 2/fatty acid-binding prote in expression; however, PPAR gamma1 is expressed. Treatment of these MM-def icient cells with a PPAR gamma ligand, troglitazone, induces C/EBP alpha ex pression and rescues the block in adipogenesis, In this regard, we also sho w that induction of C/EBP alpha gene expression by troglitazone in C3H10T1/ 2 cells ectopically expressing PPAR gamma occurs in the absence of ongoing protein synthesis, suggesting a direct transactivation of the C/EBP alpha g ene by PPAR gamma, Furthermore, ectopic expression of a dominant negative i soform of C/EBP beta (liver-enriched transcriptional inhibitory protein (LI P)) inhibits the induction of C/EBP alpha, PPAR gamma2, and adipose protein 2/fatty acid-binding protein by DEX, MM, and insulin in 3T3-L1 cells witho ut affecting the induction of PPAR gamma1 by DEX, Exposure of LIP-expressin g preadipocytes to troglitazone along with DEX, MM, and insulin induces dif ferentiation into adipocytes. Additionally, we show that sustained expressi on of C/EBP alpha in these LIP-expressing adipocytes requires constant expo sure to troglitazone. Taken together, these observations suggest that inhib ition of C/EBP beta activity not only blocks C/EBP alpha and PPAR gamma2 ex pression, but it also renders the preadipocytes dependent on an exogenous P PAR gamma ligand for their differentiation into adipocytes, We propose, the refore, an additional role for C/EBP beta in regulating PPAR gamma activity during adipogenesis, and we suggest an alternative means of inducing pread ipocyte differentiation that relies on the dexamethasone-associated inducti on of PPAR gamma1 expression.