A role for C/EBPβ in regulating peroxisome proliferator-activated receptor γ activity during adipogenesis in 3T3-L1 preadipocytes

The differentiation of 3T3-L1 preadipocytes is regulated in part by a cascade of transcriptional events involving activation of the CCAAT/enhancer-binding proteins (C/EBPs) and peroxisome proliferator-activated receptor gamma (PPAR gamma) by dexamethasone (DEX), 3-isobutyl-1-methylxanthine (MIX), and insulin. In this study, we demonstrate that exposure of 3T3-L1 preadipocytes 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 protein expression; however, PPAR gamma1 is expressed. Treatment of these MM-deficient cells with a PPAR gamma ligand, troglitazone, induces C/EBP alpha expression and rescues the block in adipogenesis, In this regard, we also show 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 gene by PPAR gamma, Furthermore, ectopic expression of a dominant negative isoform of C/EBP beta (liver-enriched transcriptional inhibitory protein (LIP)) 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 without affecting the induction of PPAR gamma1 by DEX, Exposure of LIP-expressing preadipocytes to troglitazone along with DEX, MM, and insulin induces differentiation into adipocytes. Additionally, we show that sustained expression of C/EBP alpha in these LIP-expressing adipocytes requires constant exposure to troglitazone. Taken together, these observations suggest that inhibition of C/EBP beta activity not only blocks C/EBP alpha and PPAR gamma2 expression, but it also renders the preadipocytes dependent on an exogenous PPAR gamma ligand for their differentiation into adipocytes, We propose, therefore, an additional role for C/EBP beta in regulating PPAR gamma activity during adipogenesis, and we suggest an alternative means of inducing preadipocyte differentiation that relies on the dexamethasone-associated induction of PPAR gamma1 expression.