Downregulation of repressive CUP/AP-2 isoforms during adipocyte differentiation

Activation of transcription of the C/EBPalpha (CCAAT/enhancer binding protein alpha) gene is a critical event in the differentiation of 3T3-L1 preadipocytes into adipocytes. The kinetics of this process parallels a decline of AP-2alpha protein (also referred to as CUP, C/EBP undifferentiated protein) and decreased binding of CUP/AP-2alpha to the C/EBPalpha promoter. Mutation of the CUP/AP-2 binding sites in the C/EBPalpha promoter results in increased C/EBPalpha expression. Based on these findings, it appears that decline in AP-2alpha expression is an important early event in the adipocyte differentiation program. In the studies presented here, we identify three mRNAs that encode the repressive CUP/AP-2alpha isoforms expressed in undifferentiated 3T3-L1 preadipocytes. We demonstrate that the kinetics of the decline of these isoforms' expression over the course of differentiation parallels both the decrease in CUP/AP-2alpha DNA binding activity and the increase in C/EBPalpha protein observed in previous studies.