CONTROL OF ADIPOCYTE DIFFERENTIATION BY CCAAT ENHANCER BINDING PROTEIN-ALPHA (C/EBP-ALPHA)/

Upon differentiation of 3T3-L1 preadipocytes into adipocytes transcrip tion of many adipose-specific genes is coordinately activated, A diffe rentiation-induced factor, later identified as C/EBP alpha, binds to a nd transactivates the promoters of these genes, Vector-directed expres sion of antisense C/EBP alpha RNA in preadipocytes blocked expression of C/EBP alpha, as well as adipose-specific mRNAs, and also prevented cytoplasmic triglyceride accumulation. Rescue of the 'adipocyte phenot ype' was accomplished by transfection of the antisense cells with a co mplementary sense C/EBP alpha RNA expression vector, Using an IPTG-ind ucible double-vector LacSwitch C/EBP alpha expression system, it was f ound that differentiation can be induced without exogenous hormone ind ucers. These findings indicate that C/EBP alpha is not only required, but is sufficient, to trigger differentiation of 3T3-L1 preadipocytes. The C/EBP alpha gene promoter possesses a C/EBP binding site through which C/EBP alpha autoactivates its own expression, A nuclear protein referred to as CUP (C/EBP undifferentiated protein) that binds to a bi partite element in the C/EBP alpha promoter just 5' to the C/EBP bindi ng site has been purified and characterized, During differentiation of preadipocytes, expression of CUP activity decreases as expression of C/EBP alpha increases. Evidence suggests that a CUP-containing protein complex bridges between the CUP (repression) and C/EBP (autoactivatio n) elements in the promoter and may maintain the C/EBP alpha gene in t he repressed state prior to differentiation.