The effect of platelet-derived growth factor and adipogenic hormones on the expression of CCAAT/enhancer-binding proteins in 3T3-L1 cells in serum-free conditions.

In the absence of serum and serum substitutes, insulin at high doses together with platelet-derived growth factor BB (PDGF BB), corticosterone, and 3-isobutyl-1-methyl-xanthine is required to stimulate differentiation of 3T3-L1 preadipocytes. Under these conditions the differentiating cells express the CCAAT/enhancer-binding proteins (C/EBP) C/EBPdelta, C/EBPbeta, and C/EBP alpha with a similar sequence as described for differentiating cells in the presence of serum. The major differences detected under serum-free conditions are as follows: (a) PDGF BB is the major stimulator of the expression of the C/EBPbeta isoform liver activator protein (LAP). (b) The expression of LAP is also increased in preconfluent, proliferating cells due to the treatment with PDGF BB. (c) A small protein of 20 kDa (p20C/EBPbeta is detected with the anti-C/EBPbeta antibody. It is synthesized at high levels in such cells, which subsequently express high levels of the differentiation markers C/EBP alpha and glycerol-3-phosphate dehydrogenase. (d) Treatment of cells with fibroblast growth factor-2 (bFGF) in addition to adipogenic hormones results in differentiation and C/EBP alpha expression only to a very moderate extent as compared to treatment with PDGF BB but leads to a strong expression of both C/EBPbeta and C/EBPdelta.