Down-regulation of platelet-derived growth factor receptor expression during terminal differentiation of 3T3-L1 pre-adipocyte fibroblasts.

The transcription and expression of platelet-derived growth factor (PDGF) receptors (PDGFRs) is down-regulated as a consequence of entry into the replicative cell cycle (Vaziri, C., and Faller, D. V. (1995) Mol. Cell. Biol. 15, 1244-1253). In this study, we have investigated the expression of PDGFRs during terminal differentiation, a process in which cells exit from the cell cycle. When treated with appropriate hormonal stimuli, 3T3-L1 fibroblasts initiate a differentiation program resulting in conversion to lipid-accumulating, adipocyte-like cells. Pre-adipocytes express amounts of PDGFalphaR and PDGFbetaR mRNA and protein that are similar to levels expressed in other murine 3T3 fibroblasts. In contrast, the expression of both alpha and beta receptor transcripts is greatly reduced in differentiated 3T3-L1 cells. The loss of PDGFR mRNA following induction of differentiation precedes morphological conversion as well as the induction of many adipocyte-specific genes. The amounts of cell surface PDGFR protein diminish in parallel with the mRNA levels during differentiation, as shown by Western blotting and PDGF-binding assays. The reduced expression of PDGFRs does not reflect a general down-regulation of growth factor receptors, as expression of the type 1 FGFR is unaffected by terminal differentiation. The PDGFbetaR promoter drives strong expression of a luciferase reporter gene in pre-adipocytes, but not in differentiated cells, indicating that the decrease in PDGFR expression following induction of differentiation is a transcriptionally regulated event. Decreased PDGFR expression in differentiated cells is associated with impaired biological responsiveness to PDGF, as shown by reduced activation of mitogen-activated protein-kinase following PDGF stimulation, and decreased chemotactic responsiveness to PDGF. Our data suggest that PDGFR down-regulation is an important mechanism for reducing PDGF-responsiveness in terminally differentiated 3T3-L1 cells.