Isoform-specific induction of the low-density lipoprotein receptor gene by platelet-derived growth factor

We investigated the effect of recombinant platelet-derived growth factor (PDGF) isomers on low-density lipoprotein (LDL) receptor gene expression and compared this with two indexes of cell growth response: expression of the immediate early gene c-myc and the rate of DNA synthesis. In human skin fibroblasts and NIH 3T3 cells, the PDGF-BB homodimer was more effective in inducing the LDL receptor gene and cell growth response compared with the PDGF-AA homodimer. The second messenger pathways utilized by PDGF receptors for enhancing LDL receptor gene response could, however, be dissociated from those utilized for enhancing c-myc gene response and were insensitive to inhibitors of tyrosine phosphorylation. Inhibition of tyrosine kinase activity inhibited c-myc gene response to PDGF-BB at 10(-8) M but had little effect on LDL receptor gene response. Such inhibition increased expression of the LDL receptor gene in the presence of the PDGF-AA isomer. Our results indicate that the response of the LDL receptor gene to PDGF isoforms reflects cellular growth response. However, different transduction pathways are utilized for PDGF activation of the c-myc and LDL receptor genes in mesenchymal cells.