Interrelationships of platelet-derived growth factor isoform-induced changes in c-fos expression, intracellular free calcium, and mitogenesis

Both increases in c-fos proto-oncogene expression and intracellular free calcium (Ca2+]i) have been implicated as necessary components of the signal transduction pathway by which platelet-derived growth factor (PDGF) stimulates DNA synthesis in cultured BALB/c3T3 fibroblasts. To determine the interrelationship between PDGF-induced increases in c-fos proto-oncogene expression and Ca2+]i, purified, recombinant BB and AA homodimeric isoforms of PDGF were used to evaluate the dose-response relationships and mechanisms of growth factor-induced changes in these two parameters as well as DNA synthesis. Concentration-dependent increases in Ca2+]i, c-fos expression, and 3H]thymidine incorporation were observed with both BB and AA PDGF isoforms. BB PDGF was consistently more potent and efficacious than the AA isoform in eliciting a given response. The Ca2+]i dependency of PDGF-induced increases in c-fos expression and DNA synthesis was determined by pretreatment of cells with agents that inhibit increases in Ca2+]i: BAPTA, Quin-2, and TMB-8. Under these conditions, PDGF-induced DNA synthesis was blocked, whereas c-fos expression was enhanced. Conversely, in cells made deficient in protein kinase C (PKC) activity by prolonged treatment with phorbol ester, BB and AA PDGF-induced c-fos expression was inhibited by 75-80%, while PDGF-induced increases in Ca2+]i and DNA synthesis were unaffected or enhanced. Additionally, the PKC-independent component of PDGF-stimulated c-fos expression was found to be independent of increases in Ca2+]i. These data suggest that 1) both BB and AA PDGF isoforms elicit alterations in Ca2+]i and c-fos proto-oncogene expression through the same or similar mechanisms in BALB/c3T3 fibroblasts, 2) PDGF-stimulated increases in Ca2+]i are not required for c-fos expression, and 3) distinct pathways regulate PDGF-induced c-fos expression and mitogenesis, with c-fos expression being substantially PKC-dependent yet Ca2+]i-independent, while mitogenesis is Ca2+]i-dependent yet PKC-independent.