Abstract
Polypeptide growth factors, such as platelet-derived growth factor (PDGF), promote the reinitiation of DNA synthesis and cell growth through multiple intracellular signaling pathways that converge in the nucleus to regulate the activity of transcription factors, thereby controlling the expression of growth-promoting genes. Among them, the AP-1 (activating protein-1) family of transcription factors, including c-Fos and c-Jun family members, plays a key role, as AP-1 activity is potently activated by PDGF and is required to stimulate cell proliferation. However, the nature of the pathways connecting PDGF receptors to AP-1 is still poorly defined. In this study, we show that PDGF regulates AP-1 by stimulating the expression and function of c-Fos through extracellular signal-regulated kinase (ERK). The latter involves the direct phosphorylation by ERK of multiple residues in the carboxyl-terminal transactivation domain of c-Fos, which results in its increased transcriptional activity. Interestingly, the phosphorylation of c-Fos by ERK was required for the ability of PDGF and serum to stimulate the activity of c-Fos as well as AP-1-dependent transcription. Furthermore, we provide evidence that the ERK-dependent activation of c-Fos is an integral component of the mitogenic pathway by which PDGF regulates normal and aberrant cell growth.