14-3-3γ interacts with and is phosphorylated by multiple protein kinase C isoforms in PDGF-stimulated human vascular smooth muscle cells

It has recently been demonstrated that some members of the 14-3-3 protein family play an important role in signal transduction leading to cellular proliferation. We have previously shown that expression of 14-3-3γ is induced by growth factors in human vascular smooth muscle cells (VSMC). In this study, we cloned the human homolog of 14-3-3γ and observed many potential phosphorylation sites, suggesting the potential for post- translational modification. In VSMC treated with platelet-derived growth factor (PDGF), 14-3-3γ protein was expressed and phosphorylated in an activation-dependent manner. Platelet-derived growth factor-induced phosphorylation could be inhibited by phosphokinase C (PKC) inhibitory compounds, and 14-3-3γ could be phosphorylated in the absence of PDGF by compounds that activate PKC. We also demonstrated interaction between 14-3- 3β and several PKC isoforms (α, β, θ, and δ), implicating these PKC family isoforms as the kinases responsible for PDGF-induced 14-3-3γ phosphorylation. We found that 14-3-3γ interacted with the signal transduction protein Raf-1, suggesting that 14-3-3γ provides a link between this protein and PKC. Thus, 14-3-3γ may represent a signal transduction protein that is regulated transcriptionally and post-transcriptionally by growth factors.