The ε Subtype of Protein Kinase C Is Required for Cardiomyocyte Connexin-43 Phosphorylation
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Gap junctions (GJs), composed of connexins, are intercellular channels ensuring electric and metabolic coupling between cardiomyocytes. We have shown previously that an endogenous mitogenic and cardioprotective protein, fibroblast growth factor-2 (FGF-2), decreases cardiomyocyte GJ permeability by stimulating phosphorylation of connexin-43 (Cx43). Identifying the kinase(s) phosphorylating cardiac Cx43 may thus provide a way of modulating cardiac intercellular communication. Because FGF-2 activates receptors linked to protein kinase C (PKC) and mitogen-activated protein kinase, we first investigated participation of these enzymatic systems in Cx43 phosphorylation. The inhibitor PD98059 blocked activation of mitogen-activated protein kinase, but it did not prevent the FGF-2 effects on GJs. In contrast, the PKC inhibitor chelerythrine blocked the effects of FGF-2 on Cx43 phosphorylation and permeability. Because the epsilon-isoform of PKC localizes to plasma membrane sites, we examined whether it is directly involved in the FGF-2-induced Cx43 phosphorylation. In nonstimulated myocytes, PKCepsilon displayed a discontinuous pattern of localization at intercellular contact sites and partial colocalization with Cx43. Treatment with FGF-2 or phorbol 12-myristate 13-acetate induced a more continuous pattern of PKCepsilon distribution, whereas the anti-Cx43 staining appeared to overlap extensively with that of PKCepsilon. In immunoprecipitation experiments using specific anti-Cx43 antibodies, PKCepsilon but not PKCalpha coprecipitated with Cx43. FGF-2 increased levels of coprecipitated PKCepsilon, suggesting increased association between PKCepsilon and Cx43 on stimulation. Transient gene transfer and overexpression of cDNAs coding for truncated or mutated dominant-negative forms of PKCepsilon decreased cardiomyocyte Cx43 phosphorylation significantly. We conclude that PKC mediates the FGF-2-induced effects on cardiac GJs and that PKCepsilon likely interacts with and phosphorylates cardiac Cx43 at sites of intercellular contact.
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