Opposing effects of δ- and ζ-protein kinase C isozymes on cardiac fibroblast proliferation: use of isozyme-selective inhibitors

Neonatal primary cardiac fibroblasts in defined medium continue to proliferate. Here, we show that phorbol ester inhibited and transforming growth factor-β1 (TGFβ1) stimulated this fibroblast proliferation. Cardiac fibroblasts contain six protein kinase C (PKC) isozymes: α-, δ-, ε-, βI-, βII-, and ζ-PKC. To evaluate the effect of different PKC isozymes on the proliferation of these cells, we used isozyme-selective PKC inhibitors. Inhibition of endogenous δ-PKC with δV1-1, an isozyme-selective translocation inhibitor, resulted in increased basal thymidine incorporation by 58 ± 12% of control cells, but did not affect TGFβ1-induced cell growth. Inhibition of endogenous ζ-PKC in neonatal rat cardiac fibroblasts with ζ-pseudosubstrate, a selective inhibitor for the atypical PKC isozymes, revealed an opposite effect; this inhibitor reduced basal growth to 45 ± 11% and TGFβ1-induced growth to 61 ± 10%. Other isozyme-specific inhibitors used in this study did not alter basal or TGFβ1-stimulated fibroblast growth. Taken together, our data provide evidence that δ-PKC inhibits and ζ-PKC stimulates proliferation of neonatal rat cardiac fibroblasts.