Protein kinase C-mediated inhibition of cyclin A expression in human vascular endothelial cells

Proliferation of cultured human vascular endothelial cells may be negatively regulated by the protein kinase C (PKC) pathway, because phorbol 12-myristate, 13-acetate (PMA) inhibits DNA synthesis and cell population doubling in PKC-retaining cells, but not in cells depleted of PKC by a long-term exposure to PMA. We investigated the mechanism through which PKC arrests the cell cycle with regard to cyclin A, which has been reported to play a key role in G₁/S progression activating CDK2. Cyclin A mRNA was elevated from late G₁ in accorance with the protein expression, which reached the maximal level during the S phase. PMA added at late G₁ potently reduced the levels of cyclin A mRNA and the protein in concentration-dependent manners parallel to its effect on the proliferation. However, it failed to inhibit the expression in PKC-depleted cells. The mRNA reduction by PMA was due to inhibition of the transcription. The PMA effects were mimicked by multiple doses of 1,2-dioctanoylglycerol. These findings suggest that PKC inhibits G₁/S progression through suppression of cyclcin A gene transcription in endothelial cells.