Protein kinase C-mediated bidirectional regulation of endothelial cell growth

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In human umbilical vein endothelial cells, the protein kinase C (PKC) stimulation during the early G1 phase leads to potentiations in growth factor-stimulated DNA synthesis, the activation of cdc2 and cdk2 cyclin-dependent kinases, and the mRNA expression of cdc2, cyclins A,D1 and E,but not cdk2 or cdk4. Conversely, the PKC stimulation in the late G1 phase completely inhibits DNA synthesis, the activation of cyclin-dependent kinases, and the mRNA expression of the same set of molecules except cyclin D1. Further, we found that the PKC stimulation bimodally regulates the message levels of E2F1 and B-myb, which are transcription factors implicated in the control of the mammalian cell cycle progression. These results indicate that the PKC signal transduction pathway, depending on the timing of activation in the G1 phase, either positively or negatively regulates the message level of growth-regulating genes that are crucial for the G1 to S phase progression.