Protein Kinase C δ Stimulates Apoptosis by Initiating G₁ Phase Cell Cycle Progression and S Phase Arrest*

Abstract

Overexpression of protein kinase C δ (PKCδ) stimulates apoptosis in a wide variety of cell types through a mechanism that is incompletely understood. PKCδ-deficient cells are impaired in their response to DNA damage-induced apoptosis, suggesting that PKCδ is required to mount an appropriate apoptotic response under conditions of stress. The mechanism through which it does so remains elusive. In addition to effects on cell survival, PKCδ elicits pleiotropic effects on cellular proliferation. We now provide the first evidence that the ability of PKCδ to stimulate apoptosis is intimately linked to its ability to stimulate G₁ phase cell cycle progression. Using an adenoviral-based expression system to express PKCα,-δ, and -ϵ in epithelial cells, we demonstrate that a modest increase in PKCδ activity selectively stimulates quiescent cells to initiate G₁ phase cell cycle progression. Rather than completing the cell cycle, PKCδ-infected cells arrest in S phase, an event that triggers caspase-dependent apoptotic cell death. Apoptosis was preceded by the activation of cell cycle checkpoints, culminating in the phosphorylation of Chk-1 and p53. Strikingly, blockade of S phase entry using the phosphatidylinositol 3-kinase inhibitor LY294002 prevented checkpoint activation and apoptosis. In contrast, inhibitors of mitogen-activated protein kinase cascades failed to prevent apoptosis. These findings demonstrate that the biological effects of PKCδ can be extended to include positive regulation of G₁ phase cell cycle progression. Importantly, they reveal the existence of a novel, cell cycle-dependent mechanism through which PKCδ stimulates cell death.