S-Phase-specific activation of PKCα induces senescence in non-small cell lung cancer cells

Protein kinase C (PKC) has been widely implicated in positive and negative control of cell proliferation. We have recently shown that treatment of non-small cell lung cancer (NSCLC) cells with phorbol 12-myristate 13-acetate (PMA) during G(1) phase inhibits the progression into S phase, an effect mediated by PKC delta-induced up-regulation of the cell cycle inhibitor p21(Cip1). However, PMA treatment in asynchronously growing NSCLC cells leads to accumulation of cells in G(2)/M. Studies in post-G(1) phases revealed that PMA induced an irreversible G(2)/M cell cycle arrest in NSCLC cells and conferred morphological and biochemical features of senescence, including elevated SA-beta-Gal activity and reduced telomerase activity. Remarkably, this effect was phase-specific, as it occurred only when PKC was activated in S, but not in G(1) phase. Mechanistic analysis revealed a crucial role for the classical PKC alpha isozyme as mediator of the G(2)/M arrest and senescence, as well as for inducing p21(Cip1) an obligatory event for conferring the senescence phenotype. In addition to the unappreciated role of PKC isozymes, and specifically PKC alpha, in senescence, our data introduce the paradigm that discrete PKCs trigger distinctive responses when activated in different phases of the cell cycle via a common mechanism that involves p21(Cip1) up-regulation.