The α isoform of protein kinase C mediates phorbol ester-induced growth inhibition and p21 cip1 induction in HC11 mammary epithelial cells

To clarify the roles of specific isoforms of PKC in regulating growth and cell cycle progression of the HC11 mammary epithelial cell line, we investigated the effects of activating endogenous PKC isoforms with the phorbol ester tumor promoter TPA, and also the effects of TPA on genetically engineered cells containing increased levels of individual PKC isoforms. We found that TPA treatment of HC11 cells induced a transient cell cycle arrest in G0/G1. Western blot analyses of the TPA treated cells provided evidence that the endogenous PKCα present in these cells mediated these effects. Indeed, derivatives of the HC11 cell line that inducibly overexpress an exogenous PKCα or ectopic PKCβ 1 exhibited more marked growth inhibition by TPA than control cells. Immunohistochemical staining of cells following treatment with TPA revealed selective translocation of PKCα into the nucleus, whereas PKCβ 1 remained in the cytoplasm. The transient arrest of HC11 cells following treatment with TPA was associated with marked induction of both p21 cip1 mRNA and protein. This induction was exaggerated in the derivatives that overexpressed either PKCα or PKCβ 1 . Therefore, in mouse mammary epithelial cells activation of the endogenous PKCα can transiently arrest cells in G0/G1 which may be due, at least in part, to induction of the transcription of p21 cip1 .