The alpha isoform of protein kinase C mediates phorbol ester-induced growth inhibition and p21cip1 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 PKC alpha present in these cells mediated these effects. Indeed, derivatives of the HC11 cell line that inducibly overexpress an exogenous PKC alpha or ectopic PKC beta 1 exhibited more marked growth inhibition by TPA than control cells. Immunohistochemical staining of cells following treatment with TPA revealed selective translocation of PKC alpha into the nucleus, whereas PKC beta 1 remained in the cytoplasm. The transient arrest of HC11 cells following treatment with TPA was associated with marked induction of both p21cip1 mRNA and protein. This induction was exaggerated in the derivatives that overexpressed either PKC alpha or PKC beta 1. Therefore, in mouse mammary epithelial cells activation of the endogenous PKC alpha can transiently arrest cells in G0/G1 which may be due, at least in part, to induction of the transcription of p21cip1.