Phorbol ester-induced G1 arrest in BALB/MK-2 mouse keratinocytes is mediated by δ and η isoforms of protein kinase C

We investigated the possible negative regulation of the cell cycle by protein kinase C (PKC) isoforms in synchronously grown BALB/MK-2 mouse keratinocytes, in which PKC isoforms were overexpressed hg using the adenovirus vector Ax. Cells at the G1/S boundary of the cell cycle were the most sensitive to the inhibitory effect of 12-O-tetradecanoylphorbol-13-acetate (TPA), a PKC agonist, resulting in GI arrest. TPA-induced inhibition of DNA synthesis was augmented by overexpression of the eta and delta isoforms, but rescued by the dominant-negative and antisense eta isoforms. In contrast, the alpha and zeta isoforms showed no effect on DNA synthesis with or without TPA treatment. Immunoblotting indicated cell cycle-dependent expression of the eta isoform, being highest in cells at the G1/S boundary. The present study provides evidence that the eta and delta isoforms of PKC are involved in negative regulation of cell cycle at the G1/S boundary in mouse keratinocytes.