The Protein Kinase Cδ Catalytic Fragment Is Critical for Maintenance of the G2/M DNA Damage Checkpoint

Protein kinase C delta (PKC delta) is an essential component of the intrinsic apoptotic program. Following DNA damage, such as exposure to UV radiation, PKC delta is cleaved in a caspase-dependent manner, generating a constitutively active catalytic fragment (PKC delta-cat), which is necessary and sufficient for keratinocyte apoptosis. We found that in addition to inducing apoptosis, expression of PKC delta-cat caused a pronounced G(2)/M cell cycle arrest in both primary human keratinocytes and immortalized HaCaT cells. Consistent with a G(2)/M arrest, PKC delta-cat induced phosphorylation of Cdk1 (Tyr(15)), a critical event in the G(2)/M checkpoint. Treatment with the ATM/ATR inhibitor caffeine was unable to prevent PKC delta-cat-induced G(2)/M arrest, suggesting that PKC delta-cat is functioning downstream of ATM/ATR in the G(2)/M checkpoint. To better understand the role of PKC delta and PKC delta-cat in the cell cycle response to DNA damage, we exposed wild-type and PKC delta null mouse embryonic fibroblasts (MEFs) to UV radiation. Wild-type MEFs underwent a pronounced G(2)/M arrest, Cdk1 phosphorylation, and induction of apoptosis following UV exposure, whereas PKC delta null MEFs were resistant to these effects. Expression of PKC delta-green fluorescent protein, but not caspase-resistant or kinase-inactive PKC delta, was able to restore G(2)/M checkpoint integrity in PKC delta null MEFs. The function of PKC delta in the DNA damage-induced G(2)/M cell cycle checkpoint may be a critical component of its tumor suppressor function.