Identification of the region required for the antiapoptotic function of the cyclin kinase inhibitor, p21

The CDK inhibitor, p21, exhibits an antiapoptotic or proapoptotic effect, in addition to its anti-proliferative effect, depending on the conditions. To define the apoptosis-regulatory function of p21, we constructed cells that stably express C-terminal deletion mutants of p21 (full length 164 aa), 1–157, 1–147 or 1–128, and evaluated the apoptotic response of these cells. The AnnexinV positive cell fraction after γ-irradiation did not increase in cells expressing 1–157. Consistently, an increase of caspase3 activity or the active form of caspase3 was not observed in cells expressing 1–157, but was prominent in cells expressing 1–128 and 1–147. Further, the activity of caspase9 was suppressed in γ-irradiated cells expressing 1–157. The antiapoptotic effect of 1–157 was weaker in Fas-induced apoptosis. Our data indicate that the 1–157 region of p21 inhibits apoptosis caused by γ-irradiation by reducing the activity of caspase9 and caspase3, and the 148–157 region is critical for its inhibiting activity.