Modulation of nitric oxide-evoked apoptosis by the p53-downstream target p21(WAF1/CIP1)

When produced in excess, the inflammatory mediator nitric oxide (NO) attenu ates cell-cycle progression at the G1 phase in tight correlation with p21(W AF1/CIP1) expression, provokes accumulation of the tumor suppressor p53, an d initiates apoptosis/necrosis as judged on cell accumulation in the sub-G1 phase, To verify the role of p21(WAF1/CIP1) in modulating cell-cycle arres t vs. apoptosis, we transfected stably antisense p21(WAF1/CIP1)-encoding pl asmids, Following NO exposure, accummulation of p21(WAF1/CIP1), but not p53 , was largely attenuated in antisense p21(WAF1/CIP1) tansfectants. Moreover , the G1 cell-cycle arrest was abrogated, and cells were sensitized toward apoptosis compared with parent macrophages, In contrast, antisense eliminat ion of p53 attenuated p53 as well as p21(WAF1/CIP1) expression, abolished t he G1 cell-cycle arrest, and prevented apoptosis. We conclude that p21(WAF1 /CIP1) is a downstream target of p53 in macrophages that modulate the sensi tivity toward the immune-modulator NO.