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

When produced in excess, the inflammatory mediator nitric oxide (NO) attenuates cell-cycle progression at the G1 phase in tight correlation with p21(WAF1/CIP1) expression, provokes accumulation of the tumor suppressor p53, and 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 arrest vs. apoptosis, we transfected stably antisense p21(WAF1/CIP1)-encoding plasmids. Following NO exposure, accumulation of p21(WAF1/CIP1), but not p53, was largely attenuated in antisense p21(WAF1/CIP1) transfectants. Moreover, the G1 cell-cycle arrest was abrogated, and cells were sensitized toward apoptosis compared with parent macrophages. In contrast, antisense elimination of p53 attenuated p53 as well as p21(WAF1/CIP1) expression, abolished the G1 cell-cycle arrest, and prevented apoptosis. We conclude that p21(WAF1/CIP1) is a downstream target of p53 in macrophages that modulate the sensitivity toward the immune-modulator NO.