P21(WAF1) INDUCES PERMANENT GROWTH ARREST AND ENHANCES DIFFERENTIATION, BUT DOES NOT ALTER APOPTOSIS IN PC12 CELLS

p21(WAF1) cyclin-dependent kinase inhibitor has been implicated in the control of proliferation, differentiation, and death in, various cell lines, To further examine p21 regulation of the transitions between t hese cellular processes, an inducible p21 vector (lac operon system) w as transfected into the rat pheochromocytoma (PC12) neural cell line, Induction of p21 led to permanent growth arrest, as evidenced by cell counts, FACS analysis, and thymidine incorporation, This arrest was ma intained, even after removal of the inducing signal (IPTG). Northern a nalysis revealed that endogenous p21 mRNA increased following IPTG rem oval, which may be responsible for the continued growth arrest despite the decrease in ectopic p21 expression, p21 overexpression did not di rectly lead to a differentiated phenotype; however, differentiation in response to nerve growth factor (NGF) was greatly accelerated. To exa mine effects on cell, death, and specifically test the hypothesis that apoptosis caused by withdrawal of trophic support results from inappr opriate entry into cell cycle, serum was removed from proliferating an d p21-arrested PC12 cells, The rate of apoptotic death was not affecte d by p21, nor was it effective in altering the extent of death followi ng other apoptotic stimuli.