SELECT ALTERATIONS IN PROTEIN-KINASES AND PHOSPHATASES DURING APOPTOSIS OF DIFFERENTIATED PC12 CELLS

The involvement of cell cycle-regulatory proteins in apoptosis of neur onally differentiated PC12 cells induced by the removal of nerve growt h factor and serum was examined. Three major findings are presented. ( 1) Cdc2 kinase protein levels increased fivefold in apoptotic PC12 cel ls by day 3 of serum and nerve growth factor deprivation. Histone H1 k inase activity was increased significantly in p13(suc1) precipitates o f apoptotic PC12 cells, which was due to increased activation and/or e xpression of cdc2 kinase. (2) The protein levels of cyclin-dependent k inase 4, cyclin D, and proliferating cell nuclear antigen that are nor mally expressed in the cell cycle were increased during neuronal PC12 cell apoptosis. (3) The levels of the catalytic subunit, but not the r egulatory subunit of the calcium/calmodulin-dependent protein phosphat ase 2B, decreased significantly concomitant with a significant decreas e in protein phosphatase 2B activity early in the apoptotic process. P rotein phosphatase 2A activity decreased slightly but significantly af ter 3 days of serum and nerve growth factor deprivation, and no altera tions in protein phosphatase 1 were observed during the apoptotic proc ess. These data demonstrate that certain cell cycle-regulatory protein s are inappropriately expressed and that alterations in specific phosp horylation events, as indicated by the increase in histone H1 kinase a ctivity and the decrease in protein phosphatase 2B activity, are most likely occurring during apoptosis of PC12 cells. These observations su pport the hypothesis that apoptosis may be due in part to a nondividin g cell's uncoordinated attempt to reenter and progress through the cel l cycle.