NERVE GROWTH-FACTOR REGULATES THE EXPRESSION AND ACTIVITY OF P33(CDK2) AND P34(CDC2) KINASES IN PC12 PHEOCHROMOCYTOMA CELLS

In the absence of serum, nerve growth factor (NGF) promotes the surviv al and differentiation of the PC12 pheochromocytoma cell line. In the presence of serum, NGF acts primarily as a differentiation factor and negative regulator of cell cycling. To investigate NGF control of cell cycling, we have analyzed the regulation of cyclin dependent kinases during PC12 cell differentiation. NGF treatment leads to a reduction i n the steady-state protein levels of p33(cdk2) and p34(cdc2), two key regulators of cell cycle progression. The decrease in p33(cdk2) and p3 4(cdc2) coincides with a decrease in the enzymatic activity of cyclinA -p34(cdc2), cyclinB-p34(cdc2), cyclinE-p33(cdk2), and cyclinA-p33(cdk2 ) kinases. The decline in p33(cdk2) and p34(cdc2) kinase activity in r esponse to NGF is accelerated in cells that overexpress the p140(trk) NGF receptor, suggesting that the timing of the down- regulation is de pendent on the level of p140(trk) and the strength of the NGF signal. The level of cyclin A, a regulatory subunit of p33(cdk2) and p34(cdc2) , is relatively constant during PC12 differentiation. Nevertheless, th e DNA binding activity of the cyclinA-associated transcription factor E2F/DP decreases. Thus, NGF down-regulates the activity of cyclin depe ndent kinases and cyclin-transcription factor complexes during PC12 di fferentiation.