The CDK inhibitor p21WAF1/Cip1 is induced through a p300-dependent mechanism during NGF-mediated neuronal differentiation of PC12 cells.

The block of cell proliferation elicited by the addition of nerve growth factor (NGF) to exponentially-growing PC12 cells results, in part, from the inhibition of cyclin D1-associated kinase activity by p21WAF1/CIP1. NGF treatment of PC12 cells provokes the accumulation of p21 mRNA, due to transcriptional activation of the p21 promoter in a p53-independent manner. Transient expression of a mutated form of the adenovirus E1A protein (E1A dCR2), which retains its capacity to bind the transcriptional co-activator p300, completely abolishes the NGF-mediated stimulation of p21 promoter activity. This phenomenon can be reversed by ectopic expression of p300, suggesting that p300 is necessary for the induction of p21 by NGF. In addition, stable expression of E1A dCR2 in PC12 cells results in the inhibition of the NGF response, i.e. it prevents activation of the p21 promoter, cell cycle arrest, and neuronal differentiation. The signalling pathway from the TrkA receptor via the MAP kinase pathway is not altered in these cells. Together, these data indicate that p300 could play a pivotal role in the triggering of the anti-mitogenic effect of NGF and of neuronal differentiation.