Early and late effects of NGF may be mediated by different pathways in transdifferentiating chromaffin cells

Nerve growth factor (NGF) causes cultured chromaffin cells to extend neurites and, and after about two weeks of exposure, to ‘transdifferentiate’ into mature sympathetic neurons. The molecular events leading to these responses are not fully understood, but one possible mediator of NGF's actions is protein kinase C (PKC), which can be directly activated by phorbol esters, including phorbol myristate acetate (PMA). Chronic exposure to PMA mimics the early effects of NGF, that is, it elicits the outgrowth of neurites and an enhanced rate of proliferation. However, while the initial responses to NGF and PMA are similar, after 10 days in culture striking differences become apparent: (1) cells in PMA fail to differentiate into sympathetic neuron-like cells and appear to remain in a transitional state. Even after more than 5 weeks in PMA, cells appear morpholigically the same as those grown in PMA for only one week; cells fail to form a dense neuritic network or to exhibit the somatic hypertrophy characteristics of sympathetic neurons. (2) Cells continue to proliferate for at least 4 weeks in PMA, while cells in NGF become postmitotic after about two weeks. (3) While NGF supports and causes the further neuronal differentiation of cells grown in PMA, PMA cannot support NGF_dependent cells. These results suggest that different second messenger systems may be operating in the early and late effects of NGF.