Nerve growth factor receptor signaling in proliferation of normal adult rat chromaffin cells

Adult rat chromaffin cells may proliferate or extend neurites when stimulat ed by nerve growth factor (NGF) but their response is predominantly prolife rative, making them a unique model for studying how mitogenic specificity i s achieved. We examined contributions of the NGF receptors trk and p75 and of the major NGF signaling pathways res proliferation versus neurite outgro wth. The type of initial NGF response does not correlate with intensity of immunoreactivity for trk or p75. However, proliferation is initiated at low er NGF concentrations than neurite outgrowth, suggesting that it requires a less intense signal. Mitogenic cooperativity between receptors at low NGF concentrations is suggested by inhibitory effects of p75-blocking antibodie s, but responses to trk-agonist antibody indicate that trk activation alone can induce proliferation. NGF-induced phosphorylation of ras-mediated mito gen-activated protein kinases (MAPK) Erk1 and Erk2 is as prolonged in norma l chromaffin cells as in PC12 cells, where NGF is neuritogenic. Trk-agonist antibody, which is as mitogenic as NGF but less neuritogenic, causes equal ly prolonged but less intense ERK phosphorylation. The MAPK kinase(MEK-1) i nhibitor PD98059 partially inhibits Erk phosphorylation and does not inhibi t chromaffin cell proliferation, while depolarization selectively inhibits proliferation without blocking Erk phosphorylation. Proliferation is marked ly reduced by the phosphoinositol-3 (PI-3) kinase inhibitor LY294002 while downregulation of protein kinase C (PKC) causes no change. These findings s uggest that low-level, rather than short-duration, stimulation of NGF signa ling pathways causes NGF to be mitogenic. Ras-mediated MAPK activation may be more critical in neurite outgrowth than in proliferation and PI-3 kinase may be the major mitogenic determinant.