TrkA induces differentiation but not apoptosis in C6-2B glioma cells

Nerve growth factor (NGF) binds to the TrkA tyrosine kinase and the p75 neu rotrophin receptors. Depending upon which receptor is activated, NGF can in duce differentiation or apoptosis. C6-2B glioma cells express the p75 recep tor, but NGF decreases their growth only when TrkA is introduced (C6trk). I t is unclear, however, whether TrKA reduces C6-2B cell growth by apoptosis or differentiation. To examine which mechanisms account for the anti-prolif erative effect of NGF in these cells, we first analyzed whether NGF causes apoptosis by flow cytometry, two-site immunoassay and in situ TUNEL. None o f these methods indicated that C6trk undergo apoptosis, Additional apoptoti c markers, such as Bcl-2, Bar, Bad, p53, caspase 3, and NF-kappaB were also used. C6trk cells exhibited lower levels of Bcl-2 compared with the parent al C6 mock cells, but no changes in the levels of other apoptotic proteins. Moreover, NGF increased AP-1 binding activity in C6trk cells, suggesting t hat NGF may induce differentiation. We then examined whether TrkA changes t he glioma phenotype. In C6trk cells, but not in C6mock cells, NGF enhanced the levels of neuron-specific enolase as well as the levels of A2B5 and 2', 3'-cyclic nucleotide 3'-phosphodiesterase, markers for oligodendrocytes, w ithout affecting the expression of other neuronal markers. Our data suggest that the antiproliferative properties of TrkA may rely on its ability to i nduce differentiation of C6 cells from undifferentiated glioma to oligodend rocytes. (C) 2001 Wiley-Liss, Inc.