Extended ceramide exposure activates the trkA receptor by increasing receptor homodimer formation

Binding of nerve growth factor (NGF) to the trkA tyrosine kinase receptor r esults in receptor homodimer formation, transphosphorylation, and kinase ac tivation that supports neuronal differentiation and survival. We have shown previously that short-term exposure of PC12 cells to brain-derived neurotr ophic factor or to C2-ceramide activates a signaling pathway that results i n serine phosphorylation of the trkA intracellular domain and reduces the a bility of trkA to respond to NGF. Here we demonstrate that extended C2-cera mide exposure dramatically increases NGF-induced trkA activation and furthe r show that C2-ceramide augments trkA tyrosine phosphorylation even in the absence of neurotrophin. This increase in trkA receptor phosphotyrosine is reflected in increased activation of both Erk1 and Erk2 and of the catalyti c subunit of phosphatidylinositol S-kinase. The C2-ceramide-mediated increa se in tyrosine phosphorylation is blocked completely by the trk kinase inhi bitor K252A, indicating that this increase results from an effect of C2-cer amide on trkA kinase activity. Consistent with this, crosslinking studies s how that C2-ceramide promotes the formation of active trkA receptor homodim ers. Together, these data suggest that chronic C2-ceramide treatment increa ses trkA activation by altering properties of the plasma membrane, which fa vors the formation of trkA homodimers.