Neurotrophins induce sphingomyelin hydrolysis: Modulation by co-expression of p75^NTR with Trk receptors

We examined neurotrophin-induced sphingomyelin hydrolysis in cells expressing solely the low affinity neurotrophin receptor, p75^NTR, and in PC12 cells that co-express p75^NTR and Trk receptors. Nerve growth factor (NGF), brain-derived neurotrophic factor, neurotrophin-3 (NT-3), and NT-5 stimulated sphingomyelin hydrolysis with similar kinetics in p75^NTR-NIH-3T3 cells. Although brain-derived neurotrophic factor (10 ng/ml) was slightly more potent than NGF at inducing sphingomyelin hydrolysis, NT-3 and NT-5 induced significant hydrolysis (30-35%) at 0.1 to 1 ng/ml in p75^NTR-NIH-3T3 cells. NT-3 did not induce sphingomyelin hydrolysis in Trk C-NIH-3T3 cells nor in cells expressing a mutated p75^NTR containing a 57-amino acid cytoplasmic deletion, thus demonstrating the role of p75^NTR in this signal transduction pathway. In p75^NTR-NIH-3T3 cells, neurotrophin-induced sphingomyelin hydrolysis 1) localized to an internal pool of sphingomyelin, 2) was not a consequence of receptor internalization, and 3) showed no specificity with respect to the molecular species of sphingomyelin hydrolyzed. In contrast to cells expressing solely p75^NTR, NGF (100 ng/ml) did not induce sphingomyelin hydrolysis in PC12 cells. Interestingly, NT-3 (10 ng/ml) induced the same extent of sphingomyelin hydrolysis in PC12 cells as was apparent in p75^NTR-NIH-3T3 cells. However, in the presence of NGF, NT-3 was unable to induce sphingomyelin hydrolysis, raising the possibility that Trk was modulating p75^NTR-dependent sphingomyelin hydrolysis. Inhibition of Trk tyrosine kinase activity with 200 nM K252a enabled both NGF and NT-3 in the presence of NGF to induce sphingomyelin hydrolysis. These data support that p75^NTR serves as a common signaling receptor for neurotrophins through induction of sphingomyelin hydrolysis and that cross-talk pathways exist between Trk and p75^NTR-dependent signaling pathways.