Inhibition of Phosphorylation of TrkB and TrkC and Their Signal Transduction by α2-Macroglobulin

Abstract: Monoamine-activated α₂-macroglobulin (α₂M) was shown to reduce the dopamine concentration in corpus striatum of adult rat brains and inhibit other neuronal functions in vivo and in vitro. As brain-derived neurotrophic factor, neurotrophin-4, and neurotrophin-3 are important neurotrophic factors for dopaminergic neurons, the effect of monoamine-activated α₂M on signal transduction by trkB and trkC was investigated. The results show that monoamine-activated α₂M binds to trkB and inhibits brain-derived neurotrophic factor/neurotrophin-4-promoted autophosphorylation of trkB in a dose-dependent manner in both trkB-expressing NIH3T3 (NIH3T3-trkB) and human neuroblastoma SH-SY5Y cells. Monoamine-activated α₂M also blocks tyrosine phosphorylation of phospholipase C-γ1 and extracellular signal-regulated protein kinase (ERK)-1, which are key intracellular proteins involved in trkB signal transduction. Similarly, monoamine-activated α₂M inhibits tyrosine phosphorylation of neurotrophin-3-induced trkC and its signal transduction in a dose-dependent manner in NIH3T3 cells expressing trkC (NIH3T3-trkC). In contrast to monoamine-activated α₂M, normal α₂M has little or no significant inhibitory effect on the phosphorylation of trkB and trkC. In addition, the retinoic acid-promoted tyrosine phosphorylation of phospholipase C-γ1, ERK-1, and/or ERK-2 in SH-SY5Y cells was unaffected by monoamine-activated α₂M; this suggests that the inhibitory effect of activated α₂M on the neurotrophin-stimulated phosphorylation of intracellular signalling proteins may be specific. Taken together, the data indicate that activated α₂M is a pan-trk inhibitor, which by virtue of its binding to trk receptors may block trk-mediated signal transduction in dopaminergic neurons and lead to reduction of dopamine concentration in corpus striatum.