Transactivation of platelet-derived growth factor receptor type β: Mechanisms and potential relevance in neurobiology
Kruk, Jeffrey Stephen
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In the absence of ligand, certain growth factor receptors can be activated via G protein-coupled receptor (GPCR) activation in a process termed transactivation. Serotonin (5-HT) receptors can transactivate the receptor tyrosine kinase (RTK) platelet-derived growth factor (PDGF) β receptors in smooth muscle cells, but it is not known if similar pathways occur in neuronal cells. Here, it is shown that 5-HT can transiently increase the phosphorylation of PDGFβ receptors in a time- and concentration-dependent manner in SH-SY5Y neuroblastoma cells. This transactivation pathway was pertussis-toxin sensitive, and was dependent on phospholipase C activity, intracellular calcium signaling and subsequent protein kinase C activation. Exogenous application of non-lethal concentrations of H2O2 induced the phosphorylation of PDGFβ receptors in a concentration-dependent fashion, similar to that observed with 5-HT. Further investigation revealed reactive oxygen species (ROS) production as a necessary component in the transactivation pathway, as scavenging ROS eliminated PDGFβ receptor phosphorylation. NADPH oxidase was determined to be the likely source of ROS given that the NADPH oxidase inhibitors diphenyleneiodonium chloride and apocynin abrogated PDGFβ receptor transactivation. The role of Src tyrosine kinase was also investigated, and its location in this signaling cascade was determined to be downstream of calcium signaling, but upstream of NADPH oxidase activity. In addition, the activation of ERK1/2 in this system was elucidated to be independent of PDGFβ receptor transactivation. Interestingly, 5-HT also transactivated TrkB receptors, another RTK whose function is implicated in clinical depression. Expectedly, the enzymes in this mechanism were consistent with those revealed in 5-HT-to-PDGFβ receptor signaling. This cross-talk between 5-HT and RTKs such as TrkB and PDGFβ receptors identifies a potentially important signaling link between the serotonergic system and neurotrophic factor signaling in neurons that could have implications in mental health disorders including depression. Furthermore, although transactivation pathways are commonly initiated by a GPCR, recent reports have demonstrated that selective serotonin reuptake inhibitors (SSRIs) were able to block 5-HT-induced transactivation of PDGFβ receptors, suggesting that in addition to GPCRs, monoamine transporters may also be involved in RTK transactivation. SH-SY5Y cells pretreated with the SSRI fluoxetine blocked 5-HT-induced transactivation of the PDGFβ receptors, but not PDGF-induced PDGFβ receptor activation. Upon further examination, it was discovered that during the pretreatment period, fluoxetine itself was transiently transactivating the PDGFβ receptor via 5-HT2 receptors. By the end of the pretreatment period, the effects of fluoxetine on PDGFβ receptor phosphorylation had returned to baseline, and a subsequent transactivating stimulus (5-HT) failed to “re-transactivate” the PDGFβ receptor. Additional investigations demonstrated that 5-HT pretreatment can block dopamine-induced PDGFβ receptor transactivation, but not PDGF-induced PDGFβ receptor activation. This is the first demonstration of the heterologous desensitization of an RTK via a transactivation pathway, and this phenomenon is specific for transactivation pathways because in all cases the PDGFβ receptor ligand PDGF-BB was able to directly stimulate receptor activity in spite of GPCR agonist pretreatment. Heterologous desensitization in transactivation signaling reveals a previously unknown short-term “blackout” period wherein no further transactivation signaling can occur to potentially exploit the mitogenic effects of RTK activation.