RESUMO
Fibroblast growth factor receptors (FGFR) are widely expressed in many tissues and cell types, and the temporal expression of these receptors and their ligands play important roles in the control of development. There are four FGFR family members, FGFR-1-4, and understanding the ability of these receptors to transduce signals is central to understanding how they function in controlling differentiation and development. We have utilized signal transduction by FGF-1 in PC12 cells to compare the ability of FGFR-1 and FGFR-3 to elicit the neuronal phenotype. In PC12 cells FGFR-1 is much more potent in the induction of neurite outgrowth than FGFR-3. This correlated with the ability of FGFR-1 to induce robust and sustained activation of the Ras-dependent mitogen-activated protein kinase pathways. In contrast, FGFR-3 could not induce strong sustained Ras-dependent signals. In this study, we analyzed the ability of FGFR-3 to induce the expression of sodium channels, peripherin, and Thy-1 in PC12 cells because all three of these proteins are known to be induced via Ras-independent pathways. We determined that FGFR-3 was capable of inducing several Ras-independent gene expression pathways important to the neuronal phenotype to a level equivalent of that induced by FGFR-1. Thus, FGFR-3 elicits phenotypic changes primarily though activation of Ras-independent pathways in the absence of robust Ras-dependent signals.
Assuntos
Glicoproteínas de Membrana , Proteínas Tirosina Quinases , Proteínas Proto-Oncogênicas p21(ras)/fisiologia , Receptores de Fatores de Crescimento de Fibroblastos/metabolismo , Transdução de Sinais , Animais , Sobrevivência Celular , Fator 1 de Crescimento de Fibroblastos , Fator 2 de Crescimento de Fibroblastos/farmacologia , Proteínas de Filamentos Intermediários/biossíntese , Proteínas de Filamentos Intermediários/genética , Metaloproteinase 3 da Matriz/biossíntese , Metaloproteinase 3 da Matriz/genética , Mutação , Proteínas do Tecido Nervoso/biossíntese , Proteínas do Tecido Nervoso/genética , Neurônios/efeitos dos fármacos , Neurônios/metabolismo , Células PC12 , Técnicas de Patch-Clamp , Periferinas , Proteínas Proto-Oncogênicas p21(ras)/genética , RNA Mensageiro/biossíntese , Ratos , Receptores Proteína Tirosina Quinases/metabolismo , Receptor Tipo 1 de Fator de Crescimento de Fibroblastos , Receptor Tipo 3 de Fator de Crescimento de Fibroblastos , Canais de Sódio/biossíntese , Canais de Sódio/genética , Canais de Sódio/metabolismo , Antígenos Thy-1/biossíntese , Antígenos Thy-1/genética , Ativação TranscricionalRESUMO
The functional properties of most sodium channels are too similar to permit identification of specific sodium channel types underlying macroscopic current. Such discrimination would be particularly advantageous in the nervous system in which different sodium channel family isoforms are coexpressed in the same cell. To test whether members of the mu-conotoxin family can discriminate among known neuronal sodium channel types, we examined six toxins for their ability to block different types of heterologously expressed sodium channels. PIIIA mu-conotoxin blocked rat brain type II/IIA (rBII/IIA) and skeletal muscle sodium current at concentrations that resulted in only slight inhibition of rat peripheral nerve (rPN1) sodium current. Recordings from variant lines of PC12 cells, which selectively express either rBII/IIA or rPN1 channel subtypes, verified that the differential block by PIIIA also applied to native sodium current. The sensitivity to block by PIIIA toxin was then used to discriminate between rBII/IIA and rPN1 sodium currents in NGF-treated PC12 cells in which both mRNAs are induced. During the first 24 hr of NGF-treatment, PN1 sodium channels accounted for over 90% of the sodium current. However, over the ensuing 48 hr period, a sharp rise in the proportion of rBII/IIA sodium current occurred, confirming the idea, based on previous mRNA measurements, that two distinct sodium channel types appear sequentially during neuronal differentiation of PC12 cells.
