Non-genomic stimulation of tyrosine phosphorylation cascades by 1,25(OH)2D3 by VDR-dependent and -independent mechanisms in muscle cells

Abstract

Studies with different cell types have shown that modulation of various of the fast as well as long-term responses to 1,25(OH)2D3 depends on the activation of tyrosine kinase pathways. Recent investigations of our laboratory have demonstrated that 1,25(OH)2D3 rapidly stimulates in muscle cells tyrosine phosphorylation of PLC-γ and the growth-related proteins MAPK and c-myc. We have now obtained evidence using antisense technology indicating that VDR-dependent activation of Src mediates the fast stimulation of tyrosine phosphorylation of c-myc elicited by the hormone. This non-genomic action of 1,25(OH)2D3 requires tyrosine phosphorylation of the VDR. Immunoprecipitation under native conditions coupled to Western blot analysis revealed 1,25(OH)2D3-dependent formation of complexes between Src and the VDR and c-myc. However, the activation of MAPK by the hormone was only partially mediated by the VDR and required in addition increased PKC and intracellular Ca2+. Following its phosphorylation, MAPK translocates into the nucleus where it regulates c-myc transcription. Altogether these results indicate that tyrosine phosphorylation plays a role in the stimulation of muscle cell growth by 1,25(OH)2D3. Data were also obtained involving tyrosine kinases and the VDR in hormone regulation of the Ca2+ messenger system by mediating the stimulation of store-operated calcium (SOC; TRP) channels. Congruent with this action, 1,25(OH)2D3 induces a rapid translocation of the VDR to the plasma cell membrane which can be blocked by tyrosine kinase inhibitors. Of mechanistic relevance, an association between the VDR and TRP proteins with the participation of the scaffold protein INAD was shown.