Non-genomic stimulation of tyrosine phosphorylation cascades by 1,25(OH)(2)D(3) by VDR-dependent and -independent mechanisms in muscle cells.

Studies with different cell types have shown that modulation of various of the fast as well as long-term responses to 1,25(OH)(2)D(3) depends on the activation of tyrosine kinase pathways. Recent investigations of our laboratory have demonstrated that 1,25(OH)(2)D(3) rapidly stimulates in muscle cells tyrosine phosphorylation of PLC-gamma 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)(2)D(3) requires tyrosine phosphorylation of the VDR. Immunoprecipitation under native conditions coupled to Western blot analysis revealed 1,25(OH)(2)D(3)-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 Ca(2+). 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)(2)D(3). Data were also obtained involving tyrosine kinases and the VDR in hormone regulation of the Ca(2+) messenger system by mediating the stimulation of store-operated calcium (SOC; TRP) channels. Congruent with this action, 1,25(OH)(2)D(3) 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.