Activation of Raf-mitogen-activated protein kinase signaling pathway by 1,25-dihydroxyvitamin D-3 in normal human keratinocytes

The biologic effects of the vitamin D hormone 1,25-dihydroxyvitamin D-3 (1,25(OH)(2)D-3) are believed to be mediated by an intracellular vitamin D receptor, which after ligand binding acts as a transcription factor modulating expression of a variety of genes, Besides having a well-known role in calcium metabolism, this hormone is an important regulator of proliferation in a majority of normal and neoplastic cells, Keratinocytes provide a convenient model for investigating the growth-related effects of vitamin D in normal cells, Growth of keratinocytes may be either stimulated or inhibited by 1,25(OH)(2)D-3, depending on the degree of cell differentiation. We show here that 1,25(OH)(2)D-3 stimulates DNA synthesis via sequential activation of Raf and the mitogen-activated protein kinase, Activation of these kinases is independent on protein and mRNA synthesis and is preceded by rapid tyrosine phosphorylation of an adaptor protein p66(Shc) and formation of a complex between p66(Shc), a bridging molecule Grb2, and a Ras activator, mSos. Vitamin D receptor protein associates with She, indicating that this steroid hormone is able to signal through the transcription-independent pathways similar to those used by peptide hormones and cytokines.