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pubmed:abstractText |
Our laboratory has previously demonstrated that 1,25-dihydroxyvitamin D3 (1,25[OH]2D3) rapidly stimulated polyphosphoinositide (PI) hydrolysis, raised intracellular Ca2+, and activated two Ca2+-dependent protein kinase C (PKC) isoforms, PKC-alpha and -betaII in the rat large intestine. We also showed that the direct addition of 1,25(OH)2D3 to isolated colonic membranes failed to stimulate PI hydrolysis, but required secosteroid treatment of intact colonocytes, suggesting the involvement of a soluble factor. Furthermore, this PI hydrolysis was restricted to the basal lateral plasma membrane of these cells. In the present studies, therefore, we examined whether polyphosphoinositide-phospholipase C-gamma (PI-PLC-gamma), a predominantly cytosolic isoform of PI-PLC, was involved in the hydrolysis of colonic membrane PI by 1,25(OH)2D3. This isoform has been shown to be activated and membrane-associated by tyrosine phosphorylation. We found that 1,25(OH)2D3 caused a significant increase in the biochemical activity, particulate association, and the tyrosine phosphorylation of PLC-gamma, specifically in the basal lateral membranes. This secosteroid also induced a twofold increase in the activity of Src, a proximate activator of PLC-gamma in other cells, with peaks at 1 and 9 min in association with Src tyrosine dephosphorylation. 1,25(OH)2D3 also increased the physical association of activated c-Src with PLC-gamma. In addition, Src isolated from colonocytes treated with 1,25(OH)2D3, demonstrated an increased ability to phosphorylate exogenous PLC-gamma in vitro. Inhibition of 1,25(OH)2D3-induced Src activation by PP1, a specific Src family protein tyrosine kinase inhibitor, blocked the ability of this secosteroid to stimulate the translocation and tyrosine phosphorylation of PLC-gamma in the basolateral membrane (BLM). Src activation was lost in D deficiency, and was reversibly restored with the in vivo repletion of 1,25(OH)2D3. These studies demonstrate for the first time that 1,25(OH)2D3 stimulates PLC-gamma as well as c-Src in rat colonocytes, and indicate that PLC-gamma is a direct substrate of secosteroid-activated c-Src in these cells.
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