Intestinal cell phosphate uptake and the targeted knockout of the 1,25D3-MARRS receptor/PDIA3/ERp57.

We have crossed ERp57(flx/flx) mice with commercially available mice expressing villin-driven cre-recombinase. Enterocytes isolated from 3- to 4-wk-old littermate (LM) male mice responded to 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] with enhanced phosphate uptake relative to corresponding controls within 1 min of addition, whereas in cells from targeted knockout (KO) mice, the response was severely blunted. Unlike chick enterocytes, mouse enterocytes did not respond to phorbol ester with enhanced phosphate uptake. However, forskolin, which does not stimulate phosphate uptake in chick intestinal cells, did so in enterocytes isolated from either young male LM or KO mice. Intestinal cells isolated from young female LM mice also responded to 1,25(OH)₂D₃ with enhanced phosphate uptake within 5 min of hormone addition, whereas cells from KO mice did not. Forskolin also stimulated phosphate uptake in enterocytes from young female KO or LM mice. As with intestinal cells from adult male chickens or rats, cells from adult (8 wk) male LM mice lost the ability to respond to 1,25(OH)₂D₃) with enhanced phosphate uptake. In contrast, intestinal cells from adult female LM mice did respond with enhanced phosphate uptake within 1 min of steroid hormone addition relative to corresponding controls, and the magnitude of the effect was greater than that observed in enterocytes of young females. Cells isolated from young or adult male or female LM mice failed to respond to 1,25(OH)₂D₃ with enhanced protein kinase C activity. Finally, we have previously reported that mouse enterocytes have cell surface vitamin D receptor; however preincubation of such cells with anti-vitamin D receptor antibodies demonstrated that the classical receptor is not involved in the rapid 1,25(OH)₂D₃-stimulated uptake of phosphate.