Estrogen Controls Expression and Bioresponse of 1,25-dihydroxyvitamin D Receptors in the Rat Colon

Estrogen receptors are extensively expressed in the gastrointestinal tract, however their physiological role is not clear yet. Estrogen and 1,25-dihydroxyvitamin D [1,25(OH)2D3] apparently share common activities in the intestine such as growth-suppressing effects on the colonic mucosa and positively influence intestinal calcium absorption. In view of our previous studies showing up-regulation of vitamin D receptors (VDR) in the duodenal mucosa and in osteoblasts, the present study was designed to address a possible interaction between estrogen and the vitamin D endocrine system in the colonic mucosa. Three groups of female rats were studied: sham operated ('Sham'), ovariectomized ('OVX'), and ovariectomized estrogen-treated ('OVX+E'). VDR gene expression was assessed by Northern blot analysis, VDR protein expression was assessed by ligand-binding assays, and Western-blotting. Endogenous 1,25(OH)2D3 bioactivity in colonic mucosal extracts was assessed by alkaline phosphatase activity and calbindin-9kD mRNA expression. Northern blots revealed marked increase in band intensity corresponding with the VDR mRNA product in 'Sham' or 'OVX+E' vs. 'OVX'. In ligand-binding experiments, 1,25(OH)2D3 was shown to bind specifically to a single class of receptors in extracts obtained from each of the groups (Kd--0.03 nM). The maximal VDR binding capacity of colonic mucosal extracts was 203 +/- 23 fmol/mg protein in 'Sham', 362 +/- 41 in 'OVX+E' and 102 +/- 15 in 'OVX' ('Sham' or 'OVX+E' vs. 'OVX', p < 0.001). Western-blot analysis also revealed higher VDR protein expression in the estrogen-exposed animals. Alkaline phosphatase activity and calbindin-9kD mRNA expression were significantly higher in colonic mucosal extracts from estrogen-exposed rats. Estrogen increases VDR gene transcript level, protein expression and endogenous 1,25(OH)2D3 bioactivity in colonic mucosa, which may suggest that some of the estrogen activities in the colonic mucosa, such as its growth-suppressing effect, could be mediated, at least in part, by an increase in colonic mucosa responsiveness to endogenous 1,25(OH)2D3.