Effect of 1,25,28-trihydroxyvitamin D2 and 1,24,25-trihydroxyvitamin D3 on intestinal calbindin-D9K mRNA and protein: Is there a correlation with intestinal calcium transport?

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Abstract
Although analogs and metabolites of vitamin D have been tested for their calciotropic activity, very little information has been available concerning the effects of these compounds on gene expression. In this study one analog of vitamin D, 1,25,28-trihydroxyvitamin D2 [1,25,28-(OH)3D2], and one metabolite, 1,24,25-trihydroxyvitamin D3 [1,24,25-(OH)3D3], were tested for their effect on intestinal calbindin-D9K mRNA and protein as well as for their effect on intestinal calcium absorption and bone calcium mobilization. These compounds were also evaluated for their ability to compete for rat intestinal 1,25-(OH)2D3 receptor sites and to induce differentiation of human leukemia (HL-60) cells as indicated by reduction of nitro blue tetrazolium. In vivo studies involved intrajugular injection of 12.5 ng 1,25-(OH)2D3 or test compound to vitamin D-deficient rats and sacrifice after 18 h. 1,25,28-Trihydroxyvitamin D2 had no effect on intestinal calcium absorption, bone calcium mobilization, or intestinal calbindin-D9K protein and mRNA. Competitive binding to 1,25-(OH)2D3 receptors was 0.8% of that observed using 1,25-(OH)2D3. However, 20- and 40-fold higher doses of 1,25,28-(OH)3D2 (250 and 500 ng) resulted in significant inductions in calbindin-D9K protein and mRNA (3.5 to 7.4-fold), although doses as high as 800 ng were found to have no effect on intestinal calcium absorption or bone calcium mobilization. 1,25,28-Trihydroxyvitamin D2, although lacking in calciotropic activity, was found to induce differentiation of HL-60 cells at high concentrations [ED50 = 15 × 10−8 M compared to ED50 = 2.5 × 10−8 M for 1,25-(OH)2D3]. 1,24,25-Trihydroxyvitamin D3 was 93% as active as 1,25-(OH)2D3 in stimulating intestinal calcium transport but was relatively inactive in stimulating bone calcium mobilization. Competitive binding to the 1,25-(OH)2D3 receptor was 8% of that observed using 1,25-(OH)2D3. Although 1,24,25-(OH)3D3 was 93% as active as 1,25-(OH)2D3 in stimulating intestinal calcium absorption, this compound was found to be 50% as active as 1,25-(OH)2D3 in stimulating calbindin-D9K protein and mRNA. The lack of a direct correlation between calbindin protein and mRNA and intestinal calcium transport after 1,24,25-(OH)3D3 administration or after administration of high doses of 1,25,28-(OH)3D2 suggests that factors in addition to calbindin are involved, at least in part, in vitamin D-regulated intestinal calcium transport.
Funding Information
  • NIH (DK-38961)