EFFECT OF 1,25,28-TRIHYDROXYVITAMIN-D2 AND 1,24,25-TRIHYDROXYVITAMIN-D3 ON INTESTINAL CALBINDIN-D(9K) MESSENGER-RNA AND PROTEIN - IS THERE A CORRELATION WITH INTESTINAL CALCIUM-TRANSPORT

Although analogs and metabolites of vitamin D have been tested for the ir calciotropic activity, very little information has been available c oncerning the effects of these compounds on gene expression. In this s tudy one analog of vitamin D2 1,25,28-trihydroxyvitamin D2 [1,25,28-(O H)3D2], and one metabolite, 1,24,25-trihydroxyvitamin D3, [1,24,25-(OH )3D3], were tested for their effect on intestinal calbindin-D9K mRNA a nd protein as well as for their effect on intestinal calcium absorptio n and bone calcium mobilization. These compounds were also evaluated f or their ability to compete for rat intestinal 1,25-(OH)2D3 receptor s ites and to induce differentiation of human leukemia (HL-60) cells as indicated by reduction of nitro blue tetrazolium. In vivo studies invo lved intrajugular injection of 12.5 ng 1,25-(OH)2D3 or test compound t o vitamin D-deficient rats and sacrifice after 18 h. 1,25,28-Trihydrox yvitamin D2 had no effect on intestinal calcium absorption, bone calci um mobilization, or intestinal calbindin-D9K Protein and mRNA. Competi tive 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)3D 2 (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 i n calciotropic activity, was found to induce differentiation of HL-60 cells at high concentrations [ED50 = 15 x 10(-8) M compared to ED50 = 2.5 x 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 obser ved 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 compou nd was found to be 50% as active as 1,25-(OH)2D3 in stimulating calbin din-D9K protein and mRNA. The lack of a direct correlation between cal bindin 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 transpor t.