To extend our previous observations on the regulation of CaBP biosynthesis by 1,25-dihydroxyvitamin D3, we have studied the specific mRNA encoding this protein in vitamin D-deficient and in vitamin D-repleted rats as well as the rate of its induction after a single injection of 1,25(OH)2D3 to vitamin D-deficient animals. The CaBP-mRNA was quantified by translation in a cell-free reticulocyte lysate system. CaBP-mRNA activity and cytoplasmic CaBP (measured by radioimmunoassay), dramatically decreased in rats previously fed a vitamin D-free diet for 5 weeks but neither parameter was zero. In vitamin D-deficient rats, a single injection of 1,25(OH)2D3 led to an increase in CaBP-mRNA activity within 2 h. This CaBP-mRNA activity peaked at about 4-6 h and thereafter declined to low value by 48 h. and the changes in mRNA activity always preceded the changes in cytosolic CaBP concentration. These results indicate that the induction of CaBP biosynthesis results from a 1,25(OH)2D3-induced increase in the levels of total cellular CaBP-mRNA activity and are, therefore, consistent with a transcriptional regulation of CaBP biosynthesis by 1,25(OH-2D3. This study also shows that the production of many other proteins seem to be under the control of vitamin D3.
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