Regulation of calbindin-D28K gene expression in the chick intestine: effects of serum calcium status and 1,25-dihydroxyvitamin D3.

Calbindin-D28K is a member of a superfamily of calcium binding proteins that share a common avidity for the divalent calcium ion. The ambient concentration of calcium in the blood circulation is thought to orchestrate the release of parathyroid hormone and calcitonin and to govern the activity of renal 1-hydroxylase and thereby synthesis of 1,25-dihydroxyvitamin D3. We report here the results of experiments designed to assess the possible contribution of dietary calcium status upon calbindin-D28K gene expression in the intestine of vitamin D-deficient chicks. The actions of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] and dietary calcium intake upon intestinal calbindin-D28K and calbindin-D28K mRNA were monitored by ELISA and dot-blot hybridization analyses, respectively. Vitamin D3-deficient chicks were fed either a calcium-supplemented diet (3% w/w) or a diet containing low calcium (0.4% w/w). These dietary manipulations evoked a highly significant change in serum calcium status. However, the levels of calbindin-D28K protein and its corresponding mRNA were unaffected. Administration of 1,25-(OH)2D3 (1-16 nmol per animal) to both "normocalcemic" and hypocalcemic vitamin D-deficient chicks resulted in an equivalent stimulation of duodenal calbindin-D28K accumulation of calbindin-D28K mRNA. Intestinal calbindin-D28K was stimulated 20- to 28-fold (above control levels) by 6-8 nmol 1,25-(OH)2D3 in both dietary treatment groups when measured 48 h after the single injection. Hence, despite the existence of a relatively large difference in serum calcium levels, the molecular actions of 1,25-(OH)2D3 in the vitamin D-deficient animal are apparently well insulated from serum calcium chemistry. These observations support the notion that, in the absence of vitamin D3, the calcium ion per se is unable to modulate the calbindin-D28K gene in vivo.