Regulation of gene expression and intestinal calcium absorption: Role of CAT1, calbindin D9k, and VDR

Transcellular calcium (Ca) absorption is primarily regulated by genomic action of 1,25(OH)2 D3 through vitamin D receptor (VDR). In an attempt to better understand the molecular mechanisms of transcellular Ca absorption, we investigated the role of CaT1, calbindin D9k and VDR in the process of Ca absorption in mouse. We found that duodenal CaT1 mRNA is very responsive to the changes in serum 1,25(OH)2 D3 caused by diet and 1,25(OH)2 D3 injection. Coordinate regulation of duodenal CaT1, calbindin D9k, and Ca absorption in response to short-term changes of dietary Ca, and rapid induction of CaT1 preceeding Ca absorption after 1,25(OH)2 D3 injection suggest an essential role for 1,25(OH)2 D3-mediated expression of CaT1 in Ca absorption. However, in VDR knockout (KO) mice on a 2.0% Ca diet, Ca absorption was increased by 13% while CaT1 mRNA was \u3c1% of the levels seen in WT fed a 0.5% Ca diet. Our data suggest that it is not likely that CaT1 is a rate-limiting determinant in Ca absorption. Calbindin D9k has been proposed to function as a ferry to facilitate the diffusion of Ca in the cytosol. In KO mice fed a 0.5% Ca diet, we did not see high Ca absorption based on their relative high calbindin D9k protein. Moreover, with 1,25(OH)2 D3 injection, Ca absorption proceeded the induction of calbindin D9k mRNA. Our data does not support the role of calbindin D9k as a rate-limiting determinant in intestinal Ca absorption. However, it does not eliminate the possibility that this protein acts as an intracellular buffer to prevent the cytotoxicity during active Ca absorption. Reduced VDR levels have been proposed to be responsible for intestinal resistance to 1,25(OH)2 D3 in aging. Compared to VDR heterozygous (VDR +/−) mice containing 50% of VDR protein, we found the slope of the relationship between Ca absorption and circulating 1,25(OH)2 D3 was steeper in normal mice, suggesting blunt responsiveness of intestine in VDR +/− mice. Our data support the hypothesis that reduced VDR levels account for the intestinal resistance to 1,25(OH)2 D3