The control of intestinal calcium absorption is important for human health in two ways. First, the risk of osteoporotic hip fracture is higher in women with low calcium absorption efficiency and this may be due to age-associated calcium malabsorption or intestinal resistance to 1,25(OH)2 vitamin D3 (1,25(OH)2 D, the primary regulator of intestinal calcium absorption). Second, a significant barrier to the use of vitamin D analogs as pro-differentiating agents in cancer treatment is that they stimulate intestinal calcium absorption and cause hypercalcemia. Our long-term objective is to clarify the mechanisms used by 1,25(OH)2 D to promote calcium absorption and to utilize this information to improve calcium absorption in people with low fractional calcium absorption and to aid in the design of vitamin D analogs that can be used as non-calcemic cancer therapeutics. New research shows that 1,25(OH)2 D rapidly activates second messenger and kinases pathways including the MAP kinases and their upstream activators; inhibition of these kinases blunts 1,25(OH)2 D-mediated gene transcription indicating the classical and non-classical vitamin D signaling pathways interact. The goal of the proposed research is to determine how the 1,25(OH)2 D-mediated transcriptional activation of intestinal calcium absorption is influenced by the basal or induced activity of the MAP kinases ERK1 and 2.
The specific aims of this project are to: (1) Identify the nVDR-mediated genomic pathways controlling intestinal calcium absorption that are modulated by 1,25(OH)2-induced activation of ERK1 and 2, and (2) Establish the protein-protein interactions necessary for 1,25(OH)2 D-mediated gene expression that are promoted by 1,25(OH)2 D-induced ERK1 and 2 activity. We will accomplish these aims by studying the effect of 1,25(OH)2 D in a well-characterized cell culture model (Caco-2 cells) and in the small intestine of mice. Biological actions of 1,25(OH)2 D will be studied in the presence of activators and inhibitors of protein kinases (pharmacologic inhibitors, dominant negative kinases) and the rapid actions of vitamin D (vitamin D analogs), nVDR action and function will be studied with cellular imaging, reporter genes, multi-hybrid assays, and chromatin immunoprecipitation (CHIP) assays. Elucidating the mechanism of this vitamin D signal pathway cross-talk will provide the foundation for controlled modulation of intestinal calcium absorption, e.g. when vitamin D resistance associated with aging or estrogen deficiency is present.
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