1α-Hydroxylase and innate immune responses to 25-hydroxyvitamin D in colonic cell lines

Vitamin D-insufficiency is a prevalent condition in populations throughout the world, with low serum levels of 25-hydroxyvitamin D (25OHD) linked to a variety of human health concerns including cancer, autoimmune disease and infection. Current data suggest that 25OHD action involves localized extrarenal conversion to 1,25-dihydroxyvitamin (1,25(OH)2D) via tissue-specific expression of the enzyme 25-hydroxyvitamin D-1 alpha-hydroxylase (1 alpha-hydroxylase). In cells such as macrophages, expression of 1 alpha-hydroxylase is intimately associated with toll-like receptor (TLR) recognition of pathogens. However, this mechanism may not be exclusive to extra-renal generation of 1,25(OH)2D. To investigate the relationship between TLR-mediated pathogen recognition and vitamin D-induced antibacterial activity, intracrine responses to 25OHD metabolism were explored in vitro using the established colonic cell lines Caco-2 and Caco-2 clone BBe. Analysis of antibacterial factors such as cathelicidin (LL37) and beta-defensin-4 (DEFB4) was carried out following co-treatment with TLR ligands. Data indicate that, unlike macrophages. Caco-2 and BBe colonic cell lines are unresponsive to TLR-induced 1 alpha-hydroxylase. Alternative activators of 1 alpha-hydroxylase such as transforming growth factor beta were also ineffective at priming intracrine responses to 25OHD. Thus, in common with other barrier sites such as the skin or placenta, colonic epithelial cells may require specific factors to initiate intracrine responses to vitamin D. (C) 2010 Elsevier Ltd. All rights reserved.