A variant form of the 1,25-dihydroxyvitamin D3 receptor with low apparent hormone affinity in cultured monkey kidney cells (LLC-MK2). A model for tissue resistance to vitamin D.

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Chandler et al. (Chandler, J.S., Chandler, S.K., Pike, J.W., and Haussler, M.R. (1984) J. Biol. Chem. 259, 2214-2222) previously demonstrated that 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) caused the induction of 25-hydroxyvitamin D3-24-hydroxylase (24-hydroxylase) in a rhesus monkey kidney cell line (LLC-MK2) apparently deficient in the high affinity 1,25-(OH)2D3 receptor. We have re-examined this phenomenon and report here that 24-hydroxylase induction is mediated by a receptor variant in LLC-MK2 cells with low hormone affinity. Dose response analysis showed that in contrast to LLC-PK1 (a typical receptor-positive cell line), the LLC-MK2 line was less sensitive to 1,25-(OH)2D3 by 2 orders of magnitude. Employing optimal concentrations of 1,25-(OH)2D3 for 24-hydroxylase induction in each cell type, the early time courses of this bioresponse were identical in LLC-MK2 and LLC-PK1 and were consistent with a nuclear action of hormone-receptor complexes. Moreover, the rank order of potency of vitamin D3 congeners as inducers of 24-hydroxylase activity in LLC-MK2 cells agreed well with their relative affinity for the 1,25-(OH)2D3 receptor. An examination of 1,25-(OH)2D3 receptor content via DNA-cellulose chromatography in LLC-MK2 cells incubated at ligand concentrations 10-25-fold higher than the normal 2 nM revealed a minimum of 1600 receptor-like molecules/LLC-MK2 cell. These results show that LLC-MK2 cells possess a variant receptor form with apparent low affinity for 1,25-(OH)2D3. This system should serve as a model for clinical syndromes characterized by the requirement for massive doses of vitamin D to prevent rickets.