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

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 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.