REGULATION OF VITAMIN-D-RESPONSIVE GENE-EXPRESSION BY FLUORINATED ANALOGS OF CALCITRIOL IN RAT OSTEOBLASTIC ROB-C26 CELLS

We compared the activation of vitamin D-responsive genes by 24,24-difl uorocalcitriol [F-2-1 alpha,25(OH)(2)D-3] and 26,26,26,27,27,27-hexafl uorocalcitriol [F-6-1 alpha,25(OH)(2)D-3] with that by calcitriol [1 a lpha,25(OH)(2)D-3] in rat osteoblastic ROB-C26 cells. F-2-1 alpha,25(O H)(2)D-3 and F-6-1 alpha,25(OH)(2)D-3 were ten times more potent than 1 alpha,25(OH)(2)D-3 in inducing the expression of 1 alpha, 25(OH)(2)D -3 -24-hydroxylase (24-OHase) mRNA 6 h after adding vitamin D compound s. The lower affinity of these two fluorinated analogs compared with t hat of la,25(OH),D, for vitamin D binding protein in serum (serum DEP) seemed to be partly involved in their increased ability to activate t he 24-OHase gene. A time course study revealed that the expression of the 24-OHase and osteopontin mRNAs in the cells incubated with 1 alpha ,25(OH)(2)D-3 and F-2-1 alpha,25(OH)(2)D-3 attained maximal levels at 6 h for 24-OHase mRNA and 18 h for osteopontin mRNA, the both decrease d thereafter. On the contrary, F-6-1 alpha,25(OH)(2)D-3 increased the expression of 24-OHase and osteopontin exponentially until 72 h. While F(2)1 alpha,25(OH)(2) [1 beta-H-3]D-3 was catabolized quickly by ROB- C26 cells, F-6-1 alpha,25(OH)(2) [1 beta-H-3]D-3 was slowly and quanti tatively converted into putative 26,26,26,27,27,27 -hexafluoro-23S-hyd roxy [1 beta-H-3] calcitriol {F-6-1 alpha,23S,25(OH)(3) [1 beta-H-3]D- 3}. This may explain why the time-course profiles of the accumulation of mRNAs for 24-OHase and osteopontin differed in the cells exposed to the fluorinated analogs. In addition to the longer retention, unknown up-regulating mechanisms appeared to be involved in the exponential a ctivation of the 24-OHase and osteopontion genes induced by F-6-1 alph a,25(OH)(2)D-3.