MICROTUBULES MEDIATE CELLULAR 25-HYDROXYVITAMIN-D-3 TRAFFICKING AND THE GENOMIC RESPONSE TO 1,25-DIHYDROXYVITAMIN-D-3 IN NORMAL HUMAN MONOCYTES

The genomic actions of 1,25-dihydroxyvitamin D-3 (1,25(OH)(2)D-3) are mediated by the intracellular vitamin D receptor (VDR). Although immunocytochemistry has shown that disruption of microtubular assembly prevents nuclear access of the sterol-VDR complex, the role of microtubules in the response to 1,25(OH)(2)D-3 has not been studied in viable cells. Our studies examined this interaction in normal human monocytes. Monocytes convert 25(OR)D-3 to 1,25(OH)(2)D-3 and to 24-hydroxylated metabolites more polar than 1,25(OH)(2)D-3. Microtubule disruption totally abolished the ability of exogenous 1,25(OH)(2)D-3 to suppress its own synthesis and to induce 24-hydroxylase mRNA and activity, without affecting either total 1,25(OH)(2)D-3 uptake or maximal 1,25(OH)(2)D(-)3VDR binding. Thus, intact microtubules are essential for 1,25(OH)(2)D-3-dependent modulation of gene transcription. Interestingly, microtubule disruption also decreased monocyte 1,25(OH)(2)D-3 synthesis, not by decreasing the V-max of monocyte mitochondrial 1 alpha-hydroxylase but through an increase in the K-m for 25(OH)D-3. We examined 25(OH)D-3 transport. Microtubule disruption did not affect total cellular 25(OH)D-3 uptake but reduced its intracellular trafficking to the mitochondria. Thus, microtubules participate in intracellular 25(OH)D-3 transport, and their integrity determines normal 1,25(OH)(2)D-3 synthesis.