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 immun ocytochemistry has shown that disruption of microtubular assembly prev ents nuclear access of the sterol-VDR complex, the role of microtubule s in the response to 1,25(OH)(2)D-3 has not been studied in viable cel ls. Our studies examined this interaction in normal human monocytes. M onocytes convert 25(OR)D-3 to 1,25(OH)(2)D-3 and to 24-hydroxylated me tabolites more polar than 1,25(OH)(2)D-3. Microtubule disruption total ly abolished the ability of exogenous 1,25(OH)(2)D-3 to suppress its o wn synthesis and to induce 24-hydroxylase mRNA and activity, without a ffecting 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, microtub ule disruption also decreased monocyte 1,25(OH)(2)D-3 synthesis, not b y decreasing the V-max of monocyte mitochondrial 1 alpha-hydroxylase b ut 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 mitochond ria. Thus, microtubules participate in intracellular 25(OH)D-3 transpo rt, and their integrity determines normal 1,25(OH)(2)D-3 synthesis.