Microtubules mediate cellular 25-hydroxyvitamin D3 trafficking and the genomic response to 1,25-dihydroxyvitamin D3 in normal human monocytes. 
The genomic actions of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) 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)2D3 has not been studied in viable cells. Our studies examined this interaction in normal human monocytes. Monocytes convert 25(OH)D3 to 1,25(OH)2D3 and to 24-hydroxylated metabolites more polar than 1,25(OH)2D3. Microtubule disruption totally abolished the ability of exogenous 1,25(OH)2D3 to suppress its own synthesis and to induce 24-hydroxylase mRNA and activity, without affecting either total 1,25(OH)2D3 uptake or maximal 1,25(OH)2D3-VDR binding. Thus, intact microtubules are essential for 1,25(OH)2D3-dependent modulation of gene transcription. Interestingly, microtubule disruption also decreased monocyte 1,25(OH)2D3 synthesis, not by decreasing the Vmax of monocyte mitochondrial 1 alpha-hydroxylase but through an increase in the Km for 25(OH)2D3. We examined 25(OH)D3 transport. Microtubule disruption did not affect total cellular 25(OH)D3 uptake but reduced its intracellular trafficking to the mitochondria. Thus, microtubules participate in intracellular 25(OH)D3 transport, and their integrity determines normal 1,25(OH)2D3 synthesis.