A role for the cytoskeleton in renal vitamin D metabolism

Conversion of circulating 25-hydroxyvitamin D3 (25(OH)D3) to its active metabolite 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) occurs in the renal tubule mitochondrion. Recent reports have implicated the cytoskeleton in certain other steroid metabolizing cells as a mediator of a rate-limiting mitochondrial transport step. Whilst the activity of the renal converting enzyme, a typical steroid hydroxylase, is known to be regulated closely by a number of well studied factors, no information is available to indicate whether an analogous transport step is relevant to the regulation of vitamin D metabolism. Cytochalasin B and vinblastine were used as chemical antagonists of the microfilamentous and microtubular elements of the cytoskeleton. Both agents inhibited the conversion of 25(OH)D3 to 1,25(OH)2D3 by isolated vitamin D-deficient chick renal tubules in a dose-dependent manner. At the concentrations required to inhibit 25(OH)D3-1 alpha-hydroxylase activity in whole cells, these agents inhibited neither isolated mitochondrial 1,25(OH)2D3 production, nor 24,25(OH)2D3 synthesis by vitamin D-replete tubules. The cytoskeletal antagonists were found to increase the content of labelled 1,25(OH)2D3 and 25(OH)D3 in a mitochondrial fraction prepared by Percoll fractionation of tubule cells pre-exposed to the antagonists and labelled 25(OH)D3 substrate. The data suggest that disruption of the cytoskeleton may result in inhibition of transport of newly synthesised 1,25(OH)2D3 out of the mitochondrion and through the cell, and accumulating 1,25(OH)2D3 may oppose its further synthesis. This is consistent with a transport process mediated by the cytoskeleton being involved in the regulation of renal vitamin D metabolism.