Enhancement of the production of 1,25-dihydroxyvitamin D3, in chick kidney mitochondria by an extramitochondrial factor

Inhibitors of 25-hydroxyvitamin D3-1alpha-hydroxylase: thiavitamin D analogs and biological evaluation

Six A-ring analogs of 1alpha,25-dihydroxyvitamin D3 (1, 1alpha,25-(OH)2-D3) 3-deoxy-3-thia-1alpha,25-(OH)2-D3 (3), 3-deoxy-3-thia-1alpha,25-(OH)2-D3-3alpha-oxide (6), 3-deoxy-3-thia-1alpha,25-(OH)2-D3-3beta-oxide (7) and the 5,6-trans counterparts 5, 8, and 9, respectively--were tested for their ability to inhibit 25-hydroxy-D3-1alpha-hydroxylase (1-OH-ase) in vitro in mitochondria isolated from kidneys of vitamin D deficient chicks. The six analogs were also evaluated in terms of their ability to bind to the chicken intestinal nuclear receptor (VDR) in comparison to the natural hormone 1alpha,25-(OH)2-D3. Analog 7 is not only the best inhibitor of the 1-OH-ase but it also binds effectively to the chick intestinal receptor. It is established that vitamin D analogs must have a 1alpha oxygen group for effective inhibition of the 1-OH-ase. This functional group is also needed for effective binding to the chick intestinal VDR.

Effect of three A-ring analogs of 1 alpha,25-dihydroxyvitamin D3 on 25-OH-D3-1 alpha-hydroxylase in isolated mitochondria and on 25-hydroxyvitamin D3 metabolism in cultured kidney cells

Three A-ring analogs of 1 alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3)--2-nor-1,3-seco-1,25(OH)2D3 (2-nor analog), 2-oxa-3-deoxy-25-OH-D3 (2-oxa analog), and A-homo-3-deoxy-3,3-dimethyl-2,4-dioxa-25-OH-D3 (A-homo analog)--were tested for their ability to inhibit 25-OH-D3-1 alpha-hydroxylase (1 alpha-hydroxylase) in isolated mitochondria and to alter 25-OH-D3 metabolism in cultured chick kidney cells. The 2-nor and 2-oxa analogs were relatively potent (Kis of 60 and 30 nM, respectively, compared with 170 nM for 1,25(OH)2D3), whereas the A-homo analog was completely ineffective in inhibiting 1 alpha-hydroxylase activity. In contrast, all three analogs were able to repress 1 alpha-hydroxylase and induce 24-hydroxylase activity in cultured chick kidney cells, suggesting that this process is not one of direct action in the mitochondria, but is more likely to be a receptor-mediated one.

25-Hydroxyvitamin D3 metabolism in mitochondria from primary renal cultures

Previous in vitro studies concerning the renal metabolism of 25-hydroxyvitamin D3 (25(OH)D3) to form 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) and 24,25R-dihydroxyvitamin D3 (24,25(OH)2D3) have utilized intact cell systems. In reflecting upon the possible mechanisms by which hormonally induced changes in the production of 1,25(OH)2D3 and 24,25(OH)2D3 may be brought about, we asked whether altered mitochondrial hydroxylase activities can quantitatively account for changes in the total cellular output of these steroids. Our objective was to delineate between extramitochondrial processes (e.g. altered substrate delivery), and those events restricted to the renal mitochondria (altered hydroxylase activities). We have examined the effect of pretreating primary cultures of chick kidney cells with either 1,25(OH)2D3 or parathyroid hormone (PTH) on 25(OH)D3-hydroxylase activities present in subsequently isolated mitochondria. Pretreatment with 10(-7) M 1,25(OH)2D3 reduced 1 alpha-hydroxylase activity in both cells and mitochondria to approximately 60% of control values by 1 h, and to 25-30% by 2 h. The effect of PTH (10 ng/ml) in both mitochondrial and whole cell preparations was an approximate 40% increase in measured 1 alpha-hydroxylase activity. 10 microM forskolin (FSK) elicited an approximate 2-fold increase in 1,25(OH)2D3 production. Reciprocal effects were observed with respect to 24-hydroxylase activity in both whole cell and mitochondrial preparations in response to exogenous 1,25(OH)2D3, PTH, and FSK. The findings demonstrate that these hormones initiate intracellular events which lead directly to altered 25(OH)D3 1 alpha- and 24-hydroxylase activities within the renal mitochondria.