Parathyroid hormone stimulation of the renal 25-hydroxyvitamin D-1α-hydroxylase -: Effect of age and free radicals

The capacity of parathyroid hormone (PTH) to increase serum 1,25(OH)(2)D levels declines with age in both rats and humans. In young rats, PTH stimulates renal 1,25(014)(2)D production and increases mRNA levels for the terminal mitochondrial P450 of the 1 alpha-hydroxylase complex (CYP27B1 or CYP1 alpha). However, in older rats PTH increases mRNA levels but not 1,25(OH)(2)D production. This suggests that in old animals there is either decreased CYP1 alpha protein levels in response to PTH or that the protein produced lacks functionality. The CYP1 alpha protein is located on the inner mitochondrial membrane, the site of increased free radical production with age. To study these possibilities, we examined the effect of PTH and free radicals on CYP1 alpha expression in a model system-AOK-B50 renal tubular cells. PTH increased CYP1 alpha mRNA and protein in a similar time-dependent manner, suggesting that CYP1 alpha protein levels were largely regulated by mRNA levels. The effect of free radicals was determined by preincubation with hydrogen peroxide (H2O2), a standard model for studying free radical damage. H2O2 inhibited PTH-stimulated CYP1 alpha protein levels and 1,25(OH)(2)D production in a dose dependent manner. However, 1,25(OH)(2)D production was more sensitive to H2O2 than was CYP1 alpha protein levels. This suggests that the catalytic activity of the CYP1 alpha protein may be reduced by free radical damage in these cells. Future studies will focus on detecting oxidative damage in this model system and in vivo. (c) 2006 Elsevier Ltd. All rights reserved.