LOCAL-ACTION OF EXOGENOUS GROWTH-HORMONE AND INSULIN-LIKE GROWTH-FACTOR-I ON DIHYDROXYVITAMIN-D PRODUCTION IN LLC-PK1 CELLS

Previous in vivo studies have shown that growth hormone (GH) affects v itamin D and mineral metabolism. Insulin-like growth factor-I (IGP-I) was recently reported to be a regulator of renal 1,25-dihydroxyvitamin D-3 (1,25-(OH)(2)D-3) production, suggesting that it mediates the eff ects of GH on vitamin D metabolism. However, there is no direct eviden ce to support this. The present study was designed to investigate the in vitro effects of GH and IGF-I on the renal production of 1,25-(OH)( 2)D-3 and 24,25-dihydroxyvitamin D-3 (24,25-(OH)(2)D-3) in a pig kidne y cell line, LLC-PK1. Confluent cells were preincubated in serum-free medium with hormone (GH or IGF-I) or vehicle, and then incubated with 25-[H-3]OHD3. The levels of 1,25-[H-3](OH)(2)D-3 and 24,25-[H-3](OH)(2 )D-3 produced were determined after lipid extraction and HPLC purifica tion. Production of 1,25-(OH)(2)D-3 and 24,25-(OH)(2)D-3 was increased after both IGF-I and GH preincubation in a dose-dependent manner. Sig nificant increases were found after preincubation with 13 nmol/l IGF-I (1,25-(OH)(2)D-3, 1.8-fold; 24,25-(OH)(2)D-3, 1.8-fold) or 0.9 or 9 n mol/l GH (1,25-(OH)(2)D-3, 1.3-fold and 1.5-fold; 24,25-(OH)(2)D-3, 1. 4-fold and 1.5-fold respectively). Furthermore, the effect of 9 nmol/l GH on 1,25-(OH)(2)D-3 and 24,25-(OH)(2)D-3 production was blocked in the presence of IGF-I receptor monoclonal antibody These results confi rm that IGE-I acts on renal tubules, resulting in induction of I,25-(O H)(2)D-3 and 24,25-(OH)(2)D-3 production, and the findings suggest tha t GH stimulates 1,25-(OH)2D3 and 24,25-(OW)2D3 production by increasin g local IGP-I production in the kidney.