Local Action of Phosphate Depletion and Insulin-like Growth Factor 1 on in Vitro Production of 1,25-dihydroxyvitamin D by Cultured Mammalian Kidney Cells

The hormonal form of vitamin D, 1,25(OH)2D, is synthesized mostly in proximal renal tubular cells. Experimental and clinical studies suggest that the growth hormone may be involved in growth-related fluctuations of plasma 1,25(OH)2D and in the increase of 1,25(OH)2D induced by in vivo phosphate deprivation, an action possibly mediated by insulin-like growth factor 1 (IGF 1). We tested the effects of phosphate depletion and IGF 1 addition on 1,25(OH)2D3 production in cultured kidney cells: opossum kidney (OK) cells, LLC-PK 1, and rabbit's proximal tubular cells. Confluent cell monolayers were preincubated in various phosphate concentrations, in the presence and absence of IGF 1. Then, 5 nM of [3H]25 (OH)D3 or 2 microM of 25 (OH)D3 were added to the medium and the cells were incubated for a further 120 min. The amount of biosynthesized 1,25(OH)2D3 in lipid extracts was determined after two different straight phase high performance liquid chromatographies. The experiment showed the following: (a) LLC-PK 1 and rabbit's cells expressed a detectable ability to synthesize 1,25(OH)2D3, while OK cells did not. (b) Partial or total phosphate deprivation increased the amount of 1,25(OH)2D3 produced, respectively in LLC-PK 1 and in rabbit's cells. (c) IGF 1 (25 ng/ml) increased 1,25(OH)2D3 production in rabbit's cells, particularly in phosphate-free medium (1.6-fold), and in LLC-PK 1 cells, in partial phosphate depletion (2.75-fold in 1 mM phosphate, P = 0.015, n = 5, and 3.2-fold in 0.5 mM phosphate, P = 0.043, n = 4). Our findings demonstrate a local action of phosphate depletion and of IGF 1 on 1,25-dihydroxyvitamin D3 production.