Evidence for interference of vitamin D with PTH/PTHrP receptor expression in opossum kidney cells

Vitamin D counters the phosphaturic action of parathyroid hormone (PTH) in rats in vivo. The present study was undertaken to examine this interaction using monolayers of Opossum kidney (OK) cells. ³²P uptake, cAMP generation, PTH/PTHrP receptor mRNA expression and intracellular Ca²⁺ [Ca²⁺](i) were measured in (1) control cells, (2) cells exposed to PTH, (3) cells pretreated with 1,25-dihydroxyvitamin D₃ [1,25(OH)2D₃], and (4) 1,25(OH)₂D₃- pretreated cells exposed to PTH. ³²P uptakes were in (1) 5.00±0.20 (mean ±SE), in (2) 2.30±0.14 (P<0.001 versus group 1), in (3) 4.80±0.24 (P NS versus group 1) and in (4) 3.70±0.20 (P<0.001 versus group 2) nmol P(i)/(mg·prot 10 mm). cAMP levels were in (1) 10±3, in (2) 210±8, in (3) 12±4, and in (4) 122±12 pmol cAMP/mg protein (P<0.001 versus group 2). PTH/PTHrP receptor mRNA expression was in relative units: (1) 100±0, (2) 99.5±6.2, (3) 68.7±2.6 (P<0.001 versus group 1), and (4) 34.8±3.3 (P<0.001 versus group 1). In groups 2 and 4 PTH induced equal transient increments in [Ca²⁺](i). These experiments demonstrate that the effect of vitamin D on phosphate transport is associated with a commensurate diminution in PTH/PTHrP receptor gene expression and PTH-induced cAMP formation but not with Ca²⁺ transients. Vitamin D per se does not affect ³²P uptake or cAMP generation while it slightly decreased PTH/PTHrP receptor gene expression. These observations demonstrate that: (1) 1.25(OH)₂D₃ directly antagonizes the effects of PTH on ³²P uptake in OK cells, (2) this effect is mediated via inhibition of PTH-induced activation of AC/cAMP system, (3) the diminution in PTH-induced cAMP formation may stem at least in part from a decrease in the expression of PTH/PTHrP receptor mRNA.