Altered Phosphorylation of a 91-kDa Protein in Particulate Fractions of Rat Kidney after Protracted 1,25-Dihydroxyvitamin D3or Estrogen Treatment

1,25-Dihydroxyvitamin D3[1,25(OH)2D3] treatment in vitamin D-deficient (−D) rats results in a dose-dependent decrease in phosphorylation of a 91-kDa protein (PP-D91) in particulate fractions of the kidney. This recently reported 1,25(OH)2D3effect was examined in detail herein. In contrast to the pattern expected of a rapid signal transduction event, time course (4 h–7 days) experiments demonstrated that PP-D91 phosphorylation was not decreased until 3–5 days 1,25(OH)2D3treatment, resulting in a 61 ± 3% (P< 0.01,n= 3) decrease in PP-D91 phosphorylation by 7 days. These effects paralleled increases in plasma calcium from 9.3 ± 0.6 to 13.9 ± 0.7 mg/dl after 0 vs 7 days 1,25(OH)2D3treatment, respectively. Subcellular fractionation demonstrated that the renal PP-D91 was predominantly localized and 1,25(OH)2D3-regulated in crude mitochondrial and microsomal fractions. Further, PP-D91 was present and 1,25(OH)2D3-regulated in enriched preparations of both proximal and distal renal tubule segments. Tissue distribution studies demonstrated that the PP-D91 was predominantly present and 1,25(OH)2D3regulated in the kidney, although low levels of a vitamin D-independent phosphorylated band of similar size were observed in the lung and heart. In contrast to 1,25(OH)2D3, estradiol-17B treatment (1 mg/day × 7 day) significantly (P< 0.01) increased PP-D91 phosphorylation in kidney of both −D and +D rats (increased 118.5 ± 10.6 and 81.9 ± 6.3%, respectively). Phosphoamino acid analysis after PP-D91 phosphorylation, isolation, and proteolysis indicated that these hormones alter32P incorporation into phosphoserine residues. In conclusion, the 1,25(OH)2D3effect to reduce PP-D91 phosphorylation in particulate fractions of the rat kidney is a protracted, tissue-specific effect which parallels elevated plasma calcium levels in this model. Moreover, renal PP-D91 phosphorylation is differentially regulated by 1,25(OH)2D3vs E2treatment and occurs on phosphoserine residues. The parallel between decreased PP-D91 phosphorylation and 1,25(OH)2D3-induced hypercalcemia may suggest a role for PP-D91 in the renal response to hypervitaminosis D.