Regulation of the 1b isoform of the plasma membrane calcium pump by 1,25-dihydroxyvitamin D-3 in rat osteoblast-like cells

The first isogene of the plasma membrane calcium pump (PM3CA1) is expressed on the apical plasma membrane of osteoblasts, but its regulation by 1,25-d ihydroxyvitamin D-3 [1,25(OH)(2)D-3] has not been studied in this cell type . We studied 1,25(OH)(2)D-3 effects on PMCA1 function, protein, messenger R NA (mRNA), and isoform expression in osteoblasts. Of seven rat and human im mortalized osteoblast-like cell lines studied. PMCA1 mRNA expression was co nfirmed in all. Only ROS 17/2.8 cells expressed measurable PMCA1 protein by Western analysis. Immunocytochemistry indicated that PMCA1 was expressed p redominantly on the plasma membrane of ROS 17/2.8 cells. The 1,25(OH)(2)D-3 but not 24,25-dihydrosyvitamin D-3 [24,25(OH)(2)D-3] treatment of confluen t ROS 17/2.8 cells resulted in an approximate 3- to 5-fold dose-dependent i ncrease in PMCA1 expression of message and protein as assessed by Western a nd Northern analysis and vesicular Ca-45 uptake of membrane vesicles. 1,25( OH)(2)D-3 had no effect on PMCA1 posttranscriptional splicing. The Ib isofo rm of PMCA was expressed under all experimental conditions. 1,25(OH)(2)D-3 favored increased expression of the 5.5 kilobases (kb over the 7.5-kb PMCA1 b transcript, with a 2-fold proportional increase in the smaller transcript relative to the larger transcript evident at the highest dose of 1,25(OH)( 2)D-3 studied. The resultant proportional increase in the smaller 5.5-kb tr anscript may increase mRNA stability and account for the increase in PMCA1b protein and function with 1,25(OH)(2)D-3. These data provide evidence for the role of 1,25(OH)(2)D-3 and PMCA1b in the regulation of calcium transpor t in bone cells.