EXTRACELLULAR CALCIUM (CA-O(2-SENSING RECEPTOR IN A MURINE BONE-MARROW-DERIVED STROMAL CELL-LINE (ST2) - POTENTIAL MEDIATOR OF THE ACTIONS OF CA-O(2+) ON THE FUNCTION OF ST2 CELLS()))

The calcium-sensing receptor (CaR) is a G protein-coupled receptor tha t plays key roles in extracellular calcium ion (Ca-o(2+)) homeostasis by mediating the actions of Ca-o(2+), on parathyroid gland and kidney. Bone marrow stromal cells support the formation of osteoclasts from t heir progenitors as well as the growth of hematopoietic stem cells by secreting humoral factors and through cell to cell contact. Stromal ce lls also have the capacity to differentiate into bone-forming osteobla sts. Bone resorption by osteoclasts probably produces substantial loca l increases in Ca-o(2+). that could provide a signal for stromal cells in the immediate vicinity, leading us to determine whether such strom al cells express the CaR. In this study, we used the murine bone marro w-derived, stromal cell line, ST2. Both immunocytochemistry and Wester n blot analysis, using an antiserum specific for the CaR, detected CaR protein in ST2 cells. We also identified CaR transcripts in ST2 cells by Northern analysis using a CaR-specific probe and by RT-PCR with Ca R-specific primers, followed by nucleotide sequencing of the amplified products. Exposure of ST2 cells to high Ca-o(2+). (4.8 mM) or to the polycationic CaR agonists, neomycin (300 mu M) or gadolinium (100 mu M ), stimulated both chemotaxis and DNA synthesis in ST2 cells. Therefor e, taken together, our data strongly suggest that the bone marrow-deri ved stromal cell line, ST2, possesses both CaR protein and messenger R NA that are very similar if not identical to those in parathyroid and kidney. Furthermore, as ST2 cells have the potential to differentiate into osteoblasts, the CaR in stromal cells could participate in bone t urnover by stimulating the proliferation and migration of such cells t o sites of bone resorption as a result of local, osteoclast-mediated r elease of Ca-o(2+), and, thereafter, initiating bone formation after t heir differentiation into osteoblasts.