The calcium-sensing receptor (CaR) is a G protein-coupled receptor that plays key roles in extracellular calcium ion (Ca-0(2+)) homeostasis by enabling parathyroid, kidney, and other cells to directly "sense" changes in Ca-0(2+). In multiple myeloma-associated bone disease, myeloma cells could raise the level of Ca-0(2+) within their immediate vicinity in the bone marrow microenvironment, through their known capacity to cause bone destruction by stimulating osteoclastic bone resorption. Thus if myeloma cells expressed the CaR, they might sense these locally elevated levels of Ca-0(2+), which could, in turn, potentially modify their function(s) in ways that could contribute to myeloma bone disease or other aspects of the pathophysiology of this disabling hematological malignancy. In this study, we examined the expression of the CaR in three myeloma cell lines, human U266, IM-9, and RPMI8226 cells. CaR protein was present in all three cell lines as assessed by immunocytochemistry and Western blot analysis using a monoclonal antibody specific for the CaR. Moreover, the use of reverse transcription-polymerase chain reaction (RT-PCR) with CaR-specific primers, followed by nucleotide sequencing of the amplified products, also identified CaR transcripts in the three cell lines. Exposure to known polycationic agonists of the CaR, including high Ca-0(2+) (2.5 mM), neomycin, and gadolinium (Gd3+) as well as a specific CaR activator, NPS 8467, augmented cell proliferation in all three cell lines. RT-PCR revealed that U266 cells, but not IM-9 cells or RPMI8226 cells, expressed interleukin-6 (IL-6), the expression of which was not enhanced by treatments with CaR agonists. Therefore, taken together, our data first document the fact that the myeloma cell lines, U266, IM-9, and RPMI8226, all express CaR protein and mRNA. Moreover, the CaR expressed on myeloma cells could sense the locally high levels of Ca-0(2+) in the vicinity of sites of osteoclastic bone resorption and stimulate their proliferation in an IL-6-independent manner. These processes may result in promoting further growth of the tumor and aggravating the associated bone disease. (C) 2002 Elsevier Science (USA). All rights reserved.