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Division of Endocrinology (J.T.-H., N.C., S.Y., D.K., P.R., E.M.B.), Diabetes and Hypertension, Department of Medicine and Membrane Biology Program, Brigham and Women’s Hospital and Harvard Medical School, 02115 Boston, Massachusetts; and Osteoporosis and Bone Metabolic Unit (J.T.-H., P.S.), Department of Clinical Biochemistry and Endocrinology, Copenhagen University Hospital, Hvidovre DK-2650, Denmark
Address all correspondence and requests for reprints to: Jacob Tfelt-Hansen, Division of Endocrinology, Diabetes and Hypertension, Department of Medicine and Membrane Biology Program, Brigham and Women’s Hospital and Harvard Medical School, 221 Longwood Avenue, Boston, Massachusetts 02115. E-mail: jtfelt{at}rics.bwh.harvard.edu.
Using H-500 rat Leydig cancer cells as a model of humoral hypercalcemia of malignancy (HHM), we previously showed that high Ca2+ induces PTH-related peptide (PTHrP) secretion via the calcium-sensing receptor (CaR) and mitogen- and stress-activated kinases, e.g. MAPK kinase 1 (MEK1), p38 MAPK, and stress-activated protein kinase 1/c-Jun N-terminal kinase. Because cellular proliferation is a hallmark of malignancy, we studied the role of the CaR in regulating the proliferation of H-500 cells. Elevated Ca2+ has a mitogenic effect on these cells that is mediated by the CaR, because the calcimimetic NPS R-467 also induced proliferation. Inhibition of phosphatidylinositol 3-kinase (PI3K) and p38 MAPK but not MEK1 abolished the mitogenic effect. Activation of PI3K by elevated Ca2+ was documented by phosphorylation of its downstream kinase, protein kinase B. Because protein kinase B activation promotes cell survival, we speculated that elevated Ca2+ might protect H-500 cells against apoptosis. Using terminal uridine deoxynucleotidyl nick end labeling staining, we demonstrated that high Ca2+ (7.5 mM) and NPS R-467 indeed protect cells against apoptosis induced by serum withdrawal compared with low Ca2+ (0.5 mM). Because the CaR induces PTHrP secretion, it is possible that the mitogenic and antiapoptotic effects of elevated Ca2+ could be indirect and mediated via PTHrP. However, blocking the type 1 PTH receptor with PTH (7–34) peptide did not alter either high Ca2+-induced proliferation or protection against apoptosis. Taken together, our data show that activation of PI3K and p38 MAPK but not of MEK1/ERK by the CaR promotes proliferation of H-500 cells as well as affords protection against apoptosis. These effects are likely direct without the involvement of PTHrP in an autocrine mode.
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