Regulation of Proliferation in JEG-3 Cells by a 500-kDa Ca 2+ Sensor and Parathyroid Hormone-Related Protein

Abstract

JEG-3 cells are derived from human trophoblasts and demonstrated to express a 500-kDa Ca 2+ sensing protein, which elicits biphasic elevations of cytoplasmic Ca 2+ concentrations ([Ca 2+] i) and mediates Ca 2+ regulation of parathyroid hormone-related protein (PTHrP) release from placental cytotrophoblasts. Cytocentrifuged JEG-3 cells were immunostained by monoclonal and polyclonal antiserum toward PTHrP(1-34) and (38-64). Elevation of external Ca 2+ from 0.5 to 3.0 mM induced only a sluggish rise in [Ca 2+] i and no stimulation of cAMP production despite a more than twofold elevation of PTHrP(1-34) release. Monoclonal antibodies recognizing functionally discrepant epitopes of the Ca 2+ sensor protein substantiated uncoupling of this sensor in the Ca 2+-regulated PTHrP release. Exogenous activation of protein kinase C by a phorbol ester strongly augmented the secretion of PTHrP(1-34), whereby uncoupling of the Ca 2+ sensor was partially reversed. This functional differentiation was associated with reduced [ 3H]thymidine incorporation in JEG-3 cells. Proliferation of these cells was inhibited by 71% upon rise of extracellular Ca 2+ from 0.5 to 3.0 mM, and this inhibition was abolished by antibody-mediated interference with the Ca 2+ sensor function. PTHrP(1-86) and PTH(1-34) at concentrations up to 10 −7 M decreased proliferation and stimulated the cAMP content of JEG-3 cells. The findings support concomitant Ca 2+ sensor and PTH/PTHrP receptor expression in JEG-3 cells, and that Ca 2+ inhibits proliferation by actions on the Ca 2+ sensor as well as by stimulation of PTHrP release possibly mediating autocrine growth inhibition.