Extracellular calcium-sensing receptor expression and its potential role in regulating parathyroid hormone-related peptide secretion in human breast cancer cell lines

Metastasis of breast cancer to bone occurs with advanced disease and produc es substantial morbidity. Secretion of PTH-related peptide (PTHrP) from bre ast cancer cells is thought to play a key role in osteolytic metastases and is increased by transforming growth factor-beta (TGF beta), which is relea sed from resorbed bone. Elevated extracellular calcium (Ca-o(2+)) also stim ulates PTHrP secretion from various normal and malignant cells, an action t hat could potentially be mediated by the Ca-o(2+)-sensing receptor (CaR) or iginally cloned from the parathyroid gland. Indeed, we previously showed th at both normal breast ductal epithelial cells and primary breast cancers ex press the CaR. In this study we investigated whether the MCF-7 and MDA-MB-2 31 human breast cancer cell lines express the CaR and whether CaR agonists modulate PTHrP secretion. Northern blot analysis and RT-PCR revealed bona f ide CaR transcripts, and immunocytochemistry and Western analysis with a sp ecific anti-CaR antiserum demonstrated CaR protein expression in both breas t cancer cell lines. Furthermore, elevated Ca-o(2+) and the polycationic Ca R agonists, neomycin and spermine, stimulated PTHrP secretion dose dependen tly, with maximal, 2.1- to 2.3-fold stimulation. In addition, pretreatment of MDA-MB-231 cells overnight with TGF beta1 (0.2, 1, or 5 ng/ml) augmented both basal and high Ca-o(2+)-stimulated PTHrP secretion. Thus, in PTHrP-se creting breast cancers metastatic to bone, the CaR could potentially partic ipate in a vicious cycle in which PTHrP-induced bone resorption raises the levels of Ca-o(2+) and TGF beta within the bony microenvironment, which the n act in concert to evoke further PTHrP release and worsening osteolysis.