A role for the extracellular calcium-sensing receptor in β-cell proliferation

Abstract

The extracellular calcium-sensing receptor (CaR) is a 7-transmembrane spanning G-protein-coupled receptor commonly associated with the regulation of systemic Ca2+-homeostasis. However, it is becoming increasingly apparent that the CaR has diverse effects outside this primary role. Numerous studies have now investigated the role of the CaR on proliferation. In the colon, activation of the calcium receptor up regulates adherens junction proteins, and a loss of CaR expression is associated with abnormal differentiation and progression of carcinomas [1]. In keratinocytes, inactivation of CaR expression has been shown to suppress the assembly of the cell adhesion associated epithelial-cadherin (ECAD) and - catenin-PI3K complex [2]. Pancreatic β-cells express the CaR [3]. From our own studies, pancreatic β-cells demonstrate a close association between adherens junction proteins and secretory granules [4]. Neutralising ECAD-mediated cell adhesion decreases glucose-evoked synchronicity in Ca2+-signals between adjacent cells within islets [5]. Together, these findings suggest an intriguing link between cell-cell adhesion, CaR expression and proliferation in pancreatic β-cells. Elevated [Ca2+]e can activate p38 and p42/44 MAPkinases [6, 7] and increase cell proliferation in pancreatic β-cells [8]. In the current study, we have examined whether specific activation of the CaR, using the calcimimetic R568, enhances -cell proliferation at physiologically appropriate concentrations of extracellular calcium. The involvement of MAPK signalling cascade in transducing signals from the CaR was be determined using the MEK/ERK inhibitors PD098059 and SU1498. Down-regulation of the CaR induces down-regulation of adheren junction proteins and reduces proliferative capacity in skin, colon and pancreatic -cells. In the current study, we have assessed the effect of reduced CaR expression on proliferative capacity when the cells are grown as either 2-dimensional monolayers or 3- dimensional pseudoislets, to ascertain whether cell configuration and the degree of cell-cell contact, alters CaR expression and ultimately the -cell turnover. Cellcell adhesion was reduced further by immuno-neutralising ECAD in order to test the effect on CaR-expression and calcium-mediated proliferation.