A bstractThe calcium sensing receptor (C aSR ) plays an im portant role for sensing local changes in the extracellular calcium concentration ([C a 2+ ] o ) in bone rem odeling. A lthough the function of C aSR is know n, the regulatory m echanism of C aSR rem ains controversial. W e report here the regulatory effect of caveolin on C aSR function as a process of C aSR regulation by using the hum an osteosarcom a cell line (Saos-2). The intracellular calcium concentration ([C Keyw ords: antisense oligodeoxynucleotides; calcium sensing receptor; caveolae; caveolin-1; confocal microscopy; osteosarcoma cell linea
IntroductionExtracellular calcium is essential for a number of vital processes, including bone mineralization, blood coagulation, regulation of enzymatic activity, and the modulation of permeability and excitability of the plasma membranes. For these reasons, the calcium concentration in extracellular fluids is under strict control by a complex homeostatic system that includes the bones, kidney, intestines, parathyroid and thyroid glands (Brown, 1991). The extracellular calcium sensing receptor (CaSR) is an essential component of this system for regulating parathyroid hormone secretion, Ca 2+ excretion by the kidney and bone remodeling. The CaSR belongs to the type III family of G-proteincoupled receptors, which comprises the metabotropic glutamate receptor and putative vomeronasal organ receptors (Brown et al., 1993). Several lines of evidence have suggested that an increase in local Calcium sensing receptor form s com plex with and is up-regulated by caveolin-1 in cultured hum an osteosarcom a (Saos-2) cells 92 Exp. Mol. Med. Vol. 37(2), [91][92][93][94][95][96][97][98][99][100] 2005 calcium concentration in the resorption lacunae suppresses osteoclastic bone-resorbing activity through an increase in intracellular calcium; this is mediated through a ryanodine-like receptor (Zaidi et al., 1989;1991;1993;Adebanjo et al., 1994;Shin et al., 2003) and the CaSR regulates osteoclastic bone resorption (Kameda et al., 1998).Caveolae are plasma membrane organelles that are characterized by a low solubility in Triton X-100 and these are the organelles where specific lipid and protein components are subcompartmentalized (Kurzchalia and Parton, 1999). Caveolin (Cav), a 21-to 24-kDa integral membrane protein, is a principal component of the caveolae membrane and it exists as several isoforms (Cav-1, -2, and -3) (Okamoto et al., 1998;Kurzchalia and Parton, 1999). Recent studies have shown that functional proteins including receptors are localized in the caveolae by being anchored through the caveolins (Li et al., 1995; GarciaCardena et al., 1996;Couet et al., 1997;Lee et al., 2001;Cha et al., 2004). In bone, there is no evidence of the interaction between caveolin and CaSR, and for the regulatory effect of caveolin on CaSR function.The purpose of present study, therefore, is to demonstrate the relationship between CaSR and caveolins in the osteosarcoma cell line (Saos-2). Our results show that CaSR protein is co-locali...