The mTORC2 Regulator homer1 Modulates Protein Levels and Sub-Cellular Localization of the CaSR in Human Osteoblasts.

We recently found that in human osteoblasts Homer1 complexes to CaSR and mediates AKT initiation via mTORC2 leading to beneficial effects in osteoblasts including -catenin stabilization and mTORC1 activation (doi: 10.1074/jbc.RA118.006587). Herein we further investigated the relationship between Homer1 and CaSR and demonstrate a link between the protein levels of CaSR and Homer1 in human osteoblasts in primary culture. Thus, when siRNA was used to suppress the CaSR, we observed upregulated Homer1 levels and when siRNA was used to suppress Homer1 we observed downregulated CaSR protein levels using immunofluorescence staining of cultured osteoblasts as well as western blot analyses of cell protein extracts. This finding was confirmed in vivo as the bone cells from osteoblast specific CaSR(-/-) mice showed increased Homer1 expression compared to wild-type. Furthermore, when the commonly used osteosarcoma cell lines MG63 and SAOS-2 were compared to primary osteoblasts, higher levels of Homer1 protein were associated with increased protein levels of the CaSR as well as mTOR and Rictor. CaSR and Homer1 protein were both expressed in osteocytes embedded in the long bones of wild-type mice, and immunofluorescent studies of these cells revealed that Homer1 protein sub-cellular localization was markedly altered in the osteocytes of CaSR(-/-) mice compared to wt. The study identifies additional roles for Homer1 in the control of the protein level and subcellular localization of CaSR in cells of the osteoblast lineage, in addition to its established role of mTORC2 activation downstream of the receptor.