In vitro effects of G-Protein coupled receptor 120 agonist on osteoblast differentiation and activity in osteosarcoma cells

Abstract

Osteoblasts are important bone cells that play a central role in the formation and mineralization of bone. The extracellular-signal-related kinase (ERK) and protein kinase B (AKT) pathways play a role in osteoblast differentiation by regulating the activity of runt-related transcription factor 2 (Runx2). Osterix and Runx2 are major osteoblastic transcriptional factors. Runt-related transcription factor 2 is responsible for the upregulation of osteoblast specific genes including but not limited to: receptor activator of nuclear factor kB ligand (RANKL), osteoprotegerin (OPG), and alkaline phosphatase (ALP). G-protein coupled receptors (GPCRs) govern many physiological changes in cells. G-protein coupled receptor 120 (GPR120) has gained attention as a target for the development of drugs for several human diseases because of its effects on inflammatory-associated diseases. It has further been shown to affect the formation and function of bone cells; however, the exact mechanisms of the processes are not fully understood. In this study, various methods were carried out to establish the outcome of a GPR120 agonist (TUG-891) on the formation and function of osteoblasts in human osteosarcoma MG-63 cell line. MG-63 cells were exposed to TUG-891 [0.1-100 µM] in osteogenic medium (OM). Undifferentiated cells were used to investigate TUG-891's impact on cell viability using resazurin assay. Thereafter, TUG-891 was applied to the cells for 7, 14 or 21 days. Later, the following assessments were carried out: Osteoblast differentiation using the ALP activity, calcium mineralisation using alizarin red staining, protein expression (ERK, pERK, AKT, and pAKT) using western blot, and gene expression (ALP, Osterix, RANKL, OPG, BSP and Runx2) using quantitative polymerase chain reaction (q-PCR). TUG-891 had no significant influence on viability of the cells. ALP activity was significantly increased after 7 days (P<0.01) and 21 days (P<0.0001) of differentiation compared to the OM baseline control. TUG-891 significantly decreased alizarin red S staining in the calcium mineralisation assay after 14 days (P<0.0001) and 21 days (P<0.001) when contrasted to the OM baseline control. Gene expression was tested for 7 and 14 days. There were no significant effects in gene expression observed for any of the tested genes (ALP, Osterix, RANKL, OPG, BSP and Runx2) after day 7 or 14. However, TUG-891 increase gene expression after day 7 and decreased the expression after 14 days compared to OM baseline control. TUG-891 induced no significant effect on ERK and AKT phosphorylation after 15 minutes, 60 minutes, 4 hours, or 24 hours compared to the baseline control. GPR120 siRNA experiments were carried out for 15 minutes. TUG-891 showed no significant differences in the relative ERK and AKT phosphorylation in the siRNA cells that were used as controls and the GPR120 siRNA cells in comparison to the OM baseline control. To conclude, TUG-891 reveals negative changes to gene expression and alizarin red staining. This effect may be regulated by the ERK and AKT signalling pathway. However, no significant changes were seen since only preliminary data was presented. These findings may reveal that the GPR120 agonist, TUG-891, may have negative effects on bone health through inhibition of osteoblast activity.