Recent Advance in Regulatory Effect of GRP120 on Bone Metabolism

The link between fatty acids and bone metabolism is complex and can be direct and indirect. This link has been reported in different types of bone cells and various stages of bone metabolism. G-protein coupled receptor 120 (GPR120), also called free fatty acid receptor 4 (FFAR4), is a member of the recently discovered G protein-coupled receptor family that can interact with both long-chain saturated fatty acids (C14-C18) and long-chain unsaturated fatty acids (C16-C22). Research shows that GPR120 regulates processes in different types of bone cells, directly or indirectly affecting bone metabolism. Our research reviewed the literature on the effects of GPR120 on bone marrow mesenchymal stem cells (BMMSCs), osteoblasts, osteoclasts, and chondrocytes, focusing on the research findings regarding the mechanism by which GPR120 alters specific bone metabolic diseases-osteoporosis and osteoarthritis. The data reviewed here provide a basis for clinical and basic research into the role of GPR120 on bone metabolic diseases.

GPR120 (FFAR4) is preferentially expressed in pancreatic delta cells and regulates somatostatin secretion from murine islets of Langerhans

AIMS/HYPOTHESIS

The NEFA-responsive G-protein coupled receptor 120 (GPR120) has been implicated in the regulation of inflammation, in the control of incretin secretion and as a predisposing factor influencing the development of type 2 diabetes by regulation of islet cell apoptosis. However, there is still considerable controversy about the tissue distribution of GPR120 and, in particular, it remains unclear which islet cell types express this molecule. In the present study, we have addressed this issue by constructing a Gpr120-knockout/β-galactosidase (LacZ) knock-in (KO/KI) mouse to examine the distribution and functional role of GPR120 in the endocrine pancreas.

METHODS

A KO/KI mouse was generated in which exon 1 of the Gpr120 gene (also known as Ffar4) was replaced in frame by LacZ, thereby allowing for regulated expression of β-galactosidase under the control of the endogenous GPR120 promoter. The distribution of GPR120 was inferred from expression studies detecting β-galactosidase activity and protein production. Islet hormone secretion was measured from isolated mouse islets treated with selective GPR120 agonists.

RESULTS

β-galactosidase activity was detected as a surrogate for GPR120 expression exclusively in a small population of islet endocrine cells located peripherally within the islet mantle. Immunofluorescence analysis revealed co-localisation with somatostatin suggesting that GPR120 is preferentially produced in islet delta cells. In confirmation of this, glucose-induced somatostatin secretion was inhibited by a range of selective GPR120 agonists. This response was lost in GPR120-knockout mice.

CONCLUSIONS/INTERPRETATION

The results imply that GPR120 is selectively present within the delta cells of murine islets and that it regulates somatostatin secretion.

Co-localisation and secretion of glucagon-like peptide 1 and peptide YY from primary cultured human L cells

AIMS/HYPOTHESIS

Targeting the secretion of gut peptides such as glucagon-like peptide 1 (GLP-1) and peptide YY (PYY) is a strategy under development for the treatment of diabetes and obesity, aiming to mimic the beneficial alterations in intestinal physiology that follow gastric bypass surgery. In vitro systems are now well established for studying the mouse enteroendocrine system, but whether these accurately model the human gut remains unclear. The aim of this study was to establish and characterise human primary intestinal cultures as a model for assessing GLP-1 and PYY secretion in vitro.

METHODS

Fresh surgical biopsies of human colon were digested with collagenase to generate primary cultures from which GLP-1 and PYY secretion were assayed in response to test stimuli. GLP-1 and PYY co-localisation were assessed by flow cytometry and immunofluorescence microscopy.

RESULTS

GLP-1 and PYY were found localised in the same cells and the same secretory vesicles in human colonic tissue samples. GLP-1 release was increased to 2.6-fold the control value by forskolin + isobutylmethylxanthine (10 μmol/l each), 2.8-fold by phorbol myristate acetate (1 μmol/l) and 1.4-fold by linoleic acid (100 μmol/l). PYY release was increased to 2.0-, 1.8- and 1.3-fold by the same stimuli, respectively. Agonists of G-protein-coupled receptor (GPR)40/120 and G-protein-coupled bile acid receptor 1 (GPBAR1) each increased GLP-1 release to 1.5-fold, but a GPR119 agonist did not significantly stimulate secretion.

CONCLUSIONS/INTERPRETATION

Primary human colonic cultures provide an in vitro model for interrogating the human enteroendocrine system, and co-secrete GLP-1 and PYY. We found no evidence of PYY-specific cells not producing GLP-1. GLP-1 secretion was enhanced by small molecule agonists of GPR40/120 and GPBAR1.