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    • Fatty acids have been repeatedly

    2022-01-17

    Fatty acids have been repeatedly shown to increase the responsiveness of pancreatic islets to glucose both in vitro and in vivo[6]. The recent identification of GPR40 as a receptor for free fatty acids that is localized to the pancreatic islet cells has therefore stimulated interest in obtaining small molecule modulators of its function as glucose-dependent insulin secretagogues for the treatment of Type II Diabetes [7], [8], [9].
    BIOLOGY OF GPR40 GPR40 is a member of a family G-protein-coupled receptors that sense free fatty acids [7], [8], [9], [10]. The other members of the family, GPR41 and GPR43, respond to short chain fatty acids while GPR40 prefers long chain fatty acids (LCFA, >C6). The potency of LCFA effects on GPR40 increases in parallel with increasing chain length and degree of unsaturation [8]. Among the common naturally occurring fatty acids, docosahexaenoic CCT241533 receptor (DHA) shows the most potent effects displaying an EC50 of approximately 1μM in CHO cells expressing GPR40 [8]. Expression of GPR40 was initially reported in the pancreas and small intestine of rats [8], and pancreas and the brain in humans [7]. Expression in both humans and rodents is highest in the pancreas, and is particularly concentrated in the pancreatic islets [11]. The high expression of GPR40 in pancreatic islets in humans was verified from samples taken from patients undergoing pancreatectomy and was found to be comparable to that of targets of clinically used antidiabetic agents such as the KATP channel and the GLP-1 receptor [12]. Within the islet, expression of GPR40 mRNA is highest in insulin-secreting beta cell lines such as the mouse insulinoma (MIN6) line [8]. Reports conflict as to whether GPR40 is present in other islet cell types such as glucagon-secreting alpha cells [8], [13]. When pancreatic beta cell lines such as MIN6 are treated with LCFA, insulin secretion is stimulated in a glucose-dependent manner. The effects of LCFA are evident when the medium contains 5.5mM glucose, and is near maximal at 11mM glucose [8]. The effects of LCFA on MIN6 cells are GPR40-mediated since pre-treatment of MIN6 cells with GPR40-specific small interfering RNA (siRNA) nearly abolishes the effect of LCFA on glucose-dependent insulin secretion while leaving GLP-1-stimulated insulin secretion unchanged [8]. Signal transduction from the GPR40 receptor occurs primarily through Gαq. When treated with long chain fatty acids MIN6, Chinese hamster ovary (CHO), and human embryonic kidney (HEK293) cells overexpressing GPR40 show an increase in intracellular calcium ions ([Ca2+]i) [7], [8], [14]. This increase in [Ca2+]i is attenuated by a phospholipase C (PLC) inhibitor, consistent with a Gαq-coupled pathway [14]. The products of PLC, inositol triphosphate (IP3) and diacylglycerols (DAG), can each further propagate signaling through GPR40. IP3 increases [Ca2+]i by triggering release of calcium ions from stores in the endoplasmic reticulum. DAG, either alone or in combination with increased [Ca2+]i can activate certain isoforms of protein kinase C (PKC). Furthermore, it has been reported that GPR40 can additionally signal through Gαs and thus leading to an increase in intracellular cAMP levels and activation of protein kinase A (PKA) [15] (Figure 1). To understand how activation of GPR40 can amplify glucose-stimulated insulin secretion, the signaling pathway of GPR40 must be considered in the context of overall control of insulin release from the beta cell. Once taken into the beta cell by the glucose transporter GLUT2, glucose triggers insulin release through a complex series of events starting with initiation of glycolysis and oxidative phosphorylation leading to an increase of the ATP/ADP ratio [16]. This leads to the triggering of an action potential through the closure of KATP channels. The initial depolarization then leads to opening of L-type voltage-dependent calcium channels (VDCC) that propagate the action potential. Calcium flux though the L-type VDCC has been shown to be tightly coupled to insulin secretion [17]. Insulin release ceases when the action potential is dissipated by opening of voltage-gated potassium channels allowing potassium ions to exit the cell [16]. PKC and PKA, both of which are potentially activated by GPR40 signaling, can phosphorylate the ion channels described above that regulate insulin release from the beta cell. However, the exact mechanism by which GPR40 exerts its effects in a glucose-dependent manner remains to be elucidated.