The Free Fatty Acid Receptor GPR40 Generates Excitement in Pancreatic β-Cells

Free fatty acids (FFAs) provide an important energy source and also act as signaling molecules. Accumulating evidence suggests that FFAs exert a variety of physiological responses via an emerging family of G protein-coupled transmembrane receptors. GPR40 and GPR120 are activated by medium- and long-chain FFAs, whereas GPR41 and GPR43 can be activated by short-chain FFAs (1–5). GPR40, which is preferentially expressed in pancreatic β-cells, mediates the majority of the effects of FFAs on insulin secretion (3, 6–8). Blood glucose concentration is the most important regulator of insulin secretion from the pancreatic β-cell. The β-cell is electrically excitable and generates action potentials when exposed to insulin-releasing glucose concentrations. The voltage-gated Ca2+ and K+ currents that underlie β-cell electrical activity have been described in some detail (9). As in other excitable cells, the outward voltage-gated K+ current keeps the action potential short and thus limits the period of Ca2+ influx and insulin secretion (Fig. 1). Thus, voltage-gated K+ channels potentially represent important regulators of insulin secretion. Regulation of insulin secretion by glucose and GPR40. Glucose metabolism in glycolysis and Krebs cycle leads to generation of ATP at the expense of ADP. The resulting increase in the ATP-to-ADP ratio causes closure of the ATP-sensitive K+-channels (K-ATP), cell membrane depolarization (Depol.), and stimulation of Ca2+ influx through voltage-dependent Ca2+-channels (VDCC). The resulting increase in [Ca2+]i is the trigger signal for exocytosis of the insulin-containing secretory granules. Opening of voltage-gated K+ (Kv) channels in response to membrane depolarization will repolarize the β-cell, close the VDCC, and limit Ca2+ influx. Binding of FFA to GPR40 leads to IP3 production, activation of intracellular IP3 receptors (IP3R), and mobilization of intracellular Ca2+ from the endoplasmic reticulum (ER). GPR40 activation also stimulates Ca2+ influx through VDCC. The resulting increase in [Ca2+]i stimulates insulin secretion. Binding of FFA to GPR40 also produces an increase in intracellular cAMP levels, which antagonizes the activity of Kv channels further enhancing Ca2+ influx. The dotted line indicates that it is not yet established whether GPR40 activation by FFA directly stimulates cAMP production. FFAs are known to have pleiotropic effects on the pancreatic β-cell. Although acute administration of FFAs stimulates insulin release, chronic exposure to high levels of FFAs results in the impairment of β-cell function and secretory capacity, a phenomenon recognized as lipotoxicity (10). It has always been assumed that, to exert a stimulatory effect on insulin release, FFAs must be transported across the plasma membrane into the cell and metabolized into long-chain fatty acyl-coenzyme A (11, 12). However, this assumption has recently been challenged and it is now clear that FFAs amplify glucose-dependent insulin secretion in a GPR40-dependent manner (1, 3, 6, 8). It is against this background that the report by Feng et al. (13) published in this issue of Endocrinology should be considered. The authors show that the unsaturated FFA linoleic acid (C18:2) reduces the voltage-gated K+ current in rat pancreatic β-cells through a GPR40-mediated increase in cAMP levels and protein kinase A activity, leading to enhanced β-cell excitability and insulin secretion. The K+ current generated by voltage-gated K+ channels is composed of mainly two subtypes, a fast transient current and a slow inactivating delayed rectifying current (14). Feng et al. (13) establish that the effects of linoleic acid on voltage-gated K+ current is due to inhibition of delayed rectifying K+ channels, which comprise the majority (>95%) of the total voltage-gated K+ current in β-cells. Using small interfering RNA expected to selectively silence GPR40 expression, Feng et al. (13) elegantly show that the ability of linoleic acid to antagonize voltage-gated K+ currents is mediated via GPR40. This is further supported by the observation that linoleic acid did not affect voltage-gated K+ currents in GH3 cells, which do not express GPR40. Finally, methyl linoleic acid, which has a similar structure to linoleic acid but does not bind to GPR40, did not affect K+ currents in β-cells. Previous research suggests that FFA binding to GPR40 activates phospholipase C-mediated hydrolysis of phosphatidylinositol 4,5-bisphosphate into inositol trisphosphate (IP3), which mobilizes intracellular Ca2+ from the endoplasmic reticulum (Fig. 1) (15, 16). The FFA-evoked increase in the intracellular Ca2+ concentration ([Ca2+]i) also involves enhanced Ca2+ influx through voltage-gated Ca2+ channels (15–18) (Fig. 1). Regardless of the mechanism, the FFA-induced [Ca2+]i increase is only observed in the presence of elevated glucose levels (and, therefore, already elevated [Ca2+]i levels), and inhibition of Ca2+ influx suppresses FFA stimulation of insulin release (15). These findings shed light on the mechanism(s) by which FFAs increase [Ca2+]i and thus enhance insulin secretion. The data by Feng et al. (13) suggest that the insulinotropic effect of FFAs may be mediated in part by an inhibition of delayed rectifying voltage-gated K+ channels, leading to enhanced action potential amplitude and duration. Indeed, FFAs enhanced glucose-stimulated electrical activity in vitro (18) and stimulated in vivo β-cell electrical activity in fasted mice (19). Feng et al. (13) further demonstrate that the effect of FFA is mediated by protein kinase A activation and that linoleic acid produced an increase in intracellular cAMP levels. There is precedence for this scenario, and glucagon-like peptide-1, which stimulates insulin secretion by increasing intracellular cAMP levels, has been reported to also inhibit the voltage-gated K+ current (20). At this stage it remains unresolved precisely how linoleic acid-induced activation of GPR40 stimulates cAMP production in β-cells. It is tempting to speculate that the observed increase in intracellular cAMP levels produced with linoleic acid could be secondary to elevations in [Ca2+]i and subsequent activation of Ca2+-dependent adenylate cyclase isoform(s) in the β-cell. This could be tested using pharmacological inhibitors of intracellular Ca2+ release channels. Interestingly, the activation of cAMP signaling pathway by Gqα-coupled receptors may be a more general phenomenon that is observed not only for GPR40 but also for muscarinic receptors in β-cells (21, 22). GPR40 and related receptors have also been implicated in the control of cell growth and survival via activation of the ERK and phosphatidylinositol 3-kinase/protein kinase B (Akt) signaling pathways (23–25). Future studies will have to address to what extent these FFA-induced signaling pathways and changes in intracellular cAMP levels contribute to the antiapoptotic and proliferative effects of GPR40 in β-cells. It is also essential to examine the potential beneficial role of GPR40-mediated cAMP increases in modulation of β-cell function under long-term FFA exposure. Paradoxically, under chronic conditions, β-cells from GPR40-deficient mice are protected from lipotoxicity, whereas overexpression of GPR40 in β-cells leads to impaired β-cell function, hypoinsulinemia, and diabetes (8). Clearly, the identification of GPR40 selective non-FFA agonists and antagonists would help resolve this apparent controversy. This begs further research in this exciting but fledgling area and could open the door to exciting ideas in research and treatment. I thank Patrik Rorsman for critical review of the manuscript. Free intracellular Ca2+ concentration; free fatty acid; inositol trisphosphate.