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Docking tangible virtual libraries can reveal unexpected chemotypes that complement the structures of biological targets. Seeking new agonists for the cannabinoid-1 receptor (CB1R), we docked 74 million tangible molecules, prioritizing 46 high ranking ones forde novosynthesis and testing. Nine were active by radioligand competition, with > 50% radioligand displacement, a 20% hit-rate. Structure-based optimization of one of the most potent of these (Ki= 731 nM) led to‘3234, a 1.9 nM binder and a full CB1 agonist. A cryo-EM structure of the‘3234-CB1-Gi1complex confirmed its docked pose, providing a template for further optimization. The new agonist was strongly analgesic especially against thermal pain, with a 10-fold therapeutic window over sedation and no observable catalepsy or conditioned place preference or aversion. These findings suggest that new cannabinoid chemotypes may be able to disentangle the characteristic “tetrad” side-effects from its desired analgesic effect, supporting the further development of cannabinoids as pain therapeutics.

The neuropeptide Urocortin 2 (UCN2) acts as a ligand for the G protein-coupled receptor (GPCR) corticotropin-releasing hormone receptor 2 (CRHR2) expressed in the brain and peripheral metabolic tissues. UCN2 has been reported to improve or worsen insulin sensitivity and glucose uptake in skeletal muscle and have opposing effects on glucose tolerance in vivo. In this report, we examined the acute and chronic effect of UCN2 on glucose metabolism and signaling pathways downstream of CRHR2. Consistent with previous reports, we found that acute dosing of UCN2 induced systemic insulin resistance and hyperglycemia in mice and skeletal muscle. Inversely, chronic elevation of UCN2 by injection with adenovirus encoding UCN2 (UCN2.AAV) resolved metabolic complications, improving glucose tolerance. Phosphoproteomic analysis of acutely treated skeletal muscle revealed dephosphorylation of IRS1 and AKT1S1, which was entirely reversed in UCN2.AAV skeletal muscle. Interestingly, pharmacological studies showed that all human isoforms of CRHR2 recruit Gs, as well as Gi and β-Arrestin, in response to stimulation with UCN2. However, Gi and β-Arrestin recruitment occurs at UCN2 concentrations 10-fold higher than Gs recruitment. Furthermore, pre-treating cells with UCN2 led to internalization of CRHR2 and dampened ligand-dependent increases in cAMP. Consistent with the in vivo results, treatment of mouse soleus muscle with UCN2 ex vivo showed AKT1S1 and IRS1 dephosphorylation and decreased glucose uptake in response to insulin; these responses were blunted when the muscle was pre-incubated with UCN2. These studies demonstrate that exposure to high, chronic concentrations of UCN2 desensitizes CRHR2, thus blocking the effects of acute UCN2, and improving insulin sensitivity, in skeletal muscle and systemically. On the other hand, acute treatment with UCN2 activates CRHR2 through recruitment of Gs which leads to blunted insulin signaling and glucose uptake. These results provide mechanistic insights into how UCN2 regulates insulin sensitivity and glucose metabolism in skeletal muscle and in vivo. Importantly, a working model was derived from these results that unifies the contradictory metabolic effects of UCN2.

A large-scale meta-analysis has recently identified Protease-activated receptor 2 (PAR2) gene expression to be significantly associated with resistance to immune checkpoint blockade (ICB) in cancer patients and preclinical models. PAR2 and its ligands (proteases) are indeed upregulated in different cancer types and are expressed by various cells in the tumor microenvironment. In cancer cells, the PAR2 receptor controls cell migration, proliferation, survival, and expression of inflammatory cytokines. In immune cells, it influences the infiltration and phenotype of macrophages and T cells. Therefore, PAR2 represents a promising therapeutic target in oncology and immuno-oncology. A novel series of potent and selective PAR2 inhibitors has been developed at Domain Therapeutics. In vitro experiments demonstrated unique properties of our PAR2 small molecule antagonists when compared to those of competitors. The antagonists inhibit pathogenic signaling pathways (i.e. Gz, G13, Gq, G14 and G15 protein activation as well as intracellular calcium) but not βarrestin2 recruitment, potentially reducing the risks of drug resistance. Furthermore, they maintain high potency and insurmountability in conditions that mimic the tumor microenvironment (high concentration of proteases and acidic pH). Finally, their pharmacokinetic properties are compatible with a once-a-day oral administration and demonstrate no signs of in vivo toxicity except at high doses (>500 mg/kg). Proof-of-concept experiments showed that PAR2 antagonists prevented PAR2-mediated resistance to Gefitinib in vitro and increased the potency of anti-PD1 therapy in vivo in pre-clinical syngeneic mouse models. Immunohistochemistry analyses from cancer patient biopsies confirmed that high expression levels of PAR2 in cancer and stromal cells within the tumor microenvironment significantly associates with the patient overall survival. In conclusion, new potent and selective negative allosteric modulators of PAR2 have been developed, they demonstrate strong potency by alleviating the resistance to both chemo- and immunotherapy in cancer models. These findings confirm the high value of PAR2 as a therapeutic target and demonstrates the relevance of small molecule inhibitors targeting this receptor to treat cancer. Citation Format: Thibaut Brugat, Francesco Bergami, Baptiste Rugeri, Aurélie Janvier, Edith Steinberg, Luc Baron, Mandy Recolet, Xavier Wirth, Meriem Semache, Antoine Mousson, Camille Dietsch, Quentin Ruet, Orphée Blanchard, Célia Jacoberger-Foissac, Isabelle Cousineau, Maleck Kadiri, Anne-Laure Blayo, Christel Franchet, Stanislas Mayer, John Stagg, Nathalie Lenne, Stephan Schann. Novel biased PAR2 inhibitors with best-in-class properties reduce resistance to both chemotherapy and immunotherapy in oncology models. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4961.

Summary G protein-coupled receptors are key signaling molecules and major targets for pharmaceuticals. The concept of ligand-dependent biased signaling raises the possibility of developing drugs with improved efficacy and safety profiles, yet translating this concept to native tissues remains a major challenge. Whether drug activity profiling in recombinant cell-based assays, traditionally used for drug discovery, has any relevance to physiology is unknown. Here we focused on the mu opioid receptor, the unrivalled target for pain treatment and also the key driver for the current opioid crisis. We selected a set of clinical and novel mu agonists, and profiled their activities in transfected cell assays using advanced biosensors and in native neurons from knock-in mice expressing traceable receptors endogenously. Our data identify Gi-biased agonists, including buprenorphine, and further show highly correlated drug activities in the two otherwise very distinct experimental systems, supporting in vivo translatability of biased signaling for mu opioid drugs., Graphical Abstract, Highlights • BRET sensors profiled MOR signaling and trafficking responses in HEK293 cells • MOR-Venus knock-in mice were created to monitor MOR trafficking in DRG neurons • MOR trafficking responses to opioids were correlated between HEK cells and neurons • Of the 10 opioid drugs tested, most remarkable were TRV130, PZM21, and buprenorphine, Biological Sciences; Physiology; Molecular Biology; Neuroscience; Bioengineering; Cell Biology

Brain functions rely on neurotransmitters that mediate communication between billions of neurons. Disruption of this communication can result in a plethora of psychiatric and neurological disorders. In this work, we combine molecular dynamics simulations, live-cell biosensor and electrophysiological assays to investigate the action of the neurotransmitter dopamine at the dopaminergic D2 receptor (D2R). The study of dopamine and closely related chemical probes reveals how neurotransmitter binding translates into the activation of distinct subsets of D2R effectors (i.e.: Gi2, GoB, Gz and β-arrestin 2). Ligand interactions with key residues in TM5 (S5.42) and TM6 (H6.55) in the D2R binding pocket yield a dopamine-like coupling signature, whereas exclusive TM5 interaction is typically linked to preferential G protein coupling (in particular GoB) over β-arrestin. Further experiments for serotonin receptors indicate that the reported molecular mechanism is shared by other monoaminergic neurotransmitter receptors. Ultimately, our study highlights how sequence variation in position 6.55 is used by nature to fine-tune β-arrestin recruitment and in turn receptor signaling and internalization of neurotransmitter receptors., Neurotransmitter contacts within the receptor binding site differentially contribute to the overall functional response: transmembrane helix (TM) 5 contacts promote G protein coupling whereas concerted TM5–TM6 contacts enhance β-arrestin recruitment.

Protease-activated receptor-2 (PAR2) is involved in inflammatory responses and pain, therefore representing a promising therapeutic target for the treatment of immune-mediated inflammatory diseases. However, as for other GPCRs, PAR2 can activate multiple signaling pathways and those involved in inflammatory responses remain poorly defined. Here, we describe a new selective and potent PAR2 inhibitor (I-287) that shows functional selectivity by acting as a negative allosteric regulator on Gαq and Gα12/13 activity and their downstream effectors, while having no effect on Gi/o signaling and βarrestin2 engagement. Such selective inhibition of only a subset of the pathways engaged by PAR2 was found to be sufficient to block inflammation in vivo. In addition to unraveling the PAR2 signaling pathways involved in the pro-inflammatory response, our study opens the path toward the development of new functionally selective drugs with reduced liabilities that could arise from blocking all the signaling activities controlled by the receptor., Avet et al. characterize I-287, an inhibitor to protease-activated receptor 2 using BRET-assays. They find that I-287 selectively inhibits Gαq and Gα12/13 without affecting the activation of Gi/o or the recruitment of βarrestin2 and that it blocks inflammation in vitro and in vivo.

GPR88 is an orphan G-protein coupled receptor originally characterized as a striatal-enriched transcript and is a potential target for neuropsychiatric disorders. At present, gene knockout studies in the mouse have essentially focused on striatal-related functions and a comprehensive knowledge of GPR88 protein distribution and function in the brain is still lacking. Here, we first created Gpr88-Venus knock-in mice expressing a functional fluorescent receptor to fine-map GPR88 localization in the brain. The receptor protein was detected in neuronal soma, fibers and primary cilia depending on the brain region, and remarkably, whole-brain mapping revealed a yet unreported layer-4 cortical lamination pattern specifically in sensory processing areas. The unique GPR88 barrel pattern in L4 of the somatosensory cortex appeared three days after birth and persisted into adulthood, suggesting a potential function for GPR88 in sensory integration. We next examined Gpr88 knockout mice for cortical structure and behavioral responses in sensory tasks. Magnetic resonance imaging of live mice revealed abnormally high fractional anisotropy, predominant in somatosensory cortex and caudate putamen, indicating significant microstructural alterations in these GPR88-enriched areas. Further, behavioral analysis showed delayed responses in somatosensory-, visual- and olfactory-dependent tasks, demonstrating a role for GPR88 in the integration rather than perception of sensory stimuli. In conclusion, our data show for the first time a prominent role for GPR88 in multisensory processing. Because sensory integration is disrupted in many psychiatric diseases, our study definitely positions GPR88 as a target to treat mental disorders perhaps via activity on cortical sensory networks.

In pancreatic β-cells, glucose induces the binding of the transcription factor pancreatic duodenal homeobox-1 (PDX-1) to the insulin gene promoter to activate insulin gene transcription. At low glucose levels, glycogen synthase kinase 3β (GSK3β) is known to phosphorylate PDX-1 on C-terminal serine residues, which triggers PDX-1 proteasomal degradation. We previously showed that the serine/threonine Per-Arnt-Sim domain-containing kinase (PASK) regulates insulin gene transcription via PDX-1. However, the mechanisms underlying this regulation are unknown. In this study, we aimed to identify the role of PASK in the regulation of PDX-1 phosphorylation, protein expression, and stability in insulin-secreting cells and isolated rodent islets of Langerhans. We observed that glucose induces a decrease in overall PDX-1 serine phosphorylation and that overexpression of WT PASK mimics this effect. In vitro, PASK directly phosphorylates GSK3β on its inactivating phosphorylation site Ser(9). Overexpression of a kinase-dead (KD), dominant negative version of PASK blocks glucose-induced Ser(9) phosphorylation of GSK3β. Accordingly, GSK3β Ser(9) phosphorylation is reduced in islets from pask-null mice. Overexpression of WT PASK or KD GSK3β protects PDX-1 from degradation and results in increased PDX-1 protein abundance. Conversely, overexpression of KD PASK blocks glucose-induction of PDX-1 protein. We conclude that PASK phosphorylates and inactivates GSK3β, thereby preventing PDX-1 serine phosphorylation and alleviating GSK3β-mediated PDX-1 protein degradation in pancreatic β-cells.

Background: The glucose sensor PAS kinase (PASK) plays a fundamental role in pancreatic islet physiology. Results: A single amino acid substitution in the human PASK kinase domain stimulates enzyme activity and increases insulin secretion at low glucose. Conclusion: A rare, naturally occurring mutation in PASK modulates insulin release in man. Significance: We provide direct genetic evidence for a role for PASK in controlling insulin secretion in man., PAS kinase (PASK) is a glucose-regulated protein kinase involved in the control of pancreatic islet hormone release and insulin sensitivity. We aimed here to identify mutations in the PASK gene that may be associated with young-onset diabetes in humans. We screened 18 diabetic probands with unelucidated maturity-onset diabetes of the young (MODY). We identified two rare nonsynonymous mutations in the PASK gene (p.L1051V and p.G1117E), each of which was found in a single MODY family. Wild type or mutant PASKs were expressed in HEK 293 cells. Kinase activity of the affinity-purified proteins was assayed as autophosphorylation at amino acid Thr307 or against an Ugp1p-derived peptide. Whereas the PASK p.G1117E mutant displayed a ∼25% increase with respect to wild type PASK in the extent of autophosphorylation, and a ∼2-fold increase in kinase activity toward exogenous substrates, the activity of the p.L1051V mutant was unchanged. Amino acid Gly1117 is located in an α helical region opposing the active site of PASK and may elicit either: (a) a conformational change that increases catalytic efficiency or (b) a diminished inhibitory interaction with the PAS domain. Mouse islets were therefore infected with adenoviruses expressing wild type or mutant PASK and the regulation of insulin secretion was examined. PASK p.G1117E-infected islets displayed a 4-fold decrease in glucose-stimulated (16.7 versus 3 mm) insulin secretion, chiefly reflecting a 4.5-fold increase in insulin release at low glucose. In summary, we have characterized a rare mutation (p.G1117E) in the PASK gene from a young-onset diabetes family, which modulates glucose-stimulated insulin secretion.

Prolonged exposure of pancreatic beta cells to excessive levels of glucose and fatty acids, referred to as glucolipotoxicity, is postulated to contribute to impaired glucose homeostasis in patients with type 2 diabetes. However, the relative contribution of defective beta cell function vs diminished beta cell mass under glucolipotoxic conditions in vivo remains a subject of debate. We therefore sought to determine whether glucolipotoxicity in rats is due to impaired beta cell function and/or reduced beta cell mass, and whether older animals are more susceptible to glucolipotoxic condition.Wistar rats (2 and 6 months old) received a 72 h infusion of glucose + intravenous fat emulsion or saline control. In vivo insulin secretion and sensitivity were assessed by hyperglycaemic clamps. Ex vivo insulin secretion, insulin biosynthesis and gene expression were measured in isolated islets. Beta cell mass and proliferation were examined by immunohistochemistry.A 72 h infusion of glucose + intravenous fat emulsion in 2-month-old Wistar rats did not affect insulin sensitivity, insulin secretion or beta cell mass. In 6-month-old rats by contrast it led to insulin resistance and reduced insulin secretion in vivo, despite an increase in beta cell mass and proliferation. This was associated with: (1) diminished glucose-stimulated second-phase insulin secretion and proinsulin biosynthesis; (2) lower insulin content; and (3) reduced expression of beta cell genes in isolated islets.In this in vivo model, glucolipotoxicity is characterised by an age-dependent impairment of glucose-regulated beta cell function despite a marked increase in beta cell mass.

Prolonged exposure of pancreatic beta-cells to elevated levels of glucose and fatty acids adversely affects insulin secretion and gene expression. Aim To examine whether the GLP-1 agonist exenatide or the inhibitor of the GLP-1-degrading enzyme dipeptidyl peptidase 4 (DPP-4) sitagliptin rescue insulin gene expression in rats infused for 72 h with glucose + Intralipid, independently from their glucose-lowering action. Methods Wistar rats were infused alternatively with glucose or Intralipid for cycles of 4 h each for a total of 72 h. The animals received exenatide (5 μg/kg/day IV) or sitagliptin (5 mg/kg/day IV) continuously starting 4 days prior to and continuing throughout the 3-day infusion period. Results Plasma glucose, fatty acids, insulin and C-peptide levels were unaffected by exenatide or sitagliptin treatment during the infusion period. Insulin mRNA levels increased in response to the glucose infusion, but this increase was abolished in islets from rats receiving glucose + Intralipid. Neither exenatide nor sitagliptin administration rescued insulin mRNA in glucose + Intralipid infused rats. Conclusions Neither a GLP-1 agonist nor a DPP-4 inhibitor, at doses that do not alter blood glucose levels, prevented the inhibition of insulin gene expression in this in vivo model of glucolipotoxicity.

OBJECTIVE Prolonged exposure of pancreatic β-cells to simultaneously elevated levels of fatty acids and glucose (glucolipotoxicity) impairs insulin gene transcription. However, the intracellular signaling pathways mediating these effects are mostly unknown. This study aimed to ascertain the role of extracellular-regulated kinases (ERKs)1/2, protein kinase B (PKB), and Per-Arnt-Sim kinase (PASK) in palmitate inhibition of insulin gene expression in pancreatic β-cells. RESEARCH DESIGN AND METHODS MIN6 cells and isolated rat islets were cultured in the presence of elevated glucose, with or without palmitate or ceramide. ERK1/2 phosphorylation, PKB phosphorylation, and PASK expression were examined by immunoblotting and real-time PCR. The role of these kinases in insulin gene expression was assessed using pharmacological and molecular approaches. RESULTS Exposure of MIN6 cells and islets to elevated glucose induced ERK1/2 and PKB phosphorylation, which was further enhanced by palmitate. Inhibition of ERK1/2, but not of PKB, partially prevented the inhibition of insulin gene expression in the presence of palmitate or ceramide. Glucose-induced expression of PASK mRNA and protein levels was reduced in the presence of palmitate. Overexpression of wild-type PASK increased insulin and pancreatic duodenal homeobox-1 gene expression in MIN6 cells and rat islets incubated with glucose and palmitate, whereas overexpression of a kinase-dead PASK mutant in rat islets decreased expression of insulin and pancreatic duodenal homeobox-1 and increased C/EBPβ expression. CONCLUSIONS Both the PASK and ERK1/2 signaling pathways mediate palmitate inhibition of insulin gene expression. These findings identify PASK as a novel mediator of glucolipotoxicity on the insulin gene in pancreatic β-cells.

Fat cell lipolysis, the cleavage of triglycerides and release of fatty acids and glycerol, evolved to enable survival during prolonged food deprivation but is paradoxically increased in obesity, in which a surfeit of all energy metabolites is found. Essential, previously-unsuspected components have been discovered in the lipolytic machinery, at the protective interface of the lipid droplet surface and in the signaling pathways that control lipolysis. At least two adipocyte lipases are important for controlling lipolysis, hormone-sensitive lipase (HSL) and adipocyte triglyceride lipase (ATGL). Perilipin (PLIN) and possibly other proteins of the lipid droplet surface are master regulators of lipolysis, protecting or exposing the triglyceride core of the droplet to lipases. The prototypes for hormonal lipolytic control are beta adrenergic stimulation and suppression by insulin, both of which affect cyclic AMP levels and hence the protein kinase A-mediated phosphorylation of HSL and PLIN. Newly-recognized mediators of lipolysis include atrial natriuretic peptide, cyclic GMP, the ketone body 3-hydroxybutyrate, AMP kinase and mitogen-activated kinases. Lipolysis must be interpreted in its physiological context since similar rates of basal or stimulated lipolysis occur under different conditions and by different mechanisms. Age, sex, anatomical site, genotype and species differences are each important variables. Manipulation of lipolysis has therapeutic potential in several inborn errors and in the metabolic syndrome that frequently complicates obesity.

OBJECTIVE—The G-protein–coupled receptor GPR40 is expressed in pancreatic β-cells and is activated by long-chain fatty acids. Gene deletion studies have shown that GPR40 mediates, at least in part, fatty acid–amplification of glucose-induced insulin secretion (GSIS) but is not implicated in GSIS itself. However, the role of GPR40 in the long-term effects of fatty acids on insulin secretion remains controversial. This study aimed to test the hypothesis that GPR40 plays a role in insulin secretion after high-fat feeding. RESEARCH DESIGN AND METHODS—GPR40 knockout (KO) mice on a C57BL/6 background and their wild-type (WT) littermates were fed a high-fat diet (HFD) for 11 weeks. Glucose tolerance, insulin tolerance, and insulin secretion in response to glucose and Intralipid were assessed during the course of the diet period. RESULTS—GPR40 KO mice had fasting hyperglycemia. They became as obese, glucose intolerant, and insulin resistant as their WT littermates given HFD and developed a similar degree of liver steatosis. Their fasting blood glucose levels increased earlier than those of control mice during the course of the HFD. The remarkable increase in insulin secretory responses to intravenous glucose and Intralipid seen in WT mice after HFD was of much lower magnitude in GPR40 KO mice. CONCLUSIONS—GPR40 plays a role not only in fatty acid modulation of insulin secretion, but also in GSIS after high-fat feeding. These observations raise doubts on the validity of a therapeutic approach based on GPR40 antagonism for the treatment of type 2 diabetes.

In white adipose tissue, lipolysis can occur by hormone-sensitive lipase (HSL)-dependent or HSL-independent pathways. To study HSL-independent lipolysis, we placed HSL-deficient mice in conditions of increased fatty acid flux: β-adrenergic stimulation, fasting, and dietary fat loading. Intraperitoneal administration of the β3-adrenergic agonist CL-316243 caused a greater increase in nonesterified fatty acid level in controls (0.33 ± 0.05 mmol/l) than in HSL−/−mice (0.12 ± 0.01 mmol/l, P < 0.01). Similarly, in isolated adipocytes, lipolytic response to CL-316243 was greatly reduced in HSL−/−mice compared with controls. Fasting for ≤48 h produced normal mobilization and oxidation of fatty acids in HSL−/−mice, as judged by similar values of respiratory quotient and oxygen consumption as in HSL+/+controls. In isolated adipocytes, lipolysis in the absence of β-adrenergic stimulation was 1.9-fold greater in HSL−/−than in HSL+/+cells ( P < 0.05), increasing to 6.5-fold after fasting ( P < 0.01). After 6 wk of a fat-rich diet containing 31.5% of energy as lipid, weight gain of HSL−/−mice was 4.4-fold less than in HSL+/+mice ( P < 0.01), and total abdominal fat mass was 5.2-fold lower in HSL−/−than in HSL+/+mice ( P < 0.01). In white adipose tissue, HSL is essential for normal acute β-adrenergic-stimulated lipolysis and permits normal triglyceride storage capacity in response to dietary fat loading. However, HSL-independent lipolysis can markedly increase during fasting, both in isolated adipocytes and in intact mice, and can mediate a normal flux of fatty acids during fasting.

The pancreatic beta cell is equipped with a highly sophisticated machinery to precisely sense the metabolic status of the organism and secrete the exactly appropriate amount of insulin to maintain blood glucose levels in a very narrow range. When the Metabolic Syndrome develops, insulin resistance imposes an additional burden on the beta cell, which then hypersecretes insulin to meet the demand. In the majority of individuals, the beta cell can sustain this additional workload and maintain normoglycemia. In a subset of predisposed individuals however, this compensatory response eventually fails and diabetes develops. Once diabetes is established, beta-cell function continues to deteriorate over time. The molecular and cellular mechanisms underlying beta-cell failure are not fully understood, although several hypotheses have been proposed. Amongst these, glucolipotoxicity; defective mitochondrial metabolism and oxidative stress; inflammation; amyloid deposits, disruption of autophagic flux; endoplasmic reticulum stress; beta-cell dedifferentiation and exhaustion from chronic hypersecretion probably contribute to some extent, perhaps at various stages of the disease progression and differently between individuals. Thus, beta-cell failure is likely mediated by a number of interrelated and complex mechanisms, which is reflected in the inability of the current therapeutic options to significantly slow down disease progression. This provides a strong argument for early interventions aimed at preventing the functional demise of pancreatic beta cells in the Metabolic Syndrome.

Background: Type 2 diabetes is characterized by pancreatic b-cell dysfunction and is associated with low-grade inflammation. Recent observations suggest that the signalling cascade activated by lipopolysaccharides (LPS) binding to Toll-Like Receptor 4 (TLR4) exerts deleterious effects on pancreatic b-cell function; however, the molecular mechanisms of these effects are incompletely understood. In this study, we tested the hypothesis that LPS alters insulin gene expression via TLR4 and nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kB) in islets. Methodology/Principal Findings: A 24-h exposure of isolated human, rat and mouse islets of Langerhans to LPS dosedependently reduced insulin gene expression. This was associated in mouse and rat islets with decreased mRNA expression of pancreas-duodenum homebox-1 (PDX-1) and mammalian homologue of avian MafA/l-Maf (MafA). Accordingly, LPS exposure also decreased glucose-induced insulin secretion. LPS repression of insulin, PDX-1 and MafA expression, as well as its inhibition of insulin secretion, were not observed in islets from TLR4-deficient mice. LPS inhibition of b-cell gene expression in rat islets was prevented by inhibition of the NF-kB pathway, but not the p38 mitogen-activated protein kinase (p38 MAPK) pathway. Conclusions/Significance: Our findings demonstrate that LPS inhibit b-cell gene expression in a TLR4-dependent manner and via NF-kB signaling in pancreatic islets, suggesting a novel mechanism by which the gut microbiota might affect pancreatic b-cell function.

The concept of glucolipotoxicity refers to the combined, deleterious effects of elevated glucose and fatty acid levels on pancreatic beta-cell function and survival. Significant progress has been made in recent years towards a better understanding of the cellular and molecular basis of glucolipotoxicity in the beta cell. The permissive effect of elevated glucose on the detrimental actions of fatty acids stems from the influence of glucose on intracellular fatty acid metabolism, promoting the synthesis of cellular lipids. The combination of excessive levels of fatty acids and glucose therefore leads to decreased insulin secretion, impaired insulin gene expression, and beta-cell death by apoptosis, all of which probably have distinct underlying mechanisms. Recent studies from our laboratory have identified several pathways implicated in fatty acid inhibition of insulin gene expression, including the extracellular-regulated kinase (ERK1/2) pathway, the metabolic sensor Per-Arnt-Sim kinase (PASK), and the ATF6 branch of the unfolded protein response. We have also confirmed in vivo in rats that the decrease in insulin gene expression is an early defect which precedes any detectable abnormality in insulin secretion. While the role of glucolipotoxicity in humans is still debated, the inhibitory effects of chronically elevated fatty acid levels has been clearly demonstrated in several studies, at least in individuals genetically predisposed to developing type 2 diabetes. It is therefore likely that glucolipotoxicity contributes to beta-cell failure in type 2 diabetes as well as to the decline in beta-cell function observed after the onset of the disease.

OBJECTIVE—Prolonged exposure of isolated islets of Langerhans to elevated levels of fatty acids, in the presence of high glucose, impairs insulin gene expression via a transcriptional mechanism involving nuclear exclusion of pancreas-duodenum homeobox-1 (Pdx-1) and loss of MafA expression. Whether such a phenomenon also occurs in vivo is unknown. Our objective was therefore to ascertain whether chronic nutrient oversupply inhibits insulin gene expression in vivo. RESEARCH DESIGN AND METHODS—Wistar rats received alternating 4-h infusions of glucose and Intralipid for a total of 72 h. Control groups received alternating infusions of glucose and saline, saline and Intralipid, or saline only. Insulin and C-peptide secretion were measured under hyperglycemic clamps. Insulin secretion and gene expression were assessed in isolated islets, and β-cell mass was quantified by morphometric analysis. RESULTS—Neither C-peptide secretion nor insulin sensitivity was different among infusion regimens. Insulin content and insulin mRNA levels were lower in islets isolated from rats infused with glucose plus Intralipid. This was associated with reduced Pdx-1 binding to the endogenous insulin promoter, and an increased proportion of Pdx-1 localized in the cytoplasm versus the nucleus. In contrast, MafA mRNA and protein levels and β-cell mass and proliferation were unchanged. CONCLUSIONS—Cyclical and alternating infusions of glucose and Intralipid in normal rats inhibit insulin gene expression without affecting insulin secretion or β-cell mass. We conclude that fatty acid inhibition of insulin gene expression, in the presence of high glucose, is an early functional defect that may contribute to β-cell failure in type 2 diabetes.

Hydrolysis of triglycerides is central to energy homeostasis in white adipose tissue (WAT). Hormone-sensitive lipase (HSL) was previously felt to mediate all lipolysis in WAT. Surprisingly, HSL-deficient mice show active HSL-independent lipolysis, suggesting that other lipase(s) also mediate triglyceride hydrolysis. To clarify this, we used functional proteomics to detect non-HSL lipase(s) in mouse WAT. After cell fractionation of intraabdominal WAT, most non-HSL neutral lipase activity is localized in the 100,000 × g infranatant and fat cake fractions. By oleic acid-linked agarose chromatography of infranatant followed by elution in a 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid gradient, we identified two peaks of esterase activity using p-nitrophenyl butyrate as a substrate. One of the peaks contained most of the lipase activity. In the corresponding fractions, gel permeation chromatography and SDS-PAGE, followed by tandem mass spectrometric analysis of excised Coomassie Blue-stained peptides, revealed carboxylesterase 3 (triacylglycerol hydrolase (TGH); EC 3.1.1.1). TGH is also the principle lipase of WAT fat cake extracts. Partially purified WAT TGH had lipase activity as well as lesser but detectable neutral cholesteryl ester hydrolase activity. Western blotting of subcellular fractions of WAT and confocal microscopy of fibroblasts following in vitro adipocytic differentiation are consistent with a distribution of TGH to endoplasmic reticulum, cytosol, and the lipid droplet. TGH is responsible for a major part of non-HSL lipase activity in WAT in vitro and may mediate some or all HSL-independent lipolysis in adipocytes.

The transcription factor Pancreatic and Duodenal Homeobox-1 (PDX-1) plays a major role in the development and function of pancreatic β−cells and its mutation results in diabetes. In adult β−cells, glucose stimulates transcription of the insulin gene in part by regulating PDX-1 expression, stability and activity. Glucose is also thought to modulate PDX-1 nuclear translocation but in vitro studies examining nucleo-cytoplasmic shuttling of endogenous or ectopically expressed PDX-1 in insulin-secreting cell lines have led to conflicting results. Here we show that endogenous PDX-1 undergoes translocation from the cytoplasm to the nucleus in response to glucose in dispersed rat islets but not in insulin-secreting MIN6, HIT-T15, or INS832/13 cells. Interestingly, however, we found that a PDX-1-GFP fusion protein can shuttle from the cytoplasm to the nucleus in response to glucose stimulation in HIT-T15 cells. Our results suggest that the regulation of endogenous PDX-1 sub-cellular localization by glucose is observed in primary islets and that care should be taken when interpreting data from insulin-secreting cell lines.