Thiazolidinediones induce osteocyte apoptosis by a G protein-coupled receptor 40-dependent mechanism

Minireview: nuclear receptor regulation of osteoclast and bone remodeling

Osteoclasts are bone-resorbing cells essential for skeletal remodeling and regeneration. However, excessive osteoclasts often contribute to prevalent bone degenerative diseases such as osteoporosis, arthritis, and cancer bone metastasis. Osteoclast dysregulation is also associated with rare disorders such as osteopetrosis, pycnodysostosis, Paget's disease, and Gorham-Stout syndrome. The nuclear receptor (NR) family of transcription factors functions as metabolic sensors that control a variety of physiological processes including skeletal homeostasis and serves as attractive therapeutic targets for many diseases. In this review, we highlight recent findings on the new players and the new mechanisms for how NRs regulate osteoclast differentiation and bone resorption. An enhanced understanding of NR functions in osteoclastogenesis will facilitate the development of not only novel osteoprotective medicine but also prudent strategies to minimize the adverse skeletal effects of certain NR-targeting drugs for a better treatment of cancer and metabolic diseases.

Minireview: Challenges and opportunities in development of PPAR agonists

The clinical impact of the fibrate and thiazolidinedione drugs on dyslipidemia and diabetes is driven mainly through activation of two transcription factors, peroxisome proliferator-activated receptors (PPAR)-α and PPAR-γ. However, substantial differences exist in the therapeutic and side-effect profiles of specific drugs. This has been attributed primarily to the complexity of drug-target complexes that involve many coregulatory proteins in the context of specific target gene promoters. Recent data have revealed that some PPAR ligands interact with other non-PPAR targets. Here we review concepts used to develop new agents that preferentially modulate transcriptional complex assembly, target more than one PPAR receptor simultaneously, or act as partial agonists. We highlight newly described on-target mechanisms of PPAR regulation including phosphorylation and nongenomic regulation. We briefly describe the recently discovered non-PPAR protein targets of thiazolidinediones, mitoNEET, and mTOT. Finally, we summarize the contributions of on- and off-target actions to select therapeutic and side effects of PPAR ligands including insulin sensitivity, cardiovascular actions, inflammation, and carcinogenicity.

Diet and gene interactions influence the skeletal response to polyunsaturated fatty acids

Diets rich in omega-3s have been thought to prevent both obesity and osteoporosis. However, conflicting findings are reported, probably as a result of gene by nutritional interactions. Peroxisome proliferator-activated receptor-gamma (PPARγ) is a nuclear receptor that improves insulin sensitivity but causes weight gain and bone loss. Fish oil is a natural agonist for PPARγ and thus may exert its actions through the PPARγ pathway. We examined the role of PPARγ in body composition changes induced by a fish or safflower oil diet using two strains of C57BL/6J (B6); i.e. B6.C3H-6T (6T) congenic mice created by backcrossing a small locus on Chr 6 from C3H carrying 'gain of function' polymorphisms in the Pparγ gene onto a B6 background, and C57BL/6J mice. After 9months of feeding both diets to female mice, body weight, percent fat and leptin levels were less in mice fed the fish oil vs those fed safflower oil, independent of genotype. At the skeletal level, fish oil preserved vertebral bone mineral density (BMD) and microstructure in B6 but not in 6T mice. Moreover, fish oil consumption was associated with an increase in bone marrow adiposity and a decrease in BMD, cortical thickness, ultimate force and plastic energy in femur of the 6T but not the B6 mice. These effects paralleled an increase in adipogenic inflammatory and resorption markers in 6T but not B6. Thus, compared to safflower oil, fish oil (high ratio omega-3/-6) prevents weight gain, bone loss, and changes in trabecular microarchitecture in the spine with age. These beneficial effects are absent in mice with polymorphisms in the Pparγ gene (6T), supporting the tenet that the actions of n-3 fatty acids on bone microstructure are likely to be genotype dependent. Thus caution must be used in interpreting dietary intervention trials with skeletal endpoints in mice and in humans.

Activation of free fatty acid receptor 1 improves hepatic steatosis through a p38-dependent pathway

Hepatic steatosis is highly correlated with insulin resistance and diabetes. Although, it has been demonstrated that activation of free fatty acid receptor 1 (FFAR1) by agonists showed benefits for the improvement of diabetes, the effects of FFAR1 agonists on hepatic steatosis were unknown. In this study, a high fat diet (HFD)-induced hepatic steatosis animal model was utilized to evaluate the effects of an FFAR1 agonist, GW9508, on hepatic lipid accumulation, and HepG2 hepatoma cells were also used to clarify the possible mechanisms. Administration of GW9508 significantly decreased the hepatic lipid accumulation with decreased expressions of lipogenesis-related proteins in HFD mice. Knockdown of hepatic Ffar1 by lentiviral vectors containing short hairpin RNA targeted to Ffar1 diminished the effect of GW9508 in HFD mice. In addition, GW9508 decreased oleic acid-induced lipid accumulation in HepG2 cells by decreases in the expression of lipogenesis-related proteins. Moreover, GW9508 downregulated the expression of sterol regulatory element-binding protein 1 (SREBP1) through a p38-dependent pathway, whereas knockdown of Ffar1 in HepG2 cells diminished the effect of GW9508 on the decrease in SREBP1. Considering all these results together, GW9508 exerts a therapeutic effect to improve hepatic steatosis through a p38-dependent pathway. Thus, investigation of chemicals that act on FFAR1 might be a new strategy for the treatment of hepatic steatosis.

Energy restriction mimetic agents to target cancer cells: comparison between 2-deoxyglucose and thiazolidinediones

The use of energy restriction mimetic agents (ERMAs) to selectively target cancer cells addicted to glycolysis could be a promising therapeutic approach. Thiazolidinediones (TZDs) are synthetic agonists of the nuclear receptor peroxisome proliferator-activated receptor (PPAR)γ that were developed to treat type II diabetes. These compounds also display anticancer effects which appear mainly to be independent of their PPARγ agonist activity but the molecular mechanisms involved in the anticancer action are not yet well understood. Results obtained on ciglitazone derivatives, mainly in prostate cancer cell models, suggest that these compounds could act as ERMAs. In the present paper, we introduce how compounds like 2-deoxyglucose target the Warburg effect and then we discuss the possibility that the PPARγ-independent effects of various TZD could result from their action as ERMAs.

Do thiazolidinediones still have a role in treatment of type 2 diabetes mellitus?

Thiazolidinediones have been introduced in the treatment of type 2 diabetes mellitus (T2DM) since the late 1990s. Although troglitazone was withdrawn from the market a few years later due to liver toxicity, both rosiglitazone and pioglitazone gained widespread use for T2DM treatment. In 2010, however, due to increased risk of cardiovascular events associated with its use, the European Medicines Agency recommended suspension of rosiglitazone use and the Food and Drug Administration severely restricted its use. Thus pioglitazone is the only thiazolidinedione still significantly employed for treating T2DM and it is the only molecule of this class still listed in the American Diabetes Association-European Association for the Study of Diabetes 2012 Position Statement. However, as for the other thiazolidinediones, use of pioglitazone is itself limited by several side effects, some of them potentially dangerous. This, together with the development of novel therapeutic strategies approved in the last couple of years, has made it questionable whether or not thiazolidinediones (namely pioglitazone) should still be used in the treatment of T2DM. This article will attempt to formulate an answer to this question by critically reviewing the available data on the numerous advantages and the potentially worrying shortcomings of pioglitazone treatment in T2DM.

Exendin-4 protects pancreatic beta cells from palmitate-induced apoptosis by interfering with GPR40 and the MKK4/7 stress kinase signalling pathway

AIMS/HYPOTHESIS

The mechanisms of the protective effects of exendin-4 on NEFA-induced beta cell apoptosis were investigated.

METHODS

The effects of exendin-4 and palmitate were evaluated in human and murine islets, rat insulin-secreting INS-1E cells and murine glucagon-secreting alpha-TC1-6 cells. mRNA and protein expression/phosphorylation were measured by real-time RT-PCR and immunoblotting or immunofluorescence, respectively. Small interfering (si)RNAs for Ib1 and Gpr40 were used. Cell apoptosis was quantified by two independent assays. Insulin release was assessed with an insulin ELISA.

RESULTS

Exposure of human and murine primary islets and INS-1E cells, but not alpha-TC1-6 cells, to exendin-4 inhibited phosphorylation of the stress kinases, c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK), and prevented apoptosis in response to palmitate. Exendin-4 increased the protein content of islet-brain 1 (IB1), an endogenous JNK blocker; however, siRNA-mediated reduction of IB1 did not impair the ability of exendin-4 to inhibit JNK and prevent apoptosis. Exendin-4 reduced G-protein-coupled receptor 40 (GPR40) expression and inhibited palmitate-induced phosphorylation of mitogen-activated kinase kinase (MKK)4 and MKK7. The effects of exendin-4 were abrogated in the presence of the protein kinase A (PKA) inhibitors, H89 and KT5720. Knockdown of GPR40, as well as use of a specific GPR40 antagonist, resulted in diminished palmitate-induced JNK and p38 MAPK phosphorylation and apoptosis. Furthermore, inhibition of JNK and p38 MAPK activity prevented palmitate-induced apoptosis.

CONCLUSIONS/INTERPRETATION

Exendin-4 counteracts the proapoptotic effects of palmitate in beta cells by reducing GPR40 expression and inhibiting MKK7- and MKK4-dependent phosphorylation of the stress kinases, JNK and p38 MAPK, in a PKA-dependent manner.

Free fatty acid (FFA) and hydroxy carboxylic acid (HCA) receptors

Saturated and unsaturated free fatty acids (FFAs), as well as hydroxy carboxylic acids (HCAs) such as lactate and ketone bodies, are carriers of metabolic energy, precursors of biological mediators, and components of biological structures. However, they are also able to exert cellular effects through G protein-coupled receptors named FFA1-FFA4 and HCA1-HCA3. Work during the past decade has shown that these receptors are widely expressed in the human body and regulate the metabolic, endocrine, immune and other systems to maintain homeostasis under changing dietary conditions. The development of genetic mouse models and the generation of synthetic ligands of individual FFA and HCA receptors have been instrumental in identifying cellular and biological functions of these receptors. These studies have produced strong evidence that several FFA and HCA receptors can be targets for the prevention and treatment of various diseases, including type 2 diabetes mellitus, obesity, and inflammation.

Optimal bone mechanical and material properties require a functional glucagon-like peptide-1 receptor

Bone is permanently remodeled by a complex network of local, hormonal, and neuronal factors that affect osteoclast and osteoblast biology. Among these factors, a role for gastrointestinal hormones has been proposed based on the evidence that bone resorption dramatically falls after a meal. Glucagon-like peptide-1 (GLP1) is one of these gut hormones, and despite several reports suggesting an anabolic effect of GLP1, or its stable analogs, on bone mass, little is known about the effects of GLP1/GLP1 receptor on bone strength. In this study, we investigated by three-point bending, quantitative X-ray microradiography, microcomputed tomography, qBEI, and FTIRI bone strength and bone quality in male Glp1r knockout (Glp1r KO) mice when compared with control WT animals. Animals with a deletion of Glp1r presented with a significant reduction in ultimate load, yield load, stiffness, and total absorbed and post-yield energies when compared with WT animals. Furthermore, cortical thickness and bone outer diameter were significantly decreased in deficient animals. The mineral quantity and quality were not significantly different between Glp1r KO and WT animals. On the other hand, the maturity of the collagen matrix was significantly reduced in deficient animals and associated with lowered material properties. Taken together, these data support a positive effect of GLP1R on bone strength and quality.

Modulating GPR40: therapeutic promise and potential in diabetes

The class A G-protein-coupled receptor GPR40 is predominantly expressed in pancreatic beta cells and plays a major part in fatty acid amplification of glucose-induced insulin secretion. GPR40 agonists are being developed for the treatment of type 2 diabetes. Preclinical studies have shown that GPR40 activation improves glucose control, and recent Phase II trials provided proof-of-concept for this approach. The pharmacology of GPR40 is only partially understood but recent findings suggest that full agonism of the receptor could, in addition to stimulating insulin release, engage the enteroinsular axis. Much remains to be discovered regarding the biology of the receptor to inform the development of GPR40-based drugs.

The fatty acid receptor FFA1/GPR40 a decade later: how much do we know?

Glucose homeostasis requires the highly coordinated regulation of insulin secretion by pancreatic β cells. This is primarily mediated by glucose itself, but other nutrients, including free fatty acids (FFAs), potentiate the insulinotropic capacity of glucose. A decade ago, the seven-transmembrane domain receptor (7TMR) GPR40 was demonstrated to be predominantly expressed in β cells and activated by long-chain FFAs. This discovery added a new dimension to our understanding of FFA-mediated control of glucose homeostasis. Furthermore, GPR40 has drawn considerable interest as a novel therapeutic target to enhance insulin secretion in type 2 diabetes. However, our understanding of the biology of GPR40 remains incomplete and its physiological role controversial. Here we summarize the current state of knowledge and emerging concepts regarding the role of GPR40 in regulating glucose homeostasis.

The future of thiazolidinedione therapy in the management of type 2 diabetes mellitus

Since their approval, thiazolidinediones (TZDs) have been used extensively as insulin-sensitizers for the management of type 2 diabetes mellitus (T2DM). Activation of peroxisomal proliferator-activated receptor gamma (PPARγ) nuclear receptors by TZDs leads to a vast spectrum of metabolic and antiinflammatory effects. In the past decade, clinicians and scientists across the fields of metabolism, diabetes, liver disease (NAFLD), atherosclerosis, inflammation, infertility, and even cancer have had high hopes about the potential for TZDs to treat many of these diseases. However, an increasing awareness about undesirable "off-target" effects of TZDs have made us rethink their role and be more cautious about the long-term benefits and risks related to their use. This review examines the most relevant work on the benefits and risks associated with TZD treatment, with a focus on the only PPARγ agonist currently available (pioglitazone), aiming to offer the reader a balanced overview about the current and future role of TZDs in the management of insulin-resistant states and T2DM.

Fatty acid receptor Gpr40 mediates neuromicrovascular degeneration induced by transarachidonic acids in rodents

OBJECTIVE

Nitro-oxidative stress exerts a significant role in the genesis of hypoxic-ischemic (HI) brain injury. We previously reported that the ω-6 long chain fatty acids, transarachidonic acids (TAAs), which are nitrative stress-induced nonenzymatically generated arachidonic acid derivatives, trigger selective microvascular endothelial cell death in neonatal neural tissue. The primary molecular target of TAAs remains unidentified. GPR40 is a G protein-coupled receptor activated by long chain fatty acids, including ω-6; it is highly expressed in brain, but its functions in this tissue are largely unknown. We hypothesized that TAAs play a significant role in neonatal HI-induced cerebral microvascular degeneration through GPR40 activation.

APPROACH AND RESULTS

Within 24 hours of a HI insult to postnatal day 7 rat pups, a cerebral infarct and a 40% decrease in cerebrovascular density was observed. These effects were associated with an increase in nitrative stress markers (3-nitrotyrosine immunoreactivity and TAA levels) and were reduced by treatment with nitric oxide synthase inhibitor. GPR40 was expressed in rat pup brain microvasculature. In vitro, in GPR40-expressing human embryonic kidney (HEK)-293 cells, [(14)C]-14E-AA (radiolabeled TAA) bound specifically, and TAA induced calcium transients, extracellular signal-regulated kinase 1/2 phosphorylation, and proapoptotic thrombospondin-1 expression. In vivo, intracerebroventricular injection of TAAs triggered thrombospondin-1 expression and cerebral microvascular degeneration in wild-type mice, but not in GPR40-null congeners. Additionally, HI-induced neurovascular degeneration and cerebral infarct were decreased in GPR40-null mice.

CONCLUSIONS

GPR40 emerges as the first identified G protein-coupled receptor conveying actions of nonenzymatically generated nitro-oxidative products, specifically TAAs, and is involved in (neonatal) HI encephalopathy.

Glucose-dependent insulinotropic polypeptide receptor deficiency leads to modifications of trabecular bone volume and quality in mice

A role for the gastro-intestinal tract in controlling bone remodeling is suspected since serum levels of bone remodeling markers are affected rapidly after a meal. Glucose-dependent insulinotropic polypeptide (GIP) represents a suitable candidate in mediating this effect. The aim of the present study was to investigate the effect of total inhibition of GIP signaling on trabecular bone volume, microarchitecture and quality. We used GIP receptor (GIPR) knockout mice and investigated trabecular bone volume and microarchitecture by microCT and histomorphometry. GIPR-deficient animals at 16 weeks of age presented with a significant (20%) increase in trabecular bone mass accompanied by an increase (17%) in trabecular number. In addition, the number of osteoclasts and bone formation rate was significantly reduced and augmented, respectively in these animals when compared with wild-type littermates. These modifications of trabecular bone microarchitecture are linked to a remodeling in the expression pattern of adipokines in the GIPR-deficient mice. On the other hand, despite significant enhancement in bone volume, intrinsic mechanical properties of the bone matrix was reduced as well as the distribution of bone mineral density and the ratio of mature/immature collagen cross-links. Taken together, these results indicate an increase in trabecular bone volume in GIPR KO animals associated with a reduction in bone quality.

The free fatty acid receptor G protein-coupled receptor 40 (GPR40) protects from bone loss through inhibition of osteoclast differentiation

The mechanisms linking fat intake to bone loss remain unclear. By demonstrating the expression of the free fatty acid receptor G-coupled protein receptor 40 (GPR40) in bone cells, we hypothesized that this receptor may play a role in mediating the effects of fatty acids on bone remodeling. Using micro-CT analysis, we showed that GPR40(-/-) mice exhibit osteoporotic features suggesting a positive role of GPR40 on bone density. In primary cultures of bone marrow, we showed that GW9508, a GRP40 agonist, abolished bone-resorbing cell differentiation. This alteration of the receptor activator of NF-κB ligand (RANKL)-induced osteoclast differentiation occurred via the inhibition of the nuclear factor κB (NF-κB) signaling pathway as demonstrated by decrease in gene reporter activity, inhibitor of κB kinase (IKKα/β) activation, inhibitor of κB (IkBα) phosphorylation, and nuclear factor of activated T cells 1 (NFATc1) expression. The GPR40-dependent effect of GW9508 was confirmed using shRNA interference in osteoclast precursors and GPR40(-/-) primary cell cultures. In addition, in vivo administration of GW9508 counteracted ovariectomy-induced bone loss in wild-type but not GPR40(-/-) mice, enlightening the obligatory role of the GPR40 receptor. Then, in a context of growing prevalence of metabolic and age-related bone disorders, our results demonstrate for the first time in translational approaches that GPR40 is a relevant target for the design of new nutritional and therapeutic strategies to counter bone complications.

GPR40: a therapeutic target for mediating insulin secretion (review)

G-protein-coupled receptor 40 (GPR40), known as free fatty acid receptor 1, is mainly expressed in pancreatic β-cells and activated by medium- and long-chain fatty acids. Increasing evidence indicates that the activation of GPR40 in cells causes insulin secretion, and GPR40 has become an attractive therapeutic target for type 2 diabetes. Recently, certain novel GPR40 agonists have been identified that regulate glucose-stimulated insulin secretion, leading to the development of new drugs for the treatment of type 2 diabetes. In this review, we focus on progress in the physiological role of GPR40 and potential drugs targeting GPR40 over the past decade.