Dual effects of the non-esterified fatty acid receptor ‘GPR40’ for human health

G protein-coupled receptor 40 (GPR40), a receptor for diverse non-esterified fatty acids, is expressed predominantly in the wide variety of neurons of the central nervous system and β-cells in the pancreatic islets. Since deorphanization of GPR40 in 2003, the past decade has seen major advances in our understanding of its role in the insulin secretion. However, there is still a great deal to be elucidated about the role of GPR40 in the brain, because the latter shows the most abundant GPR40 mRNA expression among the human tissues. Since a substantial expression of GPR40 is also seen in the hypothalamus, ‘brain-lipid sensing’ might be involved in the control of insulin secretion and energy balance. The preceding experiments using monkeys after transient global brain ischemia, have highlighted implication of GPR40 for amplifying adult hippocampal neurogenesis. Although GPR40-mediated intracellular signaling was recently found to result in phosphorylation of cAMP response element-binding protein (CREB) necessary for the neuronal differentiation and synaptic plasticity, the signaling cascade is still incompletely understood. Furthermore, in response to conjugated linoleic acids or trans isomers of arachidonic acid, GPR40 was recently demonstrated in rodents to mediate lipotoxicity to β-cells, neurons, or microvessels, which result in diabetes, retinopathy, stroke, etc. However, it still remains undetermined in humans whether and how oxidized, conjugated, or excessive fatty acids evoke lipotoxicity. Although literature about GPR40 is limited especially about the brain or the brain–pancreas interaction, this review aims at summarizing beneficial as well as detrimental effects of this receptor in the brain and pancreas in response to diverse fatty acids.     

G蛋白偶联受体40研究进展

G蛋白偶联受体40(G protein-coupled receptor 40, GPR40)是一种具有7个跨膜α螺旋结构的G蛋白偶联受体, 主要分布于胰腺细胞和神经系统细胞。它能与机体内游离的中长链脂肪酸结合, 激活细胞内信号通路, 从而调节其生理功能。在胰岛细胞中, GPR40可被游离脂肪酸激活, 促使细胞内钙离子浓度升高, 进而促进胰岛素释放。根据这一机理, 以GPR40为靶点的激动剂类药物相继被开发, 用于糖尿病治疗。GPR40也参与神经发生过程, 但到目前为止其相关机制尚不清楚。文章从基因结构、表达调控、蛋白配体及应用、生理功能等方面详细介绍了GPR40的研究现状, 总结了目前研究中所遇到的问题, 并对未来的研究热点进行展望。     

Activation of Stat5 and induction of a pregnancy-like mammary gland differentiation by eicosapentaenoic and docosapentaenoic omega-3 fatty acids

The protective effect of early pregnancy against breast cancer can be attributed to the transition from undifferentiated cells in the nulliparous to the differentiated mature cells during pregnancy. Considerable evidence suggests strongly that the n-3 polyunsaturated fatty acid (PUFA) content of adipose breast tissue is inversely associated with an increased risk of breast cancer. Here, we report that there was a decrease in the n-6/n-3 PUFA ratio and a significant increase in concentration of n-3 PUFA docosapentaenoic acid and eicosapentaenoic acid in the pregnant gland. The functional role of n-3 PUFAs on differentiation was supported by the studies in the fat-1 transgenic mouse, which converts endogenous n-6 to n-3 PUFAs. Alternation of the n-6/n-3 ratio in favor of n-3 PUFA, and particularly docosapentaenoic acid, in the mammary gland of fat-1 mouse resulted in development of lobulo-alveolar-like structure and milk protein beta-casein expression, mimicking the differentiated state of the pregnant gland. Docosapentaenoic acid and eicosapentaenoic acid activated the Jak2/Stat5 signaling pathway and induced a functional differentiation with production of beta-casein. Expression of brain type fatty acid binding protein brain type fatty acid binding protein in virgin transgenic mice also resulted in a reduced ratio of n-6/n-3 PUFA, a robust increase in docosapentaenoic acid accumulation, and mammary differentiation. These data indicate a role of mammary derived growth inhibitor related gene for preferential accumulation of n-3 docosapentaenoic acid and eicosapentaenoic acid in the differentiated gland during pregnancy. Thus, alternation of n-6/n-3 fatty acid compositional ratio in favor of n-3 PUFA, and particularly docosapentaenoic acid and eicosapentaenoic acid, is one of the underlying mechanisms of pregnancy-induced mammary differentiation.     

Brain metabolism of nutritionally essential polyunsaturated fatty acids depends on both the diet and the liver

Plasma alpha-linolenic acid (alpha-LNA, 18:3n-3) and linoleic acid (LA, 18:2n-6) do not contribute significantly to the brain content of docosahexaenoic acid (DHA, 22:6n-3) or arachidonic acid (AA, 20:4n-6), respectively, and neither DHA nor AA can be synthesized de novo in vertebrate tissue. Therefore, measured rates of incorporation of circulating DHA and AA into brain exactly represent their rates of consumption by brain. Positron emission tomography (PET) has been used to show, based on this information, that the adult human brain consumes AA and DHA at rates of 17.8 and 4.6 mg/day, respectively, and that AA consumption does not change significantly with age. In unanesthetized adult rats fed an n-3 PUFA "adequate" diet containing 4.6% alpha-LNA (of total fatty acids) as its only n-3 PUFA, the rate of liver synthesis of DHA was more than sufficient to maintain brain DHA, whereas the brain's rate of DHA synthesis is very low and unable to do so. Reducing dietary alpha-LNA in the DHA-free diet led to upregulation of liver but not brain coefficients of alpha-LNA conversion to DHA and of liver expression of elongases and desaturases that catalyze this conversion. Concurrently, brain DHA loss slowed due to downregulation of several of its DHA-metabolizing enzymes. Dietary alpha-LNA deficiency also promoted accumulation of brain docosapentaenoic acid (22:5n-6), and upregulated expression of AA-metabolizing enzymes, including cytosolic and secretory phospholipases A(2) and cyclooxygenase-2. These changes, plus reduced levels of brain derived neurotrophic factor (BDNF) and cAMP response element-binding protein (CREB) in n-3 PUFA diet deficient rats, likely render their brain more vulnerable to neuropathological insults.     

Delivery and turnover of plasma-derived essential PUFAs in mammalian brain

Polyunsaturated fatty acids (PUFAs) are critical to nervous system function and structure, but their rates of incorporation from plasma into brain have not been evaluated. In the adult rat, calculations based on our model show that at least 3;-5% of esterified brain arachidonic acid (AA) and 2;-8% of esterified brain docosahexaenoic acid (DHA) are replaced daily by unesterified PUFAs in plasma. These rates, when related to unlabeled brain PUFA composition, give half-lives of 1-2 weeks for plasma-brain exchange of AA and DHA. In the human brain, the arachidonate replacement rate is 0.3% per day. Although unesterified plasma PUFA concentrations are low, their rates of incorporation into brain are sufficient to compensate for metabolic and efflux losses, so that PUFA transport from plasma into brain as a component of a lipoprotein is unnecessary. Dietary supplementation, by altering plasma unesterified PUFA concentrations, can regulate brain PUFA content and may help to treat brain diseases involving PUFA imbalance.     

N-3 polyunsaturated fatty acid consumption produces neurobiological effects associated with prevention of depression in rats after the forced swimming test

Epidemiological data and clinical trials suggest that n-3 polyunsaturated fatty acids (PUFA) eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) have preventive and therapeutic effects on depression; however, the underlying mechanism remains elusive. The present study aimed to examine the behavioral effects and antidepressant mechanism of n-3 PUFA using a forced swimming test. Eleven-week-old male Sprague-Dawley rats were fed an American Institute of Nutrition-93M diet containing 0%, 0.5% or 1% EPA and DHA relative to the total energy intake in their diet for 12 weeks (n=8 per group). Total dietary intake, body weight and hippocampus weights were not significantly different among groups. The groups administered 0.5% and 1% EPA+DHA diets had significantly higher levels of n-3 PUFA in their brain phospholipids compared to those in the control group. The immobility time was significantly decreased and the climbing time was significantly increased in the 0.5% and 1% EPA+DHA groups compared with those in the 0% EPA+DHA group. Plasma serotonin concentration and hippocampus c-AMP response element binding protein (CREB) expression were significantly increased in the 0.5% and 1% EPA+DHA groups compared with those in the 0% EPA+DHA group. Conversely, interleukin (IL)-6 expression was significantly reduced in the 0.5% and 1% EPA+DHA groups compared with that in the 0% EPA+DHA group. However, there were no dose-dependent effects of n-3 PUFA and no significant differences in expressions of IL-1β, tumor necrosis factor-α, brain-derived neurotrophic factor or phosphorylated CREB. In conclusion, long-term intake of EPA+DHA induced antidepressant-like effects in rats and overexpression of CREB via decreased IL-6 expression.     

Brain ischemia, neurogenesis, and neurotrophic receptor expression in primates

Generation of new neurons persists in the normal adult mammalian brain, with neural stem/progenitor cells residing in at least two brain regions: the subventricular zone (SVZ) of the lateral ventricle and the subgranular zone (SGZ) of the dentate gyrus (DG). Adult neurogenesis is well documented in the rodent, and has also been demonstrated in vivo in nonhuman primates and humans. Brain injuries such as ischemia affect neurogenesis in adult rodents as both global and focal ischemic insults enhance the proliferation of progenitor cells residing in SGZ or SVZ. We addressed the issue whether an injury triggered activation of endogenous neuronal precursors also takes place in the adult primate brain. We found that the ischemic insult increased the number of progenitor cells in monkey SGZ and SVZ, and caused gliogenesis in the ischemia-prone hippocampal CA1 sector. To better understand the mechanisms regulating precursor cell division and differentiation in the primate, we analyzed the expression at protein level of a panel of potential regulatory molecules, including neurotrophic factors and their receptors. We found that a fraction of mitotic progenitors were positive for the neurotrophin receptor TrkB, while immature neurons expressed the neurotrophin receptor TrkA. Astroglia, ependymal cells and blood vessels in SVZ were positive for distinctive sets of ligands/receptors, which we characterized. Thus, a network of neurotrophic signals operating in an autocrine or paracrine manner may regulate neurogenesis in adult primate SVZ. We also analyzed microglial and astroglial proliferation in postischemic hippocampal CA1 sector. We found that proliferating postischemic microglia in adult monkey CA1 sector express the neurotrophin receptor TrkA, while activated astrocytes were labeled for nerve growth factor (NGF), ligand for TrkA, and the tyrosine kinase TrkB, a receptor for brain derived neurotrophic factor (BDNF). These results implicate NGF and BDNF as regulators of postischemic glial proliferation in adult primate hippocampus.     

CREB and cAMP response element-mediated gene expression in the ischemic brain

Cerebral ischemia triggers robust phosphorylation of cAMP response element-binding protein (CREB) and CRE-mediated gene expression in neurons. Glutamate receptor activation and subsequent calcium influx may activate CREB shortly after ischemia. CREB activation leads to expression of genes encoding neuroprotective molecules, such as the antiapoptotic protein Bcl-2, and contributes to survival of neurons after ischemic insult. Recent studies have suggested that CREB may be involved in acquisition of ischemic tolerance, a phenomenon that occurs after sublethal ischemic stress. CREB activation is also involved in the survival of newborn neurons in the dentate gyrus of the hippocampus after ischemia. Therefore, CREB-related therapeutics may be promising for brain protection and endogenous neurogenesis and could promote functional recovery in ischemic stroke patients. This minireview summarizes our current understanding for the role of CREB in regulating CRE-mediated gene expression during cerebral ischemia.     

High dietary fish oil alters the brain polyunsaturated fatty acid composition

Feeding adult rats a 17% corn-oil diet for 8 weeks did not change brain polyunsaturated fatty acids (PUFA) compared to rats fed 2.2% corn oil (with 2.2% lard added). When the corn-oil diet was supplemented with 14.5% cod liver oil or 12.5% salmon oil, the fatty acid composition of brain PUFA was significantly altered, even if alpha-tocopherol was added to the salmon-oil diet. Comparing salmon-oil- and cod-liver-oil-fed animals with corn-oil-fed animals, arachidonic acid 22:4(n-6) and 22:5(n-6) were reduced, and 20:5(n-3), 22:5(n-3) and 22:6(n-3) were increased. Liver fatty acids were also significantly altered. Thus, the brain is not protected against a large excess of very-long-chain n-3 PUFA, which increase n-3/n-6 ratio and could lead to abnormal function, and which might be difficult to reverse.     

CREB activity maintains the survival of cingulate cortical pyramidal neurons in the adult mouse brain

Cyclic AMP-responsive element binding protein (CREB) activity is known to contribute to important neuronal functions, such as synaptic plasticity, learning and memory. Using a microelectroporation technique to overexpress dominant negative mutant CREB (mCREB) in the adult mouse brain, we found that overexpression of mCREB in the forebrain cortex induced neuronal degeneration. Our findings suggest that constitutively active CREB phosphorylation is important for the survival of mammalian cells in the brain.     

Hypothalamic GPR40 Signaling Activated by Free Long Chain Fatty Acids Suppresses CFA-Induced Inflammatory Chronic Pain

GPR40 has been reported to be activated by long-chain fatty acids, such as docosahexaenoic acid (DHA). However, reports studying functional role of GPR40 in the brain are lacking. The present study focused on the relationship between pain regulation and GPR40, investigating the functional roles of hypothalamic GPR40 during chronic pain caused using a complete Freund''s adjuvant (CFA)-induced inflammatory chronic pain mouse model. GPR40 protein expression in the hypothalamus was transiently increased at day 7, but not at days 1, 3 and 14, after CFA injection. GPR40 was co-localized with NeuN, a neuron marker, but not with glial fibrillary acidic protein (GFAP), an astrocyte marker. At day 1 after CFA injection, GFAP protein expression was markedly increased in the hypothalamus. These increases were significantly inhibited by the intracerebroventricular injection of flavopiridol (15 nmol), a cyclin-dependent kinase inhibitor, depending on the decreases in both the increment of GPR40 protein expression and the induction of mechanical allodynia and thermal hyperalgesia at day 7 after CFA injection. Furthermore, the level of DHA in the hypothalamus tissue was significantly increased in a flavopiridol reversible manner at day 1, but not at day 7, after CFA injection. The intracerebroventricular injection of DHA (50 µg) and GW9508 (1.0 µg), a GPR40-selective agonist, significantly reduced mechanical allodynia and thermal hyperalgesia at day 7, but not at day 1, after CFA injection. These effects were inhibited by intracerebroventricular pretreatment with GW1100 (10 µg), a GPR40 antagonist. The protein expression of GPR40 was colocalized with that of β-endorphin and proopiomelanocortin, and a single intracerebroventricular injection of GW9508 (1.0 µg) significantly increased the number of neurons double-stained for c-Fos and proopiomelanocortin in the arcuate nucleus of the hypothalamus. Our findings suggest that hypothalamic GPR40 activated by free long chain fatty acids might have an important role in this pain control system.