Jacqueline O'Dowd

Roles of GPR41 and GPR43 in leptin secretory responses of murine adipocytes to short chain fatty acids

GPR41 is reportedly expressed in murine adipose tissue and mediates short chain fatty acid (SCFA)‐stimulated leptin secretion by activating Gαi. Here, we agree with a contradictory report in finding no expression of GPR41 in murine adipose tissue. Nevertheless, in the presence of adenosine deaminase to minimise Gαi signalling via the adenosine A1 receptor, SCFA stimulated leptin secretion by adipocytes from wild‐type but not GPR41 knockout mice. Expression of GPR43 was reduced in GPR41 knockout mice. Acetate but not butyrate stimulated leptin secretion in wild‐type mesenteric adipocytes, consistent with mediation of the response by GPR43 rather than GPR41. Pertussis toxin prevented stimulation of leptin secretion by propionate in epididymal adipocytes, implicating Gαi signalling mediated by GPR43 in SCFA‐stimulated leptin secretion.

Modulation of susceptibility to weight gain and insulin resistance in low birthweight rats by treatment of their mothers with leptin during pregnancy and lactation.

OBJECTIVES: To investigate whether administration of leptin to rats during pregnancy and lactation affects placental 11β-hydroxysteroid dehydrogenase (11β-HSD2) activity and the susceptibility of their offspring to weight gain and insulin resistance.DESIGN: Pregnant rats fed on a low-protein diet were administered leptin or saline by subcutaneous minipump from day 14 of gestation and throughout lactation. A further group was fed a normal diet and given saline. After weaning, the offspring of each group were fed on a normal diet until 6 weeks of age and then half of each group was transferred to a high-fat diet until 12 months of age.RESULTS: Plasma leptin levels were raised two-fold on days 16–18 of pregnancy in the leptin-treated dams, but, despite a constant rate of infusion, at parturition they dipped to control levels before rising again. The activity of placental 11β-HSD2 was reduced by the low-protein diet; this reduction was prevented by treating the dams with leptin. The male offspring of the saline-treated dams gained more weight and had higher plasma leptin levels on the high fat than the chow diet, but the offspring of the leptin-treated dams did not. Fasting blood glucose and intraperitoneal glucose tolerance at 6 and 12 months of age was unaffected by the high-fat diet, but only the offspring of the leptin-treated dams achieved this control without raised insulin levels.CONCLUSIONS: The rate of leptin clearance appears to increase at parturition. The administration of leptin to rats during late pregnancy and lactation makes their male offspring less susceptible to high-fat-diet-induced weight gain and insulin resistance.

Male mice that lack the G-protein-coupled receptor GPR41 have low energy expenditure and increased body fat content

SCFA are produced in the gut by bacterial fermentation of undigested carbohydrates. Activation of the Gαi-protein-coupled receptor GPR41 by SCFA in β-cells and sympathetic ganglia inhibits insulin secretion and increases sympathetic outflow, respectively. A possible role in stimulating leptin secretion by adipocytes is disputed. In the present study, we investigated energy balance and glucose homoeostasis in GPR41 knockout mice fed on a standard low-fat or a high-fat diet. When fed on the low-fat diet, body fat mass was raised and glucose tolerance was impaired in male but not female knockout mice compared to wild-type mice. Soleus muscle and heart weights were reduced in the male mice, but total body lean mass was unchanged. When fed on the high-fat diet, body fat mass was raised in male but not female GPR41 knockout mice, but by no more in the males than when they were fed on the low-fat diet. Body lean mass and energy expenditure were reduced in male mice but not in female knockout mice. These results suggest that the absence of GPR41 increases body fat content in male mice. Gut-derived SCFA may raise energy expenditure and help to protect against obesity by activating GPR41.

Prolonged treatment of genetically obese mice with conjugated linoleic acid improves glucose tolerance and lowers plasma insulin concentration: possible involvement of PPAR activation

BackgroundStudies in rodents and some studies in humans have shown that conjugated linoleic acid (CLA), especially its trans-10, cis-12 isomer, reduces body fat content. However, some but not all studies in mice and humans (though none in rats) have found that CLA promotes insulin resistance. The molecular mechanisms responsible for these effects are unclear, and there are conflicting reports on the effects of CLA on peroxisomal proliferator-activated receptor-γ (PPARγ) activation and expression. We have conducted three experiments with CLA in obese mice over three weeks, and one over eleven weeks. We have also investigated the effects of CLA isomers in PPARγ and PPARα reporter gene assays.ResultsInclusion of CLA or CLA enriched with its trans-10, cis-12 isomer in the diet of female genetically obese (lepob/lepob) mice for up to eleven weeks reduced body weight gain and white fat pad weight. After two weeks, in contrast to beneficial effects obtained with the PPARγ agonist rosiglitazone, CLA or CLA enriched with its trans-10, cis-12 isomer raised fasting blood glucose and plasma insulin concentrations, and exacerbated glucose tolerance. After 10 weeks, however, CLA had beneficial effects on glucose and insulin concentrations. At this time, CLA had no effect on the plasma TNFα concentration, but it markedly reduced the plasma adiponectin concentration. CLA and CLA enriched with either isomer raised the plasma triglyceride concentration during the first three weeks, but not subsequently. CLA enriched with its trans-10, cis-12 isomer, but not with its cis-9, trans-11 isomer, stimulated PPARγ-mediated reporter gene activity; both isomers stimulated PPARα-mediated reporter gene activity.ConclusionsCLA initially decreased but subsequently increased insulin sensitivity in lepob/lepob mice. Activation of both PPARγ and PPARα may contribute to the improvement in insulin sensitivity. In the short term, however, another mechanism, activated primarily by trans-10, cis-12-CLA, which probably leads to reduced adipocyte number and consequently reduced plasma adiponectin concentration, may decrease insulin sensitivity.

Metabolic responses to BRL37344 and clenbuterol in soleus muscle and C2C12 cells via different atypical pharmacologies and β2‐adrenoceptor mechanisms

Picomolar concentrations of the β3‐adrenoceptor agonist BRL37344 stimulate 2‐deoxyglucose uptake in soleus muscle via undefined receptors. Higher concentrations alter uptake, apparently via β2‐adrenoceptors. Effects of BRL37344 and β2‐adrenoceptor agonists are compared.

Leanness in postnatally nutritionally programmed rats is associated with increased sensitivity to leptin and a melanocortin receptor agonist and decreased sensitivity to neuropeptide Y

Background:Pups of normally nourished dams that are cross-fostered after birth to dams fed a low-protein (8% by weight) diet (postnatal low protein (PLP)) grow slower during the suckling period and remain small and lean throughout adulthood. At weaning, they have increased expression in the arcuate nucleus (ARC) of the hypothalamus of the orexigenic neuropeptide Y (NPY) and decreased expression of pro-opiomelanocortin, the precursor of anorexigenic melanocortins.Objectives And Methods:We investigated, using third ventricle administration, whether 3-month-old male PLP rats display altered sensitivity to leptin with respect to food intake, NPY and the melanocortin 3/4-receptor agonist MTII, and using in situ hybridization or laser capture microdissection of the ARC followed by RT-PCR, whether the differences observed were associated with changes in the hypothalamic expression of NPY or the leptin receptor, NPY receptors and melanocortin receptors.Results:PLP rats were smaller and had reduced percentage body fat content and plasma leptin concentration compared with control rats. Leptin (5 μg) reduced food intake over 0–48 h more in PLP than control rats (P<0.05). Submaximal doses of NPY increased the food intake less in PLP rats than in controls, whereas submaximal doses of MTII reduced the food intake more in PLP rats. Maximal responses did not differ between PLP and control rats. Leptin and melanocortin-3 receptor (MC3R) expression were increased in both ARC and ventromedial hypothalamic nuclei in PLP animals compared with the controls. MC4R, NPY Y1R, Y5R and NPY expression were unchanged.Conclusion:Postnatal undernourishment results in food intake in adult rats being more sensitive to reduction by leptin and melanocortins, and less sensitive to stimulation by NPY. We propose that this contributes to increased leptin sensitivity and resistance to obesity. Increased expression of ObRb and MC3R may partly explain these findings but other downstream mechanisms must also be involved.

Identification of a novel agonist of peroxisome proliferator-activated receptors α and γ that may contribute to the anti-diabetic activity of guggulipid in Lepob/Lepob mice.

The ethyl acetate extract of the gum of the guggul tree, Commiphora mukul (guggulipid), is marketed for the treatment of dyslipidaemia and obesity. We have found that it protects Lep(ob)/Lep(ob) mice from diabetes and have investigated possible molecular mechanisms for its metabolic effects, in particular those due to a newly identified component, commipheric acid. Both guggulipid (EC(50)=0.82 microg/ml) and commipheric acid (EC(50)=0.26 microg/ml) activated human peroxisome proliferator-activated receptor alpha (PPARalpha) in COS-7 cells transiently transfected with the receptor and a reporter gene construct. Similarly, both guggulipid (EC(50)=2.3 microg/ml) and commipheric acid (EC(50)=0.3 microg/ml) activated PPARgamma and both promoted the differentiation of 3T3 L1 preadipocytes to adipocytes. Guggulipid (EC(50)=0.66 microg/ml), but not commipheric acid, activated liver X receptor alpha (LXRalpha). E- and Z-guggulsterones, which are largely responsible for guggulipids hypocholesterolaemic effect, had no effects in these assays. Guggulipid (20 g/kg diet) improved glucose tolerance in female Lep(ob)/Lep(ob) mice. Pure commipheric acid, given orally (960 mg/kg body weight, once daily), increased liver weight but did not affect body weight or glucose tolerance. However, the ethyl ester of commipheric acid (150 mg/kg, twice daily) lowered fasting blood glucose and plasma insulin, and plasma triglycerides without affecting food intake or body weight. These results raise the possibility that guggulipid has anti-diabetic activity due partly to commipheric acids PPARalpha/gamma agonism, but the systemic bioavailability of orally dosed, pure commipheric acid appears poor. Another component may contribute to guggulipids anti-diabetic and hypocholesterolaemic activity by stimulating LXRalpha.

β2-Adrenoceptors and non-β-adrenoceptors mediate effects of BRL37344 and clenbuterol on glucose uptake in soleus muscle: studies using knockout mice

Background and purpose:  In previous work, 10 pM BRL37344 and 10 pM clenbuterol stimulated glucose uptake in mouse soleus muscle. Ten nM BRL37344 also stimulated uptake but 100 nM clenbuterol inhibited uptake. Antagonist studies suggested that the opposite effects of 10 nM BRL37344 and 100 nM clenbuterol are mediated by the β2‐adrenoceptor. BRL37344 and clenbuterol have been studied in muscles that lack β3‐, β2‐ or all three β‐adrenoceptors. Effects of β‐adrenoceptor antagonists on responses to the agonists have been studied further using muscles from wild‐type mice.

Endocrine pancreatic development: impact of obesity and diet

During embryonic development, multipotent endodermal cells differentiate to form the pancreas. Islet cell clusters arising from the pancreatic bud form the acini tissue and exocrine ducts whilst pancreatic islets form around the edges of the clusters. The successive steps of islet differentiation are controlled by a complex network of transcription factors and signals that influence cell differentiation, growth and lineage. A Westernized lifestyle has led to an increased consumption of a high saturated fat diet, and an increase in maternal obesity. The developing fetus is highly sensitive to the intrauterine environment, therefore any alteration in maternal nutrition during gestation and lactation which affects the in-utero environment during the key developmental phases of the pancreas may change the factors controlling β-cell development and β-cell mass. Whilst the molecular mechanisms behind the adaptive programming of β-cells are still poorly understood it is established that changes arising from maternal obesity and/or over-nutrition may affect the ability to maintain fetal β-cell mass resulting in an increased risk of type 2 diabetes in adulthood.

Improved glucose tolerance in acyl CoA:diacylglycerol acyltransferase 1-null mice is dependent on diet

BackgroundMice that lack acyl CoA:diacylglycerol acyltransferase (Dgat1-/- mice) are reported to have a reduced body fat content and improved glucose tolerance and insulin sensitivity. Studies so far have focussed on male null mice fed a high fat diet and there are few data on heterozygotes. We compared male and female Dgat1-/-, Dgat1+/- and Dgat1+/+ C57Bl/6 mice fed on either standard chow or a high fat diet.ResultsBody fat content was lower in the Dgat1-/- than the Dgat1+/+ mice in both experiments; lean body mass was higher in male Dgat1-/- than Dgat1+/+ mice fed on the high fat diet. Energy intake and expenditure were higher in male Dgat1-/- than Dgat1+/+ mice; these differences were less marked or absent in females. The body fat content of female Dgat1+/- mice was intermediate between that of Dgat1-/- and Dgat1+/+ mice, whereas male Dgat1+/- mice were similar to or fatter than Dgat1+/+ mice. Glucose tolerance was improved and plasma insulin reduced in Dgat1-/- mice fed on the high fat diet, but not on the chow diet. Both male and female Dgat1+/- mice had similar glucose tolerance to Dgat1+/+ mice.ConclusionThese results suggest that although ablation of DGAT1 improves glucose tolerance by preventing obesity in mice fed on a high fat diet, it does not improve glucose tolerance in mice fed on a low fat diet.