Important sites of lipogenesis in the mouse other than liver and white adipose tissue

Metabolic ripple effects - deciphering how lipid metabolism in cancer interfaces with the tumor microenvironment

Cancer cells require a constant supply of lipids. Lipids are a diverse class of hydrophobic molecules that are essential for cellular homeostasis, growth and survival, and energy production. How tumors acquire lipids is under intensive investigation, as these mechanisms could provide attractive therapeutic targets for cancer. Cellular lipid metabolism is tightly regulated and responsive to environmental stimuli. Thus, lipid metabolism in cancer is heavily influenced by the tumor microenvironment. In this Review, we outline the mechanisms by which the tumor microenvironment determines the metabolic pathways used by tumors to acquire lipids. We also discuss emerging literature that reveals that lipid availability in the tumor microenvironment influences many metabolic pathways in cancers, including those not traditionally associated with lipid biology. Thus, metabolic changes instigated by the tumor microenvironment have 'ripple' effects throughout the densely interconnected metabolic network of cancer cells. Given the interconnectedness of tumor metabolism, we also discuss new tools and approaches to identify the lipid metabolic requirements of cancer cells in the tumor microenvironment and characterize how these requirements influence other aspects of tumor metabolism.

Acetyl Co-A Carboxylase Inhibition Halts Hyperglycemia Induced Upregulation of De Novo Lipogenesis in Podocytes and Proximal Tubular Cells

The effect of glycemic stress on de novo lipogenesis (DNL) in podocytes and tubular epithelial cells is understudied. This study is aimed (A) to show the effect of glycemic stress on DNL, and (B) to assess the effect of acetyl-Co A (ACC) inhibition on halting upregulation of DNL, on the expression of other lipid regulatory genes in the DNL pathway, and on markers of fibrosis and apoptosis in podocytes and tubular epithelial cells. We used cultured mouse primary tubular epithelial cells, mouse proximal tubular (BUMPT) cells, and immortal mouse podocytes and measured their percentage of labeled 13C2-palmitate as a marker of DNL after incubation with 13C2 acetate in response to high glucose concentration (25 mM). We then tested the effect of ACC inhibition by complimentary strategies utilizing CRISPR/cas9 deletion or incubation with Acaca and Acacb GapmeRs or using a small molecule inhibitor on DNL under hyperglycemic concentration. Exposure to high glucose concentration (25 mM) compared to osmotic controlled low glucose concentration (5.5 mM) significantly increased labeled palmitate after 24 h up to 72 h in podocytes and primary tubular cells. Knocking out of the ACC coding Acaca and Acacb genes by CRISPR/cas9, downregulation of Acaca and Acacb by specific antisense LNA GapmeRs and inhibition of ACC by firsocostat similarly halted/mitigated upregulation of DNL and decreased markers of fibrosis and programmed cell death in podocytes and various tubular cells. ACC inhibition is a potential therapeutic target to mitigate or halt hyperglycemia-induced upregulation of DNL in podocytes and tubular cells.

Role of Lipogenesis Rewiring in Hepatocellular Carcinoma

Metabolic rewiring is one of the hallmarks of cancer. Altered de novo lipogenesis is one of the pivotal metabolic events deregulated in cancers. Sterol regulatory element-binding transcription factor 1 (SREBP1) controls the transcription of major enzymes involved in de novo lipogenesis, including ACLY, ACACA, FASN, and SCD. Studies have shown the increased de novo lipogenesis in human hepatocellular carcinoma (HCC) samples. Multiple mechanisms, such as activation of the AKT/mechanistic target of rapamycin (mTOR) pathway, lead to high SREBP1 induction and the coordinated enhanced expression of ACLY, ACACA, FASN, and SCD genes. Subsequent functional analyses have unraveled these enzymes' critical role(s) and the related de novo lipogenesis in hepatocarcinogenesis. Importantly, targeting these molecules might be a promising strategy for HCC treatment. This paper comprehensively summarizes de novo lipogenesis rewiring in HCC and how this pathway might be therapeutically targeted.

BAM15-mediated mitochondrial uncoupling protects against obesity and improves glycemic control

Obesity is a leading cause of preventable death worldwide. Despite this, current strategies for the treatment of obesity remain ineffective at achieving long-term weight control. This is due, in part, to difficulties in identifying tolerable and efficacious small molecules or biologics capable of regulating systemic nutrient homeostasis. Here, we demonstrate that BAM15, a mitochondrially targeted small molecule protonophore, stimulates energy expenditure and glucose and lipid metabolism to protect against diet-induced obesity. Exposure to BAM15 in vitro enhanced mitochondrial respiratory kinetics, improved insulin action, and stimulated nutrient uptake by sustained activation of AMPK. C57BL/6J mice treated with BAM15 were resistant to weight gain. Furthermore, BAM15-treated mice exhibited improved body composition and glycemic control independent of weight loss, effects attributable to drug targeting of lipid-rich tissues. We provide the first phenotypic characterization and demonstration of pre-clinical efficacy for BAM15 as a pharmacological approach for the treatment of obesity and related diseases.

Regulation and Metabolic Significance of De Novo Lipogenesis in Adipose Tissues

De novo lipogenesis (DNL) is a complex and highly regulated process in which carbohydrates from circulation are converted into fatty acids that are then used for synthesizing either triglycerides or other lipid molecules. Dysregulation of DNL contributes to human diseases such as obesity, type 2 diabetes, and cardiovascular diseases. Thus, the lipogenic pathway may provide a new therapeutic opportunity for combating various pathological conditions that are associated with dysregulated lipid metabolism. Hepatic DNL has been well documented, but lipogenesis in adipocytes and its contribution to energy homeostasis and insulin sensitivity are less studied. Recent reports have gained significant insights into the signaling pathways that regulate lipogenic transcription factors and the role of DNL in adipose tissues. In this review, we will update the current knowledge of DNL in white and brown adipose tissues with the focus on transcriptional, post-translational, and central regulation of DNL. We will also summarize the recent findings of adipocyte DNL as a source of some signaling molecules that critically regulate energy metabolism.

Regulation of fasting fuel metabolism by toll-like receptor 4

OBJECTIVE

Toll-like receptor 4 (TLR4) has been reported to induce insulin resistance through inflammation in high-fat-fed mice. However, the physiological role of TLR4 in metabolism is unknown. Here, we investigated the involvement of TLR4 in fasting metabolism.

RESEARCH DESIGN AND METHODS

Wild-type and TLR4 deficient (TLR4(-/-)) mice were either fed or fasted for 24 h. Glucose and lipid levels in circulation and tissues were measured. Glucose and lipid metabolism in tissues, as well as the expression of related enzymes, was examined.

RESULTS

Mice lacking TLR4 displayed aggravated fasting hypoglycemia, along with normal hepatic gluconeogenesis, but reversed activity of pyruvate dehydrogenase complex (PDC) in skeletal muscle, which might account for the fasting hypoglycemia. TLR4(-/-) mice also exhibited higher lipid levels in circulation and skeletal muscle after fasting and reversed expression of lipogenic enzymes in skeletal muscle but not liver and adipose tissue. Adipose tissue lipolysis is normal and muscle fatty acid oxidation is increased in TLR4(-/-) mice after fasting. Inhibition of fatty acid synthesis in TLR4(-/-) mice abolished hyperlipidemia, hypoglycemia, and PDC activity increase, suggesting that TLR4-dependent inhibition of muscle lipogenesis may contribute to glucose and lipid homeostasis during fasting. Further studies showed that TLR4 deficiency had no effect on insulin signaling and muscle proinflammatory cytokine production in response to fasting.

CONCLUSIONS

These data suggest that TLR4 plays a critical role in glucose and lipid metabolism independent of insulin during fasting and identify a novel physiological role for TLR4 in fuel homeostasis.

Seasonal variation in lipoprotein lipase and plasma lipids in physically active, normal weight humans

Adipose tissue lipoprotein lipase (ATLPL) provides free fatty acids (FFA) for storage in adipocytes, whereas in skeletal muscle LPL (SMLPL) provides FFA for oxidation. In hibernating animals, the level of SMLPL is relatively higher in summer than winter (promoting fat oxidation), whereas the opposite is seen with ATLPL. A patient-controlled study was designed to determine whether such seasonal variation occurs in normal weight humans. Eighteen subjects were studied in the summer and winter. After 2 days of a standardized diet, they underwent muscle and adipose biopsies for LPL activity, assessment of fitness by VO2 max, and determination of body composition by hydrostatic weighing. The percentages of body fat, body mass index, VO2 max, insulin, glucose, FFA, glycerol, and leptin were not affected by the season. Total cholesterol was higher in the winter than in the summer (157 +/- 5.5 vs. 148 +/- 4.2 mg/dL respectively; P = 0.03). The ATLPL activity was also higher in the winter than in the summer (4.4 +/- 0.8 vs. 2.3 +/- 0.6 nmol FFA/10(6) cells-min; P = 0.04). SMLPL activity trended to be higher in the winter than in the summer (1.9 +/- 0.5 vs. 1.0 +/- 0.1 nmol FFA/g x min; P = 0.06). In summary, ATLPL is seasonally regulated. It appears that SMLPL is similarly regulated by season. For physically active lean subjects, this increase in SMLPL may be a compensatory mechanism to help protect from seasonal weight gain.

Analysis of lines of mice selected on fat content. 4. Correlated responses in growth and reproduction

Lines of mice have been selected for 32 generations for either high or low fat content, resulting in a threefold divergence in the selection criterion (estimated fat content of males at 14 weeks of age). Male mice from both lines were dissected at a series of ages between 4 and 26 weeks and the following traits measured or estimated: body weight, fat content, lean weight, and the weights of several fatpads and internal organs. The lines appeared to have a similar underlying lean weight upon which the Fat line accumulated fat at a faster rate. This accumulation continued unabated in the Fat lines for at least 26 weeks but had effectively ceased by 8 weeks of age in the Lean. The liver and kidneys were slightly larger in the Fat line but there were no differences in the weights of heart, lung or spleen. This detailed phenotypic description of the lines complemented previous reports describing correlated changes in their physiology. The threefold divergence in estimated fat content was less than that in one of its component traits, growth of gonadal fatpad, but was greater than the divergence in other physiological indicators, i.e. the activity of lipogenic enzymes in vitro and direct measurement of lipogenic flux. Testis size in the Fat line was consistently lower than in the Lean although the Fat line was slightly more fecund, apparently due to a higher prenatal survival rate.

Rates of lipogenesis and plasma insulin concentrations in inbred lines of mice differing in fatness

Rates of lipogenesis de novo and plasma concentrations of insulin were compared during post-natal growth in two inbred lines of mice (VL/fDk (VL) and SWR/fNIMR (SWR] in which differences in growth and fatness are probably due to multiple not single gene effects. Irrespective of sex, the lipogenic rate/g was higher in the fatter VL mice in the liver and all other tissues except the head, where it was lower, and the gonadal fat pad, where it was not different. Adult mice in general had lower lipogenic rates than those measured soon after weaning. In both lines the lipogenic rate/g of tissue was higher in males in the liver and in females in the gonadal fat pad. Plasma insulin concentrations were higher in VL mice and tended to rise with age. These results demonstrate that metabolic differences associated with differences in fatness in inbred lines of mice in which fatness is controlled by more than one gene, are qualitatively but not quantitatively similar to those observed by other workers in lines of mice differing in fatness due to a single gene mutation.

Changes in lipid composition and desaturase activities of duodenal mucosa induced by dietary fat

In the present work we have studied the effects of feeding either olive or sunflower oil on lipid composition and desaturase activities of duodenal mucosa microsomes. Duodenal microsomes prepared from dogs fed the sunflower oil diet showed higher percentages of saturated, of linoleic and of n - 3 polyunsaturated fatty acids as well as lower levels of oleic, dihomo-gamma-linolenic and arachidonic acids in phosphatidylcholine and phosphatidylethanolamine than those prepared from animals fed the olive oil diet. In sphingomyelin, the dietary supplementation did not produce significant differences between the two groups. The cholesterol/phospholipid molar ratio was higher in the sunflower oil group than in the olive oil group. The in vitro delta 9-desaturase activity was higher in microsomes from the olive oil dogs. The delta 6-desaturase activity was similar in microsomes from the two groups and lower than that found for delta 9-desaturase activity. Desaturase activities were higher in duodenal microsomes than those previously found for liver microsomes.

Analysis of lines of mice selected for fat content. 2. Correlated responses in the activities of enzymes involved in lipogenesis

Estimates of the activities (Vmax) of six enzymes involved in de novo fat synthesis were made in replicated lines of mice differing in fat content. These lines had been selected high and low for 20 generations with three replicates each of Fat, Control and Lean lines and for a further eight generations high and low as an unreplicated line. The activities of ATP-citrate lyase (ACL), acetyl-CoA carboxylase (ACC), fatty acid synthetase (FAS), cytoplasmic malate dehydrogenase (MDH), malic enzyme (ME) and pyruvate kinase (PK) were determined in vitro in both liver and gonadal fatpad tissues taken at ages five and ten weeks. The activities of ACL, ACC, FAS and ME were significantly higher in the Fat than the Lean lines, and the differences were more pronounced at the earlier age and in the gonadal fatpad where activities in the Fat lines were higher by factors of 3.5, 2.4, 2.5 and 3.5 respectively. The activity of PK was unchanged in each tissue. MDH activity was significantly lower in adipose tissue in the Fat lines than the Lean lines at age ten weeks but not at age five weeks or in liver tissue. Results from replicates indicated that random genetic drift affected enzyme activities but nevertheless significant changes in activity were associated with the direction of selection. The changes in enzyme activity reported here are similar to those known to be associated with major mutations causing obesity in mice.

Tissue distribution of lipogenesis in vivo in the common murre (Uria aalge) and the domestic chicken (Gallus domesticus)

1. Total body lipogenesis was similar in the murre and the chicken. 2. The liver contributes 10.4% to whole body lipogenesis in fed murres when measured in vivo using 3H2O. 3. The liver contributes 28.0% to whole body lipogenesis in the fed chicken. 4. The lower contribution of the liver in the murre may be a consequence of the high fat diet of the murre relative to the chicken.

Tissue-specific alterations of de novo fatty acid synthesis in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-treated rats

De novo fatty acid synthesis was determined by the 3H2O method in numerous tissues and organs of TCDD-treated (125 micrograms/kg), pair-fed and free-fed male Sprague-Dawley rats to investigate if this important pathway of intermediary metabolism is altered by TCDD. Of the 12 tissues and organs examined, liver showed an increased, and interscapular brown adipose tissue (IBAT) a decreased de novo fatty acid synthesis when comparing TCDD-treated to pair-fed or free-fed control rats. De novo fatty acid synthesis was unaffected in other organs and tissues examined, with the exception that the concentration of 3H-fatty acids in plasma reflected the increased rate of synthesis seen in the liver of TCDD-treated animals. Increased de novo fatty acid synthesis in liver coincided with increased plasma triiodothyronine (T3) concentrations, whereas decreased de novo fatty acid synthesis in IBAT parallelled decreased plasma thyroxine (T4) levels. Thyroidectomy decreased de novo fatty acid synthesis, as expected, in both liver and IBAT. However, TCDD elicited no response in either of these organs in thyroidectomized rats. This finding suggests that changes observed in non-thyroidectomized rats are probably secondary effects. Indeed, known tissue-specific effects of T3 on liver and T4 on IBAT provide a likely explanation for the altered de novo fatty acid synthesis of these organs. It is suggested that increased de novo fatty acid synthesis in the liver of TCDD-treated rats might be responsible for the additional wasting away observable in these animals as compared to pair-fed controls.

Comparison of glucose and glucose plus lipid as caloric sources in parenterally fed rats

This study investigated the etiology of fat infiltration of the liver during total parenteral nutrition. We measured the content of liver lipids, serum lipids, liver lipogenic enzymes, rates of in vivo fatty acid synthesis, and carcass composition in rats during continuous intravenous (iv) and intragastric (ig) feeding of two diets containing either 100% glucose or 75% glucose-25% lipid (20% Intralipid). Two groups of orally (O) fed rats were given solid diets similar to either the glucose or glucose-lipid solution in energy and nitrogen content. All six groups of rats (285-295 g) received 230 kcal X kg-1 X day -1 and 766 mg N X kg-1 X day-1. Total liver fat was greater after feeding the glucose diet ig rather than iv. However, feeding the glucose-lipid diet ig but not iv reduced the accumulation of liver fat by 49%. There were no differences in serum glucose concentrations among the three groups fed the glucose solution. Serum glucose concentrations in iv and O rats fed either diet were not significantly different; whereas feeding the glucose-lipid solution ig lowered serum glucose compared with the 100% glucose solution. Insulin concentrations were similar among all groups. The concentrations of serum triglycerides and cholesterol were higher in the groups fed the glucose-lipid diet. The activities of the liver lipogenic enzymes and rates of fatty acid synthesis were higher in iv- and ig-fed rats receiving the glucose diet compared with the glucose-lipid diet.(ABSTRACT TRUNCATED AT 250 WORDS)

Absence of effects of dietary wheat bran on the activities of some key enzymes of carbohydrate and lipid metabolism in mouse liver and adipose tissue

1. The effects of a 100 g/kg dietary substitution of wheat bran on the body-weight gain, food consumption and faecal dry weight of mice given a high-sucrose diet and on the activities of some key enzymes of carbohydrate and lipid metabolism in liver and adipose tissue were studied. 2. Wheat bran had no effect on body-weight gain, food consumption or faecal dry weight. 3. Wheat bran had no effect on the activities of hepatic glucose-6-phosphate dehydrogenase (EC 1.1.1.49), 6-phosphogluconate dehydrogenase (EC 1.1.1.44), malate dehydrogenase (oxaloacetate-decarboxylating) (NADP+) (EC 1.1.1.40), ATP-citrate (pro-3S)-lyase (EC 4.1.3.8), pyruvate kinase (EC 2.7.1.40) and fructose-1,6-bisphosphatase (EC 3.1.3.11). The activity of hepatic 6-phosphofructokinase (EC 2.7.1.11) increased but only when expressed on a body-weight basis. 4. Wheat bran had no effect on the activities of adipose tissue glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, malate dehydrogenase (oxaloacetate-decarboxylating) (NADP+), ATP-citrate (pro-3S)-lyase, hexokinase (EC 2.7.1.1), 6-phosphofructokinase and pyruvate kinase. 5. These results suggest that unlike guar gum and bagasse, wheat bran does not change the flux through some pathways of lipogenesis in liver and adipose tissue when mice are given high-sucrose diets.

Reduced lipogenesis in rats fed a high-protein carbohydrate-free diet

Rates of fatty acid synthesis were assessed in carcass, liver, and adipose tissue from rats fed for 30 to 40 days a balanced diet (66% wt/wt carbohydrate, 17% casein, 8% fat) or a high-protein carbohydrate-free diet (70% casein, 8% fat). Despite similar body weight increases, carcass fatty acid content of rats on the high-protein (HP) diet was 13% less, and the weight of their epididymal fat pads was reduced by 29% in relation to the controls. In vivo incorporation of 3H2O into carcass fatty acids (FA) and into liver triacylglycerol (TAG) was significantly reduced in HP-fed rats. FA synthesis from 14C-acetate, glucose, or leucine and from 3H2O was also markedly decreased in liver slices from HP rats. The amount of 3H-TAG that accumulated in plasma of rats injected with triton WR 1339 to block peripheral utilization of lipoprotein corresponded in HP and control rats to only 4.1% and 5.0%, respectively, of 3H-FA recovered in carcasses from animals not treated with triton, indicating that almost all of the carcass 3H-TAG was synthesized in situ. However, on a long term basis, the reduced hepatic lipogenesis and the resulting decreased transport of TAG might affect lipid accumulation in HP rats. In vivo lipogenesis from 3H2O and in vitro FA synthesis from 3H2O and from 14C-precursors did not differ significantly in retroperitoneal and epididymal adipose tissue from HP and control rats. In both groups of animals, in vivo rates of lipogenesis were higher in retroperitoneal than in epididymal adipose tissue but still did not account for rates of FA synthesis by carcasses, suggesting the existence of other sites with higher lipogenic activity.

Control of fatty acid synthesis in lactation

Lactation results not only in an increased rate of fatty acid synthesis in the mammary gland but also in a decreased rate of fatty acid synthesis in adipose tissue and, in the rat at least, an increased rate of hepatic fatty acid synthesis. Progesterone (during pregnancy), prolactin and (in ruminants) GH are implicated in the regulation of the reciprocal changes in fatty acid synthesis in mammary gland and adipose tissue. Progesterone and prolactin, at least, appear to influence the rate of fatty acid synthesis by modulating the insulin-binding capacities of the tissues, but it is clear that steps in the mechanism of action of insulin subsequent to its binding to the receptor are also changed in adipose tissue during lactation.

Relative contribution of the main tissues and organs to body fatty acid synthesis in the rat

Tritiated water was used to measure the rate of fatty acid synthesis in the main tissues and organs of 7-week old Wistar male rats in order to determine the relative contribution of each tissue to body fatty acid synthesis. We reached the following conclusions: (a) the liver is the main site of fatty acid synthesis, it alone synthesizes 42% of the newly synthesized fatty acids in the body. (b) The dissectable white adipose tissues synthesize 27% of the fatty acids in the body. This group of tissues is heterogeneous because the mesenteric adipose tissue alone contains 40% of the labeled fatty acids present in the white adipose tissues. (c) Besides the intestines, organs other than the liver play a negligible role (2% of the total) in fatty acid synthesis. (d) The skin contributes 7% of the body fatty acid synthesis. (e) The rest of the carcass, essentially composed of the musculature and the skeleton, contributes 18% of body fatty acid synthesis and accounts for 33% of the extrahepatic tissue fatty acid synthesis.

In vivo changes in the rates of total lipid and fatty acid synthesis in liver and white adipose tissues of male rats during postweaning growth

1. In suckling rats, lipid synthesis is low in liver and adipose tissues. The rest of carcass is the major site of body lipogenesis. 2. After weaning, lipid synthesis (per g of wet wt) strongly increases in liver, perirenal and subcutaneous adipose tissues, and rest of carcass. The relative contributions of liver and both adipose tissues to body lipogenesis are about 35-40 and 6-9% respectively, but the rest of the carcass is still the main site of lipogenesis up to the age of 50 days. 3. In adult rats, lipid synthesis (per g of wet wt) remains high in liver but is strongly reduced in adipose tissues and carcass. Liver becomes the principal site of body lipogenesis.