As a matter of fat

Inflammatory pathways linking obesity and ovarian dysfunction

This review summarizes some of the recent advances in obesity research and describes how we and others have built upon these findings to better understand the impact of obesity on granulosa cells, cumulus cells and oocytes within the ovaries of obese females. Obesity is associated with lipid accumulation in non-adipose tissue cells and the induction of oxidative stress and endoplasmic reticulum stress responses that are tightly linked with systemic inflammation. Analysis of ovarian cells and fluid of obese women indicates that these same mechanisms are activated in the ovary in response to obesity. Studies in mice support this and allow further dissection of the pathways by which diet-induced obesity contributes to changes in mitochondria and the endoplasmic reticulum. These studies are in their infancy but cumulatively provide basic information about the cellular mechanisms that may lead to the impaired ovulation and reduced oocyte developmental potential that is observed in obese females.

Obesity and nutrient sensing TOR pathway in flies and vertebrates: Functional conservation of genetic mechanisms

The global prevalence of obesity has grown to epidemic proportions, and 400 million people are now considered to be obese. Excessive accumulation of dietary lipids (obesity) is a known risk factor for the development of deleterious metabolic conditions and has been strongly linked to the progression of heart disease and type 2 diabetes. Investigating the origin and effects of high-fat diet (HFD)-induced obesity and its genetic mediators is an important step in understanding the mechanisms that contribute to obesity. However, the mechanisms that underlie HFD pathophysiology have yet to be elucidated fully. Here we describe recent work in a Drosophila model to investigate the origin and genetic mechanisms that could underlie HFD-induced obesity, type 2 diabetes and cardiac dysfunction.

Lipoexpediency: de novo lipogenesis as a metabolic signal transmitter

De novo lipogenesis, the production of fats from simple precursors, is often dismissed as irrelevant to the pathobiology of obesity caused by positive energy balance due to typical high fat diets. However, emerging data implicate de novo lipogenesis in the generation of metabolic signals that alter disease risk. Exploiting this signaling pathway represents lipoexpediency. Lipoexpediency is the concept of directing fats toward benefit even in the setting of lipid overload, and represents a strategy to complement efforts aimed at improving energy balance. Optimizing lipid signals initiated by key lipogenic enzymes such as fatty acid synthase might limit morbidity in people who are unlikely to abandon the lifestyle of the sedentary gourmand.

Hydroxy monounsaturated fatty acids as agonists for peroxisome proliferator-activated receptors

The physiological and pathological role of oxidized polyunsaturated fatty acids (PUFAs) has been extensively studied, whereas those of hydroxy monounsaturated fatty acids (MUFAs) are not well understood. This study demonstrated that 11-hydroxy-(9Z)-octadecenoic acid ((9Z)-11-HOE), which was isolated from adlay seeds (Coix lacryma-jobi L. var. ma-yuen STAF.), can activate peroxisome proliferator-activated receptor (PPAR)alpha, delta and gamma in luciferase reporter assays more efficiently than (9Z)-octadecenoic acid (oleic acid), and to the same degree as linoleic acid. (9Z)-11-HOE increased the mRNA levels of UCP2 and CD36 in C2C12 myotubes and THP- 1 cells, respectively, and these effects were blocked by the PPARdelta- and gamma-specific antagonists GSK0660 and T0070907, respectively. Evaluation of the structure.activity relationship between hydroxy MUFAs and PPAR activation revealed that (9E)-11-HOE, the geometrical isomer of (9Z)-11-HOE, activated PPARs more potently than (9Z)-11-HOE, and that PPAR activation by hydroxyl MUFAs was not markedly influenced by the position of the hydroxy group or the double bond, although PPARdelta seemed to possess ligand specificity different to that of PPARalpha or gamma . Additionally, the finding that 11-hydroxy octadecanoic acid, the hydrogenated product of (9E)-11- HOE, was also capable of activating PPARs to a similar extent as (9E)-11-HOE indicates that the double bond in hydroxy MUFAs is not essential for PPAR activation. In conclusion, (9Z)-11-HOE derived from alday seeds and hydroxy MUFAs with a chain length of 16 or 18 acted as PPAR agonists. Hydroxylation of MUFAs may change these compounds from silent PPAR ligands to active PPAR agonists.

Crystal structure of Spot 14, a modulator of fatty acid synthesis

Spot 14 (S14) is a protein that is abundantly expressed in lipogenic tissues and is regulated in a manner similar to other enzymes involved in fatty acid synthesis. Deletion of S14 in mice decreased lipid synthesis in lactating mammary tissue, but the mechanism of S14's action is unknown. Here we present the crystal structure of S14 to 2.65 Å and biochemical data showing that S14 can form heterodimers with MIG12. MIG12 modulates fatty acid synthesis by inducing the polymerization and activity of acetyl-CoA carboxylase, the first committed enzymatic reaction in the fatty acid synthesis pathway. Coexpression of S14 and MIG12 leads to heterodimers and reduced acetyl-CoA carboxylase polymerization and activity. The structure of S14 suggests a mechanism whereby heterodimer formation with MIG12 attenuates the ability of MIG12 to activate ACC.

The PGC-1 cascade as a therapeutic target for heart failure

The PPARγ coactivator-1 (PGC-1) family of transcriptional coactivators, together with estrogen related receptors (ERRs), plays a key role in regulating genes involved in myocardial fuel metabolism and cardiac function. Increasing evidence implicates dysregulation of this transcriptional regulatory circuit in the metabolic and functional disturbances that presage heart failure due to common diseases such as hypertension and diabetes. Accordingly, the PGC-1/ERR axis is a plausible candidate therapeutic target for novel therapeutics aimed at reversing the energy metabolic disturbances that contribute to heart failure. This review describes the biologic actions of the PGC-1 and ERR cascade and summarizes the evidence that dysregulation of this transcriptional regulatory circuit contributes to heart failure. Potential strategies to modulate this target pathway are reviewed. This article is part of a special issue entitled "Key Signaling Molecules in Hypertrophy and Heart Failure."

Induced polymerization of mammalian acetyl-CoA carboxylase by MIG12 provides a tertiary level of regulation of fatty acid synthesis

Acetyl-CoA carboxylase (ACC), the first committed enzyme in fatty acid (FA) synthesis, is regulated by phosphorylation/dephosphorylation, transcription, and an unusual mechanism of protein polymerization. Polymerization of ACC increases enzymatic activity and is induced in vitro by supraphysiological concentrations of citrate (> 5 mM). Here, we show that MIG12, a 22 kDa cytosolic protein of previously unknown function, binds to ACC and lowers the threshold for citrate activation into the physiological range (< 1 mM). In vitro, recombinant MIG12 induced polymerization of ACC (as determined by nondenaturing gels, FPLC, and electron microscopy) and increased ACC activity by > 50-fold in the presence of 1 mM citrate. In vivo, overexpression of MIG12 in liver induced ACC polymerization, increased FA synthesis, and produced triglyceride accumulation and fatty liver. Thus, in addition to its regulation by phosphorylation and transcription, ACC is regulated at a tertiary level by MIG12, which facilitates ACC polymerization and enhances enzymatic activity.

Shedding light on the enigma of myocardial lipotoxicity: the involvement of known and putative regulators of fatty acid storage and mobilization

Excessive fatty acid (FA) uptake by cardiac myocytes is often associated with adverse changes in cardiac function. This is especially evident in diabetic individuals, where increased intramyocardial triacylglycerol (TG) resulting from the exposure to high levels of circulating FA has been proposed to be a major contributor to diabetic cardiomyopathy. At present, our knowledge of how the heart regulates FA storage in TG and the hydrolysis of this TG is limited. This review concentrates on what is known about TG turnover within the heart and how this is likely to be regulated by extrapolating results from other tissues. We also assess the evidence as to whether increased TG accumulation protects against FA-induced lipotoxicity through limiting the accumulations of ceramides and diacylglycerols versus whether it is a maladaptive response that contributes to cardiac dysfunction.

Tracing cardiac metabolism in vivo: one substrate at a time

In the myocardial cell, a series of enzyme-catalyzed reactions results in the efficient transfer of chemical energy into mechanical energy. The goals of this article are to emphasize the ability of noninvasive imaging techniques using isotopic tracers to detect the metabolic footprints of heart disease and to propose that cardiac metabolic imaging is more than a useful adjunct to current myocardial perfusion imaging studies. A strength of metabolic imaging is in the assessment of regional myocardial differences in metabolic activity, probing for 1 substrate at a time. We hope that new and developing methods of cardiac imaging will lead to the earlier detection of heart disease and improve the management and quality of life for patients afflicted with ischemic and nonischemic heart muscle disorders.

Deficiency of phosphoinositide 3-kinase enhancer protects mice from diet-induced obesity and insulin resistance

OBJECTIVE

Phosphoinositide 3-kinase enhancer A (PIKE-A) is a proto-oncogene that promotes tumor growth and transformation by enhancing Akt activity. However, the physiological functions of PIKE-A in peripheral tissues are unknown. Here, we describe the effect of PIKE deletion in mice and explore the role of PIKE-A in obesity development.

RESEARCH DESIGN AND METHODS

Whole-body PIKE knockout mice were generated and subjected to high-fat-diet feeding for 20 weeks. The glucose tolerance, tissue-specific insulin sensitivity, adipocyte differentiation, and lipid oxidation status were determined. The molecular mechanism of PIKE in the insulin signaling pathway was also studied.

RESULTS

We show that PIKE-A regulates obesity development by modulating AMP-activated protein kinase (AMPK) phosphorylation. PIKE-A is important for insulin to suppress AMPK phosphorylation. The expression of PIKE-A is markedly increased in adipose tissue of obese mice, whereas depletion of PIKE-A inhibits adipocyte differentiation. PIKE knockout mice exhibit a prominent phenotype of lipoatrophy and are resistant to high-fat diet-induced obesity, liver steatosis, and diabetes. PIKE knockout mice also have augmented lipid oxidation, which is accompanied by enhanced AMPK phosphorylation in both muscle and adipose tissue. Moreover, insulin sensitivity is improved in PIKE-A-deficient muscle and fat, thus protecting the animals from diet-induced diabetes.

CONCLUSIONS

Our results suggest that PIKE-A is implicated in obesity and associated diabetes development by negatively regulating AMPK activity.

Triacylglycerol homeostasis: insights from yeast

The endemic increase in lipid-associated disorders such as obesity and type 2 diabetes mellitus has placed triacylglycerol metabolism and its associated organelle, lipid droplets, in the spotlight of biomedical research. Key enzymes of triacylglycerol metabolism are structurally and functionally conserved between yeast and mammalian cells, and studies in yeast have contributed significantly to the understanding of their biological function(s). Based on these similarities, studies performed in yeast may provide further significant mechanistic insight into the molecular basis of triacylglycerol homeostasis and its important physiological roles in healthy and diseased cells.

Blockage of ceramide metabolism exacerbates palmitate inhibition of pro-insulin gene expression in pancreatic beta-cells

Chronic exposure to elevated levels of fatty acids (FAs) in conjunction with chronic hyperglycemia has been reported to contribute to the progressive deterioration of beta-cell function in patients with type 2 diabetes mellitus. The long-chain saturated free fatty acid (FFA) palmitate, unlike the unsaturated FFA oleate, is known to have an inhibitory effect on proinsulin gene expression through ceramide synthesis. This study was aimed at investigating whether this effect was exacerbated by the inhibition of ceramide degradation in pancreatic beta-cells and the molecular mechanism of intracellular ceramide-induced inhibition of proinsulin gene transcription in response to exposure to palmitate. We exposed insulin-secreting (INS-1) cells treated with low levels of palmitate to the ceramidase inhibitor n-oleoylethanolamine (NOE); this led to the generation of high levels of intracellular ceramide. We observed that the effects of ceramide accumulation in INS-1 cells were similar to the effects of the inhibition of this protein on proinsulin mRNA levels that are caused by the negative regulation of insulin promoter activity. In addition, we observed that ceramide accumulation induced by NOE leads to a significant decrease in the levels of activated extracellular signal-regulated kinase (ERK); the inactivation of the ERK cascade in response to palmitate stimuli is induced by protein phosphatase 2A (PP2A) activity. Based on these findings, we suggest that the aberrant accumulation of ceramide was caused by the inhibition of ceramide metabolism, which in turn leads to the inhibition of proinsulin gene expression; the inhibition of ERK cascades by PP2A serves as an important factor in the inhibitory effects of ceramide.

Reducing endoplasmic reticulum stress through a macrophage lipid chaperone alleviates atherosclerosis

Macrophages exhibit endoplasmic reticulum (ER) stress when exposed to lipotoxic signals associated with atherosclerosis, although the pathophysiological significance and the underlying mechanisms remain unknown. Here, we demonstrate that mitigation of ER stress with a chemical chaperone results in marked protection against lipotoxic death in macrophages and prevents macrophage fatty acid binding protein-4 (aP2) expression. Utilizing genetic and chemical models, we show that aP2 is the predominant regulator of lipid-induced macrophage ER stress. Lipid chaperone effects are mediated by the production of phospholipids rich in monounsaturated fatty acids and bioactive lipids that render macrophages resistant to lipid-induced ER stress. Furthermore, aP2’s impact on macrophage lipid metabolism and ER stress response is mediated by upregulation of key lipogenic enzymes by the liver X receptor. Our results demonstrate the central role for lipid chaperones in regulating ER homeostasis in macrophages in atherosclerosis and that ER responses can be modified, genetically or chemically, to protect the organism against the deleterious effects of hyperlipidemia.

Fine-tuning the lipogenic/lipolytic balance to optimize the metabolic requirements of cancer cell growth: molecular mechanisms and therapeutic perspectives

Evolving evidence suggest that metabolic requirements for cell proliferation are identical in all normal and cancer cells. HER2 oncogene-overexpressors, a highly aggressive subtype of human cancer cells, constitute one of the best examples of how malignant cells maximize their ability to acquire and metabolize nutrients in a manner conductive to proliferation rather than efficient ATP production. HER2-overexpressors optimize their requirements of rapid cancer cell growth by fine-tuning a double [lipogenic/lipolytic]-edged metabolic sword. On the one edge, HER2 oncogene overexpression triggers redundant signaling cascades to ensure that all the major enzymes involved in de novo fatty acid (FA) synthesis will facilitate aerobic glycolysis instead of oxidative phosphorylation for energy production (Warburg effect). HER2 also establishes a positive bidirectional relationship with the key lipogenic enzyme Fatty Acid Synthase (FASN) that rapidly senses and respond to any disturbance in the flux of lipogenic substrates (e.g. NADPH and acetyl-CoA) and lipogenesis end-products (i.e. palmitate). On the other edge, HER2 overexpression arranges detoxifying mechanisms by upregulating PPARgamma, a well established positive regulator role of adipogenesis and lipid storage in cell types with active lipid metabolism. PPARgamma establishes a lipogenesis/lipolysis joining-point that enables HER2-positive cancer cells to avoid endogenous palmitate toxicity while securing palmitate into fat stores to avoid palmitate feedback on FASN functioning. The ability of HER2 to supercharge lipogenesis (by activating regulatory circuits that activate and fuel the lipogenic enzyme FASN) while averting lipotoxicity (by promoting conversion and storage of excess FAs to triglycerides in a PPARgamma-dependent manner) supports the notion that best adapted cancer phenotypes are addicted to oncogenic lipid metabolism for cell proliferation and survival. It is conceptually attractive to assume that we can crash HER2-driven rapid cell proliferation by inhibiting "motor refueling" (upon blockade of lipogenic enzymes), by losing the "lipolytic brake" (upon blockade of PPARgamma) and/or by sticking the "lipogenic gas pedal" (upon supplementation with dietary FAs).