Lipoprotein lipase activities in adipose tissues and muscle in the obese Zucker rat

Is insulin resistance tissue-dependent and substrate-specific? The role of white adipose tissue and skeletal muscle

Insulin resistance (IR) refers to a reduction in the ability of insulin to exert its metabolic effects in organs such as adipose tissue (AT) and skeletal muscle (SM), leading to chronic diseases such as type 2 diabetes, hepatic steatosis, and cardiovascular diseases. Obesity is the main cause of IR, however not all subjects with obesity develop clinical insulin resistance, and not all clinically insulin-resistant people have obesity. Recent evidence implies that IR onset is tissue-dependent (AT or SM) and/or substrate-specific (glucometabolic or lipometabolic). Therefore, the aims of the present review are 1) to describe the glucometabolic and lipometabolic activities of insulin in AT and SM in the maintenance of whole-body metabolic homeostasis, 2) to discuss the pathophysiology of substrate-specific IR in AT and SM, and 3) to highlight novel validated tests to assess tissue and substrate-specific IR that are easy to perform in clinical practice. In AT, glucometabolic IR reduces glucose availability for glycerol and fatty acid synthesis, thus decreasing the esterification and synthesis of signaling bioactive lipids. Lipometabolic IR in AT impairs the antilipolytic effect of insulin and lipogenesis, leading to an increase in circulating FFAs and generating lipotoxicity in peripheral tissues. In SM, glucometabolic IR reduces glucose uptake, whereas lipometabolic IR impairs mitochondrial lipid oxidation, increasing oxidative stress and inflammation, all of which lead to metabolic inflexibility. Understanding tissue-dependent and substrate-specific IR is of paramount importance for early detection before clinical manifestations and for the development of more specific treatments or direct interventions to prevent chronic life-threatening diseases.

Hypericum perforatum L.: a medicinal plant with potential as a curative agent against obesity-associated complications

Obesity is a low-grade inflammatory disease that is getting increasingly common among adults and children and causes different complications. Insulin resistance, Type II diabetes, atherosclerosis, metabolic syndrome and hypertension are among the major health problems, that are associated with obesity. Some medications are used to treat obese individuals and metabolic surgery is recommended, if appropriate, for individuals with a BMI ≥ 40. Due to the fact that medications and metabolic surgery are not tolerated by all, researchers focus on alternative therapies. Medicinal plants comprise the most important group of these alternative treatments. Hypericum perforatum L. is the medicinal plant, which we focused on in this study. Hypericum perforatum L. has been recognized as a medicinally valuable plant for over 2000 years. It has been used for generations to treat anxiety, depression, insomnia, gastritis, hemorrhoids, wounds, and burns. Recent studies have indeed shown promising effects for the treatment of obesity. In this study, 3T3-L1 adipocytes were used to mimic the adipocyte differentiation associated with obesity in cellular terms. Lipoprotein lipase (Lpl), Diacylglycerol-O-acyltransferase 1 (Dgat1), Fatty acid synthase (Fasn) markers were used to study the lipid accumulation, and Collagen V (ColV) was used to study cell elasticity to investigate the relationship of the effects of the administration of Hypericum perforatum L. with obesity.

[Role of white adipose tissue in vascular complications due to obesity]

The contribution of white adipose tissue to the vascular complications associated with obesity is analysed in this review. White adipose tissue is an active metabolic organ and secretor of several molecules with endocrine, paracrine and autocrine actions. Weight gain produced in the obesity, induces an excess of fat, mainly in the visceral depot, which is responsible for the activation of different signalling pathways, leading to a higher production of proinflammatory cytokines. As adipocytes as infiltrated macrophages and lymphocytes and endothelial cells contribute to a chronic low grade inflammatory situation present in obesity. Moreover, the increase in adiposity activates the inflammatory response in the adipocyte themselves, as well as in the hepatocyte. Finally, proinflammatory and proatherogenic mediators produced by white adipose tissue and liver associated to immune cells generate insulin resistance in peripheral tissues and contribute to the beginning of atherogenic process.

Lipoprotein lipase: from gene to obesity

Lipoprotein lipase (LPL) is a multifunctional enzyme produced by many tissues, including adipose tissue, cardiac and skeletal muscle, islets, and macrophages. LPL is the rate-limiting enzyme for the hydrolysis of the triglyceride (TG) core of circulating TG-rich lipoproteins, chylomicrons, and very low-density lipoproteins (VLDL). LPL-catalyzed reaction products, fatty acids, and monoacylglycerol are in part taken up by the tissues locally and processed differentially; e.g., they are stored as neutral lipids in adipose tissue, oxidized, or stored in skeletal and cardiac muscle or as cholesteryl ester and TG in macrophages. LPL is regulated at transcriptional, posttranscriptional, and posttranslational levels in a tissue-specific manner. Nutrient states and hormonal levels all have divergent effects on the regulation of LPL, and a variety of proteins that interact with LPL to regulate its tissue-specific activity have also been identified. To examine this divergent regulation further, transgenic and knockout murine models of tissue-specific LPL expression have been developed. Mice with overexpression of LPL in skeletal muscle accumulate TG in muscle, develop insulin resistance, are protected from excessive weight gain, and increase their metabolic rate in the cold. Mice with LPL deletion in skeletal muscle have reduced TG accumulation and increased insulin action on glucose transport in muscle. Ultimately, this leads to increased lipid partitioning to other tissues, insulin resistance, and obesity. Mice with LPL deletion in the heart develop hypertriglyceridemia and cardiac dysfunction. The fact that the heart depends increasingly on glucose implies that free fatty acids are not a sufficient fuel for optimal cardiac function. Overall, LPL is a fascinating enzyme that contributes in a pronounced way to normal lipoprotein metabolism, tissue-specific substrate delivery and utilization, and the many aspects of obesity and other metabolic disorders that relate to energy balance, insulin action, and body weight regulation.

Hepatic and adipose tissue depot-specific changes in lipid metabolism in Late-onset Obese (LOB) rats

Transgenic Late-onset OBesity (LOB) rats slowly develop a male-specific, autosomal dominant, obesity phenotype with a specific increase in peri-renal white adipose tissue (WAT) depot and preserved insulin sensitivity (Bains et al. in Endocrinology 145:2666-2679, 2004). To better understand the remarkable phenotype of these rats, the lipid metabolism was investigated in male LOB and non-transgenic (NT) littermates. Total plasma cholesterol (C) levels were normal but total plasma triacylglycerol (TAG) (2.8-fold) and hepatic TAG content (25%) was elevated in LOB males. Plasma VLDL-C and VLDL-TAG levels were higher while plasma apoB levels were 60% lower in LOB males. Increased hepatic TAG secretion explained the increased VLDL levels in LOB males. The hepatic gene expression of FAS, SCD-1, mitochondrial (mt)GPAT, and DGAT2 was up-regulated in both old obese and young non-obese LOB rats. Lipoprotein lipase (LPL) activity in heart and epididymal white adipose tissue (WAT) was unchanged, while LPL activity was increased in peri-renal WAT (30%) and decreased in soleus muscle (40%). Moreover, FAS, SCD-1 and DGAT2 gene expression was increased in peri-renal, but not in epididymal WAT. Basal lipolysis was reduced or unchanged and beta-adrenergic stimulated lipolysis was reduced in WAT from both old obese and young non-obese LOB rats. To summarize, the obese phenotype of LOB male rats is associated with increased hepatic TAG production and secretion, a shift in LPL activity from skeletal muscle to WAT, reduced lipolytic response in WAT depots and a specific increase in expression of genes responsible for fatty acid and TAG synthesis in the peri-renal depot.

Lipid metabolism in the obese Zucker rat. Disposal of an oral [14C]triolein load and lipoprotein lipase activity

Oxidation in vivo of [14C]triolein to 14CO2 was significantly lower in obese (fa/fa) Zucker rats as compared with their lean (+/?) controls. In response to a 24 h starvation period, both lean and obese rats showed an enhanced rate of [14C]triolein oxidation. There were, however, no changes in the rate of intestinal absorption of [14C]triolein between the lean and obese animals. Conversely, the total tissular [14C]lipid accumulation was significantly higher in white adipose tissue, carcass and plasma in the obese animals, whereas that of brown adipose tissue was lower. This was associated with a marked hyperinsulinaemia and hypertriglyceridaemia in the fa/fa animals. Starvation dramatically decreased [14C]lipid accumulation in white adipose tissue of the lean Zucker rats, but had no effect in the obese rats. The lipogenic rate of the obese rats was significantly higher than that of lean rats in liver, white adipose tissue, skeletal muscle and carcass. Lipoprotein lipase activity (per g of tissue) was significantly lower in both white and brown adipose tissue of obese versus lean rats; however, total activity was higher in both tissues. Starvation significantly lowered perigenital-adipose-tissue lipoprotein lipase activity in the lean groups, and had no effect in the obese ones. These results demonstrate that the tissue capacity of exogenous lipid uptake is involved, but cannot be the only factor influencing the maintenance of obesity in these animals. Thus, in the adult fa/fa rat, the large increase in obesity is not solely dependent on a deviation of energy-producing substrate metabolism towards the storage of lipids in white fat. Other factors, such as a low rate of oxidation, a high lipogenic rate and decreased brown-adipose-tissue activity are involved in the perseverance of the obesity syndrome.