Metabolic syndrome pathophysiology: the role of adipose tissue

Abstract

The metabolic syndrome comprises a set of metabolic and physiological risk factors associated with elevated cardiovascular disease risk. The expression of each one of its major factors (hypertriglyceridemia, low high-density lipoprotein cholesterol levels, hypertension, abdominal obesity, and insulin resistance) has been found to be the result of complex interactions between genetic and environmental factors. Moreover, one of them, obesity, may play a major role in triggering the metabolic syndrome by interacting with genetic variants at candidate genes for dyslipidemia, hypertension, and insulin resistance. In support of this hypothesis, several studies at several candidate genes, mainly adipokines and perilipin, have already demonstrated the significance of these interactions; however, the information and its solidity are still very limited and in many cases, replication studies are still lacking in the literature. Therefore, more studies with better epidemiological design and standardized adiposity measures are needed to estimate the contribution of body weight and fat distribution to the genetic predisposition to the metabolic syndrome, the most common CVD risk factor in industrialized societies.

Cardiovascular diseases (CVD) are the result of complex interactions between both environmental and genetic factors.¹ Unlike the rare and severe genetic defects that cause monogenic diseases, the genetic factors that modulate the individual susceptibility to CVD in the general population are most probably common polymorphisms having modest effects at the individual level, but, because of their high allele frequencies, these polymorphisms may have associated with a significant population-attributable risk. For over two decades, we have been using the candidate gene approach for identifying genes contributing to CVD. More recently, genome-wide association (GWA) studies have opened the doors to the discovery of new genes involved in CVD risk and related risk factors without prior knowledge of their functions.² The goal behind this effort is the identification of genes and their variants involved in the multiple pathophysiological pathways leading to CVD. By doing this, we should be able to increase our understanding of the mechanisms of the disease.³ Moreover, this knowledge should give us the tools to identify individual susceptibilities and specific therapeutic interventions targeted to more personalized prevention and clinical management.⁴

A key element for our progress will be provided by genetic and molecular epidemiology involving large-scale population studies requiring close integration of genetics with more traditional epidemiological research. This is essential for a disease in which environmental factors mediate the phenotypic expression of the susceptibility genes. In fact, most of the susceptibility genes for common diseases in general and CVD in particular do not have a primary etiological role in the development of the disease, but rather act as response modifiers to exogenous factors such as stress, environment, disease, and drug intake. Therefore, a better characterization of the interactions between environmental and genetic factors constitutes a key issue in the understanding of the pathogenesis of CVD and our ability to use the knowledge on its prevention and therapy.

THE METABOLIC SYNDROME: COMPONENTS, PREVALENCE, AND THERAPIES

Hypertension, hyperlipidemia, impaired glucose tolerance, and obesity are established traditional CVD risk factors. When these risk factors cluster in one individual, CVD risk increases dramatically. This clustering of risk factors is, in fact, not a rare event but the most common cause of CVD in modern society. This combined phenotype has been known since the late 1980s as the ‘metabolic syndrome’.⁵

The precise definition of the metabolic syndrome has shifted slightly from time to time, and there have been a number of attempts to develop standardized criteria for its diagnosis. Although estimates of prevalence in different populations are highly dependent on the definition of the metabolic syndrome, the reality is that the current estimates are appalling and that the future perspective is even more alarming. Thus, age-adjusted estimates from the National Health and Nutrition Examination Survey III from 1988 to 1994 revealed that 24% of adult Americans (aged 20 years or older) had this syndrome.⁶ The prevalence of this syndrome clearly increases with age (from 6.7% among the National Health and Nutrition Examination Survey III participants aged 20–29 years to 43.0% for participants aged 60 years or older), being a crucial phenotypic trait in modulating genetic factors. Although gender has been considered as another important trait that modulates gene expression and then genetic susceptibility to this syndrome,⁷ the global prevalence of the metabolic syndrome among adult Americans differed little in men (24.0%) and women (23.4%). However, substantial differences of gender prevalence, depending on the ethnic group, were observed. Moreover, the overall prevalence of this syndrome was highest among Mexican Americans (31.9%) and lowest in African Americans (21.6%).⁶ It has been pointed out that differences in prevalence by gender or by ethnic group may be largely attributed to the definition used to diagnose the syndrome and that central obesity is the key factor. Limited data exist on the syndrome’s association with CVD morbi-mortality. It has been estimated that in the National Health and Nutrition Examination Survey III,⁸ the metabolic syndrome was associated with a higher risk of nonfatal myocardial infarction (OR, 2.01; 95% CI, 1.53–2.64) and stroke (OR, 2.16; 95% CI, 1.48–3.16). Prospectively, Lakka et al.⁹ in Finland reported a higher risk of coronary mortality associated with the metabolic syndrome (hazard ratio of 4.16; 95% CI, 1.60–10.8). Therefore, a major effort should be placed on its detection, prevention, and therapy. In terms of the treatment, we have the therapeutic tools to successfully deal with some of the individual components. Thus, we have efficient drugs to lower blood pressure; similarly, several drugs are being used to improve insulin sensitivity, and dyslipidemia can be treated with fibrates and even with statins. However, such therapeutic success has not been shared by the other major component of the metabolic syndrome, obesity and, more specifically, central obesity, which may be a key etiological factor in de-development of the underlying insulin resistance, and it may be the ‘trigger for the loaded gun’ of its genetic predisposition. Therefore, obesity may be at the root of the metabolic syndrome with the aggravated situation of being an unresolved and fast-growing problem all over the world. We have recently reviewed the current evidence supporting that many of the common genetic variants found in candidate genes for each of the individual components of the metabolic syndrome (hypertension, insulin resistance/diabetes, and dyslipidemia) are associated with higher risk phenotypes, and thus with increased disease risk, primarily when overweight and/or obesity is concurrently present.¹⁰

THE ADIPOCYTE AND THE METABOLIC SYNDROME

Adipocytes allow surplus fuel to be stored as triacylglycerol (TAG) during caloric abundance for retrieval during periods of food shortage and calorie debt (e.g., fasting, starvation, long-term exercise). Nonesterified fatty acids appearing as a result of lipolysis of TAG stores are released into circulation and oxidized mainly in skeletal muscle to provide energy. Through its TAG-storing capacity, involving a balanced lipogenic/lipolytic drive, the adipocyte could limit an abnormal increase in plasma nonesterified fatty acids, which are widely accepted as an important etiologic factor in the initiation of insulin resistance and metabolic syndrome in the obese.¹¹

Our current thought regarding the adipocyte has changed dramatically in the last few years. It is no longer viewed as a passive energy storage tissue; instead, it has been recognized to produce a number of metabolically and hormonally active substances, collectively called adipokines or adipocytokines.^12,13

These adipokines allow the adipocyte to initiate potent feedback actions in the regulation of appetite, food intake, glucose disposal, and energy expenditure. They are able to protect against the establishment of insulin resistance by actions on liver, skeletal muscle, and pancreatic function. They also contribute to the prevention or worsening of atherogenic processes. In addition, some of the factors secreted by the adipocyte exert local autocrine and paracrine actions affecting mainly adipose tissue remodeling, adipogenesis, and angiogenesis and are not found in the blood compartment.

With this recent emphasis on understanding the biological functions of the adipokines, it is important to keep in mind the inner workings of the fat cell and, more specifically, how lipid droplets are generated and maintained, which is fundamental to all the other subsequent processes.

In this regard, for several decades, there has been considerable research activity in the area of circulating neutral lipid bodies, that is, lipoproteins, with little attention devoted to intracellular neutral lipid depositions (i.e., adipocyte lipid droplets). There are several reasons for this. Plasma is readily accessible, and therefore its protein and lipid components can be characterized and measured relatively easily and their metabolic fates investigated. In contrast, as indicated earlier, the adipocyte had been considered as a mere storage of fat with few intrinsic metabolic properties. This has created a situation in which there was little knowledge about the biochemical properties and functions of the proteins present in intracellular lipid droplets. It is becoming increasingly evident that these proteins within the lipid droplets are important in lipid metabolism and energy balance.

PERILIPIN AS A CANDIDATE GENE IN HUMAN OBESITY AND OTHER METABOLIC SYNDROME-RELATED TRAITS

In the late 1980s, Londos’ laboratory first identified a heavily protein kinase A-phosphorylated protein that was named PLIN A, resulting from its physical location surrounding lipid droplets.¹⁴ Interestingly, PLIN was found only in adipocytes and in steroidogenic cells, which, similar to adipocytes, possess intracellular neutral lipid storage deposits that are metabolized by a common enzyme, termed hormone-sensitive lipase, in adipocytes and cholesterol esterase in steroidogenic cells. It was concluded at that time that PLIN A played a critical role in the hydrolysis of neutral lipids. In agreement with functional studies in cultured cells, data from animal models reported by two research groups in Plin^−/− mice, showing that the absence of perilipin resulted in leanness, increased basal lipolysis resistance to diet-induced obesity, and reversed obesity in Lepr(db/db) mice, strongly suggest that the PLIN gene may also have a major role in the etiology of obesity in humans.

The gene for human perilipin is on chromosome 15q26, in the region of a linkage locus for diabetes,¹⁵ hypertriglyceridemia,¹⁶ and obesity.¹⁷ It has been shown that the adipocyte perilipin content is reduced in obese women¹⁸ and that these women exhibited 2–4× higher basal and noradrenaline-induced rates of lipolysis in fat cells from the abdominal subcutaneous area. Moreover, adipocyte perilipin showed an inverse correlation with lipolysis rates and a positive correlation with plasma glycerol in these participants. This study also examined the effect of a polymorphism (11482G > A) in intron 6 of the PLIN locus in relation to perilipin content as well as the rates of lipolysis. Adipose tissue from women who were homozygous for the A allele at position 11482 exhibited 50–100% higher rates of lipolysis and 80% lower perilipin content. Interestingly, these adipocytes with lower perilipin content did not exhibit the blunted response to catecholamines observed in the perilipin-deficient mice.

This and other common polymorphisms at the PLIN locus have been examined for potential associations with body mass index (BMI) and/or obesity risk in population studies. Thus, we have examined four (6209T > C, 11482G > A, 13041A > G, and 14995A > T) single nucleotide polymorphisms (SNPs) in several geographically and ethnically diverse populations to determine the associations between these polymorphisms with obesity-related traits. In a random sample of Spanish participants (n = 1589 White participants),¹⁹ we found allele frequencies for the above listed SNPs ranging from 0.26 to 0.38. Two of these (6209T > C and 11482G > A) were in strong linkage disequilibrium, and the presence of one or both of these SNPs was associated with lower BMI and reduced risk of obesity only in women. In addition, the 11482G > A polymorphism also showed significant associations with fasting glucose and triglyceride concentrations. In contrast, the other two SNPs (13041A > G and 14995A > T) tended to be associated with increased BMI and risk of obesity. These polymorphisms were also examined in 734 White participants (373 men and 361 women) who were attending a residential lifestyle modification program in California²⁰ and, consistent with an earlier study, the 6209T > C and the 11482G > A polymorphisms were associated with lower BMI only in women. However, these associations did not reach statistical significance. Instead, the presence of the rare alleles at positions 13041 and 14995 was associated with increased BMI. These data suggest that the rare alleles at positions 6209 and 11482 at the 5′ end of the PLIN locus had opposite effects on BMI when compared with the minor alleles at positions 13041 and 14995, at the 3′ end of the PLIN locus.

These SNPs were also investigated in a multi-ethnic population living in Singapore comprising 2763 Chinese, 726 Malays, and 598 Asian Indians.²¹ One interesting finding was that the intragenic linkage disequilibrium structure of the PLIN locus differed between populations that were White, as compared with populations of Asian extraction. Specifically, in Whites, the 11482G > A polymorphism was in strong positive linkage disequilibrium with the 6209T > C polymorphism and negative linkage disequilibrium with the 13041A > G and 14995A > T polymorphisms.^22,23 In Chinese, Malays, and Asian Indians living in Singapore, the situation was reversed with the 11482G > A polymorphism in negative linkage disequilibrium with the 6209T > C polymorphisms and in positive linkage disequilibrium with the 13041A > G and the 14995A > T polymorphisms.²¹ Given the differences observed in the patterns of linkage disequilibrium, one would expect that the rare allele for the 11482G > A polymorphism, which was associated with lower levels of obesity in the white population, would be associated with increased levels of obesity in the Asian populations. In fact, this is exactly what was observed. Moreover, similar to the two earlier studies, significant associations were observed only in women. However, they were present only in Malays and Asian Indians, but not in Chinese. It is also worth noting that another study including 1120 individuals from France failed to find a statistically significant association between the 11482G > A polymorphism and anthropometric measures.²²

In summary, polymorphisms at the PLIN locus are associated with anthropometric measures and the risk of obesity in a gender-specific manner, in several ethnic groups from different studies including Whites (two studies),^19,20 Malays,²¹ and Asian Indians.²¹ However, the nature of the associations depends on the intragenic linkage disequilibrium structure of the PLIN locus in the various populations. In addition, in two other populations (French²² and Chinese^21,24), no statistically significant associations were observed between polymorphisms at the PLIN locus and obesity.

GENE–ENVIRONMENT INTERACTIONS AND THE PERILIPIN LOCUS

Complex traits such as obesity are a consequence of interactions between genetic and environmental factors, and such gene–environment interactions could explain the discrepant findings among populations. It could simply be that the French and Chinese are exposed to different environments when compared with Whites in Spain or California, and Malays and Asian Indians in Singapore. Do such gene–environment interactions operate in relation to the association between polymorphisms at the PLIN locus and obesity? In fact, the interaction between polymorphisms at this locus and gender has already been described in Whites and some of the Asian groups. Thus, we reported that individuals carrying the A allele for the 11482G > A polymorphism are resistant to weight loss with caloric restriction,²³ and another report showed resistance to weight gain following treatment with the peroxisome proliferator activator receptor-γ agonist rosiglitazone in individuals carrying this polymorphism.²⁵ Therefore, overweight and obese participants carrying this allele may not benefit from traditional dietary approaches to lose weight, and alternative approaches might be needed.

Up to this point, we have dealt primarily with associations between polymorphisms at the PLIN locus and their associations with the degree of obesity per se, as opposed to the downstream effects of obesity, such as insulin resistance. Several lines of evidence point to effects on chronic disease that may be independent of obesity. Although the perilipin knockout mouse is resistant to weight gain in the face of a high-fat diet, a high-fat diet still results in a decline in glucose tolerance.²⁶ Jang et al.²⁷ studied 177 obese Korean men and women who were treated with caloric restriction (−300 kcal per day) and found that the presence of the A11482 allele was associated with greater weight loss in response to caloric restriction and that this weight loss was primarily in the visceral fat compartment. As visceral obesity is generally associated with increased plasma-free fatty acid concentrations, one might expect a greater reduction in plasma-free fatty acids in these individuals. Instead, these individuals experienced an increase in plasma-free fatty acids despite a greater reduction in visceral fat mass. In addition, and given the relation between the postprandial plasma lipid and adipocyte metabolisms, we examined whether the presence of polymorphisms at the PLIN influenced postprandial lipoprotein metabolism in two white populations.²⁸ These included 88 healthy Spanish men and 271 healthy US participants (men and women) who underwent oral fat load tests in two independent studies. Our analyses showed that carriers of the minor C allele at the PLIN1 variant displayed lower postprandial concentrations of large triglyceride rich lipoproteins TAG than did participants carrying the T/T genotype. Moreover, in both populations, participants carrying the minor C and A alleles at PLIN1 and PLIN4, respectively, had significantly lower postprandial concentrations of plasma TAG and lower concentrations of small triglyceride rich lipoproteins TAG than did those who were homozygous for the major alleles at PLIN1 and PLIN4. Therefore, our data showed that the presence of the minor C and A alleles at PLIN1 and PLIN4, respectively, were associated with a lower postprandial response that may result in lower atherogenic risk.

It is important to remember that fatty acids serve not only as a source of energy but also as signaling molecules. Free fatty acids result in impaired glucose homeostasis and may increase the risk of type 2 diabetes mellitus.²⁹ To test the hypothesis that dietary fat may interact with polymorphisms at the PLIN locus to modulate diabetes-related traits, we reexamined data from the Singapore population, taking into consideration dietary macronutrient intake.³⁰ We found evidence of an interaction between dietary fat (specifically saturated fat) intake, polymorphisms at the PLIN locus (11482G > A and 114995A > T), and insulin resistance. Greater intake of energy from saturated fats was associated with increased insulin resistance among individuals who were homozygous for the rare alleles, but not individuals who carried the common allele. This effect was independent of BMI and consistent with the associations with anthropometric measures; these interactions were observed only in women.

In summary, there is strong evidence in support of the important role of the adipose tissue as a driver of the metabolic syndrome. Moreover, in addition to the better studied adipokines, other nonsecreted proteins in the adipocyte play an important role in the regulation of neutral lipid storage and their genetic variation may play a significant role in the risk of obesity, metabolic syndrome, and the response to dietary and pharmacological therapies.