Insulin resistance and cardiovascular disease (original) (raw)
The scheme shown in Figure 1 offers a simple (even simplistic) view of lipid and lipoprotein metabolism that may be useful for the purposes of this Perspective. Insulin resistance at the level of the fat cell is shown as the initiating insult, leading to increased intracellular hydrolysis of triglycerides (TGs) and release of fatty acids into the circulation. It seems likely that insulin resistance in adipose tissue results from the interaction of multiple defective or less than optimally functioning genes with an environment that leads to increased pressure on the fat cell to store energy (see Kahn and Flier, this Perspective series, ref. 7). This interaction results in a fat cell that cannot meet the demands placed upon it. Whatever the molecular or environmental basis for insulin resistance in adipose tissue, the result is that FFA uptake by fat cells is decreased and/or FFA release from fat cells is increased, confronting the liver with increased availability of energy. Indeed, the hypertriglyceridemia of insulin resistance can be viewed as a Ping-Pong match between the hepatocytes and fat cells, where VLDLs and FFAs are the Ping-Pong balls carrying energy back and forth between the liver and the adipose tissue. The human disorders of lipodystrophy, characterized by marked insulin resistance and hypertriglyceridemia due to overproduction of VLDL, can be viewed as the ultimate loss of the sites for energy storage. Indeed, the hypertriglyceridemia in a lipodystrophic mouse model that has a genetic deficiency of adipose tissue was partially reversed by surgical implantation of normal adipose tissue (8). These findings suggest that the relationship between insulin resistance/hypertriglyceridemia and obesity is a parabolic one: Both the absence of fat and the abundance of fat are associated with increased fatty acid flux to the liver and subsequent increased secretion of VLDL.
A simplified model relating insulin resistance to dyslipidemia and cardiovascular disease. Insulin resistance at the adipocyte results in increased release of fatty acids into the circulation. A similar accumulation of fatty acids could arise from defects in fatty acid transporters or intracellular binding proteins. Increased FFA flux to the liver stimulates the assembly and secretion of VLDL resulting in hypertriglyceridemia. In addition, VLDL stimulates the exchange of cholesteryl esters from both HDL and LDL for VLDL TG. ApoA-I can dissociate from TG-enriched HDL. This free apoA-I is cleared rapidly from plasma, in part by excretion through the kidney, thus reducing the availability of HDL for reverse cholesterol transport. TG-enriched LDL can undergo lipolysis and become smaller and more dense. Low levels of HDL and the presence of small dense LDL are each independent risk factors for cardiovascular disease. IR, insulin resistance; CE, cholesteryl ester; SD, small dense.
Candidate genes that could underlie the increased flow of fatty acids include regulators of fatty acid uptake and storage by adipocytes, such as those for hormone-sensitive lipase (HSL) (9), lipoprotein lipase (LPL) (10), and complement component C3a (whose proteolytic product is the acylation-stimulating protein [ASP] [ref. 11]), as well as for various fatty acid transporters and binding proteins. Although some investigators have suggested links between HSL, LPL, and ASP, and either the insulin resistance/obesity syndrome or combined hyperlipidemia (which some believe is also the result of insulin resistance), no firm evidence yet exists linking these important proteins to the insulin resistance syndrome. Indeed, separate studies of a mouse in which the gene for ASP was deleted have resulted in contradictory results regarding its role in lipid metabolism (11, 12). On the other hand, recent data from mice carrying null mutations in the gene for the fatty acid transporter CD36 are quite revealing as to the consequences of the inability of fat cells to store fatty acids as TG. These animals had increased levels of plasma FFAs and hypertriglyceridemia, indicative of the response of the liver to an increased flux of energy from the periphery (13). Of interest, CD36 knock-out mice, which lack the fatty acid transporter in both adipose and muscle tissue, are not insulin-resistant. Indeed, they seem to be more insulin-sensitive, possibly as a result of diminished uptake of fatty acids by muscle. The dissociation of insulin resistance and TG metabolism in this model suggests a direct and critical role of increased plasma fatty acid flux to the liver in the genesis of hypertriglyceridemia. This conclusion is supported by work with transgenic mice overexpressing CD36 in muscle (14): these animals have low plasma levels of both fatty acids and TG but are insulin-resistant. These findings in gene-modified mouse models were paralleled by the observation that a strain of the spontaneously hypertensive rat (SHR) had mutations in CD36 that appeared to be associated with insulin resistance (15, 16). However, a very recent report indicated that the original SHR line, which is insulin-resistant, has no defects in its CD36 gene (17). Further developments in this area of investigation are eagerly awaited.
In summary, the inability of insulin-resistant fat cells to store TG is very likely the initial step in the development of the dyslipidemia characteristic of insulin resistance. Importantly, this link between the fat cell and hepatic production of VLDL does not seem to be necessarily linked, in mouse models, to insulin resistance at the level of muscle. No candidate genes have been linked unequivocally to either the dyslipidemia or the insulin resistance in humans. Indeed, although the CD36 gene appears to be an important candidate, the data from the CD36 knock-out mice indicate to me that any CD36 defect would have to be specific to adipose tissue to cause both hypertriglyceridemia and insulin resistance.
Fatty acid flux and plasma VLDL TG levels. A large body of evidence has accumulated linking FFA availability to the assembly and secretion of VLDL by the liver (18). ApoB synthesis is constitutive under most physiologic conditions, with the assembly and secretion of apoBs regulated at the posttranslational stage by the availability of adequate core lipids (TG and cholesteryl esters) and the presence of microsomal triglyceride transfer protein (MTP) activity. In the absence of core lipids and/or MTP activity, newly synthesized apoB is inefficiently translocated across the endoplasmic reticulum membrane and undergoes cotranslational and early posttranslational ubiquitination and degradation by the proteasome (19). This regulation, which appears to derive from the complex physical structure of apoB, particularly its long, highly hydrophobic β-sheet domains, provides a means whereby the liver can respond to the stress of increased lipid synthesis by immediately incorporating that lipid into VLDL and secreting it. In the absence of such need, the very large carrier protein, apoB, can be degraded as it is synthesized. Consistent with this model, increasing FFA flux has been shown to stimulate secretion of VLDL in humans (20).
The role of hyperinsulinemia in the assembly and secretion of VLDL, independent of its function as a marker of insulin resistance, has been controversial. At the cellular level, based on in vitro studies, short-term increases in insulin levels are associated with both increased TG synthesis and inhibition of apoB secretion. The inhibition of apoB secretion apparently occurs by a posttranslational degradative pathway; the site of this degradation remains to be determined. Longer incubations of cells with high concentrations of insulin, on the other hand, may increase VLDL secretion, perhaps because an insulin-resistant state develops in the cells after chronic treatment. In vivo studies in normal humans, using short-term hyperinsulinemic clamps, have supported an acute inhibitory function for hyperinsulinemia (21, 22). Importantly, obese subjects and subjects with type 2 diabetes appear to be resistant to these effects of insulin (23).
An interesting twist on the role of insulin in regulating plasma TG levels arose from studies of the apoCIII gene. ApoCIII plays an important role in regulating lipolysis of VLDL by LPL, and apoCIII-overexpressing transgenic mice have severe hypertriglyceridemia. Studies demonstrating the presence of an insulin response element (IRE) in the apoCIII promoter, and of suppressed gene expression by insulin, suggested that a specific type of insulin resistance might exist in hypertriglyceridemic individuals found to have sequence polymorphisms in this IRE (24). However, that association has not been found to be independent of the association of the SST polymorphism in the 3′-untranslated region of apoCIII, and individuals with the SST polymorphism have higher TG levels irrespective of the status of their apoCIII IREs.
Cardiovascular disease and the dyslipidemia of insulin resistance. The available in vitro and in vivo data suggest that, as long as adequate insulin is available to prevent excessive lipid oxidation, FFA-driven lipid synthesis is the major determinant of VLDL TG secretion. Hyperinsulinemia probably is a marker of insulin resistance, rather than a major, direct contributor to the process. The basis for the rise in FFA flux in insulin resistance is probably oligogenic in nature, but whatever its ultimate cause, the rest of the dyslipidemic phenotype associated with insulin resistance follows once VLDL secretion increases (Figure 1). Hypertriglyceridemia, leading to low HDL cholesterol and increased small dense LDL particles, is due mainly to the actions of cholesteryl ester transfer protein (CETP) (25). In plasma, collisions between VLDL and HDL, in the presence of CETP, stimulate the transfer of VLDL TG to HDL in exchange for HDL cholesteryl esters. The resulting TG-enriched HDL becomes a good substrate for hepatic lipase (and possibly LPL), and the TG is hydrolyzed. This, in turn, generates a smaller HDL that must shed some of its surface, including apoA-I. ApoA-I is a small protein that can be filtered by the kidney and then degraded by renal tubular cells (26). In a similar fashion, intravascular collisions between VLDL and LDL allow for CETP-mediated exchange of VLDL TG for LDL cholesteryl esters. The succeeding hydrolysis of LDL TG generates small dense LDL particles.
Since both normoglycemic insulin-resistant individuals and patients with type 2 diabetes mellitus do not have higher LDL cholesterol levels than the general population, how does the resulting dyslipidemia increase the risk of insulin-resistant individuals to cardiovascular disease? I suggest that multiple aspects of their lipid profiles are atherogenic. First, not only are there increased levels of VLDL particles, which can enter the vessel wall and accumulate in atherosclerotic plaques (27, 28), but these VLDL are, by virtue of receiving CETP-transferred cholesteryl esters, able to deliver more cholesterol per particle to the vessel wall. Additionally, increased VLDL secretion can contribute to postprandial hyperlipidemia by providing competition for chylomicron clearance pathways; postprandial hyperlipidemia is independently associated with CAD (29). Second, reduced HDL cholesterol and apoA-I levels mean that there are fewer HDL particles engaged in cholesterol efflux from peripheral tissues, which is the first step in reverse cholesterol transport. Fewer HDL particles also mean that HDL cannot fulfill several proposed direct antiatherogenic actions at the vessel wall, including the role of HDL as an antioxidant. Krieger and colleagues recently identified scavenger receptor B1 (SRB1) (30), which appears to mediate the selective delivery of HDL cholesteryl esters to the liver (delivery of core lipid by the HDL particle without endocytosis and degradation of the whole particle), targeting that cholesterol for excretion via the biliary pathway. CETP-mediated transfer of HDL cholesteryl esters to VLDL not only may enrich an atherogenic lipoprotein with cholesterol but also can divert that cholesterol from the specific reverse cholesterol transport pathway. Theoretically, the reduced HDL cholesterol characteristic of insulin resistance could be the result of increased SRB1 expression; this is unlikely, however, because such an increase would likely reduce the risk of atherosclerosis (31). Finally, small dense LDL, first identified by Sniderman and colleagues (32) and then studied in depth by Krauss and Austin and their colleagues (33), may be more atherogenic than an equal number of larger more cholesteryl ester–rich LDL, because small dense LDL may be more liable to oxidation or may more readily penetrate and stick to the ECM of the artery wall.