Adiponectin
3. PLEIOTROPIC BIOLOGICAL FUNCTIONS OF ADIPONECTIN
Over the past several years, the functions of adiponectin have been extensively stud- ied in numerous animal models and in vitro systems. It is now appreciated that adiponectin is a multifunctional protein that regulates insulin sensitivity, energy homeo- stasis, vascular reactivity, inflammation, cell proliferation, and tissue remodeling. Thus Fig. 1. Structural features of adiponectin protein. Upper panel: the schematic diagram of the primary amino acid sequence of murine adiponectin. The numbers below correspond to the sites of hydroxy- lated prolines or lysines. Hydroxylated lysines are further glycosylated. SS, signal sequence; VR, hypervariable region. Lower panel: oligomeric complex formation of adiponectin. The disulfide bridge “S–S” is mediated by cysteine residue 39 at the hypervariable region.
far, the identified targets of adiponectin include liver, skeletal muscle, adipose tissue, heart, brain, pancreas, macrophages, and blood vessels (Fig. 2).
3.1. Adiponectin as Insulin-Sensitizing Hormone
The role of adiponectin as an important regulator of insulin sensitivity was first reported by Fruebis and colleagues in 2001 (16). The authors found that injection of a COOH-terminal globular adiponectin into mice acutely decreased postprandial blood glucose levels and enhance lipid clearance via increasing fatty acid G-oxidation in skele- tal muscles. This observation was subsequently confirmed and extended by several pharmacological studies using different forms of recombinant adiponectin. Yamauchi’s group demonstrated that chronic infusion of full-length or globular adiponectin pro- duced from E. coli significantly ameliorated insulin resistance and improved lipid profiles in both lipoatrophic diabetic mice and diet-induced obese mice (17). On the other hand, Berg’s group showed that intraperitoneal injection of full-length adiponectin expressed in mammalian cells triggered a significant and transient decrease in basal blood glucose levels by inhibiting the rates of endogenous glucose production in both wild type mice and several diabetic mouse models (18).
The chronic effects of adiponectin on insulin sensitivity and energy metabolism were also investigated in adiponectin transgenic mice or adiponectin knockout (KO) mice.
Scherer’s group generated a transgenic mouse model with approximately threefold elevation of native adiponectin oligomers (19). The authors demonstrated that hyper- adiponectinemia significantly increased lipid clearance and lipoprotein lipase activity, and enhanced insulin-mediated suppression of hepatic glucose production, thereby improving insulin sensitivity. Kadowaki’s group showed that transgenic overexpression of globular adiponectin in the genetic background of ob/ob obese mice led to partial amelioration of insulin resistance, hyperinsulinemia, and hyperglycemia (20).
Conflicting results have been obtained from adiponectin KO mice studies. Yamauchi et al.
found no impact of adiponectin depletion on insulin sensitivity under either normal chow or after 7 mo of feeding with a high-fat diet (21). In contrast, adiponectin KO mice
Fig. 2. Major target tissues and biological actions of adiponectin.
reported by Maeda et al. exhibited more severe high-fat diet induced insulin resistance and dyslipidemia, despite having normal glucose tolerance when fed with regular chow (22). Kubota et al. observed mild insulin resistance in the heterozygous adiponectin KO mice and moderate insulin resistance in the homozygous adiponectin KO mice even when fed with a regular chow (23). The latter two studies support the role of adiponectin as an endogenous insulin sensitizer in mice.
The insulin-sensitizing effect of adiponectin appears to be primarily attributed to its direct actions in skeletal muscle and liver, through the activation of AMPK and peroxi- some proliferator-activated receptor (PPAR)F(24,25). In liver, stimulation of AMPK by full-length adiponectin leads to decreased expression of gluconeogenic enzymes, such as phosphoenolpyruvate carboxykinase and glucose-6-phosphatase, which may account for its glucose-lowering effect in vivo (19,24). In skeletal muscle, activation of AMPK by globular or full-length adiponectin causes increased expression of proteins involved in fatty acid transport (such as CD36), fatty acid oxidation (such as acyl-coenzyme A oxidase) and energy dissipation (such as uncoupling protein-2), resulting in enhanced fatty acid oxidation and energy dissipation, and decreased tissue triglyceride (TG) accu- mulation. Excessive tissue TG accumulation has been proposed to be a major causative factor of insulin resistance in skeletal muscle (26). Therefore, reduction of tissue TG’s contents by adiponectin might be the major contributor to the insulin-sensitizing activ- ity of this adipokine.
In addition to liver and muscle, adiponectin can also act in an autocrine manner on adipocytes. It can antagonize the inhibitory effect of TNF-Fon insulin-stimulated glu- cose uptake (27), and blocks the release of insulin resistance-inducing factors from adipocytes (28). Furthermore, it has recently been suggested that adiponectin also acts in the brain to increase energy expenditure and cause weight loss (29).
3.2. Antiatherogenic Actions
In addition to its insulin-sensitizing effect, adiponectin possesses direct antiathero- genic properties (30,31). Both adenovirus-mediated overexpression of full-length adiponectin (32) and transgenic overexpression of globular adiponectin (20) have been shown to inhibit atherosclerotic lesion formation in the aortic sinus of apoE-deficient mice. On the other hand, disruption of the adiponectin gene results in impaired vaso- reactivity (33) and increased neointimal thickening in response to external vascular cuff injury (23,34).
Adiponectin can act directly on the vascular system to exert its vasoprotective functions.
In endothelial cells, full-length adiponectin enhances eNOS activity and increases nitric oxide (NO) production, which in turn improves endothelium-dependent vasodilation (35).
This protein also suppresses TNF-F-induced production of proinflammatory chemokines and adhesion molecules, including interleukin-8 (36), intracellular adhesion molecule-1, vascular cellular adhesion molecule-1, and E-selectin (37). Globular adiponectin has been shown to inhibit cell proliferation and suppress superoxide release induced by oxidized LDL in bovine aortic endothelial cells (38). In addition, the HMW form of adiponectin can protect endothelial cells from apoptosis by activation of AMPK (14).
In cultured aortic smooth muscle cells, adiponectin inhibits cell proliferation and migration induced by several atherogenic growth factors, including heparin-binding epi- dermal growth factor-like growth factor, platelet-derived growth factor BB, and basic
fibroblast growth factor (12,39). Adiponectin oligomers interact with these growth fac- tors and subsequently block their binding to the respective cell membrane receptors. In macrophages, adiponectin prevents lipid accumulation and suppresses foam cell transfor- mation by inhibiting expression of the class A scavenger receptors and uptake of acety- lated low density lipoprotein particles (40). It also blocks the attachment of monocytes to endothelial cells, which is the first and crucial step of atherosclerosis (41). A more recent study has shown that adiponectin increases tissue inhibitor of metalloproteinase-1 through inducing interleukin-10 expression in primary human macrophages (42).
3.3. Hepatoprotective Effects
Several recent studies suggest that adiponectin is protective against various types of liver injuries, steatosis, and fibrosis (43–46). Our group has examined the potential roles of adiponectin in alcoholic and nonalcoholic fatty liver diseases in mice (43). In both ob/ob obese mice and mice fed with a high fat–ethanol diet, chronic treatment with recombinant adiponectin dramatically alleviated hepatomegaly and steatosis (fatty liver), and also significantly attenuated inflammation and the elevated levels of serum alanine aminotransferase (ALT), a marker of liver injury. These protective effects were partly attributed to adiponectin’s ability to increase carnitine palmitoyltransferase I activity and enhance hepatic fatty acid oxidation, and to decrease hepatic lipogenesis and TNF-Fproduction (43).
Masaki et al. investigated the effect of adiponectin on liver injury induced by D- galactosamine/LPS (GalN/LPS) in KK-Ay obese mice (44). The authors found that pre- treatment with adiponectin ameliorated the GalN/LPS-induced elevation of serum AST and ALT levels, and also decreased the apoptotic and necrotic changes in hepatocytes, resulting in a marked reduction in lethality. Kamada et al. demonstrated that adiponectin KO mice were more susceptible to liver fibrosis induced by carbon tetrachloride (CCl4), whereas adenovirus-mediated overexpression of adiponectin prevented the development of this disease (45). In cultured rat hepatic stellate cells, adiponectin suppressed cell proliferation and migration and attenuated transforming growth factor (TGF)-G1- induced nuclear translocation of Smad2 (45,46). In addition, adiponectin has been shown to accelerate the apoptosis of activated hepatic stellate cells (46), and protect hepatocytes from TNF-F-induced death (47).
3.4. Protection Against Myocardial Injury
The beneficial effects of adiponectin on heart diseases have recently been demon- strated in several different animal models. Shibata et al. reported that pressure overload in adiponectin KO mice resulted in enhanced concentric cardiac hypertrophy and increased mortality that was associated with increased extracellular signal-regulated kinase (ERK) and diminished AMPK signaling in the myocardium (48). Consistent with this finding, a recent study from Liao et al. also showed the exacerbation of heart failure in adiponectin-KO mice (49). On the other hand, adenovirus-mediated supple- mentation of adiponectin attenuated cardiac hypertrophy in response to pressure over- loads in adiponectin KO, wild-type and db/db diabetic mice.
Shibata et al. also examined the role of adiponectin in myocardial remodeling in response to acute injury. They demonstrated that ischemia-reperfusion in adiponectin KO mice resulted in enlarged myocardial infarction size and apoptosis, and enhanced
TNF-F expression compared with wild-type mice (50). Adiponectin treatment dimi- nished infarction size, apoptosis, and TNF-Fproduction in both adiponectin KO mice and wild-type mice, through the activation of AMPK and induction of cyclooxygenase (COX)2-dependent synthesis of prostaglandin E2.
More recently, Takahashi et al. studied the effects of adiponectin replacement ther- apy on myocardial damage in ob/ob obese mice with acute viral myocarditis (51). The results from this study demonstrated that intraperitoneal injection of encephalomyo- carditis virus into ob/ob obese mice led to elevated cardiac weights and severe inflam- matory myocardial damage; these abnormalities were reversed following treatment with adiponectin.