The role of adipokines in relation to HIV lipodystrophy

Introduction Shortly after the introduction of antiretroviral therapy for HIV, practicing physicians began to notice insulin resistance, metabolic abnormalities, and changes in body shape, as side effects of therapy [1]. Changes in body fat distribution is a common finding in individuals with HIV infection being treated with antiretrovirals and this condition has many similarities with rare, congenital and acquired lipodystrophies, which are associated with depletion of subcutaneous fat, increased triglycerides and profound insulin resistance [2]. In addition, recent studies have shown that these patients have marked changes in circulating levels of adipocyte secreted hormones, including leptin and adiponectin, which may contribute to the noted metabolic abnormalities [3,4]. Herein we review the endocrine manifestations of HIV lipodystrophy (HIVLD) with a special emphasis on changes in levels of leptin and adiponectin, and discuss how these factors may prove to be important for the development of novel diagnostic as well as treatment strategies. Background Changes in body fat distribution are a frequent finding in individuals with HIV infection. Lipoatrophy, or decreased subcutaneous fat in the limbs together with prominent veins, loss of buttock fat and facial atrophy, can occur alone or in combination with fat accumulation, or lipohypertrophy, in other areas of the body, usually the abdomen and dorsocervical region. Central fat accumulation is usually due to visceral as opposed to subcutaneous fat and this, in association with peripheral wasting, initially led to characterization of the syndrome as ‘pseudocushing's syndrome’ [5]. The true prevalence of HIVLD has been difficult to establish in the setting of a lack of consensus regarding definition. Although prevalence rates have varied widely [1,6–8], it is estimated that together, lipohypertrophy, lipoatrophy and mixed abnormalities of body composition occur in more than 50% of antiretroviral-treated patients in cross sectional studies; these abnormalities have also been described in treatment naive individuals [5,9,10]. Prospective studies have shown that changes in body composition begin to occur shortly after initiation of combination antiretroviral therapy, with prevalence increasing with duration of therapy [11–13]. A key component of HIVLD is insulin resistance and the associated impaired glucose tolerance and type II diabetes. In addition, HIVLD has been shown to be associated with diabetes, dyslipidemia, and possibly an increased risk of cardiovascular disease [14–16]. In one study of HIV-infected individuals with fat redistribution, impaired glucose tolerance occurred in 35%, or seven times more frequently than in controls matched for age and body mass index (BMI), whereas 7% of subjects had diabetes [15]. Dyslipidemia with elevated total cholesterol, low density lipoprotein (LDL) cholesterol and triglycerides together with reduced high density lipoprotein (HDL) cholesterol is observed in up to 80% of individuals treated with antiretrovirals [14,15]. Individuals with abnormalities of body fat distribution have a higher prevalence of hypertriglyceridemia [14–16]. Similar to adults, children and adolescents with HIV have also been documented to develop lipodystrophy [17–20]. One study found a 25% prevalence of HIVLD in HIV infected children with 43% having at least one metabolic abnormality [21]. Recently, changes in circulating levels of several hormones, including leptin, adiponectin, and insulin-like growth factor (IGF)-1, have been described in individuals with HIVLD [13,22,23]. It has thus been hypothesized that these changes may contribute to the clinical features of this condition. If this hypothesis is confirmed, it may be possible to treat HIVLD and its associated endocrine manifestations by restoring these hormones to their physiologic levels. We review this aspect of endocrine pathophysiology here. Pathophysiology The lipodystrophy syndrome has been closely linked to the use of HIV protease inhibitors (PI) and more recently other highly active antiretrovirals including nucleoside analogues. Several mechanisms have been postulated to explain these adverse effects. PI have the capacity to: (i) inhibit the intrinsic activity of glucose transporter 4 (GLUT4) [24,26]; (ii) alter degradation of sterol-regulatory-element-binding-protein-1 (SREBP-1) and apolipoprotein B, resulting in lipodystrophy and increased lipid production [25–28]; (iii) inhibit the function of low density lipoprotein-receptor-related protein (LRP), leading to reduced triglyceride clearance from the circulation [29,30], while nucleoside reverse transcriptase inhibitors (NRTI) inhibit DNA polymerase gamma an enzyme essential for mitochondrial DNA replication, thus resulting in mitochondrial DNA depletion, adipocyte death and lipoatrophy [31,32]. Although the most compelling risk factor identified thus far has been the antiretroviral therapy, genetic predisposition, virally mediated mechanisms involving HIV-1 accessory proteins [33,34], altered hormonal milieu [35–39], and levels of inflammatory cytokines [40], may also contribute to this syndrome and are currently under investigation. Adipokines in HIVLD Adipose tissue, previously seen as an inert energy storage organ, is now considered to be an endocrine organ in its own right. The hormonal changes caused by increases (in lipohypertrophy) or reduced subcutaneous fat (in lipoatrophy) may be central to the metabolic abnormalities observed in HIVLD. Adiponectin Adiponectin, a 30-kDa complement C1q-related protein, is the most abundant gene product secreted by fat cells. Circulating levels of this adipokine are reduced with increasing central body fat [41]. Adiponectin plays an important role in regulating energy metabolism and inflammation and has a beneficial effect on insulin resistance through increased fatty acid oxidation in muscle and liver and through inhibiting gluceoneogenesis in animal models [42–44]. In humans, decreased plasma adiponectin levels are associated with insulin resistance, type 2 diabetes mellitus [45] and atherosclerosis [46]. Adiponectin expression is higher in subcutaneous than in visceral fat and decreased levels are observed in both animal models and humans with lipoatrophy [47] as well as central obesity (lipohypertrophy). In animal models of lipoatrophy, transgenic overexpression or administration of either leptin or adiponectin results in improvement in insulin resistance whereas administration of both hormones results in complete reversal [44,48]. Adiponectin in HIVLD In HIVLD, adiponectin levels are significantly lower in patients with fat redistribution and correlate inversely with serum triglycerides and insulin resistance, with levels being lowest in individuals with peripheral lipoatrophy and central lipohypertrophy [3,49,50]. These findings are independent of age, leptin levels, HIV medication and severity of disease [3]. Reduced adiponectin was recently found to be significantly associated with fat redistribution and insulin resistance in children [51]. Low adiponectin levels may reflect a direct toxic effect of anti-retroviral therapy on subcutaneous adipose tissue but may also simply reflect the accumulation of visceral adipose tissue [49]. Decreased adiponectin (as well as leptin expression), due to decreased adipocyte differentiation, could be involved in the observed whole body insulin resistance and the metabolic manifestations of HIVLD [52]. Therapy to increase adiponectin with PPAR-γ Thiazolidinediones (TZD), a class of insulin sensitizing agents, are agonists for the nuclear receptor transcription factor PPAR-γ, a subtype of the nuclear receptor superfamily that is found abundantly in adipose tissue. TZD, through changes in transcription, influence the expression of genes involved in adipocyte differentiation, lipid metabolism and insulin action [53]. These drugs have been shown to improve insulin sensitivity and to increase subcutaneous adipose tissue in diabetes and non-HIV lipoatrophy [53]. A possible mechanism of the beneficial effect of these drugs on insulin resistance is via increased adiponectin levels [54,55]. Studies involving adiponectin alone or a combination of leptin and adiponectin administration to humans with HIVLD have not yet been performed. There have been several randomized placebo-controlled trials of TZD (summarized in Table 1) in HIVLD. We have recently studied pioglitazone, a TZD with a more favorable effect on lipid profile, in individuals with HIV, insulin resistance and/or hyperlipidemia [56]. Pioglitazone, administered over a period of 12 months, improved insulin resistance and blood pressure and increased HDL without adverse effects on LDL cholesterol [56]. Waist circumference increased significantly with only a trend towards significance for total body fat. In contrast, fenofibrate, a PPARα agonist, did not have any effect on components of the metabolic syndrome in our study [56]. Importantly, pioglitazone, but not fenofibrate, resulted in a doubling of serum adiponectin levels. Another recent open label randomized trial compared rosiglitazone and metformin in men with HIVLD [57]. Both agents produced similar improvements in glucose metabolism but the metformin treated group had improved lipid profile and flow mediated vasodilation [57]. Compared with metformin, rosiglitazone increased both subcutaneous and visceral abdominal fat and was associated with a subjective improvement in fat redistribution in 47% of subjects [57]; this improvement has not been observed consistently in all studies (see Table 1). Currently, TZD are licensed only for use in diabetes and it is considered that there is insufficient evidence for their use in non-diabetic individuals with HIVLD.Table 1: Thiazolidinediones and HIVLD, randomized placebo-controlled trials.Resistin Resistin, another adipocyte-secreted protein, also called adipocyte-secreted factor or FIZZ-3 (found in inflammatory zone 3), was originally postulated to link obesity with diabetes [58,59]. The administration of resistin to animals resulted in impaired glucose and lipid metabolism in some studies [58,60], but these findings were challenged by subsequent reports of no associations between adipose tissue resistin expression and insulin resistance [61,62]. Findings in humans have also been highly variable. Elevated resistin levels have been noted in obese [63] and diabetic [64] subjects, and increased resistin has been associated with insulin resistance in lean and obese subjects [65]. Most studies, however, have found no association between circulating resistin levels and insulin resistance [61,63,64,66]. In a recent study of patients with the HAART-induced metabolic syndrome, resistin levels decreased after administration of rosiglitazone, an insulin sensitizer, but no correlation between resistin and insulin resistance or markers of inflammation and coagulation was found [67]. Further, we recently failed to show a significant difference between mean resistin levels of HIV subjects with and without HIVLD in a cross sectional study of HIV patients [68]. Leptin Leptin is the prototype adipokine discovered in 1994 through positional cloning of the mouse obese (ob/ob) gene [69], mutations of which result in significant hypoleptinemia and massive obesity, immune abnormalities, as well as insulin resistance and diabetes in the ob/ob mouse. Early hopes that leptin would provide a ‘magic bullet’ for the treatment of human obesity soon led to disappointment, however, with the discovery that the vast majority of obese individuals have high circulating leptin levels and thus are leptin resistant [70]. It has now become clear that leptin has a very important role to play at the other end of the spectrum, i.e., as a signal of energy deficiency [71–74]. Physiology studies in animal models and observational studies in humans have provided accumulating evidence for a role for leptin in regulating neuroendocrine, reproductive, immunological and cardiovascular systems, mainly in energy deficient states [75]. Leptin in HIVLD Lipoatrophy has been associated with low circulating leptin levels through loss of subcutaneous adipose tissue where leptin is predominantly expressed. Observational studies in adults have demonstrated that in HIVLD, as in the vast majority of humans, leptin levels are positively correlated with adipose tissue mass [4,76–78]. Lipoatrophy is associated with relative hypolepinemia, but it is now evident that metabolic, neuroendocrine and immunological effects of leptin deficiency may only become manifest once leptin falls below a threshold level [75]. The clinical cutoff for ‘hypoleptinemia’ remains to be clearly defined and may vary depending on the assay used. Several studies not enlisting separately subjects with predominant lipoatrophy, lipohypertrophy, and mixed patterns of fat redistribution have failed to find significantly lower leptin levels in subjects with HIVLD [77,79,80], but studies where subjects with clear lipoatrophy have been studied separately have demonstrated low leptin levels in this subgroup of HIVLD patients [4,81]. The etiology of the observed leptin deficiency is most likely due to absolute reduction in adipose tissue, but a direct effect of antiretrovirals on leptin gene expression cannot be excluded as a contributing factor [50]. Recently, leptin pulse dynamics were studied in relation to the morphologic changes in HIV patients and a significant association between leptin secretion and subcutaneous fat was reported [82]. Decreased subcutaneous fat correlated with reduced leptin levels after controlling for visceral fat quantity [82]. Metabolic effects In mice, leptin deficiency is associated with insulin resistance that is corrected by exogenous leptin administration prior to any observed change in body weight [83–85]. The mechanism of this direct effect of leptin on glucose metabolism may be centrally mediated via a hypothalamic relay or may be a direct effect of leptin in insulin sensitive tissues [86]. Humans with rare forms of non-HIV lipoatrophy, have demonstrated a dramatic response to exogenous leptin administration in uncontrolled studies with improved glycemic control, reduced triglycerides and reduced hepatic steatosis [87]. In addition, observational studies in subjects with HIVLD have shown that hypoleptinemia is independently associated with insulin resistance in patients with lipoatrophy, an effect not confounded by central or peripheral fat mass, suggesting a possible role for leptin in the development of metabolic changes in lipoatrophic patients [4]. This finding generated the hypothesis that leptin administration in leptin deficient, HIV positive subjects with lipoatrophy could result in correction of the metabolic disturbance, i.e., insulin resistance. Confirmation of this hypothesis could provide a proof of concept for its role in causing the syndrome [4]. We have recently completed a randomized, placebo-controlled, crossover study of leptin administration over a period of 8 weeks to seven men with HIV and lipodystrophy, all of whom had serum leptin levels < 3 ng/ml [81]. In this proof-of-concept study, leptin administration resulted in improvement in insulin resistance and lipid levels and a reduction in truncal but not peripheral fat or lean body mass [81]. Leptin and immune function In addition to the metabolic effects of leptin, it is also clear that leptin has an important role in immune regulation. Leptin effects both cell mediated and humoral immunity [88–90]. Leptin has been shown to have direct effects, ex vivo, on cells of the innate immune system; upregulating phagocytic function in macrophages [91], stimulating pro-inflammatory cytokine secretion [92], and stimulating chemotaxis in polymorphonuclear cells [93]. The presence of the leptin receptor, ObRb, on these immune cells [94] indicates that the role of leptin is likely to be direct and not mediated by other hormonal changes. These leptin-mediated changes in immune function appear to be mediated through activation of the JAK-STAT3 pathway in lymphocytes [95], but other pathways such as MAPK and PI3K may also be involved [96,97]. The importance of these findings in vivo is demonstrated by the immunodeficiency observed in ob/ob mice, which is improved by leptin administration [98]. Humans with congenital leptin deficiency have a higher incidence of infection related death in childhood than their peers due to defects in T cell number and function [99]. Moreover, improvement in their immune function has been demonstrated with leptin replacement [100]. Administration of leptin nearly restored the proliferation response and the cytokine release profile from lymphocytes of two children with congenital leptin deficiency studied in the context of an uncontrolled study [100]. While complete congenital leptin deficiency is rare in clinical practice, these important interactions appear also to be of relevance to the more common syndromes of relative leptin deficiency. In humans, leptin administered in replacement doses has been shown to prevent some of the starvation-induced changes in the immune system in response to 3 days of calorie deprivation [101]. Our group has also recently shown that leptin administration improves circulating cytokine levels in women with exercise induced amenorrhea who have chronic relative leptin deficiency [72] and studies involving leptin administration in states of calorie deprivation have demonstrated increased inflammatory and platelet responses [102]. The role played by leptin in the immune dysregulation associated with obesity, including its role in the proinflammatory state associated with this condition, remains to be fully elucidated. In contrast to observational studies [103], our recent interventional study involving leptin administration to individuals with leptin sufficiency or excess has failed to demonstrate a link between leptin and any alterations in the immune system associated with obesity [72]. Thus, on the basis of evidence available to and similar to other the role of leptin in immune is likely to be a and more important in states of leptin deficiency. the of relative leptin deficiency on immune function in men with HIVLD is Although we did not any changes in or HIV in our proof-of-concept study of leptin administration to men with HIV the study was for this [81]. Thus, this Leptin and neuroendocrine function Leptin also to play a role in regulating several neuroendocrine in This may be of relevance in the interventional studies have demonstrated that leptin administration in replacement doses in the in and levels caused by energy deprivation Observational studies have also reported in individuals with congenital leptin deficiency. Leptin replacement in these subjects results in development with increasing and levels in and in In addition, we have recently shown that leptin, administered in replacement doses to women with relative leptin deficiency and hypothalamic amenorrhea results in improvement in hormones and of suggesting that leptin is a key of function in humans Another study at the of found that in non-HIV lipodystrophy physiologic leptin replacement function in and results in a significant increase in in possibly through increasing insulin sensitivity and restoring Similar findings to the with leptin deficiency being associated with a state of hypothalamic Observational human studies have found that leptin replacement in individuals with congenital leptin deficiency results in improvement in the [100]. Our interventional studies have shown that leptin administration the reduced stimulating that in with deprivation In women with leptin significantly increased levels of and have reduced growth and in leptin the of growth and that with but the between leptin and the growth in humans has not yet been fully elucidated. Leptin deficient humans have decreased growth response to with [99]. Leptin replacement in these leptin deficient individuals has been shown to increase levels of proteins in one and uncontrolled study In our interventional studies in leptin did not prevent the in and proteins associated with but did significantly increase and protein 3 in leptin deficient women with deficiency has recently been associated with HIVLD in a study of HIV infected individuals the In this study, one of HIV infected individuals with fat redistribution had a growth response with a of growth deficiency The etiology of the growth secretion in this subgroup of patients with HIVLD is Findings from other associated with low leptin levels that leptin deficiency may be at least human growth was for use in in and the reduction in visceral that is observed with growth administration to non-HIV infected individuals with growth deficiency that alterations in growth secretion or action may play a role in HIVLD In individuals with HIVLD, doses of growth were found to truncal the subjects had glucose tolerance and very elevated levels growth in men with HIVLD the reduced abdominal fat and found that while insulin sensitivity was initially it had to after of therapy and was associated with significant improvement in serum A recent randomized, placebo-controlled study of low growth demonstrated a reduction in truncal without metabolic effects The effects of growth replacement to individuals with relative growth as defined by response to is may to increase growth levels the in individuals with HIVLD and recent studies of both growth and a novel growth factor have positive and similar administration was not only with a reduction in truncal and visceral but also an improvement in both and lipodystrophy in the limbs and abdomen with no change in or glucose tolerance treatment with both growth and growth is and there is regarding the of with relative growth who would to be fully These are areas of active investigation. The effects of leptin deficiency on neuroendocrine function in men and women with HIVLD are but an of active studies are to the lower of leptin levels for these physiologic Leptin and Recently, cross sectional studies have noted a prevalence of and in HIV patients of and the of metabolic disease in patients with lipodystrophy may be in than in patients Further, a cross sectional found that reduced density is in HIV subjects on antiretroviral therapy and is related to central and Although there has been some evidence that antiretroviral therapy plays a the mechanisms Leptin has recently been shown to increase density in leptin deficient animals and obesity, a and There is evidence of a direct effect of leptin on and development in The to which these animal and in studies to human metabolism remains reports of leptin treatment in individuals with congenital leptin deficiency and lipoatrophy have demonstrated effects of leptin in humans The association between leptin deficiency and density in subjects with HIVLD remains and metabolic abnormalities are common side effects in individuals on antiretroviral therapy recent the etiology of this condition remains There is increasing evidence that individuals with HIVLD have endocrine It is these abnormalities are a result of reduced adipose tissue mass or a reduction in expression in adipose or likely a combination of The of these endocrine in individuals with HIV infection as well as the of replacement therapy an active of evidence from animal studies and studies in individuals with non-HIV lipoatrophy that leptin replacement may result in improved insulin neuroendocrine and immune recent proof of concept studies we leptin or the to increase circulating adiponectin that these may improve insulin resistance, fat redistribution, and lipid to the may also have beneficial effects. by a from to and