Bohan Wang

Hypoxia in adipose tissue: a basis for the dysregulation of tissue function in obesity?

White adipose tissue is a key endocrine and secretory organ, releasing multiple adipokines, many of which are linked to inflammation and immunity. During the expansion of adipose tissue mass in obesity there is a major inflammatory response in the tissue with increased expression and release of inflammation-related adipokines, including IL-6, leptin, monocyte chemoattractant protein-1 and TNF-alpha, together with decreased adiponectin production. We proposed in 2004 (Trayhurn & Wood, Br J Nutr 92, 347-355) that inflammation in adipose tissue in obesity is a response to hypoxia in enlarged adipocytes distant from the vasculature. Hypoxia has now been directly demonstrated in adipose tissue of several obese mouse models (ob/ob, KKAy, diet-induced) and molecular studies indicate that the level of the hypoxia-inducible transcription factor, hypoxia-inducible factor-1 alpha, is increased, as is expression of the hypoxia-sensitive marker gene, GLUT1. Cell- culture studies on murine and human adipocytes show that hypoxia (induced by low O2 or chemically) leads to stimulation of the expression and secretion of a number of inflammation-related adipokines, including angiopoietin-like protein 4, IL-6, leptin, macrophage migration inhibitory factor and vascular endothelial growth factor. Hypoxia also stimulates the inflammatory response of macrophages and inhibits adipocyte differentiation from preadipocytes. GLUT1 gene expression, protein level and glucose transport by human adipocytes are markedly increased by hypoxia, indicating that low O2 tension stimulates glucose utilisation. It is suggested that hypoxia has a pervasive effect on adipocyte metabolism and on overall adipose tissue function, underpinning the inflammatory response in the tissue in obesity and the subsequent development of obesity-associated diseases, particularly type 2 diabetes and the metabolic syndrome.

Cellular hypoxia and adipose tissue dysfunction in obesity.

Expansion of adipose tissue mass, the distinctive feature of obesity, is associated with low-grade inflammation. White adipose tissue secretes a diverse range of adipokines, a number of which are inflammatory mediators (such as TNFalpha, IL-1beta, IL-6, monocyte chemoattractant protein 1). The production of inflammatory adipokines is increased with obesity and these adipokines have been implicated in the development of insulin resistance and the metabolic syndrome. However, the basis for the link between increased adiposity and inflammation is unclear. It has been proposed previously that hypoxia may occur in areas within adipose tissue in obesity as a result of adipocyte hypertrophy compromising effective O2 supply from the vasculature, thereby instigating an inflammatory response through recruitment of the transcription factor, hypoxic inducible factor-1. Studies in animal models (mutant mice, diet-induced obesity) and cell-culture systems (mouse and human adipocytes) have provided strong support for a role for hypoxia in modulating the production of several inflammation-related adipokines, including increased IL-6, leptin and macrophage migratory inhibition factor production together with reduced adiponectin synthesis. Increased glucose transport into adipocytes is also observed with low O2 tension, largely as a result of the up-regulation of GLUT-1 expression, indicating changes in cellular glucose metabolism. Hypoxia also induces inflammatory responses in macrophages and inhibits the differentiation of preadipocytes (while inducing the expression of leptin). Collectively, there is strong evidence to suggest that cellular hypoxia may be a key factor in adipocyte physiology and the underlying cause of adipose tissue dysfunction contributing to the adverse metabolic milieu associated with obesity.

Hypoxia induces leptin gene expression and secretion in human preadipocytes: differential effects of hypoxia on adipokine expression by preadipocytes

The effect of hypoxia on the expression and secretion of major adipokines by human preadipocytes has been examined. Hypoxia (1% O(2)) led to an increase in the HIF-1 alpha transcription factor subunit in cultured preadipocytes, as did incubation with the hypoxia mimetic CoCl(2). Leptin mRNA was essentially undetectable in preadipocytes incubated under normoxia (21% O(2)), but exposure to 1% O(2), or CoCl(2), for 4 or 24 h resulted in an induction of leptin gene expression (measured by real-time PCR). Immunoreactive leptin was not detected in the medium from normoxic preadipocytes, but was present in the medium from the hypoxic cells. Hypoxia stimulated expression of the GLUT-1 facilitative glucose transporter gene and the vascular endothelial growth factor (VEGF) gene in preadipocytes, as in adipocytes. PPAR gamma and aP2 mRNA levels, markers of adipocyte differentiation, were reduced by hypoxia in both cell types. In marked contrast to adipocytes, interleukin-6 (IL-6), angiopoietin-like protein 4, and plasminogen activator inhibitor-1 expression by preadipocytes was not stimulated by low O(2) tension. Consistent with the gene expression results, VEGF release into the medium from preadipocytes was increased by hypoxia, but there was no change in IL-6 secretion. It is concluded that hypoxia induces human preadipocytes to synthesize and secrete leptin. Preadipocytes and adipocytes differ in their responsiveness to low O(2) tension, maturation of the response to hypoxia developing on differentiation.

Hypoxia and the endocrine and signalling role of white adipose tissue

Abstract White adipose tissue is a major endocrine and signalling organ. It secretes multiple protein hormones and factors, termed adipokines (such as adiponectin, leptin, IL-6, MCP-1, TNFα) which engage in extensive cross-talk within adipose tissue and with other tissues. Many adipokines are linked to inflammation and immunity and these include cytokines, chemokines and acute phase proteins. In obesity, adipose tissue exhibits a major inflammatory response with increased production of inflammation-related adipokines. It has been proposed that hypoxia may underlie the inflammatory response in adipose tissue and evidence that the tissue is hypoxic in obesity has been obtained in animal models. Cell culture studies have demonstrated that the expression and secretion of key adipokines, including leptin, IL-6 and VEGF, are stimulated by hypoxia, while adiponectin (with an anti-inflammatory action) production falls. Hypoxia also stimulates glucose transport by adipocytes and may have a pervasive effect on cell function within adipose tissue.

PCR arrays identify metallothionein-3 as a highly hypoxia-inducible gene in human adipocytes.

Hypoxia-signalling pathway PCR arrays were used to examine the integrated response of human adipocytes to low O2 tension. Incubation of adipocytes in 1% O2 for 24 h resulted in no change in the expression of 63 of the 84 genes on the arrays, a reduction in expression of 9 genes (including uncoupling protein 2) and increased expression of 12 genes. Substantial increases (>10-fold) in leptin, angiopoietin-like protein 4, VEGF and GLUT-1 mRNA levels were observed. The expression of one gene, metallothionein-3 (MT-3), was dramatically (>600-fold) and rapidly (by 60 min) increased by hypoxia. MT-3 gene expression was also substantially induced by hypoxia mimetics (CoCl2, desferrioxamine, dimethyloxalylglycine), indicating transcriptional regulation through HIF-1. Hypoxia additionally induced MT-3 expression in preadipocytes, and MT-3 mRNA was detected in human (obese) subcutaneous and omental adipose tissue. MT-3 is a highly hypoxia-inducible gene in human adipocytes; the protein may protect adipocytes from hypoxic damage.

HIF-1α protein rather than mRNA as a marker of hypoxia in adipose tissue in obesity: focus on “Inflammation is associated with a decrease of lipogenic factors in omental fat in women,” by Poulain-Godefroy et al.

we were interested to see the article by Poulain-Godefroy et al. ([4][1]) in which a global decrease in lipogenic markers was associated with the enhancement of inflammation in human adipose tissue, a relationship accentuated in diabetic subjects. The authors further show that expression levels of

Hypoxia modulates the expression and secretion of inflammation-related adipokines in differentiated human adipocytes

Abstract Background Obesity is characterised by a state of chronic low-grade inflammation. Recently, we proposed that hypoxia may occur in enlarged adipocytes distant from the vasculature as adipose tissue mass expands, and that this drives the inflammatory response through dysregulation of inflammation-related adipokines. We have now examined the effects of low oxygen tension and chemically induced hypoxia on the production of key adipokines in differentiated human adipocytes. Methods Cultured human adipocytes (15 days post differentiation) were exposed to 1% oxygen or 100 μM cobalt(II) chloride for up to 24 h; control cells were maintained in normal levels of oxygen only. mRNA levels of key adipokines were quantified by real-time PCR. Cellular levels of the hypoxia-sensitive transcription factor HIF-1α and the secretion of adipokines into the medium were measured with ELISAs. Findings A large (7·8 fold) increase in HIF-1α protein was induced in human adipocytes after 4 h of hypoxia. The mRNA level of the facilitative glucose transporter, GLUT1, increased 14 fold by 24 h and there were increases (by 24 h) in the level of the mRNAs encoding major adipokines, including leptin (28 fold), fasting-induced adipose factor (11 fold), vascular endothelial growth factor (23 fold), interleukin 6 (4·5 fold), and migration inhibitory factor (2·5 fold). By contrast, adiponectin mRNA level fell (3 fold). Changes in mRNA level were accompanied by parallel alterations in adipokine secretion into the medium. Similar results were obtained when hypoxia was induced chemically with cobalt(II) chloride. Interpretation Hypoxia dysregulates the production of key adipokines in human adipocytes, leading to an inflammatory state. Hypoxia may underlie the development of inflammation in adipose tissue in obesity. Funding None.

Comment on: Hosagai et al. (2007) Adipose Tissue Hypoxia in Obesity and Its Impact on Adipocytokine Dysregulation: Diabetes 56:901–911

We noted with interest the article (1) where evidence for hypoxia in white adipose tissue (WAT) of two mouse models of obesity was presented. This study indicated that the expression of some adipokine genes, including leptin and plasminogen activator inhibitor-1, are increased in WAT of obese animals, that adiponectin is decreased, and that the effects mirrored those in studies on mouse 3T3-L1 cells (1). These findings are consistent with …

The Role of Hypoxia in Adipocyte Function and Dysfunction

White adipose tissue is a major endocrine and signalling organ, secreting a multiplicity of protein factors. These adipokines are involved in a range of physiological and metabolic functions. Many adipokines are linked to the inflammatory response, and the expansion of adipose tissue mass in obesity leads to a state of inflammation within the tissue. This inflammation is considered pivotal in the development of obesity-associated diseases, particularly insulin resistance and the metabolic syndrome; however, the basis for the initiation of the inflammatory response is unknown. It is proposed that inflammation in adipose tissue reflects a response to local hypoxia as tissue mass expands, large adipocytes becoming O2-deprived as their distance from the vasculature increases. Direct evidence for hypoxia in adipose tissue in obesity has now been obtained in mice. Candidate gene studies on adipocytes, both human and murine, in cell culture have shown that the expression and secretion of several key inflammation-related adipokines, including IL-6, leptin, Angplt4, and VEGF, are stimulated by low pO2. The production of adiponectin, with its anti-inflammatory and insulin-sensitising actions, is, on the other hand, inhibited. PCR arrays and microarray studies have demonstrated that the expression of a wide range of adipocyte genes is modulated by hypoxia (>1,000 hypoxia-sensitive genes). These include the facilitative glucose transporter GLUT1, the expression of which is increased in response to low O2 tension with a parallel rise in GLUT1 protein and an increase in basal glucose uptake. Correspondingly, the release of lactate is increased, consistent with a switch to anaerobic glycolysis. Synthesis of the monocarboxylate transporter, MCT1, is stimulated in adipocytes by hypoxia, providing a mechanism for the rise in lactate transport from hypoxic fat cells. Importantly, hypoxia has been shown to directly lead to the induction of insulin resistance in adipocytes. Hypoxia also affects the other cells within adipose tissue, including preadipocytes, in which leptin expression and secretion are switched on. It is proposed that hypoxia has a pervasive effect on adipocyte physiology and is central to the dysregulation of adipose tissue function that occurs in obesity, leading to the associated diseases.