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 TNF alpha, IL-1 beta, 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 O(2) 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 O(2) 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 pre-adipocytes (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.