Obesity and type 2 diabetes (T2DM) have become public health problems of epidemic proportions and much effort is being undertaken to understand the molecular mechanisms underlying the pathogenesis of these conditions to design new treatments. Obesity dramatically increases the risk of developing T2DM and patients with glucocorticoid (GC) excess (Cushing's syndrome) produce visceral obesity and diabetes, but circulating GC levels are not elevated in the vast majority of patients with typical obesity and metabolic syndrome (MS). However, many typical obesity and MS can be accounted for by abnormally increased regeneration of GCs within adipose tissue by an intracellular endoplasmic reticulum (ER) lumen-resident enzyme, 11-hydroxysteroid dehydrogenase (11-HSD1) that can generate active cortisol from inactive cortisone and thus amplifies intracellular GC action despite unaltered plasma GC levels in obesity and MS. Pre-receptor activation of GCs via 11-HSD1 is thus regarded as a common molecular etiology for obesity and MS. However, 11-HSD1 within the ER is crucially dependent on the enzyme hexose-6-phosphate dehydrogenase (H6PDH) to maintain its cofactor NADPH availability. In the ER lumen, H6PDH can metabolize glucose-6-phosphate (G6P) and generates NADPH from NADP and requires the G6P transporter to maintain its metabolic substrate G6P levels. H6PDH is thus able to couple the regulation of cellular G6P metabolism and GC signaling linked to the pathogenesis of T2DM and obesity. Indeed, we have shown that the increased adipose H6PDH expression leading to up-regulation of 11-HSD1 in obese diabetic mice is similar to that found in adipose tissue from T2DM patients. However, the functional consequences of altered adipose H6PDH expression are unknown. Using our unique transgenic mouse overexpression of H6PDH selectively in adipose tissue, we have observed that these mice have abnormally increased adipose corticosterone production and cellular G6P metabolism and exhibited the adverse metabolic phenotypes with hyperglycemia, visceral fat accumulation, hyperlipidemia, and insulin resistance. We thus hypothesize that adipose H6PDH plays an important role in the pathophysiology of obesity and insulin resistance, and can be manipulated to provide potential strategies for the treatment of MS. In this grant, we will explore the impact of adipose H6PDH on glucose homeostasis and insulin sensitivity in vivo using our unique existing H6PDH transgenic model. We will also directly examine the role of altered H6PDH expression in insulin signaling action through manipulation of H6PDH to regulation of cellular GC regeneration and G6P metabolism by using siRNA technology in intact mouse adipocyte cells. This will be facilitated by generation and evaluation of a H6PDH fat-specific knockout mouse. We believe that manipulating the role and impact of H6PDH on adipose function and homeostasis will provide new strategies that may translate into novel therapies for patients with T2DM and MS.
The number of individuals affected by T2DM and obesity is increasing at an alarming rate and are often associated with dysregulation of glucose and glucocorticoid metabolism. Much effort is therefore needed to be undertaken to identify the reasons for this disease to design new treatments. We recently reported a gene called H6PDH that can regulate cortisol hormone metabolism and glucose homeostasis. We believe that effective manipulation of H6PDH may provide a new therapeutic strategy for the treatment of T2DM and obesity.
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