Endocrine Abstracts

Background: Metabolic endotoxaemia (raised bacterial endotoxin in serum after high-fat feeding) has been shown to reduce insulin sensitivity in humans through systemic inflammation and oxidative stress. Mitochondria represent the main source of cellular reactive oxygen species and mutations in mitochondrial DNA often result in a diabetic phenotype. However, the direct cellular impact of endotoxin on mitochondrial respiration and DNA integrity, particularly within the context of type-2 diabetes (T2DM), is not known.

Methods: Morbidly obese women with T2DM (n=44) undergoing bariatric surgery consented to participate in this ethics-approved prospective study. Serum and abdominal subcutaneous adipose tissue biopsies collected before and six months after surgery were used to determine circulating endotoxin and mitochondrial gene expression, respectively. Human obese subcutaneous adipocytes ChubS7 were treated for 24 hours with (10 ng/ml; 100 ng/ml) or without endotoxin.

Results: A strong negative correlation was observed between serum LPS and mitochondrial number in adipose biopsies across all surgical cohorts (r²=−0.485, P=0.005**, n=32). Patients with lower serum endotoxin levels also exhibited greater weight, HbA1c and lipidaemia reduction in tandem with improvements in mitochondrial gene regulation. The in vitro endotoxin exposure up-regulated TNFα mRNA (P=0.004) and oxidative stress (P=0.009) whilst down-regulating activity of endogenous antioxidants superoxide dismutase (P=0.016) and catalase (P=0.008), and impairing glucose uptake (P<0.001) via inhibition of Insulin Receptor Substrate 1 protein (P<0.05). Furthermore, endotoxin resulted in 8 to 15% mitochondrial DNA deletion (P=0.008), mitochondrial protein depletion (P=0.007) and mitochondrial number reduction (P=0.034) compared with controls. This mitochondrial damage functionally manifested in a shift from aerobic to anaerobic respiration (P=0.03) and an impaired ability to cope with a seahorse stress test (P<0.01).

Conclusion: Taken together, our findings indicate that metabolic endotoxaemia drives insulin resistance in human adipose tissue, at least in part, via up-regulation of mitochondrial dysfunction, oxidative stress and inflammation.