Mitochondrial H-ATP synthase in human skeletal muscle: contribution to dyslipidaemia and insulin resistance.

Aims/hypothesis: Mitochondria are important regulators of the metabolic phenotype in type 2 diabetes. A key factor in mitochondrial physiology is the H-ATP synthase. The expression and activity of its physiological inhibitor, ATPase inhibitory factor 1 (IF1), controls tissue homeostasis, metabolic reprogramming and signalling. We aimed to characterise the putative role of IF1 in mediating skeletal muscle metabolism in obesity and diabetes.

Methods: We examined the 'mitochondrial signature' of obesity and type 2 diabetes in a cohort of 100 metabolically characterised human skeletal muscle biopsy samples. The expression and activity of H-ATP synthase, IF1 and key mitochondrial proteins were characterised, including their association with BMI, fasting plasma insulin, fasting plasma glucose and HOMA-IR. IF1 was also overexpressed in primary cultures of human myotubes derived from the same biopsies to unveil the possible role played by the pathological inhibition of the H-ATP synthase in skeletal muscle.

Results: The results indicate that type 2 diabetes and obesity act via different mechanisms to impair H-ATP synthase activity in human skeletal muscle (76% reduction in its catalytic subunit vs 280% increase in IF1 expression, respectively) and unveil a new pathway by which IF1 influences lipid metabolism. Mechanistically, IF1 altered cellular levels of α-ketoglutarate and L-carnitine metabolism in the myotubes of obese (84% of control) and diabetic (76% of control) individuals, leading to limited β-oxidation of fatty acids (60% of control) and their cytosolic accumulation (164% of control). These events led to enhanced release of TNF-α (10 ± 2 pg/ml, 27 ± 5 pg/ml and 35 ± 4 pg/ml in control, obese and type 2 diabetic participants, respectively), which probably contributes to an insulin resistant phenotype.

Conclusions/interpretation: Overall, our data highlight IF1 as a novel regulator of lipid metabolism and metabolic disorders, and a possible target for therapeutic intervention.