NO energy

Mitochondria in brown adipocyte tissue (BAT) are larger and more numerous than in other cell types; their inner mitochondrial membrane contains uncoupling protein 1 (UCP1), which diverts energy from ATP synthesis to thermogenesis. Nitric oxide (NO) is known to regulate biological functions in mature brown adipocytes, but, until now, NO's role in mitochondrial biogenesis has not been studied.

Reporting in Science, Nisoli et al. looked at mitochondrial biogenesis in primary cultures of mouse brown adipocyte precursors. Treatment with an NO donor increased the mtDNA content above levels seen in untreated cells, which were due to spontaneous differentiation of the adipocyte precursors. This increase was abolished in the presence of the NO scavenger oxyhaemoglobin, indicating that it was mediated by NO generation.

The authors next showed that this effect occurred through activation of the peroxisome proliferation-activated receptor and co-activator 1α (PGC-1α) — a principal regulator of mitochondrial biogenesis in BAT, and cardiac and skeletal muscle. Using a cyclic GMP analogue and a guanylate-cyclase inhibitor, they also showed that the biogenesis depends on cGMP. And study of mouse white-fat 3T3-L1 and human monocytic U937 cell lines revealed that the biogenesis was not restricted to brown adipocytes and their differentiation processes.

To investigate the role of endogenous NO, the authors stably transfected HeLa cells with endothelial nitric oxide synthase (eNOS) — the only isoform that is expressed in brown adipocytes and 3T3-L1 cells under experimental conditions. Induction of eNOS increased mitochondrial biogenesis; an effect that was abolished by a NOS inhibitor.

Cold exposure triggers PGC-1α expression through activation of β₃-adrenergic receptors and increases in intracellular cAMP and Ca²⁺, all of which stimulate NO production in brown adipocytes. So Nisoli et al. studied BAT functions in wild-type and eNOS^−/− mice before and after cold exposure. At both temperatures, histological analysis indicated that eNOS^−/− BAT was functionally inactive, and mitochondrial biogenesis was impaired.

When the authors looked at the control of biogenesis in the brain, liver and heart of the knockout mice, they found that deletion of eNOS was enough to reduce the number of mitochondria even in tissues that have a basal level of neuronal, and possibly inducible, NOS expression.

In eNOS^−/− mice, oxygen consumption rates — an indicator of metabolic rate — were decreased, indicating that BAT-dependent thermogenesis might be impaired. In genetic models of obesity, defective energy expenditure is involved in increased food intake and body-weight gain; eight-week-old eNOS^−/− mice showed similar food consumption but weighed more than wild-type mice. So, the increased body weight of eNOS^−/− mice could be accounted for by higher feed efficiency (weight gain/food intake) caused by defective energy expenditure.

So what does this mean? The features shown by eNOS^−/− mice — reduced mitochondrial number and energy expenditure, weight gain, insulin resistance and hypertension — are all typical of the so-called metabolic syndrome. Millions of people are estimated to have metabolic syndrome, placing them at an increased risk of developing diabetes and cardiovascular disease. However, if the results reported by Nisoli et al. are applicable to humans, then we will have “...clues for the prevention or treatment of this condition”.