Sildenafil Activates Browning of White Adipose and Improves Insulin Sensitivity

Obesity and metabolic disease result when energy intake consistently exceeds energy expenditure. One appealing new target for treatment is the activation of brown adipose tissue (BAT), an organ recently found to be functional in adult humans. Brown adipocytes selectively express uncoupling protein 1 (UCP1), which renders the inner membrane of mitochondria leaky, thereby diverting chemical energy from ATP generation to heat production. Interest in BAT has been spurred by the recognition that in addition to classical BAT depots, other brown-fat-like cells are present in the subcutaneous white adipose tissue (WAT) in animals and also in humans.These cells have structural and functional properties that resemble brown adipocytes, and they are referred to as beige or 'brite' (brown-in-white) adipocytes. Interestingly, browning of WAT can be induced in animals and humans by physiological stimuli such as cold exposure, which increases adrenergic tone, and by exercise, which selectively drives WAT browning through irisin, an exercise-induced myokine. In addition b-adrenergic drugs and other pharmacological agents,such as prostaglandins, can induce browning of white adipose tissue. More recently, one study showed that treatment of C57BL/6 mice with phosphodiesterase inhibitor sildenafil (12 mg/kg/d) for 7 d caused 4.6-fold increase in uncoupling protein-1 expression and promoted establishment of a brown fat cell-like phenotype ("browning") of WAT in vivo. Therefore, the investigators hypothesized that sildenafil can promote browning of white adipose tissue and improves insulin sensitivity in human adults.