Increased primary amine oxidase expression and activity in white adipose tissue of obese and diabetic db-/- mice

The major form of primary amine oxidase expressed in adipose tissue (AT) is encoded by AOC3 gene and is known as semicarbazide-sensitive amine oxidase, identical to vascular adhesion protein-1 (SSAO/VAP-1). Exogenous substrates of SSAO/VAP-1 (e.g. benzylamine) stimulate glucose transport in adipocytes and improve glucose tolerance when injected in diabetic rodents. Numerous reports on the circulating, soluble SSAO/VAP-1 have univocally evidenced an increase in diabetic conditions. However, only scarce studies have investigated whether obesity and/or diabetes is accompanied with variations of AOC3 expression in AT. Therefore, we compared the SSAO/VAP-1 content in different fat depots of db-/- mice (lacking leptin receptor and being hyperphagic, diabetic and obese) and db+/- littermates (normoglycemic and lean). AOC3 expression was increased in perigonadal and subcutaneous AT of db-/- mice, while the maximal velocity of benzylamine oxidation (V (max), expressed as pmoles of hydrogen peroxide produced/min/mg protein) increased only in the latter. Indeed, the relative abundance of primary amine oxidase was increased in subcutaneous AT of db-/- mice at all the levels: mRNA, protein and activity. While considering the overall capacity to oxidise amines contained in each depot, there was an increase in the hypertrophic fat pads of the obese db-/- mice, irrespective of their anatomical location, as a result of their dramatically larger mass than in lean db+/- control. Such higher amount of AT-bound primary amine oxidase warrants further studies to determine whether SSAO/VAP-1 inhibition or activation may be useful in treating metabolic diseases.