COL6A3 is regulated by leptin in human adipose tissue and reduced in obesity

Fibrosis of adipose tissue (AT) increases AT rigidity, reduces its expandability, and contributes to metabolic dysfunction. Collagen type VI, α3 (COL6A3) encodes 1 subunit of a fibrotic extracellular matrix protein highly expressed in rodent AT. Knockout of collagen VI in rodent AT led to a significant improvement in metabolic health in obese, diabetic ob/ob mice. However, it is unknown whether this collagen has the same metabolic significance in human AT. We therefore aimed to undertake a comprehensive assessment of COL6A3 in relation to human AT and obesity. Characterization of COL6A3 in human AT showed 5-fold higher expression in the stromalvascular fraction compared with adipocyte expression and significantly higher expression in subcutaneous AT (SCAT) than omental AT. In both depots, COL6A3 expression appeared to be lowered in obesity, whereas diet- and surgery-induced weight loss increased COL6A3 expression in SCAT. Leptin treatment caused a dose-dependent decrease in COL6A3 expression, although no effect was seen with insulin or glucose treatment and no difference observed in subjects with diabetes. In addition, we found that the collagen expression profile in humans differs significantly from rodents, because COL6A3 does not appear to be the predominant collagen in adipose, muscle, or liver. Our findings oppose those initially seen in rodent studies and, most importantly, demonstrate a direct regulation of COL6A3 by leptin. This highlights the importance of a paracrine leptin signaling pathway in human AT and suggests an additional mechanism by which leptin can regulate extracellular matrix composition and, with it, AT expandability.