Subcutaneous adipose tissue biology in metabolic syndrome

Metabolic syndrome (MetS) is a common global problem that comprises the cardio-metabolic cluster and predisposes to both diabetes and cardiovascular diseases. Although the pathogenic mechanisms have not been elucidated, both increased inflammation and insulin resistance play a pivotal role. It appears that both monocyte/macrophages and adipose tissue (AT) conspire to accentuate both the pro-inflammatory state and increased insulin resistance. Whilst there are scant data on visceral adipose tissue (VAT) and epicardial adipose tissue (EAT) biology, there are data on subcutaneous adipose tissue (SAT) dysregulation. There is a significant increase in macrophages and crown-like structures in the SAT of patients with MetS. With respect to adipokines, there is an increase in plasma leptin, plasminogen activator inhibitor-1, retinol-binding protein-4 (RBP-4), chemerin, serum amyloid-A, C-reactive protein (CRP), interleukin-1, -6, -8, lipopolysaccharide, fetuin A (FetA) and a decrease in adiponectin and omentin-1. All of the abnormalities in plasma were also confirmed for SAT-secreted adipokines except for adiponectin and RBP-4 which derive largely from VAT. As many of these biomediators correlate with both insulin resistance and increased inflammation, we can posit that dysregulation of SAT is detrimental and contributes to both the pathogenesis of MetS and its sequalae. Furthermore, as future directions, much work is needed with respect to VAT/EAT biology, autophagy, sirtuins, the gut microbiome, browning of AT, to further elucidate this common syndrome and identify potential therapeutic targets to forestall its serious complications.