Adiponectin: key player in the adipose tissue-liver crosstalk.

The adipose tissue has recently emerged as an important endocrine organ releasing numerous mediators including adipocytokines, classical cytokines and others. Adiponectin, one of the major products of adipocytes, is a prototypic anti-diabetic adipocytokine, the actions of which are mainly exerted by the activation of AMP-activated kinase and peroxisome proliferator-activated receptor alpha. This adipocytokine is one of the most abundant circulating (adipo)cytokines in health. Non-alcoholic fatty liver disease (NAFLD), the major reason for abnormal liver functions in the western world, is commonly associated with obesity, insulin resistance and decreased adiponectin serum levels. Adiponectin has many anti-inflammatory activities and suppresses tumour necrosis factor-alpha (TNFα), a cytokine of key importance in NAFLD. The anti-inflammatory effects of adiponectin are also exerted by induction of the anti-inflammatory cytokines interleukin-10 (IL-10) or IL-1 receptor antagonist and up-regulation of heme-oxygenase-1. Whereas the liver probably is not a relevant source of circulating adiponectin, it is a major target organ for many adiponectin effects. Adiponectin is able to regulate steatosis, insulin resistance, inflammation and fibrosis. NAFLD is also associated with decreased liver expression of the two adiponectin receptors (AdipoR1 and 2) thereby contributing to a state of hepatic adiponectin resistance. In contrast, most other liver diseases especially in advanced disease states exhibit increased adiponectin serum levels with highest levels observed in cirrhosis. Targeting adiponectin could evolve as a major treatment concept especially for fatty liver diseases in the future.