Implication of ghrelin isoforms in the improvement of nonalcoholic fatty liver disease after bariatric surgery

Bariatric surgery improves NAFLD and NASH, but the underlying mechanisms remain unknown. The present thesis shows the potential role of acylated and desacyl ghrelin in the progression of NAFLD to NASH through the amelioration of hepatic steatosis and inflammation after bariatric surgery in an experimental model of obesity as well as in patients with morbid obesity. Diet-induced obese rats developed hepatosteatosis and showed decreased circulating desacyl ghrelin without changes in acylated ghrelin concentrations. Sleeve gastrectomy and RYGB induced a dramatic reduction in desacyl ghrelin levels, whereas the acylated/desacyl ghrelin ratio was augmented in obese rats. In patients with morbid obesity and NAFLD, desacyl ghrelin levels were diminished, while circulating TNF-α and the acylated/desacyl ghrelin ratio were increased. Interestingly, six months after bariatric surgery, decreased acylated/desacyl ghrelin levels were found in morbidly obese patients. We demonstrated that bariatric surgery improved hepatic steatosis by reducing hepatic TG content and the lipogenic enzymes Mogat2 and Dgat1, and triggering AMPK-activated mitochondrial FFA β-oxidation and autophagy to a higher extent than caloric restriction in diet-induced obese rats. In addition, both sleeve gastrectomy and RYGB ameliorated hepatic inflammation, as evidenced by a decrease in portal and lobulillar CD68+ and apoptotic cells, proinflammatory JNK activation and a downregulation in the expression of inflammation-related genes (Crp, Tnf and Il6). In parallel, mitochondrial dysfunction was significantly attenuated after sleeve gastrectomy and RYGB via an increase in mitochondrial DNA amount as well as OXPHOS complexes I and II together with an amelioration of ER stress. Specifically, GRP78, spliced XBP-1, ATF4 and CHOP levels were reduced, as was phosphorylated eIF2α, following both bariatric surgical procedures. Our results show that the stimulation of primary rat hepatocytes with acylated and desacyl ghrelin significantly increased TG content, but also prompted AMPK-activated mitochondrial FFA β-oxidation and autophagy. Furthermore, acylated and desacyl ghrelin also inhibited palmitate-triggered inflammation and UPR induction through the downregulation of IRE1α, PERK and ATF6 expression as well as their downstream effectors, ATF4 and CHOP, and chaperone GRP78. In human HepG2 hepatocytes, acylated and desacyl ghrelin treatment reduced TNF-α-induced apoptosis, evidenced by lower caspase-8 and caspase-3 cleavage, as well as TUNEL-positive cells and pyroptosis, revealed by decreased caspase-1 activation and lower HMGB1 expression. Moreover, acylated ghelin suppressed TNF-α-activated hepatocyte autophagy, supported by decreased LC3B-II/I ratio and increased p62 accumulation via AMPK/mTOR, with acylated ghrelin being a protective factor against hepatocyte cell death. Thus, the decrease in the most abundant isoform, desacyl ghrelin, after bariatric surgery contributes to the reduction of lipogenesis, whereas the increased relative acylated ghrelin levels activate factors involved in mitochondrial FFA β-oxidation and autophagy as well as contribute to mitigate obesity-associated hepatic inflammation, mitochondrial dysfunction, ER stress and cell death, thereby ameliorating NAFL