Study On Structure And Function Of IL-22 As Well As Its Roles In Alcoholic Liver Diseases And Fulminant Hepatic Failure

Interleukin-22 (IL-22) is a member of the Interleukin-10 (IL-10) cytokine family and its sequence has 22% identity with human IL-10. However, unlike traditional"interleukin", IL-22 seems to be a novel type of immune mediator which is produced by activated Th22, Th1 and Th17 cells as well as natural killer cells and ILC cells. IL-22 although is produced by the above immune cells does not affect these cells. The main target of IL-22 is the parenchymal cells on several tissues including skin, liver pancreas and small intestine, where it favors the antimicrobial defense, regeneration, and protection against damage. In the epidermal inflammation such as psoriasis, IL-22 was an important pro-inflammatory mediator which played a critical role in the development of diseases. In contrast, IL-22 was demonstrated to be a protective factor in the Crohn's disease, ulcerative colitis and experimental autoimmune myocarditis (EAM). Current studies suggested that the dual nature of IL-22 likely depended on the inflammatory context, which included the kind of organ and of the presence of other cytokines. In the liver, IL-22 seems to be a survival factor for hepatocyte. Several liver disease models including toxins-induced hepatitis and hepatectomy model all demonstrated that IL-22 could increase the survival and regeneration of hepatocyte. However, the hepatoprotetive mechanisms of IL-22 still remain obscure.In this study, relationship between structure and function of IL-22 as well as its roles in the common liver diseases were thoroughly demonstrated. Based on the known crystal structure of IL-22, we used Homology Program (Insight II 2000 Package, Octane2 Graphics workstations) to simulate the extracellular three-dimensional structure of IL-22. Through analyzing the surface electricity and accessible surface area of above-mentioned domain and structural characteristics of cytokines, we anticipated that the potential receptor binding domain of IL-22 located at the N terminal of loopAB (Asn67–Val72). We constructed a series of IL-22 mutants by point mutation towards the potential receptor binding domain. By analyzing the promoting cell proliferation activity and STAT3 transcriptional activity of these IL-22 mutants, we found that the change of the region affected its functions. Combined with computer modeling and biological functions analysis, we speculated that the region might be the potential receptor binding domain of IL-22.Several toxin-induced experimental hepatitis models have demonstrated IL-22 to be a protective factor for hepatocyte. However, since various liver diseases have different pathophysiological features, which role will IL-22 play in other liver diseases and what is its corresponding mechanism? Depth study on these issues will help to expand the knowledge on functions and possible mechanisms of IL-22.Alcohol abuse is a global problem, each year about 3.2% of global deaths is related to alcohol. Alcoholic liver disease (ALD) is one of the major medical complications of alcohol abuse, and its incidence in our country is just below the viral hepatitis. Although the studies on ALD have been conducted for several decades, there are still no effective therapeutic drugs. We utilized alcoholic hepatitis in rats and acute alcohol-induced liver injury in mice to investigate the role of IL-22 in ALD respectively. Alcohol, fat diet and carbonyl iron were jointly applied to induce alcoholic hepatitis in rat. The results showed that multiple factors lead to continuously increased serum transaminase levels and serious histopathological changes. On 32 weeks, typical indications for alcoholic hepatitis including ballooned hepatocytes, neutrophils infiltration and formation of Mallory bodies were observed. Subsequently, this model was used to evaluate the hepatoprotective function of IL-22, and we found that IL-22 could significantly lower serum transaminase activities, total cholesterol and triglyceride levels and improve pathological changes of liver tissue. Moreover, during the course of treatment, we did not observe significant changes of general condition and body weight in rats. In addition, the results from acute alcohol-induced liver injury in mice indicated that acute alcohol administration caused prominent hepatic microvesicular steatosis and elevation of serum transaminase activities, induced a significant decrease in hepatic glutathione in conjunction with enhanced lipid peroxidation, and increased hepatocyte apoptosis as well as hepatic TNF-αproduction. IL-22 treatment attenuated these adverse changes induced by acute alcohol administration.Fulminant hepatic failure (FHF), induced by bacteria, viral hepatitis, alcohol and other hepatotoxic drugs, is a dramatic clinical syndrome, which remains an extremely poor prognosis and high mortality due to lack of effective preventives and therapies. D-galactosamine (GalN)/lipopolysaccharide (LPS)-induced acute lethal liver injury in mice has been widely used as experimental animal model to investigate the underlying mechanisms of clinical fulminant hepatic failure. This study examined the hepatoprotective effects of IL-22 against FHF and explored its potential mechanisms. Results showed that IL-22 pretreatment remarkably decreased the death rate, suppressed the elevation of serum ALT levels and improved histopathologic changes. In addition, IL-22 could also lessen hepatic oxidative stress, inhibit liver apoptosis and decrease the serum TNF-αlevel. And the hepatoprotective mechanisms may be related to the hepatic STAT3 activation and subsequent induction of Bcl-xl, HO-1 and Ref-1.These data indicated the protective effect of IL-22 could be due to attenuation of oxidative stress, anti-inflammation and inhibition of hepatocyte apoptosis. To further investigate the mechanisms behind the hepatoprotective effect of IL-22, we analyzed the effects of IL-22 on the apoptosis induced by TNF/ActD in HL7702 cell. The results showed that IL-22 could inhibit the apoptosis of HL7702 cells induced by TNF/ActD and the mechanism may be related to the phosphorylation of STAT3 and expression of Bcl-xL and HO-1.In summary, we used a computer homology modeling analysis to anticipate the potential receptor binding domain of IL-22. By analyzing the promoting cell proliferation activity and STAT3 transcriptional activity of IL-22 mutants, we found that the region might be the potential receptor binding domain of IL-22. In addition, we used three different models of liver diseases to investigate the hepatoprotective effects of IL-22 and found that IL-22 could protect against the above liver injury. And its hepatoprotective effects may be related to anti-inflammation, attenuation of oxidative stress and inhibition of hepatocyte apoptosis. Due to its hepatoprotective functions and limited targets, IL-22 may be an ideal therapeutic agent in the treatment of liver damage.