IL-19 Contributes to the Development of Nonalcoholic Steatohepatitis by Altering Lipid Metabolism

Interleukin (IL)-19, a member of the IL-10 family, is an anti-inflammatory cytokine produced primarily by macrophages. Nonalcoholic steatohepatitis (NASH) is a disease that has progressed from nonalcoholic fatty liver disease (NAFLD) and is characterized by inflammation and fibrosis. We evaluated the functions of IL-19 in a NAFLD/NASH mouse model using a 60% high fat diet with 0.1% methionine, without choline, and with 2% cholesterol (CDAHFD). Wild-type (WT) and IL-19 gene-deficient (KO) mice were fed a CDAHFD or standard diet for 9 weeks. Liver injury, inflammation, and fibrosis induced by CDAHFD were significantly worse in IL-19 KO mice than in WT mice. IL-6, TNF-α, and TGF-β were significantly higher in IL-19 KO mice than in WT mice. As a mechanism using an in vitro experiment, palmitate-induced triglyceride and cholesterol contents were decreased by the addition of IL-19 in HepG2 cells. Furthermore, addition of IL-19 decreased the expression of fatty acid synthesis-related enzymes and increased ATP content in HepG2 cells. The action of IL-19 in vitro suppressed lipid metabolism. In conclusion, IL-19 may play an important role in the development of steatosis and fibrosis by directly regulating liver metabolism and may be a potential target for the treatment of liver diseases.


Introduction
Interleukin (IL)-19 is a member of the IL-10 family and is produced primarily by macrophages [1]. We have previously analyzed the role of IL-19 in inflammatory bowel disease and dermatitis. In disease model mice of Crohn's disease [2], ulcerative colitis [3,4], and contact hypersensitivity [5,6], we found that IL-19 gene-deficient (KO) mice showed an exacerbation of symptoms, and IL-19 acted as an inhibitor of colon and cutaneous inflammation. In addition to these reports, there are reports on the role of IL-19 in asthma, the central nervous system, and joints [7]. However, the role of IL-19 in the liver, including liver inflammation and chronic liver disease, has not been reported at all.
Nonalcoholic fatty liver disease (NAFLD) is a liver disorder that resembles alcoholic liver disease, despite the absence of an obvious drinking history [8]. The most important

In Vitro Steatosis Assay in HepG2 Cells
HepG2 cells were purchased from Riken Cell BANK (Ibaraki, Japan) and cultured in DMEM supplemented with 10% FBS and antibiotics. Palmitate-BSA was prepared according to the method of Joshi-Barve et al. [18]. Briefly, palmitate (Sigma Chemicals, Saint Louis, MI, USA) was mixed with 10% fatty acid-free BSA for 1-day at 37 • C and yielded 8 mM palmitate-BSA. Palmitate-BSA was treated to HepG2 cells for 48 h in the presence or absence of IL-19 (PeproTech, Cranbury, NJ, USA).

Aminotransferase and Lactose Dehydrogenase Levels
Activities of alanine aminotransferase (ALT) and aspartate aminotransferase (AST) in the serum and supernatants of cultured cells were immediately measured using a Transaminase CII test WAKO (Wako Pure Chemical, Osaka, Japan), as previously described [19]. Lactose dehydrogenase (LDH) release was examined using LDH-Glo™ Cytotoxicity Assay (Promega).

Liver Histology and Immunohistochemical Analysis
The liver was fixed with 10% neutral buffered formalin and embedded in paraffin blocks. Hematoxylin and eosin (H&E) staining was performed, as previously described [20]. Azan staining was performed to assess hepatic fibrosis. CD68 staining was performed to assess macrophage infiltration, as previously described [21]. The immunoreactivity of CD68 was detected using the DAB system. Images were captured using a VS120 Virtual Slide Microscope (Olympus Corporation, Tokyo, Japan). Liver histology was assessed by quantification using Image J.

RNA Isolation and Quantitative Real-Time PCR (QPCR)
Isolated liver tissues or cells were homogenated. Total RNA was isolated using Sepasol (Nacalai Tesque, Kyoto, Japan), and isolated RNA was used to synthesize complementary cDNA using SuperScript Reverse Transcriptase (Roche, Madison, WI, USA), as previously described [22]. Quantitative real-time PCR analysis based on the intercalation of SYBR Green (Toyobo, Osaka, Japan) were performed, as described previously [23]. Primer sequences used are summarized in Supplementary Table S1. A non-regulated housekeeping gene HPRT or GAPDH served as an internal control and was used to normalize for differences in input RNA.

Statistical Analysis
Liver weight, ALT/AST quantification, QPCR, TG and cholesterol contents, PPREluciferase and SIE-luciferase activities, and ATP and LDH contents were analyzed with one-way ANOVA for non-repeated measures to detect differences among groups. The differences between groups were determined using the Tukey-Kramer test. Other data were evaluated using the two-tailed Student's t-test (unpaired) to detect differences between 2 groups. A p value of less than 0.05 was considered statistically significant.

IL-19 Expression in the Kupffer Cells
We examined the IL-19 expression in the Kupffer cells isolated from WT mice because previous reports have shown that IL-19 is expressed in macrophages [1] and microglia [25]. As shown in Figure 1, we show that F4/80 high Kupffer cells in the liver expressed IL-19.

IL-19 Expression in the Kupffer Cells
We examined the IL-19 expression in the Kupffer cells isolated from WT mice because previous reports have shown that IL-19 is expressed in macrophages [1] and microglia [25]. As shown in Figure 1, we show that F4/80 high Kupffer cells in the liver expressed IL-19.

Body and Liver Weights
We next examined the body weights of mice fed a CDAHFD diet for 9 weeks. WT mice lost body weight until week 4 but then recovered. The body weight of IL-19 KO mice decreased in the same manner as WT mice up to week 4 but then recovered more slowly than WT mice. After week 8, the body weight difference between IL-19 KO and WT mice increased, and the body weight of IL-19 KO mice was significantly reduced at week 9 ( Figure 2A). We confirmed that the food intake in IL-19 KO mice was similar to that in WT mice ( Figure 2B). Both WT and IL-19 KO mice fed the CDAHFD diet had significantly increased liver weight than those fed the standard diet (SD) ( Figure 2C). Importantly, the liver weight of IL-19 KO mice was significantly reduced at week 9 ( Figure 2C). In contrast, both spleen and kidney weights were similar in WT and IL-19 KO mice ( Figure 2C).

ALT and AST
Blood analysis of liver profiles showed that levels of liver-damaging enzymes, such as ALT and AST, were significantly increased in WT and IL-19 KO mice fed the CDAHFD diet compared to those fed the SD diet ( Figure 2D). Only ALT was significantly increased in IL-19 KO mice compared to WT mice at week 9.

Body and Liver Weights
We next examined the body weights of mice fed a CDAHFD diet for 9 weeks. WT mice lost body weight until week 4 but then recovered. The body weight of IL-19 KO mice decreased in the same manner as WT mice up to week 4 but then recovered more slowly than WT mice. After week 8, the body weight difference between IL-19 KO and WT mice increased, and the body weight of IL-19 KO mice was significantly reduced at week 9 ( Figure 2A). We confirmed that the food intake in IL-19 KO mice was similar to that in WT mice ( Figure 2B). Both WT and IL-19 KO mice fed the CDAHFD diet had significantly increased liver weight than those fed the standard diet (SD) ( Figure 2C). Importantly, the liver weight of IL-19 KO mice was significantly reduced at week 9 ( Figure 2C). In contrast, both spleen and kidney weights were similar in WT and IL-19 KO mice ( Figure 2C). The changes in body weight, food intake, organ weights, and serum ALT and AST levels. WT (n = 15) and IL-19 KO (n = 15) mice were fed a CDAHFD diet for 9 weeks. Body weight (n = 15) (A) and food intake (n = 5) (B) were monitored weekly. (C) Liver weight, spleen weight, and kidney weight were determined at week 9 (n = 5). Liver weight was also measured in the SD-fed group (n = 3). (D) The ALT and AST levels in the serum were measured (n = 5). The ALT and AST levels in the SD group was also measured as a control group (n = 3). * p < 0.05 vs. WT. # p < 0.05 vs. SD.

Liver Histology
Histopathology data showed that WT mice fed a CDAHFD diet caused predominantly middle droplet steatosis and induced infiltration of inflammatory cells, as stained with H&E ( Figure 3A). Fibrosis is less noticeable in WT mice, as stained with Azan staining ( Figure 3B). In IL-19 KO mice fed a CDAHFD diet, the size of droplets was intermediate, similar to WT mice, but steatosis was significantly weaker than WT mice ( Figure 3A). The changes in body weight, food intake, organ weights, and serum ALT and AST levels. WT (n = 15) and IL-19 KO (n = 15) mice were fed a CDAHFD diet for 9 weeks. Body weight (n = 15) (A) and food intake (n = 5) (B) were monitored weekly. (C) Liver weight, spleen weight, and kidney weight were determined at week 9 (n = 5). Liver weight was also measured in the SD-fed group (n = 3). (D) The ALT and AST levels in the serum were measured (n = 5). The ALT and AST levels in the SD group was also measured as a control group (n = 3). * p < 0.05 vs. WT. # p < 0.05 vs. SD.

ALT and AST
Blood analysis of liver profiles showed that levels of liver-damaging enzymes, such as ALT and AST, were significantly increased in WT and IL-19 KO mice fed the CDAHFD diet compared to those fed the SD diet ( Figure 2D). Only ALT was significantly increased in IL-19 KO mice compared to WT mice at week 9.

Liver Histology
Histopathology data showed that WT mice fed a CDAHFD diet caused predominantly middle droplet steatosis and induced infiltration of inflammatory cells, as stained with H&E ( Figure 3A). Fibrosis is less noticeable in WT mice, as stained with Azan staining ( Figure 3B). In IL-19 KO mice fed a CDAHFD diet, the size of droplets was intermediate, similar to WT mice, but steatosis was significantly weaker than WT mice ( Figure 3A). In addition, fibrosis was significantly stronger in IL-19 KO mice than in WT mice ( Figure 3B). Infiltration of inflammatory cells was markedly higher in IL-19 KO mice than in WT mice. Infiltration of inflammatory cells was quantified by immunostaining with CD68, a macrophage marker. There were significantly more CD68-positive cells in IL-19 KO mice than in WT mice ( Figure 3C).

IL-19 Expression and Factors Involved in NASH Progression
We analyzed the expression of IL-19 in WT mice fed a CDAHFD diet for 9 weeks. Immunostaining of liver tissues showed that IL-19 in F4/80 high Kupffer cells was clearly expressed, and the specificity of the antibody was also confirmed, as IL-19 signal was not detected in IL-19 KO mice ( Figure 4A). The QPCR results showed that feeding CDAHFD significantly increased the expression of IL-19 in WT mice compared to SD ( Figure 4B).

IL-19 Expression and Factors Involved in NASH Progression
We analyzed the expression of IL-19 in WT mice fed a CDAHFD diet for 9 weeks. Immunostaining of liver tissues showed that IL-19 in F4/80 high Kupffer cells was clearly expressed, and the specificity of the antibody was also confirmed, as IL-19 signal was not detected in IL-19 KO mice ( Figure 4A). The QPCR results showed that feeding CDAHFD significantly increased the expression of IL-19 in WT mice compared to SD ( Figure 4B).  IL-6 and TNF-α were measured as factors involved in inflammation. Both factors were significantly increased in WT and IL-19 KO mice fed the CDAHFD diet compared to those fed the SD diet. In addition, both factors were significantly increased in IL-19 KO mice compared to WT mice ( Figure 4C). TGF-β were measured as an important factor involved in fibrosis. TGF-β was significantly increased in IL-19 KO mice, but not WT mice, for those fed the CDAHFD diet compared to those fed the SD diet. In addition, TGF-β was significantly increased in IL-19 KO mice compared to WT mice ( Figure 4C).

Effect of IL-19 on In Vitro Steatosis Model in HepG2 Cells
We have shown that liver fibrosis is exacerbated in association with IL-19 gene deletion. In order to clarify the mechanism of action of IL-19, we conducted further experiments using in vitro steatosis models in HepG2 cells. Treatment of HepG2 cells with palmitate resulted in an accumulation of TG and cholesterol ( Figure 5). Treatment with IL-19 abolished the accumulation of TG ( Figure 5E). When quantified, TG content was significantly KO mice fed an SD (n = 4) or CDAHFD (n = 7) diet for 9 weeks. # p < 0.05, ## p < 0.01 vs. SD. * p < 0.05, ** p < 0.01 vs. WT. IL-6 and TNF-α were measured as factors involved in inflammation. Both factors were significantly increased in WT and IL-19 KO mice fed the CDAHFD diet compared to those fed the SD diet. In addition, both factors were significantly increased in IL-19 KO mice compared to WT mice ( Figure 4C). TGF-β were measured as an important factor involved in fibrosis. TGF-β was significantly increased in IL-19 KO mice, but not WT mice, for those fed the CDAHFD diet compared to those fed the SD diet. In addition, TGF-β was significantly increased in IL-19 KO mice compared to WT mice ( Figure 4C).

Effect of IL-19 on In Vitro Steatosis Model in HepG2 Cells
We have shown that liver fibrosis is exacerbated in association with IL-19 gene deletion. In order to clarify the mechanism of action of IL-19, we conducted further experiments using in vitro steatosis models in HepG2 cells. Treatment of HepG2 cells with palmitate resulted in an accumulation of TG and cholesterol ( Figure 5). Treatment with IL-19 abolished the accumulation of TG ( Figure 5E). When quantified, TG content was significantly increased by palmitate-BSA (Palmitate), and the palmitate-treated increase was significantly decreased by IL-19 treatment in a concentration-dependent manner ( Figure 5A,C). As well as the TG content, cholesterol content was significantly increased by palmitate and palmitate-treated increase was significantly decreased by IL-19 treatment in a concentration-dependent manner ( Figure 5B,D). as the TG content, cholesterol content was significantly increased by palmitate and palmitate-treated increase was significantly decreased by IL-19 treatment in a concentrationdependent manner ( Figure 5B,D). We have shown that IL-19 seems to be involved in fat digestion. Next, the changes in factors and enzymes involved in fatty acid synthesis and other processes were analyzed by Western blotting and QPCR. IL-19 activates STAT3 phosphorylation among the Jak-STAT pathway [7]. We found that phosphorylation of STAT3 was enhanced by IL-19 treatment in HepG2 cells ( Figure 6A, left). We also tested that this phosphorylation of STAT3 is cancelled by the JAK1/2 inhibitor Ruxolitinib (data not shown). Interestingly, we found that AMPK phosphorylation was enhanced by IL-19 treatment (Figure 6A, right). By We have shown that IL-19 seems to be involved in fat digestion. Next, the changes in factors and enzymes involved in fatty acid synthesis and other processes were analyzed by Western blotting and QPCR. IL-19 activates STAT3 phosphorylation among the Jak-STAT pathway [7]. We found that phosphorylation of STAT3 was enhanced by IL-19 treatment in HepG2 cells ( Figure 6A, left). We also tested that this phosphorylation of STAT3 is cancelled by the JAK1/2 inhibitor Ruxolitinib (data not shown). Interestingly, we found that AMPK phosphorylation was enhanced by IL-19 treatment ( Figure 6A, right). By QPCR, IL-19 treatment resulted in significant suppression of acetyl-CoA carboxylase (ACC) 1, fatty acid synthase (FASN), stearoyl-CoA desaturase (SCD) 1 and 5, sterol regulatory element-binding protein (SREBP)-1c and 2 ( Figure 6B). In contrast, there were no clear changes in ATP citrate lyase (ACLY) and CD36.  The mRNA expression levels of each factor were evaluated by QPCR (n = 6). * p < 0.05, ** p < 0.01 vs. control.
Since IL-19 was found to affect fatty acid synthesis, we therefore examined the effect of IL-19 on ATP production by β-oxidation. As shown in Figure 7A, IL-19 showed a significant increase in ATP production at 24 h in HepG2 cells, although suppression was observed at 6 h. We then proceeded to analyze the activity of PPARα, which are important in lipid metabolism in the liver [26]. PPRE-luciferase activity in HepG2 cells was increased at 24 h by IL-19 treatment, although suppression was observed at 2 h ( Figure 7B). In addition, IL-19 significantly increased PPRE-luciferase activity in a concentration-dependent manner ( Figure 7C). Thus, IL-19 can regulate β-oxidation via PPAR in HepG2 cells. Moreover, we proceeded to analyze the activity of SIE, which is a site of transcriptional regulation via STAT3. SIE-luciferase activity in HepG2 cells was persistently elevated from 2 to 24 h after IL-19 treatment ( Figure 7D). Since IL-19 was found to affect fatty acid synthesis, we therefore examined the effect of IL-19 on ATP production by β-oxidation. As shown in Figure 7A, IL-19 showed a significant increase in ATP production at 24 h in HepG2 cells, although suppression was observed at 6 h. We then proceeded to analyze the activity of PPARα, which are important in lipid metabolism in the liver [26]. PPRE-luciferase activity in HepG2 cells was increased at 24 h by IL-19 treatment, although suppression was observed at 2 h ( Figure 7B). In addition, IL-19 significantly increased PPRE-luciferase activity in a concentration-dependent manner ( Figure 7C). Thus, IL-19 can regulate β-oxidation via PPAR in HepG2 cells. Moreover, we proceeded to analyze the activity of SIE, which is a site of transcriptional regulation via STAT3. SIE-luciferase activity in HepG2 cells was persistently elevated from 2 to 24 h after IL-19 treatment ( Figure 7D).

Effect of IL-19 on the Response in Hepatocyte
In addition to the analysis in HepG2 cells, we used the primary hepatocytes to analyze some responses caused by IL-19 treatment. Primary hepatocytes were treated with palmitate and analyzed in a similar experiment as HepG2 cells. Similar to HepG2 cells, hepato-

Effect of IL-19 on the Response in Hepatocyte
In addition to the analysis in HepG2 cells, we used the primary hepatocytes to analyze some responses caused by IL-19 treatment. Primary hepatocytes were treated with palmitate and analyzed in a similar experiment as HepG2 cells. Similar to HepG2 cells, hepatocytes showed increased phosphorylation of STAT3 upon IL-19 treatment ( Figure 8A). In addition, IL-19 significantly suppressed the increase of ALT and LDH by palmitate in hepatocytes. The IL-19-treated decreases of ALT and LDH were cancelled by the JAK1/2 inhibitor Ruxolitinib ( Figure 8B). IL-19 significantly suppressed the TG content in hepatocytes ( Figure 8B).

Discussion
The diet used in this study was a custom-made diet with cholesterol added to a com-

Discussion
The diet used in this study was a custom-made diet with cholesterol added to a commercial diet (#A06071302), and this is the first report of its use in mice. After 9 weeks of feeding, WT mice showed increased liver weight, increased ALT, increased pro-inflammatory cytokines, and marked steatosis, while fibrosis was weak. These results indicate that the WT mice phenotype is in NAFL or the early stage of NASH. Importantly, IL-19 KO mice showed marked fibrosis in addition to increased liver weight, increased ALT, increased pro-inflammatory cytokines, and marked steatosis. On the other hand, AST did not show any significant difference. Due to the high absolute amount of AST, AST levels rise rapidly in acute hepatitis, including rapid necrosis of hepatocytes. As AST has a shorter half-life than ALT, ALT levels are higher in inactive chronic hepatitis. Therefore, since the present analysis is based on a chronic model, it is reasonable to suggest that there was a significant increase in ALT and no difference in AST. A further detailed comparison of the results of WT and IL-19 KO mice showed that liver weight and steatosis were milder in IL-19 KO mice than in WT mice, and increases in ALT and pro-inflammatory cytokines were worse in IL-19 KO mice than in WT mice. Generally, inflammatory cytokines and fibrosis increase as progression from NAFL to NASH in NAFLD in humans, but the degree of steatosis decreases and liver weight is reduced [27]. Thus, these results indicate that the IL-19 KO mice phenotype has progressed to NASH.
This custom-made diet first caused steatosis due to fat accumulation, followed by fibrosis. The action of IL-19 may control steatosis formation, fibrosis formation, or both. Although there are limitations to evaluating only one endpoint, the data of CD68-positive macrophages around oily cells are interesting. Since CD68-positive macrophages accumulated around fat droplets, IL-19 may be involved in metabolic abnormalities of hepatocytes. Since hepatocytes used in vitro were apparent to non-IL-19 producing cells, our results indicate that the cells producing IL-19 are Kupffer cells, the cells on which IL-19 produced in Kupffer cells acts are hepatocytes, and the result of the action is suppression of lipid metabolism. In vitro models of steatosis using HepG2 cells revealed novel effects of IL-19. IL-19 inhibited ACC1 and FASN, indicating that IL-19 inhibits fatty acid synthesis in the cells [28]. As a result, IL-19 contributes to a shift in lipid metabolism from fatty acid synthesis to promotion of β-oxidation. Furthermore, IL-19 suppressed SCD1 and SCD5, which means that saturated fatty acids do not become unsaturated fatty acids and the synthesis of triglycerides is suppressed [29]. These data suggest that IL-19 enhances β-oxidation and the digestion of fat. In addition, IL-19 inhibited cholesterol contents and suppressed SREBP-1c and SREBP-2 that stimulate HMG-CoA reductase, followed by cholesterol synthesis [30]. In addition, SREBPs promote the regulation of ACLY, ACC1, FASN, and SCD1, which are involved in fatty acid synthesis [31]. The data supporting this possibility is that IL-19 increased ATP production. The liver contains PPARα, which are important factors involved in fat metabolism. One of the key findings of this new study is that IL-19 increased PPRE-luciferase activity. When PPARα is stimulated, fatty acids are β-oxidized and become an energy source, inhibiting the synthesis of TG and VLDL [32]. We elucidated the signaling pathways that lead to the suppression of fatty acid synthesis and TG levels. First, IL-19 treatment resulted in the phosphorylation of STAT3 and increased SIE-luciferase activity. Importantly, IL-19 treatment resulted in the phosphorylation of AMPK. In the liver, AMPK inhibits fatty acid synthesis by suppressing ACC1 and SREBPs and produces energy by promoting β-oxidation [33]. Among factors and enzymes involved in the fatty acid synthesis measured, no significant change was observed for ACLY and CD36. A possible reason is that AMPK inhibits ACC but not ALCY. Although SREBPs regulated ACLY and ACC, the effect of IL-19 is stronger on ACC because AMPK is upstream of SREBPs. CD36 is a transporter of fatty acids. There was no change in CD36, suggesting that IL-19 does not affect the uptake of fatty acids. In summary, IL-19 inhibited lipogenesis in the liver.
In the present study, we found that IL-19 KO mice progressed from NAFLD to NASH. Therefore, IL-19 may play an important role in inhibiting the development of NAFLD. The data supporting this possibility is supported by in vitro results. The pathophysiology of NASH is very complex and many cell types within the liver are involved at multiple levels [34]. In whole liver, the factors measured by QPCR were IL-6, TNF-α, and TGF-β. We now focus on TGF-β. TGF-β is a widely known promoter of fibrosis in the liver [35]. Hepatic stellate cells are a key player in the development of fibrosis and are a major producer of extracellular matrices, such as collagen [36]. Hepatic stellate cells express TGF-β receptors and collagen productions are activated by TGF-β [37]. Macrophages in the liver can be divided into two types: resident Kupffer cells and bone marrowderived macrophages, which infiltrate in response to inflammation or injury. These resident Kupffer cells and bone marrow-derived macrophages produce TGF-β, which directly activates the hepatic stellate cells [38]. In addition, hepatocytes also produce TGF-β in small amounts. Therefore, it is suggested that either or all of Kupffer cells, bone marrowderived macrophages, or hepatocytes may have increased TGF-β productions in response to IL-19 gene deletion. We next turn our attention to IL-6 and TNF-α. For IL-6 and TNFα, Kupffer cells and bone marrow-derived macrophages are the major producers. The produced IL-6 and TNF-α first act on hepatocytes to promote TGF-β productions and also directly activate the hepatic stellate cells [39]. With the same considerations as TGF-β, it is suggested that Kupffer cells and bone marrow-derived macrophages have increased IL-6 and TNF-α productions in response to IL-19 gene deletion. In our previous report, bone marrow-derived macrophages from IL-19 KO mice produced significantly higher levels of IL-6 and TNF-α than WT mice after stimulation with lipopolysaccharide [4]. Therefore, it seems likely that IL-19 deficiency also induced an increase in pro-inflammatory cytokines in Kupffer cells and macrophages in the liver tissue. In this study, we clarified the role of IL-19 on lipid metabolism. Future analysis of the effect of IL-19 on hepatic stellate cells will be necessary to focus on fibrosis formation.

Conclusions
Despite the extensive role of IL-19 in various organs of the body, its role in liver diseases, especially in chronic liver diseases such as fatty liver and NASH, is completely unexplored. In conclusion, this is the first report that IL-19 KO mice exacerbated NASH progression, IL-19 inhibited steatosis and fibrosis by directly regulating liver metabolism, and IL-19 plays an important role in liver disease.

Conflicts of Interest:
The authors declare no conflict of interest.