Rapamycin attenuates liver injury caused by vinyl chloride metabolite chloroethanol and lipopolysaccharide in mice

Western diet unmasks transient low-level vinyl chloride-induced tumorigenesis; potential role of the (epi-)transcriptome

BACKGROUND & AIMS

Vinyl chloride (VC) monomer is a volatile organic compound commonly used in industry. At high exposure levels, VC causes liver cancer and toxicant-associated steatohepatitis. However, lower exposure levels (i.e., sub-regulatory exposure limits) that do not directly damage the liver, enhance injury caused by Western diet (WD). It is still unknown if the long-term impact of transient low-concentration VC enhances the risk of liver cancer development. This is especially a concern given that fatty liver disease is in and of itself a risk factor for the development of liver cancer.

METHODS

C57Bl/6 J mice were fed WD or control diet (CD) for 1 year. During the first 12 weeks of feeding only, mice were also exposed to VC via inhalation at sub-regulatory limit concentrations (<1 ppm) or air for 6 h/day, 5 days/week.

RESULTS

Feeding WD for 1 year caused significant hepatic injury, which was exacerbated by VC. Additionally, VC increased the number of tumors which ranged from moderately to poorly differentiated hepatocellular carcinoma (HCC). Transcriptomic analysis demonstrated VC-induced changes in metabolic but also ribosomal processes. Epitranscriptomic analysis showed a VC-induced shift of the modification pattern that has been associated with metabolic disease, mitochondrial dysfunction, and cancer.

CONCLUSIONS

These data indicate that VC sensitizes the liver to other stressors (e.g., WD), resulting in enhanced tumorigenesis. These data raise concerns about potential interactions between VC exposure and WD. It also emphasizes that current safety restrictions may be insufficient to account for other factors that can influence hepatotoxicity.

Hepatic stellate cell autophagy inhibits extracellular vesicle release to attenuate liver fibrosis

BACKGROUND & AIMS

Autophagy plays a crucial role in hepatic homeostasis and its deregulation has been associated with chronic liver disease. However, the effect of autophagy on the release of fibrogenic extracellular vesicles (EVs) by platelet-derived growth factor (PDGF)-stimulated hepatic stellate cells (HSCs) remains unknown. Herein, we aimed to elucidate the role of autophagy, specifically relating to fibrogenic EV release, in fibrosis.

METHODS

In vitro experiments were conducted in primary human and murine HSCs as well as LX2 cells. Small EVs were purified by differential ultracentrifugation. Carbon tetrachloride (CCl₄) or bile duct ligation (BDL) were used to induce fibrosis in our mouse model. Liver lysates from patients with cirrhosis or healthy controls were compared by RNA sequencing.

RESULTS

In vitro, PDGF and its downstream molecule SHP2 (Src homology 2-containing protein tyrosine phosphatase 2) inhibited autophagy and increased HSC-derived EV release. We used this PDGF/SHP2 model to further investigate how autophagy affects fibrogenic EV release. RNA sequencing identified an mTOR (mammalian target of rapamycin) signaling molecule that was regulated by SHP2 and PDGF. Disruption of mTOR signaling abolished PDGF-dependent EV release. Activation of mTOR signaling induced the release of multivesicular body-derived exosomes (by inhibiting autophagy) and microvesicles (by activating ROCK1 signaling). These mTOR-dependent EVs promoted in vitro HSC migration. To assess the importance of this mechanism in vivo, SHP2 was selectively deleted in HSCs, which attenuated CCl_4- or BDL-induced liver fibrosis. Furthermore, in the CCl₄ model, mice receiving circulating EVs derived from mice with HSC-specific Shp2 deletion had less fibrosis than mice receiving EVs from control mice. Correspondingly, SHP2 was upregulated in patients with liver cirrhosis.

CONCLUSION

These results demonstrate that autophagy in HSCs attenuates liver fibrosis by inhibiting the release of fibrogenic EVs.

LAY SUMMARY

During liver fibrosis and cirrhosis, activated hepatic stellate cells (HSCs) are the key cell type responsible for fibrotic tissue deposition. Recently, we demonstrated that activated HSCs release nano-sized vesicles enriched with fibrogenic proteins. In the current study, we unveil the mechanism by which these fibrogenic vesicles are released, moving a step closer to the long-term goal of therapeutically targeting this process.

Adipose tissue-liver crosstalk during pathologic changes caused by vinyl chloride metabolites in mice

Volatile organic compounds (VOCs), such as vinyl chloride (VC), can be directly toxic at high concentrations. However, we have shown that 'nontoxic' exposures to VC and its metabolite chloroethanol (CE) enhances experimental non-alcoholic fatty liver disease (NAFLD), suggesting an unpredicted interaction. Importantly, VOC exposure has been identified as a potential risk factor for the development of obesity and its sequelae in humans. As there is a known axis between adipose and hepatic tissue in NAFLD, the impact of CE on white adipose tissue (WAT) inflammation and lipolysis was investigated. Mice were administered CE (or vehicle) once, after 10 weeks of being fed high-fat or low-fat diet (LFD). CE significantly enhanced hepatic steatosis and inflammation caused by HFD. HFD significantly increased the size of epididymal fat pads, which was enhanced by CE. The relative size of adipocyte lipid droplets increased by HFD + CE, which was also correlated with increased expression of lipid-associated proteins (e.g., PLINs). CE also enhanced HFD-induced indices of WAT inflammation, and ER stress. Hepatic-derived circulating FGF21, a major modulator of WAT lipolysis, which is hypothesized to thereby regulate hepatic steatosis, was significantly increased by CE in animals fed HFD. Taken together these data support the hypothesis that environmental toxicant exposure can exacerbate the severity of NAFLD/NASH, involving the liver-adipose axis in this process. Specifically, CE enhances local inflammation and alters lipid metabolism and WAT-mediated hepatic steatosis due to changes in WAT lipolysis.