Obesity Drives STAT-1-Dependent NASH and STAT-3-Dependent HCC

摘要

class="head no_bottom_margin" id="sec1title">IntroductionPrimary liver cancer is one of the world’s deadliest cancers and the third most common cause of cancer death. Hepatocellular carcinoma (HCC) represents 85%–90% of primary liver cancers and is refractory to nearly all currently available anti-cancer therapies. Over the last 20 years, the incidence of HCC in developed countries has been increasing, doubling in the United States (). The obesity epidemic and the accompanying development of non-alcoholic fatty liver disease (NALFD), evident in >85% of all obese individuals, are thought to be key contributors to the development of HCC (). Indeed, obesity, the metabolic syndrome, and NAFLD account for 30%–40% of the increase in HCC in developed countries, and the risk of mortality from HCC in men with a body mass index (BMI) of 35–40 kg/m² is 4.5-fold times greater than in patients with normal body weight ().NAFLD has an estimated worldwide prevalence of 25.2% () and encompasses a broad spectrum of liver conditions ranging from simple steatosis, or non-alcoholic fatty liver (NAFL), to the more severe and progressive disease, non-alcoholic steatohepatitis (NASH). NASH is characterized by overt hepatic inflammation and tissue damage and ensuing reparative responses that result in fibrosis and ultimately cirrhosis, the principal cause of liver-related morbidity and mortality (). Obesity-associated NASH is currently the third leading cause for liver transplantation and is expected to surpass hepatitis C as the principal cause for liver transplantation in the developed world ().The mechanisms that underpin the progression from NAFL to NASH are complex and involve multiple insults and contributions from genetic modifiers that influence disease severity and progression (). Lipotoxicity and oxidative stress are considered central to the transition to NASH and fibrosis (). Several processes produce reactive oxygen species (ROS), including hepatic anaplerotic/cataplerotic reactions heightened in obesity, fatty acid oxidation, endoplasmic reticulum (ER) stress, inflammation, and the induction of ROS-producing NAD(P)H oxidases (, , , , , ). Such oxidative stress is thought to promote hepatocyte cell death and the ensuing inflammatory and reparative responses that lead to fibrosis and if unresolved, cirrhosis (, href="#bib36" rid="bib36" class=" bibr popnode">Ringelhan et al., 2018). ROS have been detected in liver biopsies from patients with chronic hepatitis (href="#bib47" rid="bib47" class=" bibr popnode">Tanaka et al., 2008) and in the liver parenchyma in rodent models of obesity (href="#bib14" rid="bib14" class=" bibr popnode">Gurzov et al., 2014, href="#bib32" rid="bib32" class=" bibr popnode">Park et al., 2010). Moreover, genetic studies in mice provide evidence for the increased ROS contributing to NASH and the genesis of liver cancer (href="#bib11" rid="bib11" class=" bibr popnode">Elchuri et al., 2005, href="#bib24" rid="bib24" class=" bibr popnode">Luedde et al., 2007, href="#bib30" rid="bib30" class=" bibr popnode">Neumann et al., 2003). In rodents, drugs that attenuate ROS production prevent NASH (href="#bib33" rid="bib33" class=" bibr popnode">Perry et al., 2015). At least one mechanism by which ROS may drive disease progression is through the induction of protein tyrosine kinase (PTK) signaling by oxidizing and inactivating protein tyrosine phosphatases (PTPs) (href="#bib49" rid="bib49" class=" bibr popnode">Tiganis, 2011). Our previous studies have shown that hepatic PTPs can be extensively oxidized in the livers of obese mice with NAFL (href="#bib14" rid="bib14" class=" bibr popnode">Gurzov et al., 2014), but the extent to which this might be pertinent to NASH remains unknown.A key feature of NASH is the activation of resident immune cells, in particular Kupffer cells, and the recruitment of macrophages, B cells, natural killer (NK) cells, and CD4⁺ and CD8⁺ T cells that contribute to the inflammation and the persistent cycle of tissue damage and repair (href="#bib13" rid="bib13" class=" bibr popnode">Font-Burgada et al., 2016, href="#bib36" rid="bib36" class=" bibr popnode">Ringelhan et al., 2018). The importance of liver inflammatory macrophages is underscored by studies showing that tumor necrosis factor (TNF) from macrophages is essential for NASH and fibrosis in MUP-Upa mice that exhibit increased steatosis and progress to NASH and fibrosis and HCC when fed a high-fat obesity-promoting diet (href="#bib28" rid="bib28" class=" bibr popnode">Nakagawa et al., 2014). The importance of infiltrating T cells is illustrated by the reliance on antigen presentation via MHC class I and the recruitment of activated CD8⁺ T cells for the development of NASH and pericellular fibrosis in mice fed a choline-deficient high fat diet (href="#bib55" rid="bib55" class=" bibr popnode">Wolf et al., 2014). The molecular mechanisms driving the recruitment and activation of immune cells contributing to NASH pathogenesis have remained unresolved.HCC can develop in a subset of obese NASH patients, but the precise mechanisms that give rise to HCC in some NASH patients and not others remain unresolved (href="#bib13" rid="bib13" class=" bibr popnode">Font-Burgada et al., 2016). Moreover, while HCC is typically accompanied by cirrhosis or severe fibrosis (href="#bib13" rid="bib13" class=" bibr popnode">Font-Burgada et al., 2016), there are a growing number of NAFLD patients without cirrhosis or advanced fibrosis that develop HCC (href="#bib1" rid="bib1" class=" bibr popnode">Alexander et al., 2013, href="#bib5" rid="bib5" class=" bibr popnode">Baffy et al., 2012, href="#bib27" rid="bib27" class=" bibr popnode">Mittal et al., 2015, href="#bib31" rid="bib31" class=" bibr popnode">Paradis et al., 2009, href="#bib35" rid="bib35" class=" bibr popnode">Rahman et al., 2013, href="#bib46" rid="bib46" class=" bibr popnode">Takuma and Nouso, 2010, href="#bib60" rid="bib60" class=" bibr popnode">Younes and Bugianesi, 2018). How HCC develops under these disparate conditions remains unclear. Over 28,000 somatic mutations have been identified in HCC (href="#bib39" rid="bib39" class=" bibr popnode">Schulze et al., 2015). These mutations influence the activation of tumor-promoting signaling pathways, including interleukin (IL)-6 and/or Janus-activated kinase (JAK)-signal transducer and activator of transcription (STAT) signaling pathways (href="#bib39" rid="bib39" class=" bibr popnode">Schulze et al., 2015). STAT-3 signaling is particularly important in driving the transformation of tumor progenitors and the development of HCC in rodents (href="#bib15" rid="bib15" class=" bibr popnode">He et al., 2010, href="#bib16" rid="bib16" class=" bibr popnode">He et al., 2013, href="#bib29" rid="bib29" class=" bibr popnode">Naugler et al., 2007, href="#bib32" rid="bib32" class=" bibr popnode">Park et al., 2010). Furthermore, STAT-3 is activated in the majority of human HCCs, positively correlating with tumor aggressiveness and negatively correlating with prognosis (href="#bib9" rid="bib9" class=" bibr popnode">Calvisi et al., 2006, href="#bib15" rid="bib15" class=" bibr popnode">He et al., 2010).We report that the inactivation of negative regulators of STAT-1 and STAT-3 signaling in obesity can drive the development of NASH and HCC. The oxidation and inactivation of the STAT-1 and STAT-3 phosphatase TCPTP and heightened STAT-1 and STAT-3 signaling were evident in NASH in both obese mice and humans. While heightened STAT-1 signaling was responsible for the recruitment of activated cytotoxic T cells and the ensuing NASH and fibrosis, this was not essential for HCC. Rather, TCPTP inactivation promoted HCC in obesity via STAT-3, independent of T cell recruitment and NASH and fibrosis. Our results shed light on mechanisms that may underpin the growing incidence of HCC in non-cirrhotic livers in the setting of NAFLD.