The different expression of caspase-1 in HBV-related liver disease and acts as a biomarker for acute-on-chronic liver failure

Worldwide, 257 million persons, or 3.5% of the population, were living with chronic HBV infection in 2015. The African and Western Pacific regions accounted for 68% of those infected [1]. The overall infection rate of the Chinese population is 6.1%, accounting for approximately 1/3 of HBV cases worldwide [2]. Decompensated liver cirrhosis (LC), liver failure and hepatocellular carcinoma (HCC) caused by HBV infection cause approximately 900,000 deaths per year [3]. Acute exacerbation of CHB infection is not uncommon, with a cumulative incidence of 10–30% every year [4]. Acute-on-chronic liver failure (ACLF) is a common pattern of end-stage liver disease that occurs frequently in patients with chronic hepatitis B infection, causing a large burden of disease as it progresses rapidly [5]. Since the treatment options are limited, the outcome is generally poor. Liver transplantation is sometimes used in ACLF patients, but this procedure is costly and has variable success rates. Early intervention can improve survival, and the recognition of this effect has creased new demand for early diagnostic and prognostic markers.

The inflammasomes, which are multiprotein complexes involved in the assembly of pattern recognition receptors (PRRs) in the cytoplasm, are an important part of the natural immune system. The inflammasomes can recognize pathogen-associated molecular patterns (PAMPs) and host-derived risk signalling molecules (DAMPs), recruiting and activating the pro-inflammatory protein cysteinyl aspartate specific protease (caspase)-1. Activated caspase-1 cleaves precursors of IL-1β and IL-18, producing corresponding mature cytokines and pyroptosis; pyroptosis, in turn, leads to cell lysis and the release of cytoplasmic contents to the outside of the cell, causing inflammation [6]. Therefore, caspase-1 is a key inflammatory mediator driving the host response to infection, injury and disease.

Caspase-1 plays an important role in the pathogenesis of various liver diseases in vivo or in vitro, not only as an inflammatory mediator but also as a hepatocyte apoptosis mediator. The activation of caspase-1 is strongly involved in Fas-mediated apoptosis and TNF-α-induced hepatocyte injury [7, 8], and inhibition of caspase-1 activity substantially suppresses Fas-induced death [9]. Necrotic liver cells cause eosinophil pyroptosis (a programmed inflammatory form of cell death that has been reported to occur in macrophages and DCs and CD4+ cells. IL-1β and IL-18 secretion, degranulation, and cell death can be blocked by caspase-1 inhibitors, showing that all of these processes are indeed caspase-1 dependent [10]. There are also studies demonstrating that IL-18 can also be secreted from FasL-stimulated macrophages via a caspase-1-independent pathway and contribute to acute liver injury in mice [11]. Our studies showed that caspase-1 activation favoured the development of a pro-inflammatory response through a glycogen synthase kinase 3β-dependent mechanism and led to liver tissue damage in mouse ALF induced by D-galactosamine (D-GalN)/ lipopolysaccharide (LPS) [12]. Testing the role of IL-1β formation through caspase-1 in acetaminophen-induced hepatic inflammation and liver injury, C57BL/6 mice were treated with the pan-caspase inhibitor Z-VD-fmk to block the inflammasome-mediated maturation of IL-1β during APAP overdose. The observed effect was activation of caspase-1, and the inflammasome contributed to the formation of pro-inflammatory mediators after APAP [13]. However, Qian studied the role of caspase-1 inactivation in promoting adaptive responses to oxidative stress and, more specifically, in limiting reactive oxygen species production and damage in cells and tissues where IL-1β and IL-18 are not highly expressed [14].

However, the role of inflammasomes in HBV-related liver disease remains largely obscure, and studies of caspase-1 in ACLF caused by acute exacerbation of CHB are scarce. This study aimed to demonstrate that caspase-1 is elaborately regulated in HBV-related ACLF, LC and HCC patients, which may be a novel biomarker to forecast ACLF occurrence and prognosis due to acute exacerbation of CHB infection.

The inflammasome is an important part of the natural immune system and can be used as a therapeutic target for anti-infective treatment of various inflammatory diseases [17]. HBV infection can progress to asymptomatic carrier status or to CHB, LC, HCC or ACLF, and the inflammasome plays a complex role in different stages of HBV-related liver disease. The inflammasome induces the maturation of pro-inflammatory cytokines, including IL-1β and IL-18, to participate in the innate immune response and liver tissue homeostasis. HBeAg inhibits inflammasome activation and the production of pro-inflammatory cytokines such as IL-18, thus inhibiting the antiviral immune response of the body, facilitating the chronic HBV infection state [18]. Liver fibrosis can be regulated by inflammasomes directly and indirectly [19]. The direct regulation mainly occurs through inflammasome expression in hepatic stellate cells (HSCs) to promote hepatic fibrosis, and the indirect pathway for liver fibrosis is regulated by inducing IL-1β and IL-18 secretion in hepatic macrophages to promote HSC activation [20, 21]. For HCC, the NLRP3 inflammasome components are lost or significantly downregulated in liver cancer tissues, and the greater the deficiency of inflammasomes, the more advanced the cancer progression is [22]. For HCC prognostication, the research demonstrated that the high expression of NLRP3, NLRC4, and caspase-1 in background non-tumorous liver is negatively correlated with patient survival after resection of HCC [23].

In this paper, we first found that the hepatic and serum levels of caspase-1 are differentially expressed among ACLF, LC and HCC patients who developed CHB, which is able to distinguish ACLF from LC and HCC. The hepatic expression of caspase-1 in CHB, ACLF, LC and HCC was compared. We found that caspase-1 shows a distinctive expression profile in different prognoses from CHB: compared with normal subjects, caspase-1 is downregulated in LC and HCC patients, whereas caspase-1 is upregulated in ACLF patients. According to the above interesting results, we can conclude that caspase-1 plays different roles in the pathogenesis of different prognoses derived from CHB. More significantly, there was the opposite trend between the serum levels and hepatic activity levels of caspase-1: compared with the normal control, the serum levels of caspase-1 were reduced in CHB, ACLF, LC and HCC patients and lowest in ACLF patients; in addition, the hepatic activity of caspase-1 was also lower in ACLF patients than in any of the other groups. Based on the above results, we naturally assume that caspase-1 may be differentially regulated in HBV-related end-stage liver disease.

Another novel finding in this paper is that the measurement of caspase-1 serum levels can early diagnosis and predict survival and death of HBV-related ACLF. According to the definition of ACLF by the Asian Pacific Association for the Study of the Liver (APASL), ACLF is distinct from liver cirrhosis and is caused by hepatitis B in the Asia-Pacific region. ACLF mortality is associated with organ failure and systemic inflammation [24, 25], which limits the current evaluation system, such as the Child-Pugh and Model for End-Stage Liver Disease (MELD) scores and their variants. Furthermore, due to the difference of aetiologies and the heterogeneity of existing definitions of ACLF between the Asia-Pacific region and Europe/North America, there is a call to explore other markers of prognosis that could cover the two central prognostic determinants and define outcome with better accuracy regardless of the aetiologies [26]. In our study, all ACLF cases caused by acute insult were based on CHB, which were devoid of decompensated cirrhosis. It is very doubtful that the hepatic levels of caspase-1 mRNA and protein were high while the serum level of caspase-1 was low in ACLF patients. The possible reason for this difference was that the expression of caspase-1 was increased during the development of ACLF. If the morphology of hepatocytes is intact, the synthesized caspase-1 cannot be released into the blood, leading to low levels of caspase-1 in serum. If the many hepatocytes are necrotic in the late stage of ACLF, the synthesized caspase-1 will be reduction, and the level of caspase-1 in serum will be decreased. In addition, the liver tissue in our study originated from patients in the early stage of ACLF, at which time there were no too many necrotic hepatocytes; however, for the serum of ACLF, there were come from patients in different stages of injury (including early, middle and late stages). Therefore, the above two reasons may explain the difference that a higher level in the liver and a lower level in the serum, but all these possibilities need further study.

More importantly, the expression levels of serum caspase-1 in HBV-related ACLF patients also indicate the survival and death of patients. As the first caspase identified in mammalian cells, caspase-1 not only plays an important role in the inflammatory response but also triggers pyroptosis, a newly discovered programmed cell death of inflammatory cells. Furthermore, the caspase-1 serum level has a significant relation to the indicators that can reflect liver function and the overall status of patients, such as ALT, AST, and total protein. Thus, we believe that caspase-1 may be an effective biomarker for the diagnosis and prognosis of HBV-related ACLF. However, the early diagnosis and prognostic evaluation value of caspase-1 in HBV-related ACLF patients requires further validation of large sample studies.

In conclusion, our study demonstrated that caspase-1, one of the critical components of the inflammasome, plays a critical role in the pathogenesis of ACLF derived from CHB; moreover, the serum level of caspase-1 may act as a non-invasive biomarker for early diagnosis and prognosis of ACLF caused by acute exacerbation of CHB.