Paving the TRAIL to anti‐fibrotic therapy (2016)

Potential conflict of interest: Nothing to report. This work was supported by a National Institutes of Health grant (DK63947; to G.J.G.). See Article on Page 209 Hepatic fibrosis (HF) is a nefarious feature of chronic liver injury, regardless of the etiology, and when advanced, leads to portal hypertension and the well‐recognized attendant deleterious sequela of this clinical syndrome. In its initial stages, HF can regress if the causative injury is removed, thereby preventing progression to advanced liver disease (e.g., hepatitis B, and so on). However, preventing or limiting HF by therapeutic strategies targeting the mechanisms of liver injury is currently not always feasible (e.g., primary sclerosing cholangitis, etc.). Therefore, development of effective antifibrotic drugs able to halt or even cause regression of HF would be therapeutically advantageous in many human chronic liver diseases. Fibrosis is the result of excessive deposition of extracellular matrix, including type I collagen, produced by wound‐healing myofibroblasts. The origin of the myofibroblast population in the liver has been controversial, but it is now widely accepted that the largest portion of myofibroblasts are derived from activated hepatic stellate cells (aHSCs).1 Indeed, fate‐tracing studies in different experimental models of murine HF have identified hepatic stellate cells (HSCs) as the predominant profibrogenic cell population in the liver,2 which renders this cell type the primary target for antifibrotic therapies. Consistent with this concept, deleting aHSCs either induces resolution or prevents progression of HF in mice.2 Moreover, the availability of new, more effective experimental tools, which permit the specific depletion of aHSCs in vivo by HSC‐specific promoter‐driven transgene expression, have provided a better understanding of the role of HSCs in promoting not only HF, but also liver injury.3 Therefore, depleting aHSCs is an attractive therapeutic strategy for treatment of human HF. One approach toward this goal is to develop a well‐tolerated, selective “chemotherapy” for aHSCs, resulting in their selective depletion from the liver. Many chemotherapy‐based cell‐depleting strategies induce apoptosis in their target cell population. Based on these concepts, a potential therapeutic strategy for HF would be to induce aHSC apoptosis. TRAIL (tumor necrosis factor‐related apoptosis‐inducing ligand/Apo‐2‐ligand; gene name TNFSF10) is a death ligand that can induce apoptosis in cells expressing its cognate receptors, TRAIL receptor 1 (TRAIL‐R1, also known as death receptor 4 [DR4]; gene name TNFRSF10A) and/or TRAIL‐R2 (also known as death receptor 5 [DR5]; gene name TNFRSF10B). TRAIL is abundantly expressed by cells of the innate immune system, especially natural killer (NK) cells. Because of its still enigmatic ability to selectively induce apoptosis in cancer cells in vitro, while showing no apparent toxicity to normal cells, TRAIL has been widely tested as a chemotherapeutic anticancer agent. However, despite promising results in vitro and in preclinical studies, efficacy of several recombinant TRAIL or TRAIL receptor agonists in clinical trials has been limited.4 Past observations by us reported that aHSCs in vitro up‐regulate DR5 and, to a lesser extent, DR4 and acquire sensitivity to TRAIL‐mediated apoptosis.5 Also, Gao et al. demonstrated that NK cells may delete aHSCs in murine models of liver fibrosis, in part, by a TRAIL‐dependent mechanism.6 These observations provided the premise that TRAIL may be a potential selective, proapoptotic therapeutic agent to deplete aHSCs in HF. However, whether exogenous administration of TRAIL would induce apoptosis of aHSCs in vivo required confirmation. This concept was not explored until the well‐done article by Oh et al.7 in this issue of Hepatology. These researchers now demonstrate that pharmacological depletion of activated murine HSCs in vivo is indeed possible by using an engineered recombinant human TRAIL. Human TRAIL interestingly binds to and activates murine TRAIL receptors, permitting proof‐of‐concept preclinical studies using a human reagent across species. The study by Oh et al. also confirms up‐regulation of DR4 and DR5 by hepatic myofibroblasts in vivo, confirming our previous in vitro studies, and providing a mechanism for the sensitivity of the activated human HSCs to TRAIL‐mediated apoptosis. Among the factors responsible for the disappointing results with TRAIL in human cancer trials are its short half‐life in vivo and dose limitations attributed to concerns about potential hepatotoxicity.8 Oh et al.7 have overcome these obstacles by generating a recombinant poly(ethylene glycol)(PEG)ylated human trimeric isoleucine‐zipper fused TRAIL (TRAILPEG) with markedly extended half‐life in both rodents and nonhuman primates, improved pharmacodynamics and pharmacokinetics, and reduced tendency to aggregate, which results in decreased toxicity. Although toxicity in nonhuman primates in vivo was not discussed in the study, the researchers report that TRAILPEG did not induce apoptosis of primary human hepatocytes, even at concentrations 50 times higher than those of unPEGylated TRAIL shown to kill human hepatocytes. We have also reported previously that steatotic hepatocytes display enhanced sensitivity to TRAIL in vitro and in vivo.9 Importantly, TRAILPEG was not toxic to hepatocytes treated with oleate in vitro to render them steatotic; however, in vivo preclinical studies are necessary to provide assurance that TRAILPEG is not toxic in animal models of fatty liver disease and steatohepatitis. TRAILPEG treatment rapidly induced apoptosis in activated primary human HSCs, but not quiescent HSCs, under basal conditions, likely attributed to the up‐regulation of DR4 and DR5 by the cellular activation process. More important, systemic administration of TRAILPEG ameliorated both CCl4‐induced liver fibrosis and cirrhosis in rats without any significant toxicity on the liver or other organs.7 This antifibrotic effect resulted predominantly from depleting aHSCs by TRAIL‐mediated apoptosis, although a potential effect of TRAILPEG on other cell types contributing to the fibrogenic process (e.g., macrophages, and so on) cannot be excluded and should be further investigated. Nonetheless, the work of Oh et al. validates the use of proapoptotic drugs targeting aHSCs as an effective antifibrotic therapy (Fig. 1). In particular, TRAILPEG can be considered as a potential drug candidate, given its improved stability and tolerability compared to other recombinant TRAILs and TRAIL receptor agonists previously tested in clinical trials. In addition to its antifibrotic effect, TRAILPEG has also exhibited improved antitumor activity in vivo in xenograft animal models11 and could potentially be beneficial in treatment of cirrhosis‐associated hepatocellular carcinoma by simultaneously targeting aHSCs and cancer cells. Consistent with the report by Oh et al., recent studies have also confirmed the antifibrotic effect of TRAIL by using a hyaluronic acid/TRAIL conjugate, which displays increased targeting of HSCs and stability.12 Further preclinical studies are obviously needed to evaluate the safety and effectiveness of these engineered recombinant TRAILs in more models of human HF (e.g., cholestatic liver injury), as well as optimal doses and possible off‐target effects, before they can enter in clinical trials. Nonetheless, these studies provide a solid rationale for testing TRAIL and other proapoptotic‐based therapies targeting aHSCs for treatment of human liver fibrosis. We look forward to these studies and anticipate that TRAIL therapy for currently untreatable fibrotic liver disease will become a reality.Figure 1: Liver injury results in activation of quiescent HSCs as part of the wound‐healing response. However, persistent HSC activation attributed to chronic injury leads to extensive liver fibrosis and cirrhosis. aHSCs up‐regulate the TRAIL receptors, DR4 and DR5, and acquire sensitivity to TRAIL‐mediated apoptosis. Systemic treatment with TRAILPEG, an engineered TRAIL with enhanced stability and no apparent toxicity in vivo, results in selective apoptosis and depletion of aHSCs and resolution of liver fibrosis. Author names in bold denote shared co‐first authorship.