Preventing liver fibrosis in patients with NAFLD and the road ahead

KEYWORDS:

• Fibrogenesis
• liver fibrosis
• myofibroblasts
• non-alcoholic fatty liver disease
• regressin
• therapy

1. Liver fibrosis a downstream event in the multiple faces of NAFLD

Liver fibrosis is the most frequent complication in the natural history of several chronic liver diseases [1]. However, with the success of novel therapies against hepatitis B and C, non-alcoholic fatty liver disease (NAFLD) has become the most common worldwide liver disease. Indeed, NAFLD prevalence continues to rise in both adult and pediatric populations concurrent with the growing incidence of obesity [2,3]. Thus, NAFLD remains the sole hepatopathy that could strongly increase the direct and indirect costs of health care over the next decade.

The multi-spectrum phenotype of NAFLD includes simple steatosis alone or co-occurrence with non-alcoholic steatohepatitis traits (hepatocyte ballooning and necro-inflammation), which may progress to advanced fibrosis. Therefore, liver fibrosis in NAFLD could be considered a downstream event that may precede end-stage liver disease. Accordingly, in a retrospective longitudinal study performed on 619 patients with biopsy-proven NAFLD, Angulo et al. recently demonstrated that the fibrotic stage is independently associated with long-term prognosis (mortality, liver transplantation, and liver-related events) [4]. These findings highlight that histological detection of fibrosis is truly relevant for patient management, because it could be crucial in identifying the subgroup of patients that require lifestyle monitoring and therapy in order to prevent fibrosis development and reduce liver damage progression.

Although eradication of etiological agents has shown encouraging improvement in liver fibrosis of other chronic liver diseases, currently, there is no successful therapy against liver fibrosis in NAFLD. Thus, the identification of untargeted and targeted therapies capable of reducing liver fibrosis is a hot-topic field requiring the combined research efforts of physicians and scientists during the coming years.

2. Pathomechanism dissection may speed-up translation into therapy

Whatever the etiology, liver fibrosis is characterized by an excessive deposition of type I and III fibrillar collagens that compose the extracellular matrix (ECM). This type of ECM engulfment occurs in response to a liver injury and is driven by myofibroblasts [1].

Scientific evidence suggesting that myofibroblasts are a heterogeneous cell population derived from reprogramming of different parenchymal and non-parenchymal cell types has replaced the original notion that myofibroblasts exclusively originate from hepatic stellate cells (HSCs) [1]. In fact, liver myofibroblasts may also originate from fibrocytes that initially express myeloid cell markers, portal fibroblasts, and bone-marrow-derived mesenchymal cells [5]. Moreover, during fibrosis progression, hepatocytes, liver sinusoidal cells, and Kupffer cells create a network of interactions with myofibroblasts, thus making the pathogenesis of fibrosis even more complex.

There are two main phenotype-switching events that occur during liver fibrogenesis. The first, known as epithelial-to-mesenchymal transition, occurs when cells of epithelial origin acquire mesenchymal cell features and become fully differentiated activated myofibroblasts [6]. The second switching event, known as endothelial-to-mesenchymal transition, occurs when the original myofibroblast source is an endothelial cell [7].

However, it is crucial to remember that liver fibrosis is a physiological wound-healing event that becomes problematic and clinically relevant, only when (in response to a persistent injury) the network between the cells and tissue environment is deregulated and when excessive scarring disrupts the normal architecture of the liver, resulting in the loss of organ function.

In recent years, we have seen the rise of experimental studies pointing to a better understanding of the cellular and molecular dynamics of fibrogenesis and the development of preclinical studies using new discoveries for therapy.

Unfortunately, although anti-fibrotic activity of new molecularly targeted agents and immunotherapy has been confirmed in vitro and in vivo, few treatments have been thoroughly validated in the clinic or commercialized as therapies [8].

Among the promising agents emerge those that exhibit a dual beneficial effect on NAFLD-related fibrosis and inflammation [9]. Moreover, most of these pharmacological approaches are based on the fact that gut-liver-adipose tissue exchange is crucial for steatosis exacerbation and progression to more severe liver damage (necro-inflammation, ballooning, and fibrosis) in patients with NAFLD [10,11]. Thus, pharmacological agents that inhibit or reverse these tissue impairments, reestablishing the homeostasis of the liver–adipose tissue and liver–gut interactions, could cure not only the primary disease, but also reduce fibrosis.

Recent studies have drawn attention to galectin-3 as a potential target molecule in NAFLD-related fibrosis [11]. Galectin-3 is a multifunctional β-galactoside-binding protein first described on the macrophage surface and widely associated with fibrosis in distinct tissues. Moreover, several lines of evidence have suggested that galectin-3 could be crucial for tissue-resident macrophage polarization and for consequent pro-inflammatory patterns in both adipose and liver tissue during NAFLD [11]. Encouraging preliminary results of a clinical trial with a galectin-3 receptor agonist (galactoarabino-rhamnogalacturonan polysaccharide, GR-MD-02) have supported the planning of a phase 2 clinical trial in NASH-related advanced fibrosis [12]. The biological and clinical effects of galectin-3 reduction/inhibition are enthusiastically expected.

Synthetic activators of the bile acid nuclear receptor (FXR) are currently in clinical development for the treatment of some chronic liver diseases, including NAFLD. In clinical studies, FXR agonists enhance the intestinal production of fibroblast growth factor 19 (FGF19) and its release into enterohepatic circulation, thus ameliorating liver fibrosis and inflammation [13]. However, the recent evidence suggesting that supra-pharmacological exposure levels of FGF19 may be oncogenic in the liver caution us that therapies elevating FGF19 levels should be rigorously monitored [14].

3. Regression or reduction, which is the best target?

The success of therapeutic approaches in eradicating hepatitis B and C has shown that livers may undergo fibrosis reversion following cessation of the primary cause. This encouraged the dissection of the cellular and molecular mechanisms upstream of liver fibrosis progression, in order to translate this novel information into clinical practice.

Fibrosis regression requires four major events that could be independent or strongly associated [15]. One mechanism is the cessation of chronic injury leading to the recovery of hepatocellular homeostasis, which in the case of NAFLD-related fibrosis mainly involves the removal of the metabolic challenge. An additional step is the induction of the shift from an intra-hepatic inflammation milieu to an immune cell and/or macrophage-mediated restorative phenotype, which in turn, may induce an increase of metalloprotease (MPP) activity. Transdifferentiation of pro-fibrogenic myofibroblasts into a senescent phenotype making them more susceptible to inactivation and apoptosis is a further step toward fibrosis regression. The final event is the degradation of fibrotic ECM engulfment that may be exerted by fibrolitic mediators, such as MPPs secreted by immune cells and/or HSCs.

It is noteworthy that most of the processes that could be targeted to reverse fibrosis have a final scarring reduction effect, thus suggesting that discovery of agents that revert fibrosis is the most promising approach for patients with NAFLD-related fibrosis.

Interestingly, a randomized, double-blind, placebo-controlled trial with the pan-caspase inhibitor emricasan has recently begun in patients with NAFLD-related cirrhosis and severe portal hypertension (NCT02960204). In preclinical experimental models, emricasan reduces hepatocellular apoptosis and suppresses liver injury and fibrosis [16]. In addition, the primary results in NAFLD patients are encouraging for long-term outcomes. The potential for this type of molecule in regard to fibrosis regression is high, because the inhibition of hepatocyte death may interrupt all danger signals that sustain myofibroblast activation and necro-inflammation.

The possibility of reverting liver fibrosis in NAFLD using simtuzumab has been tested in a recently closed Phase II clinical trial (NCT01672879). Simtuzumab is a monoclonal antibody that inhibits the activity of lysil oxidase-like-2, an extra-cellular amine oxidase involved in the post-translational modification of collagens and elastin in the ECM [17].

4. Challenges and end points

The significant recent progress made in our understanding of liver fibrosis pathomechanisms provides the foundation for the design of novel treatments. The discovery that liver fibrosis is reversible beyond the early stages has great clinical importance for the recovery of normal liver architecture and function. The major future challenge will be based around identifying drugs, small molecules, and/or cell-based therapies that may block and revert fibrogenesis without interfering with cellular processes and molecular pathways that may drive spontaneous hepatic wound healing. Moreover, since NAFLD-related fibrosis, more so than other etiological factors, could be a unique negative element, the curing of the primary disease and the concomitant hampering/solving of ECM deposition could pave the way for a happy ending.

Therefore, we believe, there is no doubt that targeting the reversion of HSC phenotype switching will be a crucial end point for future therapeutic approaches against NAFLD-related fibrosis.

However, the lack of noninvasive biomarkers for fibrosis should not be overlooked. In fact, the actual absence of robust noninvasive biomarkers that correlate with fibrosis stages may represent a limit to predicting the clinical benefit of novel therapeutic approaches in the NAFLD setting.

Declaraion of interest

A. Alisi and V. Nobili are supported by grants from the Italian Ministry of Health (Ricerca Corrente). The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

Additional information

Funding

This paper was not funded.