Targeted therapeutics and novel signaling pathways in non-alcohol-associated fatty liver/steatohepatitis (NAFL/NASH)

Abstract

Non-alcohol-associated fatty liver/steatohepatitis (NAFL/NASH) has become the leading cause of liver disease worldwide. NASH, an advanced form of NAFL, can be progressive and more susceptible to developing cirrhosis and hepatocellular carcinoma. Currently, lifestyle interventions are the most essential and effective strategies for preventing and controlling NAFL without the development of fibrosis. While there are still limited appropriate drugs specifically to treat NAFL/NASH, growing progress is being seen in elucidating the pathogenesis and identifying therapeutic targets. In this review, we discussed recent developments in etiology and prospective therapeutic targets, as well as pharmacological candidates in pre/clinical trials and patents, with a focus on diabetes, hepatic lipid metabolism, inflammation, and fibrosis. Importantly, growing evidence elucidates that the disruption of the gut-liver axis and microbe-derived metabolites drive the pathogenesis of NAFL/NASH. Extracellular vesicles (EVs) act as a signaling mediator, resulting in lipid accumulation, macrophage and hepatic stellate cell activation, further promoting inflammation and liver fibrosis progression during the development of NAFL/NASH. Targeting gut microbiota or EVs may serve as new strategies for the treatment of NAFL/NASH. Finally, other mechanisms, such as cell therapy and genetic approaches, also have enormous therapeutic potential. Incorporating drugs with different mechanisms and personalized medicine may improve the efficacy to better benefit patients with NAFL/NASH.

Fig. 1

Timeline of NAFL/NASH-related drug development. Drugs at different clinical stages are indicated in different colors: phase 4 drugs are marked in red, phase 3 drugs are marked in orange, phase 2 drugs are marked in yellow, phase 1 drugs are marked in brown, and preclinical drugs are marked in cyan. All colors of drugs in the following figures are the same. Created with BioRender

Fig. 2

Schematic summary of the pathogenesis and interorgan crosstalk of NAFL/NASH. Increased lipid synthesis and uptake in the liver exceeds lipid oxidation and excretion, leading to lipid accumulation and lipotoxicity, inflammatory response, cell death, and fibrosis. Besides the liver, insulin-sensitive organs, such as adipose tissue and muscle, produce adipokines and myokines, respectively, which promote inflammation and oxidative stress in the liver. The gut microbiota regulates the inflammatory response and hepatic lipid accumulation through the metabolism of PAMPs, bile acids, etc. Innate immune responses involved in NAFL/NASH include activation of resident Küpffer cells and recruitment of leukocytes (e.g., neutrophils, monocytes) to the liver. Lymphocyte-mediated adaptive immunity is an additional factor promoting liver inflammation. EVs act as drivers of inflammation in NAFL/NASH activating immune cells and HSC. In NAFL/NASH progression, lipotoxicity-induced hepatocyte death is an important driver including apoptosis, necroptosis, pyroptosis, and ferroptosis. Arrows (red) indicate upregulation and arrows (blue) indicate downregulation in NAFL/NASH. Produced with the assistance of Servier Medical Art (https://smart.servier.com). DNL de novo lipogenesis, FA fatty acid, FAO fatty acid oxidation, TG triglyceride, VLDL very-low-density lipoprotein, ER endoplasmic reticulum, MPO myeloperoxidase, NE neutrophil elastase, BAFF B cell-activating factor, TGF-β transforming growth factor beta, TNF-α tumor necrosis factor-alpha, IL interleukin, IFN interferon, CCL2 C-C motif ligand 2, TRAIL tumor necrosis factor-related apoptosis-inducing ligand, CHOP C/EBP homologous protein, RIP receptor-interacting serine-threonine kinase, MLKL mixed lineage kinase domain-like protein, NLPR3 NACHT, LRR, and PYD domains-containing protein 3, GSDMD gasdermin D, GSH glutathione, GSSG glutathione disulfide, GPX4 glutathione peroxidase 4, ROS reactive oxygen species, NAFL nonalcoholic fatty liver, EVs extracellular vesicles

Fig. 3

Glucose and lipid metabolisms and targeting drugs for NASH. Depiction of the drugs actions sites that are currently in preclinical and clinical trials, based on their primary locus of activity. Targets include those that regulate lipids and glucose homeostasis, such as GLP-1 signaling, mTOR signaling, PPAR signaling, BAs metabolism, DNL and NEFA metabolism, and gut microbiota targets in humans. Agonists are indicated with a green arrow and antagonists with a red inhibitor. Drugs at different clinical stages are as indicated. Created with BioRender. ACLY ATP-citrate lyase, ACC acetyl-coenzyme A carboxylase, FASN fatty acid synthase, SCD stearoyl-CoA desaturase, GLP glucagon-like peptide, FGF fibroblast growth factor, NEFA non-esterified fatty acid, FXR farnesoid X receptor, RXR retinoid X receptor, THR thyroid hormone receptor, mTOR mammalian target of rapamycin, PPARα/δ/γ peroxisome proliferator-activated receptors PPARα, PPARδ, and PPARγ, BAs bile acids, ChREBP carbohydrate response element-binding protein, SREBP sterol regulatory element-binding protein, TCA tricarboxylic acid, FMT fecal microbiota transplantation, OCA obeticholic acid, UDCA ursodeoxycholic acid

Fig. 4

Drugs targeting the apoptosis signaling in NASH. Inflammatory cytokines stimulate hepatocyte apoptosis through different pathways such as TRAIL signaling, Fas signaling and TNFα signaling pathway. TRAIL is a member of the TNF superfamily that can lead to the induction of apoptosis in tumors or infected cells. The Fas receptor induces an apoptotic signal by binding to FasL expressed on the surface of other cells. TNFα is a classical cytokine and its signaling pathway had been well investigated. Antagonists and inhibitors at different trial stages are as indicated. Drugs at different clinical stages are indicated in different colors. Created with BioRender. TRAIL tumor necrosis factor-related apoptosis-inducing ligand, Fas fatty acid synthetase, FADD Fas-associated with death domain protein, FasL Fas ligand, TNFα tumor necrosis factor-alpha, TNFR TNF receptor, TRAF2 TNF receptor-associated factor-2, ASK1 apoptosis signal-regulated kinase 1, JNK c-Jun N-terminal kinase, GSTM2 glutathione s-transferase mu 2, CFLAR caspase 8 and FADD-like apoptosis regulator, TNFAIP3 tumor necrosis factor-alpha-induced protein 3

Fig. 5

Drugs regulating the inflammatory response in NAFL/NASH. In NASH, extracellular PAMPs or metabolic stress activates proinflammatory signaling pathways through multiple receptors. Drugs regulate inflammation in NAFL/NASH by targeting different inflammatory pathways, such as TNF-α, TLR-IL-1R, IL-17 signaling, and caspase signaling. Drugs at different clinical stages are indicated in different colors. Created with BioRender. PAMP pathogen-associated molecular patterns, TLR Toll-like receptor, IL-R interleukin-receptor, Myd88 myeloid differentiation factor 88, IRAK interleukin-1 receptor-associated kinase 1, TRAF TNF receptor-associated factor, ASK apoptosis signal-regulating kinase, TAK transforming growth factor beta-activated kinase, JNK c-Jun N-terminal kinase, MAPK mitogen-activated protein kinase, ERK extracellular signal-regulated protein kinases, Casp caspase, AP-1 activating protein-1, NFκB nuclear factor kappa B, PTX pentoxifylline, DUSP7 dual specific phosphatase, RGS5 hepatic regulator of G protein signaling 5, TIPE2 tumor necrosis factor-alpha–induced protein 8-like 2. Drugs at different clinical stages are indicated in different colors

Fig. 6

Drugs targeting the fibrosis process in NASH. Chronic hepatocyte injury induces the activation of hepatic stellate cells (HSCs) and the recruits of immune cells, which result in the deposition and cross-linking of collagens in the extracellular matrix and eventually progress to fibrosis. TGFβ/SMAD signaling, a key pathway in the development of liver fibrosis and inflammation, activates Smad pathway and no-Smad pathway. Activation of TGF-β signaling with OSM exposure drives a cooperative STAT3/SMAD3 gene transcriptional program. OSMR/JAK-mediated STAT3 signaling promotes liver fibrosis and HSCs activation by phosphorylation of SMAD3, resulting in transcriptional activation of select STAT3/SMAD3 targets. Antagonists are indicated with a red inhibitor. Drugs at different clinical stages are as indicated. Drugs at different clinical stages are indicated in different colors. Created with BioRender. Ncst ASO nicastrin antisense oligonucleotide, CCR2/5 C-C chemokine receptor 2/5, MoMF monocyte-derived macrophages, TGF-β transforming growth factor-β, LOXL2 lysyl oxidase-like 2, HSC hepatic stellate cell, OSM oncostatin M, IL-11 interleukin-11, IL-11R IL-11 receptor, GP130 glycoprotein 130, JNK c-Jun N-terminal kinase, STAT3 signal transducer and activator of transcription 3, hsp47 Heat shock protein-47

Fig. 7

Gut–liver axis in NAFL/NASH. Unhealthy diet, such as high fat and high sugar, induces changes in the intestinal microbiome. This in turn affects alterations in metabolites, such as a decrease in beneficial SFAs and an increase in LPS, ethanol, TMA, etc. The impaired gut barrier allows increased and easier translocation of these dangerous substances to the liver, accelerating the progression of NAFL/NASH. BAs bile acids, SCFAs short-chain fatty acids, LPS lipopolysaccharide, TMA trimethylamine, IAA indole-3-acetic acid

Fig. 8

Stem cell therapy in liver fibrosis. MSCs isolated from different sources, including umbilical cord, bone marrow, placental, adipose tissue, placental, hair follicle, function to improve liver fibrosis. MSCs reduced hepatocyte damage through immune suppressive pathways, which in turn prevents HSC activation. Furthermore, MSCs increased the phagocytosis of hepatocyte debris by changing macrophage polarity, while increasing matrix metalloproteinase synthesis to remodel extracellular matrix. Created with BioRender