The TGFβ1 Receptor Antagonist GW788388 Reduces JNK Activation and Protects Against Acetaminophen Hepatotoxicity in Mice

Molecular Pathways Governing the Termination of Liver Regeneration

The liver has the unique capacity to regenerate, and up to 70% of the liver can be removed without detrimental consequences to the organism. Liver regeneration is a complex process involving multiple signaling networks and organs. Liver regeneration proceeds through three phases: the initiation phase, the growth phase, and the termination phase. Termination of liver regeneration occurs when the liver reaches a liver-to-body weight that is required for homeostasis, the so-called "hepatostat." The initiation and growth phases have been the subject of many studies. The molecular pathways that govern the termination phase, however, remain to be fully elucidated. This review summarizes the pathways and molecules that signal the cessation of liver regrowth after partial hepatectomy and answers the question, "What factors drive the hepatostat?" SIGNIFICANCE STATEMENT: Unraveling the pathways underlying the cessation of liver regeneration enables the identification of druggable targets that will allow us to gain pharmacological control over liver regeneration. For these purposes, it would be useful to understand why the regenerative capacity of the liver is hampered under certain pathological circumstances so as to artificially modulate the regenerative processes (e.g., by blocking the cessation pathways) to improve clinical outcomes and safeguard the patient's life.

The immunological mechanisms and therapeutic potential in drug-induced liver injury: lessons learned from acetaminophen hepatotoxicity

Acute liver failure caused by drug overdose is a significant clinical problem in developed countries. Acetaminophen (APAP), a widely used analgesic and antipyretic drug, but its overdose can cause acute liver failure. In addition to APAP-induced direct hepatotoxicity, the intracellular signaling mechanisms of APAP-induced liver injury (AILI) including metabolic activation, mitochondrial oxidant stress and proinflammatory response further affect progression and severity of AILI. Liver inflammation is a result of multiple interactions of cell death molecules, immune cell-derived cytokines and chemokines, as well as damaged cell-released signals which orchestrate hepatic immune cell infiltration. The immunoregulatory interplay of these inflammatory mediators and switching of immune responses during AILI lead to different fate of liver pathology. Thus, better understanding the complex interplay of immune cell subsets in experimental models and defining their functional involvement in disease progression are essential to identify novel therapeutic targets for the treatment of AILI. Here, this present review aims to systematically elaborate on the underlying immunological mechanisms of AILI, its relevance to immune cells and their effector molecules, and briefly discuss great therapeutic potential based on inflammatory mediators.

Coordinated activation of TGF-β and BMP pathways promotes autophagy and limits liver injury after acetaminophen intoxication

Ligands of the transforming growth factor-β (TGF-β) superfamily, including TGF-βs, activins, and bone morphogenetic proteins (BMPs), have been implicated in hepatic development, homeostasis, and pathophysiology. We explored the mechanisms by which hepatocytes decode and integrate injury-induced signaling from TGF-βs and activins (TGF-β/Activin) and BMPs. We mapped the spatiotemporal patterns of pathway activation during liver injury induced by acetaminophen (APAP) in dual reporter mice carrying a fluorescent reporter of TGF-β/Activin signaling and a fluorescent reporter of BMP signaling. APAP intoxication induced the expression of both reporters in a zone of cells near areas of tissue damage, which showed an increase in autophagy and demarcated the borders between healthy and injured tissues. Inhibition of TGF-β superfamily signaling by overexpressing the inhibitor Smad7 exacerbated acute liver histopathology but eventually accelerated tissue recovery. Transcriptomic analysis identified autophagy as a process stimulated by TGF-β1 and BMP4 in hepatocytes, with Trp53inp2, which encodes a rate-limiting factor for autophagy initiation, as the most highly induced autophagy-related gene. Collectively, these findings illustrate the functional interconnectivity of the TGF-β superfamily signaling system, implicate the coordinated activation of TGF-β/Activin and BMP pathways in balancing tissue reparatory and regenerative processes upon APAP-induced hepatotoxicity, and highlight opportunities and potential risks associated with targeting this signaling system for treating hepatic diseases.

Recovered Hepatocytes Promote Macrophage Apoptosis through CXCR4 after Acetaminophen-Induced Liver Injury in Mice

Acetaminophen (APAP) overdose is the main cause of acute liver failure in Western countries. The mechanism of APAP hepatotoxicity is associated with centrilobular necrosis which initiates infiltration of neutrophils, monocytes, and other leukocytes to the area of necrosis. While it has been recognized that this infiltration of immune cells plays a critical role in promoting liver repair, mechanism of immune cell clearance that is important for resolution of inflammation and the return to normal homeostasis are not well characterized. CXCR4 is a chemokine receptor expressed on hepatocytes as well as neutrophils, monocytes, and hematopoietic stem cells. CXCR4 function is dependent on its selective expression on different cell types and thus can vary depending on the pathophysiology. This study aimed to investigate the crosstalk between hepatocytes and macrophages through CXCR4 to promote macrophage apoptosis after APAP overdose. C57BL/6J mice were subjected to APAP overdose (300 mg/kg). Flow cytometry and immunohistochemistry were used to determine the mode of cell death of macrophages and expression pattern of CXCR4 during the resolution phase of APAP hepatotoxicity. The impact of CXCR4 in regulation of macrophage apoptosis and liver recovery was assessed after administration of a monoclonal antibody against CXCR4. RNAseq analysis was performed on flow cytometry sorted CXCR4+ macrophages at 72 h to confirm the apoptotic cell death of macrophages. Our data indicate that the inflammatory response is resolved by recovering hepatocytes through induction of CXCR4 on macrophages, which triggers their cell death by apoptosis at the end of the recovery phase.

Suppression of TGF-β/Smad2 signaling by GW788388 enhances DENV-2 clearance in macrophages

Dengue fever, caused by the dengue virus (DENV‐1, −2, −3, and −4), affects millions of people in the tropical and subtropical regions worldwide. Severe dengue is correlated with high viraemia and cytokine storm, such as high levels of transforming growth factor‐β1 (TGF‐β1) in the patient's serum. Here, the TGF‐β1 signaling was investigated in the context of in vitro viral clearance. Macrophages were infected with DENV‐2 at MOI 5 and treated with the TGF‐β receptor 1 and 2 inhibitor, GW788388. TGF‐β1 expression, signal transduction and viral load were evaluated 48 h after DENV‐2 infection by enzyme‐linked immunoassay, immunofluorescence, and RT‐qPCR assays. Total TGF‐β1 level was reduced in 15% after DENV‐2 infection, but the secretion of its biologically active form increased threefold during infection, which was followed by the phosphorylation of Smad2 protein. Phosphorylation of Smad2 was reduced by treatment with GW788388 and it was correlated with reduced cytokine production. Importantly, treatment led to a dose‐dependent reduction in viral load, ranging from 6.6 × 10⁵ RNA copies/ml in untreated cultures to 2.3 × 10³ RNA copies/ml in cultures treated with 2 ng/ml of GW788388. The anti‐TGF‐β1 antibody treatment also induced a significant reduction in viral load to 1.6 × 10³ RNA copies/ml. On the other hand, the addition of recombinant TGF‐β1 in infected cultures promoted an increase in viral load to 7.0 × 10⁶ RNA copies/ml. These results support that TGF‐β1 plays a significant role in DENV‐2 replication into macrophages and suggest that targeting TGF‐β1 may represent an alternative therapeutic strategy to be explored in dengue infection.

Compromised glutathione synthesis results in high susceptibility to acetaminophen hepatotoxicity in acatalasemic mice

Acatalasemia is caused by genetic defect in the catalase gene. Human achatalasemia patients are able to scavenge physiological hydrogen peroxide but are vulnerable to exogenous oxidative stress. In the present study, we used an acetaminophen-induced hepatotoxicity model in acatalasemic mice to explore this vulnerability. Interestingly, the acetaminophen-induced decrease in total glutathione levels was more prolonged in acatalasemic mice. While the subunits of glutamate-cysteine ligase, a glutathione synthase enzyme, were increased by acetaminophen in the liver of wild-type mice, their expression was lower and was further reduced by acetaminophen in acatalasemic mice. This feature was also observed in immortalized hepatocytes derived from the livers of these mice. However, when catalase was knocked down in HepG2 cells, a cultured human liver cell line, the expression of glutamate-cysteine ligase subunits was increased, suggesting that the low expression of glutamate-cysteine ligase subunits in acatalasemia may be due to other mechanism than catalase deficiency. Therefore, when other factors were investigated, it was found that transforming growth factor-β1 was up-regulated by acetaminophen in the liver of acatalasemic mice, which may inhibit the expression of glutamate-cysteine ligase subunits. The results of this study suggest a new toxic mechanism of acetaminophen-induced liver injury in patients with acatalasemia.

Effects of TGF-β1 Receptor Inhibitor GW788388 on the Epithelial to Mesenchymal Transition of Peritoneal Mesothelial Cells

We investigated the effectiveness of the transforming growth factor beta-1 (TGF-β) receptor inhibitor GW788388 on the epithelial to mesenchymal transition (EMT) using human peritoneal mesothelial cells (HPMCs) and examined the effectiveness of GW788388 on the peritoneal membrane using a peritoneal fibrosis mouse model. HPMCs were treated with TGF-β with or without GW788388. Animal experiments were conducted on male C57/BL6 mice. Peritoneal fibrosis was induced by intraperitoneal injection of chlorhexidine gluconate. GW788388 was administered by once-daily oral gavage. The morphological change, cell migration, and invasion resulted from TGF-β treatment, but these changes were attenuated by cotreatment with GW788388. TGF-β-treated HPMCs decreased the level of the epithelial cell marker and increased the levels of the mesenchymal cell markers. Cotreatment with GW788388 reversed these changes. Phosphorylated Smad2 and Smad3 protein levels were stimulated with TGF-β and the change was attenuated by cotreatment with GW788388. For the peritoneal fibrosis mice, thickness and collagen deposition of parietal peritoneum was increased, but this change was attenuated by cotreatment with GW788388. GW788388, an orally available potent TGF-β receptor type 1 inhibitor, effectively attenuated TGF-β-induced EMT in HPMCs. Cotreatment with GW788388 improved peritoneal thickness and fibrosis, and recovered peritoneal membrane function in a peritoneal fibrosis mouse model.

Lupeol ameliorates LPS/D-GalN induced acute hepatic damage by suppressing inflammation and oxidative stress through TGFβ1-Nrf2 signal pathway

Acute hepatic damage is a severe condition characterized by inflammation and oxidative stress, which is a serious threat to people's life and health. But there are few effective treatments for acute liver injury. Therefore, safe and effective therapeutic approaches for preventing acute liver damage are urgently needed. Lupeol is a natural compound, which has significant antioxidant and anti-inflammatory properties in liver disease. However, the protective mechanism of lupeol against acute liver injury remains unclear. Here, zebrafish and mutant mice were utilized to investigate the protective effects of lupeol against lipopolysaccharide (LPS)/ D-galactosamine(D-GalN) -induced liver injury and the underlying mechanisms. We found that pretreatment with lupeol attenuated the LPS/D-GalN-induced liver injury by decreasing the infiltration of inflammatory cells and reducing pro-inflammatory cytokines. We also demonstrated that lupeol could protect injured liver from oxidative stress by downregulating the expression of TGFβ1 and upregulating Nrf2. Notably, our experimental results provided the support that lupeol effectively protected against LPS/D-GalN-induced acute liver injury via suppression of inflammation response and oxidative stress, which were largely dependent on the upregulation of the Nrf2 pathway via downregulating TGFβ1.

Inhibition of thrombospondin-1 reduces glutathione activity and worsens acute liver injury during acetaminophen hepatotoxicity in mice

Acetaminophen (N-Acetyl-p-Aminophenol or APAP)-induced hepatotoxicity is the most common cause of acute liver failure in the United States and Western Europe. Previous studies have shown that TGFβ1 is elevated during APAP-induced hepatotoxicity and promotes liver injury by reducing liver regeneration while inducing hepatocyte senescence. At this time, little is known about the role of proteins that activate latent TGFβ1 and their effects during APAP-induced hepatotoxicity. Thrombospondin-1 (TSP1) is a homotrimeric protein that can not only activate latent TGFβ1 but can also interact with other proteins including Nrf2 to induce antioxidant signaling. The aim of the current study was to assess the role of thrombospondin-1 (TSP1) in both TGFβ1 activation and its contribution to APAP-induced liver injury. C57Bl/6 mice or TSP1 null mice (TSP1-/-) were administered 300 mg/kg or 600 mg/kg of APAP. TGFβ1 signaling, TSP1 expression, measures of hepatic injury, Nrf2 expression, measures of oxidative/nitrosative stress and GSH metabolism were assessed. The expression of TGFβ1, TSP1 and phosphorylation of SMAD proteins increased in APAP-treated mice compared to controls. TSP1-/- mice had reduced TGFβ1 expression and phosphorylation of SMAD proteins but increased liver injury. Hepatocyte cell death was increased in TSP1-/- mice and this was associated with decreased Nrf2 activity, decreased GSH levels and increased oxidative stress in comparison to wild-type C57Bl/6 mice. Together, these data demonstrate that elimination of TSP1 protein in APAP-treated mice reduces TGFβ1 signaling but leads to increased liver injury by reducing Nrf2 expression and GSH activity, ultimately resulting in increased cell death.

TGFβ1 orchestrates renal fibrosis following Escherichia coli pyelonephritis

Renal scarring after pyelonephritis is linked to long-term health risks for hypertension and chronic kidney disease. Androgen exposure increases susceptibility to, and severity of, uropathogenic Escherichia coli (UPEC) pyelonephritis and resultant scarring in both male and female mice, while anti-androgen therapy is protective against severe urinary tract infection (UTI) in these models. This work employed androgenized female C57BL/6 mice to elucidate the molecular mechanisms of post-infectious renal fibrosis and to determine how these pathways are altered by the presence of androgens. We found that elevated circulating testosterone levels primed the kidney for fibrosis by increasing local production of TGFβ1 before the initiation of UTI, altering the ratio of transcription factors Smad2 and Smad3 and increasing the presence of mesenchymal stem cell (MSC)-like cells and Gli1 + activated myofibroblasts, the cells primarily responsible for deposition of scar components. Increased production of TGFβ1 and aberrations in Smad2:Smad3 were maintained throughout the course of infection in the presence of androgen, correlating with renal scarring that was not observed in non-androgenized female mice. Pharmacologic inhibition of TGFβ1 signaling blunted myofibroblast activation. We conclude that renal fibrosis after pyelonephritis is exacerbated by the presence of androgens and involves activation of the TGFβ1 signaling cascade, leading to increases in cortical populations of MSC-like cells and the Gli1 + activated myofibroblasts that are responsible for scarring.

Comparison of liver regeneration after partial hepatectomy and acetaminophen-induced acute liver failure: A global picture based on transcriptome analysis

Liver regenerates following surgical removal and after drug-induced liver injury (DILI). However, most of the mechanisms of liver regeneration were identified using partial hepatectomy (PHX) model rather than using DILI models. We compared mechanisms of liver regeneration following PHX and after acetaminophen (APAP) overdose, a DILI model, using transcriptomic approach. Kinetics of hepatocyte proliferation and global gene expression profiles were studied in male C57BL/6J mice either subjected to PHX or following APAP overdose. Liver regeneration was much more synchronized after PHX as compared to APAP overdose. Transcriptomics analysis revealed activation of common upstream regulators in both models including growth factors HGF, EGF and VEGF; and cytokines IL6 and TNFα. However, magnitude of activation and temporality was significantly differed between the two models. HGF and VEGF showed similar activation between PHX and APAP but activation of EGF was significantly stronger in the APAP model. Activation of IL6 and TNFα transcriptional programs was delayed but remarkably higher in APAP. These dissimilarities could be attributed to inherent differences in the two models including significant injury and inflammation exclusively in the APAP model. This study highlights need to study mechanisms of liver regeneration after DILI separately from the mechanisms of regeneration PHX.