Increased fibroblast growth factor 21 in obesity and nonalcoholic fatty liver disease

BACKGROUND & AIMS

Fibroblast growth factor 21 (FGF21) is an hepatic protein that plays a critical role in metabolism, stimulating fatty acid oxidation in liver and glucose uptake in fat. Systemic administration to obese rodents and diabetic monkeys leads to improved glucose homeostasis and weight loss. In rodents, FGF21 increases with fasting and consumption of a ketogenic diet (KD). In humans, FGF21 correlates with body mass index (BMI), but studies evaluating other parameters show inconsistent results. We examined FGF21 serum levels in lean and obese individuals and in response to dietary manipulation. We also evaluated FGF21 serum levels and liver messenger RNA (mRNA) expression in nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH).

METHODS

Serum FGF21 was measured after an overnight fast in individuals with BMI ranging from normal to obese. Volunteers fasted for 16 or 72 hours and then ate a standard meal. Another group consumed KD for 12 days. Serum FGF21 and hepatic mRNA expression were measured in obese individuals with NAFLD or NASH.

RESULTS

There was a positive correlation between BMI and FGF21. There was no change in FGF21 in response to a short fast or KD. A nonstatistically significant fall in FGF21 levels was seen after a 72-hour fast. Hepatic FGF21 mRNA expression was significantly elevated in NAFLD, which correlated with a substantial increase in serum FGF21. In NASH, serum FGF21 but not liver mRNA was increased.

CONCLUSIONS

FGF21 correlates with BMI and may be a novel biomarker for NAFLD, but is not nutritionally regulated in humans.

Schwann cell autophagy, myelinophagy, initiates myelin clearance from injured nerves

Although Schwann cell myelin breakdown is the universal outcome of a remarkably wide range of conditions that cause disease or injury to peripheral nerves, the cellular and molecular mechanisms that make Schwann cell-mediated myelin digestion possible have not been established. We report that Schwann cells degrade myelin after injury by a novel form of selective autophagy, myelinophagy. Autophagy was up-regulated by myelinating Schwann cells after nerve injury, myelin debris was present in autophagosomes, and pharmacological and genetic inhibition of autophagy impaired myelin clearance. Myelinophagy was positively regulated by the Schwann cell JNK/c-Jun pathway, a central regulator of the Schwann cell reprogramming induced by nerve injury. We also present evidence that myelinophagy is defective in the injured central nervous system. These results reveal an important role for inductive autophagy during Wallerian degeneration, and point to potential mechanistic targets for accelerating myelin clearance and improving demyelinating disease.

Salermide, a Sirtuin inhibitor with a strong cancer-specific proapoptotic effect

Sirtuin 1 (Sirt1) and Sirtuin 2 (Sirt2) belong to the family of NAD+ (nicotinamide adenine dinucleotide-positive)-dependent class III histone deacetylases and are involved in regulating lifespan. As cancer is a disease of ageing, targeting Sirtuins is emerging as a promising antitumour strategy. Here we present Salermide (N-{3-[(2-hydroxy-naphthalen-1-ylmethylene)-amino]-phenyl}-2-phenyl-propionamide), a reverse amide with a strong in vitro inhibitory effect on Sirt1 and Sirt2. Salermide was well tolerated by mice at concentrations up to 100 muM and prompted tumour-specific cell death in a wide range of human cancer cell lines. The antitumour activity of Salermide was primarily because of a massive induction of apoptosis. This was independent of global tubulin and K16H4 acetylation, which ruled out a putative Sirt2-mediated apoptotic pathway and suggested an in vivo mechanism of action through Sirt1. Consistently with this, RNA interference-mediated knockdown of Sirt1, but not Sirt2, induced apoptosis in cancer cells. Although p53 has been reported to be a target of Sirt1, genetic p53 knockdowns showed that the Sirt1-dependent proapoptotic effect of Salermide is p53-independent. We were finally able to ascribe the apoptotic effect of Salermide to the reactivation of proapoptotic genes epigenetically repressed exclusively in cancer cells by Sirt1. Taken together, our results underline Salermide's promise as an anticancer drug and provide evidence for the molecular mechanism through which Sirt1 is involved in human tumorigenesis.

Metabolomic Identification of Subtypes of Nonalcoholic Steatohepatitis

BACKGROUND & AIMS

Nonalcoholic fatty liver disease (NAFLD) is a consequence of defects in diverse metabolic pathways that involve hepatic accumulation of triglycerides. Features of these aberrations might determine whether NAFLD progresses to nonalcoholic steatohepatitis (NASH). We investigated whether the diverse defects observed in patients with NAFLD are caused by different NAFLD subtypes with specific serum metabolomic profiles, and whether these can distinguish patients with NASH from patients with simple steatosis.

METHODS

We collected liver and serum from methionine adenosyltransferase 1a knockout (MAT1A-KO) mice, which have chronically low levels of hepatic S-adenosylmethionine (SAMe) and spontaneously develop steatohepatitis, as well as C57Bl/6 mice (controls); the metabolomes of all samples were determined. We also analyzed serum metabolomes of 535 patients with biopsy-proven NAFLD (353 with simple steatosis and 182 with NASH) and compared them with serum metabolomes of mice. MAT1A-KO mice were also given SAMe (30 mg/kg/day for 8 weeks); liver samples were collected and analyzed histologically for steatohepatitis.

RESULTS

Livers of MAT1A-KO mice were characterized by high levels of triglycerides, diglycerides, fatty acids, ceramides, and oxidized fatty acids, as well as low levels of SAMe and downstream metabolites. There was a correlation between liver and serum metabolomes. We identified a serum metabolomic signature associated with MAT1A-KO mice that also was present in 49% of the patients; based on this signature, we identified 2 NAFLD subtypes. We identified specific panels of markers that could distinguish patients with NASH from patients with simple steatosis for each subtype of NAFLD. Administration of SAMe reduced features of steatohepatitis in MAT1A-KO mice.

CONCLUSIONS

In an analysis of serum metabolomes of patients with NAFLD and MAT1A-KO mice with steatohepatitis, we identified 2 major subtypes of NAFLD and markers that differentiate steatosis from NASH in each subtype. These might be used to monitor disease progression and identify therapeutic targets for patients.

Extracellular matrix and liver disease

SIGNIFICANCE

The extracellular matrix (ECM) is a dynamic microenvironment that undergoes continuous remodeling, particularly during injury and wound healing. Chronic liver injury of many different etiologies such as viral hepatitis, alcohol abuse, drug-induced liver injury, obesity and insulin resistance, metabolic disorders, and autoimmune disease is characterized by excessive deposition of ECM proteins in response to persistent liver damage.

CRITICAL ISSUES

This review describes the main collagenous and noncollagenous components from the ECM that play a significant role in pathological matrix deposition during liver disease. We define how increased myofibroblasts (MF) from different origins are at the forefront of liver fibrosis and how liver cell-specific regulation of the complex scarring process occurs.

RECENT ADVANCES

Particular attention is paid to the role of cytokines, growth factors, reactive oxygen species, and newly identified matricellular proteins in the regulation of fibrillar type I collagen, a field to which our laboratory has significantly contributed over the years. We compile data from recent literature on the potential mechanisms driving fibrosis resolution such as MF' apoptosis, senescence, and reversal to quiescence.

FUTURE DIRECTIONS

We conclude with a brief description of how epigenetics, an evolving field, can regulate the behavior of MF and of how new "omics" tools may advance our understanding of the mechanisms by which the fibrogenic response to liver injury occurs.

Murine double minute 2 regulates Hu antigen R stability in human liver and colon cancer through NEDDylation

Hu antigen R (HuR) is a central RNA-binding protein regulating cell dedifferentiation, proliferation, and survival, which are well-established hallmarks of cancer. HuR is frequently overexpressed in tumors correlating with tumor malignancy, which is in line with a role for HuR in tumorigenesis. However, the precise mechanism leading to changes in HuR expression remains unclear. In the liver, HuR plays a crucial role in hepatocyte proliferation, differentiation, and transformation. Here, we unraveled a novel mean of regulation of HuR expression in hepatocellular carcinoma (HCC) and colon cancer. HuR levels correlate with the abundance of the oncogene, murine double minute 2 (Mdm2), in human HCC and colon cancer metastases. HuR is stabilized by Mdm2-mediated NEDDylation in at least three lysine residues, ensuring its nuclear localization and protection from degradation.

CONCLUSION

This novel Mdm2/NEDD8/HuR regulatory framework is essential for the malignant transformation of tumor cells, which, in turn, unveils a novel signaling paradigm that is pharmacologically amenable for cancer therapy.

HuR/methyl-HuR and AUF1 regulate the MAT expressed during liver proliferation, differentiation, and carcinogenesis

BACKGROUND & AIMS

Hepatic de-differentiation, liver development, and malignant transformation are processes in which the levels of hepatic S-adenosylmethionine are tightly regulated by 2 genes: methionine adenosyltransferase 1A (MAT1A) and methionine adenosyltransferase 2A (MAT2A). MAT1A is expressed in the adult liver, whereas MAT2A expression primarily is extrahepatic and is associated strongly with liver proliferation. The mechanisms that regulate these expression patterns are not completely understood.

METHODS

In silico analysis of the 3' untranslated region of MAT1A and MAT2A revealed putative binding sites for the RNA-binding proteins AU-rich RNA binding factor 1 (AUF1) and HuR, respectively. We investigated the posttranscriptional regulation of MAT1A and MAT2A by AUF1, HuR, and methyl-HuR in the aforementioned biological processes.

RESULTS

During hepatic de-differentiation, the switch between MAT1A and MAT2A coincided with an increase in HuR and AUF1 expression. S-adenosylmethionine treatment altered this homeostasis by shifting the balance of AUF1 and methyl-HuR/HuR, which was identified as an inhibitor of MAT2A messenger RNA (mRNA) stability. We also observed a similar temporal distribution and a functional link between HuR, methyl-HuR, AUF1, and MAT1A and MAT2A during fetal liver development. Immunofluorescent analysis revealed increased levels of HuR and AUF1, and a decrease in methyl-HuR levels in human livers with hepatocellular carcinoma (HCC).

CONCLUSIONS

Our data strongly support a role for AUF1 and HuR/methyl-HuR in liver de-differentiation, development, and human HCC progression through the posttranslational regulation of MAT1A and MAT2A mRNAs.

Role of Aramchol in steatohepatitis and fibrosis in mice

Nonalcoholic steatohepatitis (NASH) is the advanced form of nonalcoholic fatty liver disease (NAFLD) that sets the stage for further liver damage. The mechanism for the progression of NASH involves multiple parallel hits, including oxidative stress, mitochondrial dysfunction, inflammation, and others. Manipulation of any of these pathways may be an approach to prevent NASH development and progression. Arachidyl‐amido cholanoic acid (Aramchol) is presently in a phase IIb NASH study. The aim of the present study was to investigate Aramchol's mechanism of action and its effect on fibrosis using the methionine‐ and choline‐deficient (MCD) diet model of NASH. We collected liver and serum from mice fed an MCD diet containing 0.1% methionine (0.1MCD) for 4 weeks; these mice developed steatohepatitis and fibrosis. We also collected liver and serum from mice receiving a control diet, and metabolomes and proteomes were determined for both groups. The 0.1MCD‐fed mice were given Aramchol (5 mg/kg/day for the last 2 weeks), and liver samples were analyzed histologically. Aramchol administration reduced features of steatohepatitis and fibrosis in 0.1MCD‐fed mice. Aramchol down‐regulated stearoyl‐coenyzme A desaturase 1, a key enzyme involved in triglyceride biosynthesis and the loss of which enhances fatty acid β‐oxidation. Aramchol increased the flux through the transsulfuration pathway, leading to a rise in glutathione (GSH) and the GSH/oxidized GSH ratio, the main cellular antioxidant that maintains intracellular redox status. Comparison of the serum metabolomic pattern between 0.1MCD‐fed mice and patients with NAFLD showed a substantial overlap. Conclusion: Aramchol treatment improved steatohepatitis and fibrosis by 1) decreasing stearoyl‐coenyzme A desaturase 1 and 2) increasing the flux through the transsulfuration pathway maintaining cellular redox homeostasis. We also demonstrated that the 0.1MCD model resembles the metabolic phenotype observed in about 50% of patients with NAFLD, which supports the potential use of Aramchol in NASH treatment. (Hepatology Communications 2017;1:911–927)

Excess S-adenosylmethionine reroutes phosphatidylethanolamine towards phosphatidylcholine and triglyceride synthesis

Methionine adenosyltransferase 1A (MAT1A) and glycine N-methyltransferase (GNMT) are the primary genes involved in hepatic S-adenosylmethionine (SAMe) synthesis and degradation, respectively. Mat1a ablation in mice induces a decrease in hepatic SAMe, activation of lipogenesis, inhibition of triglyceride (TG) release, and steatosis. Gnmt-deficient mice, despite showing a large increase in hepatic SAMe, also develop steatosis. We hypothesized that as an adaptive response to hepatic SAMe accumulation, phosphatidylcholine (PC) synthesis by way of the phosphatidylethanolamine (PE) N-methyltransferase (PEMT) pathway is stimulated in Gnmt(-/-) mice. We also propose that the excess PC thus generated is catabolized, leading to TG synthesis and steatosis by way of diglyceride (DG) generation. We observed that Gnmt(-/-) mice present with normal hepatic lipogenesis and increased TG release. We also observed that the flux from PE to PC is stimulated in the liver of Gnmt(-/-) mice and that this results in a reduction in PE content and a marked increase in DG and TG. Conversely, reduction of hepatic SAMe following the administration of a methionine-deficient diet reverted the flux from PE to PC of Gnmt(-/-) mice to that of wildtype animals and normalized DG and TG content preventing the development of steatosis. Gnmt(-/-) mice with an additional deletion of perilipin2, the predominant lipid droplet protein, maintain high SAMe levels, with a concurrent increased flux from PE to PC, but do not develop liver steatosis.

CONCLUSION

These findings indicate that excess SAMe reroutes PE towards PC and TG synthesis and lipid sequestration.

Targeting Hepatic Glutaminase 1 Ameliorates Non-alcoholic Steatohepatitis by Restoring Very-Low-Density Lipoprotein Triglyceride Assembly

Non-alcoholic steatohepatitis (NASH) is characterized by the accumulation of hepatic fat in an inflammatory/fibrotic background. Herein, we show that the hepatic high-activity glutaminase 1 isoform (GLS1) is overexpressed in NASH. Importantly, GLS1 inhibition reduces lipid content in choline and/or methionine deprivation-induced steatotic mouse primary hepatocytes, in human hepatocyte cell lines, and in NASH mouse livers. We suggest that under these circumstances, defective glutamine fueling of anaplerotic mitochondrial metabolism and concomitant reduction of oxidative stress promotes a reprogramming of serine metabolism, wherein serine is shifted from the generation of the antioxidant glutathione and channeled to provide one-carbon units to regenerate the methionine cycle. The restored methionine cycle can induce phosphatidylcholine synthesis from the phosphatidylethanolamine N-methyltransferase-mediated and CDP-choline pathways as well as by base-exchange reactions between phospholipids, thereby restoring hepatic phosphatidylcholine content and very-low-density lipoprotein export. Overall, we provide evidence that hepatic GLS1 targeting is a valuable therapeutic approach in NASH.

S-adenosylmethionine regulates cytoplasmic HuR via AMP-activated kinase

BACKGROUND & AIMS

After liver injury, hepatic S-adenosylmethionine (SAM) content decreases, and the blockage this molecule imposes on hepatocyte proliferation is released, facilitating liver regeneration. This activity of SAM is important for normal liver function because mice deficient in hepatic SAM display abnormal liver regeneration and develop hepatocellular carcinoma. How SAM regulates hepatocyte growth is unclear, but because SAM blocks hepatocyte growth factor (HGF)-induced cyclin D1 expression and DNA synthesis without affecting HGF-induced extracellular signal-regulated kinase phosphorylation, the mitogen-activated protein kinase (MAPK) pathway is probably not the target.

METHODS

The effects of SAM on AMPK, HuR localization were assessed in rat hepatocytes after HGF, AICAR, and SAM treatment.

RESULTS

We show here that HGF and 5-aminoimidazole-4-carboxamide-riboside (AICAR), an activator of AMP-activated protein kinase (AMPK), induce the phosphorylation of AMPK in hepatocytes and that SAM blocks this process. We also show that HGF- and AICAR-induced AMPK activation stimulate the transport from nucleus to cytoplasm of HuR, an RNA-binding protein that increases the half-life of target mRNA such as cyclin A2, and that SAM blocks this process. We found that, in hepatocytes, AICAR increases HuR binding to cyclin A2 messenger RNA (mRNA) as well as the expression and stability of this mRNA and that SAM blocks these events. Consistently, we found that AICAR induces hepatocyte proliferation and that SAM blocks this effect. Finally, we found that liver AMPK phosphorylation, cytoplasmic HuR, and binding of HuR to HuR-target mRNA and the steady-state levels of these mRNA are increased in knockout mice deficient in hepatic SAM.

CONCLUSIONS

Our results yield novel insights about the mechanism by which SAM inhibits cell-cycle progression in the liver.

p38 MAPK mediates the regulation of α1(I) procollagen mRNA levels by TNF-α and TGF-β in a cell line of rat hepatic stellate cells1

The role of members of the mitogen-activated protein kinase (MAPK) family on tumor necrosis factor alpha (TNF-alpha)-mediated down-regulation of col1a1 gene was studied. TNF-alpha increased extracellular-regulated kinase and Jun-N-terminal kinase phosphorylation, but these effects were not related to its inhibitory effect on alpha1(I) procollagen (col1a1) mRNA levels. Phosphorylation of p38 MAPK was decreased in response to TNF-alpha, and the specific p38 MAPK inhibitor SB203580 mimicked the effect of TNF-alpha on col1a1 mRNA levels. Transforming growth factor beta (TGF-beta) increased p38 MAPK phosphorylation and SB203580 prevented the induction of col1a1 mRNA levels by TGF-beta. These results suggest that p38 MAPK plays an important role in regulating the expression of col1a1 in hepatic stellate cells in response to cytokines.

5'-methylthioadenosine modulates the inflammatory response to endotoxin in mice and in rat hepatocytes

5'-methylthioadenosine (MTA) is a nucleoside generated from S-adenosylmethionine (AdoMet) during polyamine synthesis. Recent evidence indicates that AdoMet modulates in vivo the production of inflammatory mediators. We have evaluated the anti-inflammatory properties of MTA in bacterial lipopolysaccharide (LPS) challenged mice, murine macrophage RAW 264.7 cells, and isolated rat hepatocytes treated with pro-inflammatory cytokines. MTA administration completely prevented LPS-induced lethality. The life-sparing effect of MTA was accompanied by the suppression of circulating tumor necrosis factor-alpha (TNF-alpha), inducible NO synthase (iNOS) expression, and by the stimulation of IL-10 synthesis. These responses to MTA were also observed in LPS-treated RAW 264.7 cells. MTA prevented the transcriptional activation of iNOS by pro-inflammatory cytokines in isolated hepatocytes, and the induction of cyclooxygenase 2 (COX2) in RAW 264.7 cells. MTA inhibited the activation of p38 mitogen-activated protein kinase (MAPK), c-jun phosphorylation, inhibitor kappa B alpha (IkappaBalpha) degradation, and nuclear factor kappaB (NFkappaB) activation, all of which are signaling pathways related to the generation of inflammatory mediators. These effects were independent of the metabolic conversion of MTA into AdoMet and the potential interaction of MTA with the cAMP signaling pathway, central to the anti-inflammatory actions of its structural analog adenosine. In conclusion, these observations demonstrate novel immunomodulatory properties for MTA that may be of value in the management of inflammatory diseases.

Human antigen R contributes to hepatic stellate cell activation and liver fibrosis

RNA-binding proteins (RBPs) play a major role in the control of messenger RNA (mRNA) turnover and translation rates. We examined the role of the RBP, human antigen R (HuR), during cholestatic liver injury and hepatic stellate cell (HSC) activation. HuR silencing attenuated fibrosis development in vivo after BDL, reducing liver damage, oxidative stress, inflammation, and collagen and alpha smooth muscle actin (α-SMA) expression. HuR expression increased in activated HSCs from bile duct ligation mice and during HSC activation in vitro, and HuR silencing markedly reduced HSC activation. HuR regulated platelet-derived growth factor (PDGF)-induced proliferation and migration and controlled the expression of several mRNAs involved in these processes (e.g., Actin, matrix metalloproteinase 9, and cyclin D1 and B1). These functions of HuR were linked to its abundance and cytoplasmic localization, controlled by PDGF, by extracellular signal-regulated kinases (ERK) and phosphatidylinositol 3-kinase activation as well as ERK/LKB1 (liver kinase B1) activation, respectively. More important, we identified the tumor suppressor, LKB1, as a novel downstream target of PDGF-induced ERK activation in HSCs. HuR also controlled transforming growth factor beta (TGF-β)-induced profibrogenic actions by regulating the expression of TGF-β, α-SMA, and p21. This was likely the result of an increased cytoplasmic localization of HuR, controlled by TGF-β-induced p38 mitogen-activated protein kinase activation. Finally, we found that HuR and LKB1 (Ser428) levels were highly expressed in activated HSCs in human cirrhotic samples.

CONCLUSION

Our results show that HuR is important for the pathogenesis of liver fibrosis development in the cholestatic injury model, for HSC activation, and for the response of activated HSC to PDGF and TGF-β.

Silencing hepatic MCJ attenuates non-alcoholic fatty liver disease (NAFLD) by increasing mitochondrial fatty acid oxidation

Nonalcoholic fatty liver disease (NAFLD) is considered the next major health epidemic with an estimated 25% worldwide prevalence. No drugs have yet been approved and NAFLD remains a major unmet need. Here, we identify MCJ (Methylation-Controlled J protein) as a target for non-alcoholic steatohepatitis (NASH), an advanced phase of NAFLD. MCJ is an endogenous negative regulator of the respiratory chain Complex I that acts to restrain mitochondrial respiration. We show that therapeutic targeting of MCJ in the liver with nanoparticle- and GalNAc-formulated siRNA efficiently reduces liver lipid accumulation and fibrosis in multiple NASH mouse models. Decreasing MCJ expression enhances the capacity of hepatocytes to mediate β-oxidation of fatty acids and minimizes lipid accumulation, which results in reduced hepatocyte damage and fibrosis. Moreover, MCJ levels in the liver of NAFLD patients are elevated relative to healthy subjects. Thus, inhibition of MCJ emerges as an alternative approach to treat NAFLD.

Non-alcoholic fatty liver (NAFLD) disease causes degeneration of the liver, affects about 25% of people globally, and has no approved treatment. Here, the authors show that the therapeutic siRNA-driven silencing of MCJ in the liver is an effective and safe treatment for NAFLD in multiple mouse models.

Fatty liver and fibrosis in glycine N-methyltransferase knockout mice is prevented by nicotinamide

Deletion of glycine N-methyltransferase (GNMT), the main gene involved in liver S-adenosylmethionine (SAM) catabolism, leads to the hepatic accumulation of this molecule and the development of fatty liver and fibrosis in mice. To demonstrate that the excess of hepatic SAM is the main agent contributing to liver disease in GNMT knockout (KO) mice, we treated 1.5-month-old GNMT-KO mice for 6 weeks with nicotinamide (NAM), a substrate of the enzyme NAM N-methyltransferase. NAM administration markedly reduced hepatic SAM content, prevented DNA hypermethylation, and normalized the expression of critical genes involved in fatty acid metabolism, oxidative stress, inflammation, cell proliferation, and apoptosis. More importantly, NAM treatment prevented the development of fatty liver and fibrosis in GNMT-KO mice. Because GNMT expression is down-regulated in patients with cirrhosis, and because some subjects with GNMT mutations have spontaneous liver disease, the clinical implications of the present findings are obvious, at least with respect to these latter individuals. Because NAM has been used for many years to treat a broad spectrum of diseases (including pellagra and diabetes) without significant side effects, it should be considered in subjects with GNMT mutations.

CONCLUSION

The findings of this study indicate that the anomalous accumulation of SAM in GNMT-KO mice can be corrected by NAM treatment leading to the normalization of the expression of many genes involved in fatty acid metabolism, oxidative stress, inflammation, cell proliferation, and apoptosis, as well as reversion of the appearance of the pathologic phenotype.

L-methionine availability regulates expression of the methionine adenosyltransferase 2A gene in human hepatocarcinoma cells: role of S-adenosylmethionine

In mammals, methionine adenosyltransferase (MAT), the enzyme responsible for S-adenosylmethionine (AdoMet) synthesis, is encoded by two genes, MAT1A and MAT2A. In liver, MAT1A expression is associated with high AdoMet levels and a differentiated phenotype, whereas MAT2A expression is associated with lower AdoMet levels and a dedifferentiated phenotype. In the current study, we examined regulation of MAT2A gene expression by l-methionine availability using HepG2 cells. In l-methionine-deficient cells, MAT2A gene expression is rapidly induced, and methionine adenosyltransferase activity is increased. Restoration of l-methionine rapidly down-regulates MAT2A mRNA levels; for this effect, l-methionine needs to be converted into AdoMet. This novel action of AdoMet is not mediated through a methyl transfer reaction. MAT2A gene expression was also regulated by 5'-methylthioadenosine, but this was dependent on 5'-methylthioadenosine conversion to methionine through the salvage pathway. The transcription rate of the MAT2A gene remained unchanged during l-methionine starvation; however, its mRNA half-life was significantly increased (from 100 min to more than 3 h). The effect of l-methionine withdrawal on MAT2A mRNA stabilization requires both gene transcription and protein synthesis. We conclude that MAT2A gene expression is modulated as an adaptive response of the cell to l-methionine availability through its conversion to AdoMet.

Deregulated neddylation in liver fibrosis

Hepatic fibrosis is a global health problem currently without effective therapeutic approaches. Even though the ubiquitin-like posttranslational modification of neddylation, that conjugates Nedd8 (neural precursor cell expressed developmentally downregulated) to specific targets, is aberrant in many pathologies, its relevance in liver fibrosis (LF) remained unexplored. Our results show deregulated neddylation in clinical fibrosis and both in mouse bileductligation- and CCl₄ -induced fibrosis. Importantly, neddylation inhibition, by using the pharmacological inhibitor, MLN4924, reduced liver injury, apoptosis, inflammation, and fibrosis by targeting different hepatic cell types. On one hand, increased neddylation was associated with augmented caspase 3 activity in bile-acid-induced apoptosis in mouse hepatocytes whereas neddylation inhibition ameliorated apoptosis through reduction of expression of the Cxcl1 and Ccl2 chemokines. On the other hand, chemokine receptors and cytokines, usually induced in activated macrophages, were reduced after neddylation inhibition in mouse Kupffer cells. Under these circumstances, decreased hepatocyte cell death and inflammation after neddylation inhibition could partly account for reduction of hepatic stellate cell (HSC) activation. We provide evidence that augmented neddylation characterizes activated HSCs, suggesting that neddylation inhibition could be important for resolving LF by directly targeting these fibrogenic cells. Indeed, neddylation inhibition in activated HSCs induces apoptosis in a process partly mediated by accumulation of c-Jun, whose cullin-mediated degradation is impaired under these circumstances.

CONCLUSION

Neddylation inhibition reduces fibrosis, suggesting neddylation as a potential and attractive therapeutic target in liver fibrosis. (Hepatology 2017;65:694-709).

Evidence for LKB1/AMP-activated protein kinase/ endothelial nitric oxide synthase cascade regulated by hepatocyte growth factor, S-adenosylmethionine, and nitric oxide in hepatocyte proliferation

S-adenosylmethionine (SAMe) is involved in numerous complex hepatic processes such as hepatocyte proliferation, death, inflammatory responses, and antioxidant defense. One of the most relevant actions of SAMe is the inhibition of hepatocyte proliferation during liver regeneration. In hepatocytes, SAMe regulates the levels of cytoplasmic HuR, an RNA-binding protein that increases the half-life of target messenger RNAs such as cyclin D1 and A2 via inhibition of hepatocyte growth factor (HGF)-mediated adenosine monophosphate-activated protein kinase (AMPK) phosphorylation. Because AMPK is activated by the tumor suppressor kinase LKB1, and AMPK activates endothelial nitric oxide (NO) synthase (eNOS), and NO synthesis is of great importance for hepatocyte proliferation, we hypothesized that in hepatocytes HGF may induce the phosphorylation of LKB1, AMPK, and eNOS through a process regulated by SAMe, and that this cascade might be crucial for hepatocyte growth. We demonstrate that the proliferative response of hepatocytes involves eNOS phosphorylation via HGF-mediated LKB1 and AMPK phosphorylation, and that this process is regulated by SAMe and NO. We also show that knockdown of LKB1, AMPK, or eNOS with specific interference RNA (iRNA) inhibits HGF-mediated hepatocyte proliferation. Finally, we found that the LKB1/AMPK/eNOS cascade is activated during liver regeneration after partial hepatectomy and that this process is impaired in mice treated with SAMe before hepatectomy, in knockout mice deficient in hepatic SAMe, and in eNOS knockout mice.

CONCLUSION

We have identified an LKB1/AMPK/eNOS cascade regulated by HGF, SAMe, and NO that functions as a critical determinant of hepatocyte proliferation during liver regeneration after partial hepatectomy.

S-adenosylmethionine levels regulate the schwann cell DNA methylome

Axonal myelination is essential for rapid saltatory impulse conduction in the nervous system, and malformation or destruction of myelin sheaths leads to motor and sensory disabilities. DNA methylation is an essential epigenetic modification during mammalian development, yet its role in myelination remains obscure. Here, using high-resolution methylome maps, we show that DNA methylation could play a key gene regulatory role in peripheral nerve myelination and that S-adenosylmethionine (SAMe), the principal methyl donor in cytosine methylation, regulates the methylome dynamics during this process. Our studies also point to a possible role of SAMe in establishing the aberrant DNA methylation patterns in a mouse model of diabetic neuropathy, implicating SAMe in the pathogenesis of this disease. These critical observations establish a link between SAMe and DNA methylation status in a defined biological system, providing a mechanism that could direct methylation changes during cellular differentiation and in diverse pathological situations.

Activation of LKB1-Akt pathway independent of phosphoinositide 3-kinase plays a critical role in the proliferation of hepatocellular carcinoma from nonalcoholic steatohepatitis

LKB1, originally considered a tumor suppressor, plays an important role in hepatocyte proliferation and liver regeneration. Mice lacking the methionine adenosyltransferase (MAT) gene MAT1A exhibit a chronic reduction in hepatic S-adenosylmethionine (SAMe) levels, basal activation of LKB1, and spontaneous development of nonalcoholic steatohepatitis (NASH) and hepatocellular carcinoma (HCC). These results are relevant for human health because patients with liver cirrhosis, who are at risk to develop HCC, have a marked reduction in hepatic MAT1A expression and SAMe synthesis. In this study, we isolated a cell line (SAMe-deficient [SAMe-D]) from MAT1A knockout (MAT1A-KO) mouse HCC to examine the role of LKB1 in the development of liver tumors derived from metabolic disorders. We found that LKB1 is required for cell survival in SAMe-D cells. LKB1 regulates Akt-mediated survival independent of phosphoinositide 3-kinase, adenosine monophosphate protein-activated kinase (AMPK), and mammalian target of rapamycin complex (mTORC2). In addition, LKB1 controls the apoptotic response through phosphorylation and retention of p53 in the cytoplasm and the regulation of herpesvirus-associated ubiquitin-specific protease (HAUSP) and Hu antigen R (HuR) nucleocytoplasmic shuttling. We identified HAUSP as a target of HuR. Finally, we observed cytoplasmic staining of p53 and p-LKB1(Ser428) in a NASH-HCC animal model (from MAT1A-KO mice) and in liver biopsies obtained from human HCC derived from both alcoholic steatohepatitis and NASH.

CONCLUSION

The SAMe-D cell line is a relevant model of HCC derived from NASH disease in which LKB1 is the principal conductor of a new regulatory mechanism and could be a practical tool for uncovering new therapeutic strategies.

SUMOylation regulates LKB1 localization and its oncogenic activity in liver cancer

Background

Even though liver kinase B1 (LKB1) is usually described as a tumor suppressor in a wide variety of tissues, it has been shown that LKB1 aberrant expression is associated with bad prognosis in Hepatocellular Carcinoma (HCC).

Methods

Herein we have overexpressed LKB1 in human hepatoma cells and by using histidine pull-down assay we have investigated the role of the hypoxia-related post-translational modification of Small Ubiquitin-related Modifier (SUMO)ylation in the regulation of LKB1 oncogenic role. Molecular modelling between LKB1 and its interactors, involved in regulation of LKB1 nucleocytoplasmic shuttling and LKB1 activity, was performed. Finally, high affinity SUMO binding entities-based technology were used to validate our findings in a pre-clinical mouse model and in clinical HCC.

Findings

We found that in human hepatoma cells under hypoxic stress, LKB1 overexpression increases cell viability and aggressiveness in association with changes in LKB1 cellular localization. Moreover, by using site-directed mutagenesis, we have shown that LKB1 is SUMOylated by SUMO-2 at Lys178 hampering LKB1 nucleocytoplasmic shuttling and fueling hepatoma cell growth. Molecular modelling of SUMO modified LKB1 further confirmed steric impedance between SUMOylated LKB1 and the STe20-Related ADaptor cofactor (STRADα), involved in LKB1 export from the nucleus. Finally, we provide evidence that endogenous LKB1 is modified by SUMO in pre-clinical mouse models of HCC and clinical HCC, where LKB1 SUMOylation is higher in fast growing tumors.

Interpretation

Overall, SUMO-2 modification of LKB1 at Lys178 mediates LKB1 cellular localization and its oncogenic role in liver cancer.

Fund

This work was supported by grants from NIH (US Department of Health and Human services)-R01AR001576-11A1 (J.M.M and M.L.M-C.), Gobierno Vasco-Departamento de Salud 2013111114 (to M.L.M.-C), ELKARTEK 2016, Departamento de Industria del Gobierno Vasco (to M.L.M.-C), MINECO: SAF2017–87301-R and SAF2014–52097-R integrado en el Plan Estatal de Investigación Cientifica y Técnica y Innovación 2013–2016 cofinanciado con Fondos FEDER (to M.L.M.-C and J.M.M., respectively), BFU2015–71017/BMC MINECO/FEDER, EU (to A.D.Q. and I.D.M.), BIOEF (Basque Foundation for Innovation and Health Research): EITB Maratoia BIO15/CA/014; Instituto de Salud Carlos III:PIE14/00031, integrado en el Plan Estatal de Investigación Cientifica y Técnica y Innovacion 2013–2016 cofinanciado con Fondos FEDER (to M.L.M.-C and J.M.M), Asociación Española contra el Cáncer (T.C.D, P·F-T and M.L.M-C), Daniel Alagille award from EASL (to T.C.D), Fundación Científica de la Asociación Española Contra el Cancer (AECC Scientific Foundation) Rare Tumor Calls 2017 (to M.L.M and M.A), La Caixa Foundation Program (to M.L.M), Programma di Ricerca Regione-Università 2007–2009 and 2011–2012, Regione Emilia-Romagna (to E.V.), Ramón Areces Foundation and the Andalusian Government (BIO-198) (A.D.Q. and I.D.M.), ayudas para apoyar grupos de investigación del sistema Universitario Vasco IT971–16 (P.A.), MINECO:SAF2015–64352-R (P.A.), Institut National du Cancer, FRANCE, INCa grant PLBIO16–251 (M.S.R.), MINECO - BFU2016–76872-R to (E.B.). Work produced with the support of a 2017 Leonardo Grant for Researchers and Cultural Creators, BBVA Foundation (M.V-R). Finally, Ciberehd_ISCIII_MINECO is funded by the Instituto de Salud Carlos III. We thank MINECO for the Severo Ochoa Excellence Accreditation to CIC bioGUNE (SEV-2016-0644). Funding sources had no involvement in study design; in the collection, analysis, and interpretation of data; in the writing of the report; and in the decision to submit the paper for publication.

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Overexpression of LKB1 in human hepatoma cells during hypoxic stress induces deregulated cell growth and survival.

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SUMO-2 modifications of LKB1 at Lys178 occur in human hepatoma cells hampering its nucleocytoplasmic shuttling.

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LKB1 SUMOylation is augmented in pre-clinical mouse models and clinical HCC, being a hallmark of more aggressive HCC tumors.

Stratification and therapeutic potential of PML in metastatic breast cancer

Patient stratification has been instrumental for the success of targeted therapies in breast cancer. However, the molecular basis of metastatic breast cancer and its therapeutic vulnerabilities remain poorly understood. Here we show that PML is a novel target in aggressive breast cancer. The acquisition of aggressiveness and metastatic features in breast tumours is accompanied by the elevated PML expression and enhanced sensitivity to its inhibition. Interestingly, we find that STAT3 is responsible, at least in part, for the transcriptional upregulation of PML in breast cancer. Moreover, PML targeting hampers breast cancer initiation and metastatic seeding. Mechanistically, this biological activity relies on the regulation of the stem cell gene SOX9 through interaction of PML with its promoter region. Altogether, we identify a novel pathway sustaining breast cancer aggressiveness that can be therapeutically exploited in combination with PML-based stratification.

The L-α-Lysophosphatidylinositol/G Protein-Coupled Receptor 55 System Induces the Development of Nonalcoholic Steatosis and Steatohepatitis

BACKGROUND AND AIMS

G protein-coupled receptor (GPR) 55 is a putative cannabinoid receptor, and l-α-lysophosphatidylinositol (LPI) is its only known endogenous ligand. Although GPR55 has been linked to energy homeostasis in different organs, its specific role in lipid metabolism in the liver and its contribution to the pathophysiology of nonalcoholic fatty liver disease (NAFLD) remains unknown.

APPROACH AND RESULTS

We measured (1) GPR55 expression in the liver of patients with NAFLD compared with individuals without obesity and without liver disease, as well as animal models with steatosis and nonalcoholic steatohepatitis (NASH), and (2) the effects of LPI and genetic disruption of GPR55 in mice, human hepatocytes, and human hepatic stellate cells. Notably, we found that circulating LPI and liver expression of GPR55 were up-regulated in patients with NASH. LPI induced adenosine monophosphate-activated protein kinase activation of acetyl-coenzyme A carboxylase (ACC) and increased lipid content in human hepatocytes and in the liver of treated mice by inducing de novo lipogenesis and decreasing β-oxidation. The inhibition of GPR55 and ACCα blocked the effects of LPI, and the in vivo knockdown of GPR55 was sufficient to improve liver damage in mice fed a high-fat diet and in mice fed a methionine-choline-deficient diet. Finally, LPI promoted the initiation of hepatic stellate cell activation by stimulating GPR55 and activation of ACC.

CONCLUSIONS

The LPI/GPR55 system plays a role in the development of NAFLD and NASH by activating ACC.

HuR/ELAVL1 drives malignant peripheral nerve sheath tumor growth and metastasis

Cancer cells can develop a strong addiction to discrete molecular regulators, which control the aberrant gene expression programs that drive and maintain the cancer phenotype. Here, we report the identification of the RNA-binding protein HuR/ELAVL1 as a central oncogenic driver for malignant peripheral nerve sheath tumors (MPNSTs), which are highly aggressive sarcomas that originate from cells of the Schwann cell lineage. HuR was found to be highly elevated and bound to a multitude of cancer-associated transcripts in human MPNST samples. Accordingly, genetic and pharmacological inhibition of HuR had potent cytostatic and cytotoxic effects on tumor growth, and strongly suppressed metastatic capacity in vivo. Importantly, we linked the profound tumorigenic function of HuR to its ability to simultaneously regulate multiple essential oncogenic pathways in MPNST cells, including the Wnt/β-catenin, YAP/TAZ, RB/E2F, and BET pathways, which converge on key transcriptional networks. Given the exceptional dependency of MPNST cells on HuR for survival, proliferation, and dissemination, we propose that HuR represents a promising therapeutic target for MPNST treatment.