The Role of Bone Morphogenetic Protein Signaling in Non-Alcoholic Fatty Liver Disease

Lrig3-deficient mice exhibit strain-specific alterations in liver fat accumulation, intestinal morphology, and middle ear inflammation

The transmembrane protein leucine-rich repeats and immunoglobulin-like domains 3 (LRIG3) regulates fat metabolism and bone morphogenetic protein (BMP) signaling. Lrig3-deficient mice exhibit impaired development of the snout and the inner ear lateral canal, neural defects, and cardiac hypertrophy in adulthood. However, no thorough and unbiased analysis of the physiological functions of Lrig3 has previously been performed. To address this knowledge gap, we performed histopathological examination of 42 tissues and organs from 1-year-old female C57BL/6JBomTac and 129S1-U mice with different Lrig3 genotypes. Among the scored pathologies, three were significantly associated with Lrig3 genotype: spontaneous macrovesicular hepatocellular degeneration (hepatocellular steatosis) was less prevalent in Lrig3-deficient C57BL/6JBomTac mice, whereas dilated or flaccid ileum and otitis media were more common in Lrig3-deficient 129S1-U mice. To further investigate hepatic steatosis phenotypes, 8-week-old C57BL/6JBomTac mice of both sexes and different Lrig3 genotypes were subjected to consuming a high-fat diet (HFD) for 8 weeks. The HFD regimen led to relatively few cases of hepatocellular steatosis, with no significant differences among the genotypes; however, female Lrig3-deficient mice presented reduced microvesicular hepatocellular degeneration compared with their wild-type littermates. This study revealed that Lrig3 regulates liver fat accumulation, intestinal morphology, and middle ear inflammation in a mouse strain-dependent manner.

Inhibition of ALK3-mediated signalling pathway protects against acetaminophen-induced liver injury

Acetaminophen (APAP)-induced liver injury is one of the most prevalent causes of acute liver failure (ALF). We assessed the role of the bone morphogenetic protein (BMP) type I receptors ALK2 and ALK3 in APAP-induced hepatotoxicity. The molecular mechanisms that regulate the balance between cell death and survival and the response to oxidative stress induced by APAP was assessed in cultured human hepatocyte-derived (Huh7) cells treated with pharmacological inhibitors of ALK receptors and with modulated expression of ALK2 or ALK3 by lentiviral infection, and in a mouse model of APAP-induced hepatotoxicity. Inhibition of ALK3 signalling with the pharmacological inhibitor DMH2, or by silencing of ALK3, showed a decreased cell death both by necrosis and apoptosis after APAP treatment. Also, upon APAP challenge, ROS generation was ameliorated and, thus, ROS-mediated JNK and P38 MAPK phosphorylation was reduced in ALK3-inhibited cells compared to control cells. These results were also observed in an experimental model of APAP-induced ALF in which post-treatment with DMH2 after APAP administration significantly reduced liver tissue damage, apoptosis and oxidative stress. This study shows the protective effect of ALK3 receptor inhibition against APAP-induced hepatotoxicity. Furthermore, findings obtained from the animal model suggest that BMP signalling might be a new pharmacological target for the treatment of ALF.

Alcohol-induced increase in BMP levels promotes fatty liver disease in male prediabetic stage Otsuka Long-Evans Tokushima Fatty rats

Alcohol consumption exacerbates liver abnormalities in animal models, but whether it increases the severity of liver disease in early diabetic or prediabetic rats is unclear. To investigate the molecular mechanisms underlying alcohol-induced liver steatosis or hepatitis, we used a prediabetic animal model. Otsuka Long-Evans Tokushima Fatty (OLETF) and Long-Evans Tokushima Fatty (LETO) rats were pair-fed with an ethanol-containing liquid diet for 6 weeks. Compared with controls, OLETF and LETO rats displayed more pronounced liver steatosis and higher plasma levels of serum glutamic oxaloacetic transaminase (SGOT) and serum glutamate pyruvate transaminase (SPGT), indicating liver injury. Ethanol-fed LETO (Pd-L-E) rats showed mild liver steatosis and no inflammation compared with ethanol-fed OLETF (Pd-O-E) rats. Although precursor and active SREBP-1 levels in the liver of ethanol-fed OLETF rats significantly increased compared with control diet-fed OLETF rats (Pd-O-C), those of Pd-L-E rats did not. Bone morphogenetic protein (BMP) and TGF-β1 balance in Pd-O-E rats was significantly altered because BMP signaling was upregulated by inducing BMP2, BMP4, BMP7, BMP9, Smad1, and Smad4, whereas TGF-β1, Smad3, and Erk were downregulated. Activation of TGF-β/Smad signaling inhibited BMP2 and BMP9 expression and increased epithelial-mesenchymal transition (EMT) marker levels (Hepcidin, Snail, and Twist) in the liver of LETO rats. Livers of ethanol-fed OLETF rats showed increased levels of vimentin, FSP-1, α-SMA, MMP1, MMP13, and collagen III compared with rats of other groups, whereas EMT marker levels did not change. Thus, BMP exerted anti- and/or pro-fibrotic effects in ethanol-fed rats. Therefore, BMP and TGF-β, two key members of the TGF-β superfamily, play important but diverse roles in liver steatosis in young LETO and OLETF rats.

Deletion of adipocyte prohibitin 1 exacerbates high-fat diet-induced steatosis but not liver inflammation and fibrosis

Adipose tissue dysfunction is closely associated with the development and progression of nonalcoholic fatty liver disease (NAFLD). Recent studies have implied an important role of prohibitin-1 (PHB1) in adipose tissue function. In the current study, we aimed to explore the function of adipocyte PHB1 in the development and progression of NAFLD. The PHB1 protein levels in adipose tissues were markedly decreased in mice fed a high-fat diet (HFD) compared to those fed a chow diet. To explore the function of adipocyte PHB1 in the progression of NAFLD, mice with adipocyte-specific (adipo) deletion of Phb1 (Phb1^adipo-/- mice) were generated. Notably, Phb1^adipo-/- mice did not develop obesity but displayed severe liver steatosis under HFD feeding. Compared to HFD-fed wild-type (WT) mice, HFD-fed Phb1^adipo-/- mice displayed dramatically lower fat mass with significantly decreased levels of total adipose tissue inflammation, including macrophage and neutrophil number as well as the expression of inflammatory mediators. To our surprise, although liver steatosis in Phb1^adipo-/- mice was much more severe, liver inflammation and fibrosis were similar to WT mice after HFD feeding. RNA sequencing analyses revealed that the interferon pathway was markedly suppressed while the bone morphogenetic protein 2 pathway was significantly up-regulated in the liver of HFD-fed Phb1^adipo-/- mice compared with HFD-fed WT mice. Conclusion: HFD-fed Phb1^adipo-/- mice display a subtype of the lean NAFLD phenotype with severe hepatic steatosis despite low adipose mass. This subtype of the lean NAFLD phenotype has similar inflammation and fibrosis as obese NAFLD in HFD-fed WT mice; this is partially due to reduced total adipose tissue inflammation and the hepatic interferon pathway.

Hepatic damage caused by long-term high cholesterol intake induces a dysfunctional restorative macrophage population in experimental NASH

Excessive dietary cholesterol is preferentially stored in the liver, favoring the development of nonalcoholic steatohepatitis (NASH), characterized by progressive hepatic inflammation and fibrosis. Emerging evidence indicates a critical contribution of hepatic macrophages to NASH severity. However, the impact of cholesterol on these cells in the setting of NASH remains elusive. Here, we demonstrate that the dietary cholesterol content directly affects hepatic macrophage global gene expression. Our findings suggest that the modifications triggered by prolonged high cholesterol intake induce long-lasting hepatic damage and support the expansion of a dysfunctional pro-fibrotic restorative macrophage population even after cholesterol reduction. The present work expands the understanding of the modulatory effects of cholesterol on innate immune cell transcriptome and may help identify novel therapeutic targets for NASH intervention.

Bone morphogenetic protein 2 is a new molecular target linked to non-alcoholic fatty liver disease with potential value as non-invasive screening tool

BACKGROUND

Non-alcoholic fatty liver disease (NAFLD) is the commonest cause of chronic liver disease worldwide, being non-alcoholic steatohepatitis (NASH) its most clinically relevant form. Given the risks associated with taking a liver biopsy, the design of accurate non-invasive methods to identify NASH patients is of upmost importance. BMP2 plays a key role in metabolic homeostasis; however, little is known about its involvement in NAFLD onset and progression. This study aimed to elucidate the impact of BMP2 in NAFLD pathophysiology.

METHODS

Hepatic and circulating levels of BMP2 were quantified in serum and liver specimens from 115 biopsy-proven NAFLD patients and 75 subjects with histologically normal liver (NL). In addition, BMP2 content and release was determined in cultured human hepatocytes upon palmitic acid (PA) overload.

RESULTS

We found that BMP2 expression was abnormally increased in livers from NAFLD patients than in subjects with NL and this was reflected in higher serum BMP2 levels. Notably, we observed that PA upregulated BMP2 expression and secretion by human hepatocytes. An algorithm based on serum BMP2 levels and clinically relevant variables to NAFLD showed an AUROC of 0.886 (95%CI, 0.83-0.94) to discriminate NASH. We used this algorithm to develop SAN (Screening Algorithm for NASH): a SAN < 0.2 implied a low risk and a SAN ≥ 0.6 indicated high risk of NASH diagnosis.

CONCLUSION

This proof-of-concept study shows BMP2 as a new molecular target linked to NAFLD and introduces SAN as a simple and efficient algorithm to screen individuals at risk for NASH.

Deficiency of SREBP1c modulates autophagy mediated lipid droplet catabolism during oleic acid induced steatosis

Objective

Increased fatty acid and triglyceride synthesis in liver, majorly modulated by Sterol Regulator Elementing Binding Protein 1c (SREBP1c), is one of the main features of non-alcoholic fatty liver disease (NAFLD). In the present study, we aimed to identify the relation between SREBP1c and autophagy mediated lipid droplet (LD) catabolism in oleic acid (OA) induced lipid accumulation.

Methods

Increased LD formation and SREBP1c induction were identified in hepatocytes (AML12 cells) following the OA administration. SREBP1c level was reduced through siRNA against SREBP1c. The amount and the size of LDs were determined by BODIPY, while protein and mRNA expressions were identified by immunoblotting and qRT-PCR, respectively. LD-lysosome colocalization was determined with immunofluorescence.

Results

Increased LD formation and SREBP1c levels were determined at 0.06 mM OA concentration. SREBP1c silencing reduced the number of LDs, while increasing mRNA levels of PPARα. On the other hand, SREBP1c silencing in non-OA and OA treated cells enhanced autophagy mediated LD catabolism.

Conclusion

Our results implicate the effect of SREBP1c deficiency in modulating PPARα signaling and autophagy mediated LD catabolism against OA induced lipid accumulation.

Emerging Role of Bone Morphogenetic Protein 4 in Metabolic Disorders

Bone morphogenetic proteins (BMPs) are a group of signaling molecules that belong to the TGF-β superfamily. Initially discovered for their ability to induce bone formation, BMPs are known to play a diverse and critical array of biological roles. We here focus on recent evidence showing that BMP4 is an important regulator of white/beige adipogenic differentiation with important consequences for thermogenesis, energy homeostasis, and development of obesity in vivo. BMP4 is highly expressed in, and released by, human adipose tissue, and serum levels are increased in obesity. Recent studies have now shown BMP4 to play an important role not only for white/beige/brown adipocyte differentiation and thermogenesis but also in regulating systemic glucose homeostasis and insulin sensitivity. It also has important suppressive effects on hepatic glucose production and lipid metabolism. Cellular BMP4 signaling/action is regulated by both ambient cell/systemic levels and several endogenous and systemic BMP antagonists. Reduced BMP4 signaling/action can contribute to the development of obesity, insulin resistance, and associated metabolic disorders. In this article, we summarize the pleiotropic functions of BMP4 in the pathophysiology of these diseases and also consider the therapeutic implications of targeting BMP4 in the prevention/treatment of obesity and its associated complications.