Nonalcoholic Fatty Liver Disease and Sarcopenia: Where Do We Stand?

Adipokines in the Crosstalk between Adipose Tissues and Other Organs: Implications in Cardiometabolic Diseases

Adipose tissue was previously regarded as a dormant organ for lipid storage until the identification of adiponectin and leptin in the early 1990s. This revelation unveiled the dynamic endocrine function of adipose tissue, which has expanded further. Adipose tissue has emerged in recent decades as a multifunctional organ that plays a significant role in energy metabolism and homeostasis. Currently, it is evident that adipose tissue primarily performs its function by secreting a diverse array of signaling molecules known as adipokines. Apart from their pivotal function in energy expenditure and metabolism regulation, these adipokines exert significant influence over a multitude of biological processes, including but not limited to inflammation, thermoregulation, immune response, vascular function, and insulin sensitivity. Adipokines are pivotal in regulating numerous biological processes within adipose tissue and facilitating communication between adipose tissue and various organs, including the brain, gut, pancreas, endothelial cells, liver, muscle, and more. Dysregulated adipokines have been implicated in several metabolic diseases, like obesity and diabetes, as well as cardiovascular diseases. In this article, we attempted to describe the significance of adipokines in developing metabolic and cardiovascular diseases and highlight their role in the crosstalk between adipose tissues and other tissues and organs.

Causal relationship between non-alcoholic fatty liver disease and sarcopenia: a bidirectional Mendelian randomization study

Introduction

A correlation between non-alcoholic fatty liver disease and sarcopenia is demonstrated, but the causality remains unclear. Our study aims to clarify the point of genetics between non-alcoholic fatty liver disease (NAFLD) and sarcopenia at the level of gene prediction through two-sample Mendelian randomization (MR) analysis.

Methods

The study employed the two-sample MR approach to investigate the bi-directional causality between NAFLD and sarcopenia. Published summary statistics were used to obtain instrumental variables (IVs) at the genome-wide significance level.

Results

IVW analysis showed that the risk of NAFLD was reduced when walking pace was increased (OR = 0.435, 95%CI 0.240-0.789, p = 0.006); Increasing appendicular lean mass (ALM) decreased the risk of NAFLD (OR = 0.906, 95%CI 0.838-0.980, p = 0.014); Those older than 60 were more likely to suffer from NAFLD if they had low grip strength (OR = 1.411, 95%CI 1.087-1.830, p = 0.0012). In the reverse MR study, weight median analysis showed that NAFLD caused a decrease in ALM (OR = 0.953, 95%CI 0.957-0.994, p = 0.001); whereas NAFLD showed no correlation with usual walking pace or grip strength (all with p > 0.05). MR-Egger regression analysis showed that there was no horizontal pleiotropy in the SNPs (all with p > 0.05).

Conclusion

The characteristics related to sarcopenia (usual walking pace, appendicular lean mass and low hand grip strength) may play a causal role in the development of nonalcoholic fatty liver disease, although the underlying mechanisms need to be further investigated. The presence of specific single nucleotide polymorphisms (SNPs) such as rs3747207, rs429358, and rs73001065 has been identified in the PNPLA3, APOE, and MAU2 proteins. These genetic markers represent potential targets for future interventions aimed at addressing, managing, or mitigating the risk of NAFLD.

ADAR2 deficiency ameliorates non-alcoholic fatty liver disease and muscle atrophy through modulating serum amyloid A1

BACKGROUND

Non-alcoholic fatty liver disease (NAFLD) is the most common cause of chronic liver disease worldwide. Sarcopenia is a syndrome characterized by progressive and generalized loss of skeletal muscle mass and strength, which is commonly associated with NAFLD. Adenosine-to-inosine editing, catalysed by adenosine deaminase acting on RNA (ADAR), is an important post-transcriptional modification of genome-encoded RNA transcripts. Three ADAR gene family members, including ADAR1, ADAR2 and ADAR3, have been identified. However, the functional role of ADAR2 in obesity-associated NAFLD and sarcopenia remains unclear.

METHODS

ADAR2+/+/GluR-BR/R mice (wild type [WT]) and ADAR2-/-/GluR-BR/R mice (ADAR2 knockout [KO]) were subjected to feeding with standard chow or high-fat diet (HFD) for 20 weeks at the age of 5 weeks. The metabolic parameters, hepatic lipid droplet, grip strength test, rotarod test, muscle weight, fibre cross-sectional area (CSA), fibre types and protein associated with protein degradation were examined. Systemic and local tissues serum amyloid A1 (SAA1) were measured. The effects of SAA1 on C2C12 myotube atrophy were investigated.

RESULTS

ADAR2 KO mice fed with HFD exhibited lower body weight (-7.7%, P < 0.05), lower liver tissue weight (-20%, P < 0.05), reduced liver lipid droplets in concert with a decrease in hepatic triglyceride content (-24%, P < 0.001) and liver injury (P < 0.01). ADAR2 KO mice displayed protection against HFD-induced glucose intolerance, insulin resistance and dyslipidaemia. Skeletal muscle mass (P < 0.01), muscle strength (P < 0.05), muscle endurance (P < 0.001) and fibre size (CSA; P < 0.0001) were improved in ADAR2 KO mice fed with HFD compared with WT mice fed with HFD. Muscle atrophy-associated transcripts, such as forkhead box protein O1, muscle atrophy F-box/atrogin-1 and muscle RING finger 1/tripartite motif-containing 63, were decreased in ADAR2 KO mice fed with HFD compared with WT mice fed with HFD. ADAR2 deficiency attenuates HFD-induced local liver and skeletal muscle tissue inflammation. ADAR2 deficiency abolished HFD-induced systemic (P < 0.01), hepatic (P < 0.0001) and muscular (P < 0.001) SAA1 levels. C2C12 myotubes treated with recombinant SAA1 displayed a decrease in myotube length (-37%, P < 0.001), diameter (-20%, P < 0.01), number (-39%, P < 0.001) and fusion index (-46%, P < 0.01). Myogenic markers (myosin heavy chain and myogenin) were decreased in SAA1-treated myoblast C2C12 cells.

CONCLUSIONS

These results provide novel evidence that ADAR2 deficiency may be important in obesity-associated sarcopenia and NAFLD. Increased SAA1 might be involved as a regulatory factor in developing sarcopenia in NAFLD.

Mitochondrial involvement in sarcopenia

Sarcopenia lowers the quality-of-life for millions of people across the world, as accelerated loss of skeletal muscle mass and function contributes to both age- and disease-related frailty. Physical activity remains the only proven therapy for sarcopenia to date, but alternatives are much sought after to manage this progressive muscle disorder in individuals who are unable to exercise. Mitochondria have been widely implicated in the etiology of sarcopenia and are increasingly suggested as attractive therapeutic targets to help restore the perturbed balance between protein synthesis and breakdown that underpins skeletal muscle atrophy. Reviewing current literature, we note that mitochondrial bioenergetic changes in sarcopenia are generally interpreted as intrinsic dysfunction that renders muscle cells incapable of making sufficient ATP to fuel protein synthesis. Based on the reported mitochondrial effects of therapeutic interventions, however, we argue that the observed bioenergetic changes may instead reflect an adaptation to pathologically decreased energy expenditure in sarcopenic muscle. Discrimination between these mechanistic possibilities will be crucial for improving the management of sarcopenia.

The Roles of Non-Pharmacologic and Emerging Pharmacologic Management of Non-alcoholic Fatty Liver Disease and Sarcopenia: A Narrative Review

Non-alcoholic fatty liver disease (NAFLD) is one of the most prevalent causes of chronic liver disease worldwide which is often seen in patients with metabolic abnormalities such as those with obesity and insulin resistance. On the other hand, sarcopenia is a generalized and progressive skeletal muscle disorder characterized by low muscle strength, low muscle quality, low physical performance, or a combination of the three. Both disease entities share several underlying risk factors and pathophysiologic mechanisms. These include: (1) cardiometabolic overlaps such as insulin resistance, chronic systemic inflammation, decreased vitamin D levels, sex hormone modifications; (2) muscle-related factors such as those mitigated by myostatin signaling, and myokines (i.e., irisin); and (3) liver-dysfunction related factors such as those associated with growth hormone/insulin-like growth factor 1 Axis, hepatokines (i.e., selenoprotein P and leukocyte cell-derived chemotaxin-2), fibroblast growth factors 21 and 19 (FGF21 and FGF19), and hyperammonemia. This narrative review will examine the pathophysiologic overlaps that can explain the links between NAFLD and sarcopenia. Furthermore, this review will explore the emerging roles of nonpharmacologic (e.g., weight reduction, diet, alcohol, and smoking cessation, and physical activity) and pharmacologic management (e.g., roles of β-hydroxy-β-methylbutyrate, branched-chain amino acid supplements, and testosterone therapy) to improve care, intervention sustainability, and acceptability for patients with sarcopenia-associated NAFLD.

Comparative analysis of the relationship between four hepatic steatosis indices and muscle mass

Several studies have attempted to validate the relationship between hepatic steatosis and sarcopenia. The crucial limitation is to establish the status of hepatic steatosis by costly or invasive methods. Therefore, several models predicting non-alcoholic fatty liver disease (NAFLD) have been developed but have exhibited heterogeneous results. In this study, we aimed to review and compare four representative models and analyze their relationship with the risk of low muscle mass. Korea National Health and Nutrition Examination Surveys from 2008 to 2011 were used to confirm our hypothesis. Dual-energy X-ray absorptiometry was used to measure the amount of skeletal muscle mass. We used four hepatic steatosis indices: hepatic steatosis index (HSI), Framingham steatosis index (FSI), liver fat score (LFS), and fatty liver index (FLI). Multivariate linear and logistic regressions were used to reveal the relationship between NAFLD and low skeletal muscle index (LSMI). Pairs of FSI-FLI and HSI-FLI exhibited the best and second-best correlations among all possible pairs. The four hepatic steatosis models were associated with increased risk for LSMI. After removing the body mass index effect, HSI and FLI remained robust predictors for LSMI. NAFLD was a significant and potent risk factor for low skeletal muscle.

Sarcopenia, healthy living, and mortality in patients with chronic liver diseases

Chronic liver diseases (CLDs) are associated with increased morbidity and mortality. Sarcopenia is an important complication of CLD that can be impacted by several modifiable risk factors. Our aim was to assess the associations between healthy living, sarcopenia, and long-term outcomes among patients with CLD. We used the Third National Health and Nutrition Examination Survey data with National Death Index-linked mortality files. We used the American Heart Association's Life's Simple 7 (LS7) metrics as surrogates of healthy living. The study included 12,032 subjects (34.9% CLDs [0.5% hepatitis B virus (HBV), 1.8% hepatitis C virus (HCV), 5.7% alcohol-associated liver disease (ALD), 26.9% nonalcoholic fatty liver disease (NAFLD)] and 65.1% controls). Prevalence of sarcopenia was higher among NAFLD than other CLDs and the controls (40.7% in NAFLD, 27.2% in ALD, 22.4% in HCV, 16.8% in HBV, and 18.5% in controls; p < 0.001). Among NAFLD and ALD, patients with sarcopenia were less likely to meet ideal LS7 metrics than those without sarcopenia. During 27 years of follow-up, among 4 patients with CLDs and the controls, all-cause cumulative mortality was highest among patients with HCV (35.2%), followed by ALD (34.7%) and NAFLD (29.6%). The presence of sarcopenia was associated with higher risk of all-cause mortality only among subjects with NAFLD (hazard ratio [HR] 1.24; 95% confidence interval [CI] 1.01-1.54; p = 0.04). Among subjects with NAFLD, presence of sarcopenia was associated with higher risk of cardiovascular-specific (HR 2.28 [1.71-3.05; p < 0.01]), cancer-specific (HR 1.90 [1.37-2.65]; p < 0.01), diabetes-specific (HR 6.42 [2.87-14.36]; p < 0.01), and liver-specific mortality (HR 2.49 [1.08-5.76]; p = 0.04). The multivariable model showed that component of LS7 metrics that provided the strongest protection against sarcopenia were ideal body mass index, ideal blood pressure, ideal physical activity, and ideal glycemic control among subjects with NAFLD subjects. Conclusions: Among subjects with NAFLD, sarcopenia is associated with a higher risk of all-cause mortality and liver mortality. Attainment of ideal LS7 metrics provides protection against sarcopenia in NAFLD.

Associations of muscle mass and grip strength with severe NAFLD: A prospective study of 333,295 UK Biobank participants

BACKGROUND & AIMS

Cross-sectional studies have reported that lower muscle mass and strength are risk factors for non-alcoholic fatty liver disease (NAFLD). However, the evidence from prospective studies is limited. This study examined both the strength and pattern of the associations between these 2 physical capability markers and severe NAFLD using data from the UK Biobank study.

METHODS

A total of 333,295 participants were included in this prospective study. Grip strength was measured using a Jamar J00105 hydraulic hand dynamometer, and the Janssen equation was used to estimate skeletal muscle mass by bioelectrical impedance. Muscle mass was adjusted for body weight and all exposures were sex-standardised. Associations of muscle mass and strength with severe NAFLD (defined as hospital admission or death) were first investigated by tertile of each exposure using Cox proportional hazard models. Non-linear associations were investigated using penalised cubic splines fitted in the Cox proportional hazard models.

RESULTS

After a median follow-up of 10 years (IQR 9.3 to 10.7 years), 3,311 individuals had severe NAFLD (3,277 hospitalisations and 34 deaths). Compared with the lowest tertile of muscle mass, the risk of severe NAFLD was lower in the middle (hazard ratio 0.76; 95% CI 0.70-0.83) and the highest tertile (hazard ratio 0.46; 95% CI 0.40-0.52). Tertiles of grip strength showed a similar pattern. Non-linearity was only identified for muscle mass (p <0.001). Being on the lower tertile of grip strength and muscle mass accounted for 17.7% and 33.1% of severe NAFLD cases, respectively.

CONCLUSIONS

Lower muscle mass and grip strength were associated with higher risk of developing severe NAFLD. Interventions to improve physical capability may be protective, but this needs to be investigated in appropriately designed trials.

LAY SUMMARY

Lower muscle mass - both quantity and quality - were associated with a higher risk of severe non-alcoholic fatty liver disease. Therefore, improving muscle mass might be a protective factor against this increasing public health problem.

Frailty in Nonalcoholic Fatty Liver Cirrhosis: A Comparison with Alcoholic Cirrhosis, Risk Patterns, and Impact on Prognosis

BACKGROUND

Physical frailty increases susceptibility to stressors and predicts adverse outcomes of cirrhosis. Data on disease course in different etiologies are scarce, so we aimed to compare the prevalence and risk factors of frailty and its impact on prognosis in nonalcoholic fatty liver (NAFLD) and alcoholic (ALD) cirrhosis. Patients and Methods. Cirrhosis registry RH7 operates since 2014 and includes hospitalized patients with decompensated cirrhosis, pre-LT evaluation, or curable hepatocellular carcinoma (HCC). From the RH7, we identified 280 ALD and 105 NAFLD patients with at least 6 months of follow-up.

RESULTS

Patients with NAFLD compared with ALD were older and had a higher proportion of females, higher body mass index (BMI) and mid-arm circumference (MAC), lower MELD score, CRP, and lower proportion of refractory ascites. The liver frailty index did not differ, and the prevalence of HCC was higher (17.1 vs. 6.8%, p=0.002). Age, sex, serum albumin, and C-reactive protein (CRP) were independent predictors of frailty. In NAFLD, frailty was also associated with BMI and MAC and in ALD, with the MELD score. The Cox model adjusted for age, sex, MELD, CRP, HCC, and LFI showed that NAFLD patients had higher all-cause mortality (HR = 1.88 95% CI 1.32-2.67, p < 0.001) and were more sensitive to the increase in LFI (HR = 1.51, 95% CI 1.05-2.2).

CONCLUSION

Patients with NAFLD cirrhosis had a comparable prevalence of frailty compared to ALD. Although prognostic indices showed less advanced disease, NAFLD patients were more sensitive to frailty, which reflected their higher overall disease burden and led to higher all-cause mortality.

Adipokines, Myokines, and Hepatokines: Crosstalk and Metabolic Repercussions

Adipose, skeletal, and hepatic muscle tissues are the main endocrine organs that produce adipokines, myokines, and hepatokines. These biomarkers can be harmful or beneficial to an organism and still perform crosstalk, acting through the endocrine, paracrine, and autocrine pathways. This study aims to review the crosstalk between adipokines, myokines, and hepatokines. Far beyond understanding the actions of each biomarker alone, it is important to underline that these cytokines act together in the body, resulting in a complex network of actions in different tissues, which may have beneficial or non-beneficial effects on the genesis of various physiological disorders and their respective outcomes, such as type 2 diabetes mellitus (DM2), obesity, metabolic syndrome, and cardiovascular diseases (CVD). Overweight individuals secrete more pro-inflammatory adipokines than those of a healthy weight, leading to an impaired immune response and greater susceptibility to inflammatory and infectious diseases. Myostatin is elevated in pro-inflammatory environments, sharing space with pro-inflammatory organokines, such as tumor necrosis factor-alpha (TNF-α), interleukin-1 (IL-1), resistin, and chemerin. Fibroblast growth factor FGF21 acts as a beta-oxidation regulator and decreases lipogenesis in the liver. The crosstalk mentioned above can interfere with homeostatic disorders and can play a role as a potential therapeutic target that can assist in the methods of diagnosing metabolic syndrome and CVD.