Potential emerging roles of the novel adipokines adipolin/CTRP12 and meteorin-like/METRNL in obesity-osteoarthritis interplay

The Function and Mechanism of Anti-Inflammatory Factor Metrnl Prevents the Progression of Inflammatory-Mediated Pathological Bone Osteolytic Diseases

Metrnl, recently identified as an adipokine, is a secreted protein notably expressed in white adipose tissue, barrier tissues, and activated macrophages. This adipokine plays a pivotal role in counteracting obesity-induced insulin resistance. It enhances adipose tissue functionality by promoting adipocyte differentiation, activating metabolic pathways, and exerting anti-inflammatory effects. Extensive research has identified Metrnl as a key player in modulating inflammatory responses and as an integral regulator of muscle regeneration. These findings position Metrnl as a promising biomarker and potential therapeutic target in treating inflammation-associated pathologies. Despite this, the specific anti-inflammatory mechanisms of Metrnl in immune-mediated osteolysis and arthritis remain elusive, warranting further investigation. In this review, we will briefly elaborate on the role of Metrnl in anti-inflammation function in inflammation-related osteolysis, arthritis, and pathological bone resorption, which could facilitate Metrnl's clinical application as a novel therapeutic strategy to prevent bone loss. While the pathogenesis of elbow stiffness remains elusive, current literature suggests that Metrnl likely exerts a pivotal role in its development.

Exerkines and osteoarthritis

Osteoarthritis (OA) is the most prevalent chronic joint disease, with physical exercise being a widely endorsed strategy in its management guidelines. Exerkines, defined as cytokines secreted in response to acute and chronic exercise, function through endocrine, paracrine, and/or autocrine pathways. Various tissue-specific exerkines, encompassing exercise-induced myokines (muscle), cardiokines (heart), and adipokines (adipose tissue), have been linked to exercise therapy in OA. Exerkines are derived from these kines, but unlike them, only kines regulated by exercise can be called exerkines. Some of these exerkines serve a therapeutic role in OA, such as irisin, metrnl, lactate, secreted frizzled-related protein (SFRP), neuregulin, and adiponectin. While others may exacerbate the condition, such as IL-6, IL-7, IL-15, IL-33, myostatin, fractalkine, follistatin-like 1 (FSTL1), visfatin, activin A, migration inhibitory factor (MIF), apelin and growth differentiation factor (GDF)-15. They exerts anti-/pro-apoptosis/pyroptosis/inflammation, chondrogenic differentiation and cell senescence effect in chondrocyte, synoviocyte and mesenchymal stem cell. The modulation of adipokine effects on diverse cell types within the intra-articular joint emerges as a promising avenue for future OA interventions. This paper reviews recent findings that underscore the significant role of tissue-specific exerkines in OA, delving into the underlying cellular and molecular mechanisms involved.

Obesity, Metabolic Syndrome, and Osteoarthritis-An Updated Review

PURPOSE OF REVIEW

Metabolic syndrome (MetS), also called the 'deadly quartet' comprising obesity, diabetes, dyslipidemia, and hypertension, has been ascertained to have a causal role in the pathogenesis of osteoarthritis (OA). This review is aimed at discussing the current knowledge on the contribution of metabolic syndrome and its various components to OA pathogenesis and progression.

RECENT FINDINGS

Lately, an increased association identified between the various components of metabolic syndrome (obesity, diabetes, dyslipidemia, and hypertension) with OA has led to the identification of the 'metabolic phenotype' of OA. These metabolic perturbations alongside low-grade systemic inflammation have been identified to inflict detrimental effects upon multiple tissues of the joint including cartilage, bone, and synovium leading to complete joint failure in OA. Recent epidemiological and clinical findings affirm that adipokines significantly contribute to inflammation, tissue degradation, and OA pathogenesis mediated through multiple signaling pathways. OA is no longer perceived as just a 'wear and tear' disease and the involvement of the metabolic components in OA pathogenesis adds up to the complexity of the disease. Given the global surge in obesity and its allied metabolic perturbations, this review aims to throw light on the current knowledge on the pathophysiology of MetS-associated OA and the need to address MetS in the context of metabolic OA management. Better regulation of the constituent factors of MetS could be profitable in preventing MetS-associated OA. The identification of key roles for several metabolic regulators in OA pathogenesis has also opened up newer avenues in the recognition and development of novel therapeutic agents.

Beyond mechanical loading: The metabolic contribution of obesity in osteoarthritis unveils novel therapeutic targets

Osteoarthritis (OA) is a prevalent progressive disease that frequently coexists with obesity. For several decades, OA was thought to be the result of ageing and mechanical stress on cartilage. Researchers' perspective has been greatly transformed when cumulative findings emphasized the role of adipose tissue in the diseases. Nowadays, the metabolic effect of obesity on cartilage tissue has become an integral part of obesity research; hoping to discover a disease-modifying drug for OA. Recently, several adipokines have been reported to be associated with OA. Particularly, metrnl (meteorin-like) and retinol-binding protein 4 (RBP4) have been recognized as emerging adipokines that can mediate OA pathogenesis. Accordingly, in this review, we will summarize the latest findings concerned with the metabolic contribution of obesity in OA pathogenesis, with particular emphasis on dyslipidemia, insulin resistance and adipokines. Additionally, we will discuss the most recent adipokines that have been reported to play a role in this context. Careful consideration of these molecular mechanisms interrelated with obesity and OA will undoubtedly unveil new avenues for OA treatment.

Biomarkers of Osteoarthritis—A Narrative Review on Causal Links with Metabolic Syndrome

Development of OA (OA) is multifactorial and is strongly associated with risk factors such as aging, trauma, metabolic disorders, and obesity. Metabolic Syndrome (MetS)-associated OA, collectively coined MetS-OA, is an increasingly recognized entity in which metabolic disorders and low-grade inflammation play a key mechanistic role in the disruption of joint homeostasis and cartilage degradation. Although there have been enormous efforts to discover biomarkers of MetS and OA, studies investigating a pathophysiological link between MetS and OA are relatively limited, and no serum blood marker has proved diagnostic so far. OA biomarkers that are necessary to discriminate and diagnose early disease remain to be elicited, explained in part by limited prospective studies, and therefore limited tools available to utilize in any prognostic capacity. Biomarker validation projects have been established by the Biomarker Consortium to determine biochemical markers demonstrating predictive validity for knee OA. Given that the metabolic constituents of MetS are treatable to varying extents, it stands to reason that treating these, and monitoring such treatment, may help to mitigate deleterious links with OA development. This narrative review will describe the current state of biomarker identification and utility in OA associated with MetS. We discuss the pathophysiological mechanisms of disease according to constituent pathologies of MetS and how identification of biomarkers may guide future investigation of novel targets.

Exercise induced meteorin-like protects chondrocytes against inflammation and pyroptosis in osteoarthritis by inhibiting PI3K/Akt/NF-κB and NLRP3/caspase-1/GSDMD signaling

The production of metrnl, a novel adipomyokine, is induced upon exercise in adipose tissue and skeletal muscle. In this study, we investigated the anti-inflammatory and antipyroptotic effects of exercise-induced metrnl producted in rats in vitro and in vivo. Forty Sprague-Dawley rats were divided randomly into five groups: control (CG), osteoarthritis (OA) with sedentary lifestyle (OAG), OA with low intensity exercise (OAL), OA with moderate intensity exercise (OAM), and OA with high intensity exercise (OAH). The correlation between the level of metrnl and OA degree was detected using ELISA, X-ray imaging, histology, and immunohistochemistry in vivo. Primary chondrocytes were preincubated with recombinant metrnl before interleukin-1β administration to verify the anti-inflammatory and antipyroptotic effects of metrnl. Western blotting and quantitative reverse transcription (qRT)-PCR were used to evaluate the differences in protein and mRNA expression between groups, respectively. Reactive oxygen species (ROS) assay, immunofluorescence, transmission electron microscopy (TEM), and flow cytometry were used to evaluate morphological changes and pyroptosis in chondrocytes. In the moderate-intensity treadmill exercise group, the severity of OA showed maximum relief and the metrnl levels had the most significant increase. Metrnl exerted its anti-inflammatory effect through the suppression of the PI3K/Akt/NF-κB pathway in IL-1β-induced OA chondrocytes, which was accompanied with the recovery of collagen II expression and the attenuation of MMP13 and ADAMTS5. Moreover, metrnl ameliorated chondrocyte pyroptosis by inhibiting the activation of the nod-like receptor protein-3/caspase-1/gasdermin D cascade. In conclusion, moderate-intensity exercise improves inflammation and pyroptosis by increasing metrnl release, which inhibits the PI3K/Akt/NF-κB and further NLRP3/caspase-1/GSDMD signaling pathways.

Meteorin-like/Metrnl, a novel secreted protein implicated in inflammation, immunology, and metabolism: A comprehensive review of preclinical and clinical studies

Meteorin-like, also known as Metrnl, Meteorin-β, Subfatin, and Cometin, is a novel secreted protein exerting pleiotropic effects on inflammation, immunology, and metabolism. Earlier research on this hormone focused on regulating energy expenditure and glucose homeostasis. Consequently, several studies attempted to characterize the molecule mechanism of Metrnl in glucose metabolism and obesity-related disorders but reported contradictory clinical results. Recent studies gradually noticed its multiple protective functions in inflammatory immune regulations and cardiometabolic diseases, such as inducing macrophage activation, angiogenesis, tissue remodeling, bone formation, and preventing dyslipidemias. A comprehensive understanding of this novel protein is essential to identify its significance as a potential therapeutic drug or a biomarker of certain diseases. In this review, we present the current knowledge on the physiology of Metrnl and its roles in inflammation, immunology, and metabolism, including animal/cell interventional preclinical studies and human clinical studies. We also describe controversies regarding the data of circulation Metrnl in different disease states to determine its clinical application better.

Elevated levels of Metrnl in rheumatoid arthritis: Association with disease activity

BACKGROUND

Meteorin-like (Metrnl) is a novel adipokine that is highly expressed in white adipose tissues. Whether Metrnl plays a role in rheumatoid arthritis (RA) remains unclear. In this study, sera from 159 RA patients, 28 osteoarthritis (OA) patients, and 50 healthy individuals were included. The serum levels of Metrnl were measured using an enzyme-linked immunosorbent assay. Clinical parameters, including disease activity score 28 (DAS28), C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), rheumatoid factor (RF), antibodies to cyclic citrulline peptide (anti-CCP), inflammatory cytokines, and blood biochemical indices were collected.

RESULTS

Metrnl levels were higher in RA patients compared to OA patients and controls. In the RA group, serum Metrnl levels were positively correlated with DAS28, RF, and CRP levels. However, in the RA group, serum Metrnl levels were not correlated with ESR, anti-CCP, immunoglobulins, and blood biochemical indices.

CONCLUSION

This study showed that Metrnl is involved in the pathogenesis of RA. Increase in serum Metrnl levels is closely related to RA activity.

Engineering Closed-Loop, Autoregulatory Gene Circuits for Osteoarthritis Cell-Based Therapies

PURPOSE OF REVIEW

Genetic engineering offers the possibility to simultaneously target multiple cellular pathways in the joints affected by osteoarthritis (OA). The purpose of this review is to summarize the ongoing efforts to develop disease-modifying osteoarthritis drugs (DMOADs) using genetic engineering, including targeting approaches, genome editing techniques, and delivery methods.

RECENT FINDINGS

Several gene circuits have been developed that reprogram cells to autonomously target inflammation, and their efficacy has been demonstrated in chondrocytes and stem cells. Gene circuits developed for metabolic disorders, such as those targeting insulin resistance and obesity, also have the potential to mitigate the impact of these conditions on OA onset and/or progression. Despite the strides made in characterizing the inflammatory environment of the OA joint, our incomplete understanding of how the multiple regulators interact to control signal transduction, gene transcription, and translation to protein limits the development of targeted disease-modifying therapeutics. Continuous advances in targeted genome editing, combined with online toolkits that simplify the design and production of gene circuits, have the potential to accelerate the discovery and clinical application of multi-target gene circuits with disease-modifying properties for the treatment of OA.

Serum Meteorin-like is associated with weight loss in the elderly patients with chronic heart failure

BACKGROUND

Unintentional weight loss (cachexia) has been associated with poor outcomes in chronic heart failure (CHF). Meteorin-like (Metrnl) is a novel myokine with protective effects on cardiovascular diseases. However, the change of Metrnl concentrations and its role in elderly patients with CHF remains unclear. The aim of this study was to evaluate the association of serum Metrnl with weight loss and outcomes in elderly patients with CHF.

METHODS

A total of 931 consecutive elderly patients (aged 60 years and older) with CHF and 135 age-matched and sex-matched control subjects were enrolled. Serum Metrnl concentration was measured by enzyme-linked immunosorbent assay. Body weight was measured at baseline and 12 months.

RESULTS

Median of serum Metrnl levels was lower in CHF patients when compared with controls [201.31 (184.95-261.16) pg/mL vs. 168.68 (103.15-197.54) pg/mL, P < 0.001]. Patients with the lowest levels of Metrnl had higher N-terminal pro brain natriuretic peptide (NT-proBNP) levels but lower left ventricular eject fraction (LVEF) and estimated glomerular filtration rate (P < 0.001). Lower serum Metrnl was associated with a higher risk of >5% weight loss from baseline to 12 months [odds ratio = 6.13, 95% confidence interval (CI) = 2.57-14.62 per log decrease; P < 0.001]. Serum Metrnl levels were decreased as LVEF decreased (P < 0.001) and were positively correlated with LVEF (r = 0.267, P < 0.001) but negatively correlated with NT-proBNP levels (r = -0.257, P < 0.001). Cox regression analysis suggested that lower serum Metrnl was associated with higher cardiovascular mortality [hazard ratio (HR) = 6.71, 95% CI = 3.41-13.18 per log decrease; P < 0.001], CHF rehospitalization (HR = 3.07, 95% CI = 1.82-5.17 per log decrease; P < 0.001), and the combined major adverse cardiac event(s) (MACEs) (HR = 5.38, 95% CI = 3.51-8.25 per log decrease; P < 0.001). The Kaplan-Meier survival curves showed that low concentration of Metrnl was a prognostic indicator of MACEs in patients with CHF.

CONCLUSIONS

Our study suggests that lower serum Metrnl level is correlated with weight loss and the severity of cardiac dysfunction in elderly patients with CHF.

Serum Metrnl levels are decreased in subjects with overweight or obesity and are independently associated with adverse lipid profile

BACKGROUND

Meteorin-like (Metrnl), a novel adipokine, is highly expressed in adipose tissue and has a beneficial effect on energy metabolism. However, data on circulating Metrnl levels in obesity are scarce and inconsistent. This study aimed to evaluate the serum levels of Metrnl in adults with obesity and its association with glucose and lipid metabolism.

METHODS

182 subjects were included in the cross-sectional study. The participants were divided into three groups according to BMI: normal (n = 95), overweight (n = 46), and obesity (n = 41). Serum Metrnl concentrations were measured by enzyme-linked immunosorbent assay.

RESULTS

Serum Metrnl levels in overweight or obese subjects were significantly lower than in the normal group. Circulating Metrnl levels were negatively correlated with TG, TC, LDL-C, and sdLDL and positively correlated with HDL-C before and after adjusting for age, sex, BMI, diabetes, HOMA-IR, and eGFR (all P < 0.05). Furthermore, logistic regression analysis indicated that compared with the highest tertile, the lowest tertile of Metrnl levels were significantly associated with the presence of hyper-TG, hyper-TC, and Hyper-LDL after full adjustment (all P for trend < 0.05).

CONCLUSIONS

Serum Metrnl levels were reduced in individuals with overweight or obesity and were independently associated with adverse lipid profile, suggesting that modifying circulating Metrnl levels may serve as a potential therapeutic target for atherogenic dyslipidemia.

The Role of Exercise, Diet, and Cytokines in Preventing Obesity and Improving Adipose Tissue

The prevalence of obesity continues to rise worldwide despite evidence-based public health recommendations. The promise to adopt a healthy lifestyle is increasingly important for tackling this global epidemic. Calorie restriction or regular exercise or a combination of the two is accepted as an effective strategy in preventing or treating obesity. Furthermore, the benefits conferred by regular exercise to overcome obesity are attributed not only to reduced adiposity or reduced levels of circulating lipids but also to the proteins, peptides, enzymes, and metabolites that are released from contracting skeletal muscle or other organs. The secretion of these molecules called cytokines in response to exercise induces browning of white adipose tissue by increasing the expression of brown adipocyte-specific genes within the white adipose tissue, suggesting that exercise-induced cytokines may play a significant role in preventing obesity. In this review, we present research-based evidence supporting the effects of exercise and various diet interventions on preventing obesity and adipose tissue health. We also discuss the interplay between adipose tissue and the cytokines secreted from skeletal muscle and other organs that are known to affect adipose tissue and metabolism.