Adipokine role in physiopathology of inflammatory and degenerative musculoskeletal diseases

Introduction

Nowadays adipocytes are not anymore considered a passive storage of fat tissue and their metabolic functions are not limited to energetic homeostasis and thermogenesis alone. Indeed, it has been widely demonstrated that adipocytes produce a complex cohort of cytokines, called adipokines, which perform endocrine, paracrine, and autocrine effects. Adipokines are neuromodulators, growth factors, proteins of the complement system, acute phase proteins, stress hormones, and proteins involved in glucose homeostasis. ¹ Circulating levels of specific adipokines correlate with the amount of adipose tissue in the human body. Changes in adipokine titers define an impairment in the balance between proinflammatory and antiinflammatory cytokines. This is in keeping with the hypothesis of a complex homeostatic function performed by adipocytes, which have also been defined as an endocrine organ. ²

It is widely known that obesity impairs the physiological balance of cytokines. Hypertrophy of adipocytes in obese individuals leads to a state of local hypoxia which subsequently causes necrosis associated changes in local immune response.^3,4 Obesity defines a shift in the anti-inflammatory/pro-inflammatory balance due to migration of monocytes to necrotic clusters in the center of adipose tissue. ⁵ The resulting low grade inflammation, whether due to obesity or to autoimmune diseases, leads to an increase in cardiovascular risk.^6,7 High-fat diet and obesity lead to CD4⁺ Foxp3⁺ regulatory T cells depletion in men, which defines reduced insulin sensitivity.^8,9 iNK (invariant natural killer) is a subset of NK lymphocytes that is widely expressed in white adipose tissue; these lymphocytes are thought to promote an anti-inflammatory effect by producing cytokines belonging to the Th2 group, such as IL-4, IL-13, ¹⁰ and IL-10, which can avoid the insulin-desensitizing effects of TNF-a on adipocytes. ¹¹ Lynch et al. ¹² observed that hypertrophic adipose tissue expressed a decreased concentration of iNK, while the number of proinflammatory macrophages increased.

A specific subset of lymphocytes, called B regulatory lymphocytes, are constitutively expressed in white adipose tissue. Their cytokine pattern includes IL-10 and TGF-beta which exert an antiinflammatory effect and in the obese population this subset is less expressed leading to a progressive proinflammatory condition. ¹³ It is important to emphasize the action performed by macrophages as scavenger cells of necrotic adipocytes, as it leads to a local increase of IL-17 and TNF-alfa. ¹⁴ Patients with autoimmune rheumatic diseases such as rheumatoid arthritis (RA), osteoarthritis (OA), systemic lupus erythematosus (SLE), systemic sclerosis (Ssc), and vasculitis among others, display a high prevalence of obesity and consequently an altered balance of adipokines toward a pro-inflammatory condition, ¹⁵ compared to general population. ¹⁶ For example, obesity has been shown to be associated with an increased incidence of RA. ¹⁷

Adipokines may play a central role in controlling the immune response and have been shown to play a crucial role importance in the pathogenesis of arthritic diseases. ¹⁸ The divergence between the adipokine concentration in serum and synovial fluid depends on many factors: the difference of molecular weight may play a role in this process, although adipokines with a similar molecular weight such as leptin and resistin showed different concentrations in the synovium and serum, which is in keeping with a local production of adipokines, as has been shown for leptin in cortical bone. ¹⁹ The distribution undergoes many adjustments influenced by multiple factors such as the presence of inflamed synovium or serum binding proteins. ²⁰

This review aims to summarize the evidences on the importance of adipokines in inflammatory and degenerative musculoskeletal diseases.

Adiponectin

Adiponectin is a 30 kDa protein characterized by a structure similar to that of the complement protein C1q secreted by adipocytes. Various forms of adiponectin have been described: the globular form, the trimeric form (low molecular weight, LMW), the hexameric form (middle molecular weight, MMW) and the multimeric (high molecular weight, HMW) adiponectin, and each of these form exerts a specific biological effect. ²¹ Adiponectin has shown anti-diabetic, anti-inflammatory and anti-atherogenic effects, ²² indeed it is a TNF-alfa inhibitor and vice versa.^23,24 But the actual role of adiponectin with respect to diseases typically featured by a high level of pro-inflammatory cytokines such as autoimmune diseases is still unclear. In addition, cardiovascular diseases, such as hypertension, have been shown to express an altered balance of adipokines including adiponectin.^25,26 In vitro studies showed anti-inflammatory effects of adiponectin, as a decreased expression of IL-8 in aortic endothelial cells stimulated by TNF-alfa was shown, ²⁷ conversely a ceased expression of the anti-inflammatory cytokine IL-10 in human macrophage, has been observed. ²⁸ In a case series of patients with sarcoidosis, high levels of serum adiponectin were observed and it was associated with clinical features such as arthritis or arthralgia, suggesting an anti-inflammatory role in attenuating the inflammatory cascade of the disease. ²⁹ In addition, exposure to adiponectin has been shown to reduce the adhesion of monocytes to endothelial cells and the conversion of macrophages to foam cells, ³⁰ thus hampering the genesis of atherosclerosis. Despite the positive in vitro effects demonstrated by adiponectin, several studies have shown that low doses of the cytokine did not show any future cardiovascular event. ³¹ Low serum levels of adiponectin have also been found in a series of cases with chronic low back pain and could take on clinical significance as a marker between the acute and the chronic phase of the disease, although more studies are needed to clarify this finding. ³² Serum adiponectin levels were found to be lower in obese subjects, conversely weight loss and reduction in the average size of adipocytes is associated with an increase in serum adiponectin levels.^33,34 The salivary glands in patients with Sjogren’s disease showed stronger expression of adiponectin and adiponectin receptor. The significance for this altered expression is still unknown, but it may be linked to the aberrant production of cytokines by the glandular cells. ³⁵

Adiponectin and systemic sclerosis

Adiponectin plays an important role in preventing the fibrotic proliferation occurring during systemic sclerosis, and this effect appears to be linked to several molecular mechanisms. As a matter of fact this cytokine suppresses Th2 CD4⁺ lymphocytes polarization and their pro-fibrotic effects; furthermore a low serum level of adiponectin has been observed in many patients with systemic sclerosis during the early stages of the disease.^46,47 Marangoni et al. ⁴⁸ observed that skin biopsies from patients with systemic sclerosis express a lower cellular phosphorylated AMPK level, which in turn reflects lower serum adiponectin levels. The expression of adiponectin in lung and gastric tissue subjected to biopsy in patients with end-stage systemic sclerosis, showed an inverse correlation with the amount of fibrosis of these tissues; while healthy controls showed strong cytokine expression, especially in the larger and thinner vessels of the lungs. ⁴⁹ This finding may be partially explained by the anti-fibrotic effect of adiponectin, as it has been shown to attenuate liver fibrosis and prevent myocardial fibrosis.^50,51 In cutaneous systemic sclerosis, a negative correlation between clinical activity and disease duration has been observed.^52,53 Evidence that IL-17 serum titer is linked to the extent of skin fibrosis ⁵⁴ implies a connection between adipokines and systemic sclerosis. Zhang et al. ⁵⁵ demonstrated that adiponectin suppresses the differentiation of Th17 and Th1 cells, while in murine models it has been observed that a deficiency of the resistin-Like Molecule (RELM) causes a decrease in the population of Th17 and Th1 cells (Figure 1).

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Figure 1.

Simplified graphic showing adiponectin implications in autoimmune diseases.

Leptin

In 1994, following the discovery of a new protein, the first of the adipokine family called leptin, ⁵⁶ many hopes raised for the obese population as this new protein seemed to be the missing link for a comprehensive understanding of the pathophysiological mechanisms and energy balance process in this population. ⁵⁷ This discovery drew the attention of the scientific community and in the following years many other adipokines were described. ⁵⁸ Leptin is a 16 kDa glycosylated protein that exerts its functions by binding to the leptin receptor, a class I cytokine receptor. This protein has two isoforms: the first is soluble and characterized by a short cytoplasmatic domain, while the second is represented in almost every organ and is involved in the JAK/STAT pathway. ⁵⁹ This cytokine plays a role in the hypothalamic–pituitary–adrenal (HPA) axis by inducing hypothalamic cells to increase anorexigenic factors and lower orexigenic factors, ⁶⁰ in order to regulate appetite and food intake. The HPA axis is responsible for the production of glucocorticoids and responds to the secretion of pro-inflammatory factors such as IL-6 or TNF-alfa. It was observed that mice with a leptin receptor deficiency showed a dramatic lowering of CD4⁺ T lymphocytes. ⁶¹ The expression of this receptor by almost all immune cells, as well as its influence on HPA axis, suggests an important role played by leptin in regulating immune system.^62,63 Recently Caso et al. ⁶⁴ showed that the serum level of leptin in patients with psoriatic arthritis (PsA) sine psoriasis correlates positively with C reactive protein and BMI, while in PsA with psoriasis the serum level of leptin correlates with IL-6 serum level but not with CRP (C reactive protein).

Visfatin

Visfatin is a novel adipokine broadly known as nicotinamide phosphoribosyltransferase (NAMPT), studied for its implications in oncology and neurology.^93,94 Visfatin is mainly expressed in visceral fat versus subcutaneous fat, but it is not limited to fat tissue as it is expressed in the lungs, kidney, bone marrow, and heart.^95,96 The homology of the highly conserved protein sequence in multiple species such as the human, mouse, dog, and invertebrate sponge suggests an important role of this adipokine,^97–99 whose expression in the cartilage and synovium ¹⁰⁰ plays a potential role in the physiopathology of musculoskeletal diseases. ¹⁰¹ Visfatin exerts a pro-inflammatory role by inducing the production of TNF-alfa, IL-6, IL1-beta in CD14⁺ monocytes and modulating the MAPK pathway and the activation of NF-kB signaling. ¹⁰² Recently the expression of visfatin following local hypoxia has been mainly linked to the activation of the Janus kinase pathway. ¹⁰³

Visfatin serum levels in RA patients has been linked to disease activity, while a decrease in visfatin levels during the first 3 months of therapy has been shown to be predictive for a long-term DAS28 improvement. ¹⁰⁴ Conversely, the serum level of visfatin has not shown any correlation with disease activity in juvenile idiopathic arthritis (JIA) despite effective treatment, but a higher visfatin level may have a potential role as a negative predictor of treatment, identifying patients more likely to cease methotrexate, which is a first line treatment in JIA; this evidence could thus lead to earlier use of second-line treatments. ¹⁰⁵ Hulejová et al. ¹⁰⁶ did not found any correlation between visfatin levels and disease activity and acute phase reactants in AS patients. Similarly, AS patients on long term treatment with infliximab did not show any significant correlation between visfatin levels and disease activity. ⁴¹ Behcet’s syndrome patients had lower serum levels of visfatin in both the active and quiescent phases of the disease compared to healthy controls, likely due to genetic suppression by pro-inflammatory cytokines such as TNF-alfa and IL-6 ¹⁰⁷ (Figure 2).

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Figure 2.

Simplified graphic showing leptin and visfatin implications in autoimmune diseases.

Chemerin-1

Chemerin, known as retinoic acid receptor responder protein 2 (RARRES2), has been shown to be expressed in various tissues. Initially it was found in stimulated cultures of psoriatic skin, ¹⁰⁸ in exudate from synovial fluids of RA patients with, ¹⁰⁹ but then it was also found in white adipose tissue. Chemerin recruits dendritic cells to migrate to visceral adipose tissue, where they produce interferon-1 which triggers the pro-inflammatory response of macrophages. ¹¹⁰ This is in keeping with the high levels of chemerin observed in autoimmune inflammatory diseases such as psoriasis, SLE, or Crohn’s disease. ¹¹¹ The chemerin receptor (ChemR23) is expressed in endothelial cells and is significantly up-regulated by pro-inflammatory cytokines ¹¹² and chemerin serum titers was associated with I-cam and E-selectin levels, ¹¹³ which in turn are expressed on endothelial cells after activation by pro-inflammatory cytokines, such as IL-1 or TNF-alfa.

Chemerin and systemic sclerosis

Recently Sawicka et al. ¹¹⁹ studied the serum levels of visfatin and chemerin in patients with systemic sclerosis in terms of clinical manifestations and duration of the disease and compared them with the levels in the healthy population. Serum levels of chemerin were significantly elevated in patients with systemic sclerosis, especially in the diffuse cutaneous subgroup, compared with the heathy population. No changes in serum levels of the cytokine were found in relation to the duration of the disease. The same was observed in patients with decreased forced vital capacity (FVC) measured by spirometry and the presence of interstitial lung disease (ILD), but interestingly, the chemerin level patients in the late stage group correlated positively with FVC when patients were divided into two groups, one early- and middle-stage group and one late-stage group; similarly, the late-stage group showed an inverse correlation between serum chemerin levels and fibrosis observed in high-resolution computed tomography (HRCT) of the lung (Figure 3).

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Figure 3.

Simplified graphic showing chemerin implications in autoimmune diseases.

Omentin

Omentin/intelectin-1 is an adipokine that was first described in Paneth cells, but subsequently was also expressed in lung, placenta, heart, and ovary.^120,121 In vitro studies demonstrated an inhibitory effect that omentin exerts through modulation of TNF-alfa phosphorylation, Janus kinase pathway activation^122,123 and NF-kB pathway through suppression of nuclear accumulation of p65, NF-kB promoter activity, and attenuation of LPS-induced inflammation in macrophages. ¹²⁴ This anti-inflammatory effect is consistent with the inverse correlation found between synovial omentin-1 and radiographic severity of osteoarthritis. ¹²⁵ On the other hand, omentin levels in the synovial fluid of patients with osteoarthritis were higher than in RA patients, ¹²⁶ suggesting a possible role of omentin in OA development. Serum levels of omentin are inversely correlated with inflammatory cytokines such as IL-6 and TNF-alfa. ¹²⁷ The determined serum levels of omentin were elevated in patients with juvenile idiopathic arhtrtis. ¹²⁸ The same evidence was found in patients with PsA ¹²⁹ compared to healthy and psoriatic controls. A recent study has shown that low omentin serum levels are associated with cardiovascular (CV) risk factors in axial spondyloarthritis (axSpA). Also, the omentin rs12409609 minor allele seems to downregulate the expression of omentin in these patients with this inflammatory arthritis. These data support a potential role of omentin as a cardiovascular risk biomarker in axSpA. ¹³⁰

Meteorin-Like

IL-41 or meteorin-like is a cytokine discovered in 2004 that was initially studied mainly for its neurotrophic role, ¹³² but it was later found that this cytokine is expressed by activated macrophages in skin and mucous membranes, while IL-17 and TNF-alfa increase its production. ¹³³ Recently Bridgewood et al. ¹³⁴ demonstrated the presence of high levels of IL-41 in the synovial fluid of patients with psoriatic arthritis and in rheumatoid arthritis, while normal levels were assessed in patients affected by osteoarthritis. They also showed that the main source of local IL-41 is the stromal cells of enthesis, although other cells may be involved in this process in inflammatory conditions.

Vaspin

Vaspin (visceral adipose tissue-derived serine protease inhibitor) is a novel cytokine, that belongs to the serine-protease inhibitor group. Initially, it was shown that the intake of vaspin defined a reduction in insulin resistance and improved glucose tolerance in obese mice. ¹³⁵ Recently, it was shown that vaspin is adipocyte-specific and that its expression in subcutaneous adipose tissue decreases with increasing fat mass ¹³⁶ and it may define a protective effect exerted on white adipose tissue, by influencing gene expression toward improved insulin resistance. ¹³⁵ Zieger et al. ¹³⁷ compared the expression of the NfKb inflammatory pathway of a native adipocyte cell line with another line overexpressing visfatin, and it was found both in vitro and in vivo that when stimulated with IL-1 beta almost no increases in IL-6 and TNF-alfa were observed in the cell line overexpressing visfatin, whereas the native cell line responded with an increase in pro-inflammatory cytokines.

Retinol Binding Protein 4

Retinol Binding protein 4 was initially identified as an adipocyte factor involved in the pathogenesis of type 2 diabetes. ¹⁴⁰ The exact role of this protein is not yet known as conflicting data on its purported are available. While Young et al. ¹⁴¹ observed a direct link between insulin resistance, obesity, and serum levels of this cytokine, other studies have not obtained the same result. ¹⁴² The serum and synovial level of RBP-4 was recently assessed for the first time in a series of patients with osteoarthritis, resulting in a high level of the cytokine either in synovial fluid or in serum, and a correlation with the level of MMP-1 was pointed out. ¹⁴³ However, in a study that included 101 patients with rheumatoid arthritis, retinol-binding protein 4 serum levels did not correlate with the presence of insulin resistance and β-cell function. Therefore, the mechanisms leading to insulin resistance in patients with this chronic inflammatory disease may be different from those occurring in obesity or diabetes. ¹⁴⁴ Interestingly, in non-diabetic patients with ankylosing spondylitis, a single infusion of the anti-TNF monoclonal antibody infliximab led to significant reduction of retinol-binding protein 4 serum levels (Figure 4). ¹⁴⁵

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Figure 4.

Schematic representation of the role of adipokines in rheumatological diseases and cardiovascular risk. Depending on the rheumatological disease, adipokines may increase or restrain the low grade inflammation.

Irisin

A recently discovered myokine/adipokine called irisin is drawing more and more attention because of multiple effects exerted on several tissues such as fat, brain, muscle, and bone. The expression of PGC1α (PPARγ coactivator-1 α) by muscle cells increases as a response to physical activity, which in turn enhances the expression of Fndc5, a membrane protein cleaved, and secreted as a hormone called irisin. This hormone influences the phenotype of adipose cells toward a “browning differentiation,” but it is believed that the beneficial effects of this adipokine go well beyond the only fat metabolism. In vivo work assessed the effects of a direct injection of the hormone in mice, resulting in reduced diet-induced obesity and improved glucose homeostasis,^146,147 although such effects could also result from browning differentiation of fat cells. Lavrova et al. ¹⁴⁸ assessed the serum level of irisin in a case series of patients affected by RA and they observed that a lower serum level is associated to a higher DAS28 score, extra-articular manifestations, and lower 25-OH vitamin D serum level. Another study disclosed significant lower levels of serum irisin in RA patients with sleep disturbances, while an inverse correlation with the duration of morning stiffness duration and the DAS28 score. ¹⁴⁹

Conclusions

Adipokines exert their biological effects by several mechanisms which are still not fully understood. The complex network and the different concentration of pro and anti-inflammatory cytokines in the serum depends on many factors such as underlying disease, ethnic group, treatment, etc. While some adipokines have shown an anti-inflammatory effect, others have been shown to increase the expression of pro-inflammatory cytokines such as IL-17 or TNF-alfa. Population heterogeneity and different methods used in the determination of cytokines could lead to conflicting data. The serum level of various adipokines has been shown to predict disease severity index, as observed for leptin in SLE or chemerin in KD. In several papers leptin has been shown to be independently correlated with the RA disease activity index, also reflecting the improvement in clinical condition after anti-TNF treatment; nevertheless, it was not true for anti-IL-6. Leptin has been shown to exert a pro-inflammatory role which in turn defines clinical worsening in SLE and in OA. This group of adipokines could become a therapeutic target in the near future, although a comprehensive understanding of the underlying pathogenic mechanisms is required. Other adipokines, such as visfatin, have been shown to predict long-term response to therapy in RA patients, when reduced serum levels were observed during the first 3 months of therapy. Adipokines may represent a great opportunity in the near future for understanding autoimmune and rheumatological diseases, for choosing more individualized and effective treatments, and as a possible follow-up tool. Further research is needed to elucidate the influence of cytokines in degenerative musculoskeletal diseases. The lack of previous papers and the limited case series are important limitations of current knowledge, especially with regard to recently discovered adipokines.