Introduction

Osteoarthritis (OA) is a progressive disease characterized by cartilage destruction and abnormal bone remodeling, resulting in joint pain and severe disability. Worldwide, millions of people suffer from this disease. Nowadays, OA is widely considered to be a joint disorder, the key pathological feature of which is cartilage destruction. Moreover, metalloproteinase (MMPs) and a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS) are major extracellular matrix proteases implicated in the degradation of cartilage aggrecan and collagen type 2, therefore exacerbating OA. In the past several decades, extensive work has been done to delineate the pathogenic mechanism(s) of OA; however, a full understanding of the initial factors for OA still needs to be developed. What has been illuminated so far is that obesity is considered to be a prominent risk factor for OA. For example, at the knee joint, obesity increases the risk of developing osteoarthritis by two- to tenfold [1–3]. Furthermore, local biomechanical factors associated with body mass index, limb alignment and quadriceps muscle strength can all influence both the onset and progression of knee osteoarthritis [4–6]. However, these factors cannot explain the association between obesity and osteoarthritis at non-load-bearing joints [7]. It is possible that beyond the mechanical loading issue caused by being overweight, more risk factors are involved in osteoarthritis caused by obesity and diabetes. Adipokines are cytokines secreted by adipose tissue and play an important role in the insulin regulation of type 2 diabetes. Moreover, adipokines such as leptin and adiponectin have been reported to contribute to chondrocyte degradation and cause osteoarthritis [8–12]. Recently, studies have increasingly reported that visfatin, which serves as a novel adipokine, also contributes to OA development. Therefore, this review will focus on outlining the role of visfatin in osteoarthritis and its molecular mechanism that causes this disease.

Visfatin serves as an adipokine

Visfatin, otherwise named nicotinamide phosphoribosyltransferase (NAmPRTase or Nampt), is also known as pre-B cell colony-enhancing factor 1 (PBEF1). Nampt/PBEF/visfatin was originally cloned as a putative cytokine shown to enhance the maturation of B cell precursors in the presence of interleukin-7 (IL-7) and stem cell factor; it was therefore named “pre-B cell colony-enhancing factor” (PBEF) [13]. It catalyzes the condensation of nicotinamide with 5-phosphoribosyl-1-pyrophosphate to yield nicotinamide mononucleotide, one step in the biosynthesis of nicotinamide adenine dinucleotide. This protein belongs to the nicotinic acid phosphoribosyltransferase (NAPRTase) family and is thought to be involved in many important biological processes, including metabolism, stress response and aging (Entrez Gene Summary for NAMPT Gene, provided by RefSeq, February 2011). In addition, it exhibits both an intracellular enzymatic activity leading to NAD synthesis and a cytokine function via binding to its hypothetical receptor. Revollo et al. determined biochemically that the mouse PBEF gene product encoded a Nampt enzyme capable of modulating intracellular NAD levels [14]. Thereafter, several studies reported the crystal structure of Nampt/PBEF/visfatin and revealed that this protein is a dimeric type II phosphoribosyltransferase enzyme and also essential for nicotinamide adenine dinucleotide (NAD) biosynthesis [15, 16]. Recently, visfatin was shown to be highly expressed in visceral adipose tissue compared with subcutaneous adipose tissue [17], and plasma levels of visfatin correlate with outcomes in patients with obesity [18]. Furthermore, accumulating evidence shows a possible association between levels of visfatin and type 2 diabetes mellitus in individuals with and without obesity [19]. Besides, circulating visfatin levels are closely correlated with WAT accumulation, and its mRNA levels increase in the course of adipocyte differentiation. Visfatin is thus recognized as an adipokine. Adipokines are mainly adipocyte-derived cytokines regulating the metabolism, and many are key regulators of insulin resistance. In the large spectrum of adipokines, visfatin is one of the most promising and interesting molecules that seems to be directly implicated in the regulation of glucose-stimulated insulin secretion in pancreatic β cells [20]. In terms of regulators, visfatin synthesis is also regulated by several factors, including glucocorticoids, TNF-a, IL-6 and growth hormone [21]. Recently, although extensive studies have focused on many aspects of the physiological and pathological roles of visfatin [22], its function in osteoarthritis is still uncertain.

High visfatin levels in OA patients

In the past decade, several studies have investigated the visfatin levels in osteoarthritis patients. Using ELISA experiments, Chen et al. found serum visfatin concentrations were much higher in OA patients compared to healthy controls. In addition, synovial fluid visfatin concentrations exceeded those in paired serum, suggesting that visfatin might be involved in the pathophysiology of OA and has local effects on the joints during the OA process [23]. This finding was also confirmed by another group. Duan et al. demonstrated visfatin levels in synovial fluid of patients with primary knee osteoarthritis (OA) were higher compared to normal persons, which was also indicated by ELISA. Besides, the visfatin level was positively correlated with the degradation biomarker of collagen II, which indicated that synovial fluid visfatin contributed to cartilage matrix degradation [24]. Furthermore, another study examined the tissular origin and conformation of visfatin in human OA joints and investigated the role of visfatin in chondrocytes and osteoblasts by studying Nampt enzymatic activity. Also using ELISA, they illuminated that visfatin formed as a homodimer in tissular explants and synovial fluid, corresponding to the enzymatically active conformation. All human OA joint tissues released visfatin (synovium 628 ± 106 ng/g tissue; subchondral bone 195 ± 26 ng/g tissue; cartilage 152 ± 46 ng/g tissue), with significantly higher levels for synovium (P < 0.0005). These data supported that in human OA patients synovium is the main location generating and restoring visfatin, therefore triggering OA [25].

Visfatin functions as an inflammatory factor to trigger OA progress

According to the association of OA disease and high visfatin levels, it is critical to illustrate whether and how visfatin causes and accelerates OA. Some adipokines such as adiponectin and leptin influenced immune and inflammatory functions. Besides these, recombinant visfatin was also demonstrated to activate human leukocytes and induce cytokine production. Moschen et al. reported that in CD14+ monocytes, visfatin induced the production of interleukin (IL)-1β, TNFα and especially IL-6 [26]. In addition, another group treated explants of porcine cartilage and meniscus with different adipokines combined with IL-1β. Among these adipokines, only visfatin but not leptin and IL-6 acted synergistically with IL-1β to increase the catabolism and production of proinflammatory mediators in porcine cartilage and meniscus, as evidenced by increased MMP activity, nitric oxide production and proteoglycan release [27]. Furthermore, Chauffier et al. demonstrated 5 μg/ml visfatin was enough to increase the release of keratinocyte chemoattractant (Kc), an IL-8 murine equivalent chemokine produced by chondrocytes, contributing to the pathophysiology of osteoarthritis [28]. Besides, in OA patients, the infrapatellar fat pad was reported to secrete more inflammatory mediators such as IL-6 and visfatin than subcutaneous adipose tissue did. This finding suggested infrapatellar fat-derived soluble mediators could contribute to pathophysiological processes in the OA knee joint [29]. Taken together, likes other inflammatory factors [30–32], visfatin contributes greatly to the destruction of cartilage and synovitis and triggers OA in patients. In terms of the mechanism, according to the literature published in the few years, several key factors are involved in this process.

Visfatin and PGE2

In humans, prostaglandin E2 (PGE2) is the most abundant prostanoid synthesized from arachidonic acid via the actions of cyclooxygenase (COX) enzymes in response to cell-specific trauma and stimuli [33–35]. Moreover, PGE2 can be stimulated by interleukins and cause cartilage degradation [36]. Recently, the role of PGE2 in visfatin-mediated OA disease was reported. Gosset et al. demonstrated the synthesis of visfatin produced by human OA chondrocytes could be increased by IL-1β, which in term triggered excessive release of PGE2. Importantly, they also reported that through PGE2, visfatin was able to induce MMP-13, ADAMTS-4 and ADAMTS-5 expression, which were considered the major causes of osteoarthritis [37]. Besides, another paper published in 2012 showed the role of visfatin in PGE2 synthesis in chondrocytes. Inhibition of visfatin activity using the APO866 inhibitor gradually decreased PGE2 release, whereas the addition of exogenous nicotinamide increased it. Furthermore, an increase of PGE2 by visfatin was demonstrated in an insulin receptor (IR)-dependent manner. Blocking IR expression with siRNA or activity using hydroxy-2-naphthalenyl methyl phosphonic acid tris acetoxymethyl ester (HNMPA-(AM) [3]) inhibitor diminished visfatin-induced PGE2 release in chondrocytes. However, RT-PCR data show visfatin could not regulate IR expression. An increase of the insulin level was also not able to advance PGE2 release, suggesting that the proinflammatory actions of visfatin in chondrocytes involved regulation of IR signaling pathways, possibly through the control of visfatin enzymatic activity [38].

Visfatin and NGF

Very recently, several groups have demonstrated the nerve growth factor (NGF) level is increased in osteoarthritis (OA) joints and is involved in pain associated with OA [39]; however,the stimuli responsible for NGF stimulation in chondrocytes are still not known. Two years ago, Pecchi et al. discovered that unstimulated human and mouse articular chondrocytes expressed low levels of NGF. The number was 19.2 ± 8.7 pg/ml for human articular chondrocytes and 13.5 ± 1.0 pg/ml for mouse articular chondrocytes. When stimulated by IL-1β and visfatin, a dose-dependent increase in NGF mRNA expression and NGF release in both human and mouse chondrocyte conditioned media were observed. The induction of NGF was abolished by Apo866, a specific inhibitor of visfatin/NAMPT enzymatic activity, or by siRNA targeting visfatin, whereas IL-1β-mediated NGF expression was not modified by siRNA targeting visfatin. These results show that IL-1β and extracellular visfatin both stimulated the expression and release of NGF by chondrocytes and thus suggested that the overexpression of visfatin and IL-1β in the OA joint might mediate OA pain via the stimulation of NGF expression and release by chondrocytes [40].

Visfatin and HIF-2α in OA progress

Hypoxia-inducible factor 2α (HIF-2α), encoded by Epas1, causes osteoarthritic cartilage destruction by regulating the expression of matrix-degrading enzymes. Recently, Yang et al. showed visfatin was a direct target of HIF-2α, determined at the mRNA and protein levels in human osteoarthritis (OA) cartilage, mouse experimental OA cartilage and primary cultured mouse chondrocytes. Visfatin protein, in turn, increased mRNA levels and activities of MMP3, MMP12 and MMP13 in chondrocytes, an action necessary for HIF-2α-induced expression of catabolic enzymes. In vivo, the authors established an experimental OA model by intra-articular injection of adenovirus Epas1 for HIF-2α overexpression. Notably, deletion of the visfatin-corresponding gene by Col2a1-cre fully rescued this OA phenotype compared to the sham group. Moreover, in an in vitro study, knocking down visfatin against the background of HIF-2α overexpression significantly reduced MMP3, MMP12 and MMP13 expression. These data suggested HIF-2α through visfatin triggered chondrocyte degradation and therefore aggravated OA progress [41]. The same group followed up and found HIF-2α activated the NAMPT-NAD+-SIRT axis in chondrocytes by increasing visfatin, which stimulated NAD+ synthesis and thereby activated SIRT family members. The activated NAMPT-SIRT pathway, in turn, promoted HIF-2α protein stability by negatively regulating its hydroxylation and 26S proteasome-mediated degradation, resulting in increased HIF-2α transcriptional activity. Among SIRT family members (SIRT1-7), SIRT2 and SIRT4 were positively associated with HIF-2α stability and transcriptional activity in chondrocytes. This reciprocal regulation was required for the expression of catabolic matrix metalloproteinases (MMP3, MMP12 and MMP13) and OA cartilage destruction caused by intra-articular injection of Ad-Epas1 and Ad-Nampt (visfatin). Therefore, the positive feedback regulation of HIF-2α and the NAMPT-NAD+-SIRT axis in articular chondrocytes was involved in OA cartilage destruction caused by HIF-2α or visfatin [42].

Visfatin and SIRT1

SIRT1 is an NAD-dependent histone deacetylase that regulates gene expression, cell differentiation and organism development. It can modulate chromatin function through deacetylation of histones and can promote alterations in the methylation of histones and DNA, leading to transcriptional repression [43]. SIRT1 also modulates proteostasis and blocks oxidative stress, inflammatory and matrix catabolic processes in chondrocytes [44]. In cartilage, SIRT1 promotes cartilage-specific gene expression [45], such as aggrecan and collagen type 2, and inhibits chondrocyte apoptosis [46, 47]. Therefore, data suggest that SIRT1 has a preventive role against the development of OA. Consistently, Dvir-Ginzberg et al. discovered that in human chondrocytes derived from OA patients, SIRT1 increased chondrogenic gene expression. However, interestingly, data also show visfatin increased the NAD level, which in turn altered SIRT1 activity. This study suggested SIRT1, visfatin and NAD may therefore have a positive function in human cartilage by elevating the expression of genes encoding cartilage extracellular matrix [48]. In this context, visfatin prevented OA progress, contrary to the knowledge that visfatin increases OA risk. These data suggest that besides having an inflammatory effect on OA, visfatin also affects chondrocyte maturation. Moreover, chondrocyte dedifferentiation is demonstrated to be one of the destructive causes of osteoarthritis. Recently, Hong et al. demonstrated that IL-1β induced dedifferentiation of articular chondrocytes by upregulation of visfatin expression, which in turn activated SIRT1, and showed the association of inflammatory cytokines with adipokine regulation [49].

Visfatin and ERK

The MAPK/ERK pathway is a chain of proteins in the cell that communicates a signal from a receptor on the cell surface to the DNA in the cell nucleus. The pathway includes many proteins, including ERK (extracellular signal-regulated kinases), which communicate by adding phosphate groups to a neighboring protein [50]. In 2012, Yammani et al. reported that pretreatment of chondrocytes with visfatin inhibited IGF-1-stimulated proteoglycan synthesis in a dose-dependent manner, which contributed to osteoarthritis. Interestingly, treatment of chondrocytes with visfatin inhibited IGF-1-induced phosphorylation of downstream signaling molecules, including insulin receptor substrate-1 (IRS1) and AKT, but was unable to inhibit IGF-1R phosphorylation. Moreover, visfatin stimulated a sustained phosphorylation of the ERK/MAPK signaling pathway. By inhibition of the ERK/MAPK signaling pathway restored IGF-1-mediated insulin receptor substrate-1 and AKT phosphorylation. Therefore, this study demonstrated that visfatin inhibited IGF-1 function in articular chondrocytes by activating the ERK/MAPK pathway independent of the IGF-1 receptor [51]. Hong et al. also showed SIRT1 was activated in a visfatin-dependent manner and activated ERK and P38 in response to IL-1β. Moreover, they found that the SIRT1-ERK complex, but not SIRT1-p38, was engaged in IL-1β-induced chondrocyte dedifferentiation via a Sox-9-mediated mechanism. It is worth noting that both visfatin and ERK signaling enhanced SIRT1-mediated chondrocyte dedifferentiation [49].

In conclusion, OA is a degenerative joint disease, affecting the whole joint structure, including the articular cartilage and synovial tissues. The novel adipokine visfatin/Nampt is released by human OA tissues in a dimeric, enzymatically active conformation and mostly by the synovium, which displays Nampt activity. Visfatin itself is regulated by IL-1β, HIF-2α and other interleukins at either the expression or enzyme activity level. Subsequently, visfatin inhibits the downstream phosphorylation of insulin receptor factors, such as IRS1 and AKT, and therefore reduces proteoglycan production, but increases MMP, NGF and PEG2 expression, aggravating OA (Fig. 1). Although extensive studies have investigated the function of visfatin in OA development, many questions still need further answers, such as which receptor mediates visfatin's intercellular translocation and how visfatin activates NAD and ERK through this specific receptor (Fig. 1).

Fig. 1
figure 1

Summary of the mechanism that visfatin induced OA P, phosphorylation. Red arrow up-regulation; Green arrow down-regulation; Blue arrow stimulation. PEG2 prostaglandin E2; NGF nerve growth factor; MMPs matrix metalloproteinases. Apo866 inhibitor of NAMPT enzymatic activity

Full size image

In this short review, we summarize visfatin's molecular mechanism for triggering OA; importantly, visfatin's Nampt activity is involved in chondrocyte degeneration. Therefore, targeting this enzymatic activity to disrupt joint tissue interactions may be novel in OA therapy.