Resultados

It is clear from the literature that NAMPT is a ubiquitously expressed pleiotropic protein that is important for life. Attempts to create NAMPT knockouts in mice result in early embryonic lethality at embryonic day 10.5 (E10.5; (Fukuhara et al., 2005, Ognjanovic and Bryant-Greenwood, 2002), suggesting an essential role in cell metabolism. In normal tissues, NAMPT is very highly expressed in the liver, adipose tissue and peripheral blood leukocytes (summarised in figure 1.6). If NAMPT is expressed relative to the organ’s metabolic activity, it is possible that organs that require NAMPT start to develop around E10.5; and therefore homozygous knockout embryos are not viable and are thus expelled. In contrast, NAMPT shows very low expression in brain tissue (Samal et al., 1994), suggesting that brain development is not affected by NAMPT deletion. NAMPT is constitutively expressed in human foetal membranes during pregnancy; however, it is overexpressed in the placenta during infection-induced pre- term labour (Ognjanovic and Bryant-Greenwood, 2002). Since its initial discovery, dysregulation of NAMPT has been continually implicated in a number of human diseases and conditions (summarised in figure 1.6).

Figure 1.6 Expression and distribution of NAMPT in the human body

A) NAMPT is ubiquitously expressed in all tissues, though it is preferentially expressed in circulating leukocytes and the liver. Coloured boxes denote expression levels of NAMPT in cells and tissues, ranging from low (yellow) to very high (red). B) Pathological conditions where elevated NAMPT levels have been reported in the organ tissues and/or peripheral blood of patients. PCOS, Polycystic Ovary Syndrome.

Otero and colleagues (2006) published the first study linking NAMPT to RA, a chronic inflammatory syndrome characterised by swelling and stiffness of the joints. They investigated plasma levels of numerous adipokines in patients with RA and compared them with healthy controls, including leptin, adiponectin, NAMPT (called visfatin) and resistin. They reported a marked increase in levels of leptin, adiponectin and NAMPT in patients with RA, indicating NAMPT as one of the biomarkers of the disease (Otero et al., 2006). At the same time, Nowell et al (2006) found that levels of NAMPT were also elevated in the serum and synovial fluid of RA patients. Analysis of human RA synovial tissue confirmed that NAMPT was immunolocalised within the synovial membrane, adipose tissue and the subintimal lining of the synovial joint. Additionally, NAMPT was shown to be upregulated in fibroblast-like synoviocytes treated with IL-6 and its soluble receptor sIL-6R. Subsequently, RT-PCR analysis revealed that NAMPT regulation by IL- 6 trans-signalling could by blocked by the inclusion of a STAT-3 inhibitor peptide. Based on these in vitro observations, wild type and IL-6 deficient mice were induced with

antigen-induced arthritis (AIA). Induction of AIA resulted in a 4-fold increase in NAMPT expression in wild-type mice, whilst little or no change was observed IL-6 deficient mice. Synovial STAT-1/3 activity was also impaired in the latter. This work demonstrated that STAT-3-dependent IL-6 trans-signalling regulates NAMPT expression, and that NAMPT is actively expressed during RA (Nowell et al., 2006).

Brentano and colleagues (2007) reported that increased levels of NAMPT in serum and synovial fluid were significantly correlated with the degree of inflammation and clinical disease severity in patients with RA. In RASFs, NAMPT was induced by TLR ligands and cytokines characteristically present in the joints of RA patients, including IL- 1β, TNFα, TLR-4 ligand LPS, and TLR-2 ligand bLP (Brentano et al., 2007). The addition of NAMPT itself to RASFs activated NF-κB and AP-1, and induced IL-6 and CXCL8 protein, and NAMPT stimulation of RA monocytes caused an upregulation of IL-6 and TNFα. Similarly, Gosset et al (2008) found that IL-1β stimulated NAMPT synthesis in human articular chondrocytes. This was associated with excessive release of the catabolic mediator PGE2, which in turn triggered the expression of MMPs and ADAMTSs; (Gosset et al., 2008).

NAMPT gene expression is elevated in synovial tissues, peripheral blood mononuclear cells (PBMC) and peripheral blood granulocytes (PBG) in patients with RA (Matsui et al., 2008). NAMPT is often described as an adipokine, a protein mediator secreted by fat cells (section 1.3.5). Rho and colleagues (2009) explored the effects of NAMPT, along with other adipokines, on radiographic joint damage in patients with RA. They compared serum concentrations of RA patients and non-diseased controls, and determined associations between adipokines and body mass index (BMI), CRP, IL-6 and TNFα and radiographic joint damage. They found that NAMPT levels were significantly higher in RA patients than controls, even after adjusting for BMI and inflammation. In addition to this, NAMPT concentrations were significantly and positively associated with the degree of radiological joint damage (Rho et al., 2009) suggesting that NAMPT is actively involved in degradative processes in RA.

Mesko and colleagues (2010) studied a database of 400 genes associated with RA, inflammatory bowel disease (IBD) and psoriasis. They identified 53 genes expressed in peripheral blood, which differed significantly between diseased and healthy individuals. Of these genes, NAMPT was identified as one of five genes that discriminate between samples from healthy controls and patients with chronic inflammatory diseases; highlighting NAMPT as a universal marker of chronic inflammation (Mesko et al., 2010). These data, combined with observations within our group, establish NAMPT as a pro- inflammatory and pro-degradative mediator of joint inflammation in RA. However, the possible mechanisms by which NAMPT exerts these effects are as yet to be resolved.