An Update on the Role of Adipose Tissues in Psoriasis

White adipose tissue is no longer considered an inert tissue mainly devoted to energy storage but is emerging as an active participant in regulating physiologic and pathologic processes, including immunity and inflammation. Macrophages are components of adipose tissue and actively participate in its activities. Furthermore, cross-talk between lymphocytes and adipocytes can lead to immune regulation. Adipose tissue produces and releases a variety of proinflammatory and anti-inflammatory factors, including the adipokines leptin, adiponectin, resistin, and visfatin, as well as cytokines and chemokines, such as TNF-alpha, IL-6, monocyte chemoattractant protein 1, and others. Proinflammatory molecules produced by adipose tissue have been implicated as active participants in the development of insulin resistance and the increased risk of cardiovascular disease associated with obesity. In contrast, reduced leptin levels might predispose to increased susceptibility to infection caused by reduced T-cell responses in malnourished individuals. Altered adipokine levels have been observed in a variety of inflammatory conditions, although their pathogenic role has not been completely clarified.

The adipokine resistin is suggested to be an important link between obesity and insulin resistance. In the present study, we assessed the impact of resistin as inflammatogenic cytokine in the setting of arthritis. In vitro experiments on human PBMC were performed to assess cytokine response and transcription pathways of resistin-induced inflammation. Proinflammatory properties of resistin were evaluated in animal model by intra-articular injection of resistin followed by histological evaluation of the joint. Levels of resistin were assessed by ELISA in 74 paired blood and synovial fluid samples of patients with rheumatoid arthritis. Results were compared with the control group comprised blood samples from 34 healthy individuals and 21 synovial fluids from patients with noninflammatory joint diseases. We now show that resistin displays potent proinflammatory properties by 1) strongly up-regulating IL-6 and TNF-alpha, 2) responding to TNF-alpha challenge, 3) enhancing its own activity by a positive feedback, and finally 4) inducing arthritis when injected into healthy mouse joints. Proinflammatory properties of resistin were abrogated by NF-kappaB inhibitor indicating the importance of NF-kappaB signaling pathway for resistin-induced inflammation. Resistin is also shown to specifically accumulate in the inflamed joints of patients with rheumatoid arthritis and its levels correlate with other markers of inflammation. Our results indicate that resistin is a new and important member of the cytokine family with potent regulatory functions. Importantly, the identified properties of resistin make it a novel and interesting therapeutic target in chronic inflammatory diseases such as rheumatoid arthritis.

The traditional role attributed to white adipose tissue is energy storage, fatty acids being released when fuel is required. The metabolic role of white fat is, however, complex. For example, the tissue is needed for normal glucose homeostasis and a role in inflammatory processes has been proposed. A radical change in perspective followed the discovery of leptin; this critical hormone in energy balance is produced principally by white fat, giving the tissue an endocrine function. Leptin is one of a number of proteins secreted from white adipocytes, which include angiotensinogen, adipsin, acylation-stimulating protein, adiponectin, retinol-binding protein, tumour neorosis factor a, interleukin 6, plasminogen activator inhibitor-1 and tissue factor. Some of these proteins are inflammatory cytokines, some play a role in lipid metabolism, while others are involved in vascular haemostasis or the complement system. The effects of specific proteins maybe autocrine or paracrine, or the site of action maybe distant from adipose tissue. The most recently described adipocyte secretory proteins are fasting-induced adipose factor, a fibrinogen-angiopoietin-related protein, metallothionein and resistin. Resistin is an adipose tissue-specific factor which is reported to induce insulin resistance, linking diabetes to obesity. Metallothionein is a metal-binding and stress-response protein which may have an antioxidant role. The key challenges in establishing the secretory functions of white fat are to identify the complement of secreted proteins, to establish the role of each secreted protein, and to assess the pathophysiological consequences of changes in adipocyte protein production with alterations in adiposity (obesity, fasting, cachexia). There is already considerable evidence of links between increased production of some adipocyte factors and the metabolic and cardiovascular complications of obesity. In essence, white adipose tissue is a major secretory and endocrine organ involved in a range of functions beyond simple fat storage.