Leptin: a metabolic hormone that functions like a proinflammatory adipokine

Initially described as a satiety factor with neuroendocrine properties, leptin has been shown to regulate immune and inflammatory processes. Mainly produced by white adipose tissue, this hormone was first known to regulate energy homeostasis by inhibiting food intake and by upregulating energy consumption. Leptin is a dual molecule: apart from its actions as a hormone involved in energy homeostasis, increasing evidence suggests that leptin is a novel proinflammatory adipocyte-derived factor that operates in the cytokine network by linking immune and inflammatory processes to the neuroendocrine system. In fact, recent findings have shown that leptin regulates and participates both in immune homeostasis and inflammatory processes not only by acting as a modulator of T-cell activity, but also by playing a key role in a host of autoimmune inflammatory conditions such as autoimmune encephalomyelitis, type 1 diabetes, bowel inflammation and articular degenerative diseases such as osteoarthritis and rheumatoid arthritis. This review will more closely address leptin's cytokine properties rather than its role as a metabolic hormone by focusing on its biological actions in inflammatory processes, specifically those related to degenerative inflammatory diseases of the joints.