Leptin in the interplay of inflammation, metabolism and immune system disorders

Key Points Leptin is an adipokine produced predominantly by adipose tissue, but also expressed in the articulation by chondrocytes and synoviocytes, and by immune cells As a consequence of its dual role as an anorexigenic and a proinflammatory factor, leptin is now considered a link between the neuroendocrine and immune systems Leptin participates in innate immunity by inhibiting natural killer cells and by inducing proliferation and activation of monocytes Leptin signalling can also regulate adaptive immunity by activating T-cell proliferation and responsiveness, and by stimulating B-cell proliferation and cytokine production Leptin can exert its proinflammatory and pro-catabolic actions on cartilage, leading to articular degeneration characteristic of osteoarthritis Leptin is involved in regulating bone mass, basal metabolism and insulin secretion, among other processes. This Review explores the role of leptin in the immune system and metabolism, with particular emphasis on its effect on autoimmune and inflammatory rheumatic diseases. Leptin is one of the most relevant factors secreted by adipose tissue and the forerunner of a class of molecules collectively called adipokines. Initially discovered in 1994, its crucial role as a central regulator in energy homeostasis has been largely described during the past 20 years. Once secreted into the circulation, leptin reaches the central and peripheral nervous systems and acts by binding and activating the long form of leptin receptor (LEPR), regulating appetite and food intake, bone mass, basal metabolism, reproductive function and insulin secretion, among other processes. Research on the regulation of different adipose tissues has provided important insights into the intricate network that links nutrition, metabolism and immune homeostasis. The neuroendocrine and immune systems communicate bi-directionally through common ligands and receptors during stress responses and inflammation, and control cellular immune responses in several pathological situations including immune-inflammatory rheumatic diseases. This Review discusses the latest findings regarding the role of leptin in the immune system and metabolism, with particular emphasis on its effect on autoimmune and/or inflammatory rheumatic diseases, such as rheumatoid arthritis and osteoarthritis.