Signaling mechanisms of cytokine receptors and their perturbances in disease.

Cytokines regulate the proliferation and differentiation of cells through their interaction with specific receptors on the surface of target cells which are coupled to intracellular signal transduction pathways. The cytokine receptor class I superfamily, characterized by structural homology in the extracellular domain, includes receptors for many interleukins and hematopoietic growth factors, but also those of growth hormone, leptin, ciliary neurotrophic factor (CNTF), oncostatin M (OSM), leukemia inhibitory factor (LIF) and cardiotrophin-1 (CT-1). The receptors for interferons are structurally distinct and have therefore been categorized separately (class II cytokine receptors). The discovery of the JAK/STAT pathway in the early 1990s has been an important step forward in deciphering cytokine mediated signaling. This pathway connects activation of the receptor complexes directly to transcription of genes. Studies of humans and mice, deficient for one of the JAKs or STATs, have revealed crucial roles of these molecules in embryonic development, blood cell formation and immune responses. In addition, recent studies have revealed some of the mechanisms that control the activation of the JAKs and STATs, which contribute to signal intensity and specificity. In this review we will summarize these recent insights and discuss their implications for a variety of pathological conditions.