Cytokine receptor signaling pathways.

Cytokines represent a diverse group of molecules that transmit intercellular signals. These signals may either be autocrine (where the same cell both produces the cytokine and responds to it) or paracrine (where the cytokine is made by one cell and acts on another). Both these situations can occur simultaneously. Cytokines use multiple signaling pathways. This review will focus on signaling by type I cytokines and in particular on signaling by the IL-2 family of cytokines, as an illustrative example. The major signaling pathway that will be discussed is the Jak-STAT pathway, although other pathways will also be reviewed. The Jak-STAT pathway is a very rapid cytosol-to-nuclear signaling pathway that underscores how quickly extracellular signals can be transmitted to the nucleus. Aspects related to cytokine redundancy, pleiotropy, and specificity will be discussed.