Intranuclear Signaling Cascades Triggered by Nuclear GPCRs

G protein-couped receptors (GPCRs) play a key role on cellular membranes, where they respond to a broad array of extracellular signals such as lipids, peptides, proteins and sensory agents. Intracellular biological responses triggered by these receptors include hormone secretion, muscle contraction, cellular metabolism a tyrosine kinase receptors transactivation. Recent results indicate that GPCRs localize to and signal also at nuclear level, thus regulating distinct signaling pathways which can also result from the integration of extracellular and intracellular stimuli. Nuclear GPCRs play a central role in many cellular processes, including regulation of gene transcription, cellular proliferation, neovascularization and RNA synthesis. On nuclear membranes and in nucleoplasm are present all the downstream signal transduction components of GPCRs, including G proteins, adenylyl cyclase, and second messengers such as Ca++, ERKs, p38MAPK and other protein kinases. Nuclear GPCRs may be constitutively active or may be activated by ligands internalized from the extracellular space or synthesized within the cell. The translocation of membrane receptors to the nucleus could be attributed to the presence of a Nuclear Localization Signal, which is present in the eighth helix or in the third intracellular loop of a limited number of GPCRs. However, several sequence motifs that do not resemble classical Nuclear Localization Signals can promote import of GPCRs. In this review we discuss the most recent results on nuclear localization and signaling of several GPCRS.