Activation of MAPKs by G Protein-Coupled Receptors

G protein-coupled receptors (GPCRs) constitute the largest family of cell-surface molecules involved in signal transmission. These receptors play key physiologic roles, with their dysfunction resulting in a number of disease states (1 ). Recently, we have learned that many of the cellular responses mediated by GPCRs do not involve the sole stimulation of conventional second-messenger-generating systems, but result from the functional integration of an intricate network of intracellular signaling pathways. Among these, the main mitogenactivated protein kinase (MAPK) pathways, those mediated by extracellular signal-regulated kinase (ERK)1-2, c-Jun N-terminal kinase (JNK), p38, and ERK5/big mitogen-activated protein kinase (BMK), are potently activated by GPCRs (2 ). GPCRs owe their name to their extensively studied interaction with heterotrimeric G proteins (α-, β-, and γ-subunits), which undergo conformational changes that lead to the exchange of guanosine 5′-diphosphate for guanosine 5′-triphosphate (GTP) bound to the α-subunit on receptor activation. Consequently, Gα- and Gβγ-subunits stimulate effector molecules, including adenylyl and guanylyl cyclases, phosphodiesterases, phospholipase A₂ (PLA₂ ) and PLC, and phosphatidylinositol-3 kinases, thereby activating or inhibiting the production of a variety of second messengers, such as cyclic adenosine monophosphate, cyclic guanosine monophosphate, diacylglycerol, inositol triphosphate (IP₃ ), and phosphatidyinositol triphosphate (PIP₃ ), arachidonic and phosphatidic acid, and promoting (Ca²⁺ ) elevation and the opening or closing of a variety of ion channels (3 ).