Melatonin MT1 and MT2 receptor ERK signaling is differentially dependent on Gi/o and Gq/11 proteins

G protein-coupled receptors (GPCRs) transmit extracellular signals into cells by activating G protein- and beta-arrestin-dependent pathways. Extracellular signal-regulated kinases (ERKs) play a central role in integrating these different linear inputs coming from a variety of GPCRs to regulate cellular functions. Here, we investigated human melatonin MT1 and MT2 receptors signaling through the ERK1/2 cascade by employing different biochemical techniques together with pharmacological inhibitors and siRNA molecules. We show that ERK1/2 activation by both receptors is exclusively G protein-dependent, without any participation of beta-arrestin1/2 in HEK293 cells. ERK1/2 activation by MT1 is only mediated though G(i/o) proteins, while MT2 is dependent on the cooperative activation of G(i/o) and G(q/11) proteins. In the absence of G(q/11) proteins, however, MT2-induced ERK1/2 activation switches to a beta-arrestin1/2-dependent mode. The signaling cascade downstream of G proteins is the same for both receptors and involves activation of the PI3K/PKC zeta/c-Raf/MEK/ERK cascade. The differential G protein dependency of MT1- and MT2-mediated ERK activation was confirmed at the level of EGR1 and FOS gene expression, two ERK1/2 target genes. G(i/o)/G(q/11) cooperativity was also observed in Neuroscreen-1 cells expressing endogenous MT2, whereas in the mouse retina, where MT2 is engaged into MT1/MT2 heterodimers, ERK1/2 signaling is exclusively G(i/o)-dependent. Collectively, our data reveal differential signaling modes of MT1 and MT2 in terms of ERK1/2 activation, with an unexpected G(i/o)/G(q/11) cooperativity exclusively for MT2. The plasticity of ERK activation by MT2 is highlighted by the switch to a beta-arrestin1/2-dependent mode in the absence of G(q/11) proteins and by the switch to a G(i/o) mode when engaged into MT1/MT2 heterodimers, revealing a new mechanism underlying tissue-specific responses to melatonin.