Melatonin MT and MT receptor ERK signaling is differentially dependent on G and G proteins.

G protein-coupled receptors (GPCRs) transmit extracellular signals into cells by activating G protein- and β-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 MT and MT 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 β-arrestin1/2 in HEK293 cells. ERK1/2 activation by MT is only mediated though G proteins, while MT is dependent on the cooperative activation of G and G proteins. In the absence of G proteins, however, MT -induced ERK1/2 activation switches to a β-arrestin1/2-dependent mode. The signaling cascade downstream of G proteins is the same for both receptors and involves activation of the PI3K/PKCζ/c-Raf/MEK/ERK cascade. The differential G protein dependency of MT - and MT -mediated ERK activation was confirmed at the level of EGR1 and FOS gene expression, two ERK1/2 target genes. G /G cooperativity was also observed in Neuroscreen-1 cells expressing endogenous MT , whereas in the mouse retina, where MT is engaged into MT /MT heterodimers, ERK1/2 signaling is exclusively G -dependent. Collectively, our data reveal differential signaling modes of MT and MT in terms of ERK1/2 activation, with an unexpected G /G cooperativity exclusively for MT . The plasticity of ERK activation by MT is highlighted by the switch to a β-arrestin1/2-dependent mode in the absence of G proteins and by the switch to a G mode when engaged into MT /MT heterodimers, revealing a new mechanism underlying tissue-specific responses to melatonin.