c-Src regulates akt signaling in response to ghrelin via β-arrestin signaling-independent and -dependent mechanisms

The aim of the present study was to identify the signaling mechanisms to ghrelin-stimulated activation of the serine/threonine kinase Akt. In human embryonic kidney 293 (HEK293) cells transfected with GHS-R1a, ghrelin leads to the activation of Akt through the interplay of distinct signaling mechanisms: an early Gi/o protein-dependent pathway and a late pathway mediated by β-arrestins. The starting point is the Gi/o-protein dependent PI3K activation that leads to the membrane recruitment of Akt, which is phosphorylated at Y by c-Src with the subsequent phosphorylation at A-loop (T308) and HM (S473) by PDK1 and mTORC2, respectively. Once the receptor is activated, a second signaling pathway is mediated by β-arrestins 1 and 2, involving the recruitment of at least β-arrestins, c-Src and Akt. This β-arrestin-scaffolded complex leads to full activation of Akt through PDK1 and mTORC2, which are not associated to the complex. In agreement with these results, assays performed in 3T3-L1 preadipocyte cells indicate that β-arrestins and c-Src are implicated in the activation of Akt in response to ghrelin through the GHS-R1a. In summary this work reveals that c-Src is crucially involved in the ghrelin-mediated Akt activation. Furthermore, the results support the view that β-arrestins act as both scaffolding proteins and signal transducers on Akt activation This work was supported by grants from Instituto de Salud Carlos III (Ministerio de Ciencia e Innovacion: PI060239, PI060705, PI070908), and Xunta de Galicia (PGIDIT05BTF20802PR, PGIDIT06PXIB918322PR, PGIDIT06PXIB918360PR). The work of M Lodeiro is funded by Instituto de Salud Carlos III (Ministerio de Ciencia e Innovacion). The work of M Theodoropoulou was supported by a research fellowship at the University of Santiago de Compostela for the Pfizer Young Investigator Fellowship. The work of JP Camina is funded by the Instituto de Salud Carlos III (Ministerio de Ciencia e Innovacion) and the Xunta de Galicia (SERGAS) through a research-staff stabilization contract SI