Thrombin receptors activate Go proteins in endothelial cells to regulate intracellular calcium and cell shape changes

Abstract

Thrombin receptors couple to Gi/o, Gq, and G12/13 proteins to regulate a variety of signal transduction pathways that underlie the physiological role of endothelial cells in wound healing or inflammation. Whereas the involvement of Gi, Gq, G12, or G13 proteins in thrombin signaling has been investigated extensively, the role of Go proteins has largely been ignored. To determine whether Go proteins could contribute to thrombin-mediated signaling in endothelial cells, we have developed minigenes that encode an 11-amino acid C-terminal peptide of Go1 proteins. Previously, we have shown that use of the C-terminal minigenes can specifically block receptor activation of G protein families (1). In this study, we demonstrate that Go proteins are present in human microvascular endothelial cells (HMECs). Moreover, we show that thrombin receptors can stimulate [35S]guanosine-5′-O-(3-thio)triphosphate binding to Go proteins when co-expressed in Sf9 membranes. The potential coupling of thrombin receptors to Go proteins was substantiated by transfection of the Go1 minigene into HMECs, which led to a blockade of thrombin-stimulated release of [Ca2+]i from intracellular stores. Transfection of the β-adrenergic kinase C terminus blocked the [Ca2+]i response to the same extent as with Go1 minigene peptide, suggesting that this Go-mediated [Ca2+]i transient was caused by Gβγ stimulation of PLCβ. Transfection of a Gi1/2 minigene had no effect on thrombin-stimulated [Ca2+]i signaling in HMEC, suggesting that Gβγ derived from Go but not Gi could activate PLCβ. The involvement of Go proteins on events downstream from calcium signaling was further evidenced by investigating the effect of Go1 minigenes on thrombin-stimulated stress fiber formation and endothelial barrier permeability. Both of these effects were sensitive to pertussis toxin treatment and could be blocked by transfection of Go1 minigenes but not Gi1/2 minigenes. We conclude that the Go proteins play a role in thrombin signaling distinct from Gi1/2 proteins, which are mediated through their Gβγ subunits and involve coupling to calcium signaling and cytoskeletal rearrangements.