Pertussis toxin-sensitive and -insensitive thrombin stimulation of Shc phosphorylation and mitogenesis are mediated through distinct pathways

Activation of both receptor tyrosine kinases (RTKs) and G protein-coupled receptors (GPCRs) result in phosphorylation of the adaptor protein Shc, providing sites of interaction for proteins in downstream signal transduction cascades. The mechanism of Shc phosphorylation and its function in G protein signaling pathways is still unclear. By examining Shc phosphorylation in response to thrombin in two cell lines, we have defined distinct pertussis toxin (PTX)-sensitive and -insensitive mechanisms by which GPCRs can stimulate tyrosine phosphorylation of Shc. By mutating the tyrosines in Shc, we show that the three sites of tyrosine phosphorylation, Y239, Y240, and Y317, are necessary for thrombin signaling in both systems. The SH2 (src homology 2) domain of Shc is also critical for signaling, but not required for phosphorylation of Shc. In both cell types, inhibition of src family member kinases by chemical inhibitors or microinjection block Shc phosphorylation and bromodeoxyuridine (BrdU) incorporation in response to thrombin. However, in the PTX-sensitive thrombin pathway, both betagamma function and the epidermal growth factor receptor (EGFR) are necessary for Shc phosphorylation and BrdU incorporation. In contrast, signaling in the PTX-insensitive pathway is not mediated through betagamma or the EGFR. Thus, while phosphorylation and function of Shc appear to be the same in both thrombin pathways, the mechanism of tyrosine kinase activation proximal to Shc is different. The differences in signaling between the two thrombin pathways may be representative of mechanisms used by other PTX-sensitive and -insensitive GPCRs to mediate specific responses. In addition, transactivation of RTKs may be a manner by which GPCRs can amplify their signal.