Transactivation of the epidermal growth factor (EGF) receptor (EGFR) has been proposed to represent an essential link between G-protein-coupled receptors and the mitogen-activated protein kinase (MAPK) pathway in various cell types. In the present work we report, in contrast, that in A431 cells bradykinin transinactivates the EGFR and stimulates MAPK activity independently of EGFR tyrosine phosphorylation. Both effects of bradykinin are mediated by a pertussis-toxin-insensitive G-protein. Three lines of evidence suggest the activation of a protein tyrosine phosphatase (PTP) by bradykinin: (i) treatment of A431 cells with bradykinin decreases both basal and EGF-induced EGFR tyrosine phosphorylation, (ii) this effect of bradykinin can be blocked by two different PTP inhibitors, and (iii) bradykinin significantly increased the PTP activity in total A431 cell lysates when measured in vitro. The transmembrane receptor PTP σ was identified as a putative mediator of bradykinin-induced downregulation of EGFR autophosphorylation. Activation of MAPK in response to bradykinin was insensitive towards AG 1478, a specific inhibitor of EGFR tyrosine kinase, but was blocked by wortmannin or bisindolylmaleimide, inhibitors of phosphatidylinositol 3-kinase (PI3-K) and protein kinase C (PKC) respectively. These results also suggest that the bradykinin-induced activation of MAPK is independent of EGFR and indicate a pathway involving PI3-K and PKC. In addition, bradykinin evokes a rapid and transient increase in Src kinase activity. Although Src does not participate in bradykinin-induced stimulation of PTP activity, inhibition of Src by 4-amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo(3,4-d)pyrimidine leads to an increase in MAPK activation by bradykinin. Our results suggest that in A431 cells the Gq/11-protein-coupled bradykinin B2 receptor may stimulate PTP activity and thereby transinactivate the EGFR, and may simultaneously activate MAPK by an alternative signalling pathway which can bypass EGFR.

No abstract

Costimulation of G protein-coupled receptors (GPCRs) may result in cross talk interactions between their downstream signaling pathways. Stimulation of GPCRs may also lead to cross talk regulation of receptor tyrosine kinase signaling and thereby to activation of mitogen-activated protein kinase (MAPK). In COS-7 cells, we investigated the interactions between two particular mitogenic receptor pathways, the endogenously expressed ␤-adrenergic receptor (␤-AR) and the transiently transfected human bradykinin (BK) B 2 receptor (B 2 R). When ␤-AR and B 2 R are costimulated, we found two different cross talk mechanisms. First, the predominantly G q protein-coupled B 2 R is enabled to activate a G i protein and, subsequently, type II adenylate cyclase. This results in augmentation of ␤-AR-mediated cyclic AMP (cAMP) accumulation by BK, which alone is unable to increase the cAMP level. Second, independently of BK-induced superactivation of the cAMP system, costimulation of ␤-AR leads to protein kinase A-mediated blockade of phospholipase C activation by BK. Thereby, the pathway from B 2 R to MAPK, which essentially involves protein kinase C activation, is selectively switched off. The MAPK activation in response to isoproterenol was not affected due to costimulation. Furthermore, in the presence of isoproterenol, BK lost its ability to stimulate DNA synthesis in COS-7 cells. Thus, our findings might establish a novel paradigm: cooperation between simultaneously activated mitogenic pathways may prevent multiple stimulation of MAPK activity and increased cell growth.

The fast paced progress in genomics yielded a multitude of novel identified genes. Therefore reliable and up scalable methods are required to identify the functions of the encoded proteins. By using protein-protein interaction screening technologies, the function of many proteins has successfully been assigned, and proved in combination with other techniques. We describe the application of an in vivo screening system for protein-protein interactions, the Ras-recruitment system (RRS). The RRS has been used extensively to identify novel interaction partners, also in cases where common systems failed. Therefore we propose the RRS as a valuable alternative interaction screening method that can widely be used among protein classes and that has been further developed by our group into a high-throughput screening system.Protein interaction analysis with Ras-recruitment system 199

No abstract