Thrombin protease-activated receptor-1 signals through Gq- and G13-initiated MAPK cascades regulating c-Jun expression to induce cell transformation

Although the ability of G protein-coupled receptors to stimulate normal and aberrant cell growth has been intensely investigated, the precise nature of the molecular mechanisms underlying their transforming potential are still not fully understood. In this study, we have taken advantage of the potent mitogenic effect of thrombin and the focus-forming activity of one of its receptors, protease-activated receptor-1, to dissect how this receptor coupled to Galpha(i), Galpha(q/11), and Galpha(12/13) transduces signals from the membrane to the nucleus to initiate transcriptional events involved in cell transformation. Using endogenous and transfected thrombin receptors in NIH 3T3 cells, ectopic expression of muscarinic receptors coupled to Galpha(q) and Galpha(i), and chimeric G protein alpha subunits and murine fibroblasts deficient in Galpha(q/11), and Galpha(12/13), we show here that, although coupling to Galpha(i) is sufficient to induce ERK activation, the ability to couple to Galpha(q) and/or Galpha(13) is necessary to induce c-jun expression and cell transformation. Furthermore, we show that Galpha(q) and Galpha(13) can initiate the activation of MAPK cascades, including JNK, p38, and ERK5, which in turn regulate the activity of transcription factors controlling expression from the c-jun promoter. We also present evidence that c-Jun and the kinases regulating its expression are integral components of the transforming pathway initiated by protease-activated receptor-1.