Pasteurella multocida toxin‐induced Gαq‐dependent mTOR‐mediated ribosomal S6 protein phosphorylation

Pasteurella multocida toxin (PMT) is a potent mitogen. We investigated the effect of PMT treatment on the mammalian target of rapamycin (mTOR), a protein kinase believed to play a key role in cell growth, proliferation, and apoptosis. Western blot analysis revealed that PMT was able to stimulate mTOR in wild type mouse embryonic fibroblasts (MEF) and in Swiss 3T3 cells as judged by a hefty increase in the phosphorylation of its downstream target, 40S ribosomal S6 protein at Ser235/236 and Ser240/244. This observation was validated by results showing that PMT was capable of inducing mTOR phosphorylation at serine 2448 and S6K1 phosphorylation at serine 389 in 3T3 cells. However, in MEF cells lacking only Gα11, PMT was able to stimulate S6 phosphorylation at Ser240/244 but not Ser235/236. In MEF cells lacking both Gαq and Gα11, we observed a decrease in S6 phosphorylation at Ser235/236 and Ser240/244 following PMT treatment. To confirm that PMT‐induced S6 phosphorylation is directly mediated through mTOR, we showed that rapamycin, a specific inhibitor of TORC1, inhibits PMT‐induced S6 phosphorylation in wild type MEF and in 3T3 cells. Our results reveal for the first time that PMT induces a Gαq‐dependent activation of mTOR which in turn activates S6k1 and leads to S6 phosphorylation. This research was supported by the Intramural Research Program of the NIH, NHLBI.