Fig. 3

Putative interactions and feedback loops in the MAPK and PI3K/mTOR signaling cascades. Putative signaling diagram of the MAPK and PI3K/mTOR pathways derived from the Western blot analysis shown Fig. 2 and literature data specified in the text. Treatment with the allosteric MEK inhibitor AZD6244 causes desphosphorylation of Erk resulting in phenotypic changes (e.g. changes in the proliferation rate, cell cycle phase distribution and radiosensitivity). However, simultaneously the MAPK pathway is activated through a feedback loop from Erk to the upper part of the signaling cascade resulting in hyperphosphorylation of MEK. Inhibiting the PI3K/mTOR pathway with NVP-BEZ235 caused a dephosphorylation of the transcription and translation regulators S6 and 4E-BP1 also inducing the aforementioned phenotypic changes. However, also for the PI3K/mTOR pathway we observed a feedback loop, as indicated by increased Akt phosphorylation after PI3K and mTOR inhibition in SNB19 cells. Furthermore, we validated a crosstalk between the two signaling cascades since treatment with the dual PI3K/mTOR inhibitor also resulted in decreased Raf-1 expression levels. Stimulation is indicated by black normal arrows, whereas inhibition is indicated by red blunt arrows