Abstract
Oncogenic Ras mutations render the protein constitutively active and promote tumorigenesis via chronic stimulation of effector pathways. In A549 lung adenocarcinoma approx. 50% of the total Ras population is constitutively active, yet these cells display only weak activation of the effectors: ERK1/2 (extracellular-signal-regulated kinase 1/2) and Akt. In order to identify key negative regulators of oncogenic Ras signalling we performed a phosphatome RNAi (RNA interference) screen in A549 cells and ranked their effects on phosphorylation of Ser473 of Akt. As expected, the tumour suppressor PTEN (phosphatase and tensin homologue deleted on chromosome 10) emerged as a leading hit: knockdown elevated Akt activation to 70% of maximal generated by acute EGF (epidermal growth factor) stimulation. Importantly, we identified other phosphatases with similar potencies including PTPN2 (T-cell protein tyrosine phosphatase; also known as TC-PTP) and PTPRJ (protein tyrosine phosphatase receptor type J; also known as DEP-1/CD148). Potentiation of Akt phosphorylation by knockdown of PTEN or PTPRJ was contingent on the presence of oncogenic K-Ras. Our data reveal a synergy between oncogene function and the loss of a tumour suppressor within the same pathway that was necessary for full effector activation since each alone failed to elicit significant Akt phosphorylation. Taken together, these data reveal potent regulators of Akt signalling which contribute to ameliorating the consequences of oncogenic K-Ras activity.