Tyrosine phosphatase inhibition triggers sustained canonical serine-dependent NFkappaB activation via Src-dependent blockade of PP2A.

Activation status of Tyr-kinase Src as well as of the transcription factor NFkappaB is a decisive criterion for the onset of cancer and in conveying radio-resistance. While the activation status of Src is Tyr phosphorylation-dependent, NFkappaB activation requires Ser phosphorylation of its cytosolic inhibitor, IkappaBalpha. Since constitutive NFkappaB activation was linked to tumor maintenance, its tight regulation is mandatory. We provide evidence that inhibition of pan-Tyr phosphatase activity by orthovanadate is translated via Src to inhibition of Ser phosphatase PP2A, thereby changing the physiologic response of the cell. In particular we unravelled a new sequence of molecular interactions linking initial activating Tyr416 phosphorylation of Src not to Tyr42-dependent phosphorylation and degradation of IkappaBalpha, but to sustained Ser177/181 phosphorylation of IkappaBalpha kinase IKKbeta following IL-1 stimulation. Consequently, sustained IKKbeta activation provides for chronic canonical IkappaBalpha degradation, thereby eliciting constitutive NFkappaB activation. As the critical translator of Tyr to Ser phosphorylation we identified Ser/Thr phosphatase PP2A. We show that the catalytic subunit PP2Ac serves as a Src substrate with Tyr307 phosphorylation leading to its catalytic inhibition. Additionally to the known survival pathways triggered by Src, Src-mediated canonical and persistent NFkappaB activation may fortify its tumorigenic effects.