Tumor suppressor MMAC/PTEN inhibits cytokine-induced NFκB activation without interfering with the IκB degradation pathway

The phosphoinositide 3-kinase (PI 3-kinase) pathway has been implicated in the activation of the proinflammatory transcription factor nuclear factor kappaB (NF kappaB). To investigate the role of this pathway in NF kappaB activation, we employed mutated in multiple advanced cancers/phosphatase and tensin homologue (MMAC/PTEN), a natural antagonist of PI 3-kinase activity. Our results show that cytokine-induced DNA binding and transcriptional activities of NF kappaB were both inhibited in a glioma cell line that was stably transfected with MMAC/PTEN. The ability of interleukin-1 (IL-1) to induce inhibitor (I kappaB) degradation or nuclear translocation of NF kappaB was, however, unaffected by MMAC/PTEN expression, suggesting that PI 3-kinase utilizes another equally important mechanism to control NF kappaB activation. It is conceivable that NF kappaB is directly phosphorylated through such a mechanism because treatment with protein phosphatase 2A significantly reduced its DNA binding activity. Moreover, IL-1-induced phosphorylation of p50 NF kappaB was potently inhibited in MMAC/PTEN-expressing cells. Whereas the mediators of NF kappaB phosphorylation remain to be identified, IL-1 was found to induce physical interactions between the PI 3-kinase target Akt kinase and the I kappaB.I kappaB kinase complex. Physical interactions between these proteins were antagonized by MMAC/PTEN consistent with their potential involvement in NF kappaB activation. Taken together, our observations suggest that PI 3-kinase regulates NF kappaB activation through a novel phosphorylation dependent mechanism.