Nuclear receptor corepressor is a novel regulator of phosphatidylinositol 3-kinase signaling

The nuclear receptor corepressor (NCoR) regulates the activities of DNA-binding transcription factors. Recent observations of its distribution in the extranuclear compartment raised the possibility that it could have other cellular functions in addition to transcription repression. We previously showed that phosphatidylinositol 3-kinase (PI3K) signaling is aberrantly activated by a mutant thyroid hormone P receptor (TR beta PV, hereafter referred to as PV) via physical interaction with p85 alpha, thus contributing to thyroid carcinogenesis in a mouse model of follicular thyroid carcinoma (TR beta(PV/PV) mouse). Since NCoR is known to modulate the actions of TR beta mutants in vivo and in vitro, we asked whether NCoR regulates PV-activated PI3K signaling. Remarkably, we found that NCoR physically interacted with and competed with PV for binding to the C-terminal SH2 (Src homology 2) domain of p85 alpha, the regulatory subunit of PI3K. Confocal fluorescence microscopy showed that both NCoR and p85 alpha were localized in the nuclear as well as in the cytoplasmic compartments. Overexpression of NCoR in thyroid tumor cells of TR beta(PV/PV) mouse reduced PI3K signaling, as indicated by the decrease in the phosphorylation of its immediate downstream effector, p-AKT. Conversely, lowering cellular NCoR by siRNA knockdown in tumor cells led to overactivated p-AKT and increased cell proliferation and motility. Furthermore, NCoR protein levels were significantly lower in thyroid tumor cells than in wild-type thyrocytes, allowing more effective binding of PV to p85 alpha to activate PI3K signaling and thus contributing to tumor progression. Taken together, these results indicate that NCoR, via protein-protein interaction, is a novel regulator of PI3K signaling and could serve to modulate thyroid tumor progression.