TNF receptor-2 signals clear-cell renal carcinoma proliferation via phosphorylated-4EBP1 and mitochondrial gene translation

Clear cell renal cell carcinoma (ccRCC), a tubular epithelial malignancy, secretes tumor necrosis factor (TNF) which signals ccRCC cells in an autocrine manner via two cell surface receptors, TNFR1 and TNFR2, to activate shared and distinct signalling pathways. Selective ligation of TNFR2 was shown to drive cell cycle entry of malignant cells via a signalling pathway involving Etk, VEGFR2, PI3K, Akt, pSer727-Stat3 and mTOR. In this study, phosphorylated-4EBP1 serine 65 (pSer65-4EBP1) is identified as a downstream target of this TNFR2 signalling pathway. pSer65-4EBP1 expression is significantly elevated relative to total 4EBP1 in ccRCC tissue compared to normal kidneys, signal intensity increasing with malignant grade. Selective ligation of TNFR2 with the mutein R2TNF increases pSer65-4EBP1 expression in organ cultures that co-localises with internalised TNFR2 in mitochondria and increases expression of mitochondrially-encoded COX (cytochrome c oxidase subunit) Cox1, as well as nuclear-encoded Cox4/5b subunits. Pharmacological inhibition of mTOR reduces both R2TNF-mediated phosphorylation of 4EBP1 and cell cycle activation in tumor cells while increasing cell death. These results signify the importance of pSer65-4EBP1 in mediating TNFR2-driven cell-cycle entry in tumor cells in ccRCC and implicate a novel relationship between the TNFR2/pSer65-4EBP1/COX axis and mitochondrial function.