NRF3-POMP-20S Proteasome Assembly Axis Promotes Cancer Development via Ubiquitin-Independent Proteolysis of p53 and Retinoblastoma Protein

Abstract

Proteasomes are essential protease complexes that maintain cellular homeostasis, and aberrant proteasomal activity supports cancer development. The regulatory mechanisms and biological function of the ubiquitin-26S proteasome has been studied extensively, while those of the ubiquitin-independent 20S proteasome system remain obscure. Herein, we show that the CNC family transcription factor NRF3 specifically enhances 20S proteasome assembly in cancer cells and 20S proteasomes contribute to colorectal cancer development through ubiquitin-independent proteolysis of the tumor suppressor p53 and Rb proteins. The NRF3 gene is highly expressed in many cancer tissues and cell-lines, and is important for cancer cell growth. In cancer cells, NRF3 upregulates assembly of the 20S proteasome by directly inducing gene expression of the 20S proteasome maturation protein POMP . Interestingly, NRF3 knockdown not only increases p53 and Rb protein levels but also increased p53 activities for tumor suppression, including cell-cycle arrest and induction of apoptosis. Furthermore, protein stability and cell viability assays using two distinct proteasome inhibitor anticancer drugs, the 20S proteasome inhibitor bortezomib and the ubiquitin activating enzyme E1 inhibitor TAK-243, which show that upregulation of the NRF3-POMP axis leads to ubiquitin-independent proteolysis of p53 and Rb, and to impaired sensitivity to bortezomib, but not TAK-243. More importantly, the NRF3-POMP axis supports tumorigenesis and metastasis, with higher NRF3/POMP expression correlating with poor prognosis in patients with colorectal or rectal adenocarcinoma. These results suggest the NRF3-POMP-20S proteasome assembly axis is significant for cancer development via ubiquitin-independent proteolysis of tumor suppressor proteins.