NRF3 activates mTORC1 arginine-dependently for cancer cell viability.

Cancer cells coordinate the mTORC1 signals and the related metabolic pathways to robustly and rapidly grow in response to nutrient conditions. Although a CNC-family transcription factor NRF3 promotes cancer development, the biological relevance between NRF3 function and mTORC1 signals in cancer cells remains unknown. Hence, we showed that NRF3 contributes to cancer cell viability through mTORC1 activation in response to amino acids, particularly arginine. NRF3 induced SLC38A9 and RagC expression for the arginine-dependent mTORC1 recruitment onto lysosomes, and it also enhanced RAB5-mediated bulk macropinocytosis and SLC7A1-mediated selective transport for arginine loading into lysosomes. Besides, the inhibition of the NRF3-mTORC1 axis impaired mitochondrial function, leading to cancer cell apoptosis. Consistently, the aberrant upregulation of the axis caused tumor growth and poor prognosis. In conclusion, this study sheds light on the unique function of NRF3 in arginine-dependent mTORC1 activation and the pathophysiological aspects of the NRF3-mTORC1 axis in cancer development.

The CNC-family transcription factor Nrf3 coordinates the melanogenesis cascade through macropinocytosis and autophagy regulation.

Melanin is a pigment produced from the amino acid L-tyrosine in melanosomes. The CNC-family transcription factor Nrf3 is expressed in the basal layer of the epidermis, where melanocytes reside, but its melanogenic function is unclear. Here, we show that Nrf3 regulates macropinocytosis and autophagy to coordinate melanogenesis cascade. In response to an exogenous inducer of melanin production, forskolin, Nrf3 upregulates the core melanogenic gene circuit, which includes Mitf, Tyr, Tyrp1, Pmel, and Oca2. Furthermore, Nrf3 induces the gene expression of Cln3, an autophagosome-related factor, for melanin precursor uptake by macropinocytosis. Ulk2 and Gabarapl2 are also identified as Nrf3-target autophagosome-related genes for melanosome formation. In parallel, Nrf3 prompts autolysosomal melanosome degradation for melanocyte survival. An endogenous melanogenic inducer αMSH also activates Nrf3-mediated melanin production, whereas it is suppressed by an HIV-1 protease inhibitor, nelfinavir. These findings indicate the significant role of Nrf3 in the melanogenesis and the anti-melanogenic potential of nelfinavir.

Nrf3 Functions Reversely as a Tumorigenic to an Antitumorigenic Transcription Factor in Obese Mice.

Tumor tissue includes cancer cells and their associated stromal cells, such as adipocytes, myocytes, and immune cells. Obesity modulates tumor microenvironment through the secretion of several inflammatory mediators by inducing adipogenesis and myogenesis. Previously, we indicated that tumor growth is promoted by a transcription factor nuclear factor erythroid 2-related factor 3 (NRF3) in human cancer cells. However, the impact of obesity on NRF3-mediated tumorigenesis remains unknown. Here we show that obesity reprograms the tumorigenic to the antitumorigenic function of Nrf3 using a diet-induced obese mouse model. Nrf3 knockdown decreased tumor growth in mice fed a normal diet (ND), whereas it reversely increased tumor growth in mice fed a high-fat diet (HFD). Then, the tumor tissues derived from Nrf3 knockdown or control cancer cells in ND- or HFD-fed mice were subjected to a DNA microarray-based analysis. Similar to the tumor formation results, the expressions of genes related to adipogenesis, myogenesis, and interferon-alpha response were reversed by obesity, implying an increase or recruitment (or both combined) of adipocytes, myocytes, and immune cells. Among these gene sets, we focused on adipocytes. We showed that Nrf3 knockdown reduced cancer cell growth in the preadipocyte culture medium, while the growth inhibitory effect of Nrf3 knockdown on cancer cells was abolished in the adipocyte culture medium. These results suggest the possibility that cancer-associated adipocytes secrete the potential reprogramming factor from the tumorigenic to the antitumorigenic function of Nrf3 in cancer cells.

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

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 have been studied extensively, while those of the ubiquitin-independent 20S proteasome system remain obscure. Here, we show that the cap 'n' collar (CNC) family transcription factor NRF3 specifically enhances 20S proteasome assembly in cancer cells and that 20S proteasomes contribute to colorectal cancer development through ubiquitin-independent proteolysis of the tumor suppressor p53 and retinoblastoma (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 the assembly of the 20S proteasome by directly inducing the gene expression of the 20S proteasome maturation protein POMP. Interestingly, NRF3 knockdown not only increases p53 and Rb protein levels but also increases 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, show that the 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 levels correlating with poor prognoses in patients with colorectal or rectal adenocarcinoma. These results suggest that the NRF3-POMP-20S proteasome assembly axis is significant for cancer development via ubiquitin-independent proteolysis of tumor suppressor proteins.