The transcription factor NRF1 (NFE2L1) activates aggrephagy by inducing p62 and GABARAPL1 after proteasome inhibition to maintain proteostasis.

The ubiquitin‒proteasome system (UPS) and autophagy are the two primary cellular pathways of misfolded or damaged protein degradation that maintain cellular proteostasis. When the proteasome is dysfunctional, cells compensate for impaired protein clearance by activating aggrephagy, a type of selective autophagy, to eliminate ubiquitinated protein aggregates; however, the molecular mechanisms by which impaired proteasome function activates aggrephagy remain poorly understood. Here, we demonstrate that activation of aggrephagy is transcriptionally induced by the transcription factor NRF1 (NFE2L1) in response to proteasome dysfunction. Although NRF1 has been previously shown to induce the expression of proteasome genes after proteasome inhibition (i.e., the proteasome bounce-back response), our genome-wide transcriptome analyses identified autophagy-related p62/SQSTM1 and GABARAPL1 as genes directly targeted by NRF1. Intriguingly, NRF1 was also found to be indispensable for the formation of p62-positive puncta and their colocalization with ULK1 and TBK1, which play roles in p62 activation via phosphorylation. Consistently, NRF1 knockdown substantially reduced the phosphorylation rate of Ser403 in p62. Finally, NRF1 selectively upregulated the expression of GABARAPL1, an ATG8 family gene, to induce the clearance of ubiquitinated proteins. Our findings highlight the discovery of an activation mechanism underlying NRF1-mediated aggrephagy through gene regulation when proteasome activity is impaired.

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.