Nuclear Receptor Nur77 Facilitates Melanoma Cell Survival under Metabolic Stress by Protecting Fatty Acid Oxidation.

Fatty acid oxidation (FAO) is crucial for cells to overcome metabolic stress by providing ATP and NADPH. However, the mechanism by which FAO is regulated in tumors remains elusive. Here we show that Nur77 is required for the metabolic adaptation of melanoma cells by protecting FAO. Glucose deprivation activates ERK2 to phosphorylate and induce Nur77 translocation to the mitochondria, where Nur77 binds to TPß, a rate-limiting enzyme in FAO. Although TPß activity is normally inhibited by oxidation under glucose deprivation, the Nur77-TPß association results in Nur77 self-sacrifice to protect TPß from oxidation. FAO is therefore able to maintain NADPH and ATP levels and prevent ROS increase and cell death. The Nur77-TPß interaction further promotes melanoma metastasis by facilitating circulating melanoma cell survival. This study demonstrates a novel regulatory function of Nur77 with linkage of the FAO-NADPH-ROS pathway during metabolic stress, suggesting Nur77 as a potential therapeutic target in melanoma.

Mannose antagonizes GSDME-mediated pyroptosis through AMPK activated by metabolite GlcNAc-6P.

Pyroptosis is a type of regulated cell death executed by gasdermin family members. However, how gasdermin-mediated pyroptosis is negatively regulated remains unclear. Here, we demonstrate that mannose, a hexose, inhibits GSDME-mediated pyroptosis by activating AMP-activated protein kinase (AMPK). Mechanistically, mannose metabolism in the hexosamine biosynthetic pathway increases levels of the metabolite N-acetylglucosamine-6-phosphate (GlcNAc-6P), which binds AMPK to facilitate AMPK phosphorylation by LKB1. Activated AMPK then phosphorylates GSDME at Thr6, which leads to blockade of caspase-3-induced GSDME cleavage, thereby repressing pyroptosis. The regulatory role of AMPK-mediated GSDME phosphorylation was further confirmed in AMPK knockout and GSDMET6E or GSDMET6A knock-in mice. In mouse primary cancer models, mannose administration suppressed pyroptosis in small intestine and kidney to alleviate cisplatin- or oxaliplatin-induced tissue toxicity without impairing antitumor effects. The protective effect of mannose was also verified in a small group of patients with gastrointestinal cancer who received normal chemotherapy. Our study reveals a novel mechanism whereby mannose antagonizes GSDME-mediated pyroptosis through GlcNAc-6P-mediated activation of AMPK, and suggests the utility of mannose supplementation in alleviating chemotherapy-induced side effects in clinic applications.

The metabolite α-KG induces GSDMC-dependent pyroptosis through death receptor 6-activated caspase-8.

Pyroptosis is a form of regulated cell death mediated by gasdermin family members, among which the function of GSDMC has not been clearly described. Herein, we demonstrate that the metabolite α-ketoglutarate (α-KG) induces pyroptosis through caspase-8-mediated cleavage of GSDMC. Treatment with DM-αKG, a cell-permeable derivative of α-KG, elevates ROS levels, which leads to oxidation of the plasma membrane-localized death receptor DR6. Oxidation of DR6 triggers its endocytosis, and then recruits both pro-caspase-8 and GSDMC to a DR6 receptosome through protein-protein interactions. The DR6 receptosome herein provides a platform for the cleavage of GSDMC by active caspase-8, thereby leading to pyroptosis. Moreover, this α-KG-induced pyroptosis could inhibit tumor growth and metastasis in mouse models. Interestingly, the efficiency of α-KG in inducing pyroptosis relies on an acidic environment in which α-KG is reduced by MDH1 and converted to L-2HG that further boosts ROS levels. Treatment with lactic acid, the end product of glycolysis, builds an improved acidic environment to facilitate more production of L-2HG, which makes the originally pyroptosis-resistant cancer cells more susceptible to α-KG-induced pyroptosis. This study not only illustrates a pyroptotic pathway linked with metabolites but also identifies an unreported principal axis extending from ROS-initiated DR6 endocytosis to caspase-8-mediated cleavage of GSDMC for potential clinical application in tumor therapy.

Nur77-activated lncRNA WFDC21P attenuates hepatocarcinogenesis via modulating glycolysis.

Hepatocellular carcinoma (HCC) is one of the leading causes of cancer-related mortality worldwide. Orphan nuclear receptor Nur77, which is low expressed in HCC, functions as a tumor suppressor to suppress HCC. However, the detailed mechanism is still not well understood. Here, we demonstrate that Nur77 could inhibit HCC development via transcriptional activation of the lncRNA WAP four-disulfide core domain 21 pseudogene (WFDC21P). Nur77 binds to its response elements on the WFDC21P promoter to directly induce WFDC21P transcription, which inhibits HCC cell proliferation, tumor growth, and tumor metastasis both in vitro and in vivo. In clinical HCC samples, WFDC21P expression positively correlated with that of Nur77, and the loss of WFDC21P is associated with worse prognosis. Mechanistically, WFDC21P could inhibit glycolysis by simultaneously interacting with PFKP and PKM2, two key enzymes in glycolysis. These interactions not only abrogate the tetramer formation of PFKP to impede its catalytic activity but also prevent the nuclear translocation of PKM2 to suppress its function as a transcriptional coactivator. Cytosporone-B (Csn-B), an agonist for Nur77, could stimulate WFDC21P expression and suppress HCC in a WFDC21P-dependent manner. Therefore, our study reveals a new HCC suppressor and connects the glycolytic remodeling of HCC with the Nur77-WFDC21P-PFKP/PKM2 axis.

Tom20 senses iron-activated ROS signaling to promote melanoma cell pyroptosis.

Iron has been shown to trigger oxidative stress by elevating reactive oxygen species (ROS) and to participate in different modes of cell death, such as ferroptosis, apoptosis and necroptosis. However, whether iron-elevated ROS is also linked to pyroptosis has not been reported. Here, we demonstrate that iron-activated ROS can induce pyroptosis via a Tom20-Bax-caspase-GSDME pathway. In melanoma cells, iron enhanced ROS signaling initiated by CCCP, causing the oxidation and oligomerization of the mitochondrial outer membrane protein Tom20. Bax is recruited to mitochondria by oxidized Tom20, which facilitates cytochrome c release to cytosol to activate caspase-3, eventually triggering pyroptotic death by inducing GSDME cleavage. Therefore, ROS acts as a causative factor and Tom20 senses ROS signaling for iron-driven pyroptotic death of melanoma cells. Since iron activates ROS for GSDME-dependent pyroptosis induction and melanoma cells specifically express a high level of GSDME, iron may be a potential candidate for melanoma therapy. Based on the functional mechanism of iron shown above, we further demonstrate that iron supplementation at a dosage used in iron-deficient patients is sufficient to maximize the anti-tumor effect of clinical ROS-inducing drugs to inhibit xenograft tumor growth and metastasis of melanoma cells through GSDME-dependent pyroptosis. Moreover, no obvious side effects are observed in the normal tissues and organs of mice during the combined treatment of clinical drugs and iron. This study not only identifies iron as a sensitizer amplifying ROS signaling to drive pyroptosis, but also implicates a novel iron-based intervention strategy for melanoma therapy.