Development of a mouse model expressing a bifunctional glutathione-synthesizing enzyme to study glutathione limitation in vivo.

Glutathione (GSH) is a highly abundant tripeptide thiol that performs diverse protective and biosynthetic functions in cells. While changes in GSH availability are associated with inborn errors of metabolism, cancer, and neurodegenerative disorders, studying the limiting role of GSH in physiology and disease has been challenging due to its tight regulation. To address this, we generated cell and mouse models that express a bifunctional glutathione-synthesizing enzyme from Streptococcus thermophilus (GshF), which possesses both glutamate-cysteine ligase and glutathione synthase activities. GshF expression allows efficient production of GSH in the cytosol and mitochondria and prevents cell death in response to GSH depletion, but not ferroptosis induction, indicating that GSH is not a limiting factor under lipid peroxidation. CRISPR screens using engineered enzymes further revealed genes required for cell proliferation under cellular and mitochondrial GSH depletion. Among these, we identified the glutamate-cysteine ligase modifier subunit, GCLM, as a requirement for cellular sensitivity to buthionine sulfoximine, a glutathione synthesis inhibitor. Finally, GshF expression in mice is embryonically lethal but sustains postnatal viability when restricted to adulthood. Overall, our work identifies a conditional mouse model to investigate the limiting role of GSH in physiology and disease.

A mouse model to study glutathione limitation in vivo.

Glutathione (GSH) is a highly abundant tripeptide thiol that performs diverse protective and biosynthetic functions in cells. While changes in GSH availability are linked to many diseases, including cancer and neurodegenerative disorders, determining the function of GSH in physiology and disease has been challenging due to its tight regulation. To address this, we generated cell and mouse models that express a bifunctional glutathione-synthesizing enzyme from Streptococcus Thermophilus (GshF). GshF expression allows efficient production of GSH in the cytosol and mitochondria and prevents cell death in response to GSH depletion, but not ferroptosis, indicating that GSH is not a limiting factor under lipid peroxidation. CRISPR screens using engineered enzymes revealed metabolic liabilities under compartmentalized GSH depletion. Finally, GshF expression in mice is embryonically lethal but sustains postnatal viability when restricted to adulthood. Overall, our work identifies a conditional mouse model to investigate the role of GSH availability in physiology and disease.

Metabolic-scale gene activation screens identify SLCO2B1 as a heme transporter that enhances cellular iron availability.

Iron is the most abundant transition metal essential for numerous cellular processes. Although most mammalian cells acquire iron through transferrin receptors, molecular players of iron utilization under iron restriction are incompletely understood. To address this, we performed metabolism-focused CRISPRa gain-of-function screens, which revealed metabolic limitations under stress conditions. Iron restriction screens identified not only expected members of iron utilization pathways but also SLCO2B1, a poorly characterized membrane carrier. SLCO2B1 expression is sufficient to increase intracellular iron, bypass the essentiality of the transferrin receptor, and enable proliferation under iron restriction. Mechanistically, SLCO2B1 mediates heme analog import in cellular assays. Heme uptake by SLCO2B1 provides sufficient iron for proliferation through heme oxygenases. Notably, SLCO2B1 is predominantly expressed in microglia in the brain, and primary Slco2b1-/- mouse microglia exhibit strong defects in heme analog import. Altogether, our work identifies SLCO2B1 as a microglia-enriched plasma membrane heme importer and provides a genetic platform to identify metabolic limitations under stress conditions.

SLC25A39 is necessary for mitochondrial glutathione import in mammalian cells.

Glutathione (GSH) is a small-molecule thiol that is abundant in all eukaryotes and has key roles in oxidative metabolism1. Mitochondria, as the major site of oxidative reactions, must maintain sufficient levels of GSH to perform protective and biosynthetic functions2. GSH is synthesized exclusively in the cytosol, yet the molecular machinery involved in mitochondrial GSH import remains unknown. Here, using organellar proteomics and metabolomics approaches, we identify SLC25A39, a mitochondrial membrane carrier of unknown function, as a regulator of GSH transport into mitochondria. Loss of SLC25A39 reduces mitochondrial GSH import and abundance without affecting cellular GSH levels. Cells lacking both SLC25A39 and its paralogue SLC25A40 exhibit defects in the activity and stability of proteins containing iron-sulfur clusters. We find that mitochondrial GSH import is necessary for cell proliferation in vitro and red blood cell development in mice. Heterologous expression of an engineered bifunctional bacterial GSH biosynthetic enzyme (GshF) in mitochondria enables mitochondrial GSH production and ameliorates the metabolic and proliferative defects caused by its depletion. Finally, GSH availability negatively regulates SLC25A39 protein abundance, coupling redox homeostasis to mitochondrial GSH import in mammalian cells. Our work identifies SLC25A39 as an essential and regulated component of the mitochondrial GSH-import machinery.

PSPC1-interchanged interactions with PTK6 and ß-catenin synergize oncogenic subcellular translocations and tumor progression.

Hepatocellular carcinoma (HCC) is one of the most lethal cancers worldwide due to metastasis. Paraspeckle component 1 (PSPC1) upregulation has been identified as an HCC pro-metastatic activator associated with poor patient prognosis, but with a lack of targeting strategy. Here, we report that PSPC1, a nuclear substrate of PTK6, sequesters PTK6 in the nucleus and loses its metastasis driving capability. Conversely, PSPC1 upregulation or PSPC1-Y523F mutation promotes epithelial-mesenchymal transition, stemness, and metastasis via cytoplasmic translocation of active PTK6 and nuclear translocation of ß-catenin, which interacts with PSPC1 to augment Wnt3a autocrine signaling. The aberrant nucleocytoplasmic shuttling of active PTK6/ß-catenin is reversed by expressing the PSPC1 C-terminal interacting domain (PSPC1-CT131), thereby suppressing PSPC1/PTK6/ß-catenin-activated metastasis to prolong the survival of HCC orthotopic mice. Thus, PSPC1 is the contextual determinant of the oncogenic switch of PTK6/ß-catenin subcellular localizations, and PSPC1-CT131 functions as a dual inhibitor of PSPC1 and PTK6 with potential for improving cancer therapy.

A New Switch for TGFß in Cancer.

The TGFß cytokine plays dichotomous roles during tumor progression. In normal and premalignant cancer cells, the TGFß signaling pathway inhibits proliferation and promotes cell-cycle arrest and apoptosis. However, the activation of this pathway in late-stage cancer cells could facilitate the epithelial-to-mesenchymal transition, stemness, and mobile features to enhance tumorigenesis and metastasis. The opposite functions of TGFß signaling during tumor progression make it a challenging target to develop anticancer interventions. Nevertheless, the recent discovery of cellular contextual determinants, especially the binding partners of the transcription modulators Smads, is critical to switch TGFß responses from proapoptosis to prometastasis. In this review, we summarize the recently identified contextual determinants (such as PSPC1, KLF5, 14-3-3ζ, C/EBPß, and others) and the mechanisms of how tumor cells manage the context-dependent autonomous TGFß responses to potentiate tumor progression. With the altered expression of some contextual determinants and their effectors during tumor progression, the aberrant molecular prometastatic switch might serve as a new class of theranostic targets for developing anticancer strategies.

PSPC1 potentiates TGF-ß-dependent metastatic dissemination.

Metastatic reprogramming toward malignant tumor progression relies on the activation of oncogenic regulators, yet the cellular determinants remain elucidated. Through identification of aberrant prognostic cancer genes, we identified paraspeckle component 1 (PSPC1) functions as a master activator of metastatic reprogramming by activating epithelial-to-mesenchymal transition (EMT), stemness and TGF-ß1 pro-metastatic switch.

PSPC1 mediates TGF-ß1 autocrine signalling and Smad2/3 target switching to promote EMT, stemness and metastasis.

Activation of metastatic reprogramming is critical for tumour metastasis. However, more detailed knowledge of the underlying mechanism is needed to enable targeted intervention. Here, we show that paraspeckle component 1 (PSPC1), identified in an aberrant 13q12.11 locus, is upregulated and associated with poor survival in patients with cancer. PSPC1 promotes tumorigenesis, epithelial-to-mesenchymal transition (EMT), stemness and metastasis in multiple cell types and in spontaneous mouse cancer models. PSPC1 is the master activator for transcription factors of EMT and stemness and accompanies c-Myc activation to facilitate tumour growth. PSPC1 increases transforming growth factor-ß1 (TGF-ß1) secretion through an interaction with phosphorylated and nuclear Smad2/3 to potentiate TGF-ß1 autocrine signalling. Moreover, PSPC1 acts as a contextual determinant of the TGF-ß1 pro-metastatic switch to alter Smad2/3 binding preference from tumour-suppressor to pro-metastatic genes. Having validated the PSPC1-Smads-TGF-ß1 axis in various cancers, we conclude that PSPC1 is a master activator of pro-metastatic switches and a potential target for anti-metastasis drugs.

Pyrimidine metabolic rate limiting enzymes in poorly-differentiated hepatocellular carcinoma are signature genes of cancer stemness and associated with poor prognosis.

Cellular metabolism of cancer cell is generally recognized to provide energy for facilitating tumor growth, but little is known about the aberrant metabolism in tumor progression and its prognostic value. Here, we applied integrated genomic approach to uncover the aberrant expression of metabolic enzymes in poorly-differentiated human hepatocellular carcinoma (HCC) for revealing targets against HCC malignancy. A total of 135 upregulated (22 are rate-limiting enzymes (RLEs)) and 362 down-regulated (77 are RLEs) metabolic genes were identified and associated with poor patient survival in large-cohorts of HCC patients in TCGA-LIHC and two other independent transcriptomic studies. Ten out of 22 upregulated RLEs in poorly-differentiated HCC are critical enzymes in pyrimidine metabolism pathways in association with stemness features by gene enrichment analysis and upregulated in ALDH1+ stem-like HCC subpopulations. By focusing on three RLEs including TK1, TYMS and DTYMK of dTTP biosynthesis pathway, expression of 3 RLEs in well-differentiated HCC cells increased ALDH1+ and spheroid stemness population but reversed by knockdown in poorly-differentiated HCC cells. Up-regulated 3 RLEs in HCC were associated with poor patient survival in multiple cohorts. Together, we identified aberrant pyrimidine pathway in poorly-differentiated HCC promotes cancer stemness served as potential theranostic target for battling HCC tumor progression.