A pro-metastatic tRNA fragment drives Nucleolin oligomerization and stabilization of its bound metabolic mRNAs.

Stress-induced cleavage of transfer RNAs (tRNAs) into tRNA-derived fragments (tRFs) occurs across organisms from yeast to humans; yet, its mechanistic underpinnings and pathological consequences remain poorly defined. Small RNA profiling revealed increased abundance of a cysteine tRNA fragment (5'-tRFCys) during breast cancer metastatic progression. 5'-tRFCys was required for efficient breast cancer metastatic lung colonization and cancer cell survival. We identified Nucleolin as the direct binding partner of 5'-tRFCys. 5'-tRFCys promoted the oligomerization of Nucleolin and its bound metabolic transcripts Mthfd1l and Pafah1b1 into a higher-order transcript stabilizing ribonucleoprotein complex, which protected these transcripts from exonucleolytic degradation. Consistent with this, Mthfd1l and Pafah1b1 mediated pro-metastatic and metabolic effects downstream of 5'-tRFCys-impacting folate, one-carbon, and phosphatidylcholine metabolism. Our findings reveal that a tRF can promote oligomerization of an RNA-binding protein into a transcript stabilizing ribonucleoprotein complex, thereby driving specific metabolic pathways underlying cancer progression.

The pupal moulting fluid has evolved social functions in ants.

Insect societies are tightly integrated, complex biological systems in which group-level properties arise from the interactions between individuals1-4. However, these interactions have not been studied systematically and therefore remain incompletely known. Here, using a reverse engineering approach, we reveal that unlike solitary insects, ant pupae extrude a secretion derived from the moulting fluid that is rich in nutrients, hormones and neuroactive substances. This secretion elicits parental care behaviour and is rapidly removed and consumed by the adults. This behaviour is crucial for pupal survival; if the secretion is not removed, pupae develop fungal infections and die. Analogous to mammalian milk, the secretion is also an important source of early larval nutrition, and young larvae exhibit stunted growth and decreased survival without access to the fluid. We show that this derived social function of the moulting fluid generalizes across the ants. This secretion thus forms the basis of a central and hitherto overlooked interaction network in ant societies, and constitutes a rare example of how a conserved developmental process can be co-opted to provide the mechanistic basis of social interactions. These results implicate moulting fluids in having a major role in the evolution of ant eusociality.

Maintaining Iron Homeostasis Is the Key Role of Lysosomal Acidity for Cell Proliferation.

The lysosome is an acidic multi-functional organelle with roles in macromolecular digestion, nutrient sensing, and signaling. However, why cells require acidic lysosomes to proliferate and which nutrients become limiting under lysosomal dysfunction are unclear. To address this, we performed CRISPR-Cas9-based genetic screens and identified cholesterol biosynthesis and iron uptake as essential metabolic pathways when lysosomal pH is altered. While cholesterol synthesis is only necessary, iron is both necessary and sufficient for cell proliferation under lysosomal dysfunction. Remarkably, iron supplementation restores cell proliferation under both pharmacologic and genetic-mediated lysosomal dysfunction. The rescue was independent of metabolic or signaling changes classically associated with increased lysosomal pH, uncoupling lysosomal function from cell proliferation. Finally, our experiments revealed that lysosomal dysfunction dramatically alters mitochondrial metabolism and hypoxia inducible factor (HIF) signaling due to iron depletion. Altogether, these findings identify iron homeostasis as the key function of lysosomal acidity for cell proliferation.

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.

Metabolic determinants of cancer cell sensitivity to canonical ferroptosis inducers.

Cancer cells rewire their metabolism and rely on endogenous antioxidants to mitigate lethal oxidative damage to lipids. However, the metabolic processes that modulate the response to lipid peroxidation are poorly defined. Using genetic screens, we compared metabolic genes essential for proliferation upon inhibition of cystine uptake or glutathione peroxidase-4 (GPX4). Interestingly, very few genes were commonly required under both conditions, suggesting that cystine limitation and GPX4 inhibition may impair proliferation via distinct mechanisms. Our screens also identify tetrahydrobiopterin (BH4) biosynthesis as an essential metabolic pathway upon GPX4 inhibition. Mechanistically, BH4 is a potent radical-trapping antioxidant that protects lipid membranes from autoxidation, alone and in synergy with vitamin E. Dihydrofolate reductase catalyzes the regeneration of BH4, and its inhibition by methotrexate synergizes with GPX4 inhibition. Altogether, our work identifies the mechanism by which BH4 acts as an endogenous antioxidant and provides a compendium of metabolic modifiers of lipid peroxidation.

A deep intronic splice mutation of STAT3 underlies hyper IgE syndrome by negative dominance.

Heterozygous in-frame mutations in coding regions of human STAT3 underlie the only known autosomal dominant form of hyper IgE syndrome (AD HIES). About 5% of familial cases remain unexplained. The mutant proteins are loss-of-function and dominant-negative when tested following overproduction in recipient cells. However, the production of mutant proteins has not been detected and quantified in the cells of heterozygous patients. We report a deep intronic heterozygous STAT3 mutation, c.1282-89C>T, in 7 relatives with AD HIES. This mutation creates a new exon in the STAT3 complementary DNA, which, when overexpressed, generates a mutant STAT3 protein (D427ins17) that is loss-of-function and dominant-negative in terms of tyrosine phosphorylation, DNA binding, and transcriptional activity. In immortalized B cells from these patients, the D427ins17 protein was 2 kDa larger and 4-fold less abundant than wild-type STAT3, on mass spectrometry. The patients' primary B and T lymphocytes responded poorly to STAT3-dependent cytokines. These findings are reminiscent of the impaired responses of leukocytes from other patients with AD HIES due to typical STAT3 coding mutations, providing further evidence for the dominance of the mutant intronic allele. These findings highlight the importance of sequencing STAT3 introns in patients with HIES without candidate variants in coding regions and essential splice sites. They also show that AD HIES-causing STAT3 mutant alleles can be dominant-negative even if the encoded protein is produced in significantly smaller amounts than wild-type STAT3.

Stabilization of the Reductase Domain in the Catalytically Self-Sufficient Cytochrome P450BM3 by Consensus-Guided Mutagenesis.

The multidomain, catalytically self-sufficient cytochrome P450 BM-3 from Bacillus megaterium (P450BM3 ) constitutes a versatile enzyme for the oxyfunctionalization of organic molecules and natural products. However, the limited stability of the diflavin reductase domain limits the utility of this enzyme for synthetic applications. In this work, a consensus-guided mutagenesis approach was applied to enhance the thermal stability of the reductase domain of P450BM3 . Upon phylogenetic analysis of a set of distantly related P450s (>38 % identity), a total of 14 amino acid substitutions were identified and evaluated in terms of their stabilizing effects relative to the wild-type reductase domain. Recombination of the six most stabilizing mutations generated two thermostable variants featuring up to tenfold longer half-lives at 50 °C and increased catalytic performance at elevated temperatures. Further characterization of the engineered P450BM3 variants indicated that the introduced mutations increased the thermal stability of the FAD-binding domain and that the optimal temperature (Topt ) of the enzyme had shifted from 25 to 40 °C. This work demonstrates the effectiveness of consensus mutagenesis for enhancing the stability of the reductase component of a multidomain P450. The stabilized P450BM3 variants developed here could potentially provide more robust scaffolds for the engineering of oxidation biocatalysts.

Anticancer activity profiling of parthenolide analogs generated via P450-mediated chemoenzymatic synthesis.

The plant-derived sesquiterpene lactone parthenolide (PTL) was recently found to possess promising anticancer activity but elaboration of this natural product scaffold for optimization of its pharmacological properties has proven challenging via available chemical methods. In this work, P450-catalyzed C-H hydroxylation of positions C9 and C14 in PTL was coupled to carbamoylation chemistry to yield a panel of novel carbamate-based PTL analogs ('parthenologs'). These compounds, along with a series of other C9- and C14-functionalized parthenologs obtained via O-H acylation, alkylation, and metal-catalyzed carbene insertion, were profiled for their cytotoxicity against a diverse panel of human cancer cell lines. These studies led to the discovery of several parthenologs with significantly improved anticancer activity (2-14-fold) compared to the parent molecule. Most interestingly, two PTL analogs with high cytotoxicity (LC50∼1-3µM) against T cell leukemia (Jurkat), mantle cell lymphoma (JeKo-1), and adenocarcinoma (HeLa) cells as well as a carbamate derivative with potent activity (LC50=0.6µM) against neuroblastoma cells (SK-N-MC) were obtained. In addition, these analyses resulted in the identification of parthenologs featuring both a broad spectrum and tumor cell-specific anticancer activity profile, thus providing valuable probes for the future investigation of biomolecular targets that can affect cell viability across multiple as well as specific types of human cancers. Altogether, these results highlight the potential of P450-mediated chemoenzymatic C-H functionalization toward tuning and improving the anticancer activity of the natural product parthenolide.

Chemoenzymatic synthesis and antileukemic activity of novel C9- and C14-functionalized parthenolide analogs.

Parthenolide is a naturally occurring terpene with promising anticancer properties, particularly in the context of acute myeloid leukemia (AML). Optimization of this natural product has been challenged by limited opportunities for the late-stage functionalization of this molecule without affecting the pharmacologically important α-methylene-γ-lactone moiety. Here, we report the further development and application of a chemoenzymatic strategy to afford a series of new analogs of parthenolide functionalized at the aliphatic positions C9 and C14. Several of these compounds were determined to be able to kill leukemia cells and patient-derived primary AML specimens with improved activity compared to parthenolide, exhibiting LC50 values in the low micromolar range. These studies demonstrate that different O-H functionalization chemistries can be applied to elaborate the parthenolide scaffold and that modifications at the C9 or C14 position can effectively enhance the antileukemic properties of this natural product. The C9-functionalized analogs 22a and 25b were identified as the most interesting compounds in terms of antileukemic potency and selectivity toward AML versus healthy blood cells.

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.

Threonine fuels glioblastoma through YRDC-mediated codon-biased translational reprogramming.

Cancers commonly reprogram translation and metabolism, but little is known about how these two features coordinate in cancer stem cells. Here we show that glioblastoma stem cells (GSCs) display elevated protein translation. To dissect underlying mechanisms, we performed a CRISPR screen and identified YRDC as the top essential transfer RNA (tRNA) modification enzyme in GSCs. YRDC catalyzes the formation of N6-threonylcarbamoyladenosine (t6A) on ANN-decoding tRNA species (A denotes adenosine, and N denotes any nucleotide). Targeting YRDC reduced t6A formation, suppressed global translation and inhibited tumor growth both in vitro and in vivo. Threonine is an essential substrate of YRDC. Threonine accumulated in GSCs, which facilitated t6A formation through YRDC and shifted the proteome to support mitosis-related genes with ANN codon bias. Dietary threonine restriction (TR) reduced tumor t6A formation, slowed xenograft growth and augmented anti-tumor efficacy of chemotherapy and anti-mitotic therapy, providing a molecular basis for a dietary intervention in cancer treatment.

Inter-organellar nucleic acid communication by a mitochondrial tRNA regulates nuclear metabolic transcription.

Efficient communication between mitochondria and the nucleus underlies homoeostatic metabolic control, though the involved mitochondrial factors and their mechanisms are poorly defined. Here, we report the surprising detection of multiple mitochondrial-derived transfer RNAs (mito-tRNAs) within the nuclei of human cells. Focused studies of nuclear-transported mito-tRNA-asparagine (mtAsn) revealed that its cognate charging enzyme (NARS2) is also present in the nucleus. MtAsn promoted interaction of NARS2 with histone deacetylase 2 (HDAC2), and repressed HDAC2 association with specific chromatin loci. Perturbation of this axis using antisense oligonucleotides promoted nucleotide biogenesis and enhanced breast cancer growth, and RNA and nascent transcript sequencing demonstrated specific alterations in the transcription of nuclear genes. These findings uncover nucleic-acid mediated communication between two organelles and the existence of a machinery for nuclear gene regulation by a mito-tRNA that restricts tumor growth through metabolic control. Highlights: Multiple mitochondrial-derived tRNAs are detected in human cell nucleiMtAsn promotes binding between NARS2 and HDAC2Metabolic alterations driven by mtAsn impact cell proliferationMtAsn inhibition releases HDAC2 to bind and transcriptionally regulate multiple nuclear genes.

Arginine limitation causes a directed DNA sequence evolution response in colorectal cancer cells.

Utilization of specific codons varies significantly across organisms. Cancer represents a model for understanding DNA sequence evolution and could reveal causal factors underlying codon evolution. We found that across human cancer, arginine codons are frequently mutated to other codons. Moreover, arginine restriction-a feature of tumor microenvironments-is sufficient to induce arginine codon-switching mutations in human colon cancer cells. Such DNA codon switching events encode mutant proteins with arginine residue substitutions. Mechanistically, arginine limitation caused rapid reduction of arginine transfer RNAs and the stalling of ribosomes over arginine codons. Such selective pressure against arginine codon translation induced a proteomic shift towards low arginine codon containing genes, including specific amino acid transporters, and caused mutational evolution away from arginine codons-reducing translational bottlenecks that occurred during arginine starvation. Thus, environmental availability of a specific amino acid can influence DNA sequence evolution away from its cognate codons and generate altered proteins.

Arginine limitation drives a directed codon-dependent DNA sequence evolution response in colorectal cancer cells.

Utilization of specific codons varies between organisms. Cancer represents a model for understanding DNA sequence evolution and could reveal causal factors underlying codon evolution. We found that across human cancer, arginine codons are frequently mutated to other codons. Moreover, arginine limitation-a feature of tumor microenvironments-is sufficient to induce arginine codon-switching mutations in human colon cancer cells. Such DNA codon switching events encode mutant proteins with arginine residue substitutions. Mechanistically, arginine limitation caused rapid reduction of arginine transfer RNAs and the stalling of ribosomes over arginine codons. Such selective pressure against arginine codon translation induced an adaptive proteomic shift toward low-arginine codon-containing genes, including specific amino acid transporters, and caused mutational evolution away from arginine codons-reducing translational bottlenecks that occurred during arginine starvation. Thus, environmental availability of a specific amino acid can influence DNA sequence evolution away from its cognate codons and generate altered proteins.

Integrative genetic analysis identifies FLVCR1 as a plasma-membrane choline transporter in mammals.

Genome-wide association studies (GWASs) of serum metabolites have the potential to uncover genes that influence human metabolism. Here, we combined an integrative genetic analysis that associates serum metabolites to membrane transporters with a coessentiality map of metabolic genes. This analysis revealed a connection between feline leukemia virus subgroup C cellular receptor 1 (FLVCR1) and phosphocholine, a downstream metabolite of choline metabolism. Loss of FLVCR1 in human cells strongly impairs choline metabolism due to the inhibition of choline import. Consistently, CRISPR-based genetic screens identified phospholipid synthesis and salvage machinery as synthetic lethal with FLVCR1 loss. Cells and mice lacking FLVCR1 exhibit structural defects in mitochondria and upregulate integrated stress response (ISR) through heme-regulated inhibitor (HRI) kinase. Finally, Flvcr1 knockout mice are embryonic lethal, which is partially rescued by choline supplementation. Altogether, our findings propose FLVCR1 as a major choline transporter in mammals and provide a platform to discover substrates for unknown metabolite transporters.

Disruption of proteome by an oncogenic fusion kinase alters metabolism in fibrolamellar hepatocellular carcinoma.

Fibrolamellar hepatocellular carcinoma (FLC) is a usually lethal primary liver cancer driven by a somatic dysregulation of protein kinase A. We show that the proteome of FLC tumors is distinct from that of adjacent nontransformed tissue. These changes can account for some of the cell biological and pathological alterations in FLC cells, including their drug sensitivity and glycolysis. Hyperammonemic encephalopathy is a recurrent problem in these patients, and established treatments based on the assumption of liver failure are unsuccessful. We show that many of the enzymes that produce ammonia are increased and those that consume ammonia are decreased. We also demonstrate that the metabolites of these enzymes change as expected. Thus, hyperammonemic encephalopathy in FLC may require alternative therapeutics.

Autoregulatory control of mitochondrial glutathione homeostasis.

Mitochondria must maintain adequate amounts of metabolites for protective and biosynthetic functions. However, how mitochondria sense the abundance of metabolites and regulate metabolic homeostasis is not well understood. In this work, we focused on glutathione (GSH), a critical redox metabolite in mitochondria, and identified a feedback mechanism that controls its abundance through the mitochondrial GSH transporter, SLC25A39. Under physiological conditions, SLC25A39 is rapidly degraded by mitochondrial protease AFG3L2. Depletion of GSH dissociates AFG3L2 from SLC25A39, causing a compensatory increase in mitochondrial GSH uptake. Genetic and proteomic analyses identified a putative iron-sulfur cluster in the matrix-facing loop of SLC25A39 as essential for this regulation, coupling mitochondrial iron homeostasis to GSH import. Altogether, our work revealed a paradigm for the autoregulatory control of metabolic homeostasis in organelles.

Nanoparticle-Mediated Delivery of Micheliolide Analogs to Eliminate Leukemic Stem Cells in the Bone Marrow.

Micheliolide (MCL) is a naturally occurring sesquiterpene lactone that selectively targets leukemic stem cells (LSCs), which persist after conventional chemotherapy for myeloid leukemias, leading to disease relapse. To overcome modest MCL cytotoxicity, analogs with ≈two-threefold greater cytotoxicity against LSCs are synthesized via late-stage chemoenzymatic C-H functionalization. To enhance bone marrow delivery, MCL analogs are entrapped within bone-targeted polymeric nanoparticles (NPs). Robust drug loading capacities of up to 20% (mg drug mg-1 NP) are obtained, with release dominated by analog hydrophobicity. NPs loaded with a hydrolytically stable analog are tested in a leukemic mouse model. Median survival improved by 13% and bone marrow LSCs are decreased 34-fold following NPMCL treatments versus controls. Additionally, selective leukemic cell and LSC cytotoxicity of the treatment versus normal hematopoietic cells is observed. Overall, these studies demonstrate that MCL-based antileukemic agents combined with bone-targeted NPs offer a promising strategy for eradicating LSCs.

Analysis of capecitabine metabolites in conjunction with digital autoradiography in a murine model of pancreatic cancer suggests extensive drug penetration through the tumor.

Previously published digital autoradiography of 3 H-labeled capecitabine reveals a near-uniform distribution of activity throughout a murine pancreatic model. This is in contrast both to 14 C-labeled gemcitabine, and established expectations, as the dense stroma of pancreatic cancer is understood to inhibit drug penetration. Capecitabine is a pro-drug for 5 FU. The positioning of the radiolabel on capecitabine leaves open the possibility that much of the autoradiographic signal is generated by nontoxic compounds. Studies were performed on tumors derived via organoid culture from a murine KPC tumor. As before, we performed autoradiography comparing 3 H capecitabine to the gemcitabine analog 18 F-FAC. The metabolism of capecitabine in this model was studied through LC-MS of tumor tissue. The autoradiographs confirmed that the 3 H label from capecitabine was much more uniformly distributed through the tumor than the 18 F from the gemcitabine analog. LC-MS revealed that approximately 75% of the molar mass of capecitabine had been converted into 5 FU or pre-5 FU compounds. The remainder had been converted into nontoxic species. Therapeutically relevant capecitabine metabolites achieve a relatively even distribution in this pancreatic cancer model, in contrast to the gemcitabine analog 18 F-FAC. In a human xenograft model, (BxPC3), the 3 H label from capecitabine was also uniformly spread across the tumor autoradiographs. However, at 2 h post-administration the metabolism of capecitabine had proceeded further and the bulk of the agent was in the form of nontoxic species.

Adaptive stimulation of macropinocytosis overcomes aspartate limitation in cancer cells under hypoxia.

Stress-adaptive mechanisms enable tumour cells to overcome metabolic constraints under nutrient and oxygen shortage. Aspartate is an endogenous metabolic limitation under hypoxic conditions, but the nature of the adaptive mechanisms that contribute to aspartate availability and hypoxic tumour growth are poorly understood. Here we identify GOT2-catalysed mitochondrial aspartate synthesis as an essential metabolic dependency for the proliferation of pancreatic tumour cells under hypoxic culture conditions. In contrast, GOT2-catalysed aspartate synthesis is dispensable for pancreatic tumour formation in vivo. The dependence of pancreatic tumour cells on aspartate synthesis is bypassed in part by a hypoxia-induced potentiation of extracellular protein scavenging via macropinocytosis. This effect is mutant KRAS dependent, and is mediated by hypoxia-inducible factor 1 (HIF1A) and its canonical target carbonic anhydrase-9 (CA9). Our findings reveal high plasticity of aspartate metabolism and define an adaptive regulatory role for macropinocytosis by which mutant KRAS tumours can overcome nutrient deprivation under hypoxic conditions.