Eric H. Ma

Metformin Antagonizes Cancer Cell Proliferation by Suppressing Mitochondrial-Dependent Biosynthesis

Metformin is a biguanide widely prescribed to treat Type II diabetes that has gained interest as an antineoplastic agent. Recent work suggests that metformin directly antagonizes cancer cell growth through its actions on complex I of the mitochondrial electron transport chain (ETC). However, the mechanisms by which metformin arrests cancer cell proliferation remain poorly defined. Here we demonstrate that the metabolic checkpoint kinases AMP-activated protein kinase (AMPK) and LKB1 are not required for the antiproliferative effects of metformin. Rather, metformin inhibits cancer cell proliferation by suppressing mitochondrial-dependent biosynthetic activity. We show that in vitro metformin decreases the flow of glucose- and glutamine-derived metabolic intermediates into the Tricarboxylic Acid (TCA) cycle, leading to reduced citrate production and de novo lipid biosynthesis. Tumor cells lacking functional mitochondria maintain lipid biosynthesis in the presence of metformin via glutamine-dependent reductive carboxylation, and display reduced sensitivity to metformin-induced proliferative arrest. Our data indicate that metformin inhibits cancer cell proliferation by suppressing the production of mitochondrial-dependent metabolic intermediates required for cell growth, and that metabolic adaptations that bypass mitochondrial-dependent biosynthesis may provide a mechanism of tumor cell resistance to biguanide activity.

Exploring structural motifs necessary for substrate binding in the active site of Escherichia coli pantothenate kinase.

The coenzyme A (CoA) biosynthetic enzymes have been used to produce various CoA analogues, including mechanistic probes of CoA-dependent enzymes such as those involved in fatty acid biosynthesis. These enzymes are also important for the activation of the pantothenamide class of antibacterial agents, and of a recently reported family of antibiotic resistance inhibitors. Herein we report a study on the selectivity of pantothenate kinase, the first and rate limiting step of CoA biosynthesis. A robust synthetic route was developed to allow rapid access to a small library of pantothenate analogs diversified at the β-alanine moiety, the carboxylate or the geminal dimethyl group. All derivatives were tested as substrates of Escherichia coli pantothenate kinase (EcPanK). Four derivatives, all N-aromatic pantothenamides, proved to be equivalent to the benchmark N-pentylpantothenamide (N5-pan) as substrates of EcPanK, while two others, also with N-aromatic groups, were some of the best substrates reported for this enzyme. This collection of data provides insight for the future design of PanK substrates in the production of useful CoA analogues.

Mitochondrial cyclophilin D regulates T cell metabolic responses and disease tolerance to tuberculosis

The regulation of a T cell metabolic program is a critical component of host tolerance to tuberculosis. Mycobacteria and metabolism Since the discovery of Mycobacterium tuberculosis (Mtb) over a century ago, great progress has been made in defining mechanisms of host resistance to tuberculosis (TB). By contrast, our understanding of how 90 to 95% of infected individuals live with chronic TB is extremely limited. Here, Tzelepis et al. examine the role of mitochondrial matrix protein cyclophilin D (CypD) in T cells using a mouse model of Mtb infection. CypD-deficient mice were more susceptible to Mtb infection in spite of enhanced Mtb-specific T cell responses, which has no impact on curbing bacterial loads but substantially increased lung immunopathology. Their findings indicate that CypD is a critical checkpoint of T cell metabolism for regulating disease tolerance in TB. Mycobacterium tuberculosis (Mtb) is one of the most ancient human pathogens, yet the exact mechanism(s) of host defense against Mtb remains unclear. Although one-third of the world’s population is chronically infected with Mtb, only 5 to 10% develop active disease. This indicates that, in addition to resistance mechanisms that control bacterial burden, the host has also evolved strategies to tolerate the presence of Mtb to limit disease severity. We identify mitochondrial cyclophilin D (CypD) as a critical checkpoint of T cell metabolism that controls the expansion of activated T cells. Although loss of CypD function in T cells led to enhanced Mtb antigen–specific T cell responses, this increased T cell response had no impact on bacterial burden. Rather, mice containing CypD-deficient T cells exhibited substantially compromised disease tolerance and succumbed to Mtb infection. This study establishes a mechanistic link between T cell–mediated immunity and disease tolerance during Mtb infection.

TORC)ing up purine biosynthesis

An enzyme complex coordinates metabolism and nucleotide production to fuel cell growth [Also see Reports by Ben-Sahra et al. and French et al.] Proliferating cells must coordinate their metabolic activities to meet the bioenergetic and biosynthetic demands of anabolic growth. Rapidly growing cells—such as cancer cells—achieve this in part by rewiring their metabolic pathways to increase flux through specific biosynthetic pathways. A more complex issue is how metabolic enzymes are organized to ensure efficient processing of metabolic intermediates. On pages 728 and 733 of this issue, Ben-Sahra et al. (1) and French et al. (2), reveal how a protein complex called mammalian/mechanistic target of rapamycin complex 1 (mTORC1) orchestrates metabolism and purine nucleotide biosynthesis to promote cell proliferation.

LKB1 deficiency in T cells promotes the development of gastrointestinal polyposis

Inflammation promotes gut polyposis Peutz–Jeghers Syndrome (PJS) causes benign polyps in the gut and a higher risk of several cancers caused by mutations in the tumor suppressor gene STK11, which encodes liver kinase B1 (LKB1). LKB1s role in this disease is thought to be related to its tumor suppressor function. Now, Poffenberger et al. show that the T cell–specific heterozygous deletion of Stk11 is sufficient to reproduce PJS symptoms in mice (see the Perspective by Hollstein and Shaw). Polyps in mice and humans are characterized by immune cell infiltration, enhanced STAT3 signaling, and increased levels of inflammatory cytokines such as interleukin-6 (IL-6). Targeting STAT3 signaling, IL-6, or T cells ameliorated the polyps, suggesting potential therapies for this disease. Science, this issue p. 406; see also p. 332 T cell–mediated inflammation promotes Peutz–Jeghers syndrome. Germline mutations in STK11, which encodes the tumor suppressor liver kinase B1 (LKB1), promote Peutz–Jeghers syndrome (PJS), a cancer predisposition syndrome characterized by the development of gastrointestinal (GI) polyps. Here, we report that heterozygous deletion of Stk11 in T cells (LThet mice) is sufficient to promote GI polyposis. Polyps from LThet mice, Stk11+/− mice, and human PJS patients display hallmarks of chronic inflammation, marked by inflammatory immune-cell infiltration, signal transducer and activator of transcription 3 (STAT3) activation, and increased expression of inflammatory factors associated with cancer progression [interleukin 6 (IL-6), IL-11, and CXCL2]. Targeting either T cells, IL-6, or STAT3 signaling reduced polyp growth in Stk11+/− animals. Our results identify LKB1-mediated inflammation as a tissue-extrinsic regulator of intestinal polyposis in PJS, suggesting possible therapeutic approaches by targeting deregulated inflammation in this disease.

Translational control in the tumor microenvironment promotes lung metastasis: Phosphorylation of eIF4E in neutrophils

Significance Our findings document mRNA translation in cells of the tumor microenvironment (TME) as a crucial factor in metastatic progression. The results underscore the importance of understanding how translation-targeting therapies affect different cell types within the TME. We provide a rationale for targeting eIF4E phosphorylation in both cancer cells and cells that comprise the TME to halt metastasis and demonstrate the efficacy of this strategy using merestinib, a small molecule targeting the mitogen-activated protein kinase integrating kinases (MNKs). Our findings raise the possibility that a combination of MNK inhibitors with immunotherapy represents a therapeutic opportunity worthy of further investigation for treating cancer metastasis. The translation of mRNAs into proteins serves as a critical regulatory event in gene expression. In the context of cancer, deregulated translation is a hallmark of transformation, promoting the proliferation, survival, and metastatic capabilities of cancer cells. The best-studied factor involved in the translational control of cancer is the eukaryotic translation initiation factor 4E (eIF4E). We and others have shown that eIF4E availability and phosphorylation promote metastasis in mouse models of breast cancer by selectively augmenting the translation of mRNAs involved in invasion and metastasis. However, the impact of translational control in cell types within the tumor microenvironment (TME) is unknown. Here, we demonstrate that regulatory events affecting translation in cells of the TME impact cancer progression. Mice bearing a mutation in the phosphorylation site of eIF4E (S209A) in cells comprising the TME are resistant to the formation of lung metastases in a syngeneic mammary tumor model. This is associated with reduced survival of prometastatic neutrophils due to decreased expression of the antiapoptotic proteins BCL2 and MCL1. Furthermore, we demonstrate that pharmacological inhibition of eIF4E phosphorylation prevents metastatic progression in vivo, supporting the development of phosphorylation inhibitors for clinical use.

Glycolytic metabolism is essential for CCR7 oligomerization and dendritic cell migration

Dendritic cells (DCs) are first responders of the innate immune system that integrate signals from external stimuli to direct context-specific immune responses. Current models suggest that an active switch from mitochondrial metabolism to glycolysis accompanies DC activation to support the anabolic requirements of DC function. We show that early glycolytic activation is a common program for both strong and weak stimuli, but that weakly activated DCs lack long-term HIF-1α-dependent glycolytic reprogramming and retain mitochondrial oxidative metabolism. Early induction of glycolysis is associated with activation of AKT, TBK, and mTOR, and sustained activation of these pathways is associated with long-term glycolytic reprogramming. We show that inhibition of glycolysis impaired maintenance of elongated cell shape, DC motility, CCR7 oligomerization, and DC migration to draining lymph nodes. Together, our results indicate that early induction of glycolysis occurs independent of pro-inflammatory phenotype, and that glycolysis supports DC migratory ability regardless of mitochondrial bioenergetics.The activation of dendritic cells (DC) is associated with a metabolic switch from oxidative to glycolytic metabolism. Here, the authors show that both strong and weak stimuli cause an immediate increase in glycolysis, but only strong stimuli induce long-term glycolytic reprogramming.