Joshua J. Gruber

Hypoxia promotes isocitrate dehydrogenase-dependent carboxylation of α-ketoglutarate to citrate to support cell growth and viability.

Citrate is a critical metabolite required to support both mitochondrial bioenergetics and cytosolic macromolecular synthesis. When cells proliferate under normoxic conditions, glucose provides the acetyl-CoA that condenses with oxaloacetate to support citrate production. Tricarboxylic acid (TCA) cycle anaplerosis is maintained primarily by glutamine. Here we report that some hypoxic cells are able to maintain cell proliferation despite a profound reduction in glucose-dependent citrate production. In these hypoxic cells, glutamine becomes a major source of citrate. Glutamine-derived α-ketoglutarate is reductively carboxylated by the NADPH-linked mitochondrial isocitrate dehydrogenase (IDH2) to form isocitrate, which can then be isomerized to citrate. The increased IDH2-dependent carboxylation of glutamine-derived α-ketoglutarate in hypoxia is associated with a concomitant increased synthesis of 2-hydroxyglutarate (2HG) in cells with wild-type IDH1 and IDH2. When either starved of glutamine or rendered IDH2-deficient by RNAi, hypoxic cells are unable to proliferate. The reductive carboxylation of glutamine is part of the metabolic reprogramming associated with hypoxia-inducible factor 1 (HIF1), as constitutive activation of HIF1 recapitulates the preferential reductive metabolism of glutamine-derived α-ketoglutarate even in normoxic conditions. These data support a role for glutamine carboxylation in maintaining citrate synthesis and cell growth under hypoxic conditions.

Ars2 Regulates Both miRNA- and siRNA- Dependent Silencing and Suppresses RNA Virus Infection in Drosophila

Intrinsic immune responses autonomously inhibit viral replication and spread. One pathway that restricts viral infection in plants and insects is RNA interference (RNAi), which targets and degrades viral RNA to limit infection. To identify additional genes involved in intrinsic antiviral immunity, we screened Drosophila cells for modulators of viral infection using an RNAi library. We identified Ars2 as a key component of Drosophila antiviral immunity. Loss of Ars2 in cells, or in flies, increases susceptibility to RNA viruses. Consistent with its antiviral properties, we found that Ars2 physically interacts with Dcr-2, modulates its activity in vitro, and is required for siRNA-mediated silencing. Furthermore, we show that Ars2 plays an essential role in miRNA-mediated silencing, interacting with the Microprocessor and stabilizing pri-miRNAs. The identification of Ars2 as a player in these small RNA pathways provides new insight into the biogenesis of small RNAs that may be extended to other systems.

Ars2 Links the Nuclear Cap-Binding Complex to RNA Interference and Cell Proliferation

Here we identify a component of the nuclear RNA cap-binding complex (CBC), Ars2, that is important for miRNA biogenesis and critical for cell proliferation. Unlike other components of the CBC, Ars2 expression is linked to the proliferative state of the cell. Deletion of Ars2 is developmentally lethal, and deletion in adult mice led to bone marrow failure whereas parenchymal organs composed of nonproliferating cells were unaffected. Depletion of Ars2 or CBP80 from proliferating cells impaired miRNA-mediated repression and led to alterations in primary miRNA processing in the nucleus. Ars2 depletion also reduced the levels of several miRNAs, including miR-21, let-7, and miR-155, that are implicated in cellular transformation. These findings provide evidence for a role for Ars2 in RNA interference regulation during cell proliferation.

Imatinib resistance associated with BCR-ABL upregulation is dependent on HIF-1alpha-induced metabolic reprograming.

As chronic myeloid leukemia (CML) progresses from the chronic phase to blast crisis, the levels of BCR-ABL increase. In addition, blast-transformed leukemic cells display enhanced resistance to imatinib in the absence of BCR-ABL-resistance mutations. In this study, we show that when BCR-ABL-transformed cell lines were selected for imatinib resistance in vitro, the cells that grew out displayed a higher BCR-ABL expression comparable to the increase seen in accelerated forms of the disease. This enhanced expression of BCR-ABL was associated with an increased rate of glycolysis but with a decreased rate of proliferation. The higher level of BCR-ABL expression in the selected cells correlated with a nonhypoxic induction of hypoxia-inducible factor-1α (HIF-1α) that was required for cells to tolerate enhanced BCR-ABL signaling. HIF-1α induction resulted in an enhanced rate of glycolysis but with reduced glucose flux through both the tricarboxylic acid cycle and the oxidative arm of the pentose phosphate pathway (PPP). The reduction in oxidative PPP-mediated ribose synthesis was compensated by the HIF-1α-dependent activation of the nonoxidative PPP enzyme, transketolase, in imatinib-resistant CML cells. In both primary cultures of cells from patients exhibiting blast transformation and in vivo xenograft tumors, use of oxythiamine, which can inhibit both the pyruvate dehydrogenase complex and transketolase, resulted in enhanced imatinib sensitivity of tumor cells. Together, these results suggest that oxythiamine can enhance imatinib efficacy in patients who present an accelerated form of the disease.

DR5 Knockout Mice Are Compromised in Radiation-Induced Apoptosis

ABSTRACT DR5 (also called TRAIL receptor 2 and KILLER) is an apoptosis-inducing membrane receptor for tumor necrosis factor-related apoptosis-inducing ligand (also called TRAIL and Apo2 ligand). DR5 is a transcriptional target of p53, and its overexpression induces cell death in vitro. However, the in vivo biology of DR5 has remained largely unexplored. To better understand the role of DR5 in development and in adult tissues, we have created a knockout mouse lacking DR5. This mouse is viable and develops normally with the exception of having an enlarged thymus. We show that DR5 is not expressed in developing embryos but is present in the decidua and chorion early in development. DR5-null mouse embryo fibroblasts expressing E1A are resistant to treatment with TRAIL, suggesting that DR5 may be the primary proapoptotic receptor for TRAIL in the mouse. When exposed to ionizing radiation, DR5-null tissues exhibit reduced amounts of apoptosis compared to wild-type thymus, spleen, Peyers patches, and the white matter of the brain. In the ileum, colon, and stomach, DR5 deficiency was associated with a subtle phenotype of radiation-induced cell death. These results indicate that DR5 has a limited role during embryogenesis and early stages of development but plays an organ-specific role in the response to DNA-damaging stimuli.

The glucose-responsive transcription factor ChREBP contributes to glucose-dependent anabolic synthesis and cell proliferation

Tumor cells are metabolically reprogrammed to fuel cell proliferation. Most transformed cells take up high levels of glucose and produce ATP through aerobic glycolysis. In cells exhibiting aerobic glycolysis, a significant fraction of glucose carbon is also directed into de novo lipogenesis and nucleotide biosynthesis. The glucose-responsive transcription factor carbohydrate responsive element binding protein (ChREBP) was previously shown to be important for redirecting glucose metabolism in support of lipogenesis in nonproliferating hepatocytes. However, whether it plays a more generalized role in reprogramming metabolism during cell proliferation has not been examined. Here, we demonstrated that the expression of ChREBP can be induced in response to mitogenic stimulation and that the induction of ChREBP is required for efficient cell proliferation. Suppression of ChREBP resulted in diminished aerobic glycolysis, de novo lipogenesis, and nucleotide biosynthesis, but stimulated mitochondrial respiration, suggesting a metabolic switch from aerobic glycolysis to oxidative phosphorylation. Cells in which ChREBP was suppressed by RNAi exhibited p53 activation and cell cycle arrest. In vivo, suppression of ChREBP led to a p53-dependent reduction in tumor growth. These results demonstrate that ChREBP plays a key role both in redirecting glucose metabolism to anabolic pathways and suppressing p53 activity.

Long-lived microRNA–Argonaute complexes in quiescent cells can be activated to regulate mitogenic responses

Cellular proliferation depends on the integration of mitogenic stimuli with environmental conditions. Increasing evidence suggests that microRNAs play a regulatory role in this integration. Here we show that during periods of cellular quiescence, mature microRNAs are stabilized and stored in Argonaute protein complexes that can be activated by mitogenic stimulation to repress mitogen-stimulated targets, thus influencing subsequent cellular responses. In quiescent cells, the majority of microRNAs exist in low molecular weight, Argonaute protein-containing complexes devoid of essential components of the RNA-induced silencing complex (RISC). For at least 3 wk, this pool of Argonaute-associated microRNAs is stable and can be recruited into RISC complexes subsequent to mitogenic stimulation. Using several model systems, we demonstrate that stable Argonaute protein-associated small RNAs are capable of repressing mitogen-induced transcripts. Therefore, mature microRNAs may represent a previously unappreciated form of cellular memory that allows cells to retain posttranscriptional regulatory information over extended periods of cellular quiescence.

Differentiated Thyroid Cancer: Focus on Emerging Treatments for Radioactive Iodine-Refractory Patients

BACKGROUND The treatment of differentiated thyroid cancer refractory to radioactive iodine (RAI) had been hampered by few effective therapies. Recently, tyrosine kinase inhibitors (TKIs) have shown activity in this disease. Clinical guidance on the use of these agents in RAI-refractory thyroid cancer is warranted. MATERIALS AND METHODS Molecular mutations found in RAI-refractory thyroid cancer are summarized. Recent phase II and III clinical trial data for TKIs axitinib, lenvatinib, motesanib, pazopanib, sorafenib, sunitinib, and vandetinib are reviewed including efficacy and side effect profiles. Molecular targets and potencies of these agents are compared. Inhibitors of BRAF, mammalian target of rapamycin, and MEK are considered. RESULTS Routine testing for molecular alterations prior to therapy is not yet recommended. TKIs produce progression-free survival of approximately 1 year (range: 7.7-19.6 months) and partial response rates of up to 50% by Response Evaluation Criteria in Solid Tumors. Pazopanib and lenvatinib are the most active agents. The majority of patients experienced tumor shrinkage with TKIs. Common adverse toxicities affect dermatologic, gastrointestinal, and cardiovascular systems. CONCLUSION Multiple TKIs have activity in RAI-refractory differentiated thyroid cancer. Selection of a targeted agent should depend on disease trajectory, side effect profile, and goals of therapy.

Association of AHSG with alopecia and mental retardation (APMR) syndrome

Alopecia with mental retardation syndrome (APMR) is a very rare autosomal recessive condition that is associated with total or partial absence of hair from the scalp and other parts of the body as well as variable intellectual disability. Here we present whole-exome sequencing results of a large consanguineous family segregating APMR syndrome with seven affected family members. Our study revealed a novel predicted pathogenic, homozygous missense mutation in the AHSG (OMIM 138680) gene (AHSG: NM_001622:exon7:c.950G>A:p.Arg317His). The variant is predicted to affect a region of the protein required for protein processing and disrupts a phosphorylation motif. In addition, the altered protein migrates with an aberrant size relative to healthy individuals. Consistent with the phenotype, AHSG maps within APMR linkage region 1 (APMR 1) as reported before, and falls within runs of homozygosity (ROH). Previous families with APMR syndrome have been studied through linkage analyses and the linkage resolution did not allow pointing out to a single gene candidate. Our study is the first report to identify a homozygous missense mutation for APMR syndrome through whole-exome sequencing.

Disruption of mesoderm formation during cardiac differentiation due to developmental exposure to 13-cis-retinoic acid

Abstract13-cis-retinoic acid (isotretinoin, INN) is an oral pharmaceutical drug used for the treatment of skin acne, and is also a known teratogen. In this study, the molecular mechanisms underlying INN-induced developmental toxicity during early cardiac differentiation were investigated using both human induced pluripotent stem cells (hiPSCs) and human embryonic stem cells (hESCs). Pre-exposure of hiPSCs and hESCs to a sublethal concentration of INN did not influence cell proliferation and pluripotency. However, mesodermal differentiation was disrupted when INN was included in the medium during differentiation. Transcriptomic profiling by RNA-seq revealed that INN exposure leads to aberrant expression of genes involved in several signaling pathways that control early mesoderm differentiation, such as TGF-beta signaling. In addition, genome-wide chromatin accessibility profiling by ATAC-seq suggested that INN-exposure leads to enhanced DNA-binding of specific transcription factors (TFs), including HNF1B, SOX10 and NFIC, often in close spatial proximity to genes that are dysregulated in response to INN treatment. Altogether, these results identify potential molecular mechanisms underlying INN-induced perturbation during mesodermal differentiation in the context of cardiac development. This study further highlights the utility of human stem cells as an alternative system for investigating congenital diseases of newborns that arise as a result of maternal drug exposure during pregnancy.