Spotlight pubs.acs.org/crt
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TYPHOID TOXIN STRUCTURE AND CELLULAR TARGETS REVEALED Salmonella enterica, a mostly food-borne bacterium transmitted through poor hygiene, has many serovars (i.e., subspecies variants) that typically cause gastroenteritis. The Typhi serovar, which causes the serious systemic infection typhoid fever, is very similar to other S. entrica serovars with the exception that it produces only a few unique virulence factors, including “typhoid toxin.” Typhoid toxin is an AB-type toxin, meaning that it consists of an active “A” component and a binding “B” component. In particular, typhoid toxin contains two A subunits (PltA, an ADP ribosyltransferase, and CdtB, a DNAase that induces cell-cycle arrest) bound to a pentamer of B subunits (PltB). Jorge Galán and co-workers have now observed that mice treated with purified typhoid toxin displayed many symptoms associated with typhoid fever ((2013) Nature, 499, 350−354). To identify the cellular targets of typhoid toxin, the researchers used affinity purification to isolate proteins in human epithelial and immune cells that bind typhoid toxin. The researchers demonstrated that binding specifity was associated with specific types of glycosylation on the target proteins. A crystal structure of typhoid toxin revealed sugar-binding pockets on the PltB subunit, as well as a unique disulfide bond between PltA and CdtB that was shown to be crucial to the complex’s toxicity. The authors suggest that the disulfide bond between PltA and CdtB is broken upon exposure to reductases within the target cell, allowing each subunit to translocate to and damage its respective intracellular targets. Heidi A. Dahlmann
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FLUSHING OUT HIV WITH SMALL MOLECULE INDUCERS
materials, they can produce scalable quantities of analogues in a modular fashion suitable for further structural tuning. Heidi A. Dahlmann
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Two of the hallmarks of cancer are the development of blood vessels (angiogenesis) to enable tumor blood supply and the ability of cancer cells to stimulate their own growth. It was wellestablished that hydrogen sulfide (H2S) stimulates angiogenesis in organs deprived of oxygen and increases cellular bioenergetics in many cell lines in vitro. A team of researchers led by Csaba Szabo and Mark R. Hellmich has now demonstrated that H2S, previously known to be elevated in patients with certain types of cancer, performs the same functions in tumors ((2013) PNAS, 110, 12474−12479). Observing that the H2S-producing enzyme cystathionineβ-synthase (CBS) was overexpressed in colon adenocarcinomaderived cell lines (such as HCT116 cells) as well as in human patient-derived tumors (but not in patient-matched nontumor colon tissue), the research team showed that by knocking down CBS expression with shRNA or by inhibiting CBS with aminooxyacetic acid (AOAA) in HCT116 cells, the cells showed diminished growth, basal cellular respiration, and ability to migrate in culture. In mouse xenograft systems, HCT116 tumors with CBS-knockdown developed fewer blood vessels than normal HCT116 tumors. Furthermore, the growth of patient-derived xenografts of a CBS-overexpressing tumor was reduced in mice treated with AOAA. The authors note that the identification of CBS-derived H2S as a tumor growth factor establishes the endogenous molecule as a potential anticancer drug target. Heidi A. Dahlmann
Treatment of patients infected with HIV, the devastating virus that kills millions every year, typically involves highly active antiretroviral therapy (HAART). HAART agents suppress plasma virus levels, slowing the progression of disease, but do not eliminate latently infected cells. During latency, HIV remains dormant in reservoirs of HIV-infected cells, such as inactive CD4+T immune cells. However, if HAART is interrupted or the reservoir cells become activated, the concomitant burst of viral production could lead to plasma viral rebound. Because of its inability to completely eradicate HIV from infected patients, HAART requires lifelong administration, which raises concerns over costs, side effects, compliance, and drug resistance. Thus, the search is on for a treatment that targets the latent viral reservoir; agents such as prostratin that induce latent HIV expression could allow the reservoir cells to be identified and selectively killed. Recently, Paul A. Wender, Jerome A. Zack, and co-workers reported the synthesis of a series of esterified prostratin derivatives ((2013) PNAS, 110, 11698−11703). The researchers demonstrated that these analogues bind more efficiently to protein kinase C (PKC), which is proposed to mediate latent virus activation, than does their parent compound prostratin. Furthermore, the analogues were up to 100-fold more potent than prostratin at activating latent HIV in a model reservoir cell line and in HIV patient-dervied CD4+T cells. The authors also note that since their synthesis is performed in relatively few steps from readily accessible starting © 2013 American Chemical Society
ENDOGENOUS H2S STIMULATES CANCER DEVELOPMENT
Published: August 21, 2013 1283
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Chemical Research in Toxicology
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Spotlight
Vuk Stambolic and co-workers ((2013) Science, 341, 395−399), the loss of PTEN (due to deletion or mutation) results in the inability of a cell to repair damage. Normally, PTEN acts as a phosphatidylinositol phosphatase that mediates the phosphatidylinositol 3-kinase (PI3K) signaling pathway in the cytoplasm. Stambolic and co-workers demonstrated that PTEN−SUMO (small ubiquitin-like modifier) conjugates accumulate in the nucleus. Cells lacking SUMO−PTEN were unable to resolve ionizing radiation-induced double strand breaks, due to what appeared to be a failure of the homologous recombination repair pathway. When mice bearing xenograft tumors were treated with both a PI3K-inhibitor and cisplatin, the growth of PTEN-deficient tumors was significantly inhibited compared to the growth of PTEN-proficient tumors. The authors suggest that the targeting of PI3K signaling in combination with the administration of genotoxic agents could be a viable strategy for attacking PTENdeficient cancers in human patients. Heidi A. Dahlmann
PROTECTING DNA WITH BRAIN GLUCOSE-6-PHOSPHATE DEHYDROGENASE
Reprinted from Jeng et al. (2013) ACS Chem. Neurosci., 4, 1123− 1132. Copyright 2013 American Chemical Society.
It is well known that reactive oxygen species (ROS) damage biological macromolecules such as lipids, proteins, and DNA and that oxidative stress can disrupt cellular processes and tissue function. The central nervous system (CNS) may be particularly vulnerable to oxidative stress since ROS can be produced during mitochondrial respiration (which is relatively high in the CNS) and facilitated by proto-oxidants present in the CNS, such as nonheme iron and catechols, including neurotransmitters. Compounding the possibility of enhanced ROS production is the fact that the CNS generally has relatively low levels of antioxidative enzymes. However, under oxidative stress, the expression of the enzyme glucose-6-phosphate dehydrogenase (G6PD) in the CNS is increased. Since G6PD activity generates the NADPH cofactor used by glutathione (GSH) reductase, which mediates the GSH peroxidase-dependent detoxification of H2O2, G6PD is considered a protective enzyme against ROS and oxidative stress. G6PD, which was previously shown to protect mouse embryos from endogenous and xenobiotic-enhanced oxidative DNA damage, has now been demonstrated by Peter G. Wells and co-workers to also protect against endogenous ROS-mediated neurodegeneration in aged mice ((2013) ACS Chem. Neurosci., 4, 1123−1132). Levels of 8-oxo-2′-deoxyguanosine (8-oxo-dG), a biomarker of oxidative stress and a potentially pathogenic lesion, in the cortex, hippocampus, and cerebellum tissues of G6PDnormal (G6PD+/+) mice were lower than the corresponding levels in G6PD-deficient (G6PD+/def) mice. Furthermore, G6PD+/def mouse tissues and cell-types containing elevated 8-oxo-dG appeared to sustain significant neurodegenerative changes. Thus, the authors suggest that hereditary G6PD deficiencies may elevate the risk for neurodegenerative diseases. Heidi A. Dahlmann
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PTEN IN THE NUCLEUS MEDIATES DNA REPAIR Treating cancer with genotoxic agents that induce cytotoxic levels of DNA damage can be frustrated by the capacity of cells to repair damaged DNA. Conversely, tumors that are DNA repairdeficient are especially susceptible to genotoxic anticancer therapies. For example, many human cancer cells harbor deletions of the PTEN (phosphate and tensin homologue on chromosome 10) tumor suppressor gene. As recently reported by 1284
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