Spotlight pubs.acs.org/acschemicalbiology
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LIPIDS MEDIATE MITOCHONDRIA-TO-CYTOSOL STRESS SIGNALING
NOVEL STRUCTURES INHIBIT MALARIA PARASITE ACROSS MULTIPLE LIFE STAGES
Adapted by permission from Macmillan Publishers Ltd.: Nature, advance online publication 07 September 2016, DOI: 10.1038/ nature19804. Adapted by permission from Macmillan Publishers Ltd.: Nature, advance online publication 07 September 2016, DOI: 10.1038/nature19804.
Malaria, a serious illness affecting millions of people each year, begins when Plasmodium protozoan parasites in the form of sporozoites are transmitted from mosquitos to humans through mosquito bites. The sporozoites migrate to the liver to multiply into merozoites that eventually move on to replicate within red blood cells. During the blood stage, in which the human host begins to suffer malaria symptoms, merozoites transform into gametocytes that get picked up by mosquitos through blood meals and combine in the mosquito gut to begin the life cycle again. Over 400 years since Western medicine began treating malaria with quinine, researchers are still scrambling to discover new compounds for combating the deadly diseasea difficult task because Plasmodium parasites continually evolve to acquire drug resistance. Thus, researchers seek compounds that kill the parasite via new modes of action. Furthermore, although most current drugs target blood-stage parasites, new drugs that also target the liver and gametocyte stages to curb the onset of illness and reduce transmission to other human hosts are desirable as well. Rising to these challenges, a large international research team headed by Stuart L. Schreiber at the Broad Institute of Harvard and MIT screened a library of over 100 000 compounds to find novel antimalarial inhibitors (Nature 2016, DOI: 10.1038/ nature19804). The team used diversity-oriented synthesis to create molecules with unique three-dimensional shapes. After selecting compounds active against blood-stage drug-resistant Plasmodium strains, which were screened to deprioritize molecules with known modes of action, the team further narrowed down the drug candidates to those active against liver- and transmission-stage parasites. A set of bicyclic azetidines that inhibited a new antimalarial target, phenylalanine tRNA synthase, were found to be effective in mouse models for each stage of malaria infection, notably when administered in a single low dosea feature that would facilitate compliance in the poor countries most plagued by the disease.
Reprinted from Cell, 166, Kim et al., 1539−1552. Copyright 2016, with permission from Elsevier.
A team of researchers set out to study molecular mechanisms behind neurodegenerative diseases such as Huntington’s, Parkinson’s, and Alzheimer’s and, in the process, discovered a previously unknown mechanism of communication between mitochondria and the cytosol (Cell 2016, 166, 1539−1552). In their recent publication, Andrew Dillin of the University of California, Berkeley and co-workers describe how lipid homeostasis mediates the upregulation of protective proteins in the cytosol in response to mitochondrial disruption, in a phenomenon dubbed the mitochondrial-to-cytosolic stress response (MCSR). Huntington’s, Parkinson’s, and Alzheimer’s diseases are all believed to be caused by misfolded proteins forming plaques that eventually destroy neuron function. Misfolded proteins can be mitigated in part by the 70 kDa heat shock protein (HSP70) family, which includes members that act in specific subcellular compartments to refold aggregated or misfolded proteins or help translocate proteins from one compartment to another. Thus, Dillin and co-workers hypothesized that the loss of one compartment-specific HSP70 would elicit unfolded protein responses in other compartments. To test this hypothesis, the research team used RNA interference to knock down 12 HSP70 proteins one at a time in C. elegans; they found that reduced expression of a single protein, the mitochondrial chaperone hsp-6, induced MCSR. Further investigation allowed the team to propose a model for MCSR in which a loss of hsp-6 activity promoted a reduction in fatty acid oxidation, causing a buildup of lipids in the cytosol that triggered the expression of cytosolic chaperone proteins, which in turn mitigated the toxicity of model Huntington’s disease protein aggregates. Notably, the accumulation of the phospholipid cardiolipin caused a reduction of the waxy sphingolipid ceramide, which normally suppresses MCSR. On the basis of these observations, the authors suggest that drugs targeting lipid metabolism and promoting MCSR may serve as potential therapeutics for treating protein-misfolding diseases.
Heidi A. Dahlmann
Heidi A. Dahlmann
Published: October 21, 2016 © 2016 American Chemical Society
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DOI: 10.1021/acschembio.6b00862 ACS Chem. Biol. 2016, 11, 2662−2664
ACS Chemical Biology
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MOLECULAR MOWER CLEARS PATH FOR IMMUNOTHERAPY
Spotlight
THERAPY PLAYS RING AROUND THE GLUCOSE
Xiao, H., et al., Proc. Natl. Acad. Sci., U.S.A., 113, 10304−10309. Copyright 2016 National Academy of Sciences, U.S.A.
Adapted with permission from M. Patra, et al., J. Am. Chem. Soc. 138, 12541−12551. Copyright 2016, American Chemical Society.
Xiao, H., et al., Proc. Natl. Acad. Sci., U.S.A., 113, 10304−10309. Copyright 2016 National Academy of Sciences, U.S.A.
Adapted with permission from M. Patra, et al., J. Am. Chem. Soc. 138, 12541−12551. Copyright 2016, American Chemical Society.
Immunotherapy, one of the hottest areas of cancer therapy research, relies on the immune system’s ability to identify and attack “non-self” entities, including tumor cells. While some immunotherapies involve ramping up the immune response to tumor-specific antigens, other immunotherapies involve blocking inhibitory signaling in the immune system, enabling immune system cells to attack tumor cells that would otherwise evade an immune response. A combination of these approaches forms the basis of a new immunotherapy strategy recently reported by Carolyn Bertozzi and co-workers at Stanford University (Proc. Natl. Acad. Sci., U.S.A. 2016, 113, 10304− 10309). The research team focused on tumor cells expressing HER-2, a cell-surface receptor upregulated in certain aggressive breast cancers, which can be treated with trastuzumab (Tras), a known HER2-targeting therapeutic antibody that recruits natural killer (NK) cells to destroy the tumor cells. However, this targeted attack can be thwarted when sialic acids attached to sugars coating the tumor cell surface bind to sialic acidbinding Ig-like lectins (Siglecs) on the NK cell surface, which inhibits the NK cell-mediated immune response. To evade this resistance mechanism, the research team covalently attached a sialic acid-cleaving enzyme to Tras to form a conjugate dubbed T-sia, which was anticipated to potentiate Tras-mediated cytotoxicity not only by reducing Siglec binding-induced immunosuppression but also by freeing up sialylated NK-activating ligands on the tumor surface. Indeed, T-sia was found to selectively desialylate HER2-positive cells and was more effective than trastuzumab alone at promoting NK-mediated cytotoxicity, especially toward tumor cells that expressed low levels of HER2 and would thus normally be less affected by trastuzumab therapy. These results indicate that the multipronged approach exemplified by T-sia could be a promising avenue for future cancer immunotherapies.
In many types of cancer, energy production by glucose metabolism is in high demand, and feeding this need for sugar is a set of glucose transporter genes, the GLUTs, which are often overexpressed. Among the best studied family members is GLUT1, and high expression levels are accompanied by a poor prognosis in numerous cancers. Some researchers have taken advantage of increased facilitative transport of glucose into cancer cells as a means to target and transport small molecule cargo into cells by backpacking on a glucose. For example, fluorescent glycoconjugates can successfully mark cancer cells in a preferential manner and anticancer drug glycoconjugates can increase the tumor-targeting potential. The structure of glucose allows conjugation at any of five hydroxyl groups, and payloads have successfully been loaded at several of these. However, a comparative screen with the full set of positional isomers has not been carried out, probably due to the formidable synthetic barriers. Recently, Patra et al. (J. Am. Chem. Soc. 2016, DOI: 10.1021/jacs.6b06937) took on this challenge, demonstrating synthesis methods to selectively install an anticancer platinum complex at each of the hydroxyl groups around glucose. With these new glucose−platinum conjugates in hand, the researchers could ask which positions were most impactful for cellular uptake by GLUTs. Decorating the 2 position of the glucose proved the most potent in killing the prostate cancer line, DU145, a cell line known to overexpress GLUT1. Further experiments employing a GLUT1 inhibitor drug or an RNAi knockdown of GLUT1 demonstrated that the toxic platinum glycocongugate targets and enters the cell via this transporter. Trials in mice with the 2-position glycoconjugate showed that it could halt breast cancer tumor growth like a standard platinum chemotherapy drug but that the intended smart targeting works in vivo; the tumor accumulates the glycoconjugate drug over the course of 24 h while other tissues show relatively constant levels.
Heidi A. Dahlmann
Jason G. Underwood 2663
DOI: 10.1021/acschembio.6b00862 ACS Chem. Biol. 2016, 11, 2662−2664
ACS Chemical Biology
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Spotlight
PROBING MECHANISM TO IMPROVE ENZYMATIC CATALYSIS
Adapted with permission from Renata, H., et al., J. Am. Chem. Soc., 138, 12527−12533. Copyright 2016, American Chemical Society.
Cytochrome P450 enzymes and similar proteins have shown promise as catalysts for an important carbon−carbon bondforming reaction: the transfer of carbene precursors. Now in a new study, researchers have used mechanistic studies to understand the side reactions that inactivate their engineered enzymes and introduce specific mutations to improve their efficiency (Renata, H. et al, J. Am. Chem. Soc. 2016, DOI: 10.1021/ jacs.6b06823) In 2014, Frances Arnold’s Caltech team engineered a highly active cyclopropanating P450 (BM3-HStar) by swapping Cys to His. But the enzyme was not perfect: over time and with increasing substrate concentrations, the enzyme became inactive. Thirty years ago, researchers had discovered that carbene precursors could serve as mechanism-based inhibitors of cytochrome P450 enzymes, which provided clues to what could be happening. To build on those earlier results, Arnold’s team studied the inactivation pathways in their engineered enzyme. Earlier studies of cytochrome P450 inhibition pointed to modifications of the heme cofactor, and LC-MS and UV−vis studies suggested both alkylation and a loss of the heme cofactor. The team also used mass spectrometry to look for side-chain alkylation in the enzyme and proteomic analysis to identify seven modified side chains. Almost all of these modified residues are far from the heme cofactor, but they may have disrupted the secondary structure of the enzyme, contributing to the inactivation. Using this information, the researchers developed seven mutants, each replacing a single side chain that had been alkylated in BM3-HStar. H92N and H100N mutants showed higher turnover, and a double mutant boosted turnover by 1.8 fold over BM3-HStar. In whole cell reactions, the enzymes were even more efficient, with nearly 3-fold higher turnover than BM3-HStar. Many efforts to improve novel enzyme activity rely on directed evolution of active site residues and high throughput screening. But that strategy can miss other types of incompatibilities between modified enzymes and nonbiological substrates. These results provide an example of how a mechanistic analysis could be useful for solving such problems. Sarah A. Webb
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DOI: 10.1021/acschembio.6b00862 ACS Chem. Biol. 2016, 11, 2662−2664
