Characterization of the Interaction of Galectin-1 with Sodium Arsenite

Chemoproteomic Approach for the Quantitative Identification of Arsenic-Binding Proteins

Arsenic is a widespread environmental contaminant, and long-term exposure to arsenic in drinking water is known to be associated with the development of many human diseases. Identification of arsenic-binding proteins is important for understanding the mechanisms underlying the toxic effects of arsenic species. Here, we developed a chemoproteomic strategy, relying on the use of a biotin-As(III) probe, stable isotope labeling by amino acids in cell culture, and liquid chromatography-tandem mass spectrometry analysis, to identify quantitatively As(III)-binding proteins. Over 400 proteins were enriched from the lysate of HEK293T cells with streptavidin beads immobilized with the biotin-As(III) probe. Competitive labeling experiments in the presence or absence of p-aminophenylarsenoxide (PAPAO) revealed 51 candidate As(III)-binding proteins, including several molecular chaperones and cochaperones, that is, HSPA4, HSPA4L, HSPH1, HOP1, FKBP51, and FKBP52. We also validated, by employing western blot analysis, the ability of HSPA4, a member of heat shock protein 70 (HSP70) family, in binding with PAPAO and sodium arsenite in vitro. Together, our work led to the identification of a number of new As(III)-interaction proteins, and our results suggest that As(III) may perturb proteostasis partly through binding directly with molecular chaperones.

Characterization of the role of protein-cysteine residues in the binding with sodium arsenite

To better characterize the interaction of protein-cysteines with sodium arsenite, arsenic-binding proteins were identified from the arsenic-resistant Chinese hamster ovary cell line SA7 using a p-aminophenylarsine oxide (PAO)-agarose matrix in combination with proteomic techniques. Twenty of the isolated arsenic-binding proteins were further peptide-mapped by MALDI-Q-TOF-MS. The binding capacity of PAO-agarose-retained proteins was then verified by re-applying Escherichia coli overexpressed recombinant proteins with various numbers of cysteine residues onto the PAO-agarose matrix. The results showed that recombinant heat shock protein 27 (HSP27, with one cysteine residue), reticulocalbin-3 (RCN3, with no cysteine residue), galectin-1 (GAL1, with six cysteine residues), but not peroxiredoxin 6 (Prdx6, with one cysteine residue but not retained by the PAO-agarose matrix), were bound to the PAO-agarose matrix. The six free cysteine residues in GAL1 were individually or double-mutated to alanine by means of site-directed mutagenesis and subjected to CD and ICP-MS analysis. The binding capacity of GAL1 for sodium arsenite was significantly attenuated in C16A, C88A and all double mutant clones. Taken together, our current data suggest that the cysteine residues in GAL1 may play a critical role in the binding of arsenic, but that in the case of RCN3 and Prdx6, this interaction may be mediated by other factors.

From an old remedy to a magic bullet: molecular mechanisms underlying the therapeutic effects of arsenic in fighting leukemia

Arsenic had been used in treating malignancies from the 18th to mid-20th century. In the past 3 decades, arsenic was revived and shown to be able to induce complete remission and to achieve, when combined with all-trans retinoic acid and chemotherapy, a 5-year overall survival of 90% in patients with acute promyelocytic leukemia driven by the t(15;17) translocation-generated promyelocytic leukemia-retinoic acid receptor α (PML-RARα) fusion. Molecularly, arsenic binds thiol residues and induces the formation of reactive oxygen species, thus affecting numerous signaling pathways. Interestingly, arsenic directly binds the C3HC4 zinc finger motif in the RBCC domain of PML and PML-RARα, induces their homodimerization and multimerization, and enhances their interaction with the SUMO E2 conjugase Ubc9, facilitating subsequent sumoylation/ubiquitination and proteasomal degradation. Arsenic-caused intermolecular disulfide formation in PML also contributes to PML-multimerization. All-trans retinoic acid, which targets PML-RARα for degradation through its RARα moiety, synergizes with arsenic in eliminating leukemia-initiating cells. Arsenic perturbs a number of proteins involved in other hematologic malignancies, including chronic myeloid leukemia and adult T-cell leukemia/lymphoma, whereby it may bring new therapeutic benefits. The successful revival of arsenic in acute promyelocytic leukemia, together with modern mechanistic studies, has thus allowed a new paradigm to emerge in translational medicine.

Insecticidal action of mammalian galectin-1 against diamondback moth (Plutella xylostella)

BACKGROUND

Previous studies showed that mammalian galectin-1 (GAL1) could interact with chitosan or chitin, one component of the peritrophic membrane (PM). This finding suggests that the PM could be a target of GAL1, which prompted the authors to explore the effect of GAL1 on larval growth and its potential mechanism.

RESULTS

The development of Plutella xylostella (L.) larvae was significantly disturbed after they were fed recombinant GAL1. The histochemical structure and immunostaining pattern suggested that GAL1 treatment resulted in dose- and time-dependent disruption of the microvilli and abnormalities in these epithelial cells. Ultrastructural studies showed that the PM was not present in the midgut of GAL1-treated insects; instead, numerous bacteria were found in the lumen area. These results indicate that the protective function of the PM was disrupted by GAL1 treatment. Moreover, in vitro data showed that GAL1 interacts with chitosan/chitin in a dose-dependent manner, and also specifically binds to the PM in vitro.

CONCLUSION

In view of the fact that the carbohydrate recognition domain of GAL1 recognises the structural motif N-acetyl lactosamine (Gal beta 1-4 GlcNAc), which is similar to that of chitin (beta-1,4 N-acetyl-D-glucosamine), it is proposed that the insecticidal mechanism of GAL1 involves direct binding with chitin to interfere with the structure of the PM.

Identification of arsenic-binding proteins in human breast cancer cells

As a cancer chemotherapeutic drug, arsenic acts on numerous intracellular signal transduction pathways in cancer cells. However, its mechanism of actions is still not fully understood. Previous studies suggest that arsenic reacts with closely spaced cysteine (Cys) residues of proteins with high Cys content and accessible sulfhydryl (SH) groups. In this study, human breast cancer cell line MCF-7 was examined as a cellular model to explore arsenic-binding proteins and the mechanism of binding. An arsenic-biotin conjugate was synthesized by coupling the pentafluorophenol ester of biotin with p-aminophenylarsenoxide. Arsenic-binding proteins were eluted with streptavidin resin from arsenic-biotin treated MCF-7 cells, separated by polyacrylamide gel electrophoresis, and identified by matrix assisted laser desorption ionization mass spectrometry (MALDI-MS). Arsenic-binding properties of two of these proteins, beta-tubulin and pyruvate kinase M2 (PKM2), were studied further in vitro and the biological consequences of this binding was evaluated. Binding assay with Western blotting confirmed binding of beta-tubulin and PKM2 by arsenic in a concentration-dependent manner. Arsenic binding inhibited tubulin polymerization, but surprisingly had no effect on PKM2 activity. Molecular modeling showed that binding of Cys(12) alone or vicinal Cys residues (Cys(12) and Cys(213)) of beta-tubulin by arsenic blocked the active site for access of GTP, which is necessary for tubulin polymerization. On the contrary, all Cys residues of PKM2 were far away from the active site of the enzyme. In summary, this study confirmed beta-tubulin and PKM2 as arsenic-binding proteins in MCF-7 cells. Functional consequence of such binding may depend on whether arsenic binding causes conformational changes or blocks active sites of target proteins.

Molecular processes in cellular arsenic metabolism

Elucidating molecular processes that underlie accumulation, metabolism and binding of iAs and its methylated metabolites provides a basis for understanding the modes of action by which iAs acts as a toxin and a carcinogen. One approach to this problem is to construct a conceptual model that incorporates available information on molecular processes involved in the influx, metabolism, binding and efflux of arsenicals in cells. This conceptual model is initially conceived as a non-quantitative representation of critical molecular processes that can be used as a framework for experimental design and prediction. However, with refinement and incorporation of additional data, the conceptual model can be expressed in mathematical terms and should be useful for quantitative estimates of the kinetic and dynamic behavior of iAs and its methylated metabolites in cells. Development of a quantitative model will be facilitated by the availability of tools and techniques to manipulate molecular processes underlying transport of arsenicals across cell membranes or expression and activity of enzymes involved in methylation of arsenicals. This model of cellular metabolism might be integrated into more complex pharmacokinetic models for systemic metabolism of iAs and its methylated metabolites. It may also be useful in development of biologically based dose-response models describing the toxic and carcinogenic actions of arsenicals.