Role of HDAC6-STAT3 in immunomodulatory pathways in Colorectal cancer cells

Colorectal cancer (CRC) is one of the most common malignant neoplasms and the second leading cause of death from tumors worldwide. Therefore, there is a great need to study new therapeutical strategies, such as effective immunotherapies against these malignancies. Unfortunately, many CRC patients do not respond to current standard immunotherapies, making it necessary to search for adjuvant treatments. Histone deacetylase 6 (HDAC6) is involved in several processes, including immune response and tumor progression. Specifically, it has been observed that HDAC6 is required to activate the Signal Transducer and Activator of Transcription 3 (STAT3), a transcription factor involved in immunogenicity, by activating different genes in these pathways, such as PD-L1. Over-expression of immunosuppressive pathways in cancer cells deregulates T-cell activation. Therefore, we focused on the pharmacological inhibition of HDAC6 in CRC cells because of its potential as an adjuvant to avoid immunotolerance in immunotherapy. We investigated whether HDAC6 inhibitors (HDAC6is), such as Nexturastat A (NextA), affected STAT3 activation in CRC cells. First, we found that NextA is less cytotoxic than the non-selective HDACis panobinostat. Then, NextA modified STAT3 and decreased the mRNA and protein expression levels of PD-L1. Importantly, transcriptomic analysis showed that NextA treatment affected the expression of critical genes involved in immunomodulatory pathways in CRC malignancies. These results suggest that treatments with NextA reduce the functionality of STAT3 in CRC cells, impacting the expression of immunomodulatory genes involved in the inflammatory and immune responses. Therefore, targeting HDAC6 may represent an interesting adjuvant strategy in combination with immunotherapy.

Heart of gold: enabling ligands for oxidative addition of haloorganics in Au(I)/Au(III) catalysed cross-coupling reactions

The field of Au-catalysis has been an area rich with new discoveries due to the unique properties of the lustrous element. In the past decade, developments in Au(I)/Au(III) cross-coupling methodology have been made possible with the use of external oxidants that facilitate the challenging oxidation of Au(I) to Au(III) in a stable and catalytically competent fashion. Until recently, Au-chemistry was not known to undergo catalytic transformations that feature oxidative addition of haloarenes like those that were made famous by transition metals such as Pd and Ni. The discovery that ligand modification could facilitate the oxidative addition of Au(I) with haloorganics to provide Au(III) intermediates that are competent in other areas of catalysis (i.e. Lewis acid catalysis) has revolutionized this field and has led to the invention of new cross-coupling methodology. The recent advances at the leading edge in the emerging field of Au(I)/Au(III) catalysis under redox-neutral conditions are highlighted.

Mechanistic Insights about the Ligand-Enabled Oxy-arylation/vinylation of Alkenes via Au(I)/Au(III) Catalysis

The mechanism of oxy-arylation/vinylation of alkenes catalyzed by the (MeDalphos)AuCl complex was comprehensively investigated by DFT. (P,N)Au(Ph)²⁺ and (P,N)Au(vinyl)²⁺ are key intermediates accounting for the activation of the alkenols and for their cyclization by outer-sphere nucleophilic attack of oxygen. The 5-exo and 6-endo paths have been computed and compared, reproducing the peculiar regioselectivity difference observed experimentally between 4-penten-1-ol, (E) and (Z)-4-hexen-1-ols. Examining the way the alkenol coordinates to gold (more η² or η¹ ) can offer, in some cases, a simple way to predict the favored path of cyclization.

C-C Cross-Couplings from a Cyclometalated Au(III) C ∧ N Complex: Mechanistic Insights and Synthetic Developments

In recent years, the reactivity of gold complexes was shown to extend well beyond π-activation and to hold promises to achieve selective cross-couplings in several C-C and C-E (E=heteroatom) bond forming reactions. Here, with the aim of exploiting new organometallic species for cross-coupling reactions, we report on the Au(III)-mediated C(sp2 )-C(sp) occurring upon reaction of the cyclometalated complex [Au(CCH2 N)Cl2 ] (1, CCH2 N=2-benzylpyridine) with AgPhCC. The reaction progress has been monitored by NMR spectroscopy, demonstrating the involvement of a number of key intermediates, whose structures have been unambiguously ascertained through 1D and 2D NMR analyses (1 H, 13 C, 1 H-1 H COSY, 1 H-13 C HSQC and 1 H-13 C HMBC) as well as by HR-ESI-MS and X-ray diffraction studies. Furthermore, crystallographic studies have serendipitously resulted in the authentication of zwitterionic Au(I) complexes as side-products arising from cyclization of the coupling product in the coordination sphere of gold. The experimental work has been paralleled and complemented by DFT calculations of the reaction profiles, providing valuable insight into the structure and energetics of the key intermediates and transition states, as well as on the coordination sphere of gold along the whole process. Of note, the broader scope of the cross-coupling at the Au(III) CCH2 N centre has also been demonstrated studying the reaction of 1 with C(sp2 )-based nucleophiles, namely vinyl and heteroaryl tin and zinc reagents. These reactions stand as rare examples of C(sp2 )-C(sp2 ) cross-couplings at Au(III).

Tuning Cyclometalated Gold(III) for Cysteine Arylation and Ligand-Directed Bioconjugation

Transition-metal-based approaches to selectively modify proteins hold promise in addressing challenges in chemical biology. Unique bioorthogonal chemistry can be achieved with preformed metal-based compounds; however, their utility in native protein sites within cells remain underdeveloped. Here, we tune the ancillary ligands of cyclometalated gold(III) as a reactive group, and the gold scaffold allows for rapid modification of a desired cysteine residue proximal to the ligand binding site of a target protein. Moreover, evidence for a ligand association mechanism toward C-S bond formation by X-crystallography is established. The observed reactivity of cyclometalated gold(III) enables the rational design of a cysteine-targeted covalent inhibitor of mutant KRAS. This work illustrates the potential of structure-activity relationship studies to tune kinetics of cysteine arylation and rational design of metal-mediated ligand affinity chemistry (MLAC) of native proteins.

Oxidative additions of alkynyl/vinyl iodides to gold and gold-catalyzed vinylation reactions triggered by the MeDalphos ligand

The hemilabile Ad2P(o-C6H4)NMe2 ligand promotes fast, quantitative and irreversible oxidative addition of alkynyl and vinyl iodides to gold. The reaction is general. It works with a broad range of substrates of various electronic bias and steric demand, and proceeds with complete retention of stereochemistry from Z and E vinyl iodides. Both alkynyl and vinyl iodides react faster than aryl iodides. The elementary step is amenable to catalysis. Oxidative addition of vinyl iodides to gold and π-activation of alkenols (and N-alkenyl amines) at gold have been combined to achieve hetero-vinylation reactions. A number of functionalized heterocycles, i.e. tetrahydrofuranes, tetrahydropyranes, oxepanes and pyrrolidines were obtained thereby (24 examples, 87% average yield). Taking advantage of the chemoselectivity for vinyl iodides over aryl iodides, sequential transformations involving first a hetero-vinylation step and then a C-N coupling, a C-C coupling or an heteroarylation were achieved from a vinyl/aryl bis-iodide substrate.

Homogeneous Gold Redox Chemistry: Organometallics, Catalysis, and Beyond

Gold redox chemistry holds the promise of unique reactivities and selectivities that are different to other transition metals. Recent studies have utilized strain release, ligand design, and photochemistry to promote the otherwise sluggish oxidative addition to Au(I) complexes. More details on the reductive elimination from Au(III) complexes have also been revealed. These discoveries have facilitated the development of gold redox catalysis and will continue to offer mechanistic insight and inspiration for other transition metals. This review highlights how research in organometallic chemistry has led to gold redox catalysis, as well as applications in materials science, bioconjugation, and radiochemical synthesis.

Gold-Catalyzed Cross-Coupling Reactions: An Overview of Design Strategies, Mechanistic Studies, and Applications

Transition-metal-catalyzed cross-coupling reactions are central to many organic synthesis methodologies. Traditionally, Pd, Ni, Cu, and Fe catalysts are used to promote these reactions. Recently, many studies have showed that both homogeneous and heterogeneous Au catalysts can be used for activating selective cross-coupling reactions. Here, an overview of the past studies, current trends, and future directions in the field of gold-catalyzed coupling reactions is presented. Design strategies to accomplish selective homocoupling and cross-coupling reactions under both homogeneous and heterogeneous conditions, computational and experimental mechanistic studies, and their applications in diverse fields are critically reviewed. Specific topics covered are: oxidant-assisted and oxidant-free reactions; strain-assisted reactions; dual Au and photoredox catalysis; bimetallic synergistic reactions; mechanisms of reductive elimination processes; enzyme-mimicking Au chemistry; cluster and surface reactions; and plasmonic catalysis. In the relevant sections, theoretical and computational studies of Au^I /Au^III chemistry are discussed and the predictions from the calculations are compared with the experimental observations to derive useful design strategies.