Hsp72 Is an Intracellular Target of the α,β-Unsaturated Sesquiterpene Lactone, Parthenolide

An Unbiased Screen Identified the Hsp70-BAG3 Complex as a Regulator of Myosin-Binding Protein C3

Variants in the gene myosin-binding protein C3 (MYBPC3) account for approximately 50% of familial hypertrophic cardiomyopathy (HCM), leading to reduced levels of myosin-binding protein C3 (MyBP-C), the protein product made by gene MYBPC3. Elucidation of the pathways that regulate MyBP-C protein homeostasis could uncover new therapeutic strategies. Toward this goal, we screened a library of 2,426 bioactive compounds and identified JG98, an allosteric modulator of heat shock protein 70 that inhibits interaction with Bcl-2-associated athanogene (BAG) domain co-chaperones. JG98 reduces MyBP-C protein levels. Furthermore, genetic reduction of BAG3 phenocopies treatment with JG-98 by reducing MYBP-C protein levels.. Thus, an unbiased compound screen identified the heat shock protein 70-BAG3 complex as a regulator of MyBP-C stability.

Marine diterpenoid targets STING palmitoylation in mammalian cells

Natural products are important sources of therapeutic agents and useful drug discovery tools. The fused macrocycles and multiple stereocenters of briarane-type diterpenoids pose a major challenge to total synthesis and efforts to characterize their biological activities. Harnessing a scalable source of excavatolide B (excB) from cultured soft coral Briareum stechei, we generated analogs by late-stage diversification and performed structure-activity analysis, which was critical for the development of functional excB probes. We further used these probes in a chemoproteomic strategy to identify Stimulator of Interferon Genes (STING) as a direct target of excB in mammalian cells. We showed that the epoxylactone warhead of excB is required to covalently engage STING at its membrane-proximal Cys91, inhibiting STING palmitoylation and signaling. This study reveals a possible mechanism-of-action of excB, and expands the repertoire of covalent STING inhibitors.

Is It Still Possible to Think about HSP70 as a Therapeutic Target in Onco-Hematological Diseases?

The search for molecules to be targeted that are involved in apoptosis resistance/increased survival and pathogenesis of onco-hematological malignancies is ongoing since these diseases are still not completely understood. Over the years, a good candidate has been identified in the Heat Shock Protein of 70kDa (HSP70), a molecule defined as “the most cytoprotective protein ever been described”. HSP70 is induced in response to a wide variety of physiological and environmental insults, allowing cells to survive lethal conditions. This molecular chaperone has been detected and studied in almost all the onco-hematological diseases and is also correlated to poor prognosis and resistance to therapy. In this review, we give an overview of the discoveries that have led us to consider HSP70 as a therapeutic target for mono- or combination-therapies in acute and chronic leukemias, multiple myeloma and different types of lymphomas. In this excursus, we will also consider HSP70 partners, such as its transcription factor HSF1 or its co-chaperones whose druggability could indirectly affect HSP70. Finally, we will try to answer the question asked in the title of this review considering that, despite the effort made by research in this field, HSP70 inhibitors never reached the clinic.

Heliangin acts as a covalent ligand of RPS2 that disrupts pre-rRNA metabolic processes in NPM1-mutated acute myeloid leukemia

Although NPM1 mutations are frequently found in acute myeloid leukemia patients, therapeutic strategies are scarce and unsuitable for those who cannot tolerate intensive chemotherapy. Here we demonstrated that heliangin, a natural sesquiterpene lactone, exerts favorable therapeutic responses in NPM1 mutant acute myeloid leukemia cells, with no apparent toxicity to normal hematogenous cells, by inhibiting their proliferation, inducing apoptosis, causing cell cycle arrest, and promoting differentiation. In-depth studies on its mode of action using quantitative thiol reactivity platform screening and subsequent molecular biology validation showed that the ribosomal protein S2 (RPS2) is the main target of heliangin in treating NPM1 mutant AML. Upon covalent binding to the C222 site of RPS2, the electrophilic moieties of heliangin disrupt pre-rRNA metabolic processes, leading to nucleolar stress, which in turn regulates the ribosomal proteins-MDM2-p53 pathway and stabilizes p53. Clinical data shows that the pre-rRNA metabolic pathway is dysregulated in acute myeloid leukemia patients with the NPM1 mutation, leading to a poor prognosis. We found that RPS2 plays a critical role in regulating this pathway and may be a novel treatment target. Our findings suggest a novel treatment strategy and lead compound for acute myeloid leukemia patients, especially those with NPM1 mutations.

Exploration of the cysteine reactivity of human inducible Hsp70 and cognate Hsc70

Hsp70s are multifunctional proteins and serve as the central hub of the protein quality control network. Hsp70s are also related to a number of diseases and have been established as drug targets. Human HspA1A (hHsp70) and HspA8 (hHsc70) are the major cytosolic Hsp70s, and they have both overlapping and distinct functions. hHsp70 contains five cysteine residues, and hHsc70 contains four cysteine residues. Previous studies have shown these cysteine residues can undergo different cysteine modifications such as oxidation or reaction with electrophiles to regulate their function, and hHsp70 and hHsc70 have different cysteine reactivity. To address the mechanism of the differences in cysteine reactivity between hHsp70 and hHsc70, we studied the factors that determine this reactivity by Ellman assay for the quantification of accessible free thiols and NMR analysis for the assessment of structural dynamics. We found the lower cysteine reactivity of hHsc70 is probably due to its lower structural dynamics and the stronger inhibition effect of interaction between the α-helical lid subdomain of the substrate-binding domain (SBDα) and the β-sheet substrate-binding subdomain (SBDβ) on cysteine reactivity of hHsc70. We determined that Gly557 in hHsp70 contributes significantly to the higher structural dynamics and cysteine reactivity of hHsp70 SBDα. Exploring the cysteine reactivity of hHsp70 and hHsc70 facilitates an understanding of the effects of redox reactions and electrophiles on their chaperone activity and regulation mechanisms, and how these differences allow them to undertake distinct cellular roles.

An Overview of NRF2-Activating Compounds Bearing α,β-Unsaturated Moiety and Their Antioxidant Effects

The surge of scientific interest in the discovery of Nuclear Factor Erythroid 2 (NFE2)-Related Factor 2 (NRF2)-activating molecules underscores the importance of NRF2 as a therapeutic target especially for oxidative stress. The chemical reactivity and biological activities of several bioactive compounds have been linked to the presence of α,β-unsaturated structural systems. The α,β-unsaturated carbonyl, sulfonyl and sulfinyl functional groups are reportedly the major α,β-unsaturated moieties involved in the activation of the NRF2 signaling pathway. The carbonyl, sulfonyl and sulfinyl groups are generally electron-withdrawing groups, and the presence of the α,β-unsaturated structure qualifies them as suitable electrophiles for Michael addition reaction with nucleophilic thiols of cysteine residues within the proximal negative regulator of NRF2, Kelch-like ECH-associated protein 1 (KEAP1). The physicochemical property such as good lipophilicity of these moieties is also an advantage because it ensures solubility and membrane permeability required for the activation of the cytosolic NRF2/KEAP1 system. This review provides an overview of the reaction mechanism of α,β-unsaturated moiety-bearing compounds with the NRF2/KEAP1 complex, their pharmacological properties, structural activity-relationship and their effect on antioxidant and anti-inflammatory responses. As the first of its kind, this review article offers collective and comprehensive information on NRF2-activators containing α,β-unsaturated moiety with the aim of broadening their therapeutic prospects in a wide range of oxidative stress-related diseases.

Cytotoxicity and molecular docking analysis of racemolactone I, a new sesquiterpene lactone isolated from Inula racemosa

CONTEXT

Traditionally, Inula racemosa Hook. f. (Asteraceae) has been reported to be effective in cancer treatment which motivated the authors to explore the plant for novel anticancer compounds.

OBJECTIVE

To isolate and characterize new cytotoxic phytoconstituents from I. racemosa roots.

MATERIALS AND METHODS

The column chromatography of I. racemosa ethyl acetate extract furnished a novel sesquiterpene lactone whose structure was established by NMR (1D/2D), ES-MS and its cytotoxic properties were assessed on HeLa, MDAMB-231, and A549 cell lines using MTT and LDH (lactate dehydrogenase) assays. Further, morphological changes were analyzed by flow cytometry, mitochondrial membrane potential, AO-EtBr dual staining, and comet assay. Molecular docking and simulation were performed using Glide and Desmond softwares, respectively, to validate the mechanism of action.

RESULTS

The isolated compound was identified as racemolactone I (compound 1). Amongst the cell lines tested, considerable changes were observed in HeLa cells. Compound 1 (IC50 = 0.9 µg/mL) significantly decreased cell viability (82%) concomitantly with high LDH release (76%) at 15 µg/mL. Diverse morphological alterations along with significant increase (9.23%) in apoptotic cells and decrease in viable cells were observed. AO-EtBr dual staining also confirmed the presence of 20% apoptotic cells. A gradual decrease in mitochondrial membrane potential was observed. HeLa cells showed significantly increased comet tail length (48.4 µm), indicating broken DNA strands. In silico studies exhibited that compound 1 binds to the active site of Polo-like kinase-1 and forms a stable complex.

CONCLUSIONS

Racemolactone I was identified as potential anticancer agent, which can further be confirmed by in vivo investigations.

Electrophilic Natural Products as Drug Discovery Tools

Electrophilic natural products (ENPs) are a rich source of bioactive molecules with tremendous therapeutic potential. While their synthetic complexity may hinder their direct use as therapeutics, they represent tools for elucidation of suitable molecular targets and serve as inspiration for the design of simplified synthetic counterparts. Here, we review the recent use of various activity-based protein profiling methods to uncover molecular targets of ENPs. Beyond target identification, these examples also showcase further development of synthetic ligands from natural product starting points. Two examples demonstrate how ENPs can progress the emerging fields of targeted protein degradation and molecular glues. Though challenges still remain in the synthesis of ENP-based probes, and in their synthetic simplification, their potential for discovery of novel mechanisms of action makes it well worth the effort.

Parthenolide Destabilizes Microtubules by Covalently Modifying Tubulin

Detyrosination of the α-tubulin C-terminal tail is a post-translational modification (PTM) of microtubules that is key for many biological processes.¹ Although detyrosination is the oldest known microtubule PTM,^2-7 the carboxypeptidase responsible for this modification, VASH1/2-SVBP, was identified only 3 years ago,^8,9 precluding genetic approaches to prevent detyrosination. Studies examining the cellular functions of detyrosination have therefore relied on a natural product, parthenolide, which is widely believed to block detyrosination of α-tubulin in cells, presumably by inhibiting the activity of the relevant carboxypeptidase(s).¹⁰ Parthenolide is a sesquiterpene lactone that forms covalent linkages predominantly with exposed thiol groups; e.g., on cysteine residues.^11-13 Using mass spectrometry, we show that parthenolide forms adducts on both cysteine and histidine residues on tubulin itself, in vitro and in cells. Parthenolide causes tubulin protein aggregation and prevents the formation of microtubules. In contrast to epoY, an epoxide inhibitor of VASH1/2-SVBP,⁹ parthenolide does not block VASH1-SVBP activity in vitro. Lastly, we show that epoY is an efficacious inhibitor of microtubule detyrosination in cells, providing an alternative chemical means to block detyrosination. Collectively, our work supports the notion that parthenolide is a promiscuous inhibitor of many cellular processes and suggests that its ability to block detyrosination may be an indirect consequence of reducing the polymerization-competent pool of tubulin in cells.

Identification of proteasome inhibitors using analysis of gene expression profiles

Inhibitors of the 20S proteasome such as bortezomib (Velcade_®) and carfilzomib (Kypriolis_®) are in clinical use for the treatment of patients with multiple myeloma and mantle cell lymphoma. In an attempt to identify novel inhibitors of the ubiquitin-proteasome system (UPS) we used the connectivity map (CMap) resource, based on alterations of gene expression profiles by perturbagens, and performed COMPARE analyses of drug sensitivity patterns in the NCI₆₀ panel. Cmap analysis identified a large number of small molecules with strong connectivity to proteasome inhibition, including both well characterized inhibitors of the 20S proteasome and molecules previously not described to inhibit the UPS. A number of these compounds have been reported to be cytotoxic to tumor cells and were tested for their ability to decrease processing of proteasome substrates. The antibiotic thiostrepton and the natural products celastrol and curcumin induced strong accumulation of polyubiquitinated proteasome substrates in exposed cells. Other compounds elicited modest increases of proteasome substrates, including the protein phosphatase inhibitor BCI-Cl and the farnesyltransferase inhibitor manumycin A, suggesting that these compounds inhibit proteasome function. Induction of chaperone expression in the absence of proteasome inhibition was observed by a number of compounds, suggesting other effects on the UPS. We conclude that the combination of bioinformatic analyses and cellular assays resulted in the identification of compounds with potential to inhibit the UPS.

Terpenoids as Potential Geroprotectors

Terpenes and terpenoids are the largest groups of plant secondary metabolites. However, unlike polyphenols, they are rarely associated with geroprotective properties. Here we evaluated the conformity of the biological effects of terpenoids with the criteria of geroprotectors, including primary criteria (lifespan-extending effects in model organisms, improvement of aging biomarkers, low toxicity, minimal adverse effects, improvement of the quality of life) and secondary criteria (evolutionarily conserved mechanisms of action, reproducibility of the effects on different models, prevention of age-associated diseases, increasing of stress-resistance). The number of substances that demonstrate the greatest compliance with both primary and secondary criteria of geroprotectors were found among different classes of terpenoids. Thus, terpenoids are an underestimated source of potential geroprotectors that can effectively influence the mechanisms of aging and age-related diseases.

Advances in chemistry and bioactivity of parthenolide

(−)-Parthenolide is a germacrane sesquiterpene lactone, available in ample amounts from the traditional medical plant feverfew (Tanacetum parthenium).

A FABP4-PPARγ signaling axis regulates human monocyte responses to electrophilic fatty acid nitroalkenes

Nitro-fatty acids (NO2-FA) are electrophilic lipid mediators derived from unsaturated fatty acid nitration. These species are produced endogenously by metabolic and inflammatory reactions and mediate anti-oxidative and anti-inflammatory responses. NO2-FA have been postulated as partial agonists of the Peroxisome Proliferator-Activated Receptor gamma (PPARγ), which is predominantly expressed in adipocytes and myeloid cells. Herein, we explored molecular and cellular events associated with PPARγ activation by NO2-FA in monocytes and macrophages. NO2-FA induced the expression of two PPARγ reporter genes, Fatty Acid Binding Protein 4 (FABP4) and the scavenger receptor CD36, at early stages of monocyte differentiation into macrophages. These responses were inhibited by the specific PPARγ inhibitor GW9662. Attenuated NO2-FA effects on PPARγ signaling were observed once cells were differentiated into macrophages, with a significant but lower FABP4 upregulation, and no induction of CD36. Using in vitro and in silico approaches, we demonstrated that NO2-FA bind to FABP4. Furthermore, the inhibition of monocyte FA binding by FABP4 diminished NO2-FA-induced upregulation of reporter genes that are transcriptionally regulated by PPARγ, Keap1/Nrf2 and HSF1, indicating that FABP4 inhibition mitigates NO2-FA signaling actions. Overall, our results affirm that NO2-FA activate PPARγ in monocytes and upregulate FABP4 expression, thus promoting a positive amplification loop for the downstream signaling actions of this mediator.

Oxidative Stress and Apoptosis of Gaeumannomyces graminis (Get) Induced by Carabrone

Carabrone is isolated from Carpesium macrocephalum Franch. et Sav, which has good fungicidal activity, especially for Gaeumannomyces graminis (Get). According to previous studies, we speculated that carabrone targets the mitochondrial enzyme complex III of Get. To elucidate the mode of action, we used carabrone to induce oxidative stress and apoptosis in Get. Incubation with carabrone reduced the burst of reactive oxygen species (ROS) and mitochondrial membrane potential, as well as phosphatidylserine release. Carabrone caused ROS accumulation in mycelia by inhibiting the activity of antioxidase enzymes, among which inhibition of glutathione reductase (GR) activity was most obvious. The catalytic center of GR consists of l-cysteine residues that react with the α-methylene-γ-butyrolactone active site of carabrone. Additionally, a positive TUNEL reaction led to diffusion of the DNA electrophoresis band and upregulation of Ggmet1 and Ggmet2. We propose that carabrone inhibits antioxidant enzymes and promotes ROS overproduction, which causes membrane hyperpermeability, release of apoptotic factors, activation of the mitochondria-mediated apoptosis pathway, and fungal cell apoptosis.

Derivatisation of parthenolide to address chemoresistant chronic lymphocytic leukaemia

Parthenolide is a natural product that exhibits anti-leukaemic activity, however, its clinical use is limited by its poor bioavailability. It may be extracted from feverfew and protocols for growing, extracting and derivatising it are reported. A novel parthenolide derivative with good bioavailability and pharmacological properties was identified through a screening cascade based on in vitro anti-leukaemic activity and calculated "drug-likeness" properties, in vitro and in vivo pharmacokinetics studies and hERG liability testing. In vitro studies showed the most promising derivative to have comparable anti-leukaemic activity to DMAPT, a previously described parthenolide derivative. The newly identified compound was shown to have pro-oxidant activity and in silico molecular docking studies indicate a prodrug mode of action. A synthesis scheme is presented for the production of amine 7 used in the generation of 5f.

Parthenolide Covalently Targets and Inhibits Focal Adhesion Kinase in Breast Cancer Cells

Parthenolide, a natural product from the feverfew plant and member of the large family of sesquiterpene lactones, exerts multiple biological and therapeutic activities including anti-inflammatory and anti-cancer effects. Here, we further study the parthenolide mechanism of action using activity-based protein profiling-based chemoproteomic platforms to map additional covalent targets engaged by parthenolide in human breast cancer cells. We find that parthenolide, as well as other related exocyclic methylene lactone-containing sesquiterpenes, covalently modify cysteine 427 of focal adhesion kinase 1 (FAK1), leading to impairment of FAK1-dependent signaling pathways and breast cancer cell proliferation, survival, and motility. These studies reveal a functional target exploited by members of a large family of anti-cancer natural products.