Epichaperome inhibition targets TP53-mutant AML and AML stem/progenitor cells.

TP 53-mutant acute myeloid leukemia (AML) remains the ultimate therapeutic challenge. Epichaperomes, formed in malignant cells, consist of heat shock protein 90 (HSP90) and associated proteins that support the maturation, activity, and stability of oncogenic kinases and transcription factors including mutant p53. High-throughput drug screening identified HSP90 inhibitors as top hits in isogenic TP53-wild-type (WT) and -mutant AML cells. We detected epichaperomes in AML cells and stem/progenitor cells with TP53 mutations but not in healthy bone marrow (BM) cells. Hence, we investigated the therapeutic potential of specifically targeting epichaperomes with PU-H71 in TP53-mutant AML based on its preferred binding to HSP90 within epichaperomes. PU-H71 effectively suppressed cell intrinsic stress responses and killed AML cells, primarily by inducing apoptosis; targeted TP53-mutant stem/progenitor cells; and prolonged survival of TP53-mutant AML xenograft and patient-derived xenograft models, but it had minimal effects on healthy human BM CD34+ cells or on murine hematopoiesis. PU-H71 decreased MCL-1 and multiple signal proteins, increased proapoptotic Bcl-2-like protein 11 levels, and synergized with BCL-2 inhibitor venetoclax in TP53-mutant AML. Notably, PU-H71 effectively killed TP53-WT and -mutant cells in isogenic TP53-WT/TP53-R248W Molm13 cell mixtures, whereas MDM2 or BCL-2 inhibition only reduced TP53-WT but favored the outgrowth of TP53-mutant cells. Venetoclax enhanced the killing of both TP53-WT and -mutant cells by PU-H71 in a xenograft model. Our data suggest that epichaperome function is essential for TP53-mutant AML growth and survival and that its inhibition targets mutant AML and stem/progenitor cells, enhances venetoclax activity, and prevents the outgrowth of venetoclax-resistant TP53-mutant AML clones. These concepts warrant clinical evaluation.

Single-Atom Fluorescence Switch: A General Approach toward Visible-Light-Activated Dyes for Biological Imaging.

Photoactivatable fluorophores afford powerful molecular tools to improve the spatial and temporal resolution of subcellular structures and dynamics. By performing a single sulfur-for-oxygen atom replacement within common fluorophores, we have developed a facile and general strategy to obtain photoactivatable fluorogenic dyes across a broad spectral range. Thiocarbonyl substitution within fluorophores results in significant loss of fluorescence via a photoinduced electron transfer-quenching mechanism as suggested by theoretical calculations. Significantly, upon exposure to air and visible light residing in their absorption regime (365-630 nm), thio-caged fluorophores can be efficiently desulfurized to their oxo derivatives, thus restoring strong emission of the fluorophores. The effective photoactivation makes thio-caged fluorophores promising candidates for super-resolution imaging, which was realized by photoactivated localization microscopy (PALM) with low-power activation light under physiological conditions in the absence of cytotoxic additives (e.g., thiols, oxygen scavengers), a feature superior to traditional PALM probes. The versatility of this thio-caging strategy was further demonstrated by multicolor super-resolution imaging of lipid droplets and proteins of interest.

Novel mitochondria-targeting compounds selectively kill human leukemia cells.

Acute myeloid leukemia (AML) is a heterogeneous group of aggressive hematological malignancies commonly associated with treatment resistance, high risk of relapse, and mitochondrial dysregulation. We identified six mitochondria-affecting compounds (PS compounds) that exhibit selective cytotoxicity against AML cells in vitro. Structure-activity relationship studies identified six analogs from two original scaffolds that had over an order of magnitude difference between LD50 in AML and healthy peripheral blood mononuclear cells. Mechanistically, all hit compounds reduced ATP and selectively impaired both basal and ATP-linked oxygen consumption in leukemic cells. Compounds derived from PS127 significantly upregulated production of reactive oxygen species (ROS) in AML cells and triggered ferroptotic, necroptotic, and/or apoptotic cell death in AML cell lines and refractory/relapsed AML primary samples. These compounds exhibited synergy with several anti-leukemia agents in AML, acute lymphoblastic leukemia (ALL), or chronic myelogenous leukemia (CML). Pilot in vivo efficacy studies indicate anti-leukemic efficacy in a MOLM14/GFP/LUC xenograft model, including extended survival in mice injected with leukemic cells pre-treated with PS127B or PS127E and in mice treated with PS127E at a dose of 5 mg/kg. These compounds are promising leads for development of future combinatorial therapeutic approaches for mitochondria-driven hematologic malignancies such as AML, ALL, and CML.

Mitochondrial metabolism as a target for acute myeloid leukemia treatment.

Acute myeloid leukemias (AML) are a group of aggressive hematologic malignancies resulting from acquired genetic mutations in hematopoietic stem cells that affect patients of all ages. Despite decades of research, standard chemotherapy still remains ineffective for some AML subtypes and is often inappropriate for older patients or those with comorbidities. Recently, a number of studies have identified unique mitochondrial alterations that lead to metabolic vulnerabilities in AML cells that may present viable treatment targets. These include mtDNA, dependency on oxidative phosphorylation, mitochondrial metabolism, and pro-survival signaling, as well as reactive oxygen species generation and mitochondrial dynamics. Moreover, some mitochondria-targeting chemotherapeutics and their combinations with other compounds have been FDA-approved for AML treatment. Here, we review recent studies that illuminate the effects of drugs and synergistic drug combinations that target diverse biomolecules and metabolic pathways related to mitochondria and their promise in experimental studies, clinical trials, and existing chemotherapeutic regimens.

Mitochondria-affecting small molecules ameliorate proteostasis defects associated with neurodegenerative diseases.

Macroautophagic recycling of dysfunctional mitochondria, known as mitophagy, is essential for mitochondrial homeostasis and cell viability. Accumulation of defective mitochondria and impaired mitophagy have been widely implicated in many neurodegenerative diseases, and loss-of-function mutations of PINK1 and Parkin, two key regulators of mitophagy, are amongst the most common causes of heritable parkinsonism. This has led to the hypothesis that pharmacological stimulation of mitophagy may be a feasible approach to combat neurodegeneration. Toward this end, we screened ~ 45,000 small molecules using a high-throughput, whole-organism, phenotypic screen that monitored accumulation of PINK-1 protein, a key event in mitophagic activation, in a Caenorhabditis elegans strain carrying a Ppink-1::PINK-1::GFP reporter. We obtained eight hits that increased mitochondrial fragmentation and autophagosome formation. Several of the compounds also reduced ATP production, oxygen consumption, mitochondrial mass, and/or mitochondrial membrane potential. Importantly, we found that treatment with two compounds, which we named PS83 and PS106 (more commonly known as sertraline) reduced neurodegenerative disease phenotypes, including delaying paralysis in a C. elegans ß-amyloid aggregation model in a PINK-1-dependent manner. This report presents a promising step toward the identification of compounds that will stimulate mitochondrial turnover.

An Automated Differential Nuclear Staining Assay for Accurate Determination of Mitocan Cytotoxicity.

The contribution of mitochondria to oncogenic transformation is a subject of wide interest and active study. As the field of cancer metabolism becomes more complex, the goal of targeting mitochondria using various compounds that inflict mitochondrial damage (so-called mitocans) is becoming quite popular. Unfortunately, many existing cytotoxicity assays, such as those based on tetrazolium salts or resazurin require functional mitochondrial enzymes for their performance. The damage inflicted by compounds that target mitochondria often compromises the accuracy of these assays. Here, we describe a modified protocol based on differential staining with two fluorescent dyes, one of which is cell-permeant (Hoechst 33342) and the other of which is not (propidium iodide). The difference in staining allows living and dead cells to be discriminated. The assay is amenable to automated microscopy and image analysis, which increases throughput and reduces bias. This also allows the assay to be used in high-throughput fashion using 96-well plates, making it a viable option for drug discovery efforts, particularly when the drugs in question have some level of mitotoxicity. Importantly, results obtained by Hoechst/PI staining assay show increased consistency, both with trypan blue exclusion results and between biological replicates when the assay is compared to other methods.

Utilizing Synergistic Potential of Mitochondria-Targeting Drugs for Leukemia Therapy.

Acute myeloid leukemia (AML) is an aggressive group of cancers with high mortality rates and significant relapse risks. Current treatments are insufficient, and new therapies are needed. Recent discoveries suggest that AML may be particularly sensitive to chemotherapeutics that target mitochondria. To further investigate this sensitivity, six compounds that target mitochondria [IACS-010759, rotenone, cytarabine, etoposide, ABT-199 (venetoclax), and carbonyl cyanide m-chlorophenylhydrazone] were each paired with six compounds with other activities, including tyrosine kinase inhibitors (midostaurin and dasatinib), glycolytic inhibitors (2-deoxy-D-glucose, 3-bromopyruvate, and lonidamine), and the microtubule destabilizer vinorelbine. The 36 resulting drug combinations were tested for synergistic cytotoxicity against MOLM-13 and OCI-AML2 AML cell lines. Four combinations (IACS-010759 with vinorelbine, rotenone with 2-deoxy-D-glucose, carbonyl cyanide m-chlorophenylhydrazone with dasatinib, and venetoclax with lonidamine) showed synergistic cytotoxicity in both AML cell lines and were selective for tumor cells, as survival of healthy PBMCs was dramatically higher. Among these drug pairs, IACS-010759/vinorelbine decreased ATP level and impaired mitochondrial respiration and coupling efficiency most profoundly. Some of these four treatments were also effective in K-562, KU812 (chronic myelogenous leukemia) and CCRF-CEM, MOLT-4 (acute lymphoblastic leukemia) cells, suggesting that these treatments may have value in treating other forms of leukemia. Finally, two of the four combinations retained high synergy and strong selectivity in primary AML cells from patient samples, supporting the potential of these treatments for patients.

Addition of Isocyanide-Containing Amino Acids to the Genetic Code for Protein Labeling and Activation.

Site-specific introduction of bioorthogonal handles into biomolecules provides powerful tools for studying and manipulating the structures and functions of proteins. Recent advances in bioorthogonal chemistry demonstrate that tetrazine-based bioorthogonal cycloaddition is a particularly useful methodology due to its high reactivity, biological selectivity, and turn-on property for fluorescence imaging. Despite its broad applications in protein labeling and imaging, utilization of tetrazine-based bioorthogonal cycloaddition has been limited to date by the requirement of a hydrophobic strained alkene reactive moiety. Circumventing this structural requirement, we report the site-specific incorporation of noncanonical amino acids (ncAAs) with a small isocyanide (or isonitrile) group into proteins in both bacterial and mammalian cells. We showed that under physiological conditions and in the absence of a catalyst these isocyanide-containing ncAAs could react selectively with tetrazine molecules via [4 + 1]-cycloaddition, thus providing a versatile bioorthogonal handle for site-specific protein labeling and protein decaging. Significantly, these bioorthogonal reactions between isocyanides and tetrazines also provide a unique mechanism for the activation of tetrazine-quenched fluorophores. The addition of these isocyanide-containing ncAAs to the list of 20 commonly used, naturally occurring amino acids expands our repertoire of reagents for bioorthogonal chemistry, therefore enabling new biological applications ranging from protein labeling and imaging studies to the chemical activation of proteins.