Membrane lipids drive formation of KRAS4b-RAF1 RBDCRD nanoclusters on the membrane

The oncogene RAS, extensively studied for decades, presents persistent gaps in understanding, hindering the development of effective therapeutic strategies due to a lack of precise details on how RAS initiates MAPK signaling with RAF effector proteins at the plasma membrane. Recent advances in X-ray crystallography, cryo-EM, and super-resolution fluorescence microscopy offer structural and spatial insights, yet the molecular mechanisms involving protein-protein and protein-lipid interactions in RAS-mediated signaling require further characterization. This study utilizes single-molecule experimental techniques, nuclear magnetic resonance spectroscopy, and the computational Machine-Learned Modeling Infrastructure (MuMMI) to examine KRAS4b and RAF1 on a biologically relevant lipid bilayer. MuMMI captures long-timescale events while preserving detailed atomic descriptions, providing testable models for experimental validation. Both in vitro and computational studies reveal that RBDCRD binding alters KRAS lateral diffusion on the lipid bilayer, increasing cluster size and decreasing diffusion. RAS and membrane binding cause hydrophobic residues in the CRD region to penetrate the bilayer, stabilizing complexes through β-strand elongation. These cooperative interactions among lipids, KRAS4b, and RAF1 are proposed as essential for forming nanoclusters, potentially a critical step in MAP kinase signal activation.

Direct Modulators of K-Ras-Membrane Interactions

Protein-membrane interactions (PMIs) are ubiquitous in cellular signaling. Initial steps of signal transduction cascades often rely on transient and dynamic interactions with the inner plasma membrane leaflet to populate and regulate signaling hotspots. Methods to target and modulate these interactions could yield attractive tool compounds and drug candidates. Here, we demonstrate that the conjugation of a medium-chain lipid tail to the covalent K-Ras(G12C) binder MRTX849 at a solvent-exposed site enables such direct modulation of PMIs. The conjugated lipid tail interacts with the tethered membrane and changes the relative membrane orientation and conformation of K-Ras(G12C), as shown by molecular dynamics (MD) simulation-supported NMR studies. In cells, this PMI modulation restricts the lateral mobility of K-Ras(G12C) and disrupts nanoclusters. The described strategy could be broadly applicable to selectively modulate transient PMIs.

RAS-dependent RAF-MAPK hyperactivation by pathogenic RIT1 is a therapeutic target in Noonan syndrome-associated cardiac hypertrophy

RIT1 is a RAS guanosine triphosphatase (GTPase) that regulates different aspects of signal transduction and is mutated in lung cancer, leukemia, and in the germline of individuals with Noonan syndrome. Pathogenic RIT1 proteins promote mitogen-activated protein kinase (MAPK) hyperactivation; however, this mechanism remains poorly understood. Here, we show that RAF kinases are direct effectors of membrane-bound mutant RIT1 necessary for MAPK activation. We identify critical residues in RIT1 that facilitate interaction with membrane lipids and show that these are necessary for association with RAF kinases and MAPK activation. Although mutant RIT1 binds to RAF kinases directly, it fails to activate MAPK signaling in the absence of classical RAS proteins. Consistent with aberrant RAF/MAPK activation as a driver of disease, we show that pathway inhibition alleviates cardiac hypertrophy in a mouse model of RIT1 mutant Noonan syndrome. These data shed light on the function of pathogenic RIT1 and identify avenues for therapeutic intervention.

Asynchronous Reciprocal Coupling of Martini 2.2 Coarse-Grained and CHARMM36 All-Atom Simulations in an Automated Multiscale Framework

The appeal of multiscale modeling approaches is predicated on the promise of combinatorial synergy. However, this promise can only be realized when distinct scales are combined with reciprocal consistency. Here, we consider multiscale molecular dynamics (MD) simulations that combine the accuracy and macromolecular flexibility accessible to fixed-charge all-atom (AA) representations with the sampling speed accessible to reductive, coarse-grained (CG) representations. AA-to-CG conversions are relatively straightforward because deterministic routines with unique outcomes are achievable. Conversely, CG-to-AA conversions have many solutions due to a surge in the number of degrees of freedom. While automated tools for biomolecular CG-to-AA transformation exist, we find that one popular option, called Backward, is prone to stochastic failure and the AA models that it does generate frequently have compromised protein structure and incorrect stereochemistry. Although these shortcomings can likely be circumvented by human intervention in isolated instances, automated multiscale coupling requires reliable and robust scale conversion. Here, we detail an extension to Multiscale Machine-learned Modeling Infrastructure (MuMMI), including an improved CG-to-AA conversion tool called sinceCG. This tool is reliable (∼98% weakly correlated repeat success rate), automatable (no unrecoverable hangs), and yields AA models that generally preserve protein secondary structure and maintain correct stereochemistry. We describe how the MuMMI framework identifies CG system configurations of interest, converts them to AA representations, and simulates them at the AA scale while on-the-fly analyses provide feedback to update CG parameters. Application to systems containing the peripheral membrane protein RAS and proximal components of RAF kinase on complex eight-component lipid bilayers with ∼1.5 million atoms is discussed in the context of MuMMI.

Recapitulation of cell-like KRAS4b membrane dynamics on complex biomimetic membranes

Understanding the spatiotemporal distribution and dynamics of RAS on the plasma membrane (PM) is the key for elucidating the molecular mechanisms of the RAS signaling pathway. Single particle tracking (SPT) experiments show that in cells, KRAS diffuses in at least three interchanging states on the cellular PM; however, KRAS remains monomeric and always shows homogeneous diffusion on artificial membranes. Here, we show for the first time on a supported lipid bilayer composed of heterogeneous lipid components that we can recapitulate the three-state diffusion of KRAS seen in cells. The use of a biologically relevant eight-lipid system opens a new frontier in the biophysical studies of RAS and other membrane associated proteins on a biomimetic system that recapitulates the complexity of a cellular PM.

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KRAS4b shows homogeneous diffusion on simple 2-lipids bilayer

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KRAS4b shows a cell-like, three-state diffusion on a complex 8-lipid bilayer

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Phase separation in lipids favors the multi-state diffusion of KRAS4b

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The complex lipid composition favors RAS nanoclustering irrespective of nucleotide state

RAS Nanoclusters: Dynamic Signaling Platforms Amenable to Therapeutic Intervention

RAS proteins are mutated in approximately 20% of all cancers and are generally associated with poor clinical outcomes. RAS proteins are localized to the plasma membrane and function as molecular switches, turned on by partners that receive extracellular mitogenic signals. In the on-state, they activate intracellular signal transduction cascades. Membrane-bound RAS molecules segregate into multimers, known as nanoclusters. These nanoclusters, held together through weak protein-protein and protein-lipid associations, are highly dynamic and respond to cellular input signals and fluctuations in the local lipid environment. Disruption of RAS nanoclusters results in downregulation of RAS-mediated mitogenic signaling. In this review, we discuss the propensity of RAS proteins to display clustering behavior and the interfaces that are associated with these assemblies. Strategies to therapeutically disrupt nanocluster formation or the stabilization of signaling incompetent RAS complexes are discussed.

Abstract LB-062: A live-cell, protein-protein interaction assay identified inhibitors of KRAS4b-G12D interaction with full-length RAF1

KRAS4b is a major driver of cancers of the pancreas, lung, and colon, and until recently was considered undruggable. However, recent discoveries have revealed that molecules can target KRAS directly, and that our understanding of KRAS is incomplete. To exploit this new appreciation of KRAS druggability and biology, we have developed several protein-protein interaction (PPI) assays to target KRAS activity in live cells. Here we report on a campaign where we screened a 20,000-compound library against full length KRAS4b-G12D and RAF1 interactions in live HEK293 cells using the NanoBRET platform. From the list of screening hits, we identified several structurally related active compounds with dose-dependent, albeit weak, inhibition of the PPI signal. We expanded these active compounds to a more complete list of structural analogs which we tested in a full dose response. From this expanded group of compounds, we identified several with low µM IC50s. Follow up with these compounds in secondary assays revealed promising biological readouts, including inhibition of downstream pERK levels in G12D mutant PANC1 cells. In summary, by using well-designed live cell assays, we have identified promising leads, which we are pursuing with mechanism of action studies.

Citation Format: John Columbus, Thomas J. Turbyville, Vanessa Wall, Dominic Esposito, David A. Barda, Sheng-Bin Peng. A live-cell, protein-protein interaction assay identified inhibitors of KRAS4b-G12D interaction with full-length RAF1 [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr LB-062.

RAS internal tandem duplication disrupts GTPase-activating protein (GAP) binding to activate oncogenic signaling

The oncogene RAS is one of the most widely studied proteins in cancer biology, and mutant active RAS is a driver in many types of solid tumors and hematological malignancies. Yet the biological effects of different RAS mutations and the tissue-specific clinical implications are complex and nuanced. Here, we identified an internal tandem duplication (ITD) in the switch II domain of NRAS from a patient with extremely aggressive colorectal carcinoma. Results of whole-exome DNA sequencing of primary and metastatic tumors indicated that this mutation was present in all analyzed metastases and excluded the presence of any other clear oncogenic driver mutations. Biochemical analysis revealed increased interaction of the RAS ITD with Raf proto-oncogene Ser/Thr kinase (RAF), leading to increased phosphorylation of downstream MAPK/ERK kinase (MEK)/extracellular signal-regulated kinase (ERK). The ITD prevented interaction with neurofibromin 1 (NF1)-GTPase-activating protein (GAP), providing a mechanism for sustained activity of the RAS ITD protein. We present the first crystal structures of NRAS and KRAS ITD at 1.65-1.75 Å resolution, respectively, providing insight into the physical interactions of this class of RAS variants with its regulatory and effector proteins. Our in-depth bedside-to-bench analysis uncovers the molecular mechanism underlying a case of highly aggressive colorectal cancer and illustrates the importance of robust biochemical and biophysical approaches in the implementation of individualized medicine.

New weapons to penetrate the armor: Novel reagents and assays developed at the NCI RAS Initiative to enable discovery of RAS therapeutics

Development of therapeutic strategies against RAS-driven cancers has been challenging due in part to a lack of understanding of the biology of the system and the ability to design appropriate assays and reagents for targeted drug discovery efforts. Recent developments in the field have opened up new avenues for exploration both through advances in the number and quality of reagents as well as the introduction of novel biochemical and cell-based assay technologies which can be used for high-throughput screening of compound libraries. The reagents and assays developed at the NCI RAS Initiative offer a suite of new weapons that could potentially be used to enable the next generation of RAS drug discovery efforts with the hope of finding novel therapeutics for a target once deemed undruggable.

Englerin A Inhibits EWS-FLI1 DNA Binding in Ewing Sarcoma Cells

High-throughput screening of extracts from plants, marine, and micro-organisms led to the identification of the extract from the plant Phyllanthus engleri as the most potent inhibitor of EWS-FLI1 induced luciferase reporter expression. Testing of compounds isolated from this extract in turn led to the identification of Englerin A (EA) as the active constituent of the extract. EA induced both necrosis and apoptosis in Ewing cells subsequent to a G2M accumulation of cells in the cell cycle. It also impacted clonogenic survival and anchorage-independent proliferation while also decreasing the proportion of chemotherapy-resistant cells identified by high ALDH activity. EA also caused a sustained increase in cytosolic calcium levels. EA appears to exert its effect on Ewing cells through a decrease in phosphorylation of EWS-FLI1 and its ability to bind DNA. This effect is mediated, at least in part, through a decrease in the levels of the calcium-dependent protein kinase PKC-βI after a transient up-regulation.

Abstract 1996: Surface proteomics of KRASG12V transfected MCF10A cells reveals molecular phenotype of a model cell line expressing oncogenic KRAS

Oncogenic RAS mutations play a major role in the most aggressive epithelial cancers in humans (i.e., pancreatic and lung cancer). Mutated K-Ras protein is found in ∼ 95% of pancreatic and ∼ 40% of lung and/or colon carcinomas, characterized by metastasis and a low average survival time after diagnosis. Plasma membrane proteins are top therapeutic targets due to their accessibility at the surface of cancer cells. In order to expand the treatment options of cancers driven by oncogenic RAS, new cell surface targets need to be identified and characterized. Here, we describe mass spectrometry based phenotyping of the KRASG12V cell surface using MCF10A-KRASG12V as a cell line model of constitutively activated KRAS. Extensive view of the MCF10A-KRASG12V surface proteome was achieved by applying concurrently targeted hydrazide-based cell surface capturing (CSC) technology and global shotgun membrane (GSM) proteomics. Our combined approach revealed 666 plasma membrane proteins unique to the MCF10A-KRASG12V cell surface. Of these, 104 were cell surface glycoproteins identified by CSC technology while 562 cell surface proteins were identified using GSM proteomics. K-Ras was exclusively identified by GSM proteomics in the membrane fraction of MCF10A-KRASG12V cells and subsequently cross-validated by Western blot (WB) analysis. Subtractive proteomics, spectral counting-based based quantitation of changes in protein abundances, and Ingenuity Pathway Analysis showed that this investigation reliably revealed expected K-Ras induced changes at the surface of MCF10A-KRASG12V cells as well as alterations in cell surface protein expression with very little prior information. This analysis uncovered a subset of eight proteins identified exclusively at the cell surface of MCF10A-KRASG12V cells by both CSC and GSM technologies. Of these, two were further cross-validated using immunofluorescence and WB analysis. Furthermore, scanning electron microscopy and functional cell assays showed extensive changes at the KRASG12V cell surface consistent with widespread epithelial to mesenchymal transformation (EMT). Taken together, this dataset greatly extends the known molecular phenotype of the MCF10A-KRASG12V cell surface and reveals the important role of EMT in the pathophysiology of the KRASG12V driven malignant transformation in MCF10A- KRASG12V model cell line.

Citation Format: Xiaoying Ye, King C. Chan, Thomas J. Turbyville, Rachel Bagni, Alexander Martinko, Juan Diaz, Jim Wells, Franck McCormick, Gordon Whiteley, Josip Blonder. Surface proteomics of KRASG12V transfected MCF10A cells reveals molecular phenotype of a model cell line expressing oncogenic KRAS. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1996. doi:10.1158/1538-7445.AM2015-1996

Abstract 1829: Comparative surface proteomics of NCI-H2122 cells reveals distinct cell surface phenotype of a metastatic NSCLC cell line expressing oncogenic KRASG12C

Oncogenic KRAS mutations are found in 40% of non-small cell lung carcinomas (NSCLC). In order to expand the treatment options for NSCLC harboring oncogenic K-Ras, new therapeutic cell surface targets need to be identified and characterized. Towards this goal we carried out comparative cell surface analysis of the NSCLC cell line H2122-KRASG12C and the BL2122 cell line (i.e., control), which has been established from the peripheral blood lymphocytes of the same NSCLC patient. Here, we describe optimized hydrazide-based glycoproteomics for mapping of the cell surface proteome of the NSCLC H2122 cell line harboring oncogenic KRASG12C. Our comparative glycoproteomics revealed 632 proteins identified by LC-MS at the surface of both H2122-KRASG12C and BL2122 cell lines. Subtractive proteomics revealed 215 proteins detected solely at the H2122-KRASG12C cell surface while 214 proteins were found germane to the cell surface of BL2122 cells. A total of 203 proteins were commonly identified at the surface of both cell lines. Spectral counting based quantitation revealed 44 proteins showing ≥ 3-fold increase in their relative concentration at the cell surface of H2122-KRASG12C cells. Subsequent meta-analysis via Ingenuity Pathway Analysis (IPA) revealed significant activation of canonical pathways known to be involved in NSCLC biology (e.g., EGFR/neuregulin, and PI3K/AKT signaling). From a subset of proteins showing significant up-regulation at the surface of H2122-KRASG12C cells, we further cross-validated CD147 using immunofluorescence analysis (IFA) and Western blotting (WB). Interestingly, subsequent IFA confirmed the over-expression of CD147 at the cell surface of pancreatic KP-3, lung H2444, and colon SW620 cancer cell lines, each harboring constitutively activated KRAS. Importantly, amongst 215 proteins identified solely at the cell surface of H2122-KRASG12C cells, proteins upstream of K-Ras, epidermal growth factor receptor (EGFR), receptor tyrosine-protein kinase erbB-2 (ERBB2), receptor tyrosine-protein kinase erbB-3 (ERBB3), and disintegrin metalloproteinase domain-containing protein 17 (ADAM17) were unambiguously identified. Using WB analysis, we first confirmed the expression K-Ras in the membrane preparation of H2122-KRASG12C cells. Interestingly, insulin-like growth factor 1 receptor, (IGF1R), and mesothelin (MSLN) were also detected exclusively at the cell surface of the H2122-KRASG12C. We further cross-validated the expression of mesothelin using WB. Taken together, present approach greatly extends the known cell surface phenotype of the NSCLC H2122-KRASG12C cells and can be readily employed as a primary proteomic screen to provide the basis for discovery and characterization of novel cell surface therapeutic targets or diagnostic assays in cells/tissues harboring oncogenic K-Ras.

Citation Format: Xiaoying Ye, Thomas J. Turbyville, Rachel Bagni, Franck McCormick, Gordon Whiteley, Josip Blonder. Comparative surface proteomics of NCI-H2122 cells reveals distinct cell surface phenotype of a metastatic NSCLC cell line expressing oncogenic KRASG12C. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1829. doi:10.1158/1538-7445.AM2015-1829

Quantitative analysis of F-actin redistribution in astrocytoma cells treated with candidate pharmaceuticals

Actin fibers (F-actin) control the shape and internal organization of cells, and generate force. It has been long appreciated that these functions are tightly coupled, and in some cases drive cell behavior and cell fate. The distribution and dynamics of F-actin is different in cancer versus normal cells and in response to small molecules, including actin-targeting natural products and anticancer drugs. Therefore, quantifying actin structural changes from high resolution fluorescence micrographs is necessary for further understanding actin cytoskeleton dynamics and phenotypic consequences of drug interactions on cells. We applied an artificial neural network algorithm, which used image intensity and anisotropy measurements, to quantitatively classify F-actin subcellular features into actin along the edges of cells, actin at the protrusions of cells, internal fibers and punctate signals. The algorithm measured significant increase in F-actin at cell edges with concomitant decrease in internal punctate actin in astrocytoma cells lacking functional neurofibromin and p53 when treated with three structurally-distinct anticancer small molecules: OSW1, Schweinfurthin A (SA) and a synthetic marine compound 23'-dehydroxycephalostatin 1. Distinctly different changes were measured in cells treated with the actin inhibitor cytochalasin B. These measurements support published reports that SA acts on F-actin in NF1(-/-) neurofibromin deficient cancer cells through changes in Rho signaling. Quantitative pattern analysis of cells has wide applications for understanding mechanisms of small molecules, because many anti-cancer drugs directly or indirectly target cytoskeletal proteins. Furthermore, quantitative information about the actin cytoskeleton may make it possible to further understand cell fate decisions using mathematically testable models.

Carminomycin I is an apoptosis inducer that targets the Golgi complex in clear cell renal carcinoma cells

Clear cell renal cell carcinoma (CCRCC) evolves due to mutations in the Von Hippel-Lindau (VHL) tumor suppressor gene. Although the loss of VHL enables survival and proliferation of CCRCC cells, it is also expected to introduce vulnerabilities that may be exploited for therapeutics discovery. To this end, we developed a high-throughput screen to identify small molecules derived from plants, microorganisms, and marine organisms to which CCRCC cells are sensitive. Screening over 8,000 compounds using this approach, we report here the identification of the microbially derived compound carminomycin I (CA) as an effective inhibitor of VHL-defective (VHL(-/-)) CCRCC cell proliferation. CA also induced apoptosis in CCRCC cells by a mechanism independent of p53 or hypoxia-inducible factor 2. We found that P-glycoprotein (P-gp) sequestered CA within the Golgi complex. Interestingly, Golgi sequestration was critical for the antiproliferative effects of CA and P-gp inhibitors abrogated this activity. Furthermore, CA induced cleavage of the Golgi protein p115 and the translocation of its C-terminal fragment to the nucleus. Finally, examination of the activity of the VHL-interacting Golgi protein, endoplasmic reticulum-Golgi intermediate compartment, ERGIC-53 showed that VHL could mediate protection from CA in CCRCC cells. Our natural product-based screening approach has revealed the P-gp-mediated localization of anticancer compounds within the Golgi in CCRCC cells as a potential strategy of targeting VHL-deficient CCRCC cells.

Synthesis of a 35-member stereoisomer library of bistramide A: evaluation of effects on actin state, cell cycle and tumor cell growth

Synthesis and preliminary biological evaluation of a 35-member library of bistramide A stereoisomers are reported. All eight stereoisomers of the C1-C13 tetrahydropyran fragment of the molecule were prepared utilizing crotylsilane reagents 9 and 10 in our [4+2]-annulation methodology. In addition, the four isomers of the C14-C18 gamma-amino acid unit were accessed via a Lewis acid mediated crotylation reaction with use of both enantiomers of organosilane 11. The spiroketal subunit of bistramide A was modified at the C39-alcohol to give another point of stereochemical diversification. The fragments were coupled by using a standard peptide coupling protocol to provide 35 stereoisomers of the natural product. These stereochemical analogues were screened for their effects on cellular actin and cytotoxicity against cancer cell lines (UO-31 renal and SF-295 CNS). The results of these assays identified one analogue, 1.21, with enhanced potency relative to the natural product, bistramide A.

Schweinfurthin A selectively inhibits proliferation and Rho signaling in glioma and neurofibromatosis type 1 tumor cells in a NF1-GRD-dependent manner

Neurofibromatosis type 1 (NF1) is the most common genetic disease affecting the nervous system. Patients typically develop many tumors over their lifetime, leading to increased morbidity and mortality. The NF1 gene, mutated in NF1, is also commonly mutated in sporadic glioblastoma multiforme (GBM). Because both NF1 and GBM are currently incurable, new therapeutic approaches are clearly needed. Natural products represent an opportunity to develop new therapies, as they have been evolutionarily selected to play targeted roles in organisms. Schweinfurthin A is a prenylated stilbene natural product that has previously shown specific inhibitory activity against brain and hematopoietic tumor lines. We show that patient-derived GBM and NF1 malignant peripheral nerve sheath tumor (MPNST) lines, as well as tumor lines derived from the Nf1-/+;Trp53-/+ (NPcis) mouse model of astrocytoma and MPNST are highly sensitive to inhibition by schweinfurthin A and its synthetic analogs. In contrast, primary mouse astrocytes are resistant to the growth inhibitory effects of schweinfurthin A, suggesting that schweinfurthin A may act specifically on tumor cells. Stable transfection of the GTPase-activating protein related domain of Nf1 into Nf1-/-;Trp53-/- astrocytoma cells confers resistance to schweinfurthin A. In addition, the profound effect of schweinfurthin A on dynamic reorganization of the actin cytoskeleton led us to discover that schweinfurthin A inhibits growth factor-stimulated Rho signaling. In summary, we have identified a class of small molecules that specifically inhibit growth of cells from both central and peripheral nervous system tumors and seem to act on NF1-deficient cells through cytoskeletal reorganization correlating to changes in Rho signaling.

A rhizosphere fungus enhances Arabidopsis thermotolerance through production of an HSP90 inhibitor

The molecular chaperone HEAT SHOCK PROTEIN90 (HSP90) is essential for the maturation of key regulatory proteins in eukaryotes and for the response to temperature stress. Earlier, we have reported that fungi living in association with plants of the Sonoran desert produce small molecule inhibitors of mammalian HSP90. Here, we address whether elaboration of the HSP90 inhibitor monocillin I (MON) by the rhizosphere fungus Paraphaeosphaeria quadriseptata affects plant HSP90 and plant environmental responsiveness. We demonstrate that MON binds Arabidopsis (Arabidopsis thaliana) HSP90 and can inhibit the function of HSP90 in lysates of wheat (Triticum aestivum) germ. MON treatment of Arabidopsis seedlings induced HSP101 and HSP70, conserved components of the stress response. Application of MON, or growth in the presence of MON, allowed Arabidopsis wild type but not AtHSP101 knockout mutant seedlings to survive otherwise lethal temperature stress. Finally, cocultivation of P. quadriseptata with Arabidopsis enhanced plant heat stress tolerance. These data demonstrate that HSP90-inhibitory compounds produced by fungi can influence plant growth and responses to the environment.

A new dihydroxanthenone from a plant-associated strain of the fungus Chaetomium globosum demonstrates anticancer activity

Bioassay-guided fractionation of a cytotoxic EtOAc extract of the fungal strain, Chaetomium globosum, inhabiting the rhizosphere of the Christmas cactus, Opuntia leptocaulis, of the Sonoran desert afforded a new dihydroxanthenone, globosuxanthone A (1), a new tetrahydroxanthenone, globosuxanthone B (2), two new xanthones, globosuxanthone C (3) and D (4), 2-hydroxyvertixanthone (5), and two known anthraquinones (6 and 7). The structures of the new compounds 1-4 were elucidated by NMR and MS techniques, and the relative stereochemistry of 1 was determined by X-ray crystallographic analysis. Of the compounds encountered, 1 was found to exhibit strong cytotoxicity against a panel of seven human solid tumor cell lines, disrupt the cell cycle leading to the accumulation of cells in either G2/M or S phase, and induce classic signs of apoptosis.

Search for Hsp90 inhibitors with potential anticancer activity: isolation and SAR studies of radicicol and monocillin I from two plant-associated fungi of the Sonoran desert

In an effort to discover small molecule inhibitors of Hsp90, we have screened over 500 EtOAc extracts of Sonoran desert plant-associated fungi using a two-stage strategy consisting of a primary cell-based heat shock induction assay (HSIA) followed by a secondary biochemical luciferase refolding assay (LRA). Bioassay-guided fractionation of extracts active in these assays derived from Chaetomium chiversii and Paraphaeosphaeria quadriseptata furnished the Hsp90 inhibitors radicicol (1) and monocillin I (2), respectively. In SAR studies, 1, 2, and their analogues, 3-16, were evaluated in these assays, and the antiproliferative activity of compounds active in both assays was determined using the breast cancer cell line MCF-7. Radicicol and monocillin I were also evaluated in a solid-phase competition assay for their ability to bind Hsp90 and to deplete cellular levels of two known Hsp90 client proteins with relevance to breast cancer, estrogen receptor (ER), and the type 1 insulin-like growth factor receptor (IGF-1R). Some inferences on SAR were made considering the crystal structure of the N-terminus of yeast Hsp90 bound to 1 and the observed biological activities of 1-16. Isolation of radicicol and monocillin I in this study provides evidence that we have developed an effective strategy for discovering natural product-based Hsp90 inhibitors with potential anticancer activity.

The anticancer activity of the fungal metabolite terrecyclic acid A is associated with modulation of multiple cellular stress response pathways

Tumors are dependent on cellular stress responses, in particular the heat shock response, for survival in their hypoxic, acidotic, and nutrient-deprived microenvironments. Using cell-based reporter assays, we have identified terrecyclic acid A (TCA) from Aspergillus terreus, a fungus inhabiting the rhizosphere of Opuntia versicolor of the Sonoran desert, as a small-molecule inducer of the heat shock response that shows anticancer activity. Further characterization suggested that TCA also affects oxidative and inflammatory cellular stress response pathways. The presence of an alpha-methylene ketone moiety suggested that TCA may form adducts with sulfhydryl groups of proteins. Reaction with labile intracellular cysteines was supported by our finding that the glutathione precursor N-acetyl-cysteine protected tumor cells from the cytotoxic effects of TCA whereas the glutathione-depleting agent buthionine sulfoximine enhanced its activity. Related sesquiterpenes have been shown to increase levels of reactive oxygen species (ROS) and to inhibit nuclear factor kappaB (NF-kappaB) transcriptional activity. To assess whether TCA could have similar activities, we used a ROS-sensitive dye and flow cytometry to show that TCA does indeed increase ROS levels in 3LL cells. When tested in cells carrying NF-kappaB reporter constructs, TCA also exhibited concentration-dependent inhibition of cytokine-induced NF-kappaB transcriptional activity. These findings suggest that TCA modulates multiple stress pathways-the oxidative, heat shock, and inflammatory responses-in tumor cells that promote their survival. Small-molecule natural products such as TCA may serve as useful probes for understanding the relationships between these pathways, potentially providing leads for the design of novel and effective anticancer drugs.

Cytotoxic constituents of Aspergillus terreus from the rhizosphere of Opuntia versicolor of the Sonoran Desert

A novel cyclopentenedione, asterredione (1), two new terrecyclic acid A derivatives, (+)-5(6)-dihydro-6-methoxyterrecyclic acid A (2) and (+)-5(6)-dihydro-6-hydroxyterrecyclic acid A (3), and five known compounds, (+)-terrecyclic acid A (4), (-)-quadrone (5), betulinan A (6), asterriquinone D (7), and asterriquinone C-1 (8), were isolated from Aspergillus terreus occurring in the rhizosphere of Opuntia versicolor, using bioassay-guided fractionation. Acid-catalyzed reaction of 2 under mild conditions afforded 4, whereas under harsh conditions 2 yielded 5 and (-)-isoquadrone (9). Catalytic hydrogenation and methylation of 4 afforded 5(6)-dihydro-terrecyclic acid A (10) and (+)-terrecyclic acid A methyl ester (11), respectively. The structures of 1-11 were elucidated by spectroscopic methods. All compounds were evaluated for cytotoxicity in a panel of three sentinel cancer cell lines, NCI-H460 (non-small cell lung cancer), MCF-7 (breast cancer), and SF-268 (CNS glioma), and were found to be moderately active. Cell cycle analysis of 2, 4, and 5 using the NCI-H460 cell line indicated that 4 is capable of disrupting the cell cycle through an apparent arrest to progression at the G(1) and G(2)/M phases in this p53 competent cell line. A pathway for the biosynthetic origin of asterredione (1) from asterriquinone D (7) is proposed.