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Luke Whitesell

Publications and source records attributed to Luke Whitesell.

At least 19 recordsLinked to original sources
PubMed2006

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.

Anthraquinones↗
PubMed2006

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.

Animals↗
PubMed2006

Heat shock protein 90: a unique chemotherapeutic target.

A large body of work spanning the past decade has identified the molecular chaperone heat shock protein 90 (Hsp90) as a critical modulator of an extensive network of cellular signaling pathways. Many of the processes overseen by Hsp90 are deregulated in tumor cells, including cell cycle control, gene transcription, and apoptotic signaling. Hsp90 inhibition offers the potential of accomplishing what most molecularly targeted anticancer therapies do not--the simultaneous disruption of multiple signaling events critical to tumor cell growth and survival. Indeed, small molecule inhibitors of Hsp90 function are actively being evaluated in the clinic as anticancer agents. In this review, we highlight the current understanding of Hsp90 biology as it relates to cancer and discuss the discovery, development, and clinical status of Hsp90 inhibitors as anticancer drugs.

Antineoplastic Agents↗
PubMed2005

Actin microfilament aggregation induced by withaferin A is mediated by annexin II.

The actin cytoskeleton supports diverse cellular processes such as endocytosis, oriented growth, adhesion and migration. The dynamic nature of the cytoskeleton, however, has made it difficult to define the roles of the many accessory molecules that modulate actin organization, especially the multifunctional adapter protein annexin II. We now report that the compound withaferin A (1) can alter cytoskeletal architecture in a previously unknown manner by covalently binding annexin II and stimulating its basal F-actin cross-linking activity. Drug-mediated disruption of F-actin organization is dependent on annexin II expression by cells and markedly limits their migratory and invasive capabilities at subcytotoxic concentrations. Given the extensive ethnobotanical history of withaferin-containing plant preparations in the treatment of cancer and inflammatory and neurological disorders, we suggest that annexin II represents a feasible, previously unexploited target for therapeutic intervention by small-molecule drugs.

Actin Cytoskeleton↗
PubMed2005

Hsp90 is essential in the filarial nematode Brugia pahangi.

The development of a compound with activity against filarial nematodes (a 'macrofilaricide') has been a long-standing goal of the World Health Organization. However, adult filariae have proved remarkably difficult to kill. To some extent this reflects a lack of understanding of key pathways and processes in filarial nematodes that may be suitable targets for chemotherapy. In this paper we show that geldanamycin (GA), a specific inhibitor of the activity of the heat shock protein 90 (Hsp90) family, kills adult worms and microfilariae (Mf) of Brugia pahangi at nanomolar concentrations. In addition, release of Mf from adult worms is inhibited within 24 h of exposure to GA and is not recoverable, demonstrating that GA effectively sterilises the worm. Similar results were obtained with a second filarial worm Acanthocheilonema viteae. In contrast GA has no effect on the free-living nematode Caenorhabditis elegans despite a high degree of conservation between the nematode Hsp90 sequences. In keeping with these findings, Brugia Hsp90 binds GA in a solid phase pull-down assay while the binding of C. elegans Hsp90 to immobilised GA is undetectable. In other eukaryotes, GA is known to bind in the N-terminal ATP pocket of Hsp90, disrupting its interactions with client proteins which are then targeted for degradation via the proteasome pathway. Thus, Hsp90 or some of its client proteins may provide novel targets for the chemotherapy of filarial infection.

Amino Acid Sequence↗
PubMed2005

Hsp90 inhibitors deplete key anti-apoptotic proteins in pediatric solid tumor cells and demonstrate synergistic anticancer activity with cisplatin.

Drugs that inhibit the function of heat shock protein 90 (Hsp90) are of interest in the treatment of pediatric cancers because these agents deplete the cellular levels of signaling molecules that are important for the growth and survival of many childhood tumors. To generate preclinical data in anticipation of clinical trials of Hsp90 inhibitors in children, we evaluated the effects of the Hsp90 inhibitor geldanamycin (GA) alone and in combination with cis-platinum (II)-diamine dichloride (cisplatin) in pediatric tumor cells. Immunoblotting demonstrated depletion of the Hsp90 client proteins AKT and the type 1 insulin-like growth factor receptor (IGF1R) in a panel of pediatric tumor cell lines after exposure to GA. Drug exposure also led to a dramatic decrease in cell survival/proliferation in MYCN-amplified and non-amplified neuroblastoma cells. Moderate inhibition of survival/proliferation was observed in RB-deficient and wild-type osteosarcoma cells. Treatment of neuroblastoma and osteosarcoma cell lines with GA in combination with cisplatin resulted in greater than additive inhibition of survival/proliferation based on median dose analysis. Exposure to this drug combination also resulted in a marked increase in nuclear fragmentation as assessed by terminal deoxynucleotide transferase-mediated UTP nick end labeling (TUNEL) analysis. Combined exposure also abrogated the ability of GA to induce a cytoprotective heat shock response and resulted in Hsp90 adduct formation. Our findings suggest that Hsp90 inhibitors may prove useful either alone or as a component of multi-drug regimens in the treatment of neuroblastoma and osteosarcoma.

Antineoplastic Agents↗
PubMed2005

HSP90 and the chaperoning of cancer.

Standing watch over the proteome, molecular chaperones are an ancient and evolutionarily conserved class of proteins that guide the normal folding, intracellular disposition and proteolytic turnover of many of the key regulators of cell growth, differentiation and survival. This essential guardian function is subverted during oncogenesis to allow malignant transformation and to facilitate rapid somatic evolution. Pharmacologically 'bribing' the essential guard duty of the chaperone HSP90 (heat-shock protein of 90 kDa) seems to offer a unique anticancer strategy of considerable promise.

Animals↗
PubMed2005

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.

3T3 Cells↗
PubMed2005

HSP90: a rising star on the horizon of anticancer targets.

Over the past decade, heat-shock protein (HSP)90 has begun to draw increasing attention as a novel anticancer target with unique features. As a molecular chaperone, HSP90 promotes the maturation and maintains the stability of a large number of conformationally labile client proteins, most of which are involved in biologic processes that are often deranged within tumor cells, such as signal transduction, cell-cycle progression and apoptosis. As a result, and in contrast to other molecular targeted therapeutics, inhibitors of HSP90 achieve their promising anticancer activity through simultaneous disruption of many oncogenic substrates within cancer cells. This review provides a brief summary of HSP90 biology and its association with cancer. It describes the discovery and development of HSP90 inhibitors as anticancer agents and their current status in the clinic. Finally, it closes with a discussion of the unique challenges confronting the further development of these agents and their prospects for the future.

Animals↗
PubMed2004

Ponicidin and oridonin are responsible for the antiangiogenic activity of Rabdosia rubescens, a constituent of the herbal supplement PC SPES.

Antiangiogenic activity has been identified in an aqueous EtOH extract of Rabdosia rubescens, a component of the dietary supplement PC SPES. Bioassay-guided fractionation using a novel in vitro human endothelial cell-based assay for angiogenesis afforded the diterpenoids ponicidin (1) and oridonin (2), with significant antiangiogenic activity at subcytotoxic concentrations, suggesting that these constituents may strongly contribute to the demonstrated clinical efficacy of PC SPES as a treatment for advanced prostate cancer.

Angiogenesis Inhibitors↗
PubMed2004

Hsp90: an emerging target for breast cancer therapy.

Rapidly evolving insights into the specific molecular genetic abnormalities that drive the growth and metastasis of breast cancer have led to the development of targeted therapeutics that do not rely on the generalized disruption of DNA metabolism and cell division for activity. Of particular interest are inhibitors of cellular signal transduction pathways involving tyrosine kinases as well as selective modulators of steroid hormone signaling, histone acetylation, angiogenesis and tumor cell apoptosis. Unique within this array of promising new agents, however, are compounds that target heat shock protein 90 (Hsp90). This molecular chaperone associates with a distinct, but surprisingly diverse, set of proteins that are referred to as Hsp90 client proteins. Hsp90 binds to these clients, and plays a key role in regulating their stability and function. Many of the proteins chaperoned by Hsp90 are involved in breast cancer progression and resistance to therapy, including the estrogen receptor, receptor tyrosine kinases of the erbB family, Akt, and mutant p53. Several small molecule inhibitors of Hsp90 have been identified that can deplete cellular levels of multiple oncogenic client proteins simultaneously by enhancing their ubiquitination and proteasome-mediated degradation. The activity of Hsp90 inhibitors has been well validated in preclinical breast cancer models, both in single-agent studies and in combination with conventional chemotherapy. One of these inhibitors, 17-allylamino, 17-demethoxygeldanamycin (17-AAG, NSC 330507) has recently completed phase I testing. The agent was well tolerated at drug exposures that were shown to cause modulation of Hsp90 client protein levels. Given the redundancy and complexity of the molecular abnormalities present in most breast cancers, the ability of Hsp90 inhibitors to alter the activity of multiple oncogenic targets may prove of unique therapeutic benefit.

Animals↗
PubMed2004

Evidence for a Novel, Caspase-8-Independent, Fas Death Domain-Mediated Apoptotic Pathway.

Certain caspase-8 null cell lines demonstrate resistance to Fas-induced apoptosis, indicating that the Fas/FasL apoptotic pathway may be caspase-8-dependent. Some reports, however, have shown that Fas induces cell death independent of caspase-8. Here we provide evidence for an alternative, caspase-8-independent, Fas death domain-mediated apoptotic pathway. Murine 12B1-D1 cells express procaspase-3, -8, and -9, which were activated upon the dimerization of Fas death domain. Bid was cleaved and mitochondrial transmembrane potential was disrupted in this apoptotic process. All apoptotic events were completely blocked by the broad-spectrum caspase inhibitor Z-VAD-FMK, but not by other peptide caspase inhibitors. Cyclosporin A (CsA), which inhibits mitochondrial transition pore permeability, blocked neither pore permeability disruption nor caspase activation. However, CsA plus caspase-8 inhibitor blocked all apoptotic events of 12B1-D1 induced by Fas death domain dimerization. Our data therefore suggest that there is a novel, caspase-8-independent, Z-VAD-FMK-inhibitable, apoptotic pathway in 12B1-D1 cells that targets mitochondria directly.

Journal Article↗
PubMed2004

Altered Hsp90 function in cancer: a unique therapeutic opportunity.

Molecular chaperones or so-called heat shock proteins serve as central integrators of protein homeostasis within cells. In performing this function, they guide the folding, intracellular disposition, and proteolytic turnover of many key regulators of cell growth, differentiation, and survival. Recent data show essential roles for the chaperones in facilitating malignant transformation at the molecular level and support the concept that their altered utilization during oncogenesis is critical to the development of human cancers. The field is evolving rapidly, but it has become apparent that chaperones can serve as biochemical buffers at the phenotypic level for the genetic instability that is characteristic of many human cancers. Chaperone proteins thus allow tumor cells to tolerate the mutation of multiple critical signaling molecules that would otherwise be lethal. Much of the recent progress in understanding the complex role of heat shock proteins in tumorigenesis has been made possible by the discovery of several natural product antitumor antibiotics that selectively inhibit the function of the chaperone Hsp90. These agents have been used as probes to define the biological functions of Hsp90 at the molecular level and to validate it as a novel target for anticancer drug action. One of these agents, 17-allylamino,17-demethoxygeldanamycin (NSC 330507) has begun phase II clinical trials, and several second-generation compounds are now in late preclinical development. The best way to use Hsp90 inhibitors as anticancer agents remains to be defined. Trials accomplished to date, however, serve as proof of principle that Hsp90 function can be modulated pharmacologically without undue toxicity in humans. Given the redundancy and complexity of the signaling pathway abnormalities present in most cancers, the ability of Hsp90 inhibitors to alter the activity of multiple aberrant signaling molecules instead of just one or two (as most current-generation molecular therapeutics have been designed to do) may prove of unique therapeutic benefit.

Animals↗
PubMed2003

Hormone-refractory breast cancer remains sensitive to the antitumor activity of heat shock protein 90 inhibitors.

PURPOSE: The antiestrogen tamoxifen (Tam) has been used as therapy against estrogen receptor (ER)-positive breast cancer for decades. Most tumors respond initially, but resistance frequently develops. The ER exists in a multiprotein complex containing the molecular chaperone heat shock protein (Hsp) 90, which is known to regulate the stability and activity of this receptor. Therefore, we investigated a ligand-independent approach to hormonal therapy that depletes cellular levels of the receptor by inhibiting the function of Hsp90. EXPERIMENTAL DESIGN: The activity of the Hsp90 inhibitor geldanamycin (GA) and its clinically relevant derivative, 17-allylamino-17-demethoxygeldanamycin (17AAG), was examined at the molecular and cellular levels using Tam-resistant MCF-7 breast cancer cells both in vitro and in tumor xenografts. RESULTS: The ER was depleted by GA in several Tam-resistant cell lines, as were other Hsp90 client proteins such as Akt and Raf-1. Unexpectedly, Tam inhibited ER depletion by GA but had no effect on destabilization of Akt or Raf-1. When SCID mice supplemented with Tam were treated with 17AAG, their tumors also showed no decrease in ER levels as measured by immunofluorescent staining and laser scanning cytometry. In these same tumors, however, decreased Akt and Raf-1 levels were observed. Drug administration also led to inhibition of tumor xenograft growth. The mechanism by which Tam inhibits GA-mediated ER depletion is unclear, but immunoprecipitation experiments showed that Tam does not inhibit the ability of GA to alter the ER-chaperone complex. CONCLUSIONS: Based on its ability to deplete the ER as well as other critical signaling molecules in Tam-resistant breast cancer, 17AAG may provide a useful alternative treatment for patients with recurrent, hormone-refractory breast cancer that should be explored further in Phase II trials. In this context, combined treatment with 17AAG and Tam should be avoided because Tam may inhibit the ability of 17AAG to deplete the ER, potentially reducing its anticancer activity.

Animals↗
PubMed2003

Design of quinolinedione-based geldanamycin analogues.

Quinoline-5,8-dione-based compounds were designed from the structure of the geldanamycin-bound Hsp-90 active site. The active site model predicted that aromatic substituents should be present at the 2-position, to take advantage of a hydrophobic pocket, and amino substituents should be present at the 6- or 7-position. COMPARE analysis revealed that the LC(50) profile of 2-phenyl-6-(2-chloroethylamino)quinoline-5,8-dione has the highest geldanamycin-like activity (0.74 correlation coefficient).

Antineoplastic Agents↗
PubMed2003

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.

Antineoplastic Agents↗
PubMed2003

Expression of a unique drug-resistant Hsp90 ortholog by the nematode Caenorhabditis elegans.

In all species studied to date, the function of heat shock protein 90 (Hsp90), a ubiquitous and evolutionarily conserved molecular chaperone, is inhibited selectively by the natural product drugs geldanamycin (GA) and radicicol. Crystal structures of the N-terminal region of yeast and human Hsp90 have revealed that these compounds interact with the chaperone in a Bergerat-type adenine nucleotide-binding fold shared throughout the gyrase, Hsp90, histidine kinase mutL (GHKL) superfamily of adenosine triphosphatases. To better understand the consequences of disrupting Hsp90 function in a genetically tractable multicellular organism, we exposed the soil-dwelling nematode Caenorhabditis elegans to GA under a variety of conditions designed to optimize drug uptake. Mutations in the gene encoding C elegans Hsp90 affect larval viability, dauer development, fertility, and life span. However, exposure of worms to GA produced no discernable phenotypes, although the amino acid sequence of worm Hsp90 is 85% homologous to that of human Hsp90. Consistent with this observation, we found that solid phase-immobilized GA failed to bind worm Hsp90 from worm protein extracts or when translated in a rabbit reticulocyte lysate system. Further, affinity precipitation studies using chimeric worm-vertebrate fusion proteins or worm C-terminal truncations expressed in reticulocyte lysate revealed that the conserved nucleotide-binding fold of worm Hsp90 exhibits the novel ability to bind adenosine triphosphate but not GA. Despite its unusual GA resistance, worm Hsp90 appeared fully functional when expressed in a vertebrate background. It heterodimerized with its vertebrate counterpart and showed no evidence of compromising its essential cellular functions. Heterologous expression of worm Hsp90 in tumor cells, however, did not render them GA resistant. These findings provide new insights into the nature of unusual N-terminal nucleotide-binding fold of Hsp90 and suggest that target-related drug resistance is unlikely to emerge in patients receiving GA-like chemotherapeutic agents.

Adenosine Triphosphate↗