Sensitivity to cisplatin and platinum-containing compounds of Schizosaccharomyces pombe rad mutants

Inhibition of Ataxia-Telangiectasia Mutated and RAD3-Related (ATR) Overcomes Oxaliplatin Resistance and Promotes Antitumor Immunity in Colorectal Cancer

Although many patients with colorectal cancer initially respond to the chemotherapeutic agent oxaliplatin, acquired resistance to this treatment remains a major challenge to the long-term management of this disease. To identify molecular targets of oxaliplatin resistance in colorectal cancer, we performed an shRNA-based loss-of-function genetic screen using a kinome library. We found that silencing of ataxia-telangiectasia mutated and RAD3-related (ATR), a serine/threonine protein kinase involved in the response to DNA stress, restored oxaliplatin sensitivity in a cellular model of oxaliplatin resistance. Combined application of the ATR inhibitor VE-822 and oxaliplatin resulted in strong synergistic effects in six different colorectal cancer cell lines and their oxaliplatin-resistant subclones, promoted DNA single- and double-strand break formation, growth arrest, and apoptosis. This treatment also increased replicative stress, cytoplasmic DNA, and signals related to immunogenic cell death such as calreticulin exposure and HMGB1 and ATP release. In a syngeneic colorectal cancer mouse model, combined administration of VE-822 and oxaliplatin significantly increased survival by promoting antitumor T-cell responses. Finally, a DNA repair gene signature discriminated sensitive from drug-resistant patients with colorectal cancer. Overall, our results highlight the potential of ATR inhibition combined with oxaliplatin to sensitize cells to chemotherapy as a therapeutic option for patients with colorectal cancer. SIGNIFICANCE: These findings demonstrate that resistance to oxaliplatin in colorectal cancer cells can be overcome with inhibitors of ATR and that combined treatment with both agents exerts synergistic antitumor effects.Graphical Abstract: http://cancerres.aacrjournals.org/content/canres/79/11/2933/F1.large.jpg.

Fingolimod (FTY720) stimulates Ca(2+)/calcineurin signaling in fission yeast

Fingolimod hydrochloride (FTY720) is the first in class of sphingosine 1-phosphate (S1P) receptor modulator approved to treat multiple sclerosis via down-regulation of G protein-coupled S1P receptor 1 by its phosphorylated form (FTY720-P). Many studies have revealed that FTY720 exerts various biological effects, including antitumor activities, angiogenesis inhibition, Ca(2+) mobilization and apoptosis, independently of S1P receptors. However, the exact mechanisms underlying their effects or signaling pathways mediated by FTY720 have not been completely established. To gain further insights into molecular mechanisms of FTY720 action, the effect of FTY720 on Ca(2+) signaling in fission yeast was analyzed. The addition of Ca(2+) enhanced the sensitivity induced by FTY720, and mutants lacking genes required for calcium homeostasis, including calcineurin and its downstream transcription factor, Ppb1-responsive zinc finger protein (Prz1), were hypersensitive to FTY720 and CaCl2. The effect of FTY720 on calcineurin signaling was monitored by utilizing a luciferase reporter construct fused to three tandem repeats of the calcineurin-dependent response element (CDRE), which gives an accurate measure of calcineurin activity. The addition of FTY720 increased calcineurin activity as well as Ca(2+) influx in a concentration-dependent manner. Notably, the FTY720-mediated Ca(2+) influx and calcineurin activation were reduced markedly by the deletion of yam8 (+) or cch1 (+) encoding putative subunits of a Ca(2+) channel. Consistently, the deletion of Pmk1 mitogen-activated protein kinase (MAPK), which plays an important role in the activation of the Yam8/Cch1 channel, markedly decreased the intracellular Ca(2+) levels upon FTY720 treatment. These results suggest that the FTY720-stimulated Ca(2+)/calcineurin signaling activation partly involves the Yam8/Cch1 channel in fission yeast.

Rad5 template switch pathway of DNA damage tolerance determines synergism between cisplatin and NSC109268 in Saccharomyces cerevisiae

The success of cisplatin (CP) based therapy is often hindered by acquisition of CP resistance. We isolated NSC109268 as a compound altering cellular sensitivity to DNA damaging agents. Previous investigation revealed an enhancement of CP sensitivity by NSC109268 in wild-type Saccharomyces cerevisiae and CP-sensitive and -resistant cancer cell lines that correlated with a slower S phase traversal. Here, we extended these studies to determine the target pathway(s) of NSC109268 in mediating CP sensitization, using yeast as a model. We reasoned that mutants defective in the relevant target of NSC109268 should be hypersensitive to CP and the sensitization effect by NSC109268 should be absent or strongly reduced. A survey of various yeast deletion mutants converged on the Rad5 pathway of DNA damage tolerance by template switching as the likely target pathway of NSC109268 in mediating cellular sensitization to CP. Additionally, cell cycle delays following CP treatment were not synergistically influenced by NSC109268 in the CP hypersensitive rad5Δ mutant. The involvement of the known inhibitory activities of NSC109268 on 20S proteasome and phosphatases 2Cα and 2A was tested. In the CP hypersensitive ptc2Δptc3Δpph3Δ yeast strain, deficient for 2C and 2A-type phosphatases, cellular sensitization to CP by NSC109268 was greatly reduced. It is therefore suggested that NSC109268 affects CP sensitivity by inhibiting the activity of unknown protein(s) whose dephosphorylation is required for the template switch pathway.

Binding of kinetically inert metal ions to RNA: the case of platinum(II)

In this chapter several aspects of Pt(II) are highlighted that focus on the properties of Pt(II)-RNA adducts and the possibility that they influence RNA-based processes in cells. Cellular distribution of Pt(II) complexes results in significant platination of RNA, and localization studies find Pt(II) in the nucleus, nucleolus, and a distribution of other sites in cells. Treatment with Pt(II) compounds disrupts RNA-based processes including enzymatic processing, splicing, and translation, and this disruption may be indicative of structural changes to RNA or RNA-protein complexes. Several RNA-Pt(II) adducts have been characterized in vitro by biochemical and other methods. Evidence for Pt(II) binding in non-helical regions and for Pt(II) cross-linking of internal loops has been found. Although platinated sites have been identified, there currently exists very little in the way of detailed structural characterization of RNA-Pt(II) adducts. Some insight into the details of Pt(II) coordination to RNA, especially RNA helices, can be gained from DNA model systems. Many RNA structures, however, contain complex tertiary folds and common, purine-rich structural elements that present suitable Pt(II) nucleophiles in unique arrangements which may hold the potential for novel types of platinum-RNA adducts. Future research aimed at structural characterization of platinum-RNA adducts may provide further insights into platinum-nucleic acid binding motifs, and perhaps provide a rationale for the observed inhibition by Pt(II) complexes of splicing, translation, and enzymatic processing.

Mutation analysis of Rad18 in human cancer cell lines and non small cell lung cancer tissues

Background

Genetic instability is known as a cause of oncogenesis. Though Rad18 is reported to function in a post replication mismatch repair system, the relation between the status of Rad18 and human tumorigenesis has not been described so far.

Methods

Mutation analysis of 34 human cancer cell lines and 32 non small cell lung cancer (NSCLC) tissues were performed by RT-PCR SSCP. Expression level of Rad18 was measured by real time RT-PCR. Stable transfectant was constructed for in vitro study.

Results

No mutation was found in both cancer cell lines and NSCLC tissues. A single nucleotide polymorphism (SNP) at codon 302 was detected in 51.5% of the cell lines and 62.5% of NSCLC tissues. Interestingly, Rad18 was homozygously deleted in a pulmonary adenocarcinoma cell line PC3. Furthermore, there was no difference in the expression level of wild type Rad18 and Rad18 with SNP. The growth, cell morphology, sensitivity to anti-cancer drugs and in vitro DNA repair activity between wild type Rad18 and Rad18 with SNP revealed to have no difference in vitro.

Conclusion

Though the frequency of SNP was tended to be higher in NSCLC patients than healthy volunteers (57.7%), as the difference was not significant, we have concluded that there is no relation between Rad18 SNP and lung cancer development.

The Schizosaccharomyces pombe checkpoint kinases Chk1 and Cds1 are important for cell survival in response to cisplatin

BACKGROUND

DNA damage checkpoints insure that the integrity of genomic DNA is faithfully maintained throughout the eukaryotic cell cycle. In the presence of damaged DNA, checkpoints are triggered to delay cell cycle progression to allow for DNA repair. In fission yeast, the kinases Chk1 and Cds1 are major components of these DNA damage checkpoint pathways. Both Chk1 and Cds1 are important for viability in the presence of several DNA damaging agents. In this study we hypothesized that Chk1 and Cds1 play a vital role in fission yeast cells ability to survive exposure to the DNA damaging agent cisplatin. Cisplatin is a potent chemotherapeutic drug that interacts with DNA and causes both inter- and intra-strand DNA cross-links.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we demonstrated that treatment with cisplatin in fission yeast causes a Chk1-dependent DNA damage signal. chk1(-) cells were sensitive to cisplatin and Chk1 was phosphorylated in response to cisplatin treatment. We also showed that a Chk1-dependent DNA damage checkpoint pathway is activated in a dose-dependent fashion in cells challenged with cisplatin. Furthermore the Cds1 checkpoint kinase was also important for viability in cisplatin challenged cells. In cds1(-) cells, cisplatin treatment reduced cell viability and this phenotype was exacerbated in a chk1(-)/cds1(-) background.

CONCLUSIONS/SIGNIFICANCE

Thus, we conclude that the concerted effort of both major checkpoint kinases in fission yeast, Chk1 and Cds1, protect cells from cisplatin induced DNA damage. These observations are significant because they suggest that various classes of inter-strand crosslinking agents may generate slightly different lesions as work by others did not observe loss of viability in cds1(-) cells treated with other crosslinking agents like nitrogen mustard.

Ataxia telangiectasia and rad3-related kinase contributes to cell cycle arrest and survival after cisplatin but not oxaliplatin

The DNA cross-linking agents cisplatin and oxaliplatin are widely used in the treatment of human cancer. Lesions produced by these agents are widely known to activate the G1 and G2 cell cycle checkpoints. Less is known about the role of the intra-S-phase checkpoint in the response to these agents. In the present study, two different cell lines expressing a dominant-negative kinase dead (kd) version of the ataxia telangiectasia and rad3-related (ATR) kinase in an inducible fashion were examined for their responses to these two platinating agents and a variety of other DNA cross-linking drugs. The expression of the kdATR allele markedly sensitized the cells to cisplatin, but not to oxaliplatin, as assessed by inhibition of colony formation, induction of apoptosis, and cell cycle analysis. Similar differences in survival were noted for melphalan (ATR dependent) and 4-hydroperoxycyclophosphamide (ATR independent). Further experiments showed that ATR function is not necessary for removal of Pt-DNA adducts. The predominant difference between the responses to the two platinum drugs was the presence of a drug-specific ATR-dependent S-phase arrest after cisplatin but not oxaliplatin. These results indicate that involvement of ATR in the response to DNA cross-linking agents is lesion specific. This observation might need to be taken into account in the development and use of ATR or Chk1 inhibitors.

Improving platinum(II)-based anticancer drug delivery using cucurbit[n]urils

Despite the synthesis of hundreds of new platinum(II) and platinum(IV)-based complexes each year as potential anticancer drugs, only three have received world-wide approval: cisplatin, carboplatin and oxaliplatin. The next big advance in platinum-based chemotherapy is not likely to come from the development of new drugs, but from the controlled and targeted delivery of already approved drugs or those in late stage clinical trials. Encapsulation of platinum drugs inside macromolecules has already demonstrated promise, and encapsulation within cucurbit[n]urils has shown particular potential. Partial or full encapsulation within cucurbit[n]urils provides steric hindrance to drug degradation by peptides and proteins, and the use of different sized cucurbit[n]urils allows for the tuning of drug release rates, cytotoxicity and toxicity.

Valproic acid affects membrane trafficking and cell-wall integrity in fission yeast

Valproic acid (VPA) is widely used to treat epilepsy and manic-depressive illness. Although VPA has been reported to exert a variety of biochemical effects, the exact mechanisms underlying its therapeutic effects remain elusive. To gain further insights into the molecular mechanisms of VPA action, a genetic screen for fission yeast mutants that show hypersensitivity to VPA was performed. One of the genes that we identified was vps45+, which encodes a member of the Sec1/Munc18 family that is implicated in membrane trafficking. Notably, several mutations affecting membrane trafficking also resulted in hypersensitivity to VPA. These include ypt3+ and ryh1+, both encoding a Rab family protein, and apm1+, encoding the mu1 subunit of the adaptor protein complex AP-1. More importantly, VPA caused vacuolar fragmentation and inhibited the glycosylation and the secretion of acid phosphatase in wild-type cells, suggesting that VPA affects membrane trafficking. Interestingly, the cell-wall-damaging agents such as micafungin or the inhibition of calcineurin dramatically enhanced the sensitivity of wild-type cells to VPA. Consistently, VPA treatment of wild-type cells enhanced their sensitivity to the cell-wall-digesting enzymes. Altogether, our results suggest that VPA affects membrane trafficking, which leads to the enhanced sensitivity to cell-wall damage in fission yeast.

Multiple repair pathways mediate tolerance to chemotherapeutic cross-linking agents in vertebrate cells

Cross-linking agents that induce DNA interstrand cross-links (ICL) are widely used in anticancer chemotherapy. Yeast genetic studies show that nucleotide excision repair (NER), Rad6/Rad18-dependent postreplication repair, homologous recombination, and cell cycle checkpoint pathway are involved in ICL repair. To study the contribution of DNA damage response pathways in tolerance to cross-linking agents in vertebrates, we made a panel of gene-disrupted clones from chicken DT40 cells, each defective in a particular DNA repair or checkpoint pathway, and measured the sensitivities to cross-linking agents, including cis-diamminedichloroplatinum (II) (cisplatin), mitomycin C, and melphalan. We found that cells harboring defects in translesion DNA synthesis (TLS), Fanconi anemia complementation groups (FANC), or homologous recombination displayed marked hypersensitivity to all the cross-linking agents, whereas NER seemed to play only a minor role. This effect of replication-dependent repair pathways is distinctively different from the situation in yeast, where NER seems to play a major role in dealing with ICL. Cells deficient in Rev3, the catalytic subunit of TLS polymerase Polzeta, showed the highest sensitivity to cisplatin followed by fanc-c. Furthermore, epistasis analysis revealed that these two mutants work in the same pathway. Our genetic comprehensive study reveals a critical role for DNA repair pathways that release DNA replication block at ICLs in cellular tolerance to cross-linking agents and could be directly exploited in designing an effective chemotherapy.

Molecular alterations of cells resistant to platinum drugs: role of PKCalpha

Development of resistance to platinum compounds may involve not only overexpression of defence mechanisms but also alterations in cellular response to the drug-induced genotoxic stress. To investigate the cellular bases of response to platinum compounds, we examined the profile of gene expression of ovarian carcinoma cells exhibiting sensitivity (A2780) or resistance (A2780/BBR3464) to platinum compounds. Using display PCR, we found that acquisition of resistance to the multinuclear platinum complex BBR3464 was associated with modulation of several transcripts, including up-regulation of the major substrate of protein kinase C (PKC), the myristoylated alanine-rich C kinase substrate (MARCKS). This feature was associated with PKCalpha down-regulation. To explore the role of PKCalpha in cellular sensitivity to platinum compounds, resistant cells were transfected with a PKCalpha-containing vector. PKCalpha-overexpressing resistant cells exhibited a decrease in sensitivity to cisplatin, whereas no significant change in sensitivity to BBR3464 was observed. A number of approaches designed to modulate the function or expression of PKCalpha support that the isoenzyme may play a role in determining resistance only to cisplatin but not to BBR3464, which is known to activate a different pathway of cell response. In conclusion, in spite of PKCalpha down-regulation in our model, its regulatory function was not apparently implicated in the development of resistance to platinum compounds and the present results do not support a general role of PKCalpha as a determinant of the resistance status.

Homologous recombination is a highly conserved determinant of the synergistic cytotoxicity between cisplatin and DNA topoisomerase I poisons

Phase I and II clinical trails are currently investigating the antitumor activity of cisplatin and camptothecins (CPTs; DNA topoisomerase I poisons), based on the dramatic synergistic cytotoxicity of these agents in some preclinical models. However, the mechanistic basis for this synergism is poorly understood. By exploiting the evolutionary conservation of DNA repair pathways from genetically tractable organisms such as budding and fission yeasts to mammalian cells, we demonstrate that the synergism of CPT and cisplatin requires homologous recombination. In yeast and mammalian cell lines defective for RAD52 and XRCC2/3, respectively, the combination of these agents proved antagonistic, while greater than additive activity was evident in isogenic wild-type cells. Homologous recombination appears to mediate a similar interaction of X-rays and CPT, but antagonizes the synergism of cytarabine (Ara-C) with CPT. These findings suggest that homologous recombination comprises an evolutionarily conserved determinant of cellular sensitivity when CPTs are used in combination with other therapeutics.

Development of resistance to a trinuclear platinum complex in ovarian carcinoma cells

BBR3464 is a trinuclear platinum complex that exhibits a potent cytotoxicity and efficacy against cisplatin-resistant tumors. To better understand the determinants of cellular resistance to BBR3464, we selected a resistant ovarian carcinoma cell line after exposure to the complex. The resistant cells (A2780/BBR3464) exhibited a high level of resistance to the selecting agent, but a marginal cross-resistance to cisplatin. Although cellular accumulation of BBR3464 was similar in parental and in resistant cells, DNA platination was decreased in A2780/BBR3464 cells, suggesting a reduced drug accessibility to DNA. This behavior reflected a partial drug inactivation at cytoplasmic level, as a consequence of increased levels of nucleophilic molecules including metallothioneins and human neurofilament low, but not glutathione. A2780/BBR3464 cells also exhibited a reduced susceptibility to apoptosis, which was consistent with reduced expression of Bax, and an alteration of DNA mismatch repair system, as reflected by lack of expression of MLH1 and PMS2, which could impair the recognition/repair of DNA lesions. Whereas both platinum drugs induced G2/M arrest in the parental cells, BBR3464, but not cisplatin, caused a late G1 arrest of resistant cells. Cisplatin induced an appreciable increase of p21(WAF1) levels in both models, in contrast to BBR3464 that produced a substantial upregulation of p21(WAF1) only in parental cells. An inverse relationship with p21(WAF1) modulation was found for CHK1 in parental cells treated with both agents and in resistant cells treated with cisplatin. This pattern of response is consistent with a regulatory loop involving p53 and p21(WAF1) at G2 checkpoint. In contrast, no modulation of CHK1 was found in A2780/BBR3464 treated with the triplatinum compound. These findings, indicating a different activation of regulatory pathways at DNA damage checkpoints in response to cisplatin and BBR3464, support an altered ability of resistant cells to recognize or tolerate sublethal lesions induced by BBR3464.

Tumour-inhibiting platinum complexes--state of the art and future perspectives

Thirty years after the onset of the first clinical studies with cisplatin, the development of antineoplastic platinum drugs continues to be a productive field of research. This article reviews the current preclinical and clinical status, including a discussion of the molecular basis for the activity of the parent drug cisplatin and platinum drugs of the second and third generation, in particular their interaction with DNA. Further emphasis is laid on the development of third generation platinum drugs with activity in cisplatin-resistant tumours, particularly on chelates containing 1,2-diaminocyclohexane (DACH) and on the promising and more recently evolving field of non-classic ( trans- and multinuclear) platinum complexes. The development of oral platinum drugs and drug targeting strategies using liposomes, polymers or low-molecular-weight carriers in order to improve the therapeutic index of platinum chemotherapy are also covered.

Allelic variants of the human glutathione S-transferase P1 gene confer differential cytoprotection against anticancer agents in Escherichia coli

The polymorphic human GSTP1 gene locus encodes proteins that differentially metabolize electrophilic substrates, including, many chemotherapeutic agents used in clinical cancer therapy. In this study, we used XL1-Blue MRF strain, transformed with phagemid expression vectors carrying cDNAs of three GSTP1 alleles, to investigate the cytoprotective abilities of the different GSTP1 alleles against four clinically active anticancer agents, namely, carboplatin, cisplatin, thiotepa, and 4-hydroperoxyifosfamide. Following induction of protein expression with isopropyl-beta-d-thiogalactoside, the cells were treated with each drug for 3 h (1 h for 4-hydroperoxyifosfamide). Surviving fractions were determined and used to compute a cytoprotective factor for each allele against each drug. The results showed all the GSTP1 alleles to be cytoprotective, albeit, to different degrees. For cisplatin and carboplatin, the allele was most protective, with CPs of 5.58 and 3.76, respectively, compared with 1.21 and 1.61 for and 2.50 and 2.79 for. In contrast, protection against thiotepa was highest for the allele, with a cytoprotective factor of 1.56, compared to 1.32 for and 1.1 for. For 4-hydroperoxyifosfamide, the CP for and was the same, 1.45, compared with 1.18 for. These data demonstrate significant differences in the ability of the different GSTP1 alleles to protect against the cytotoxicity of electrophilic anticancer agents. The level of protection differs significantly between different GSTP1 alleles, and between different anticancer agents. The optimized prokaryotic system described provides a useful and rapid tool for pharmacogenetic analysis of the effects of genes on drug sensitivity.

The novel trinuclear platinum complex BBR3464 induces a cellular response different from cisplatin

BBR3464 is a new platinum-based drug non cross-resistant with cisplatin. To characterise the cellular basis of BBR3464 cytotoxicity as opposed to cisplatin, we performed a comparative study of the two drugs in cisplatin-resistant neuroblastoma and astrocytoma cells. In both model systems, BBR3464 proved to be more potent than cisplatin and was able to overcome cisplatin resistance. The higher potency exhibited by BBR3464 correlated with an increased cellular platinum accumulation and DNA-adduct formation. At equitoxic doses, BBR3464 induced apoptosis to a lesser extent than cisplatin and failed to overcome the decreased susceptibility to cisplatin-induced apoptosis in cisplatin-resistant cells. Cell cycle analysis showed a dose-dependent G2/M arrest by BBR3464. In astrocytoma cells, cisplatin treatment resulted in the upregulation of p53, p21 and bax, while only p21 induction was observed after BBR3464 treatment. In cisplatin-resistant cells, the reduced sensitivity to cisplatin paralleled a resistance to the induction of p53/p21 pathway by cisplatin, while the same doses of BBR3464 induced p21 to a similar extent in the resistant cells as in the parental cells. In conclusion, BBR3464 induces a cellular response that is different from cisplatin, supporting the view that the two drugs act through different mechanisms. Our data indicate that BBR3464 may be a promising agent in the treatment of tumours unresponsive to cisplatin and with a non-functional p53.

Multiple pathways of recombination define cellular responses to cisplatin

BACKGROUND

Cisplatin is a DNA-damaging drug used for treatment of testicular tumors. The toxicity of cisplatin probably results from its ability to form DNA adducts that inhibit polymerases. Blocked replication represents a particular challenge for tumor cells, which are committed to unremitting division. Recombination provides a mechanism by which replication can proceed despite the presence of lesions and therefore could be significant for managing cisplatin toxicity.

RESULTS

Recombination-deficient Escherichia coli mutants were strikingly sensitive to cisplatin when compared with the parental strain. Our data identified both daughter-strand gap and double-strand break recombination pathways as critical for survival following treatment with cisplatin. Although it is established that nucleotide excision repair (NER) significantly protects against cisplatin toxicity, most recombination-deficient strains were as sensitive to the drug as the NER-deficient uvrA mutant. Recombination/NER deficient double mutants were more sensitive to cisplatin than the corresponding single mutants, suggesting that recombination and NER pathways play independent roles in countering cisplatin toxicity. Cisplatin was a potent recombinogen in comparison with the trans isomer and canonical alkylating agents. Mitomycin C, which like cisplatin, forms DNA cross-links, was also recombinogenic at minimally toxic doses.

CONCLUSIONS

We have demonstrated that all of the major recombination pathways are critical for E. coli survival following treatment with cisplatin. Moreover, recombination pathways act independently of NER and are of equal importance to NER as genoprotective systems against cisplatin toxicity. Taken together, these results shed new light on how cells survive and succumb to this widely used anticancer drug.

Antifungal activities of antineoplastic agents: Saccharomyces cerevisiae as a model system to study drug action

Recent evolutionary studies reveal that microorganisms including yeasts and fungi are more closely related to mammals than was previously appreciated. Possibly as a consequence, many natural-product toxins that have antimicrobial activity are also toxic to mammalian cells. While this makes it difficult to discover antifungal agents without toxic side effects, it also has enabled detailed studies of drug action in simple genetic model systems. We review here studies on the antifungal actions of antineoplasmic agents. Topics covered include the mechanisms of action of inhibitors of topoisomerases I and II; the immunosuppressants rapamycin, cyclosporin A, and FK506; the phosphatidylinositol 3-kinase inhibitor wortmannin; the angiogenesis inhibitors fumagillin and ovalicin; the HSP90 inhibitor geldanamycin; and agents that inhibit sphingolipid metabolism. In general, these natural products inhibit target proteins conserved from microorganisms to humans. These studies highlight the potential of microorganisms as screening tools to elucidate the mechanisms of action of novel pharmacological agents with unique effects against specific mammalian cell types, including neoplastic cells. In addition, this analysis suggests that antineoplastic agents and derivatives might find novel indications in the treatment of fungal infections, for which few agents are presently available, toxicity remains a serious concern, and drug resistance is emerging.