Abstract 1851: Selective targeting of aberrant ETS transcription factors using uniquely modified mithramycin analogs

Background: Recent studies demonstrated that the aberrant oncogenic activity of ETS transcription factor fusions such as EWS-FLI1 and EWS-ERG in Ewing sarcoma can be targeted with the natural product mithramycin (MTM), which is known to have poor pharmacology. We hypothesized that the pharmacological properties of MTM can be ameliorated by rational semisynthetic modifications that improve its pharmacokinetic (PK) liabilities and expand its efficacy toxicity window. Further, we reasoned that the MTM core of each analog can facilitate DNA docking while the substitutions may interact uniquely with oncogenic ETS transcriptional complexes in different cancers. Here we present the pharmacologic properties of two analogs and their unique transcriptome profiles in TC32 Ewing sarcoma and VCaP prostate cancer cells, both of which depend on the aberrant oncogenic activity of an ETS gene fusion.

Methods: MTM analogs were generated by semisynthetic conjugation to the 3-side chain of MTM. Cytotoxicity (GI50) was assessed using the resazurin assay following 72 hr incubation with test compounds (0.3nM-10uM). PK properties were assessed in athymic nu/nu mice after a bolus tail-vein injection and concentrations were assessed in plasma using LC/MS/MS methods. The maximum tolerated dose (MTD) was estimated based on weight loss and blood hematology. In vivo efficacy was assessed in subcutaneous xenografts established, which were dosed SIDx5 at the respective MTD or fractions of the MTD. Transcriptomic profiles were assessed in cells exposed to their respective 72 hr GI50 for 24 hr and in livers of mice that were dosed SIDx5.

Results: Here we demonstrate that select modifications displayed unique cytotoxicity and efficacy Ewing sarcoma (EWS-FLI1) and VCaP prostate (TMPRSS2-ERG) cancer cell lines. Analogues with bulky amino acid substitutions in the 3-side chain of MTM had significantly improved PK in mice, as compared to MTM. Select analogs displayed selective efficacy in xenograft models of Ewing sarcoma at doses ranging as low as 1/3 of the MTD. Significantly, analogues of MTM displayed significantly different transcriptomic profiles in each cell line, which suggests that these molecules have unique properties despite sharing a core DNA-binding structure with MTM.

Conclusions: MTM analogues with bulky hydrophobic amino acid substitutions on the 3-side chain showed the greatest improvement in PK properties and efficacy. Presumably PK are affected due to increased protein binding and an increase in the pKa, which leads to a lower ionized fraction and decreased uptake in the liver by organic anion transporters. This increase in plasma exposure may lead to increased drug partition in the tumor, as compared to MTM, and, therefore, improved efficacy. Transcriptomic data suggest that the analogs have unique transcriptional effects and therefore unlikely to manifest the same toxicity profile as MTM.

Citation Format: Markos Leggas, Ambika Dudhate, Jurgen Rohr. Selective targeting of aberrant ETS transcription factors using uniquely modified mithramycin analogs [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1851.

Broadening the ecology of fear: non-lethal effects arise from diverse responses to predation and parasitism

Research on the 'ecology of fear' posits that defensive prey responses to avoid predation can cause non-lethal effects across ecological scales. Parasites also elicit defensive responses in hosts with associated non-lethal effects, which raises the longstanding, yet unresolved question of how non-lethal effects of parasites compare with those of predators. We developed a framework for systematically answering this question for all types of predator-prey and host-parasite systems. Our framework reveals likely differences in non-lethal effects not only between predators and parasites, but also between different types of predators and parasites. Trait responses should be strongest towards predators, parasitoids and parasitic castrators, but more numerous and perhaps more frequent for parasites than for predators. In a case study of larval amphibians, whose trait responses to both predators and parasites have been relatively well studied, existing data indicate that individuals generally respond more strongly and proactively to short-term predation risks than to parasitism. Apart from studies using amphibians, there have been few direct comparisons of responses to predation and parasitism, and none have incorporated responses to micropredators, parasitoids or parasitic castrators, or examined their long-term consequences. Addressing these and other data gaps highlighted by our framework can advance the field towards understanding how non-lethal effects impact prey/host population dynamics and shape food webs that contain multiple predator and parasite species.

Mithramycin and Analogs for Overcoming Cisplatin Resistance in Ovarian Cancer

Ovarian cancer is a highly deadly malignancy in which recurrence is considered incurable. Resistance to platinum-based chemotherapy bodes a particularly abysmal prognosis, underscoring the need for novel therapeutic agents and strategies. The use of mithramycin, an antineoplastic antibiotic, has been previously limited by its narrow therapeutic window. Recent advances in semisynthetic methods have led to mithramycin analogs with improved pharmacological profiles. Mithramycin inhibits the activity of the transcription factor Sp1, which is closely linked with ovarian tumorigenesis and platinum-resistance. This article summarizes recent clinical developments related to mithramycin and postulates a role for the use of mithramycin, or its analog, in the treatment of platinum-resistant ovarian cancer.

Abstract 2954: Mithramycin analogues disrupt ETS transcription factor DNA binding

Introduction: Ewing sarcoma, prostate cancer, and leukemia are a few examples where ETS transcription factors drive tumorigenesis. The transcription factors EWS-FLI1 and EWS-ERG are common translocations in Ewing sarcoma and bind DNA at GGAA repeats leading to expression of genes that drive tumor growth. Pharmacologic inhibition of EWS-FLI1 with mithramycin (MTM) was shown to inhibit expression of downstream genes and tumor growth in mice. But despite this specific inhibitory activity, MTM has a narrow therapeutic window with hematologic and hepatic toxicity attributed to displacement of the ubiquitously acting Sp1 transcription factor. Thus, a synthetic effort was initiated to develop MTM analogues with reduced toxicity and increased specificity for ETS binding sites. Structural studies informed the design of MTM analogues that may stabilize transcriptional complexes leading to the disruption of transcriptional activity and DNA damage. In vitro cytotoxicity assays demonstrated that MTM analogues have significantly higher cytotoxicity in EWS-ETS expressing cell lines. Here we present mechanistic evidence for the differences in biochemical activity among MTM and its novel analogues.

Methods: Qualitative interactions between drug-DNA-protein were assessed and optimized by electrophoretic mobility shift assays (EMSA). Time-resolved fluorescence energy transfer (TR-FRET) assays were used to quantitatively determine ERG displacement from DNA in the presence of MTM and analogues. Expression of proteins indicating DNA damage (c-PARP, γ-H2AX) and phosphorylation at the C-terminal domain (CTD) of RNAPII was determined by western blot following drug treatments in ETS and non ETS expressing cell lines.

Results: Using TR-FRET, we observed that MTM displaced DNA bound ERG more potently and in a concentration dependent manner as compared to MTM analogues. As compared to MTM, treatment with MTM analogues resulted in higher expression of DNA damage markers, γ-H2AX and c-PARP, specifically in cell lines containing EWS-ETS translocations in a concentration dependent manner.

Conclusion: These studies provide insights regarding differences among MTM and analogues in DNA binding and interactions with DNA associated proteins in the presence and absence of EWS-ETS expression. Our results suggest that MTM analogues may bind and stabilize transcriptional complexes. These differences will provide the basis for structure activity relationships and for the development of analogues with decreased in vivo toxicity. Future work will incorporate co-immunoprecipitation studies to determine if physical protein interactions are being disrupted by MTM and analogues and cellular thermal shift assays to directly probe drug interactions with EWS-ETS proteins and with RNAPII.

Note: This abstract was not presented at the meeting.

Citation Format: Reiya C. Hayden, Caixia Hou, Prithiba Mitra, Abhisek Mandal, Jurgen Rohr, Jon Thorson, Oleg Tsodikov, Markos Leggas. Mithramycin analogues disrupt ETS transcription factor DNA binding [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2954.

Abstract B043: Mithramycin-SA analogues with reduced toxicity for the treatment of ETS transcription factor-driven tumors

Introduction: Chromosomal translocations involving the ETS family of transcription factors are common in Ewing sarcoma, prostate cancer, and leukemia. These translocations lead in overexpression of aberrant ETS transcription factors, which drive tumorigenesis. Mithramycin (MTM) inhibits EWS-FLI1, the most common ETS-related transcription factor in Ewing sarcoma, presumably through interference at its DNA binding sites on promoter regions. However, MTM has a narrow therapeutic window marked by severe liver and hematologic toxicities. These are likely the result of interference with the ubiquitously expressed SP1 transcription factor. Here, we sought to develop analogues with specificity toward ETS transcription factors and reduced interaction with SP1. Methods: Using MTM-SA and semisynthetic approaches a series of analogues were obtained. To determine specificity toward cells expressing ETS-related chromosomal translocations, MTM-SA analogues were screened for growth inhibition in a panel of Ewing sarcoma cell lines (n=8), all expressing EWS-ETS translocations, and compared to a panel of non-Ewing cell lines (n=9) that do not express these translocations. Luciferase reporter constructs were developed to evaluate interference with EWS-FLI1 and SP1 regulated genes following MTM-SA analogue treatment. Select analogues were tested in vivo to identify the maximum tolerated dose and determine pharmacokinetics (PK). Results: In growth inhibition assays, several MTM-SA analogues resulted in > 10-fold specificity toward Ewing sarcoma cell lines vs. MTM. Luciferase reporter assays identified two analogues, MTM-SA-Tryptophan-A2 and MTM-SA-Tryptophan-A10, with 10-fold reduced inhibition of SP1 but similar inhibition of EWS-FLI1, as compared to MTM. Additionally, MTM-SA-Phe analogue has lower toxicity, despite longer plasma half-life. Conclusion: These data show that MTM-SA analogues are cytotoxic against tumor cell lines expressing aberrant ETS transcription factors. Further, their reduced interference with the ubiquitously expressed SP1 transcription factor, as well as their improved PK, may result in a wider therapeutic window.

Citation Format: Markos Leggas, Joseph Eckenrode, Prithiba Mitra, Amit Jha, Shaimaa Salem, Abhisek Mandal, Jon Thorson, Jurgen Rohr. Mithramycin-SA analogues with reduced toxicity for the treatment of ETS transcription factor-driven tumors [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr B043.

Disruption of de Novo Adenosine Triphosphate (ATP) Biosynthesis Abolishes Virulence in Cryptococcus neoformans

Opportunistic fungal pathogens such as Cryptococcus neoformans are a growing cause of morbidity and mortality among immunocompromised populations worldwide. To address the current paucity of antifungal therapeutic agents, further research into fungal-specific drug targets is required. Adenylosuccinate synthetase (AdSS) is a crucial enzyme in the adeosine triphosphate (ATP) biosynthetic pathway, catalyzing the formation of adenylosuccinate from inosine monophosphate and aspartate. We have investigated the potential of this enzyme as an antifungal drug target, finding that loss of function results in adenine auxotrophy in C. neoformans, as well as complete loss of virulence in a murine model. Cryptococcal AdSS was expressed and purified in Escherichia coli and the enzyme's crystal structure determined, the first example of a structure of this enzyme from fungi. Together with enzyme kinetic studies, this structural information enabled comparison of the fungal enzyme with the human orthologue and revealed species-specific differences potentially exploitable via rational drug design. These results validate AdSS as a promising antifungal drug target and lay a foundation for future in silico and in vitro screens for novel antifungal compounds.

Synthesis of Psoralidin derivatives and their anticancer activity: First synthesis of Lespeflorin I1

Synthetic scheme for the preparation of a number of different derivatives of anticancer natural product Psoralidin is described. A convergent synthetic approach is followed using simple starting materials like substituted phenyl acetic esters and benzoic acids. The developed synthetic route leads us to complete the first synthesis of an analogous natural product Lespeflorin I₁, a mild melanin synthesis inhibitor. Preliminary bioactivity studies of the synthesized compounds are carried out against two commonly used prostate cancer cell lines. Results show that the bioactivity of the compounds can be manipulated by the simple modification of the functional groups.

Recurrent activating mutations of CD28 in peripheral T-cell lymphomas

Peripheral T-cell lymphomas (PTCLs) comprise a heterogeneous group of mature T-cell neoplasms with a poor prognosis. Recently, mutations in TET2 and other epigenetic modifiers as well as RHOA have been identified in these diseases, particularly in angioimmunoblastic T-cell lymphoma (AITL). CD28 is the major co-stimulatory receptor in T cells which, upon binding ligand, induces sustained T-cell proliferation and cytokine production when combined with T-cell receptor stimulation. We have identified recurrent mutations in CD28 in PTCLs. Two residues-D124 and T195-were recurrently mutated in 11.3% of cases of AITL and in one case of PTCL, not otherwise specified (PTCL-NOS). Surface plasmon resonance analysis of mutations at these residues with predicted differential partner interactions showed increased affinity for ligand CD86 (residue D124) and increased affinity for intracellular adaptor proteins GRB2 and GADS/GRAP2 (residue T195). Molecular modeling studies on each of these mutations suggested how these mutants result in increased affinities. We found increased transcription of the CD28-responsive genes CD226 and TNFA in cells expressing the T195P mutant in response to CD3 and CD86 co-stimulation and increased downstream activation of NF-κB by both D124V and T195P mutants, suggesting a potential therapeutic target in CD28-mutated PTCLs.

Abstract 2628: Mithramycin analogs with reduced toxicity for EWS-FLI1 targeting

Introduction: EWS-FLI1 and related chromosomal translocations are prevalent in Ewing sarcoma and play a major role in modulating oncogenic transcription. Development of drugs that affect EWS-FLI1 oncoprotein function may lead to successful treatment for these patients. Mithramycin (MTM) was shown to inhibit transcriptional targets of EWS-FLI1, but it has a narrow therapeutic window attributed to its nonspecific toxicities. To overcome this, semisynthetic methods were developed to generate MTM analogs with unique pharmacologic properties. Mechanistic and pharmacologic studies are presented here.

Methods: Studies were conducted using MTM and lead analogs (mithramycin-SK (MTM-SK), mithramycin-SA-tryptophan (MTM-SA-Trp), and mithramycin-SA-phenylalanine (MTM-SA-Phe)). EWS-FLI1 promoter occupancy was investigated using chromatin immunoprecipitation real-time PCR (ChIP-RTPCR). The effect of drug treatment on expression of genes controlled by EWS-FLI1 was evaluated by quantitative real-time PCR (qRT-PCR). The effect of treatment on cell cycle distribution was also compared among analogs. In vitro efficacy was evaluated by estimating GI50 parameters (72-hr). In addition, the maximum tolerated dose (MTD) and the effect of treatment on plasma total-calcium were used to assess relative toxicity in mice.

Results: EWS-FLI1 promoter occupancy upstream from Nr0b1, Tgfbr2, and Rcor1 genes was evaluated in Ewing sarcoma cells (TC32) by ChIP-RTPCR. MTM and MTM-SA-Trp analog destabilized FLI1 binding to all three promoters and MTM-SA-Trp was shown to be the most destabilizing. Comparatively, MTM-SK appears to mostly stabilize FLI1. Additionally, qRTPCR showed that MTM and its analogs efficiently down-regulated mRNA expression in a dose-dependent manner (rank-order of efficiency: MTM-SA-Trp>MTM = MTM-SK). These data were in accord with the in vitro cytotoxicity data that show MTM-SA-Trp has relatively higher potency (lower GI50) among Ewing cell lines (n = 8) as compared to other analogs. Furthermore, the effect of drug treatment appears to lead to differences in cell-cycle progression. MTM and MTM-SK treated TC32 cells were primarily in G1/G2 phase, whereas MTM-SA-Trp treated cells showed increased S-phase accumulation. Compared to MTM, mice tolerated significantly higher single doses of the MTM analogs. Repeated dosing showed similar results except that MTM-SA-Trp was tolerated at lower doses. MTM treatment caused an acute drop in total-calcium, which also occurred with MTM-SA-Trp and MTM-SA-Phe analogs three days later within the two-week treatment. Comparatively, MTM-SK caused an increase in total-calcium. In all cases, total-calcium concentrations returned to baseline within two weeks following treatment.

Conclusion: The work presented here demonstrates the ability to design more specific and less toxic analogs of MTM. Development of such analogs could lead to successful treatments of Ewing sarcoma.

Citation Format: Joseph Eckenrode, Jamie Horn, Jhong-Min Chen, Jurgen Rohr, Markos Leggas. Mithramycin analogs with reduced toxicity for EWS-FLI1 targeting. [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 2628. doi:10.1158/1538-7445.AM2015-2628

Activation and silencing of secondary metabolites in Streptomyces albus and Streptomyces lividans after transformation with cosmids containing the thienamycin gene cluster from Streptomyces cattleya

Activation and silencing of antibiotic production was achieved in Streptomyces albus J1074 and Streptomyces lividans TK21 after introduction of genes within the thienamycin cluster from S. cattleya. Dramatic phenotypic and metabolic changes, involving activation of multiple silent secondary metabolites and silencing of others normally produced, were found in recombinant strains harbouring the thienamycin cluster in comparison to the parental strains. In S. albus, ultra-performance liquid chromatography purification and NMR structural elucidation revealed the identity of four structurally related activated compounds: the antibiotics paulomycins A, B and the paulomenols A and B. Four volatile compounds whose biosynthesis was switched off were identified by gas chromatography-mass spectrometry analyses and databases comparison as pyrazines; including tetramethylpyrazine, a compound with important clinical applications to our knowledge never reported to be produced by Streptomyces. In addition, this work revealed the potential of S. albus to produce many others secondary metabolites normally obtained from plants, including compounds of medical relevance as dihydro-β-agarofuran and of interest in perfume industry as β-patchoulene, suggesting that it might be an alternative model for their industrial production. In S. lividans, actinorhodins production was strongly activated in the recombinant strains whereas undecylprodigiosins were significantly reduced. Activation of cryptic metabolites in Streptomyces species might represent an alternative approach for pharmaceutical drug discovery.

Amalgamation of nucleosides and amino acids in antibiotic biosynthesis: discovery of an L-threonine:uridine-5'-aldehyde transaldolase

The lipopeptidyl nucleoside antibiotics represented by A-90289, caprazamycin, and muraymycin are structurally highlighted by a nucleoside core that contains a nonproteinogenic β-hydroxy-α-amino acid named 5'-C-glycyluridine (GlyU). Bioinformatic analysis of the biosynthetic gene clusters revealed a shared open reading frame encoding a protein with sequence similarity to serine hydroxymethyltransferases, resulting in the proposal that this shared enzyme catalyzes an aldol-type condensation with glycine and uridine-5'-aldehyde to furnish GlyU. Using LipK involved in A-90289 biosynthesis as a model, we now functionally assign and characterize the enzyme responsible for the C-C bond-forming event during GlyU biosynthesis as an l-threonine:uridine-5'-aldehyde transaldolase. Biochemical analysis revealed this transformation is dependent upon pyridoxal-5'-phosphate, the enzyme has no activity with alternative amino acids, such as glycine or serine, as aldol donors, and acetaldehyde is a coproduct. Structural characterization of the enzyme product is consistent with stereochemical assignment as the threo diastereomer (5'S,6'S)-GlyU. Thus this enzyme orchestrates C-C bond breaking and formation with concomitant installation of two stereocenters to make a new l-α-amino acid with a nucleoside side chain.

Purification and structure elucidation of the by-product of new regulator of antibiotic production and differentiation of Streptomyces

Streptomyces globisporus 1912, a producer of the antitumor antibiotic landomycin E, forms the new low-molecular signaling molecule N-methylphenylalanyl-dehydrobutyrine diketopiperazine (BDD) and its complex and unstable by-product which restore, like the A-factor in Streptomyces griseus 773, landomycin E and streptomycin biosynthesis, and sporulation of the defective mutants S. globisporus 1912-B2 and S. griseus 1439, respectively. Here, we report the purification and structure elucidation of two compounds with R(f)0.8 by HPLC, LC/MS and 1HMR analysis. These compounds have m/z 338 and 384, accordingly, and each of them is presented by two stereoisomers containing BDD in their structure. A hypothesis explaining the composition and regulatory properties of these unstable compounds is presented.

Inhibition of Sp1-dependent transcription and antitumor activity of the new aureolic acid analogues mithramycin SDK and SK in human ovarian cancer xenografts

OBJECTIVE

Increased activity of Sp family of transcription factors is a frequent and critical event in cancer development and progression. Genes governing tumor growth, invasion and angiogenesis are regulated by Sp factors, like Sp1, Sp3 or Sp4, and are frequently over-expressed in tumors. Targeting Sp factors has been explored as a therapeutic approach. Mithramycin (MTM) is a natural antibiotic that binds DNA and inhibit Sp1-dependent transcription. New analogues, named MTM-SDK and MTM-SK, were recently obtained by genetic engineering of the MTM biosynthetic pathway and have demonstrated improved transcriptional and antiproliferative activity in ovarian cancer cell lines in vitro. In the present study we evaluated the activity of the new compounds in human ovarian cancer xenografts.

METHODS

Expression of Sp1 and target proteins in ovarian cancer specimens and tumor xenografts was assessed by immunohistochemistry. Drug-induced silencing of Sp1-regulated genes in cells and tumor xenograft samples was assessed by quantitative RT-PCR. Toxicity and antitumor activity of the compounds were investigated in healthy and tumor-bearing immunocompromised mice, respectively.

RESULTS

Expression of Sp1 was frequently increased in human epithelial ovarian cancers. MTM-SDK and MTM-SK acted as potent inhibitors of Sp1-dependent transcription both in vitro and in tumor xenografts. Both compounds were well tolerated even after prolonged administration and delayed growth of ovarian tumor xenografts. MTM-SDK was particularly effective against orthotopic tumors leading to a significant increase of survival and delay of tumor progression.

CONCLUSIONS

MTM-SDK and MTM-SK show relevant activity in vivo and represent interesting candidates for treatment of ovarian cancers.

Properties of lanK-based regulatory circuit involved in landomycin biosynthesis in Streptomyces cyanogenus S136

LanK is TetR-like regulatory protein recently shown to regulate the export and glycosylation of landomycins in Streptomyces cyanogenus S136. Here, several properties of the lanK-mediated regulation were deciphered. LanK seems to function as oligomer as evident from experiments in vitro. In vivo, it is able to recognize various landomycins with altered aglycon structure and the minimal concentration of landomycin A sensed by LanK lies in low nanomolar range. Coexpression studies showed that the positive regulatory gene lanI upregulates lanK-dependent lan genes once the negative LanK-regulation is cancelled. Gene lanK can be useful for the construction of biosensor strains for sensitive and specific identification of producers of landomycin-like molecules with long glycosidic chains.

Abstract 4043: Targeting microRNA for the prevention and treatment of prostate cancer

MicroRNAs (miRs) are small 20-24 nucleotide non-coding RNA molecules that are involved in post-transcriptional regulation of gene expression in multicellular organisms by affecting both stability and translation of mRNAs. The expression and activity of miRs vary in specific cell types and disease states. Several groups have investigated and determined the strong association between specific expression patterns of miRs and various stages of prostate cancer. Recent findings have highlighted the strong correlation between overexpression of miRs (21, 210, 125b, 221 and 222) and cancer development and progression. In this study we determined whether modulation of miR expression using a natural compound suppresses PCa growth. Previously we published that psoralidin inhibits prostate tumor growth and progression in both in vitro and in vivo models. While studying the expression patterns of a panel of miRs in control and psoralidin treated prostate cancer cells we found that psoralidin significantly inhibits expression of two specific miRNAs, miR-21 and miR-210 in turn resulting in the inhibition of EGFR-, MAPK- and PI3K-mediated prosurvival signaling and induction of JNK-mediated apoptosis in prostate cancer cells. Currently, we are overexpressing and inhibiting these two miRs to determine whether it modulates psoralidin-mediated effects on prostate cancer cells. The outcome of our study may provide encouraging data that may enable us to exploit miRs as therapeutic targets for the treatment of various malignancies.

Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4043.

Abstract 2491: Biphasic effect of psoralidin on cell cycle regulatory markers in hormone refractory prostate cancer

Cell cycle deregulation has been strongly associated with the pathogenesis of prostate cancer (PCa). Overexpression of cell cycle regulatory proteins (cyclin D1-25%, cyclin A-35% and cyclin B-75%) have been reported in clinical PCa samples when compared to normal prostate tissue. Based on this, we rationalized that downregulation of cell cycle regulatory molecules may play an important role in chemoprevention/therapy of PCa. We previously published that psoralidin; a natural compound effectively suppresses PCa growth both in vitro and in vivo. Hence, in this study we determined whether psoralidin alters cell cycle regulatory proteins in PCa cells. To determine this we performed cell cycle analysis using PI staining and also studied the expression patterns and activity of various cell cycle regulatory markers using Western blot analysis and kinase assays respectively in two hormone refractory prostate cancer (HRPC) cells (PC-3 and DU-145) following treatment with or without psoralidin in dose dependent manner. Psoralidin has biphasic effect on PCa cells where at a lower concentration (40µM) caused a G0/G1 cell cycle arrest whereas at a higher concentration (60µM) caused a G2/M cell cycle arrest. Psoralidin at lower concentration induced expression of cyclin dependent kinase inhibitors (p27, p57, p21, p16) in PC-3 and DU-145 cells whereas at higher concentration upregulation of p18 was observed in both HRPC cells. Additionally, we found that at lower concentration cyclins (E & E2 and H) and cdks (6 and 9) are downregulated whereas at higher concentration psoralidin downregulated expression of cyclins (D1, A, E & E2, B1, D3 and H) and cdks (6, 7, 2 and 9) in both HRPC cells. At lower concentration psoralidin inhibits expression of cdc-2 and cdc-25B whereas at higher concentration cdc-2, −25A and −25B are downregulated in both PC-3 and DU-145 cells. Also, psoralidin inhibited phosphorylation of Rb at both lower and higher concentration in HRPC cells causing irreversible cell cycle arrest resulting in the induction of apoptosis. These results clearly highlight the potential of psoralidin as a therapeutic agent for PCa. Currently, we are investigating the effect of psoralidin on cell cycle markers using both xenograft and TRAMP models. The outcome of our study may enable bringing psoralidin to the mainstream of medicine for prevention and/or therapy for PCa.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2491.

Abstract 4387: Reactive oxygen species-mediated cell death by psoralidin in prostate cancer cells

Modulating reactive oxygen species (ROS) levels may lead to either cell proliferation or death based on the cell type. It is well established that in cancer cells the basal levels of ROS is significantly higher when compared to their normal counterparts. Thus, identifying non-toxic small molecules which induces ROS specifically in cancer cells may be of enormous importance in cancer therapy. We have previously shown that psoralidin, a natural compound derived from Psoralea corylifolia specifically induces apoptosis in prostate cancer (PCa) cells (PC-3, DU-145, LNCaP and C4-2B). In this study we dissected the role of ROS in psoralidin-induced cell death in PCa. Our results suggested that psoralidin significantly induces ROS generation in both androgen-responsive (LNCaP) and -refractory (PC-3, DU-145 and C4-2B) PCa cells when compared to normal prostate epithelial cells (PzHPv7); leading to loss of mitochondrial membrane potential and release of cytochrome-c, caspase activation (9 and 3) and induction of apoptosis. In addition, induction of ROS downregulates pAkt expression and kinase activity, NF-κB (p65, p50 and p55) and mTOR (pp70S6K and p4E-BP1)-mediated pro-survival signaling in PCa cells. On the other hand, inhibition of ROS generation by N-acetylcysteine (NAC) prevented psoralidin-mediated anticancer effects on PCa. Collectively, our results suggest that psoralidin-mediated ROS generation in PCa cells causes inhibition of survival/proliferation and induction of apoptosis. Further investigation of the mechanism of action of psoralidin may facilitate initiation of clinical trials using psoralidin for the treatment of PCa.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4387.

Abstract 4140: Inhibition of mTOR signaling by psoralidin in breast cancer

Breast carcinoma is the most common malignancy in women and is the second leading cause of cancer-related deaths in the United States. Mammalian target of rapamycin (mTOR) is a major effector of the PI3Kinase/Akt pro-survival signaling pathway and has received significant attention as a therapeutic target for the treatment of various malignancies. Rapamycin and its analogs are specific mTOR antagonists; however their toxicity hampers wide use in the clinical setting for the treatment of breast cancer. This compels development of new strategies for the treatment of advanced, triple and double negative breast cancers. Psoralidin is one of the major active ingredients in Psoralea corylifolia, herbal plant that is extensively used in Asian and African traditional medicines. In our study we tested the effects of psoralidin on the mTOR pathway in estrogen-responsive (MCF-7) and -refractory (MDA 231) breast cancer cells. Our results suggest that psoralidin inhibits phoshporlyation of mTOR resulting in the inhibition of the phosphorylation of mTOR effectors (p70S6K and 4EBP1) in both MCF-7 and MDA 231 cells. Interestingly, there was no significant down regulation of pAkt expression in either cell lines following treatment with psoralidin. While dissecting the effect of psoralidin on epithelial-mesenchymal transition (EMT), we found that psoralidin inhibits expression of β-catenin with a concomitant increase in the expression of E-cadherin in both estrogen-responsive and -refractory breast cancers cells. In addition, treatment with psoralidin effectively inhibits the growth of both estrogen-responsive (MCF-7) and -refractory (MDA 231) breast cancer cells and induces apoptosis without causing significant toxicity to normal breast epithelial cells (MCF-10A). Currently we are pursuing studies using xenograft models to determine the effect of psoralidin in estrogen-responsive and -refractory breast cancers animal models. The outcome of this study may provide a mechanistic validation of psoralidin as a potent compound that may be used for the treatment of breast cancer.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4140.

Anti-Mullerian hormone indicates early ovarian decline in fragile X mental retardation (FMR1) premutation carriers: a preliminary study

BACKGROUND

Women who carry the fragile X mental retardation (FMR1) premutation are at risk for fragile X-associated primary ovarian insufficiency. Past studies have shown that carriers who are still cycling have increased levels FSH compared with non-carriers. As anti-Mullerian hormone (AMH) has been shown as an excellent marker of ovarian decline, we examined AMH levels among premutation carriers to characterize their ovarian function.

METHODS

We determined the level of FSH and AMH in serum samples collected during early follicular phase from women who carried longer FMR1 repeat alleles (defined as >or=70 repeats, n = 40) and those with shorter repeat alleles (<70 repeats, n = 75), identified by DNA analysis. Comparisons were made stratified by age and carrier status.

RESULTS

For all age groups, AMH levels were significantly lower among longer repeat allele carriers compared to shorter repeat allele carriers (P = 0.002, 0.006 and 0.020 for women ages 18-30, 31-40 and 41-50 years, respectively). In contrast, increased FSH indicative of early ovarian decline was only evident for longer repeat allele carriers aged 31-40 years (P = 0.089, 0.001 and 0.261 for women ages 18-30, 31-40 and 41-50 years, respectively).

CONCLUSIONS

These preliminary data suggest that AMH levels indicate early ovarian decline among women with longer FMR1 repeat alleles; moreover, AMH appears to be a better marker than FSH in identifying this early decline.

Surprising production of a new urdamycin derivative by S. fradiae Delta urdQ/R

A strain (S. fradiae Delta urdQ/R) with mutations in urdQ and urdR encoding a dTDP-hexose-3,4-dehydratase and a dTDP-hexose-4-ketoreductase, respectively, produces a new urdamycin analogue (urdamycin X) with changes in the polyketide structure. The structure of urdamycin X has been elucidated by NMR spectroscopy. Urdamycin X was not detectable, even in small amounts, in either S. fradiae Delta urdQ, in S. fradiae DeltaurdR or in S. fradiae A0, a mutant lacking all glycosyltransferase genes. Complementation of S. fradiae Delta urdQ/R restored urdamycin A production indicating that the mutations did not cause any polar effect.

Crystallization and X-ray diffraction properties of Baeyer-Villiger monooxygenase MtmOIV from the mithramycin biosynthetic pathway in Streptomyces argillaceus

The Baeyer-Villiger monooxygenase MtmOIV from Streptomyces argillaceus is a 56 kDa FAD-dependent and NADPH-dependent enzyme that is responsible for the key frame-modifying step in the biosynthesis of the natural product mithramycin. Crystals of MtmOIV were flash-cooled and diffracted to 2.69 A resolution using synchrotron radiation on beamline SER-CAT 22-ID at the Advanced Photon Source. Crystals of MtmOIV are monoclinic and light-scattering data reveal that the enzyme forms dimers in solution. The rotation function suggests the presence of two dimers in the asymmetric unit. L-Selenomethionine-incorporated MtmOIV has been obtained. Structural solution combining molecular-replacement phases and anomalous phases from selenium is in progress.

DNA-binding properties of cosmomycin D, an anthracycline with two trisaccharide chains

Cosmomycin D (CosD) is the major constituent fraction isolated from a culture of Streptomyces olindensis ICB20. The ability of this compound to intercalate with double-stranded DNA was studied by gel mobility shift assays and electrospray ionization mass spectrometry (ESI-MS). ESI-MS experiments showed that the complex of CosD with 16-mer double-stranded DNA was at least as stable as a complex of daunorubicin with the same DNA sequence. This is the first study showing DNA binding properties of an anthracycline containing a beta-rhodomycinone aglycone chromophore O-linked to two trisaccharide chains.

Combining sugar biosynthesis genes for the generation of L- and D-amicetose and formation of two novel antitumor tetracenomycins

L- and D-stereoisomers of amicetose were generated by combining sugar biosynthesis genes from four different antibiotic gene clusters and both sugars were transferred to the elloramycin aglycone by the sugar flexible ElmGT glycosyltransferase.

Field measurements and modeling of dilution in the wake of a US navy frigate

A field measurement and computer modeling effort was made to assess the dilution field of pulped waste materials discharged into the wake of a US Navy frigate. Pulped paper and fluorescein dye were discharged from the frigate's pulper at known rates. The subsequent particle and dye concentration field was then measured throughout the wake by a following vessel using multiple independent measures. Minimum dilution of the pulped paper reached 3.2 x 10(5) within 1900 m behind the frigate, or about 8 min after discharge. Independent measures typically agreed within 25% of one another and within 20% of model predictions. Minimum dilution of dye reached 2.3 x 10(5) at a down-wake distance of approximately 3500 m, or roughly 15 min. Comparison to model measurements were again within 20%. The field test was not only successful at characterizing wake dilution under one set of at-sea conditions, but was successful at validating the computer model used for assessing a wide range of ships and conditions.

Elucidation of the function of two glycosyltransferase genes (lanGT1 and lanGT4) involved in landomycin biosynthesis and generation of new oligosaccharide antibiotics

BACKGROUND

The genetic engineering of antibiotic-producing Streptomyces strains is an approach that became a successful methodology in developing new natural polyketide derivatives. Glycosyltransferases are important biosynthetic enzymes that link sugar moieties to aglycones, which often derive from polyketides. Biological activity is frequently generated along with this process. Here we report the use of glycosyltransferase genes isolated from the landomycin biosynthetic gene cluster to create hybrid landomycin/urdamycin oligosaccharide antibiotics.

RESULTS

Production of several novel urdamycin derivatives by a mutant of Streptomyces fradiae Tü2717 has been achieved in a combinatorial biosynthetic approach using glycosyltransferase genes from the landomycin producer Streptomyces cyanogenus S136. For the generation of gene cassettes useful for combinatorial biosynthesis experiments new vectors named pMUNI, pMUNII and pMUNIII were constructed. These vectors facilitate the construction of gene combinations taking advantage of the compatible MunI and EcoRI restriction sites.

CONCLUSIONS

The high-yielding production of novel oligosaccharide antibiotics using glycosyltransferase gene cassettes generated in a very convenient way proves that glycosyltransferases can be flexible towards the alcohol substrate. In addition, our results indicate that LanGT1 from S. cyanogenus S136 is a D-olivosyltransferase, whereas LanGT4 is a L-rhodinosyltransferase.

Deoxysugar Methylation during Biosynthesis of the Antitumor Polyketide Elloramycin by Streptomyces olivaceus

The anthracycline-like polyketide drug elloramycin is produced by Streptomyces olivaceus Tü2353. Elloramycin has antibacterial activity against Gram-positive bacteria and also exhibits antitumor activity. From a cosmid clone (cos16F4) containing part of the elloramycin biosynthesis gene cluster, three genes (elmMI, elmMII, and elmMIII) have been cloned. Sequence analysis and data base comparison showed that their deduced products resembled S-adenosylmethionine-dependent O-methyltransferases. The genes were individually expressed in Streptomyces albus and also coexpressed with genes involved in the biosynthesis of l-rhamnose, the 6-deoxysugar attached to the elloramycin aglycon. The resulting recombinant strains were used to biotransform three different elloramycin-type compounds: l-rhamnosyl-tetracenomycin C, l-olivosyl-tetracenomycin C, and l-oleandrosyl-tetracenomycin, which differ in their 2'-, 3'-, and 4'-substituents of the sugar moieties. When only the three methyltransferase-encoding genes elmMI, elmMII, and elmMIII were individually expressed in S. albus, the methylating activity of the three methyltransferases was also assayed in vitro using various externally added glycosylated substrates. From the combined results of all of these experiments, it is proposed that methyltransferases ElmMI, ElmMII, and ElmMIII are involved in the biosynthesis of the permethylated l-rhamnose moiety of elloramycin. ElmMI, ElmMII, and ElmMIII are responsible for the consecutive methylation of the hydroxy groups at the 2'-, 3'-, and 4'-position, respectively, after the sugar moiety has been attached to the aglycon.

Identification of a sugar flexible glycosyltransferase from Streptomyces olivaceus, the producer of the antitumor polyketide elloramycin

BACKGROUND

Elloramycin is an anthracycline-like antitumor drug related to tetracenomycin C which is produced by Streptomyces olivaceus Tü2353. Structurally is a tetracyclic aromatic polyketide derived from the condensation of 10 acetate units. Its chromophoric aglycon is glycosylated with a permethylated L-rhamnose moiety at the C-8 hydroxy group. Only limited information is available about the genes involved in the biosynthesis of elloramycin. From a library of chromosomal DNA from S. olivaceus, a cosmid (16F4) was isolated that contains part of the elloramycin gene cluster and when expressed in Streptomyces lividans resulted in the production of a non-glycosylated intermediate in elloramycin biosynthesis, 8-demethyl-tetracenomycin C (8-DMTC).

RESULTS

The expression of cosmid 16F4 in several producers of glycosylated antibiotics has been shown to produce tetracenomycin derivatives containing different 6-deoxysugars. Different experimental approaches showed that the glycosyltransferase gene involved in these glycosylation events was located in 16F4. Using degenerated oligoprimers derived from conserved amino acid sequences in glycosyltransferases, the gene encoding this sugar flexible glycosyltransferase (elmGT) has been identified. After expression of elmGT in Streptomyces albus under the control of the erythromycin resistance promoter, ermEp, it was shown that elmG can transfer different monosaccharides (both L- and D-sugars) and a disaccharide to 8-DMTC. Formation of a diolivosyl derivative in the mithramycin producer Streptomyces argillaceus was found to require the cooperative action of two mithramycin glycosyltransferases (MtmGI and MtmGII) responsible for the formation of the diolivosyl disaccharide, which is then transferred by ElmGT to 8-DMTC.

CONCLUSIONS

The ElmGT glycosyltransferase from S. olivaceus Tü2353 can transfer different sugars into the aglycon 8-DMTC. In addition to its natural sugar substrate L-rhamnose, ElmGT can transfer several L- and D-sugars and also a diolivosyl disaccharide into the aglycon 8-DMTC. ElmGT is an example of sugar flexible glycosyltransferase and can represent an important tool for combinatorial biosynthesis.

The mtmVUC genes of the mithramycin gene cluster in Streptomyces argillaceus are involved in the biosynthesis of the sugar moieties

Mithramycin is a glycosylated aromatic polyketide produced by Streptomyces argillaceus, and is used as an antitumor drug. Three genes (mtmV, mtmU and mtmC) from the mithramycin gene cluster have been cloned, and characterized by DNA sequencing and by analysis of the products that accumulate in nonproducing mutants, which were generated by insertional inactivation of these genes. The mtm V gene codes for a 2,3-dehydratase that catalyzes early and common steps in the biosynthesis of the three sugars found in mithramycin (D-olivose, D-oliose and D-mycarose); its inactivation caused the accumulation of the nonglycosylated intermediate premithramycinone. The mtmU gene codes for a 4-ketoreductase involved in D-oliose biosynthesis, and its inactivation resulted in the accumulation of premithramycinone and premithramycin A , the first glycosylated intermediate which contains a D-olivose unit. The third gene, mtmC, is involved in D-mycarose biosynthesis and codes for a C-methyltransferase. Two mutants with lesions in the mtmC gene accumulated mithramycin intermediates lacking the D-mycarose moiety but containing D-olivose units attached to C-12a in which the 4-keto group is unreduced. This suggests that mtmC could code for a second enzyme activity, probably a D-olivose 4-ketoreductase, and that the glycosyltransferase responsible for the incorporation of D-olivose (MtmGIV) shows some degree of flexibility with respect to its sugar co-substrate, since the 4-ketoanalog is also transferred. A pathway is proposed for the biosynthesis of the three sugar moieties in mithramycin.

The NDP-sugar co-substrate concentration and the enzyme expression level influence the substrate specificity of glycosyltransferases: cloning and characterization of deoxysugar biosynthetic genes of the urdamycin biosynthetic gene cluster

BACKGROUND

Streptomyces fradiae is the principal producer of urdamycin A. The antibiotic consists of a polyketide-derived aglycone, which is glycosylated with four sugar components, 2x D-olivose (first and last sugar of a C-glycosidically bound trisaccharide chain at the 9-position), and 2x L-rhodinose (in the middle of the trisaccharide chain and at the 12b-position). Limited information is available about both the biosynthesis of D-olivose and L-rhodinose and the influence of the concentration of both sugars on urdamycin biosynthesis.

RESULTS

To further investigate urdamycin biosynthesis, a 5.4 kb section of the urdamycin biosynthetic gene cluster was sequenced. Five new open reading frames (ORFs) (urdZ3, urdQ, urdR, urdS, urdT) could be identified each one showing significant homology to deoxysugar biosynthetic genes. We inactivated four of these newly allocated ORFs (urdZ3, urdQ, urdR, urdS) as well as urdZ1, a previously found putative deoxysugar biosynthetic gene. Inactivation of urdZ3, urdQ and urdZ1 prevented the mutant strains from producing L-rhodinose resulting in the accumulation of mainly urdamycinone B. Inactivation of urdR led to the formation of the novel urdamycin M, which carries a C-glycosidically attached D-rhodinose at the 9-position. The novel urdamycins N and O were detected after overexpression of urdGT1c in two different chromosomal urdGT1c deletion mutants. The mutants lacking urdS and urdQ accumulated various known diketopiperazines.

CONCLUSIONS

Analysis of deoxysugar biosynthetic genes of the urdamycin biosynthetic gene cluster revealed a widely common biosynthetic pathway leading to D-olivose and L-rhodinose. Several enzymes responsible for specific steps of this pathway could be assigned. The pathway had to be modified compared to earlier suggestions. Two glycosyltransferases normally involved in the C-glycosyltransfer of D-olivose at the 9-position (UrdGT2) and in conversion of 100-2 to urdamycin G (UrdGT1c) show relaxed substrate specificity for their activated deoxysugar co-substrate and their alcohol substrate, respectively. They can transfer activated D-rhodinose (instead of D-olivose) to the 9-position, and attach L-rhodinose to the 4A-position normally occupied by a D-olivose unit, respectively.

Sjögren's syndrome and granulomatous panniculitis

An 83-year-old Cambodian woman presented with multiple painful erythematous nodules on the lower legs, forearms and wrists. Subsequent history and investigations revealed a granulomatous lobular panniculitis associated with Sjögren's syndrome. This is a rare cutaneous association of Sjögrens syndrome with only three similar case reports in the literature.

Generation of hybrid elloramycin analogs by combinatorial biosynthesis using genes from anthracycline-type and macrolide biosynthetic pathways

Elloramycin and oleandomycin are two polyketide compounds produced by Streptomyces olivaceus Tü2353 and Streptomyces antibioticus ATCC11891, respectively. Elloramycin is an anthracycline-like antitumor drug and oleandomycin a macrolide antibiotic. Expression in S. albus of a cosmid (cos16F4) containing part of the elloramycin biosynthetic gene cluster produced the elloramycin non-glycosylated intermediate 8-demethyl-tetracenomycin C. Several plasmid constructs harboring different gene combinations of L-oleandrose (neutral 2,6-dideoxyhexose attached to the macrolide antibiotic oleandomycin) biosynthetic genes of S. antibioticus that direct the biosynthesis of L-olivose, L-oleandrose and L-rhamnose were coexpressed with cos16F4 in S. albus. Three new hybrid elloramycin analogs were produced by these recombinant strains through combinatorial biosynthesis, containing elloramycinone or 12a-demethyl-elloramycinone (= 8-demethyl-tetracenomycin C) as aglycone moiety encoded by S. olivaceus genes and different sugar moieties, coded by the S. antibioticus genes. Among them is L-olivose, which is here described for the first time as a sugar moiety of a natural product.

Characterization of two polyketide methyltransferases involved in the biosynthesis of the antitumor drug mithramycin by Streptomyces argillaceus

A DNA chromosomal region of Streptomyces argillaceus ATCC 12596, the producer organism of the antitumor polyketide drug mithramycin, was cloned. Sequence analysis of this DNA region, located between four mithramycin glycosyltransferase genes, showed the presence of two genes (mtmMI and mtmMII) whose deduced products resembled S-adenosylmethionine-dependent methyltransferases. By independent insertional inactivation of both genes nonproducing mutants were generated that accumulated different mithramycin biosynthetic intermediates. The M3DeltaMI mutant (mtmMI-minus mutant) accumulated 4-demethylpremithramycinone (4-DPMC) which lacks the methyl groups at carbons 4 and 9. The M3DeltaM2 (mtmMII-minus mutant) accumulated 9-demethylpremithramycin A3 (9-DPMA3), premithramycin A1 (PMA1), and 7-demethylmithramycin, all of them containing the O-methyl group at C-4 and C-1', respectively, but lacking the methyl group at the aromatic position. Both genes were expressed in Streptomyces lividans TK21 under the control of the erythromycin resistance promoter (ermEp) of Saccharopolyspora erythraea. Cell-free extracts of these clones were precipitated with ammonium sulfate (90% saturation) and assayed for methylation activity using different mithramycin intermediates as substrates. Extracts of strains MJM1 (expressing the mtmMI gene) and MJM2 (expressing the mtmMII gene) catalyzed efficient transfer of tritium from [(3)H]S-adenosylmethionine into 4-DPMC and 9-DPMA3, respectively, being unable to methylate other intermediates at a detectable level. These results demonstrate that the mtmMI and mtmMII genes code for two S-adenosylmethionine-dependent methyltransferases responsible for the 4-O-methylation and 9-C-methylation steps of the biosynthetic precursors 4-DPMC and 9-DPMA3, respectively, of the antitumor drug mithramycin. A pathway is proposed for the last steps in the biosynthesis of mithramycin involving these methylation events.

Function of glycosyltransferase genes involved in urdamycin A biosynthesis

BACKGROUND

Urdamycin A, the principle product of Streptomyces fradiae Tü2717, is an angucycline-type antibiotic. The polyketide-derived aglycone moiety is glycosylated at two positions, but only limited information is available about glycosyltransferases involved in urdamycin biosynthesis.

RESULTS

To determine the function of three glycosyltransferase genes in the urdamycin biosynthetic gene cluster, we have carried out gene inactivation and expression experiments. Inactivation of urdGT1a resulted in the predominant accumulation of urdamycin B. A mutant lacking urdGT1b and urdGT1c mainly produced compound 100-2. When urdGT1c was expressed in the urdGT1b/urdGT1c double mutant, urdamycin G and urdamycin A were detected. The mutant lacking all three genes mainly accumulated aquayamycin and urdamycinone B. Expression of urdGT1c in the triple mutant led to the formation of compound 100-1, whereas expression of urdGT1a resulted in the formation of compound 100-2. Co-expression of urdGT1b and urdGT1c resulted in the production of 12b-derhodinosyl-urdamycin A, and co-expression of urdGT1a, urdGT1b and urdGT1c resulted in the formation of urdamycin A.

CONCLUSIONS

Analysis of glycosyltransferase genes of the urdamycin biosynthetic gene cluster led to an unambiguous assignment of each glycosyltransferase to a certain biosynthetic saccharide attachment step.

Two new tailoring enzymes, a glycosyltransferase and an oxygenase, involved in biosynthesis of the angucycline antibiotic urdamycin A in Streptomyces fradiae Tü2717

Urdamycin A, the principal product of Streptomyces fradiae Tu2717, is an angucycline-type antibiotic and anticancer agent containing C-glycosidically linked D-olivose. To extend knowledge of the biosynthesis of urdamycin A the authors have cloned further parts of the urdamycin biosynthetic gene cluster. Three new ORFs (urdK, urdJ and urdO) were identified on a 3.35 kb fragment, and seven new ORFs (urdL, urdM, urdJ2, urdZl, urdGT2, urdG and urdH) on an 8.05 kb fragment. The deduced products of these genes show similarities to transporters (urdJ and urdJ2), regulatory genes (urdK), reductases (urdO), cyclases (urdL) and deoxysugar biosynthetic genes (urdG, urdH and urdZ1). The product of urdM shows striking sequence similarity to oxygenases (N-terminal sequence) as well as reductases (C-terminal sequence), and the deduced amino acid sequence of urdGT2 resembles those of glycosyltransferases. To determine the function of urdM and urdGT2, targeted gene inactivation experiments were performed. The resulting urdM deletion mutant strains accumulated predominantly rabelomycin, indicating that UrdM is involved in oxygenation at position 12b of urdamycin A. A mutant in which urdGT2 had been deleted produced urdamycin I, urdamycin J and urdamycin K instead of urdamycin A. Urdamycins I, J and K are tetracyclic angucyclinones lacking a C-C connected deoxysugar moiety. Therefore UrdGT2 must catalyse the earliest glycosyltransfer step in the urdamycin biosynthetic pathway, the C-glycosyltransfer of one NDP-D-olivose.

Characterization of two glycosyltransferases involved in early glycosylation steps during biosynthesis of the antitumor polyketide mithramycin by Streptomyces argillaceus

A 2,580-bp region of the chromosome of Streptomyces argillaceus, the producer of the antitumor polyketide mithramycin, was sequenced. Analysis of the nucleotide sequence revealed the presence of two genes (mtmGIII and mtmGIV) encoding proteins that showed a high degree of similarity to glycosyltransferases involved in the biosynthesis of various antibiotics and antitumor drugs. Independent insertional inactivation of both genes produced mutants that did not synthesize mithramycin but accumulated several mithramycin intermediates. Both mutants accumulated premithramycinone, a non-glycosylated intermediate in mithramycin biosynthesis. The mutant affected in the mtmGIII gene also accumulated premithramycin A1, which contains premithramycinone as the aglycon unit and a D-olivose attached at C-12a-O. These experiments demonstrate that the glycosyltransferases MtmGIV and MtmGIII catalyze the first two glycosylation steps in mithramycin biosynthesis. A model is proposed for the glycosylation steps in mithramycin biosynthesis.

Analysis of two chromosomal regions adjacent to genes for a type II polyketide synthase involved in the biosynthesis of the antitumor polyketide mithramycin in Streptomyces argillaceus

Mithramycin is an aromatic antitumour polyketide synthesized by Streptomyces argillaceus. Two chromosomal regions located upstream and downstream of the locus for the mithramycin type II polyketide synthase were cloned and sequenced. Analysis of the sequence revealed the presence of eight genes encoding three oxygenases (mtmOI, mtmOII and mtmOIII), three reductases (mtmTI, mtmTII and mtmTIII), a cyclase (mtm Y) and an acyl CoA ligase (mtmL). The three oxygenase genes were each inactivated by gene replacement. Inactivation of one of them (mtmOII) generated a non-producing mutant, while inactivation of the other two (mtmOl and mtmOIII) did not affect the biosynthesis of mithramycin. The mtmOII gene may code for an oxygenase responsible for the introduction of oxygen atoms at early steps in the biosynthesis of mithramycin leading to 4-demethylpremithramycinone. One of the reductases may be responsible for reductive cleavage of an intermediate from an enzyme and another for the reduction of a keto group in the side-chain of the mithramycin aglycon moiety. A hypothetical biosynthetic pathway showing in particular the involvement of oxygenase MtmOII and of various other gene products in mithramycin biosynthesis is proposed.

Oxidative cleavage of premithramycin B is one of the last steps in the biosynthesis of the antitumor drug mithramycin

BACKGROUND

Mithramycin is a member of the clinically important aureolic acid group of antitumor drugs that interact with GC-rich regions of DNA nonintercalatively. These drugs contain a chromophore aglycon that is derived from condensation of ten acetate units (catalyzed by a type II polyketide synthase). The aglycones are glycosylated at two positions with different chain length deoxyoligosaccharides, which are essential for the antitumor activity. During the early stages of mithramycin biosynthesis, tetracyclic intermediates of the tetracycline-type occur, which must be converted at later stages into the tricyclic glycosylated molecule, presumably through oxidative breakage of the fourth ring.

RESULTS

Two intermediates in the mithramycin biosynthetic pathway, 4-demethyl-premithramycinone and premithramycin B, were identified in a mutant lacking the mithramycin glycosyltransferase and methyltransferase genes and in the same mutant complemented with the deleted genes, respectively. Premithramycin B contains five deoxysugars moieties (like mithramycin), but contains a tetracyclic aglycon moiety instead of a tricyclic aglycon. We hypothesized that transcription of mtmOIV (encoding an oxygenase) was impaired in this strain, preventing oxidative breakage of the fourth ring of premithramycin B. Inactivating mtmOIV generated a mithramycin nonproducing mutant that accumulated premithramycin B instead of mithramycin. In vitro assays demonstrated that MtmOIV converted premithramycin B into a tricyclic compound.

CONCLUSIONS

In the late stages of mithramycin biosynthesis by Strepyomyces argillaceus, a fully glycosylated tetracyclic tetracycline-like intermediate (premithramycin B) is converted into a tricyclic compound by the oxygenase MtmOIV. This oxygenase inserts an oxygen (Baeyer-Villiger oxidation) and opens the resulting lactone. The following decarboxylation and ketoreduction steps lead to mithramycin. Opening of the fourth ring represents one of the last steps in mithramycin biosynthesis.

The structure of mithramycin reinvestigated

A reinvestigation of the structure of mithramycin, the principal product of Streptomyces argillaceus ATCC 12956, is reported. The structure elucidation was carried out with mithramycin decaacetate (4) using 2D NMR methods, including TOCSY, HMBC, and HSQC experiments. The work resulted in structure 3being confirmed for mithramycin.