Drug inhibitors of RNA polymerase II transcription

Transcription and Translation Inhibitors in Cancer Treatment

Transcription and translation are fundamental cellular processes that govern the protein production of cells. These processes are generally up regulated in cancer cells, to maintain the enhanced metabolism and proliferative state of these cells. As such cancerous cells can be susceptible to transcription and translation inhibitors. There are numerous druggable proteins involved in transcription and translation which make lucrative targets for cancer drug development. In addition to proteins, recent years have shown that the "undruggable" transcription factors and RNA molecules can also be targeted to hamper the transcription or translation in cancer. In this review, we summarize the properties and function of the transcription and translation inhibitors that have been tested and developed, focusing on the advances of the last 5 years. To complement this, we also discuss some of the recent advances in targeting oncogenes tightly controlling transcription including transcription factors and KRAS. In addition to natural and synthetic compounds, we review DNA and RNA based approaches to develop cancer drugs. Finally, we conclude with the outlook to the future of the development of transcription and translation inhibitors.

Anthracyclines Suppress Both NADPH Oxidase- Dependent and -Independent NETosis in Human Neutrophils

Neutrophil extracellular traps (NETs) are cytotoxic DNA-protein complexes that play positive and negative roles in combating infection, inflammation, organ damage, autoimmunity, sepsis and cancer. However, NETosis regulatory effects of most of the clinically used drugs are not clearly established. Several recent studies highlight the relevance of NETs in promoting both cancer cell death and metastasis. Here, we screened the NETosis regulatory ability of 126 compounds belonging to 39 classes of drugs commonly used for treating cancer, blood cell disorders and other diseases. Our studies show that anthracyclines (e.g., epirubicin, daunorubicin, doxorubicin, and idarubicin) consistently suppress both NADPH oxidase-dependent and -independent types of NETosis in human neutrophils, ex vivo. The intercalating property of anthracycline may be enough to alter the transcription initiation and lead NETosis inhibition. Notably, the inhibitory doses of anthracyclines neither suppress the production of reactive oxygen species that are necessary for antimicrobial functions nor induce apoptotic cell death in neutrophils. Therefore, anthracyclines are a major class of drug that suppresses NETosis. The dexrazoxane, a cardioprotective agent, used for limiting the side effects of anthracyclines, neither affect NETosis nor alter the ability of anthracyclines to suppress NETosis. Hence, at correct doses, anthracyclines together with dexrazoxane could be considered as a therapeutic candidate drug for suppressing unwanted NETosis in NET-related diseases.

Structural basis of transcription inhibition by antibiotic streptolydigin

Streptolydigin (Stl) is a potent inhibitor of bacterial RNA polymerases (RNAPs). The 2.4 A resolution structure of the Thermus thermophilus RNAP-Stl complex showed that, in full agreement with the available genetic data, the inhibitor binding site is located 20 A away from the RNAP active site and encompasses the bridge helix and the trigger loop, two elements that are considered to be crucial for RNAP catalytic center function. Structure-based biochemical experiments revealed additional determinants of Stl binding and demonstrated that Stl does not affect NTP substrate binding, DNA translocation, and phosphodiester bond formation. The RNAP-Stl complex structure, its comparison with the closely related substrate bound eukaryotic transcription elongation complexes, and biochemical analysis suggest an inhibitory mechanism in which Stl stabilizes catalytically inactive (preinsertion) substrate bound transcription intermediate, thereby blocking structural isomerization of RNAP to an active configuration. The results provide a basis for a design of new antibiotics utilizing the Stl-like mechanism.

Human RNA polymerase II is partially blocked by DNA adducts derived from tumorigenic benzo[c]phenanthrene diol epoxides: relating biological consequences to conformational preferences

Environmental polycyclic aromatic hydrocarbons (PAHs) are metabolically activated to diol epoxides that can react with DNA, resulting in covalent modifications to the bases. The (+)- and (-)-3,4-dihydroxy-1,2-epoxy-1,2,3,4-tetrahydro-benzo[c]phenanthrene (anti-BPhDE) isomers are diol epoxide metabolites of the PAH benzo[c]phenanthrene (BPh). These enantiomers readily react with DNA at the N6 position of adenine, forming bulky (+)-1R- or (-)-1S-trans-anti-[BPh]-N6-dA adducts. Transcription-coupled nucleotide excision repair clears such bulky adducts from cellular DNA, presumably in response to RNA polymerase transcription complexes that stall at the bulky lesions. Little is known about the effects of [BPh]-N6-dA lesions on RNA polymerase II, hence, the behavior of human RNA polymerase II was examined at these adducts. A site-specific, stereochemically pure [BPh]-N6-dA adduct was positioned on the transcribed or non-transcribed strand of a DNA template with a suitable promoter for RNA polymerase II located upstream from the lesion. Transcription reactions were then carried out with HeLa nuclear extract. Each [BPh]-dA isomer strongly impeded human RNA polymerase II progression when it was located on the transcribed strand; however, a small but significant degree of lesion bypass occurred, and the extent of polymerase blockage and bypass was dependent on the stereochemistry of the adduct. Molecular modeling of the lesions supports the idea that each adduct can exist in two orientations within the polymerase active site, one that permits nucleotide incorporation and another that blocks the RNA polymerase nucleotide entry channel, thus preventing base incorporation and causing the polymerase to stall or arrest.

Protein synthesis inhibitory activity in culture filtrates from new strains of Streptomyces isolated from Brazilian tropical soils

AIMS

To investigate the effect of the culture supernatants from three newly isolated Streptomyces strains, 221, 235 and 606 on eukaryotic cells.

METHODS AND RESULTS

Cell lines were treated with the culture filtrates and assayed for protein synthesis by metabolic labelling, followed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis analysis. RNA synthesis was investigated by [5-3H]uridine incorporation. The three culture filtrates presented a strong inhibitory activity, reducing total protein synthesis of different eukaryotic cell lines by more than 85%. No effect on cellular RNA synthesis was detected. The culture filtrates did not affect the growth of the prokaryotic cells tested.

CONCLUSIONS

These new Streptomyces strains, recently isolated from Brazilian tropical soils, produce molecule(s) with inhibitory activity specific to eukaryote protein synthesis.

SIGNIFICANCE AND IMPACT OF THE STUDY

Streptomyces strains 221, 235 and 606, probably representing new species, might produce new bioactive compound(s), and can be used as valuable tools to study the protein synthesis pathway in eukaryotes.

Selective alterations in rat cardiac mRNA induced by doxorubicin: possible subcellular mechanisms

Doxorubicin (Adriamycin, ADR) is an effective antineoplastic agent with a major side effect of dilated cardiomyopathy. Previously we showed ADR selectively decreased alpha cardiac (alpha c) actin mRNA in the rat heart when compared to other mRNAs examined in heart and skeletal muscle. The present study determined if this effect was selective for mRNAs within the thin filament, related to inhibitory effects on mitochondrial transcription, and modified by pretreatment with the cardioprotective chelating agent ICRF-187. Adult Sprague-Dawley rats received ADR at 8 mg/kg intraperitoneally (ip) with or without pretreatment with ICRF-187 given at 80 mg/kg ip. After 3 days, rats were killed and myocardial RNA was extracted, electrophoresed, transferred to nitrocellulose, and hybridized with the [32]cDNA probes alpha c actin, troponin C (TnC), BamHI fragment of mouse mitochondria (MM), and glyceraldehyde-3-phosphate dehydrogenase (G3PD). Results showed a major depressive effect of ADR on rat myocardial alpha c actin mRNA. No depression of the other mRNAs examined (TnC, MM, or G3PD) was seen. ICRF-187 did not modify the effect. We conclude that the ADR-induced decrease in alpha c actin mRNA was: (1) selective within the thin filament; (2) not related to inhibitory effects on mitochondrial transcription; and (3) not related to free radical formation. Possible subcellular mechanisms are discussed.

Transcription factor IIA of wheat and human function similarly with plant and animal viral promoters

Eucaryotic transcription initiation by RNA polymerase II involves protein:DNA interactions during the formation of a transcription complex. In addition to RNA polymerase II there are at least five other general transcription factors necessary for initiation with the adenovirus major late promoter. One of these, TFIIA, is involved in the earliest events during transcription complex assembly. We have purified TFIIA from wheat germ and characterized it in an in vitro transcription system. Wheat TFIIA is a single polypeptide of Mr approximately 35 kd which functionally replaces human (HeLa) TFIIA to form a wheat/HeLa transcription system. [This polypeptide can be eluted from a SDS-polyacrylamide gel, refolded to a native conformation, and will function as wheat TFIIA in the heterologous system.] The heterologous system requires a lower optimal incubation temperature than the HeLa system. Biochemical characterization, using the adenovirus major late promoter, indicates that transcription reaction parameters for both wheat and HeLa TFIIA are similar but the kinetics of transcription for both TFIIAs are somewhat dissimilar. A plant viral promoter, the cauliflower mosaic virus 35S promoter, accurately and efficiently directs in vitro transcription in both the wheat/HeLa and HeLa systems with identical transcription kinetics. We conclude that TFIIA function has been conserved during evolution.

Early events of RNA polymerase II transcription initiation

We have investigated the earliest stages of assembly of an RNA polymerase II transcription complex. General transcription factors from HeLa cells were partially purified and assayed using the adenovirus-2 major late promoter. Preincubation of either all the transcription factors (TF) with the DNA or only the subset consisting of TFIIA, TFIID, and DNA overcame the 15-20 min lag normally observed. The kinetics demonstrate that TFIIA first interacts with the template over a 5 min. period, and then TFIID interacts with the IIA:DNA complex over a 2 min. period. The remainder of the necessary transcription factors then interact with the IIA:IID:DNA complex. There are apparently interactions between IIA and IID, as a pre-incubation of these factors (without DNA) overcomes the lag period. Both IIA:DNA and IIA:DNA:IID interactions are temperature sensitive, resulting in slower kinetics at 0 degree C. Thus, the kinetics of transcription involve activation processes in addition to DNA binding.