👤 MyatNoeZin Myint

Aberrant activation of the PI3K pathway is one of the commonest oncogenic events in human cancer. AKT is a key mediator of PI3K oncogenic function, and as such, it has been intensively pursued as a therapeutic target. Despite the high frequency of AKT activation in... Show less

To overcome drug resistance and reduce the side effects of cisplatin, a widely used antineoplastic agent, major efforts have been made to develop next generation platinum-based anticancer drugs. Because cisplatin-DNA adducts block RNA polymerase II unless removed by transcription-coupled excision repair, compounds that react similarly but elude repair are desirable. The monofunctional platinum agent pyriplatin displays antitumor activity in mice, a cytotoxicity profile in cell cultures distinct from that of cisplatin, and a unique in vitro transcription inhibition mechanism. In this study, we incorporated pyriplatin globally or site specifically into luciferase reporter vectors to examine its transcription inhibition profiles in live mammalian cells. Monofunctional pyriplatin reacted with plasmid DNA as efficiently as bifunctional cisplatin and inhibited transcription as strongly as cisplatin in various mammalian cells. Using repair-defective nucleotide excision repair (NER)-, mismatch repair-, and single-strand break repair-deficient cells, we show that NER is mainly responsible for removal of pyriplatin-DNA adducts. These findings reveal that the mechanism by which pyriplatin generates its antitumor activity is very similar to that of cisplatin, despite the chemically different nature of their DNA adducts, further supporting a role for monofunctional platinum anticancer agents in human cancer therapy. This information also provides support for the validity of the proposed mechanism of action of cisplatin and provides a rational basis for the design of more potent platinum anticancer drug candidates using a monofunctional DNA-damaging strategy. Show less

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. Show less

We have investigated the processing of site-specific Pt-DNA cross-links in live mammalian cells to enhance our understanding of the mechanism of action of platinum-based anticancer drugs. The activity of platinum drugs against cancer is mediated by a combination of processes including cell entry, drug activation, DNA-binding, and transcription inhibition. These drugs bind nuclear DNA to form Pt-DNA cross-links, which arrest key cellular functions, including transcription, and trigger a variety of responses, such as repair. Mechanistic investigations into the processing of specific Pt-DNA cross-links are critical for understanding the effects of platinum-DNA damage, but conventional in vitro techniques do not adequately account for the complex and intricate environment within a live cell. With this limitation in mind, we developed a strategy to study platinum cross-links on plasmid DNAs transfected into live mammalian cells based on luciferase reporter vectors containing defined platinum-DNA lesions that are either globally or site-specifically incorporated. Using cells with either competent or deficient nucleotide excision repair systems, we demonstrate that Pt-DNA cross-links impede transcription by blocking passage of the RNA polymerase complex and that nucleotide excision repair can remove the block and restore transcription. Results are presented for approximately 3800-base pair plasmids that are either globally platinated or carry a single 1,2-d(GpG) or 1,3-d(GpTpG) intrastrand cross-link formed by either cis-{Pt(NH(3))(2)}(2+) or cis-{Pt(R,R-dach)}(2+), where {Pt(NH(3))(2)}(2+) is the platinum unit conveyed by cisplatin and carboplatin and R,R-dach is the oxaliplatin ligand, R,R-1,2-diaminocyclohexane. Show less