Note for: Molecular mechanisms of etoposide

Note for: Molecular mechanisms of etoposide

(doi: 10.17179/excli2015-561)

TopoII - introducing transient breaks in the DNA molecule.

involved in fundamental biological processes such as DNA replication, transcription, DNA repair and chromatin remodeling. A tyrosine in the active site of the enzyme initiates the first transesterification and forms a covalent adduct with the phosphate in the backbone of the DNA molecule. TopoII can perform three kinds of reactions while DNA relaxation is in common to TopoI, catenation/decatenation and knotting/unknotting are TopoII specific.

Etoposide poisons the TopoII cleavage complexes (TopoIIcc) and inhibits the second step of the reaction (i.e. DNA religation). Mammals have two TopoII isoenzymes, TopoIIa and beta that are differently regulated during cell growth.

TopoIIalpha is a proliferation marker and is greatly elevated in tumor cells, beta isoenzyme is present in proliferating as well as postmitotic cells (cell not devide anymore - might help during transcription).

-TopoIIalpha -- cell cycle events such as DNA replication and chromosome segregation.

-TopoIIbeta has been implicated in transcription and is associated with developmental and differentiation programs.

TopoII isoenzymes are targets of etoposide, the relative contributions of TopoIIα and TopoIIbeta to the chemotherapeutic effects have yet to be resolved.

However, it is still unclear whether the two isoenzymes play different roles in tumor-cell killing in response to etoposide or more in general to TopoII based chemotherapy.

TopoIIalpha is not expressed appreciably in quiescent cells, etoposide targeting of TopoIIbeta in differentiated tissues, such as cardiac cells, could account for much of the off-target toxicity of the drug.

contains Alu sequences, corresponding to putative recognition sites of TopoII-mediated DNA cleavage and chromosome scaffold/matrix attachment regions (SAR/MAR).

TopoIIbeta is the isoform responsible for etoposide-induced carcinogenesis.

in cell-culture models, etoposide-induced DNA sequence rearrangements and DSBs are also found to be primarily TopoIIbeta-dependent. By contrast, etoposide cytotoxicity in tumor cells expressing both isoenzymes appears to be mainly TopoIIalpha-dependent.

in order to reduce the risk of treatment-related secondary malignancies. This task can be helped by the recent resolution of the crystal structure of a large fragment of human TopoIIbeta in a ternary complex with DNA and etoposide.

Another important therapeutic issue is the development of etoposide resistance -- This effect arises from the ability of MDM2 protein to bind and target TopoII for degradation.

These observations in yeast reveal a role of the MRN complex and CtIP in the cell resistance to a clinically important group of anticancer drugs.

By analogy with the enzyme that removes TopoI-DNA adducts (tyrosine phosphodiesterase TDP1), it has been renamed tyrosine phosphodiesterase 2 (TDP2) and represents the major 5’-TDP activity in vertebrate cells.

DSBs primarily activate ATM, processing of DSBs by specific endonucleases in S and G2 phases can produce extended single-stranded DNA regions, resulting in ATR activation indicating that both ATM and ATR kinases may be engaged at the same lesion.

As expected from its ability to induce DSBs, etoposide triggers the activation of ATM and of its downstream kinase Chk2. Mutations of the ATM kinase, as in the case of ataxia telangiectasia (AT) patients, result in hypersensitivity to etoposide.

The focal accumulation of DNA repair factors, including MRN complex and γH2AX is a key cytological signature of the DNA damage response. Although these foci have been extensively studied by light microscopy, little is known about their ultrastructure.

electron spectroscopic imaging (LM/eSI) – tool to look at subnucleolar level!

Chromatin organization and chromosome territories may be actually targets of etoposide treatment.

Another connection between etoposide, chromatin organization and gene expression has been recently reported by Huang and coworkers. These authors showed the ability of etoposide and other topoisomerase inhibitors to un-silence gene expression by repressing transcription of cis acting non-coding RNA.

Binding of E2F-4 to etoposide inhibits gene transcription mediated by the heterodimeric E2F-4/DP complexes in the nucleus.

It is known that high etoposide concentrations can trigger caspase-mediated apoptosis which mainly occurs through the cytochrome c/caspase 9 pathway.

etoposide treatment induces the formation of DSBs also in non-replicating cells thus perturbing gene transcription.

Although autophagy can be seen as a cellular pro-survival mechanism, it can also promote non-apoptotic programmed cell death.

Etoposide is a well-known trigger of apoptotic pathways but recent findings suggest its involvement in autophagic pathways as well.

DNA damaging agents such as etoposide may induce an autophagy-associated ATP

surge that protects the cells and contributes to drug resistance.

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