Tirapazamine: a hypoxia-activated topoisomerase II poison

Area of Science:

• Molecular Biology
• Cancer Research
• Biochemistry

Background:

• Tirapazamine (TPZ) is a hypoxia-selective cytotoxin with demonstrated anti-cancer activity.
• TPZ generates DNA damage, including double-strand breaks (DSBs), under hypoxic conditions.
• Previous findings suggested a protein association with TPZ-induced DSBs, prompting investigation into topoisomerase II (topo II) involvement.

Purpose of the Study:

• To investigate the role of topoisomerase II (topo II) in the DNA damage and cytotoxicity induced by Tirapazamine (TPZ).
• To determine if TPZ functions as a topoisomerase II poison.

Main Methods:

• Assessed topo II activity in nuclear extracts from human lung cancer cells treated with TPZ or etoposide under hypoxia.
• Utilized catalytic inhibitors of topo II (merbarone, aclarubicin) to examine their effect on TPZ- and etoposide-induced DNA damage and cell kill.
• Investigated TPZ-induced DNA damage in a small cell lung cancer cell line with low topo IIalpha levels.
• Examined the covalent binding of topo IIalpha to DNA and the stability of cleavable complexes after TPZ treatment.

Main Results:

• TPZ treatment under hypoxia significantly reduced nuclear topo II activity.
• Pretreatment with topo II catalytic inhibitors abrogated both DNA DSBs and cell kill induced by TPZ or etoposide.
• TPZ- and etoposide-mediated DSBs were reduced in cells with low topo IIalpha levels.
• TPZ induced covalent binding of topo IIalpha to DNA, forming stable cleavable complexes.

Conclusions:

• Tirapazamine (TPZ) exerts its cytotoxic effect, at least in part, by poisoning topoisomerase II (topo II).
• TPZ acts as a tumor-specific topo II poison due to its activation under hypoxic conditions characteristic of solid tumors.