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Mechanism of melphalan crosslink enhancement by misonidazole pretreatment
Abstract:
Sensitization of Chinese hamster ovary cells to melphalan (L-PAM) toxicity by prior treatment with misonidazole (MISO, 5 mM, 2 hr, hypoxic conditions, 37 degrees C) is associated with increased levels of DNA crosslinks believed to be the critical lesion for bifunctional alkylating agent toxicity. Enhanced L-PAM crosslinking of DNA could occur by a variety of mechanisms in MISO-pretreated cells including: (1) increased transport or binding of L-PAM, (2) decreased repair of L-PAM monoadducts which would allow more time for their conversion to crosslinks, (3) decreased crosslink repair (unhooking of one arm), or (4) chemical modification of the DNA structure, presumably by bound MISO derivatives, such that crosslink formation is facilitated. Previous studies have eliminated mechanisms (1) and (3). Mechanism (4) was investigated by following MISO-pretreatments of whole cells with L-PAM treatments of the isolated DNA from these cells. This was accomplished by using a modification of the alkaline elution assay for DNA crosslink measurement in which a 1 hr treatment with L-PAM (0-12 micrograms/ml) was inserted between the cell lysis steps and DNA elution procedure. Treatment of bare DNA with L-PAM modeled very well the crosslinking behavior in whole cells although it was somewhat more efficient (more crosslinks at a given L-PAM dose). In the presence of double stranded DNA and absence of repair systems during and after the L-PAM exposure, it was determined that MISO-pretreatments did not increase the crosslinking efficiency of L-PAM (mechanism [4] above). Inhibition of repair of L-PAM monoadducts (mechanism [2] above) still remains as a possible means for crosslink enhancement by MISO-pretreatment.
Insights
Misonidazole (MISO) pretreatment enhances melphalan (L-PAM) DNA crosslinking in cells. This study found MISO does not directly alter DNA to increase L-PAM crosslinking, suggesting repair inhibition is the likely mechanism.
Area of Science:
- Molecular Biology
- Cancer Research
- Drug Development
Background:
- Melphalan (L-PAM) is a bifunctional alkylating agent used in cancer therapy.
- L-PAM toxicity is linked to DNA crosslinks, which are critical lesions.
- Misonidazole (MISO) sensitizes cells to L-PAM, increasing DNA crosslink levels.
Purpose of the Study:
- To investigate the mechanisms by which MISO pretreatment enhances L-PAM-induced DNA crosslinking.
- To determine if MISO directly modifies DNA structure to facilitate L-PAM crosslinking.
- To evaluate the role of DNA repair inhibition in MISO-mediated sensitization.
Main Methods:
- Chinese hamster ovary cells were pretreated with MISO under hypoxic conditions.
- L-PAM was administered to whole cells or to isolated DNA.
- DNA crosslinking was measured using a modified alkaline elution assay.
Main Results:
- MISO pretreatment did not increase L-PAM crosslinking efficiency when applied to isolated DNA, ruling out direct DNA modification by MISO.
- L-PAM treatment of isolated DNA mimicked whole-cell crosslinking behavior, though with higher efficiency.
- The study eliminated mechanisms involving increased L-PAM transport/binding and decreased crosslink repair.
Conclusions:
- MISO pretreatment does not directly alter DNA to enhance L-PAM crosslinking.
- Inhibition of L-PAM monoadduct repair remains a plausible mechanism for MISO-induced sensitization.
- Further research is needed to elucidate the precise role of DNA repair pathways.