Novobiocin inhibition of DNA excision repair may occur through effects on mitochondrial structure and ATP metabolism, not on repair topoisomerases

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Abstract
Novobiocin inhibits DNA topoisomerases. It also inhibits excision repair of DNA photodamage, blocking both repair synthesis and the earlier step of incision at u.v. damage sites (as measured by the accumulation of DNA strand breaks in u.v.-irradiated interphase cells treated with DNA synthesis inhibitors such as hydroxyurea or cytosine arabinoside). It has been supposed, therefore, that novobiocin affects repair by blocking a putative topoisomerase step prior to incision. But we find that novobiocin also has a marked dose- and time-dependent effect on mitochondria: in cells exposed to novobiocin, mitochondria swell and their cristae become disrupted, and the intracellular ATP:ADP ratio is lowered, though the membrane potential is maintained as judged by rhodamine 123 fluorescence. Mitotic cells are more resistant to mitochondrial disruption by novobiocin than are interphase cells. This correlates with a relative resistance of u.v.-irradiated mitotic cells to the inhibition of incision by novobiocin. The chromosomal decondensation that results from the accumulation of DNA breaks due to incision when u.v.-irradiated mitotic cells are treated with hydroxyurea and cytosine arabinoside is largely suppressed by novobiocin. Furthermore, the suppression of induced strand break accumulation is partly due to a suppression by novobiocin of the uptake and phosphorylation of cytosine arabinoside; breaks accumulated in u.v.-irradiated cells in the presence of aphidicolin, an inhibitor of DNA polymerase alpha that does not require phosphorylation, are less novobiocin-sensitive. We conclude that the effects of novobiocin on excision repair are more likely to be due to a non-specific effect on ATP metabolism than to a specific effect on a repair-related topoisomerase.

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