Camptothecin induces degradation of DNA topoisomerase I in fibroblasts. (A) Presence of DNA toposiomerase I in the immunocomplexes described in 1B was confirmed by Western blotting analyses. (B) Fibroblasts described in 1B were treated with 25 μM camptothecin or 25 μM β-lapachone, and DNA topoisomerase I present in the lysates was detected by Western blotting. The same blots were stripped and reprobed with anti-β-tubulin antibodies as a loading control. Relative ratios of DNA topoisomerase I to tubulin levels are indicated. All the data shown here are representative of at least two independent experiments. CPT = camptothecin, Topo I = DNA topoisomerase I, IP = immunoprecipitation, IB = immunoblotting, and β-LP = β-lapachone.