Abstract DNA topoisomerase I (Top1) inhibition by camptothecin derivatives can impair the hypoxia-induced cell transcriptional response. In the present work, we determined molecular aspects of the mechanism of camptothecin's effects on hypoxia-inducible factor-1α (HIF-1α) activity in human cancer cells. In particular, we provide evidence that low concentrations of camptothecin, without interfering with HIF-1α mRNA levels, can reduce HIF-1α protein expression and activity. As luciferase assays demonstrated the involvement of the HIF-1α mRNA 3′ untranslated region in camptothecin-induced impairment of HIF-1α protein regulation, we performed microarray analysis to identify camptothecin-induced modification of microRNAs (miRNA) targeting HIF-1α mRNA under hypoxic-mimetic conditions. The selected miRNAs were then further analyzed, demonstrating a role for miR-17-5p and miR-155 in HIF-1α protein expression after camptothecin treatments. The present findings establish miRNAs as key factors in a molecular pathway connecting Top1 inhibition and human HIF-1α protein regulation and activity, widening the biologic and molecular activity of camptothecin derivatives and the perspective for novel clinical interventions. Mol Cancer Ther; 13(1); 239–48. ©2013 AACR.