Abstract
PURPOSE: This study was to investigate whether the topoisomerase (Top) I inhibitor topotecan and the Top II inhibitor etoposide could modulate the hypoxia-induced HIF-1alpha expression in non-small cell lung cancer (NSCLC) cell lines. METHODS: Hypoxic conditions were maintained in a humidified airtight anaerobic incubator flushed with a mixture of gas consisting of 1% O(2), 5% CO(2) and 94% N(2). The expressions of HIF-1alpha and Akt phosphorylation were measured by Western blotting or quantitative reverse transcription-polymerase chain reaction. Small interfering RNA treatment was done to inhibit the expressions of Top I and IIalpha. Constitutively active akt was expressed by transient transfection using pUSEamp(+)/myr Akt. RESULTS: The HIF-1alpha was increased and this peaked at 9 h in hypoxic conditions. Both topotecan and etoposide in a dose- and time-dependent manner inhibited the accumulation of hypoxia-induced HIF-1alpha protein. Interestingly, the daily addition of these drugs at a lower concentration could inhibit the HIF-1alpha expression more effectively than a single treatment, which shows that their effects are schedule-dependent. This down-regulation of HIF-1alpha was associated with proteosomal degradation and decreased Akt phosphorylation. Top I and Top IIalpha were required for the inhibitory effect of topotecan and etoposide, respectively. CONCLUSION: Both Top I and II inhibitors could suppress the HIF-1alpha expression in a schedule-dependent manner, and this suggests that these drugs might be useful to overcome the therapeutic resistance induced by tumor hypoxia in NSCLC.