Picropodophyllin inhibits the growth of pemetrexed-resistant malignant pleural mesothelioma via microtubule inhibition and IGF-1R-, caspase-independent pathways

BACKGROUND: Acquired pemetrexed resistance leads to treatment interruption in patients with malignant pleural mesothelioma (MPM). Insulin-like growth factor-I receptor (IGF-1R) inhibitor is a candidate for treating pemetrexed-naïve MPM. However, the efficacy of cytotoxic and targeted drugs in acquired-pemetrexed resistant MPM is unclear. We explored anticancer drugs, including the IGF-1R inhibitor picropodophyllin, against acquired pemetrexed-resistant MPMs. METHODS: Acquired pemetrexed-resistant human MPM cell lines, named as H2452/PEM and 211H/PEM after their parental lines H2452 and 211H, respectively, were established by exposure to pemetrexed in vitro. Picropodophyllin and siRNA for IGF-1R knockdown were used to evaluate the efficacy of IGF-1R inhibition. Immunofluorescence was used to evaluate microtubule localization. The efficacy of picropodophyllin was evaluated in 3-dimensional MPM models. RESULTS: The acquired pemetrexed-resistant MPM lines retained their resistance after the removal of culture treatment. IGF-1R levels in H2452/PEM cells were higher than those in H2452 cells but not in 211H/PEM cells compared to the respective parental line. Picropodophyllin induced sub-G1 arrest in H2452/PEM cells but induced G2/M phase arrest in 211H/PEM cells, leading to caspase-independent cell death in the two acquired pemetrexed-resistant MPM lines. Although picropodophyllin inhibited phosphorylation of IGF-1R, specific inhibition of IGF-1R by RNA interference did not reduce the viability of pemetrexed-resistant MPM lines. Additionally, picropodophyllin reduced the viability of both IGF-1R knockdown pemetrexed-resistant MPM cells. Picropodophyllin was cytotoxic in acquired-pemetrexed-resistant MPM lines because of inhibition of microtubule formation and induction of aberrant mitosis. Moreover, combination treatment with picropodophyllin and vinorelbine synergistically affected the pemetrexed-resistant MPM lines but not the parental lines. Furthermore, we observed a similar efficacy of picropodophyllin in 3-dimensional pemetrexed-resistant MPM models. CONCLUSIONS: Picropodophyllin may offer novel therapeutic properties for treating acquired pemetrexed-resistant MPM. Targeting tubulin may be an important strategy in the treatment of MPM after the discontinuation of pemetrexed.