Figure 8. Combination of 17-AAG and lestaurtinib further inhibits MAST1 activity and cisplatin-resistant tumor growth.

(A and B) Combination effect of 17-AAG and lestaurtinib on expression and activity of MAST1. KB-3-1^cisR and A549^cisR cells were treated with 17-AAG (100 nM) and lestaurtinib (100 nM) in the presence of sublethal doses of cisplatin for 24 hours. The kinase activity of MAST1 was assessed by phospho-MEK1 S217/S221 level (A) and ADP-Glo Kinase assay (B). Effect of combinatorial treatment with 17-AAG and lestaurtinib on cell viability (C) and cisplatin sensitivity (D). (E) Combination index (CI) plots are shown for diverse cisplatin-resistant cancer cell lines. (F–H) Effect of cisplatin treatment with the combination of 17-AAG and lestaurtinib on tumor growth of lung cancer PDX mice. Mice were administered 17-AAG, lestaurtinib, and cisplatin from 28 days after xenograft. Tumor volume (F) and tumor weight (G) are shown. (H and I) The kinase activity of MAST1 in PDX tumors. Data shown are representative of 3 (A, C–E, and H) and 2 (B and I) independent biological experiments. Data are mean ± SD from 3 technical replicates for B–D and I. Error bars indicate SEM (F) and SD (G) in 5 mice/group. Statistical analysis was performed by 2-way ANOVA for F and 1-way ANOVA for all the rest. *P < 0.05; **P < 0.01; ***P < 0.005. (J) Proposed model for the stability regulation of MAST1 in human cancers. Left: Hsp90B binds to and stabilizes MAST1, which reactivates MEK1 in the absence of cRaf upon cisplatin treatment and promotes cisplatin-resistant tumor growth. Right: Loss of hsp90B induces ubiquitination of MAST1 at K317 and K545 by CHIP, which triggers its degradation. Combinatorial treatment with 17-AAG and lestaurtinib further diminishes MAST1 kinase activity and attenuates cisplatin-resistant tumor growth.