Background: Inhibitors of the mammalian target of rapamycin (mTOR) might become a novel tool to treat advanced prostate cancer. However, chronic drug exposure may trigger resistance, limiting the utility of mTOR inhibitors.
Methods: Metastatic potential of PC3 prostate cancer cells, susceptible (PC3(par)) or resistant (PC3(res)) to the mTOR-inhibitor RAD001 was investigated. Adhesion to vascular endothelium or immobilised collagen, fibronectin and laminin was quantified. Motility, migration and invasion were explored by modified Boyden chamber assay. Integrin α and β subtypes were analysed by flow cytometry, western blotting and real-time PCR. Integrin-related signalling, EGFr, Akt, p70S6kinase and ERK1/2 activation were determined.
Results: Adhesion was reduced, whereas motility, migration and invasion were enhanced in PC3(res). The α2 and β1 integrin subtypes were dramatically elevated, integrins α1 and α6 were lowered, whereas α5 was nearly lost in PC3(res). Activation of the Akt signalling pathway was strongly upregulated in these cells. Treating PC3(par) cells with RAD001 reduced motility, migration and invasion and deactivated Akt signalling. Blocking studies revealed that α2 and β1 integrins significantly trigger the motile behaviour of the tumour cells.
Conclusion: Chronic RAD001 treatment caused resistance development characterised by distinct modification of the integrin-expression profile, driving prostate cancer cells towards high motility.