Abstract
Mammalian target of rapamycin (mTOR) inhibitors represent the standard of care for metastatic renal cell carcinoma (RCC). However, treatment outcomes are relatively poor, suggesting a potential problem with tolerating mTOR inhibitors. The aim of this study was to establish everolimus-resistant sublines and to compare their molecular characteristics with those of their counterparts. Human-derived RCC, Caki-2, and 786-O cells were continuously exposed to everolimus at 1 μM, and the established resistant sublines were designated as Caki/EV and 786/EV, respectively. Cellular characteristics were compared between both cells. Caki/EV and 786/EV cells showed a decrease in sensitivity to everolimus as well as other mTOR inhibitors. Expression of mTOR and its effectors exhibited no alteration in resistant sublines and their counterparts. However, phosphorylation of S6K, an index of mTOR activity, decreased in resistant sublines. PCR array analysis of mTOR signaling pathway-related factors indicated that the expression of INSR, TP53, and IGFBP3 increased in Caki/EV cells, whereas that of TELO2, HRAS, and SGK1 was up-regulated in 786/EV cells. The levels of DDIT4, DEPTOR, HIF1A, and PLD1 mRNAs decreased in both cell lines. The novel everolimus-resistant Caki/EV and 786/EV cells exhibited cross-resistance to other mTOR inhibitors and decreased mTOR activity. Furthermore, down-regulation of DDIT4, DEPTOR, HIF1A, and PLD1 may contribute to everolimus resistance.
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