Inhibitors of the
mammalian target of rapamycin (mTOR) have improved the treatment of
renal cell carcinoma (RCC). However, chronic
drug exposure may trigger resistance, limiting the utility of these agents. The metastatic behavior of RCC cells, susceptible (RCC(par)) or resistant (RCC(res)) to the mTOR inhibitor
temsirolimus, was investigated. Adhesion to vascular endothelium or immobilized
collagen and
fibronectin was quantified. Chemotactic motility was evaluated with a modified Boyden chamber assay.
Integrin α and β subtype receptors were analyzed by flow cytometry and Western blot analysis. The physiological relevance of the
integrins was then determined by blocking studies and
small interfering RNA knockdown. Adhesion to endothelial cells and to
fibronectin (not to
collagen) and chemotaxis were enhanced in RCC(res) compared to RCC(par). RCC(res) detached from
fibronectin and motile activity further increased under
retreatment with low-dosed
temsirolimus. α5
integrin was diminished inside the cell and at the cell surface, whereas the β3 subtype was reduced intracellularly but elevated at the plasma membrane. In RCC(par), blocking α5 surface receptors enhanced RCC-
collagen but reduced RCC-
fibronectin interaction, whereas the opposite was true for RCC(res). Chemotaxis of RCC(par) but not of RCC(res) was strongly diminished by the α5 antibody. Blocking β3 significantly lowered chemotaxis with stronger effects on RCC(res), compared to RCC(par). Importantly, β3 knockdown reduced chemotaxis of RCC(par) but upregulated the motile behavior of RCC(res).
Temsirolimus resistance is characterized by quantitative alterations of
integrin α5 and β3 expression, coupled to functional changes of the
integrin molecules, and forces a switch from RCC adhesion to RCC migration.