Metastatic renal cell carcinomas (mRCC) are highly vascularized tumors that are a paradigm for the treatment with antiangiogenesis drugs targeting the vascular endothelial growth factor (VEGF) pathway. The available drugs increase the time to progression but are not curative and the patients eventually relapse. In this study we have focused our attention on the molecular mechanisms leading to resistance to sunitinib, the first line treatment of mRCC. Because of the anarchic vascularization of tumors the core of mRCC tumors receives only suboptimal concentrations of the drug. To mimic this in vivo situation, which is encountered in a neoadjuvant setting, we exposed sunitinib-sensitive mRCC cells to concentrations of sunitinib below the concentration of the drug that gives 50% inhibition of cell proliferation (IC50). At these concentrations, sunitinib accumulated in lysosomes, which downregulated the activity of the lysosomal protease CTSB (cathepsin B) and led to incomplete autophagic flux. Amino acid deprivation initiates autophagy enhanced sunitinib resistance through the amplification of autolysosome formation. Sunitinib stimulated the expression of ABCB1 (ATP-binding cassette, sub-family B [MDR/TAP], member 1), which participates in the accumulation of the drug in autolysosomes and favor its cellular efflux. Inhibition of this transporter by elacridar or the permeabilization of lysosome membranes with Leu-Leu-O-methyl (LLOM) resensitized mRCC cells that were resistant to concentrations of sunitinib superior to the IC50. Proteasome inhibitors also induced the death of resistant cells suggesting that the ubiquitin-proteasome system compensates inhibition of autophagy to maintain a cellular homeostasis. Based on our results we propose a new therapeutic approach combining sunitinib with molecules that prevent lysosomal accumulation or inhibit the proteasome.
Keywords: Leu-Leu-O-Methyl; angiogenesis; elacridar; lysosome; proteasome inhibitors; renal cell carcinoma; resistance; sunitinib.