RESUMO
The ATP-binding cassette transporter breast cancer resistance protein (BCRP/ABCG2) is supposed to be a major determinant of the multidrug resistance phenotype of tumors by extruding chemically diverse cytostatic drugs out of tumor cells. BCRP physically and possibly also functionally interacts with caveolin-1 (CAV1, encoded by Cav1), an integral membrane protein of lipid rafts important for signal transduction and membrane trafficking. Moreover, Cav1 is linked to an aggressive phenotype of cancer cells in various tumors. We therefore investigated whether Cav1 plays a functional role in the regulation of BCRP transport activity and in the resistance against chemotherapeutics that are BCRP substrates. As a cell model, we used the BCRP overexpressing cell line MDCKII-BCRP and the corresponding parental cell line MDCKII as a control. Cav1 expression was down-regulated using retrovirus-mediated RNA interference technology. BCRP activity was assessed by pheophorbide A efflux assay and the resistance towards cytostatic drugs was measured by proliferation assays. Efficient knockdown of Cav1 reduced Cav1 expression by 85-95% and BCRP activity by 35%. Concurrently, it reduced resistance towards the BCRP substrate mitoxantrone but not towards vincristine, a chemotherapeutic that is not extruded by BCRP. Western blot analysis of gradient ultracentrifugation fractions and immunofluorescence demonstrates that BCRP localization within the plasma membrane was largely unaltered in Cav1-deficient cells compared to controls. The diminished BCRP function after Cav1 knockdown is, thus, likely mediated by alterations in protein-protein interactions and suggests a positive regulation of BCRP function by CAV1.
