Cancer-type organic anion transporting polypeptide 1B3 (Ct-OATP1B3) is localized in lysosomes and mediates resistance against kinase inhibitors

Cancer-type organic anion transporting polypeptide 1B3 (Ct-OATP1B3), a splice variant of the hepatic uptake transporter OATP1B3 (liver-type; Lt-OATP1B3), is expressed in several tumor entities including colorectal carcinoma (CRC) and breast cancer. In CRC, high OATP1B3 expression has been associated with reduced progression-free and overall survival. Several kinase inhibitors used for antitumor treatment are substrates and/or inhibitors of OATP1B3 (e.g. encorafenib, vemurafenib). The functional importance of Ct-OATP1B3 has not been elucidated so far. HEK293 cells stably overexpressing Ct-OATP1B3 protein were established and compared with control cells. Confocal laser scanning microscopy, immunoblot, and proteomics-based expression analysis demonstrated that Ct-OATP1B3 protein is intracellularly localized in lysosomes of stably-transfetced cells. Cytotoxicity experiments showed that cells recombinantly expressing the Ct-OATP1B3 protein were more resistant against the kinase inhibitor encorafenib compared to control cells [e.g. encorafenib (100 µM) survival rates: 89.5% vs. 52.8%]. In line with these findings, colorectal cancer DLD1 cells endogenously expressing Ct-OATP1B3 protein had poorer survival rates when the OATP1B3 substrate bromosulfophthalein (BSP) was coincubated with encorafenib or vemurafenib compared to the incubation with the kinase inhibitor alone. This indicates a competitive inhibition of Ct-OATP1B3-mediated uptake into lysosomes by BSP. Accordingly, mass spectrometry-based drug analysis of lysosomes showed a reduced lysosomal accumulation of encorafenib in DLD1 cells additionally exposed to BSP. These results demonstrate that Ct-OATP1B3 protein is localized in the lysosomal membrane and can mediate transport of certain kinase inhibitors into lysosomes revealing a new mechanism of resistance. Significance Statement We describe the characterization of a splice variant of the liver-type uptake transporter OATP1B3 expressed in several tumor entities. This variant is localized in lysosomes mediating resistance against kinase inhibitors which are substrates of this transport protein by transporting them into lysosomes and thereby reducing the cytoplasmic concentration of these antitumor agents. Therefore, the expression of the Ct-OATP1B3 protein is associated with a better survival of cells revealing a new mechanism of drug resistance.