Multidrug resistance mediated by ABCB1 P-glycoprotein and related ATP-binding cassette (ABC) transporters is an impediment to successful cancer therapy. ABCB5 is a novel human P-glycoprotein, which has recently been demonstrated to function as a rhodamine efflux transporter and has been shown to regulate cell fusion in physiological skin progenitor cells as a determinant of membrane potential. Here, we have examined the potential role of ABCB5 as a chemotherapeutic drug efflux transporter and mediator of chemoresistance in human malignant melanoma. Using ABCB5 gene-specific primers and RT-PCR and ABCB5-specific mAb and immunohistochemistry, we found ABCB5 to be expressed in all of a series of five clinical human patient-derived melanomas, including both primary tumors and metastases. Indirect surface immunostaining and flow cytometry analysis of cultured human G3361 melanoma cells revealed ABCB5 to be preferentially expressed on a subset of hyperpolarized, CD133+ progenitor phenotype-expressing cancer cells among heterogeneous tumor cultures. ABCB5 blockade by means of ABCB5-specific mAb significantly reversed doxorubicin resistance of G3361 melanoma cells, resulting in a 43% reduction in the LD50 from 4 mcM to 2.3 mcM doxorubicin (P<0.05). Our results identified ABCB5-mediated doxorubicin efflux transport as the underlying mechanism of resistance, since ABCB5 blockade significantly enhanced intracellular drug accumulation in the ABCB5-expressing tumor subpopulation (P<0.05). Consistent with this novel ABCB5 function and mechanism in doxorubicin resistance, gene expression levels of the transporter across a panel of human cancer cell lines used by the National Cancer Institute for drug screening (NCI-60) determined by real time PCR correlated significantly with tumor resistance to doxorubicin (r=0.44, p=0.016). Our results define a novel role of ABCB5 as a drug resistance transporter in malignant melanoma cells. Moreover, our findings show that these chemoresistant tumor cells among cancer bulk populations can be specifically targeted via ABCB5 to enhance cytotoxic efficacy.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]