Mechanisms of chemoresistance in CD133-expressing GBM cells: the role drug efflux transporters

Glioblastoma Multiforme (GBM) is the most common and lethal adult primary tumor of the central nervous system. Despite current chemotherapy with concomitant surgical resection and radiotherapy, GBM 5-year survival is ~3%. A better understanding of the cellular pathways in tumor proliferation and survival is required for the development of new targeted treatments. Although debatable, CD133 (Prominin-1) has been used as a cell surface marker to enrich for cells exhibiting a primitive phenotype. We have isolated CD133-expressing cells from U87 and T98G GBM cell lines, then and confirmed the dose-dependent chemoresistant properties of these to Temozolomide (TMZ). The mechanisms which underlie this chemoresistance are currently unknown. TMZ is an O6-methylguanyl alkylating agent, resistance to TMZ may arise through a number of methods. We explored cell cycle regulation in CD133-expressing GBM and have confirmed no difference in the cycling status of these. ATP-binding drug efflux pumps can bind to compounds such as TMZ and transport them in to the extracellular space. MDR1 (ABCB1) and BCRP (ABCG2) are two such transporters known to be upregulated in GBM. Gene expression analysis confirms increased expression of both MDR1 and BCRP in CD133 GBM cells. Inhibition of MDR1 and BCRP by Cyclosporin A (CyA) resulted in increased cell death when treated with TMZ. Knockdown by siRNA of both transporters confirmed the importance of these in GBM chemoresistance. Taken together, our data indicates that CD133 GBM resistance to TMZ is not due to cell cycle detention but rather to MDR1/ABCG2 upregulation.