Up‐regulation of breast cancer resistance protein expression in hepatoblastoma following chemotherapy: A study in patients and in vitro

Aim: Hepatoblastoma (HB), the most common pediatric malignant liver tumor, is treated with chemotherapy to facilitate surgical resection. Previous studies suggest that HB acquires chemoresistance via increased expression of multidrug resistance protein 1 (MDR1, ABCC1). There is no well established evidence that this also occurs in the clinical setting and little is known about the effects of chemotherapeutic treatments on HB in situ. Methods: Clinical and histopathological features and expression patterns of ABC transporters in diagnostic needle biopsies from 7 HBs taken before chemotherapy were compared with those in surgically resected tumors. To understand the mechanism s leading to chemoresistance we also investigated the involvement of hypoxia on protein expression and functional activity of drug transporters (BCRP and MDR1) in cultures of HepG2 human HB cells. Results: We found that chemotherapeutical treatment of HBs led to an increased expression of the breast cancer resistance protein (BCRP, ABCG2) in all patients studied. There was no change in the expression pattern of MDR1 or other ABC transporters. Chemotherapy‐induced specific vascular abnormalities associated with areas of necrosis and fibrosis were seen in all cases, suggesting tumor hypoxia. The observations of increased BCRP expression in hypoxic areas of three‐dimensional HepG2 aggregates and the enhanced BCRP function in monolayer cultures of HepG2 cells under hypoxic conditions, support a role for hypoxia in enhanced BCRP expression. Conclusions: Chemotherapeutical treatment of HB leads to vascular alterations that modify the tumor microenvironment, and increased BCRP expression in which hypoxia might play a role. No evidence was found for upregulation of MDR1 in HBs as suggested from previous experimental studies.