Dexamethasone treatment of a canine, but not human, tumour cell line increases chemoresistance independent of P-glycoprotein and multidrug resistance-related protein expression.

Glucocorticoids are often used in veterinary cancer patients because of their anti-inflammatory actions, appetite-stimulating effects, ability to decrease nausea and vomiting associated with some chemotherapy agents, and, in some instances, for their cytotoxic actions on susceptible tumour cells. Veterinary oncologists may not consider the possibility that the use of glucocorticoids may adversely affect response to chemotherapy. There is evidence that glucocorticoids can up-regulate the expression of multidrug resistance genes in some tissues. Whether or not glucocorticoid-induced expression of multidrug resistance proteins occurs in tumour cells is not presently known. The purpose of this study was to determine if dexamethasone induces P-glycoprotein (P-gp) or multidrug resistance-related protein 1 (MRP1) in tumour cell lines. A canine osteosarcoma cell line (OS2.4) and a human myeloid leukaemia cell line 60 (HL60) were treated in culture with dexamethasone. The presence of a glucocorticoid receptor was confirmed in both cell lines by reverse-transcriptase polymerase chain reaction. Western blots for P-gp and MRP1 expression were performed on vehicle-treated and dexamethasone-treated cells. Sensitivity towards several chemotherapeutic drugs (cisplatin (cis-diamminedichloroplatinum), doxorubicin, methotrexate and vincristine) was determined by 3-(4,5-dimthylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. While dexamethasone treatment of OS2.4 cells increased the resistance to cisplatin and methotrexate, an increase in P-gp or MRP1 expression was not observed. Dexamethasone-treated HL60 cells did not develop chemoresistance and did not show increased expression of P-gp or MRP1.