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Multidrug resistance (MDR) is a major cause leading to chemotherapy failure. Recent studies indicate that drug resistance can be rapidly induced by some soluble factors, such as cytokines, chemokines, growth factors and cell adhesion factors in tumor microenvironment. Osteopontin (OPN), an extracellular matrix protein, has a functional RGD domain for binding to integrin. Here we found that OPN expression was up-regulated by hypoxic condition in PC-3 prostate tumor cells. OPN increased the mRNA and protein expression of p-glycoprotein (P-gp), a subfamily of ATP-binding cassette transporter (ABC transporter), in a concentrationand time-dependent manner. The increase of P-gp transporter by OPN was mediated by binding to αvβ3 integrin. Daunomycin (DUN), a chemotherapeutic agent with autofluorescence, was used to evaluate the pump activity and OPN was found to increase the drug pumping-out activity. OPN inhibited DUN-induced cell death, which was antagonized by αvβ3 monoclonal antibody. Long-term treatment with DUN further enhanced the expression of OPN. Knockdown of endogenous OPN potentiated the DUN-induced apoptosis of PC-3 cells. Furthermore, knockdown of OPN also enhanced the cell death caused by other drugs, including paclitaxel, doxorubicin, actinomycin-D and rapamycin, which are also the P-gp substrate. The animal studies also showed that OPN knockdown enhanced the cytotoxic action of DUN. These results indicate that This article has not been copyedited and formatted. The final version may differ from this version. Molecular Pharmacology Fast Forward. Published on February 22, 2013 as DOI: 10.1124/mol.112.082339 at A PE T Jornals on M ay 9, 2017 m oharm .aspeurnals.org D ow nladed from