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Cyclooxygenase-2 Inhibitors Reverse Chemoresistance Phenotype in Medullary Thyroid Carcinoma by a Permeability Glycoprotein-Mediated Mechanism

Section of Endocrinology, Department of Biomedical Sciences and Advanced Therapies, University of Ferrara, Via Savonarola 9, 44100 Ferrara, Italy

Address all correspondence and requests for reprints to: Prof. Ettore C. degli Uberti, Section of Endocrinology, Department of Biomedical Sciences and Advanced Therapies, University of Ferrara, Via Savonarola 9, 44100 Ferrara, Italy. E-mail: ti8{at}unife.it.

Objective: Medullary thyroid carcinoma (MTC) is a highly chemoresistant malignant neoplasia deriving from parafollicular C cells. Chemotherapy failure has been ascribed, at least in part, to the overexpression by MTC of the multidrug resistance 1 (MDR1) gene, encoding a transmembrane glycoprotein [permeability glycoprotein (P-gp)] that antagonizes intracellular accumulation of cytotoxic agents. P-gp expression and function in a rat model have been demonstrated to depend on cyclooxygenase (COX)-2 isoform levels, which are found elevated in many human cancers. The aim of our study was to investigate the role of the COX-2 pathway in modulating chemoresistance.

Design and Results: We investigated P-gp and COX-2 expression and then evaluated the sensitizing effects of COX-2 inhibitors on the cytotoxic effects of doxorubicin in the presence or in the absence of prostaglandin E₂ in primary cultures and in a human MTC cell line, TT. Moreover, P-gp function has been studied. Our data show that TT cells express both MDR1 and COX-2 and that rofecoxib, a selective COX-2 inhibitor, sensitizes TT cells to the cytotoxic effects of doxorubicin, reducing P-gp expression and function.

Conclusions: Our data suggest that these effects are mediated by a mechanism not involving the generation of prostaglandin E₂, possibly implicating the synthesis of other COX-2 products.

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