We studied effects of 2-chloro-N(6)-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (Cl-IB-MECA) on apoptosis induction in the K562/Dox cell line, which overexpressed P-glycoprotein (P-gp, ABCB1, MDR1). We found that the K562/Dox cell line was significantly more resistant to Cl-IB-MECA than the maternal cell line K562, which did not express P-gp. Although both cell lines expressed the A3 adenosine receptor (A3AR), cytotoxic effects of Cl-IB-MECA were not prevented by its selective antagonist MRS1523 (3-propyl-6-ethyl-5-[(ethylthio)carbonyl]-2 phenyl-4-propyl-3-pyridine carboxylate). Analysis of cell extracts revealed that the intracellular level of Cl-IB-MECA was significantly lower in the K562/Dox cell line than in the maternal cell line K562. The downregulation of P-gp expression using shRNA targeting ABCB1 gene led to increased intracellular level of Cl-IB-MECA and restored cell sensitivity to this drug. Similarly, valspodar (PSC-833), a specific inhibitor of P-gp, restored sensitivity of the K562/Dox cell line to Cl-IB-MECA with concomitant increase of intracellular level of Cl-IB-MECA in the resistant cell line, while it affected cytotoxicity of Cl-IB-MECA in the sensitive cell line only marginally. An enzyme based assay provided evidence for interaction of P-gp with Cl-IB-MECA. We further observed that cytotoxic effects of Cl-IB-MECA could be augmented by activation of extrinsic cell death pathway by Apo-2L (TRAIL) but not FasL or TNF-α. Our results revealed that Cl-IB-MECA induced an increase in expression of TRAIL receptors in K562 cells, which could sensitize cells to apoptosis induction via an extrinsic cell death pathway. Importantly, these effects were inversely related to P-gp expression. In addition, MRS1523 did not affect Cl-IB-MECA induced expression of TRAIL receptors.

Department of Biology Faculty of Medicine and Dentistry Palacky University Olomouc Olomouc Czech Republic

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