Inhibition of RARα2 and its downstream signaling pathways decreases drug resistance induced by overexpression of RARα2. (A-B) Western blots show the expression of Bcl-2, Bcl-xl, and Mcl-1 in RARα2-OE OCI-MY5 and ARP1 cells (A) and in RARα2-shRNA ARK1 and KMS11 cells (B). (C) Flow cytometry shows the effect of COX-2 inhibitor, CAY10404, in cell membrane pump efflux induced by overexpression of RARα2 in OCI-MY5 and ARP1 cells. (D) The ABC transporter inhibitor MK-571 blocks RARα2-induced drug resistance in ARP1 cells. Clonogenic assay shows the effect of bortezomib or combination of MK571 and bortezomib on the clonogenic formation in RARα2-overexpressing ARP1 cells compared with the control (magnification ×40).