RA blocks cell proliferation and colony formation in both drug-sensitive and drug-resistant human OS cells. (A) The human OS HOS, KHOSR, U2OS, U2OSR and MG63 cell lines were treated with vehicle (0.1% DMSO) or the indicated concentration of RA for 48 h, and cell viability was measured by CCK8 assay. (B) OS cells (HOS and KHOSR) were plated in six-well plates and treated with vehicle or the indicated concentration of RA for 48 h, cells were then maintained in fresh medium for 10-14 days. Colonies were fixed and stained with 0.1% crystal violet. (C) HOS, KHOSR, U2OS, and U2OSR cells were treated with the indicated concentration of RA for 48 h, and the expression of the proliferation marker PCNA was analyzed by immunoblotting. β-actin was used as a loading control. *P<0.05, ** P<0.01 compared with vehicle control.