Objective To investigate the regulatory effect of circular RNA ASAP1(circASAP1)targeting microRNA-4500(miR-4500)on the proliferation and apoptosis of osteosarcoma cells.Methods Human osteosarcoma cells(Saos-2)were selected as the experimental subjects and cul-tured in DMEM medium containing 10％fetal bovine serum while maintaining 5％CO2.The expres-sions of circASAP1 and miR-4500 in osteosarcoma tissues were detected using RT-qPCR.Saos-2 cells were transfected with si-NC(40 nmol/L),si-circASAP1(40 nmol/L),pcDNA(0.4 μg),pcDNA-circASAP1(0.4 μg),miR-NC(40 nmol/L),miR-4500 mimics(40 nmol/L),si-circASAP1+anti-miR-NC(40 nmol/L),and si-circASAP1+anti-miR-4500(40 nmol/L),respectively.Cell proliferation and apoptosis were assessed using CCK-8,colony formation assays,and flow cytometry.The interaction between circASAP1 and miR-4500 was analyzed using a dual-luciferase reporter assay.Results Compared with adjacent non-tumor tissues,the expression of circASAP1 was significantly upregulated,while that of miR-4500 was significantly downregulated in osteosarcoma tissues(P＜0.001).Compared with the si-NC group,the si-circASAP1 group exhibited significantly decreased circASAP1 expression,optical density(OD)values,and the number of cell colonies(P＜0.001).Compared with the si-NC group,the si-circASAP1 group showed significantly upregulated levels of cleaved-caspase3 protein,cleaved-caspase9 protein,and the apoptosis rate(P＜0.001).StarBase search revealed specific binding sequences between miR-4500 and circASAP1.Co-transfection of miR-4500 mimics and WT-circASAP1 significantly reduced the relative luciferase activity of the cells[(0.34±0.03)versus(0.95±0.06),t=27.280,P＜0.001].The expression of miR-4500 in the pcDNA-circASAP1 group was significantly lower than that in the pcDNA group,whereas the ex-pression of miR-4500 in the si-circASAP1 group was significantly higher than that in the si-NC group(P＜0.001).Compared with the miR-NC group,the miR-4500 group exhibited significantly in-creased miR-4500 expression,cleaved-caspase3 protein levels,cleaved-caspase9 protein levels,and the apoptosis rate,while the OD values and the number of colonies significantly decreased(P＜0.001).Compared with the si-circASAP1+anti-miR-NC group,the si-circASAP1+anti-miR-4500 group showed significantly decreased miR-4500 expression,cleaved-caspase3 protein levels,cleaved-caspase9 protein levels,and the apoptosis rate,while the OD values and the number of col-onies significantly increased(P＜0.001).Conclusion The expression of circASAP1 is increased,while that of miR-4500 is decreased in osteosarcoma tissues.Moreover,circASAP1 promotes the proliferation and inhibits the apoptosis of osteosarcoma cells by targeting miR-4500.