Aim To explore the cellular mechanisms of free cholesterol(FC)-induced macrophage apoptosis by activating unfolded protein response(UPR).Methods Macrophages were harvested from the peritonea of female C57BL6/J mice,then incubated in DMEM and 1% fetal bovine serum(FBS) medium alone,or medium containing acetyl-low density lipoprotein(ac-LDL) and compound 58035(free cholesterol-loading conditions),or medium containing ac-LDL and compound 58035 plus U18666A respectively.Western-blot was used to determine expression of C/EBP homology protein(CHOP).The apoptosis percentages of macrophages were detected with flow cytometry(FCM) after the staining of Annexin-V and propidium(PI).Results Macrophages incubated in DMEM and 1% FBS alone had no expression of CHOP and showed very little evidence of cell apoptosis.After incubated with medium containing ac-LDL and compound 58035 in our cell culture model,the expression of CHOP increased notably and about 21.83%±2.47% apoptosis of the cells had been detected.But apoptosis was diminished markedly in free cholesterol-loaded plus U18666A and also no CHOP expression had been detected.Conclusions Excess intracellular FC accumulation plays an important role in macrophage apoptosis,and UPR is involved in this process.But U18666A can block FC transport into endoplasmic reticulum(ER) to activate the CHOP and subsequent apoptosis.