Abstract: Objective　To investigate the influence of homocysteine(Hcy) on the expressions of ATP-binding cassette transporter A1(ABCA1) and acylcoenzyme A: cholesterol acyltransferase-1 (ACAT1) in the formation of THP-1 monocytederived foam cells. Methods　THP-1 monocytes were cultured with phorbol myristate acetate(PMA) and oxidized low density lipoprotein(ox-LDL) so as to copy foam cell model， and then intervened by 50，100，200 and 500 μmol/L Hcy and 100 μmol/L Hcy+folic acid+vitamin B₁₂(Vit B₁₂) for 72 h. The control group(not intervened by Hcy) was set. The formation of foam cells was analyzed by the oil red O staining. The intracellular total cholesterol(TC)，free cholesterol(FC) and cholesterol ester(CE) of foam cells were measured by enzymatic end point determination in order to observe the effects of Hcy on foam cell CE efflux. Fluorescence quantitative reverse transcription polymerase chain reaction(RT-PCR) detected the expressions of ABCA1 mRNA and ACAT1 mRNA. Western blotting tested the expressions of ABCA1 protein and ACAT1 protein. Results　After the intervention of Hcy， oil red O staining showed that the formation of foam cells was increased significantly， but not in doseeffect relationship. The positive percentage of experimental group (50, 100, 200 and 500 μmol/L Hcy and 100 μmol/L Hcy+folic acid+Vit B₁₂) was higher than that of control group (P<0.05,P<0.01). The 100 μmol/L Hcy group was the most significant one(P<0.01). The outflow of foam intracellular TC，FC and CE was reduced significantly compared with control group(P<0.05), and the 100 μmol/L Hcy group was the most significant one (P<0.01). The 100 μmol/L Hcy+folic acid+Vit B₁₂ group showed that the formation of foam cells was decreased， whereas the cholesterol efflux was increased compared with 100 μmol/L Hcy group. RT-PCR showed that the expression of ABCA1 mRNA was decreased, but ACAT1 mRNA was increased，and the 100 μmol/L Hcy group was the most significant effect one(P<0.01). Western blotting showed that the expressions of ABCA1 protein and ACAT1 protein were consistent with mRNA expression. Conclusions　Hcy reduces the expression of ABCA1，in the contrast，the expression of ACAT1 is increased. Both of them promote the formation of foam cells.

Key words: Homocysteine, ATPbinding cassette transporter A1, Acylcoenzyme A: cholesterol acyltransferase-1