The Effect Of Sphingomyelin Synthase (sms) Activity On The Secretion Of Apolipoprotein A-i And Lipids And The Content Of Lipids In Huh7 Cells

As the most common cardiovascular disease, Atherosclerosis (Atherosclerosis, AS) is detrimental to human health. Recent studies found that the sphingomyelin (SM) accumulate in the atherosclerotic plaques. Epidemiological investigation showed that the increased plasma sphingomyelin (SM) is a independent risk factor of AS. Sphingomyelin synthase (SMS) is one of the key enzymes involved in the sphingomyelin synthesis process. Apo A-I (apolipoprotein A-I) is an apolipoprotein which can combine with the cholesterol, triglyceride, phospholipids and other lipid material to form HDL (high density lipoprotein). From the peripheral tissue, HDL can transport cholesterol back to the liver, namely, reverse cholesterol transport (RCT), a process which can reduce the incidence of atherosclerosis. So, the plasma HDL-C level is negatively correlated with the incidence of atherosclerosis. As a membrane receptor of cholesterol transport, Apo A-I can interact with the ABCA1 and accept the efflux cholesterol to form nascent HDL. So, the content of plasma Apo A-I is a negative correlator with the occurrence of atherosclerosis.The purpose of the present study is to investigate whether there is a relationship between SM content and cholesterol content, Apo A-I secretion.We constructed Tet-off-SMS system which can be regulated by DOX. In this Tet-off-SMS system, the expression of SMS-FLAG and SMS activity can be accurately regulated by different dose DOX (0,0.01 and 100ng/ml) with a dose dependent (P<0.05), while Apo A-I secretion can not. However, the Apo A-I secretion was significantly decreased in the higher SMS activity compared with control in Huh7 cell (P<0.05), while the mRNA expression level of Apo A-I exhibited no significant difference (P>0.05). That Apo A-I secretion was increased while Apo A-I mRNA had no significant difference in Huh7 cell when the SMS activity was inhibited by 105 compound, indicated that lower SMS activity is beneficial to Apo A-I secretion.The cellular content of cholesterol and SM and cholesterol in the medium increased in the higher SMS activity Huh7 cells (P<0.05), in the 105 compound treatment group (lower SMS activity) the results was opposite (P<0.05).In order to study the possible mechanism for those results, studies for the membrane proteins which are associated with the cholesterol efflux showed that the mRNA and protein expression of ABCA1, ABCG1 and SR-BI are all increased in the higher SMS activity of Huh7 cells, but decreased in the lower SMS activity system. The expression of HMG-CoA reductase reduced in the higher SMS activity and increased in lower SMS activity system through the semi-quantitative RT-PCR analysis (P<0.05)This study shows that Apo A-I secretion, the cellular contents and secretion of cholesterol and sphingomyelin can be regulated by the activity of SMS; There have a positive association between the cellular cholesterol content and sphingomyelin content which can be anchored by themselves; 105 compound, as an innovative designed compound, is a potential inhibitor for SMS.