DNA Topoisomerase Ⅱ Inhibitors Induce Macrophage ABCA1Expression And Cholesterol Efflux-An LXR-dependent Mechanism

Atherosclerosis is one of the main pathogens for coronary heart disease (CHD). Accumulation of cholesterol/lipids stimulates macrophage/foam cells differentiation. The foam cells is a basic component of atherosclerotic plaques. Therefore, it is critical to prevent the development of atherosclerosis by regulating macrophage cholesterol homeostasis. The reverse cholesterol transport (RCT) is an important process in cholesterol metabolism and inhibit the prevalence of CHD.ABCA1is a member of ATP-binding cassette transporters family. ABCA1has been demonstrated to protect cardiovascular system in both humans and animal models. By using the energy from the hydrolysis of ATPs, ABCA1positively activates efflux of free cholesterol and phospholipids from macrophages, the critical and the initial step in RCT. The ABCA1-mediated cholesterol efflux also plays a vital role in the formation of nascent HDL and cholesterol metabolism. Expression of ABCA1is mainly regulated at the transcriptional level, in particular, by liver X receptor (LXR), a ligand-activated transcription factor.Both etoposide and teniposide are DNA topoisomerase Ⅱ inhibitors and used in clinic to treat patients with cancers. They promote formation of the covalent enzyme-DNA complex and stabilize the complex. The functions of DNA topoisomerase Ⅱ and its inhibitors in cancer biology and immunology have been well investigated. However, little is known if DNA topoisomerase II inhibitors can play an important role in other fields. It has been reported that etoposide reduces atherosclerosis in rabbits indicating other functions than anti-tumorigenesis. However, the underlying anti-atherogenic mechanisms are not clear. We hypothesized that DNA topoisomerase Ⅱ inhibitors might be able to activate macrophage ABCA1expression and cholesterol efflux, thereby inhibiting macrophage/foam cell differentiation, and inhibit lesion development. In our study, we found that both etoposide and teniposide(another DNA Topo Ⅱ inhibitor) significantly upregulated ABCA1protein expression which was associated with increased macrophage free cholesterol efflux. Both etoposide and teniposite also increased ABCA1mRNA expression indicating the activation occurred at the transcriptional level. In deed, we observed that etoposide and teniposide induced ABCA1promoter activity in an LXR dependent manner. The reulst of EMSA demonstrated that etoposide and teniposide enhanced the formation of LXRE-LXR/RXR complex. In addition, we observed that other LXR target genes, such as ABCG1(another member of ABC family) and FASN (the molecule for fatty acid synthesis), were also induced by etoposide and teniposide. In vivo, we observed that administration of etoposide and teniposide enhanced cholesterol reverse transport which further support the impotance of DNA TopⅡ inhibitors in anti-atherosclerosis.In this thesis, we also determined the effect of MEK1/2inhibitors (some of MEK1/2inhibitor are in clinical trial studies for cancer treatment), PD98059and U0126, on macrophage ABCG1expression both in vitro and in vivo. We observe that U0126induced ABCG1expression which was also associated with increased macrophage reverse cholesterol transport in vivo. In addition, we determined increased macrophage ABCG1expression in atherosclerotic lesion area by administration of U0126.In summary, our study demonstrates that etoposide, teniposide and other anti-cancer agents can induce macrophage ABCA1/G1expression and cholesterol efflux thereby reducing lipid accumulation and foam cell formation. Such function can contribute to the anti-atherogenic properties of these agents. Our study not only unveils the novel role of anti-cancer medicines, but also supplies the new theory to support the potential application of the medicines for treatment of patients with other diseases.