Thyromimetics and statins: influence on reverse cholesterol transport & atherosclerosis

Atherosclerosis is still the leading cause of death in Western societies, with hypercholesterolemia being a main risk factor for its development. Currently, statin treatment represents the standard for the treatment of hypercholesterolemia. However, the recommended goal of plasma LDL cholesterol = 70 mg/dl is currently not achieved in all patients at high cardiovascular risk. Therefore, the search for alternative and/or additive pharmacological approaches is still of utmost importance. Liver-selective thyroid hormone analogs (thyromimetics, TM) were shown to lower plasma cholesterol in several animal models, and maybe most promising, to exert similar effects in humans also. In humans, the TM eprotirome was not only shown to reduce plasma LDL cholesterol when used as monotherapy, but also to further lower plasma cholesterol by up to 30% when used as add-on therapy to statins (N Engl J Med. 2010 Mar 11;362(10):906-16). Using different animal models, our group was the first to show that selective TM prevent atherosclerosis development. Up to date, TM were believed to act mainly by promoting the plasma clearance of LDL cholesterol through upregulation of the hepatic LDL receptor. However, in our experiments in apoE KO mice TM lowered plasma LDL cholesterol and inhibited atherosclerosis without affecting the hepatic expression of the LDL receptor. Instead, we found an upregulation of hepatic CYP7A1 and ABCG5/G8, major players of bile acid synthesis and biliary sterol secretion. In addition, TM were recently shown to induce the hepatic expression of ABCB11, the limiting step in bile flow. Taken together, these findings point to a major lipid-lowering activity of TM through activation of the biliary sterol metabolism; however, this has not been investigated yet. The fact that eprotirome, when used in adjunction to ongoing statin therapy, led to a further significant, dose- dependent reduction in LDL cholesterol suggested additive effects for these two drug classes. However, the underlying mechanisms have not investigated yet. By treating mice deficient in CYP7A1, ABCG5/G8 and ABCB11 with eprotirmoe, we plan to study the impact of the biliary sterol metabolism on the lipid-lowering action of TM. Besides standard plasma and tissue cholesterol measurements we plan to study the reverse cholesterol transport (measurement of [3H]-cholesterol in plasma, liver and feces over 48 hours after i.p. injection of [3H]-cholesterol - labeled macrophages), since it provides further insights into the kinetics of cholesterol in mice over a significant time-period. We also plan to perform atherosclerosis studies in apoE KO mice on Western type diet treated with the TM eprotirome together with atorvastatin. It is currently not possible to assess whether selective TM will reduce cardiovascular morbidity and mortality in humans. However, recent data from animal studies by our laboratory heavily suggest atheroprotective effects for TM. Our experiments will help to further understand the lipid-lowering mechanisms of TM, and to anticipate possible additive atheroprotective effects when used in combination with statins.

Athersosclerosis still represents the main cause of death in industrialized countries, and lipoprotein metabolism is closely interrelated with the initiation and progression of this disease. The two most abundant lipoproteins in the plasma are low-density lipoproteins (LDL) and high-density lipoproteins (HDL). Targeting aspects of their metabolism is one of the main interventions for preventing and treating atherosclerotic cardiovascular disease. The current state-of-the-art treatment for hypercholesterolemia is limited to the use of statins. Any novel approach to efficiently lower LDL cholesterol and to promote the antiatherosclerotic mechanism mediated by HDL particles, i.e. the Reverse Cholesterol Transport (RCT) is expected to counteract atherosclersosis and its clinical sequelae including myocardial infarction and stroke. We and others have recently shown that liver- and/or ß1-selective thyroid hormone analogs, so-called selective thyromimetics (TM), significantly lower plasma LDL cholesterol and promote RCT. Moreover, our group was the first to show that selective TM significantly prevent atherosclerosis development in hyperlipidemic rabbits as well as in cholesterol-fed apoE knockout mice. With the current project we aimed at investigating whether eprotirome, the lead compound of TM, would exhibit antiatherosclerotic effects in mice, as it rapidly entered phase II and III clinical trials and was shown to efficiently lower LDL cholesterol in humans on top of statins. To better characterize this compound and to anticipate its putative anti-atherosclerotic effects, we studied its impact on RCT and on atherosclerosis development in mice. Similarly to what we previously had observed with another selective TM, eprotirome promoted RCT in wild-type mice. However, it failed to lower LDL cholesterol and to promote RCT in dyslipidemic mice, which translated in failure to protect from atherosclerosis development. During the ongoing work on this project, two reports were made public showing that the compound specifically investigated in this application, namely eprotirome, had deleterious effects on joints in an animal study and on liver function in a study performed in hypercholesterolemic patients. Our negative findings in a mouse model of dyslipidemia critically contribute to the field, as they represent an additional important point not to implement eprotirome into the treatment of dyslipidemia into the clinical routine. We hope that our studies will also help for the design and development of novel TM which could enter the clinical routine, as these negative results may help to better specify the prerequisites of such a compound, and as we previously showed that this drug class may definitively be used to counteract atherosclerosis development.