The role of hepatic SURF4 in the lipid metabolism and the development of atherosclerosis
Creators: Shen, Yishi
Publisher: name: University of Alberta Library
Year: 2023

<p>Atherosclerotic cardiovascular disease (ASCVD) is the leading cause of morbidity and<br>
mortality worldwide. Long-term elevated plasma levels of low-density lipoprotein cholesterol<br>
(LDL-C) are the major risk factors for developing atherosclerotic lesions, which can eventually<br>
lead to myocardial infarction and stroke. Lowering plasma LDL-C levels remains the main way<br>
to prevent CVD. LDL is removed from the circulation mainly by the hepatic LDL receptor.<br>
Mutations in this receptor cause familial hypercholesterolemia, which is characterized by<br>
elevated plasma levels of LDL-C and increased risk of CVD. Statins, the most-prescribed drug<br>
lowering plasma cholesterol levels, increase the expression of LDL receptor and reduce<br>
cardiovascular events by 20% to 40%. However, about 15% of people treated with statins show<br>
statin intolerance and require alternative therapies to lower LDL-C levels. Another option is<br>
Proprotein convertase subtilisin/kexin type 9 (PCSK9), which promotes degradation of LDL<br>
receptors. Recently approved PCSK9 inhibitors can effectively reduce plasma LDL-C levels,<br>
but the treatment is expensive. PCSK9 siRNA therapy may be more affordable, but it is still too<br>
expensive for all eligible patients to use for primary prevention. Furthermore, the lipid-lowering<br>
effects of both statins and PCSK9 inhibitors depends mainly on enhancing clearance of LDL<br>
through increasing expression of LDL receptor. LDL is produced through catabolism of very<br>
low-density lipoprotein (VLDL), a triglyceride-rich lipoprotein that is made exclusively by the<br>
liver. Triglycerides (TG) on circulating VLDL are hydrolyzed by lipoprotein lipase (LPL),<br>
leading to formation of intermediate-density lipoprotein that can then be converted to LDL. The<br>
drugs Mipomersen (inhibitor of apolipoprotein B100 (apoB100)) and Lomitapide (inhibitor of<br>
microsomal triglyceride transfer protein (MTP)) can reduce VLDL production and secretion,<br>
lowering plasma levels of LDL-C. However, both drugs have severe adverse effects such as hepatic lipid accumulation, liver damage and diarrhea. Thus, the need to identify novel<br>
therapeutic targets to reduce plasma cholesterol levels is urgent. This thesis aims to investigate<br>
the role of hepatic Surfeit locus 4 (Surf4), a cargo receptor located in the endoplasmic reticulum<br>
(ER) membrane, in regulating plasma lipid levels and the development of atherosclerosis by<br>
mediating PCSK9 and VLDL secretion from hepatocytes.<br>
In chapter 3, I investigated the role of Surf4 in PCSK9 secretion in vitro and in vivo. It has been<br>
reported that Surf4 mediates overexpressed PCSK9 secretion in HEK293T cells, which were<br>
derived from human embryonic kidney cells. Considering endogenous PCSK9 is mainly<br>
expressed and secreted by the liver in normal physiological condition, I determined the role of<br>
Surf4 in mediating PCSK9 secretion in human hepatoma derived cell lines, Huh7 and HepG2<br>
cells. Interestingly, I found that knockdown of SURF4 did not inhibit endogenous PCSK9<br>
secretion; instead, deficiency of SURF upregulated PCSK9 expression in both Huh7 and HepG2<br>
cells. On the other hand, knockdown of hepatic Surf4 in wild type mice (C57BL6/J) has no<br>
effects on PCSK9 expression and secretion. Although the results from in vitro and in vivo<br>
studies were completely consistent, these findings indicate a negligible role of hepatic SURF4<br>
in endogenous PCSK9 secretion from the liver and cultured hepatocytes.<br>
In chapter 4, we generated Surf4 hepatocyte-specific knockout (Surf4LKO) mice and found that<br>
plasma levels of total cholesterol (TC), TG, non-high-density lipoprotein cholesterol (HDL-C),<br>
and apoB were significantly reduced in Surf4LKO mice compared to Surf4Flox mice. I also found<br>
that Surf4 coimmunoprecipitated and colocalized with apoB100, and SURF4 silencing reduced<br>
secretion of apoB100 in cultured human hepatoma cells. Furthermore, knockdown of hepatic<br>
Surf4 in LDL receptor knockout (Ldlr-/-) mice significantly reduced TG secretion, plasma<br>
levels of TC, non-HDL-C, and apoB, and ameliorated the development of atherosclerosis. However, both Surf4LKO mice and Ldlr-/- mice with hepatic Surf4 knockdown displayed similar<br>
levels of liver lipids and plasma alanine aminotransferase (ALT) activity as their control mice,<br>
indicating that inhibition of hepatic Surf4 does not cause notable liver damage. Expression of<br>
stearoyl-CoA desaturase-1 (SCD1) was also reduced in the liver of Surf4 knockdown mice,<br>
suggesting a reduction in de novo lipogenesis. These findings suggest that deficiency of hepatic<br>
Surf4 reduced VLDL secretion, plasma lipid levels, and the risk of atherosclerosis without<br>
causing significant hepatic lipid accumulation or liver damage.<br>
In chapter 5, I investigated the role of hepatic SURF4 in lipoprotein metabolism and the<br>
development of atherosclerosis in another commonly used mouse model of atherosclerosis,<br>
apolipoprotein E knockout (apoE-/-) mice. I found that, in apoE-/- mice fed a regular chow diet,<br>
knockdown of hepatic Surf4 expression significantly reduced TG secretion and plasma levels<br>
of non-HDL-C and TG without causing obvious hepatic lipid accumulation or liver damage.<br>
When hepatic Surf4 was knocked down in apoE-/- mice fed the Western-type diet (WTD), I<br>
observed a significant reduction in plasma levels of non-HDL-C, but not TG. Knockdown of<br>
hepatic Surf4 did not increase hepatic TC and TG levels or cause liver damage, but significantly<br>
diminished atherosclerotic lesions in apoE-/- mice fed with WTD. Therefore, these findings,<br>
together with the results from Ldlr-/- mice, indicate the potential of hepatic Surf4 inhibition as<br>
a novel therapeutic strategy to reduce plasma lipid levels and the risk of ASCVD.<br>
In summary, the work performed during this Ph.D. thesis will shed light on the novel target of<br>
lowering plasma levels of cholesterol and TG and reducing the risk of atherosclerosis without<br>
causing obvious hepatic lipid accumulation. </p>