TEM-879; No. of Pages 7 Review The regulation of ApoB metabolism by insulin 1 2 1 Mary E. Haas , Alan D. Attie , and Sudha B. Biddinger 1 Division of Endocrinology, Boston Children’s Hospital, Harvard Medical School, Boston, MA 02115, USA 2 Department of Biochemistry, University of Wisconsin-Madison, Madison, WI 53706, USA The leading cause of death in diabetic patients is cardio- are not capable of catabolizing cholesterol. They must vascular disease. Apolipoprotein B (ApoB)-containing therefore return excess cholesterol to the liver, which lipoprotein particles, which are secreted and cleared can secrete cholesterol into the bile, either directly or after by the liver, are essential for the development of athero- conversion to bile acids [5]. sclerosis. Insulin plays a key role in the regulation of Lipoproteins can be divided into classes that vary in ApoB. Insulin decreases ApoB secretion by promoting their apolipoproteins, lipid composition and, therefore, ApoB degradation in the hepatocyte. In parallel, insulin function. Apolipoprotein B is a structural protein that is promotes clearance of circulating ApoB particles by the an integral component of chylomicrons, as well as very low- liver via the low-density lipoprotein receptor (LDLR), density lipoprotein (VLDL), intermediate-density lipopro- LDLR-related protein 1 (LRP1), and heparan sulfate pro- tein (IDL), and low-density lipoprotein (LDL) particles. teoglycans (HSPGs). Consequently, the insulin-resistant Chylomicrons are secreted by the intestine and enable state of type 2 diabetes (T2D) is associated with in- the transport of dietary triglycerides to other tissues after creased secretion and decreased clearance of ApoB. feeding. They contain a truncated form of ApoB, referred to Here, we review the mechanisms by which insulin con- as ApoB48 because it is 48% of the full-length protein [6]. trols the secretion and uptake of ApoB in normal and Chylomicrons are lipolyzed within the vasculature, releas- diabetic livers. ing their fatty acids to peripheral cells, particularly the adipose tissue, for storage. When most of the triglycerides Cardiovascular disease in diabetes have been removed, the chylomicron remnant is cleared by Most individuals with diabetes ultimately die of cardiovas- the liver [5]. cular disease (CVD). Hyperglycemia is a central feature of VLDL, which is secreted by the liver, also transports the diabetic state, and it has been suggested that hyper- triglycerides. In humans, VLDL contains only ApoB100, glycemia promotes cardiovascular disease by increasing the full-length protein, whereas either ApoB100 or ApoB48 inflammation, oxidative stress, and the formation of ad- is present in the VLDL of mice and some other rodents [6]. vanced glycosylation end products (for a review, see [1]). As the VLDL particle delivers triglycerides to other tissues However, the normalization of serum glucose levels does it is successively lipolyzed into an IDL and then LDL not consistently reduce the risk of CVD in diabetic patients particle. LDL-cholesterol can be taken up by cells that [2,3]. A more important factor may be derangements in require exogenous cholesterol. However, most LDL is lipoprotein metabolism. Although diabetic patients are not cleared by the liver [5]. usually hypercholesterolemic, they do show abnormalities The other major class of lipoproteins is high-density in lipoprotein kinetics and composition. Moreover, statin lipoprotein (HDL). HDL does not contain ApoB, and high drugs, which reduce serum cholesterol, effectively reduce HDL levels are correlated with decreased risk of athero- CVD risk in diabetic patients [4]. We review here the sclerosis. HDL can remove excess cholesterol from periph- effects of insulin on the hepatic metabolism of ApoB- eral tissues, such as endothelial cells and macrophages, containing lipoprotein particles, as these are essential and return it to the liver for excretion from the body, a for the development of atherosclerosis. process known as reverse cholesterol transport [7]. Insulin plays a central role in coordinating lipoprotein Overview of lipoprotein metabolism metabolism. In the fed state, triglycerides and other nutri- The lipoprotein system allows the intercellular transport ents are available from the gut. In this case, high insulin of cholesterol and triglycerides, which serve distinct roles levels act on the adipocyte to promote triglyceride uptake in the body. Triglycerides are an important source of and inhibit free fatty acid release. This promotes the storage energy. Cholesterol, by contrast, is a structural element of triglycerides for later use. In fasting, this process is of membranes as well as a precursor for steroids and bile reversed: insulin levels fall, and free fatty acids are released acids. In contrast to triglycerides, most mammalian tissues from the adipocyte and delivered to the liver. The liver re- esterifies the fatty acids to triglycerides and secretes them Corresponding author: Biddinger, S.B. ([email protected]). as VLDL [7]. Moreover, the ability of the adipose to take up Keywords: apolipoprotein B; insulin; diabetes; very low-density lipoprotein (VLDL); triglycerides is impaired in the absence of insulin, and the cardiovascular disease; selective insulin resistance. triglycerides are therefore preferentially utilized by other 1043-2760/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tem.2013.04.001 tissues, such as skeletal muscle and liver [7,8]. Trends in Endocrinology and Metabolism xx (2013) 1–7 1 TEM-879; No. of Pages 7 Review Trends in Endocrinology and Metabolism xxx xxxx, Vol. xxx, No. x Regulation of VLDL production by which insulin regulates ApoB (Figure 2). Although Within the hepatocyte, insulin exerts its effects via a com- insulin does not alter ApoB mRNA levels [12], it inhibits plex signaling cascade [9]. Some of the major branches of this ApoB translation by promoting the trafficking of ApoB cascade are shown in Figure 1. Upon binding insulin, the mRNA into P-bodies, aggregates of translationally re- insulin receptor recruits the insulin receptor substrate (IRS) pressed mRNAs [13,14]. The effects of insulin on ApoB proteins, leading to activation of phosphatidylinositide translation require signaling through PI3K and mTORC1, 3-kinase (PI3K), which mediates many of the metabolic and may involve the association of RNA-binding proteins 0 effects of insulin. PI3K activates Akt, which inhibits the with the ApoB mRNA 5 -untranslated region [15]. transcription factor FoxO1 by phosphorylating it. Phosphor- As ApoB is translated, it is lipidated by microsomal ylated FoxO1 is excluded from the nucleus, and is therefore triglyceride transfer protein (MTTP). MTTP adds triglyc- unable to activate transcription of its targets, including the erides to the nascent ApoB during its cotranslational gluconeogenic genes. Akt also stimulates glycogen synthesis translocation into the lumen of the endoplasmic reticulum by inhibiting glycogen synthase kinase 3 (GSK3). Further- [16]. This produces a dense, lipid-poor, pre-VLDL particle. more, Akt activates lipogenesis, in part via the mammalian The expression of MTTP is driven by FoxO1, and is thus target of rapamycin complex 1 (mTORC1) signaling com- inhibited by insulin [17]. Because ApoB must be lipidated plex, which is necessary for the induction of the lipogenic by MTTP to escape proteosomal degradation [16], insulin transcription factor sterol regulatory element binding pro- may promote ApoB degradation by decreasing expression tein (SREBP)-1c [10]. Consequently, in the hepatocyte, of MTTP. Consistent with this, insulin-resistant rodents insulin suppresses glucose production and stimulates the show increased amounts of nuclear FoxO1, Mttp mRNA, production of glycogen and triglycerides. It also acutely and MTTP protein, as well as increased ApoB secretion inhibits the secretion of ApoB [11]. [17,18]. However, acute insulin treatment in rodents does The synthesis and secretion of ApoB are complex. Sur- not decrease either Mttp mRNA or MTTP activity [19]; this prisingly, most ApoB protein is degraded before secretion, may due to the long half-life of Mttp mRNA (4.4 days) and the amount of ApoB secreted is determined largely by [20]. ApoB may also be regulated by ER60, an endoplasmic the proportion of newly synthesized polypeptide that reticulum resident chaperone and protease that binds to escapes degradation [6]. There are multiple mechanisms and degrades ApoB when overexpressed [21]; interesting- ly, ER60 is decreased and ApoB secretion is increased in the insulin-resistant fructose-fed hamster [22]. A second lipidation step, in which the bulk of lipids are Insulin added to ApoB, occurs independently of MTTP [11,23]. receptor This maturation step can be inhibited by insulin/PI3K IRS1,2 Shc signaling [24]. Although the exact mechanisms are un- known, one potential mediator is ApoCIII, an apolipopro- Cell growth, PI3K ERK tein secreted by the liver and to a lesser extent by the proliferaon intestine. Individuals with a null allele of APOCIII mani- Glycogen Akt GSK3 fest a 45% reduction in serum triglycerides and a 27% synthesis decrease in coronary heart disease risk [25]. ApoCIII is best known for its ability to inhibit lipoprotein lipase and SREBP-1c mTORC1 FoxO1 Gluconeogenesis hepatic lipase, and therefore lipolysis of triglycerides car- MTTP ried on chylomicrons and VLDL [26], but it has also been Lipogenesis Sorlin ApoCIII suggested to act intracellularly to promote