Can modulators of apolipoproteinB biogenesis serve as an alternate target for cholesterol-lowering drugs?

Abstract

Understanding the molecular defects underlying cardiovascular disease is necessary for the development of therapeutics. The most common method to lower circulating lipids, which reduces the incidence of cardiovascular disease, is statins, but other drugs are now entering the clinic, some of which have been approved. Nevertheless, patients cannot tolerate some of these therapeutics, the drugs are costly, and/or the treatments are approved for only rare forms of disease. Efforts to find alternative treatments have focused on other factors, such as apolipoproteinB (apoB), which transports cholesterol in the blood stream. The levels of apoB are regulated by endoplasmic reticulum (ER) associated degradation as well as by a post ER degradation pathway in model systems, and we suggest that these events provide novel therapeutic targets. We discuss first how cardiovascular disease arises and how cholesterol is regulated, and then summarize the mechanisms of action of existing treatments for cardiovascular disease. We then review the apoB biosynthetic pathway, focusing on steps that might be amenable to therapeutic interventions.

Keywords: ApolipoproteinB; Autophagy; Cardiovascular disease; Cholesterol; Endoplasmic Reticulum Associated Degradation (ERAD); Very Low Density Lipoprotein (VLDL).

Figure 1. Lipoprotein Particles of Blood Plasma

The four main classes of plasma lipoproteins are HDL, LDL, VLDL, and chylomicrons. HDLs are the smallest particles and are the most dense. Chylomicrons are the largest particle and are the least dense. LDL and VLDL contain apoB100 as their major apolipoprotein while chylomicrons in humans contain apoB48. HDL contains apoAI and apoAII as the major apolipoproteins. VLDL is synthesized in the liver whereas chylomicrons are synthesized in the small intestine. LDL is a product of VLDL modifications made in circulation. HDL is made in both tissues.

Figure 2. Proposed Structural Domains and Modifications of ApoB

(A) The predicted apoB domain structure contains a 27 amino acid signal sequence (SS), which targets apoB to the ER membrane and allows for translocation or entry into the ER, and a β-barrel domain, which is directly followed by α-helix, β-sheet, α-helix motifs, and is interspersed by 2 lipid association domains. The naturally occurring isoforms, apoB100 and apoB48, and the hypolipidemic truncation, apoB29, are depicted using vertical blue lines. The LDL receptor binding region is found in the β2 domain and is indicated with a horizontal green line. The 16 glycosylated asparagine residues in apoB are indicated with triangles. Of the 25 cysteines, only 16 participate in disulfide bond formation (red circles) while the remaining 9 are free sulfhydryls (open circles). (B) Computer simulated model of proposed apoB structure (left) alone and (right) in a lipoprotein. The image is reproduced with permission from [49]. (C) A representation of a lipoprotein particle. The apolipoprotein wraps around the circumference of the lipoprotein which consists of phospholipids, free cholesterol, triglycerides, and cholesteryl esters.

Figure 3. ApoB is Metabolically Regulated by ERAD

(A) ApoB is required for the assembly of a pre-VLDL. (1) ApoB is co-translationally translocated into the ER where the MTP complex attaches lipids (purple diamonds) to apoB. (2) This results in a pre-VLDL particle that contains apoB. (3) ApoB exits the ER in a (perhaps non-canonical) COPII coated vesicle. (4) The pre-VLDL can undergo further maturation in the Golgi apparatus. (5) If the particle fully matures, it will be secreted and enter the bloodstream to deliver lipids. If the particle does not fully mature, it can also be sent to the lysosome for degradation by post-ER presecretory proteolysis. (B) If lipids are limiting or MTP function is blunted, then apoB fails to become lipidated. Translocation slows, exposing large cytoplasmic loops. These loops can be acted upon by chaperones (e.g. Hsp70 and Hsp90) and the ubiquitination machinery. Another chaperone, Hsp110, protects apoB from degradation (see text for details). Once the cytoplasmically-disposed peptide loop is ubiquitinated, apoB is retrotranslocated back through the Sec61 complex and targeted to the proteasome for degradation.