Lipoproteins assembly

Shelness GS, Sellers JA Very-low-density lipoprotein assembly and secretion. Curr Opin Lipidol 2001 12 151. 

J. Luchoomun and M. M. Hussain, Assembly and secretion of chylomicrons by differentiated Caco-2 cells. Nascent triglycerides and preformed phospholipids are preferentially used for lipoprotein assembly, J. Biol. Chem. 274 (1999) 19565-19572. 

Hussain MM, Fatma S, Pan X, Iqbal J (2005) Intestinal lipoprotein assembly. Curr Opin Lipidol 16 281-285... 

Hussain MM, Shi J, Dreizen P (2003) Microsomal triglyceride transfer protein and its role in apoB-lipoprotein assembly. J Lipid Res 44 22-32... 

S.O. Olofsson, G. Bjursell, K. Bostrom, P. Carlsson, J. Elovson, A.A. Protter, M.A. Reuben and G. Bondjers, Apolipoprotein B structure, biosynthesis and role in the lipoprotein assembly process, Atherosclerosis 68 (1987) 1-17. 

Fig. 9. RNA editing, (a) Unedited apolipoprotein B mRNA is translated to yield ApoB-100, a 4536-amino acid long polypeptide with structural domains for lipoprotein assembly and receptor binding functions (b) translation of the edited mRNA yields the shorter ApoB-48 which lacks the receptor binding domain.

Lipoprotein Assembly and Microsomal Triglyceride Transfer Protein... 

In ABL, an early step in apoB lipoprotein assembly shared by intestinal and liver cells is defective. The net result is near absence of all plasma apoB lipoproteins. ApoB synthesis from a mRNA transcript occurs, but its successful assembly into the mature lipoprotein particle does not. The inability to assemble apoB into lipoproteins was shown to be due to a defect in the mttp gene in affected individuals (Wetterau et al., 1992). Its translational product is an 894-amino acid, 97-kd, polypeptide that exists in the ER complexed with a 55-kd protein disulfide isomerase which is believed to maintain solubility, physiologic activity, and ER retention of the 97-kd peptide. The heterodimeric complex of the 97-kd and 55-kd subunits is referred to as microsomal triglyceride transfer protein (MTP) (Wetterau et al., 1992). 

Several functions of MTP have been identified all have been implicated in coordinating successful lipoprotein assembly (Fig. 27-1). MTP transfers lipids between vesicles in vitro, and this activity is likely to be its major function. MTP can pick up lipids from membrane (step A) or vesicles and droplets (step B) and transfer them to the nascent apoB. In addition, the lipid transfer activity of MTP has been implicated in the accretion of neutral lipids from the cytosol into the ER lumen (step C). Compounds that inhibit in vitro transfer activity of MTP decrease apoB secretion by cells, indicating that this activity is essential for apoB lipoprotein secretion. Apart from transferring lipids, MTP has been shown to interact physically with apoB (step D). This activity... 

The ultimate dependence of vitamin E for transport by liver and intestinal cells via apoB lipoproteins may explain the severe vitamin E deficiency observed in ABL. In contrast, apoB lipoprotein assembly is required only for the mobilization of dietary vitamin A by the intestinal cells liver distributes the endogenous vitamin A to other tissues by retinol-binding protein. Unlike vitamins E and A, dietary vitamin K may only partially depend on apoB lipoproteins for its transport across the intestinal epithelial cells and tissue targeting and may explain why bleeding diathesis is rarely observed in ABL. 

Rader, D.J., Brewer, H.B. Abetalipoproteinemia New insights into lipoprotein assembly and vitamin E metabolism from a rare genetic disease. X Amer. Med. Ass. 1993 270 865-869... 

Lipoproteins are assembled in two organs, the small intestine and the liver. The lipoproteins assembled in the intestine contain the lipids assimilated from the diet. These lipoproteins, called chylomicrons, leave the enterocyte and enter the bloodstream via the Lymphatic system. The lipoproteins assembled in the liver contain lipids originating from the bloodstream and from de novo synthesis in the liver. The term de novo simply means "newly made from simple components" as opposed to "acquired from the diet" or "recycled from preexisting complex components." These lipoproteins, called very-low-dcnslty lipoproteins (VLDLs), are secreted from the liver into the bloodstream. The liver also synthesizes and secretes other Lipoproteins called high-density Lipoproteins (HDLs), which interact with the chylomicrons and VLDLs in the bloodstream and promote their maturation and function. The data in Table 6-4 show that chylomicrons contain a small proportion of protein, whereas HDLs have a relatively high protein content. Of greater interest is the identity and function of the proteins that constitute these particles. These proteins confer specific properties to lipoprotein particles, as detailed later in this chapter. 

Another independent approach to the mapping of apoB on the lipoprotein surface arose unexpectedly during studies on the mechanism of lipoprotein assembly in hepatocyte cell lines, and this will be described next. 

Fig. 14. Cotranslational assembly of a lipoprotein from the inner leaflet of the ER biiayer and apoB. In this model, translation of the C-terminal portion of apoB proceeds on a membrane-bound ribosome, while translocation and lipoprotein assembly occur on the luminal side of the ER. The N-terminal portion of apoB is believed to be embedded in the inner leaflet of the bilayer, where it nucleates the formation of an oil droplet from the supersaturated ER membranes. As the hydrophobic inner surface of apoB attempts to surround the oil droplet, it bulges into the lumen, as depicted here. On completion of translation, the two ends of apoB become free to meet, which would automatically result in the detachment of the lipoprotein from the bilayer. (Not drawn to scale.)...

Cardiomyocytes are known to express HSL [68, 78, 79 see below] and the hormone-responsiveness of myocardial lipolysis suggests that HSL is probably responsible. In addition, the heart can synthesize and secrete apoB-containing lipoproteins [3]. If, as apparently with the liver, endogenous TAG is mobilized for lipoprotein assembly via a lipolytic event the question arises can HSL fulfil this function in the heart or is another lipase involved in a mechanism identical to that in the liver It has been proposed that mobilization of endogenous cardiac TAG for lipoprotein secretion provides the heart with a safety valve to dispose of excess lipid [3] and thus protects against lipotoxicity [65]. Lipoapoptosis observed in the heart leads to the development of heart failure [80, 81]. If, as apparently with skeletal muscle [82], excess TAG accumulation plays a role in insuhn resistance, hpoprotein secretion may enable cardiac muscle insuhn sensitivity in the face of an excessive flux of FA. 

Excess cholesterol can also be metabolized to CE. ACAT is the ER enzyme that catalyzes the esterification of cellular sterols with fatty acids. In vivo, ACAT plays an important physiological role in intestinal absorption of dietary cholesterol, in intestinal and hepatic lipoprotein assembly, in transformation of macrophages into CE laden foam cells, and in control of the cellular free cholesterol pool that serves as substrate for bile acid and steroid hormone formation. ACAT is an allosteric enzyme, thought to be regulated by an ER cholesterol pool that is in equilibrium with the pool that regulates cholesterol biosynthesis. ACAT is activated more effectively by oxysterols than by cholesterol itself, likely due to differences in their solubility. As the fatty acyl donor, ACAT prefers endogenously synthesized, monounsaturated fatty acyl-CoA. 

The type of lipoprotein assembled and secreted by the liver is very low-density lipoprotein (VLDL). It contains a high ratio of triglyceride to cholesterol. The main apoprotein in VLDL is apo-Bl 00 which is derived from the same gene as the chylomicron-specific apo-B48. The latter apoprotein is slightly shorter because the mRNA in the intestinal cells is posttranscriptionally altered to introduce a premature stop codon. 

Spring, D.J., Chen-Liu, L.W., Chatterton, J.E., Elovson, J. and Schumaker, V.N. (1992) J. Biol Chem., W, 14839-14845. Lipoprotein assembly. Apolipoprotein B size determines lipoprotein core circumference. 

Gordon, D.A., Jamil, H., Gregg, R.E., Olofsson, S-O. Boren, J. (19%) J. Biol Chem., 271, 33047-33053. Inhibition of the microsomal triglyceride transfer protein blocks the first step of apolipoprotein B lipoprotein assembly but not the addition of bulk core lipids in the second step. 

VLDL Very low density lipoprotein a triglyceride-rich lipoprotein assembled in the endoplasmic reticulum and Golgi of hepatocytes and then secreted into the bloodstream. While in the circulation, it gives rise to IDL and then LDL after it loses triglyceride. 

What is the function of the lipoprotein Why does the lipoprotein have such an unusual amino-terminal structure Why is the lipoprotein assembled in the outer membrane in two different forms Since 7.5 x 10 molecules of the lipoprotein exist in a cell, they must occupy a substantial portion of the outer membrane. It has been suggested that one possible function of the bound form is to connect the outer membrane with the peptidoglycan layer. However, the finding that the free form exists in twice the abundance as the bound form suggests another function of the lipoprotein. 

---