Heterogeneity of lipoprotein particles in hepatic Golgi fractions

Golgi pH and Ion Homeostasis in Health and Disease

Maintenance of the main Golgi functions, glycosylation and sorting, is dependent on the unique Golgi pH microenvironment that is thought to be set by the balance between the rates of V-ATPase-mediated proton pumping and its leakage back to the cytoplasm via an unknown pathway. The concentration of other ions, such as chloride, potassium, calcium, magnesium, and manganese, is also important for Golgi homeostasis and dependent on the transport activity of other ion transporters present in the Golgi membranes. During the last decade, several new disorders have been identified that are caused by, or are associated with, dysregulated Golgi pH and ion homeostasis. Here, we will provide an updated overview on these disorders and the proteins involved. We will also discuss other disorders for which the molecular defects remain currently uncertain but which potentially involve proteins that regulate Golgi pH or ion homeostasis.

Golgi pH, Ion and Redox Homeostasis: How Much Do They Really Matter?

Exocytic and endocytic compartments each have their own unique luminal ion and pH environment that is important for their normal functioning. A failure to maintain this environment - the loss of homeostasis - is not uncommon. In the worst case, all the main Golgi functions, including glycosylation, membrane trafficking and protein sorting, can be perturbed. Several factors contribute to Golgi homeostasis. These include not only ions such as H⁺, Ca²⁺, Mg²⁺, Mn²⁺, but also Golgi redox state and nitric oxide (NO) levels, both of which are dependent on the oxygen levels in the cells. Changes to any one of these factors have consequences on Golgi functions, the nature of which can be dissimilar or similar depending upon the defects themselves. For example, altered Golgi pH homeostasis gives rise to Cutis laxa disease, in which glycosylation and membrane trafficking are both affected, while altered Ca²⁺ homeostasis due to the mutated SCPA1 gene in Hailey-Hailey disease, perturbs various protein sorting, proteolytic cleavage and membrane trafficking events in the Golgi. This review gives an overview of the molecular machineries involved in the maintenance of Golgi ion, pH and redox homeostasis, followed by a discussion of the organelle dysfunction and disease that frequently result from their breakdown. Congenital disorders of glycosylation (CDGs) are discussed only when they contribute directly to Golgi pH, ion or redox homeostasis. Current evidence emphasizes that, rather than being mere supporting factors, Golgi pH, ion and redox homeostasis are in fact key players that orchestrate and maintain all Golgi functions.

Triglyceride-rich lipoproteins and plasma lipid transport

This memoir provides a history of the triglyceride-rich lipoproteins of blood plasma over the last half-century. As precursors of low-density lipoproteins and in their own right, triglyceride-rich lipoproteins are essential to the formation of atherosclerotic plaques and to consequent ischemic vascular disease. The author recounts research at the National Heart Institute during 1953 to 1956 and continuing thereafter at the University of California San Francisco. Emphasis is placed on key insights arising from investigations of human disease, the interplay of fatty acid and triglyceride-transport involving the liver, small intestine, adipose tissue and muscle, and the role of the liver in the synthesis and catabolism of atherogenic lipoproteins.

Biogenesis of cytoplasmic lipid droplets: from the lipid ester globule in the membrane to the visible structure

The cytoplasmic lipid droplet (CLD) and very low-density lipoprotein are generated from the lipid ester synthesized in the endoplasmic reticulum. The lipid ester deposited between the two membrane leaflets is supposed to bulge toward the cytoplasm to make a nascent CLD, but its size must be below the resolution limit of conventional techniques and the detectable CLD should only form after acquisition of additional lipid esters. The CLD is different from vesicular organelles in that the internal content is highly hydrophobic and the shape is invariably spherical. Due to its unique characteristics, quantitative discordance between the surface and the volume may occur in the growth and/or involution processes of the CLD. The possibility that these processes may give rise to the structural and functional diversities of the CLD is discussed.

Altered hepatic lipid status and apolipoprotein A-I metabolism in mice lacking phospholipid transfer protein

The effect of PLTP deficiency on hepatic lipid status and apolipoprotein A-I (apoA-I) biosynthesis in PLTP knockout (PLTP-KO) mice was investigated. PLTP-KO mice exhibited a marked reduction in HDL levels, but also increased triglycerides (TG), phospholipids (PL), and cholesterol in very-low-density lipoproteins (VLDL). Both male and female PLTP-KO mice displayed increased hepatic PL and decreased TG, and in the females, increased hepatic cholesterol was also detected. Primary hepatocytes from PLTP-KO mice displayed a different PL molecular species composition to the wild type (WT) controls, with prominent changes being a reduction of long chain fatty acid-containing and an increase of medium chain mono- or di-unsaturated fatty acid containing PL species. Cultured PLTP-KO hepatocytes synthesized and secreted apoA-I in similar quantities as the WT cells. However, the apoA-I secreted by PLTP-KO hepatocytes contained less choline PL, differing also in phosphatidylcholine/sphingomyelin ratio and fatty acyl species composition when compared to apoA-I from WT hepatocytes. Furthermore, the PLTP-KO-derived PL-deficient apoA-I was less stable in the hepatocyte culture medium than that produced by WT cells. These results demonstrate a complex regulatory role of PLTP in serum and liver lipid homeostasis, as well as in the formation of nascent apoA-I-PL complexes from the liver.

Hydrolysis of minor glycerophospholipids of plasma lipoproteins by human group IIA, V and X secretory phospholipases A2

We investigated the hydrolysis of the minor glycerophospholipids of human HDL(3), total HDL and LDL using human group IIA, V and X secretory phospholipases A(2) (sPLA(2)s). For this purpose we employed the enzyme and substrate concentrations and incubation times optimized for hydrolysis of phosphatidylcholine (PtdCho), the major glycerophospholipid of plasma lipoproteins. In contrast to PtdCho, which was readily hydrolyzed by group V and X sPLA(2)s, and to a lesser extent by group IIA sPLA(2), the minor ethanolamine, inositol and serine glycerophospholipids exhibited marked resistance to hydrolysis by all three sPLA(2)s. Thus, when PtdCho was hydrolyzed about 80%, the ethanolamine and inositol glycerophospholipids reached a maximum of 40% hydrolysis. The hydrolysis of phosphatidylserine (PtdSer), which was examined to a more limited extent, showed similar resistance to group IIA, V and X sPLA(2)s, although the group V sPLA(2) attacked it more readily than group X sPLA(2) (52% versus 39% hydrolysis, respectively). Surprisingly, the group IIA sPLA(2) hydrolysis remained minimal at 10-15% for all minor glycerophospholipids, and was of the order seen for the PtdCho hydrolysis by group IIA sPLA(2) at the 4-h digestion time. All three enzymes attacked the oligo- and polyenoic species in proportion to their mole percentage in the lipoproteins, although there were exceptions. There was evidence of a more rapid destruction of the palmitoyl compared to the stearoyl arachidonoyl glycerophospholipids. Overall, the characteristics of hydrolysis of the molecular species of the lipoprotein-bound diradyl GroPEtn, GroPIns and GroPSer by group V and X sPLA(2)s differed significantly from those observed with lipoprotein-bound PtdCho. As a result, the acidic inositol and serine glycerophospholipids accumulated in the digestion residues of both LDL and HDL, and presumably increased the acidity of the residual particles. An accumulation of the ethanolamine glycerophospholipids in the sPLA(2) digestion residues also had not been previously reported. These results further emphasize the diversity in the enzymatic activity of the group IIA, V and X sPLA(2)s. Since these sPLA(2)s possess comparable tissue distribution, their combined activity may exacerbate their known proinflammatory and proatherosclerotic function.

Current understanding of the role of high-density lipoproteins in atherosclerosis and senescence

Numerous epidemiologic and interventional studies revealed that high-density lipoprotein (HDL) is an important risk factor for coronary heart disease. There are several well documented HDL functions that may account for the antiatherogenic effects of this lipoprotein. The best recognized of these is the capacity of HDL to transport cholesterol from the periphery to the liver, and thereby prevent cholesterol deposition in the arterial wall. Further properties of HDL that may also be antiatherogenic include its potent anti oxidative and anti-inflammatory action. In addition, HDL seems to be involved in processes related to senescence at both the cellular and whole-organism level. Both protein components of HDL (such as apolipoprotein A-I) and its lipid components (such as, lysosphingolipids) appear to mediate the antiatherogenic and anti-aging effects of HDL. The purpose of this review is to summarize the novel functions of HDL that may protect from atherosclerosis and senescence.

ApoA-I Lipidation in Primary Mouse Hepatocytes

The liver is the major site of both apolipoprotein A-I (apoA-I) synthesis and ATP-binding cassette transporter A1 (ABCA1) expression. Here, we compare the lipidation with cholesterol and phospholipid of newly synthesized human apoA-I (hapoA-I) using adenoviral vector-mediated endogenous expression or exogenously added hapoA-I in wild type and ABCA1-null hepatocytes. Hepatocytes were labeled with [3H]cholesterol (delivered with LDL or methyl-beta-cyclodextrin), [3H]mevalonate, or [3H]choline. ABCA1 deficiency decreased apoA-I phospholipidation by 80%, but acquisition of de novo synthesized and exogenous cholesterol only decreased by 40-60%. The transfer of de novo synthesized cholesterol to apoA-I was decreased at all time points, but that of exogenously delivered cholesterol was independent of ABCA1 activity at the early time points. Progesterone does not affect apoA-I synthesis or its lipidation but inhibited the early phase of apoA-I cholesterol lipidation in both wild type and ABCA1-null hepatocytes. Fast protein liquid chromatography analysis of medium lipoproteins confirmed that with ABCA1 deficiency, the proportion of secreted high density lipoprotein-associated apoA-I and cholesterol decreased by about 50%. The very low density lipoprotein (VLDL)/LDL size fraction also contained a significant level of cholesterol in ABCA1 deficiency, consistent with the result of immunoprecipitations showing the presence of lipoproteins with both apoA-I and murine apoB. ApoA-I lipidation with newly synthesized cholesterol in ABCA1-null hepatocytes was significantly decreased by brefeldin A and monensin. In conclusion, we demonstrate that: (i) whereas most hepatic phospholipidation of apoA-I is mediated by ABCA1, acquisition of cholesterol depends on active transfer from intracellular compartments by ABCA1-dependent and -independent pathways, both sensitive to progesterone and (ii) there is separate regulation of phospholipid and cholesterol lipidation of apoA-I in hepatocytes.

ATP-binding cassette transporter AI and its role in HDL formation

PURPOSE OF REVIEW

ATP-binding cassette transporter AI (ABCA1)-mediated assembly of phospholipid and free cholesterol with apoA-I plays an important role in HDL biogenesis. This review focuses on recent progress in ABCA1-mediated HDL formation and regulation of ABCA1 expression.

RECENT FINDINGS

Studies of hepatic ABCA1 overexpression suggest that the liver is a major site for HDL formation. Lipidation of apoA-I by ABCA1 increases its potential for reverse cholesterol transport based on the following findings: (1) apoA-I/lipid complexes formed by ABCA1 are better acceptors of cellular lipid via non-ABCA1-mediated efflux pathways than lipid-free apoA-I in vitro and (2) lipidation of apoA-I prevents it from rapidly associating with plasma HDL in vivo, resulting in more available nascent pre-beta HDL for cellular lipid efflux. Several novel regulatory mechanisms for ABCA1 at the post-transcriptional level have been identified recently. Interaction of apoA-I with ABCA1 prevents phosphorylation of a sequence rich in proline, glutamic acid, serine and threonine in a cytoplasmic domain of ABCA1, resulting in less degradation by calpain proteolysis and increased surface expression of ABCA1. In addition, destabilization and decreased cellular surface expression of ABCA1 protein by unsaturated fatty acids have been identified.

SUMMARY

Initial lipidation of apoA-I by hepatic ABCA1 is critical for plasma HDL formation because it enables pre-beta HDL to function more efficiently as a cholesterol acceptor for other pathways of cholesterol efflux in the reverse cholesterol transport pathway and prevents apoA-I from rapidly associating with preexisting plasma HDL particles, resulting in greater availability of pre-beta HDL particles for cholesterol efflux.

Quality control in the apoA-I secretory pathway

From a total of 47 known apolipoprotein A-I (apoA-I) mutations, only 18 are linked to low plasma HDL apoA-I concentrations, and 78% of these map to apoA-I helices 6 and 7 (residues 143-186). Gene transfer and transgenic mouse studies have shown that several helix 6 apoA-I mutations have reduced hepatic HDL production. Our objective was to examine the impact of helix 6 modifications on intracellular biosynthetic processing and secretion of apoA-I. Cells were transfected with wild-type or mutant apoA-I, radiolabeled with [(35)S]Met/Cys, and then placed in unlabeled medium for up to 4 h. Results show that >90% of newly synthesized wild-type apoA-I was secreted by 60 min. Over the same length of time, only 20% of helix 6 deletion mutant (Delta 6 apoA-I) was secreted, whereas 80% remained cell associated. Microscopic and biochemical studies revealed that cell-associated Delta 6 apoA-I was located predominantly within the cytoplasm as lipid-protein inclusions, whereas wild-type apoA-I was localized in the endoplasmic reticulum/Golgi. Results using other helix deletions or helix 6 substitution mutations indicated that only complete removal of helix 6 resulted in massive cytoplasmic accumulation. These data suggest that alterations in native apoA-I conformation can lead to aberrant trafficking and accumulation of apolipoprotein-phospholipid structures. Thus, conformation-dependent alterations in intracellular trafficking and turnover may underlie the reduced plasma HDL concentrations observed in individuals harboring deletion mutations within helix 6.

The lipidation by hepatocytes of human apolipoprotein A-I occurs by both ABCA1-dependent and -independent pathways

The pathways of hepatic intra- and peri-cellular lipidation of apolipoprotein A-I (apoA-I) were studied by infecting primary mouse hepatocytes from either apoA-I-deficient or ABCA1-deficient mice with a recombinant adenovirus expressing the human apoA-I (hapoA-I) cDNA (endo apoA-I) or incubating the hepatocytes with exogenously added hapoA-I (exo apoA-I) and examining the hapoA-I-containing lipoproteins formed. The cells, maintained in serum-free medium, were labeled with [(3)H]choline, and the cell medium was separated by fast protein liquid chromatography or immunoprecipitated to quantify labeled choline phospholipids specifically associated with hapoA-I. With the apoA-I-deficient hepatocytes, the high density lipoprotein fraction formed with endo apoA-I contained proportionally more phospholipids than that formed with exo apoA-I. However, the lipoprotein size and electrophoretic mobility and phospholipid profiles were similar for exo apoA-I and endo apoA-I. Taken together, these data demonstrate that a significant proportion of hapoA-I is secreted from hepatocytes in a phospholipidated state but that hapoA-I is also phospholipidated peri-cellularly. With primary hepatocytes from ABCA1-deficient mice, the expression and net secretion of adenoviral-generated endogenous apoA-I was unchanged compared with control mice, but (3)H-phospholipids associated with endo apoA-I and exo apoA-I decreased by 63 and 25%, respectively. The lipoprotein size and electrophoretic migration and their phospholipid profiles remained unchanged. In conclusion, we demonstrated that intracellular and peri-cellular lipidation of apoA-I represent distinct and additive pathways that may be regulated independently. Hepatocyte expression of ABCA1 is central to the lipidation of newly synthesized apoA-I but also contributes to the lipidation of exogenous apoA-I. However, a significant basal level of phospholipidation occurs in the absence of ABCA1.

Nascent lipidated apolipoprotein B is transported to the Golgi as an incompletely folded intermediate as probed by its association with network of endoplasmic reticulum molecular chaperones, GRP94, ERp72, BiP, calreticulin, and cyclophilin B

We have previously demonstrated that endoplasmic reticulum (ER)-resident molecular chaperones interact with apolipoprotein B-100 (apoB) during its maturation. The initial stages of apoB folding occur while it is bound to the ER membrane, where it becomes partially lipidated to form a primordial intermediate. We determined whether this intermediate is dependent on the assistance of molecular chaperones for its subsequent folding steps. To that end, microsomes were prepared from HepG2 cells and luminal contents were subjected to KBr density gradient centrifugation. Immunoprecipitation of apoB followed by Western blotting showed that the luminal pool floated at a density of 1.12 g/ml and, like the membrane-bound pool, was associated with GRP94, ERp72, BiP, calreticulin, and cyclophilin B. Except for calreticulin, chaperone/apoB ratio in the lumen was severalfold higher than that in the membrane, suggesting a role for these chaperones both in facilitating the release of the primordial intermediate into the ER lumen and in providing stability. Subcellular fractionation on sucrose gradients showed that apoB in the Golgi was associated with the same array of chaperones as the pool of apoB recovered from heavy microsomes containing the ER, except that chaperone/apoB ratio was lower. KBr density gradient fractionation showed that the major pool of luminal apoB in the Golgi was recovered from 1.02 < d < 1.08 g/ml, whereas apoB in ER was recovered primarily from 1.08 < d < 1.2 g/ml. Both fractions were associated with the same spectrum of chaperones. Together with the finding that GRP94 was found associated with sialylated apoB, we conclude that correct folding of apoB is dependent on the assistance of molecular chaperone, which play multiple roles in its maturation throughout the secretory pathway including distal compartments such as the trans-Golgi network.

Identification of a novel antigen on the apical surface of rat alveolar epithelial type II and Clara cells

Here we describe a monoclonal antibody (MMC4) that recognizes a novel antigen on the apical surface of rat alveolar epithelial type II and Clara cells in the lung, proximal tubule epithelial cells in the kidney, and villus epithelial cells in the small intestine. Biochemical analysis showed that the MMC4 antigen was sensitive to heating and proteinase K digestion and that it is distributed in the detergent-rich phase after Triton X-114 phase separation. These data suggest that the MMC4 antigen is an integral membrane protein. Glycerol gradient sedimentation identified two forms of the MMC4 antigen: one with a sedimentation coefficient of 10.1 and one with a sedimentation coefficient of 1.66, suggesting that the antigen may be part of a multiprotein complex. During rat development (fetal day 16 to adult), the MMC4 antigen increased 12-fold in the lung and 200-fold in the kidney. In the intestine, the MMC4 antigen increased 150-fold by neonatal day 1 and then decreased to adult values. Our data demonstrate that the MMC4 antigen is unlike known type II cell- and Clara cell-associated proteins. The MMC4 monoclonal antibody will be useful as a marker of epithelial cell phenotype in development and injury studies.

Mobilisation of triacylglycerol stores

Triacylglycerol (TAG) is an energy dense substance which is stored by several body tissues, principally adipose tissue and the liver. Utilisation of stored TAG as an energy source requires its mobilisation from these depots and transfer into the blood plasma. The means by which TAG is mobilised differs in adipose tissue and liver although the regulation of lipid metabolism in each of these organs is interdependent and synchronised in an integrated manner. This review deals principally with the mechanism of hepatic TAG mobilisation since this is a rapidly expanding area of research and may have important implications for the regulation of plasma very-low-density lipoprotein metabolism. TAG mobilisation plays an important role in fuel selection in non-hepatic tissues such as cardiac muscle and pancreatic islets and these aspects are also reviewed briefly. Finally, studies of certain rare inherited disorders of neutral lipid storage and mobilisation may provide useful information about the normal enzymology of TAG mobilisation in healthy tissues.

Cell and molecular biology of the assembly and secretion of apolipoprotein B-containing lipoproteins by the liver

Triglycerides are one of the most efficient storage forms of free energy. Because of their insolubility in biological fluids, their transport between cells and tissues requires that they be assembled into lipoprotein particles. Genetic disruption of the lipoprotein assembly/secretion pathway leads to several human disorders associated with malnutrition and developmental abnormalities. In contrast, patients displaying inappropriately high rates of lipoprotein production display increased risk for the development of atherosclerotic cardiovascular disease. Insights provided by diverse experimental approaches describe an elegant biological adaptation of basic chemical interactions required to overcome the thermodynamic dilemma of producing a stable emulsion vehicle for the transport and tissue targeting of triglycerides. The mammalian lipoprotein assembly/secretion pathway shows an absolute requirement for: (1) the unique amphipathic protein: apolipoprotein B, in a form that is sufficiently large to assemble a lipoprotein particle containing a neutral lipid core; and, (2) a lipid transfer protein (microsomal triglyceride transfer protein-MTP). In the endoplasmic reticulum apolipoprotein B has two distinct metabolic fates: (1) entrance into the lipoprotein assembly pathway within the lumen of the endoplasmic reticulum; or, (2) degradation in the cytoplasm by the ubiquitin-dependent proteasome. The destiny of apolipoprotein B is determined by the relative availability of individual lipids and level of expression of MTP. The dynamically varied expression of cholesterol-7alpha-hydroxylase indirectly influences the rate of lipid biosynthesis and the assembly and secretion lipoprotein particles by the liver.

Changes in the morphology and phosphatase cytochemistry of the Golgi region of hepatocytes during the acute phase response to inflammation

BACKGROUND

During the acute phase response to inflammation, the Golgi apparatus of rat hepatocytes processes an increased quantity of glycoproteins, in the form of acute phase reactants.

METHODS

The compartmental organization of the hepatocyte Golgi of control and 24 hour inflamed rats was studied, using transmission electron microscopic techniques, including cytochemistry, to detect nicotinamide adenine dinucleotide phosphatase (NADPase), thiamine pyrophosphatase (TPPase), and cytidine monophosphatase (CMPase) activity.

RESULTS

In inflamed rats, individual Golgi stacks were enlarged, but retained their organization into four compartments: 1) a phosphatase negative, perforated cis-element, 2) two mid-saccules which sometimes were positive for NADPase, 3) one or occasionally two NADPase and TPPase positive trans-saccules, and 4) a tubulovesicular trans-Golgi network (TGN) which was NADPase reactive and contained a spotty TPPase reaction product. Two of these compartments were noticeably altered in response to inflammation. The two mid-saccules were consistently and uniformly dilated. The TGN was altered to the point of being difficult to recognize and had acquired CMPase reactivity. In control rats the TGN consisted of anastomosing tubules forming cage-like structures; secretory granules containing lipoprotein particles pinched off from these. In inflamed rats, most of the cage-like TGN structures had been replaced with an extensive vesicular syncytium which produced secretory granules with a granulofilamentous content.

CONCLUSIONS

In hepatocytes from inflamed rats an apparent switch had occurred in the type of secretory material processed by the Golgi apparatus. Furthermore, the inflammation-induced increase in the size of individual Golgi stacks apparently was not due to a parallel increase in size of all Golgi saccules. Rather, saccules within given Golgi compartments responded in a characteristic and specific manner to the increase in glycoprotein processing that occurs during inflammation.

Insulin regulation of triacylglycerol-rich lipoprotein synthesis and secretion

This review has considered a number of observations obtained from studies of insulin in perfused liver, hepatocytes, transformed liver cells and in vivo and each of the experimental systems offers advantages. The evaluation of insulin effects on component lipid synthesis suggests that overall, lipid synthesis is positively influenced by insulin. Short-term high levels of insulin through stimulation of intracellular degradation of freshly translated apo B and effects on synthesis limit the ability of hepatocytes to form and secrete TRL. The intracellular site of apo B degradation may involve membrane-bound apo B, cytoplasmic apo B and apo B which has entered the ER lumen. How insulin favors intracellular apo B degradation is not known. An area of recent investigation is in insulin-stimulated phosphorylation of intracellular substrates such as IRS-1 which activates insulin specific cellular signaling molecules [245]. Candidate molecules to study insulin action on apo B include IRS-1 and SH2-containing signaling molecules. Insulin dysregulation in carbohydrate metabolism occurs in non-insulin-dependent diabetes mellitus due to an imbalance between insulin sensitivity of tissue and pancreatic insulin secretion (reviewed in Refs. [307,308]). Insulin resistance in the liver results in the inability to suppress hepatic glucose production; in muscle, in impaired glucose uptake and oxidation and in adipose tissue, in the inability to suppress release of free FA. This lack of appropriate sensitivity towards insulin action leads to hyperglycemia which in turn stimulates compensatory insulin secretion by the pancreas leading to hyperinsulinemia. Ultimately, there may be failure of the pancreas to fully compensate, hyperglycemia worsens and diabetes develops. The etiology of insulin resistance is being intensively studied for the primary defect may be over secretion of insulin by the pancreas or tissue insulin resistance and both of these defects may be genetically predetermined. We suggest that, in addition to effects in carbohydrate metabolism, insulin resistance in liver results in the inability of first phase insulin to suppress hepatic TRL production which results in hypertriglyceridemia leading to high levels of plasma FA which accentuate insulin resistance in other target organs. As recently reviewed [17,254] the role of insulin as a stimulator of hepatic lipogenesis and TRL production has been long established. Several lines of evidence support that insulin is stimulatory to the production of hepatic TRL in vivo. First, population based studies support a positive relationship between plasma insulin and total TG and VLDL [253]. Second, there is a strong association between chronic hyperinsulinemia and VLDL overproduction [309].(ABSTRACT TRUNCATED AT 400 WORDS)

Membrane and secretory proteins are transported from the Golgi complex to the sinusoidal plasmalemma of hepatocytes by distinct vesicular carriers

From rat livers labeled in vivo for 30 min with [35S] cys-met, we have isolated two classes of vesicular carriers operating between the Golgi complex and the basolateral (sinusoidal) plasmalemma. The starting preparation is a Golgi light fraction (GLF) isolated by flotation in a discontinuous sucrose density gradient and processed through immunoisolation on magnetic beads coated with an antibody against the last 11 aa. of the pIgA-R tail. GLF and the ensuing subfractions (bound vs nonbound) were lysed, and the lysates processed through immunoprecipitation with anti-pIgA-R and anti-albumin antibodies followed by radioactivity counting, SDS-PAGE, and fluorography. The recovery of newly synthesized pIgA-R was > 90% and the distribution was 90% vs 10% in the bound vs nonbound subfractions, respectively. Albumin radioactivity was recovered to approximately 80%, with 20% and 80% in bound vs nonbound subfractions, respectively. Other proteins studied were: (a) secretory-apolipoprotein-B, prothrombin, C3 component of the complement, and caeruloplasmin; (b) membrane-transferrin receptor, EGR-receptor, asialoglycoprotein receptor, and the glucose transporter. In all the experiments we have performed, the secretory proteins distributed up to 85% in the nonbound subfraction (large secretory vacuoles), whereas the membrane proteins were segregated up to 95% in the bound subfraction (small vesicular carriers). These results suggest that in hepatocytes, membrane and secretory proteins are transported from the Golgi to the basolateral plasmalemma by separate vesicular carriers as in glandular cells capable of constitutive and regulated secretion.

Biosynthesis of lipoprotein: location of nascent apoAI and apoB in the rough endoplasmic reticulum of chicken hepatocytes

Our previous studies showed that in hepatic RER of young chickens, nascent apoAI is not associated with lipoprotein particles and only becomes part of these lipoprotein structures in the Golgi. In this study, we have used three different methodologies to determine the locations of apoAI and apoB in the RER and compared them to that of albumin. Immunoelectron microscopic examination of the RER cell fractions showed that both apoAI and apoB were associated only with the RER membrane whereas albumin was located both within the lumen and on the limiting membrane of the vesicles. To examine the possibility of membrane integration of nascent apoAI and apoB in the RER, we administered L-[3H]leucine to young chickens for 10 min, isolated RER, treated this cell fraction with buffers of varying pH, and measured the release of radioactive albumin, apoAI, and apoB. The majority of nascent apoAI (64%), nascent apoB (100%), and nascent albumin (97%) was released from RER vesicles at pH 11.2, suggesting that, like albumin, apolipoproteins are not integrated within the membrane. To determine if nascent apoproteins are exposed to the cytoplasmic surface, we administered L-[3H]leucine to young chickens and at various times isolated RER and Golgi cell fractions. Radioactive RER and Golgi cell fractions were treated with exogenous protease and the percent of nascent apoAI and apoB accessible to proteolysis was determined and compared to that of albumin. At 5, 10, and 20 min of labeling, 35-56% of nascent apoAI and 60-75% of apoB in RER were degraded, while albumin was refractive to this treatment. At all times both apolipoproteins and albumin present in Golgi cell fractions were protected from proteolysis. These biochemical and morphological findings indicate that apoAI and apoB are associated with the rough microsomal membrane and are partially exposed to the cytoplasmic surface at early stages of secretion. They may later enter the luminal side of the ER and, on entering the Golgi, form lipoprotein particles.

Properties of phosphatidylethanolamine-containing phospholipid-apolipoprotein complexes modified by lecithin-cholesterol acyltransferase

The effect of the inclusion of phosphatidylethanolamine (PE), a phospholipid with unusual packing properties, on the substrate properties of protein-lipid complexes toward lecithin-cholesterol acyltransferase (LCAT) has been studied. Recombinant particles of apolipoprotein A-I with dimyristoylphosphatidylcholine (DMPC), dilauroylphosphatidylethanolamine (DLPE) and cholesterol were prepared at a molar ratio of 1:140:14 (A-I/DMPC/cholesterol) or 1:70:70:14 (A-I/DMPC/DLPE/cholesterol); the efficiency of cholesterol incorporation into complexes containing phosphatidylethanolamine was found to be very pH-dependent, with enhanced cholesterol incorporation at elevated pH values. By incubating the complexes with either purified human LCAT or the d greater than 1.21 g/ml fraction of rat serum as a source of LCAT activity, it was found that a high degree of cholesterol esterification could be achieved with either complex; however, the DLPE-containing complex possessed a much smaller Stokes' diameter than the DMPC-only particle despite compositional similarities between these complexes. With respect to particle diameter the DLPE-containing particles behaved more like complexes prepared with egg yolk lecithin than did complexes prepared with DMPC alone. When human LDL was added to the incubations to provide a source of additional cholesterol, the products were markedly different. Concomitant with an increased cholesteryl ester core was an increase in the protein stoichiometry in both types of particles, from 2 to 3 or 4 apo A-I per particle. The proportion of DLPE to DMPC in the products was reduced from 1:1 to 0.3:1, reflecting a preferential hydrolysis of PE by LCAT, and the Stokes' diameters of the DMPC-only and the DLPE-containing complexes were closely similar. We conclude that the presence of elevated proportions of certain phospholipid species may significantly alter both the physical properties of the particles and their substrate properties with regard to reactions with enzymes of lipid metabolism.

The assembly of lipids into lipoproteins during secretion

The process of assembly and secretion of lipoproteins is discussed with particular reference to the role of lipids. The majority of circulating lipoproteins is produced by the liver (80%) with the remainder being supplied by the intestine. The liver secretes both very low density lipoproteins and high density lipoproteins, but the assembly and secretion of these two types of particles may follow different routes. The major lipid components of lipoproteins are triacylglycerols, cholesterol, cholesterol esters and phospholipids. The biosynthesis of these lipids occurs on membranes of the endoplasmic reticulum, with many of the enzymes also being present in the Golgi; the roles of these two subcellular organelles in the assembly of lipoproteins are discussed. There appears to be a compartmentalization of lipids in cells, such that defined pools, often those newly-synthesized, are preferred, or even required, for lipoprotein assembly. The process of hepatic very low density lipoprotein secretion appears to be regulated by the supply of lipids. Indeed, the synthesis of new lipid may be a major driving force in lipoprotein assembly and secretion.

Effect of phosphatidylethanolamine on the properties of phospholipid-apolipoprotein complexes

Plasma high density lipoproteins (HDL) are synthesized in intestinal mucosal cells and hepatocytes and are secreted into the blood. Factors influencing the structure and function of these HDL, such as lipid and protein composition, are poorly understood. It appears, however, that intracellular, discoidal HDL are enriched, relative to plasma HDL, in phosphatidylethanolamine (PE), a phospholipid known to generate unusual, nonbilayer structures of putative physiological significance. Although incubation of dimyristoylphosphatidylcholine (DMPC) with apolipoprotein A-I at the gel-liquid crystalline phase transition temperature results in the spontaneous formation of lipid-protein complexes, the presence of proportionately small amounts of PE prevents the formation of such complexes, suggesting that PE profoundly alters the phase properties of the phospholipid bilayers. However, by using a detergent-mediated method for the formation of PE-rich model nascent HDL from phospholipids and apolipoprotein A-I, lipid-protein complexes containing as much as 75% DLPE could be formed, thus demonstrating that the presence of PE causes a kinetic, rather than a thermodynamic, barrier to spontaneous complex formation. The products contained a DLPE:DMPC molar ratio similar to that of the initial incubation mixture; however, as the mole percentage of DLPE increased, the products became less heterogeneous, the buoyant density of the products increased, and the Stokes diameter of the products decreased. Similar results were obtained when dimyristoylphosphatidylethanolamine (DMPE) and dipalmitoylphosphatidylethanolamine (DPPE) were employed in lieu of DLPE. Electron microscopy of complexes containing DLPE and DMPC at a 1:1 molar ratio showed that these particles possessed a discoidal, bilayer morphology similar to that seen with complexes containing only phosphatidylcholine.(ABSTRACT TRUNCATED AT 250 WORDS)

Assembly and secretion of hepatic very-low-density lipoprotein

In contrast to water-soluble fuels such as glucose or ketone bodies, the use of lipids as an energy source for tissues has required the development of complex structures for their transport through the aqueous plasma. In the case of endogenously synthesized triacylglycerol this is achieved by the assembly and secretion of hepatic VLDL which provides the necessary stability in an aqueous medium. An essential component of this assembly process is apo B. Dietary changes which require an increase in hepatic VLDL secretion appear to be accompanied by increases in the availability of functional apo B. Interesting questions relate to: (a) the intracellular site(s) of triacylglycerol association with apo B, and (b) the mechanism(s) by which the availability of functional apo B at this site responds to metabolic and hormonal signals which reflect dietary status and, thus, the need to secrete triacylglycerol. As regards the latter, although in some cases changes in apo B synthesis occur in response to VLDL secretion hepatic apo B mRNA levels appear to be quite stable in vitro. Intracellular switching of apo B between the secretory and degradative pathways may be important in controlling VLDL assembly and post-translational modifications of the apoprotein may also play a role by influencing its ability to bind to triacylglycerol. Transport is not the only problem associated with the utilization of a concentrated energy source such as triacylglycerol and the complex problems of waste product disposal and recycling have to be dealt with. In the case of triacylglycerol, potentially toxic waste products include atherogenic remnants and LDL. The overall problem, then, in the long-term, involves the development of a 'safe' means of utilizing triacylglycerol and this requirement accounts for much of the complexity of plasma lipoprotein metabolism. In this area, the rat could teach the human a few tricks. One of these appears to be the utilization of hepatic apo B48 rather than apo B100 for VLDL assembly in response to increases in the extrahepatic utilization of hepatically synthesized triacylglycerol. Under these conditions, the remnants of hepatic triacylglycerol utilization by peripheral tissues are cleared from the plasma much more readily via a process which seems to involve the cycling of more triacylglycerol back to the liver than that which occurs in humans. The means by which this is achieved, though, are obscure and may involve a chylomicron remnant receptor, the nature of which, itself, remains controversial.(ABSTRACT TRUNCATED AT 400 WORDS)

Expression and secretion of chicken apolipoprotein AI in transfected COS cells

A full-length chicken apolipoprotein A-I (apoAI) cDNA has been cloned into an expression vector, pRSVapoAI. This plasmid was transfected into a monkey kidney (COS-1) cell line in order to study apolipoprotein-lipid assembly. Chicken apoAI is the major apolipoprotein of chicken high-density lipoprotein (HDL), which is less complex in apolipoprotein content than the HDL of human plasma. The transient transfected COS-1 cells synthesized and secreted authentic plasma apoAI. Under serum-free medium conditions, COS cells secreted only proapoAI. A small portion (15%) of the secreted apoAI floated at a density 1.07-1.20 g/ml. Upon incubation with fetal bovine serum at 10 degrees C, a majority of the apoAI was recovered in the HDL density (1.06-1.20 g/ml) region. Secreted apoAI was labeled when transfected COS cells were incubated with [U-14C]palmitate, but the incorporation of radioactivity was not the result of fatty acid acylation through ester bond formation. These results indicate that heterologous COS-1 cells are capable of synthesizing and secreting apoAI, and that intracellular association of apoAI with lipids is not necessary for secretion.

Nascent very low density lipoproteins from rat hepatocytic Golgi fractions are enriched in phosphatidylethanolamine

The phospholipid composition of nascent very low density lipoproteins (VLDL) of rat hepatocytic Golgi fractions differs greatly from that of plasma VLDL. The phospholipids of nascent VLDL contain about four times more phosphatidylethanolamine (PE) than plasma VLDL, whereas plasma VLDL contain considerably more sphingomyelin. Thus, the ratio of PE to sphingomyelin differs by a factor of about 12 between nascent Golgi VLDL and circulating plasma VLDL. It is evident from these data that the PE/sphingomyelin ratio of VLDL can be used to estimate endosomal contamination of hepatocytic Golgi fractions.

Expression of apolipoprotein B epitopes in lipoproteins. Relationship to conformation and function

The immunochemical properties of apolipoprotein (apo) B have been studied in very low density lipoprotein (VLDL)1 (Sf 100 to 400), VLDL2 (Sf 60 to 100), VLDL3 (Sf 20 to 60), different intermediate density lipoprotein (IDL), and low density lipoprotein (LDL) subfractions isolated from patients with type IV hypertriglyceridemia. In these lipoproteins, we characterized the association of apo B with other apolipoproteins and the expression and immunoreactivity of several apo B epitopes close to the apo B receptor binding sites (3F5, 4G3, 3A8, and 5E11) and of other epitopes located on the apo B100-B48 common region (1D1 and 2D8). Immunoprecipitation showed that the proportion of lipoprotein particles expressing each apo B epitope increased from VLDL1 to LDL2; this was more apparent with 3A8 and 5E11 than with 3F5. The VLDL that were negative for apo E epitopes (60% or more of the total) were enriched in apo C. The lipoprotein particles containing apo E and/or apo C-III decreased progressively from VLDL1 (30% and 85%, respectively) to LDL2 (10% and 25%, respectively). Similar observations were made for apo C-I and apo D, demonstrating that apolipoprotein heterogeneity is greatest in the lightest lipoproteins. By competitive radioimmunoassay, the epitope for 4G3 was equally immunoreactive in each lipoprotein subclass, and the affinity constant (Ka) of 4G3 for different lipoproteins showed little variation. In contrast, both immunoreactivity and Ka of 3A8 and 5E11 increased progressively and significantly with the increasing density of the lipoprotein subclasses. This phenomenon is correlated with the increasing binding affinity of apo B in these lipoprotein subclasses to the LDL receptor of fibroblasts. We conclude that, as the apo B-containing lipoproteins become smaller, the conformation of specific regions of apo B is modified: in the receptor binding domain, the conformation of epitope 4G3, which is mapped between residues 2980 and 3080, remains constant, while that of 3A8 and 5E11 (residues 3441 to 3568) changes progressively. We propose the theory that the change in conformation in the domain spanning residues 3441 and 3568 allows the maximum expression of epitopes 3A8 and 5E11 and of the receptor binding site.

Hepatocytic lipoprotein receptors and intracellular lipoprotein catabolism

Hepatocytes, as the major site of synthesis and terminal catabolism of plasma lipoproteins, exert the major regulatory influence on the concentration of atherogenic lipoproteins in blood plasma and may thereby influence the rate of atherogenesis. The LDL receptor on the microvillous sinusoidal surface of hepatocytes mediates the catabolism of remnants of triglyceride-rich lipoproteins and LDL. Binding of VLDL remnants to the receptor, mediated by apo E, is of very high affinity and presumably multivalent, whereas binding of LDL, mediated by apo B-100, is monovalent and of lower affinity, accounting for the much longer residence time of the latter in the blood. The magnitude of the influx of lipoprotein particles into hepatocytic endosomal compartments dwarfs that of other macromolecules undergoing receptor-mediated endocytosis and terminal catabolism in lysosomes of these cells. The intracellular compartments and processing steps in hepatocytic lipoprotein uptake and degradation are essentially the same as those described for other ligands in the liver and other cells. Receptors with bound lipoproteins migrate into coated pits which become coated vesicles. These vesicles uncoat and fuse to form CURL vesicles and tubules near the cell surface where most receptors are recycled, presumably via receptor-rich appendages that become separated from the vesicles. CURL vesicles become mature MVBs as they migrate to the Golgi/bile canalicular pole of hepatocytes, where they fuse with putative Golgi-derived primary lysosomes and are transformed into heterophagic secondary lysosomes. MVBs also contain a receptor-rich appendage that may recycle some receptors directly to the cell surface or through adjacent Golgi compartments. Dilated ends of trans-Golgi cisternae contain nascent VLDL undergoing packaging for secretion following their synthesis and assembly in the endoplasmic reticulum. Because these "forming secretory vesicles" resemble remnant-filled MVBs, occur in a similar location in the Golgi area of hepatocytes and coisolate in centrifugal fractions of liver homogenates, there has been considerable confusion about the identity of these compartments. With the aid of specific endocytic and exocytic markers, highly purified and morphologically intact endosomal and Golgi compartments can now be obtained from rat liver homogenates. The availability of these and similar fractions of defined purity should facilitate investigation of the hepatocytic processing of endocytosed and secreted macromolecules. Although chylomicron remnants are also taken up by receptor-mediated endocytosis, the nature of the hepatocytic remnant receptor remains elusive.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence that during very low density lipoprotein assembly in rat hepatocytes most of the triacylglycerol and phospholipid are packaged with apolipoprotein B in the Golgi complex

Rat liver lipids were labelled by an intraportal injection of [3H]palmitic acid followed by isolation of rough and smooth endoplasmic reticulum or 'cis' or 'trans'-enriched Golgi fractions. The preparations were separated into membrane and contents and the apolipoprotein B of the content fractions was immunoprecipitated. More than 90% of the labelled triacylglycerol and phospholipid secreted into the blood immunoprecipitated with apolipoprotein B. Under the same experimental conditions 8, 12, 27 and 59% of the lipids of the rough, smooth, 'cis-Golgi' and 'trans-Golgi' contents, respectively, were immunoprecipitated. Thus, the 'trans-Golgi' region appears to be the major intracellular site of assembly of apolipoprotein B with triacylglycerol and phospholipid.

Regulation of intestinal and hepatic apoprotein synthesis after chronic fat and cholesterol feeding

Although diet influences levels of lipoproteins and their corresponding apoproteins, its effects on the molecular regulation of apoprotein synthesis are relatively unknown. Male Sprague-Dawley rats were fed an atherogenic diet containing cholesterol and propylthiouracil (PTU). Intestinal apo AI and AIV mRNA concentrations were decreased by the atherogenic diet, but apo AI and AIV synthesis was increased in vitro (organ explants) and in vivo (polysome runoff), consistent with regulation at the translational level. In contrast, hepatic apo E mRNA concentration and synthesis were increased after the atherogenic diet, consistent with pretranslational regulation. The response to cholesterol feeding for hepatic apo AI and E showed a third pattern of regulation, in which synthesis increased and mRNA content remained stable or fell, again suggesting translational control, but polysome runoff synthesis was unchanged. The apparent importance of translational regulation in the intestine is consistent with the necessity for the tissue to respond rapidly to changes in intraluminal content.

Apolipoprotein B: structure, biosynthesis and role in the lipoprotein assembly process

The complete amino acid sequence of the liver-synthesized apolipoprotein B (apoB) species, apoB 100, has been derived from cloned cDNA. The protein consists of 4536 amino acids (+ a 27 amino acid signal sequence). Cysteine is clustered in the N-terminal 1/10 of the protein, suggesting the presence of a stabilized tertiary structure in this part of the molecule. Three types of structure are suggested to be of importance for the binding of the protein to lipids; (i) hydrophobic sequences with a high probability for beta-sheet structure, (ii) strict amphipathic beta-sheets, and (iii) amphipathic alfa-helices. An apoB 100 molecule is completed within 10-14 min and secreted after approximately 30 min, 1/3 of which is due to the transfer through the endoplasmic reticulum (ER), while 2/3 is spent in the Golgi apparatus. ApoB 100 is co-translationally N-glycosylated and 25% of the oligosaccharide chains is processed in the Golgi compartment. Other posttranslational modifications that have been discussed include covalent acylation and phosphorylation. It has also been suggested that the lipid moiety of the apoB 100 lipoproteins are modified during the passage through the Golgi apparatus. The site of lipoprotein assembly is suggested to be separated from the site of apoB 100 synthesis, and apoB 100 appears to be co-translationally bound to the ER membrane and from this transferred to the ER lumen. Based on these observations a model for the assembly of apoB 100 lipoproteins is discussed in this paper. The intestinal derived apoB species, apoB 48, has a molecular mass of 210 kDa and appears to correspond to the N-terminal 48% of apoB 100. The mechanism by which apoB 48 is formed is still not known. Available data indicate that the protein is formed within the intestinal cells, these data also argue against the possibility that apoB 48 is formed by posttranslational proteolysis of apoB 100. The formation of a separate apoB 48 mRNA by alternative splicing has been suggested, based on the observation of a 7 kb mRNA which corresponds to the 5' portion of the apoB 100 mRNA. However, the most abundant apoB mRNA species found in the intestine have a size that corresponds to that of the apoB 100 mRNA, furthermore the observation that apoB 48 appears to terminate in a 7.5 kb exon that appears to lack alternative splice sites, does not favour the possibility of alternative splicing.

The Fao cell. A tissue culture model for lipoprotein synthesis and secretion. II. Modulation by lipid depletion and supplementation

We have shown that the Fao cell, a differentiated rat hepatoma line, is an excellent model for the study of the synthesis of lipoproteins (Scarino, M L & Howell, K E, Exp cell res 170 (1987) 1 [1]. Here we demonstrate that variation of the lipid composition of the growth medium significantly modulates the composition and quantity of particles formed. Three growth conditions were compared: normal, lipid-depleted, and lipid-supplemented. The synthesis of both the protein and lipid moieties of the lipoproteins was quantitated using the radioactive metabolic precursors [35S]methionine and [14C]acetate. The total secretion of the cells was collected and fractionated into four density classes equivalent to plasma lipoproteins and a bottom fraction equivalent to plasma proteins. Each density class was evaluated for the apoprotein distribution after separation by SDS-PAGE and for lipid distribution and composition after lipid extraction. ApoE accounts for approx. 15% of the total protein synthesized and is the major apoprotein. The amount synthesized remains relatively constant under all growth conditions. In contrast, the amount of apoB synthesis varies over 600-fold. In lipid-depleted conditions, only 0.01 times the normal amount was synthesized, while in lipid-supplemented conditions 6.2 times the normal amount was synthesized. ApoB was associated with the lighter fraction; therefore the modulation increased the quantity of low-density particles formed. A similar but far less pronounced variation of the heavier particles and the apoA-I concentration was obtained. Under lipid-depleted conditions, 0.75 times the normal amount was synthesized, while under lipid-supplemented conditions 2.6 times the normal quantity was synthesized.