Inhibition of apolipoprotein E degradation in a post-Golgi compartment by a cysteine protease inhibitor

Microglial P2X4 receptors promote ApoE degradation and contribute to memory deficits in Alzheimer's disease

Numerous evidences support that microglia contributes to the progression of Alzheimer's disease. P2X4 receptors are ATP-gated channels with high calcium permeability, which are de novo expressed in a subset of reactive microglia associated with various pathological contexts, contributing to microglial functions. P2X4 receptors are mainly localized in lysosomes and trafficking to the plasma membrane is tightly regulated. Here, we investigated the role of P2X4 in the context of Alzheimer's disease (AD). Using proteomics, we identified Apolipoprotein E (ApoE) as a specific P2X4 interacting protein. We found that P2X4 regulates lysosomal cathepsin B (CatB) activity promoting ApoE degradation; P2rX4 deletion results in higher amounts of intracellular and secreted ApoE in both bone-marrow-derived macrophage (BMDM) and microglia from APPswe/PSEN1dE9 brain. In both human AD brain and APP/PS1 mice, P2X4 and ApoE are almost exclusively expressed in plaque-associated microglia. In 12-month-old APP/PS1 mice, genetic deletion of P2rX4 reverses topographical and spatial memory impairment and reduces amount of soluble small aggregates of Aß1-42 peptide, while no obvious alteration of plaque-associated microglia characteristics is observed. Our results support that microglial P2X4 promotes lysosomal ApoE degradation, indirectly altering Aß peptide clearance, which in turn might promotes synaptic dysfunctions and cognitive deficits. Our findings uncover a specific interplay between purinergic signaling, microglial ApoE, soluble Aß (sAß) species and cognitive deficits associated with AD.

Isoform-dependent lysosomal degradation and internalization of apolipoprotein E requires autophagy proteins

The human apolipoprotein E4 isoform (APOE4) is the strongest genetic risk factor for late-onset Alzheimer's disease (AD), and lysosomal dysfunction has been implicated in AD pathogenesis. We found, by examining cells stably expressing each APOE isoform, that APOE4 increases lysosomal trafficking, accumulates in enlarged lysosomes and late endosomes, alters autophagic flux and the abundance of autophagy proteins and lipid droplets, and alters the proteomic contents of lysosomes following internalization. We investigated APOE-related lysosomal trafficking further in cell culture, and found that APOE from the post-Golgi compartment is degraded through autophagy. We found that this autophagic process requires the lysosomal membrane protein LAMP2 in immortalized neuron-like and hepatic cells, and in mouse brain tissue. Several macroautophagy-associated proteins were also required for autophagic degradation and internalization of APOE in hepatic cells. The dysregulated autophagic flux and lysosomal trafficking of APOE4 that we observed suggest a possible novel mechanism that might contribute to AD pathogenesis. This article has an associated First Person interview with the first author of the paper.

Bifunctional small molecules that mediate the degradation of extracellular proteins

Targeted protein degradation (TPD) has emerged as a promising therapeutic strategy. Most TPD technologies use the ubiquitin-proteasome system, and are therefore limited to targeting intracellular proteins. To address this limitation, we developed a class of modular, bifunctional synthetic molecules called MoDE-As (molecular degraders of extracellular proteins through the asialoglycoprotein receptor (ASGPR)), which mediate the degradation of extracellular proteins. MoDE-A molecules mediate the formation of a ternary complex between a target protein and ASGPR on hepatocytes. The target protein is then endocytosed and degraded by lysosomal proteases. We demonstrated the modularity of the MoDE-A technology by synthesizing molecules that induce depletion of both antibody and proinflammatory cytokine proteins. These data show experimental evidence that nonproteinogenic, synthetic molecules can enable TPD of extracellular proteins in vitro and in vivo. We believe that TPD mediated by the MoDE-A technology will have widespread applications for disease treatment.

Perilipin-2 is critical for efficient lipoprotein and hepatitis C virus particle production

In hepatocytes, PLIN2 is the major protein coating lipid droplets (LDs), an organelle the hepatitis C virus (HCV) hijacks for virion morphogenesis. We investigated the consequences of PLIN2 deficiency on LDs and on HCV infection. Knockdown of PLIN2 did not affect LD homeostasis, likely due to compensation by PLIN3, but severely impaired HCV particle production. PLIN2-knockdown cells had slightly larger LDs with altered protein composition, enhanced local lipase activity and higher β-oxidation capacity. Electron micrographs showed that, after PLIN2 knockdown, LDs and HCV-induced vesicular structures were tightly surrounded by ER-derived double-membrane sacs. Strikingly, the LD access for HCV core and NS5A proteins was restricted in PLIN2-deficient cells, which correlated with reduced formation of intracellular HCV particles that were less infectious and of higher density, indicating defects in maturation. PLIN2 depletion also reduced protein levels and secretion of ApoE due to lysosomal degradation, but did not affect the density of ApoE-containing lipoproteins. However, ApoE overexpression in PLIN2-deficient cells did not restore HCV spreading. Thus, PLIN2 expression is required for trafficking of core and NS5A proteins to LDs, and for formation of functional low-density HCV particles prior to ApoE incorporation.This article has an associated First Person interview with the first author of the paper.

Apolipoprotein E in lipoprotein metabolism, health and cardiovascular disease

Apolipoprotein E (apoE), a 34 kDa circulating glycoprotein of 299 amino acids, predominantly synthesised in the liver, associates with triglyceride-rich lipoproteins to mediate the clearance of their remnants after enzymatic lipolysis in the circulation. Its synthesis in macrophages initiates the formation of high density-like lipoproteins to effect reverse cholesterol transport to the liver. In the nervous system apoE forms similar lipoproteins which perform the function of distributing lipids amongst cells. ApoE accounts for much of the variation in plasma lipoproteins by three common variants (isoforms) that influence low-density lipoprotein concentration and the risk of atherosclerosis. ApoE2 generally is most favourable and apoE4 least favourable for cardiovascular and neurological health. The apoE variants relate to different amino acids at positions 112 and 158: cysteine in both for apoE2, arginine at both sites for apoE4, and respectively cysteine and arginine for apoE3 that is viewed as the wild type. Paradoxically, under metabolic stress, homozygosity for apoE2 may result in dysbetalipoproteinaemia in adults owing to impaired binding of remnant lipoproteins to the LDL receptor and related proteins as well as heparan sulphate proteoglycans. This highly atherogenic condition is also seen with other mutations in apoE, but with autosomal dominant inheritance. Mutations in apoE may also cause lipoprotein glomerulopathy. In the central nervous system apoE binds amyloid β-protein and tau protein and fragments may incur cellular damage. ApoE4 is a strong risk factor for the development of Alzheimer's disease. ApoE has several other physiological effects that may influence health and disease, including supply of docosahexaenoic acid for the brain and modulating immune and inflammatory responses. Genotyping of apoE may have application in disorders of lipoprotein metabolism as well as glomerulopathy and may be relevant to personalised medicine in understanding cardiovascular risk, and the outcome of nutritional and therapeutic interventions. Quantitation of apoE will probably not be clinically useful. ApoE is also of interest as it may generate peptides with biological function and could be employed in nanoparticles that may allow crossing of the blood-brain barrier. Therapeutic options may emerge from these newer insights.

S-Adenosylmethionine increases circulating very-low density lipoprotein clearance in non-alcoholic fatty liver disease

BACKGROUND & AIMS

Very-low-density lipoproteins (VLDLs) export lipids from the liver to peripheral tissues and are the precursors of low-density-lipoproteins. Low levels of hepatic S-adenosylmethionine (SAMe) decrease triglyceride (TG) secretion in VLDLs, contributing to hepatosteatosis in methionine adenosyltransferase 1A knockout mice but nothing is known about the effect of SAMe on the circulating VLDL metabolism. We wanted to investigate whether excess SAMe could disrupt VLDL plasma metabolism and unravel the mechanisms involved.

METHODS

Glycine N-methyltransferase (GNMT) knockout (KO) mice, GNMT and perilipin-2 (PLIN2) double KO (GNMT-PLIN2-KO) and their respective wild type (WT) controls were used. A high fat diet (HFD) or a methionine deficient diet (MDD) was administrated to exacerbate or recover VLDL metabolism, respectively. Finally, 33 patients with non-alcoholic fatty-liver disease (NAFLD); 11 with hypertriglyceridemia and 22 with normal lipidemia were used in this study.

RESULTS

We found that excess SAMe increases the turnover of hepatic TG stores for secretion in VLDL in GNMT-KO mice, a model of NAFLD with high SAMe levels. The disrupted VLDL assembly resulted in the secretion of enlarged, phosphatidylethanolamine-poor, TG- and apoE-enriched VLDL-particles; special features that lead to increased VLDL clearance and decreased serum TG levels. Re-establishing normal SAMe levels restored VLDL secretion, features and metabolism. In NAFLD patients, serum TG levels were lower when hepatic GNMT-protein expression was decreased.

CONCLUSIONS

Excess hepatic SAMe levels disrupt VLDL assembly and features and increase circulating VLDL clearance, which will cause increased VLDL-lipid supply to tissues and might contribute to the extrahepatic complications of NAFLD.

Regulation of endogenous apolipoprotein E secretion by macrophages

Apolipoprotein E has critical roles in the protection against atherosclerosis and is understood to follow the classical constitutive secretion pathway. Recent studies have indicated that the secretion of apoE from macrophages is a regulated process of unexpected complexity. Cholesterol acceptors such as apolipoprotein A-I, high density lipoprotein, and phospholipid vesicles can stimulate apoE secretion. The ATP binding cassette transporter ABCA1 is involved in basal apoE secretion and in lipidating apoE-containing particles secreted by macrophages. However, the stimulation of apoE secretion by apoA-I is ABCA1-independent, indicating the existence of both ABCA1-dependent and -independent pathways of apoE secretion. The release of apoE under basal conditions is also regulated, requiring intact protein kinase A activity, intracellular calcium, and an intact microtubular network. Mathematical modeling of apoE turnover indicates that whereas some pools of apoE are committed to either secretion or degradation, other pools can be diverted from degradation toward secretion. Targeted inhibition or stimulation of specific apoE trafficking pathways will provide unique opportunities to regulate the biology of this important molecule.

Cellular determinants of hepatitis C virus assembly, maturation, degradation, and secretion

Intracellular infectious hepatitis C virus (HCV) particles display a distinctly higher buoyant density than do secreted virus particles, suggesting that the characteristic low density of extracellular HCV particles is acquired during viral egress. We took advantage of this difference to examine the determinants of assembly, maturation, degradation, and egress of infectious HCV particles. The results demonstrate that HCV assembly and maturation occur in the endoplasmic reticulum (ER) and post-ER compartments, respectively, and that both depend on microsomal transfer protein and apolipoprotein B, in a manner that parallels the formation of very-low-density lipoproteins (VLDL). In addition, they illustrate that only low-density particles are efficiently secreted and that immature particles are actively degraded, in a proteasome-independent manner, in a post-ER compartment of the cell. These results suggest that by coopting the VLDL assembly, maturation, degradation, and secretory machinery of the cell, HCV acquires its hepatocyte tropism and, by mimicry, its tendency to persist.

The Proprotein Convertase SKI-1/S1P

Subtilisin kexin isozyme-1 (SKI-1) represents the first mammalian member of secretory subtilisin-like processing enzymes that cleaves after nonbasic residues. It is synthesized as an inactive precursor that undergoes three sequential autocatalytic processing steps of its N-terminal prosegment and an ectodomain shedding at a site near the transmembrane domain. The various cellular functions of SKI-1 emphasize the need to understand the sites of its activation and shedding. We have previously shown that SKI-1 undergoes autocatalytic shedding at the sequence KHQKLL(953) downward arrow, resulting in a membrane-bound stump called St-1 (amino acids 954-1052). However, little is known about the cellular localization of SKI-1 or its shed forms. In the present study, we have further identified a smaller C-terminal fragment St-2 generated closer to the transmembrane domain. By sequencing and mass spectrometric analysis, the start site and the molecular mass of St-2 were determined. Site-directed mutagenesis revealed the critical amino acid involved in this novel process. Mutation of Met(990) to M990A, M990I, and M990L failed to generate St-2, suggesting an internal alternate translation event at Met(990), as confirmed by an in vitro transcription/translation assay. Confocal microscopy defined the subcellular localization of SKI-1 and its fragments. The data show that most of membrane-bound SKI-1 and its stumps St-1 and St-2 localize to the Golgi and can enter the endosomal/lysosomal compartments but do not sort to the cell surface. Deletion studies showed that the transmembrane domain of SKI-1 determines its trafficking. Finally, rSt-1 and rSt-2 seem to affect the processing of ATF6 by SKI-1, but cellular stress does not regulate the production of St-2.

Distinction between processing of normal and mutant complement C3 within human skin fibroblasts

Inherited C3 deficiency may result from mutations in the C3 gene affecting transcription or translation (type I deficiency). We described a type II C3 deficiency caused by a mutation yielding an abnormal non-secreted C3. The post-translational processing of mutant and normal C3 was analyzed in fibroblasts grown from skin biopsies. Mutant C3 is located mainly in the endoplasmic reticulum (ER), whereas normal C3 is seen evenly distributed throughout the cytoplasm. Most of the mutant C3 is degraded within the cell, and only a small fraction (around 8%) is secreted after 20 h chase. Processing of C3 at 19 degrees C was reduced in normal fibroblasts but completely blocked in mutant fibroblasts. ATP depletion blocked processing of normal proC3 to C3. In contrast, the mutant proC3 was partly degraded in ATP-depleted cells, yet its complete degradation and secretion were blocked. Intracellular degradation of the mutant C3 was not inhibited by NH4Cl, thus excluding cleavage within lysosomes. These results demonstrate that the type II mutant C3 studied here is retained in the ER probably by a quality contol machinery that identifies abnormal protein folding. Consequently, it is destined to undergo a two-step intracellular degradation; an initial ATP-independent step followed by an ATP-dependent step.

O-Glycosylation of C-terminal tandem-repeated sequences regulates the secretion of rat pancreatic bile salt-dependent lipase

Amino acid sequences rich in Pro, Glu, Ser, and Thr (PEST) are common to rapidly degraded proteins (Rogers, S., Wells, R. & Rechsteiner, M. (1986) Science 234, 364-368). On pancreatic bile salt-dependent lipase (BSDL), PEST sequences are present in the C-terminal region of the enzyme to which is associated the O-glycosylation. We have postulated that the O-glycosylation of BSDL may contribute to mask PEST sequences and to trigger the secretion of this enzyme instead of its delivery into a degradative pathway (Bruneau, N., and Lombardo, D. (1995) J. Biol. Chem. 270, 13524-13523). To further examine the role of the O-linked glycosylation on BSDL metabolism, rat pancreatic BSDL cDNA was stably transfected into two Chinese hamster ovary (CHO) cell lines, the CHO K1 wild-type line and the O-glycosylation defective CHO ldlD line. In these latter cells, O-glycosylation can be reversibly modulated by culture conditions. Results indicate that the rate of BSDL synthesis by transfected CHO K1 or CHO ldlD cells reflects, independently of culture conditions, the amount of mRNA specific for BSDL present in these transfected cells. Nevertheless, the rate of secretion of the enzyme depends upon cell culture conditions and increases with the cell capability to O-glycosylate C-terminal tandem-repeated sequences. Immunoprecipitation experiments performed on cell lysates suggested that a rapid degradation of BSDL occurred particularly when transfected CHO ldlD cells were cultured under non-permissive conditions. We further showed that BSDL secreted by CHO ldlD cells grown under non-permissive conditions that normally prevent O-glycosylation incorporated galactose and was reactive with peanut agglutinin, which recognizes the core structure of O-linked glycans. We concluded that the BSDL expressed by CHO ldlD cells grown under non-permissive conditions was rapidly degraded but a fraction of the enzyme was allowed to O-glycosylate and consequently was secreted.

Degradation of distinct assembly forms of immunoglobulin M occurs in multiple sites in permeabilized B cells

Protein degradation is essential for quality control which retains and eliminates abnormal, unfolded, or partially assembled subunits of oligomeric proteins. The localization of this nonlysosomal pre-Golgi degradation to the endoplasmic reticulum (ER) has been mostly deduced from kinetic studies and carbohydrate analyses, while direct evidence for degradation within the ER has been provided by in vitro reconstitution of this process. In this article, we took advantage of the transport incompetence of permeabilized cells to directly demonstrate that the selective degradation of secretory IgM (sIgM) in B lymphocytes is transport-dependent. We show that, upon permeabilization of the plasma membrane with either streptolysin O or digitonin, sIgM is not degraded unless transport is allowed. Nevertheless, upon complete reduction of interchain disulfide bonds with thiols, the free mu heavy chains are degraded by a transport-independent quality control mechanism within the ER. This latter degradation is nonselective to the secretory heavy chain mus, and the membrane heavy chain mum, which is normally displayed on the surface of the B cell, is also eliminated. Moreover, the degradation of free mus is no longer restricted to B lymphocytes, and it takes place also in the ER of plasma cells which normally secrete polymers of sIgM. Conversely, when assembled with the light chain, the degradation is selective to sIgM, is restricted to B lymphocytes, and is a transport-dependent post-ER event.

Cell surface proteoglycans modulate net synthesis and secretion of macrophage apolipoprotein E

Using a macrophage cell line that constitutively expresses a human apolipoprotein E (apoE) cDNA, we have investigated the post-translational metabolism of endogenously produced apoE. Inhibition of lysosomal or cysteine proteases led to significant inhibition of apoE degradation but did not increase apoE secretion, indicating that cellular degradation is not limiting for apoE secretion in macrophages. Treatment of macrophages with inhibitors of proteoglycan synthesis (4-methylumbelliferyl-beta-D-xyloside) or sulfation (sodium chlorate) enhanced the release of apoE from cells and significantly attenuated the increase in secretion produced by incubation with phosphatidylcholine vesicles (PV). These observations suggested that a significant fraction of the apoE retained by cells (and released by incubation with PV) was associated with proteoglycans. Treatment of cells with exogenous heparinase led to a greater than 4-fold increase in apoE secretion and similarly attenuated the response to PV, suggesting that apoE was trapped in an extracellular proteoglycan matrix. This conclusion was confirmed in studies showing that PV could enhance the release of apoE from cells during an incubation at 4 degrees C, but this enhanced release was abolished in proteoglycan-depleted cells. Incubation with lactoferrin at 4 or 37 degrees C produced a similar decrement in cellular apoE, again indicating the existence of a cell surface pool of apoE. Pulse-chase studies showed that the apoE trapped in the proteoglycan matrix was susceptible to rapid cellular degradation such that net synthesis of apoE (secreted plus cell-associated) was increased significantly in proteoglycan-depleted cells compared with control cells as early as 45 min during a chase period.

The possible place of cathepsins and cystatins in the puzzle of Alzheimer disease: a review

Lysosomal proteinases (cathepsins) and their endogenous inhibitors (cystatins) have been found to be closely associated with senile plaques, cerebrovascular amyloid deposits, and neurofibrillary tangles in Alzheimer disease (AD). Further, profound changes in the lysosomal system seem to be an early event in "at-risk" neurons of AD brains. There is an ongoing controversy as to whether lysosome-associated proteolytic mechanisms are causally related to the development and/or further progression of the disease. The present article deals with some arguments "pro" and "contra" an involvement of the endosomal/lysosomal pathway in amyloidogenesis as a cardinal process in AD. Other putative targets of acidic proteinases and their natural inhibitors in the pathogenesis of AD (such as formation of neurofibrillary tangles and regulation of apolipoprotein E) are also discussed.

Localization of a domain in apolipoprotein E with both cytostatic and cytotoxic activity

Apoliprotein E (apoE) is a potent suppressor of interleukin 2- (IL2-) dependent T lymphocyte proliferation. In this study, we have used a range of monomeric and dimeric peptides encompassing amino acids 130-169 in human apoE to locate a region with both cytostatic and cytotoxic effects on IL2-dependent T lymphocytes. Monomeric peptides representing residues 130-149 or 130-155 inhibited the proliferation of the cells without causing loss of cell viability. However, cytostasis by a peptide representing the extended 130-169 domain or dimeric peptides of amino acids 141-155 or 141-149 was accompanied by potent cytotoxic activity. These results suggest that residues 141-149, which include the overlap between the functional peptides, are responsible for cytostasis and cytotoxicity. Complete ablation of both activities by the polyanionic agent heparin highlighted the important contribution of the positively charged amino acids in the 141-149 region to peptide bioactivity. Furthermore, the bioactive apoE peptides also had a relatively high helical content, suggesting that alpha-helical content is necessary for bioactivity. Cytotoxic apoE peptides were characterized by a high density of polar face positively charged residues together with a high nonpolar face hydrophobicity. This conclusion is supported by the reduced hydrophobicity and polar face positive charge density of the significantly less active E2(130-169) peptide. The cytotoxic apoE peptides are structurally similar to previously characterized class L lytic peptides. They do not, however, exert their cytotoxic activity by destabilizing membrane bilayers as is the case with the class L peptides, as evidenced by their minimal hemolytic activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular degradation of type I collagen and fibronectin in human lung fibroblasts: evidence against degradation in pre-lysosomal compartments

Fibroblasts degrade about 15% of newly synthesised collagen within the cell before it can be secreted. When the helical structure of collagen is disrupted, about 30% is degraded intracellularly. To determine if collagen degradation occurs in a pre-lysosomal compartment, the passage of type 1 collagen out of the endoplasmic reticulum or Golgi was inhibited by incubating human lung fibroblasts with brefeldin A or monensin. In both cases, the type I collagen retained within the cell was stable over a 20 h period. Disrupting the helical structure of collagen with cis-hydroxyproline, 2,2'-bipyridyl or ethyl 3,4-dihydroxybenzoate did not alter the stability of type I collagen in brefeldin or monensin-treated cells. Incubating permeabilised cells in the presence of GTP gamma S (guanosine 5'-(3-O-thio)triphosphate), which blocks transport out of the endoplasmic reticulum, also resulted in the stable retention of type I collagen. Addition of dithiothreitol to permeabilised cells failed to initiate intracellular degradation. Similar results were obtained with fibronectin. Both normal fibronectin and fibronectin in which canavanine replaced arginine were stable for 20 h in cells treated with brefeldin A or monensin. The degradation of native collagen is sensitive to inhibition by a cell-permeable cysteine proteinase inhibitor (ALLN) but is insensitive to chloroquine (which raises the pH of acidic intracellular compartments), whereas the degradation of abnormal collagen was sensitive to both ALLN and chloroquine. These results argue against the intracellular degradation of collagen or fibronectin in a pre-lysosomal compartment.

Gene expression and cellular content of cathepsin D in Alzheimer's disease brain: evidence for early up-regulation of the endosomal-lysosomal system

In Alzheimer's disease brains, more than 90% of pyramidal neurons in lamina V and 70% in lamina III displayed 2- to 5-fold elevated levels of cathepsin D (Cat D) mRNA by in situ hybridization compared with neurologically normal controls. Most of these cells appeared histologically normal. The less vulnerable nonpyramidal neuron population in lamina IV had relatively normal message levels. Neuronal populations expressing more Cat D mRNA also displayed quantitatively increased Cat D immunoreactive protein. Cat D mRNA expression was only moderately increased in astrocytes. Degenerating neurons exhibited intense immunoreactivity but lowered Cat D mRNA levels. The upregulation of Cat D synthesis and accumulation of hydrolase-laden lysosomes indicate an early activation of the endosomal-lysosomal system in vulnerable neuronal populations, possibly reflecting early regenerative or repair processes. These abnormalities also represent a basis for altered regulation of amyloid precursor protein processing.

Intracellular apoprotein B degradation is suppressed by decreased albumin concentration in Hep G2 cells

It is generally accepted that hepatic secretion of apoprotein (apo) B-containing lipoproteins is substantially increased in nephrosis. To elucidate the mechanisms for the oversecretion of apo B, we investigated the effect of a various concentration of albumin on apo B kinetics in the absence or presence of oleate in Hep G2 cells. Hep G2 cells were labeled with [3H]-leucine in leucine-free medium containing 0, 1.5, 3.0 or 4.5% BSA for 180 minutes, and the secreted radiolabeled apo B, apo A1 and albumin were isolated by immunoprecipitation and counted. The secretions of apo B and albumin were suppressed by BSA (bovine serum albumin) in a dose-dependent manner, but the secretion of apo A1 was not suppressed significantly. Oleate (0.4 mM) increased the rate of apo B secretion by 2.5-fold when oleate was bound to 1.5% BSA, but at higher concentrations of BSA (3.0 or 4.5%), apo B secretion was less responsive to oleate. A pulse-chase study indicated that early apo B degradation was significantly suppressed in cells incubated with lower concentrations of BSA (0 or 1.5% BSA), thereby rapidly stimulating apo B secretion. Oleate (0.4 mM) potently inhibited apo B degradation when oleate was bound to 1.5% BSA, whereas the inhibition was not observed when oleate was bound to 4.5% BSA. Intracellular albumin synthesis was stimulated in BSA-free medium, but intracellular decay of albumin was essentially unaffected by concentration of BSA. Similar to BSA, a higher concentration of dextran (3.0 or 4.5%) reduced apo B secretion, and this was the result of increased early apo B degradation in the cells. These results indicate that reduced albumin suppresses intracellular apo B degradation, and the inhibition of apo B degradation by oleate is manifested only at a low concentration of albumin. Therefore, the present study suggests that free fatty acids bound to low concentration of albumin in the circulating plasma play an important role on hepatic oversecretion of apo B-containing lipoprotein in hypoalbuminemic state, such as nephrotic syndrome.

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)