Liver regrowth and apolipoprotein B secretion by rat hepatocytes following partial hepatectomy

Insulin dependent apolipoprotein B degradation and phosphatidylinositide 3-kinase activation with microsomal translocation are restored in McArdle RH7777 cells following serum deprivation

Previous studies in rat hepatocytes demonstrated that insulin-dependent apolipoprotein (apo) B degradation (IDAD) is lost when cells are maintained for 3 d under enriched culture conditions. Loss of IDAD correlates with increased expression of protein tyrosine phosphatase 1B (PTP1B) known to be associated with resistance to insulin signaling in the liver. McArdle RH7777 hepatoma (McA) cells cultured in serum containing medium are resistant to IDAD; demonstrate a 30% increase in apo B secretion, and express increased levels of PTP1B protein and mRNA. In addition, insulin-stimulated Class I phosphatidylinositide 3-kinase (PI3K) activity of anti-pY immunoprecipitates is severely blunted. IDAD resistance in McA cells correlates with diminished translocation of insulin-stimulated pY-IRS1 to intracellular membranes. Incubation of McA cells with RK682, a protein tyrosine phosphatase inhibitor, is sufficient to restore IDAD in resistant McA cells. Overall, results further support the importance of Class I PI3K activity in IDAD, and suggest that loss of this activity is sufficient to cause resistance. Although other factors are involved in downstream events including sortilin binding to apo B, autophagy, and lysosomal degradation, loss of signal generation and reduced localization of Class I PI3K to intracellular membranes plays a significant role in IDAD resistance.

Acute suppression of apo B secretion by insulin occurs independently of MTP

Secretion of apolipoprotein (apo) B-containing lipoproteins by the liver depends mainly upon apo B availability and microsomal triglyceride transfer protein (MTP) activity and is subject to insulin regulation. Hepatic MTP mRNA expression is negatively regulated by insulin which correlates with inhibition of apo B secretion suggesting that insulin might suppress apo B secretion through an MTP-dependent mechanism. To investigate this possibility, we examined the acute effect of insulin on hepatic MTP expression and activity levels in vivo utilizing apobec-1(-/-) mice. Insulin did not significantly alter hepatic MTP mRNA levels or lipid transfer activity 2h following injection, but suppressed expression of genes important in gluconeogenesis. To study the specific role of MTP, we expressed human MTP (hMTP) in primary rat hepatocytes using adenoviral gene transfer. Increased expression of hMTP resulted in a 47.6±17.9% increase in total apo B secreted. Incubation of hepatocytes with insulin suppressed apo B secretion by 50.1±10.8% in cells over-expressing hMTP and by 53.0±12.4% in control transfected hepatocytes. Results indicate that even under conditions of increased hepatic apo B secretion mediated by MTP, responsiveness of hepatocytes to insulin to suppress apo B secretion is maintained.

Interleukin-6 mediates hepatic hypersecretion of apolipoprotein B

Obesity and type 2 diabetes are associated with insulin resistance (IR), increased circulating proinflammatory cytokines, and hypertriglyceridemia, the latter being caused by overproduction of hepatic very low density lipoprotein (VLDL). One cytokine strongly linked with development of hepatic IR is interleukin-6 (IL-6). Our objective was to evaluate IL-6 effects on hepatic apolipoprotein B (apoB) and VLDL secretion and to examine possible linkages between cytokine signaling and insulin-suppressive effects on lipoprotein secretion. Of the cytokines examined, only IL-6 stimulated secretion of apoB-containing lipoproteins in a dose-dependent manner. Both B100 and B48 secretion were significantly increased in VLDL and in lipoproteins with a density >1.019 g/ml. The ability of insulin to suppress hepatic apoB secretion was maintained in hepatocytes treated with IL-6. Pulse-chase studies indicated that enhanced apoB synthesis was the primary mechanism for increased lipoprotein secretion, which corresponded with higher abundance of apoB mRNA. Because IL-6 did not alter the decay rate of apoB mRNA transcripts, results support that increased apoB mRNA levels are the result of enhanced apob gene transcription. Increased apoB-lipoprotein secretion was also detected with oncostatin M (OSM), supporting involvement of the signal-transducing protein, gp130. Increased suppressor of cytokine signaling (SOCS) 3 expression negated IL-6 and OSM effects and significantly reduced cellular apoB mRNA abundance. We conclude that IL-6 favors secretion of apoB-containing lipoproteins by increasing availability of apoB through changes in apob gene transcription. These changes may contribute to hypersecretion of VLDL associated with obesity, particularly under conditions where SOCS3 is not overexpressed to an extent capable of overcoming IL-6-stimulated apob gene transcription.

PI3-kinase activity modulates apo B available for hepatic VLDL production in apobec-1-/- mice

Insulin regulates hepatic VLDL production by activation of phosphatidylinositide 3-kinase (PI3-kinase) which decreases apo B available for lipid assembly. The current study evaluated the dependence of the VLDL apolipoprotein B (apo B) pathway on PI3-kinase activity in vivo. VLDL production was examined in B100 only, apo B mRNA editing catalytic subunit 1 (apobec-1(-/-)) mice, using the Triton WR 1339 method. Glucose injection suppressed VLDL triglyceride production by 28% in male and by 32% in female mice compared with saline-injected controls. When wortmannin was injected to inhibit PI3-kinase, VLDL triglyceride production was increased by 52% in males and by 89% in females, and VLDL B100 levels paralleled triglyceride changes. Pulse-chase experiments in primary mouse hepatocytes showed that wortmannin increased net freshly synthesized B100 availability by >35%. To test whether physiological insulin resistance produced equivalent effects to wortmannin, we studied male apobec-1(-/-) mice who became hyperlipidemic on being fed a fructose-enriched diet. Fructose-fed apobec-1(-/-) mice had significantly higher VLDL triglyceride and B100 production rates compared with chow-fed mice, and rates were refractile to glucose or wortmannin. Hepatic VLDL triglyceride and B100 production in wortmannin-injected chow-fed mice equaled that observed in fructose-fed mice. Together, results suggest in vivo and in vitro that wortmannin-sensitive PI3-kinases maintain a basal level of VLDL suppression that is sensitive to changes in activation and that can increase VLDL production when PI3-kinase is inhibited to levels similar to those induced by insulin resistance.

Serum lipid and lipoprotein alterations represent recovery of liver function after hepatectomy

BACKGROUND

The assessment of liver function during human liver regeneration is necessary to prevent unexpected liver failure and to prepare for further treatment. We selected patients prospectively and measured serum lipid and lipoprotein levels to identify which lipids and lipoproteins could represent recovery of liver function in human liver regeneration.

METHODS

Thirty selected patients who underwent hepatectomy were divided into three groups depending on the serum hyaluronate (HA) level and the type of liver resection.

RESULTS

We found three patterns of lipid and lipoprotein alterations after hepatectomy. Among the lipids and lipoproteins examined, the serum beta-lipoprotein and low-density lipoprotein (LDL) levels were significantly different among the groups at 7 days after hepatectomy. The alteration of the apolipoprotein (Apo) B level was similar to that of LDL. The LDL level was correlated with both beta-lipoprotein and Apo B before hepatectomy (r=0.653 and 0.894, respectively) and at 7 days after hepatectomy (r=0.841 and 0.943, respectively).

CONCLUSION

Serum HA before hepatectomy can reflect postoperative liver function depending on the type of liver resection. Recovery of the beta-lipoprotein and LDL levels can reflect the recovery of liver function in human liver regeneration within the early period in association with the Apo B level.

Altered triglyceride-rich lipoprotein production in Zucker diabetic fatty rats

Triglyceride-rich lipoprotein (TRL) production was studied in Zucker diabetic fatty (ZDF) rats, a model of insulin-resistant type 2 diabetes progression. TRL production was measured in vivo by blocking catabolism with Triton WR-1339. Ten-week ZDF rats are hyperinsulinemic with increased TRL production [both triglyceride and apolipoprotein B (apoB)]. Twenty-week ZDF rats are insulinopenic, and TRL production is similar to lean controls. Insulin infusion suppresses glucose and free fatty acids in 10- and 20-wk ZDF rats. Increased TRL production is not reduced by insulin in 10-wk rats; however, at 20 wk, TRL production is suppressed by insulin. In vitro studies with hepatocytes derived from 10-wk ZDF rats showed minimal insulin dose effects on apoB secretion compared with the response and sensitivity of hepatocytes derived from 20-wk ZDF and control lean rats. Hepatic sterol regulatory-binding protein (SREBP)-1c mRNA levels are increased at 10 wk but return to control levels at 20 wk. ApoB mRNA levels are similar to lean controls at 10 and 20 wk. The following two mechanisms for hypertriglyceridemia associated with hyperinsulinemia are suggested: increased TRL synthesis and loss of TRL suppression. Increased triglyceride production in hyperinsulinemic rats likely relates to increased expression of SREBP-1c, whereas increased apoB production involves posttranscriptional processes.

Mutations in the PCSK9 gene in Norwegian subjects with autosomal dominant hypercholesterolemia

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is at a locus for autosomal dominant hypercholesterolemia, and recent data indicate that the PCSK9 gene is involved in cholesterol biosynthesis. Mutations within this gene have previously been found to segregate with hypercholesterolemia. In this study, DNA sequencing of the 12 exons of the PCSK9 gene has been performed in 51 Norwegian subjects with a clinical diagnosis of familial hypercholesterolemia where mutations in the low-density lipoprotein receptor gene and mutation R3500Q in the apolipoprotein B-100 gene had been excluded. Two novel missense mutations were detected in the catalytic subdomain of the PCSK9 gene. Two patients were heterozygotes for D374Y, and one patient was a double heterozygote for D374Y and N157K. D374Y segregated with hypercholesterolemia in the two former families where family members were available for study. Our findings support the notion that mutations in the PCSK9 gene cause autosomal dominant hypercholesterolemia.

Insulin regulates protein kinase CbetaII alternative splicing in multiple target tissues: development of a hormonally responsive heterologous minigene

Cells respond to external signals like insulin to alter metabolic pathways in response to varying physiological environments. Insulin stimulates the protein kinase C beta (PKCbeta) isozymes and preferentially switches the expression to PKCbetaII isozyme, which is shown to have a crucial role in glucose uptake, cellular proliferation, and differentiation. We have developed an insulin-responsive PKCbetaII heterologous minigene to identify cis-elements in vivo in eukaryotes by cloning the PKCbetaII exon and its flanking intronic sequences into the splicing vector pSPL3. The transfected minigene mimicked the endogenous insulin response of PKCbetaII alternative splicing in five distinct cell types, i.e. L6 skeletal muscle, 3T3-L1 pre-adipocytes, HepG2 human hepatoma cells, A10 vascular smooth muscle cells, and murine embryonic fibroblasts within 30 min of insulin stimulation. Sequential deletions of the flanking introns in the minigene demonstrated that insulin regulated elements within the 5'-intron flanking the PKCbetaII exon. Mutational studies indicated the SRp40 binding site promotes splice site selection. In these cases, splicing appears to be regulated by a phosphatidylinositol 3-kinase signaling pathway because LY294002 and wortmannin, its specific inhibitors, blocked exon inclusion. Cotransfection with constitutively active Akt2 kinase mimicked insulin action. Signal-dependent regulation of splicing by insulin is unique from tissue-specific and developmentally regulated mechanisms previously reported and serves as a prototype for studies of alternative splicing involving protein phosphorylation.

Structure and interfacial properties of human apolipoprotein A-V

Apolipoprotein A-V (apoA-V), the newest member of the plasma apolipoprotein family, was recently discovered by comparison of the mouse and human genomes. Studies in rodents and population surveys of human apoA-V polymorphisms have noted a strong effect of apoA-V on plasma triglyceride levels. Toward the elucidation of the biologic function of apoA-V, we used spectroscopic and surface chemistry techniques to probe its structure and interfacial activity. Computer-assisted sequence analysis of apoA-V predicts that it is very hydrophobic, contains a significant amount of alpha-helical secondary structure, and probably is composed of discrete structural regions with varying degrees of lipid affinity. Fluorescence spectroscopy of recombinant human apoA-V provided evidence of tertiary folding, and light scattering studies indicated that apoA-V transforms dimyristoylphosphatidylcholine vesicles into discoidal complexes with an efficiency similar to that of apoA-I. Surface chemistry techniques revealed that apoA-V displays high affinity, low elasticity, and slow binding kinetics at hydrophobic interfaces, properties we propose may retard triglyceride-rich particle assembly. Metabolic labeling and immunofluorescence studies of COS-1 cells transfected with human apoA-V demonstrated that apoA-V is poorly secreted, remains associated with the endoplasmic reticulum, and does not traffic to the Golgi. Given that overexpression of the apoA-V gene lowers plasma triglycerides in mice, these data together suggest that apoA-V may function intracellularly to modulate hepatic VLDL synthesis and/or secretion.

The secretory proprotein convertase neural apoptosis-regulated convertase 1 (NARC-1): liver regeneration and neuronal differentiation

Seven secretory mammalian kexin-like subtilases have been identified that cleave a variety of precursor proteins at monobasic and dibasic residues. The recently characterized pyrolysin-like subtilase SKI-1 cleaves proproteins at nonbasic residues. In this work we describe the properties of a proteinase K-like subtilase, neural apoptosis-regulated convertase 1 (NARC-1), representing the ninth member of the secretory subtilase family. Biosynthetic and microsequencing analyses of WT and mutant enzyme revealed that human and mouse pro-NARC-1 are autocatalytically and intramolecularly processed into NARC-1 at the (Y,I)VV(V,L)(L,M) downward arrow motif, a site that is representative of its enzymic specificity. In vitro peptide processing studies andor Ala substitutions of the P1-P5 sites suggested that hydrophobicaliphatic residues are more critical at P1, P3, and P5 than at P2 or P4. NARC-1 expression is highest in neuroepithelioma SK-N-MCIXC, hepatic BRL-3A, and in colon carcinoma LoVo-C5 cell lines. In situ hybridization and Northern blot analyses of NARC-1 expression during development in the adult and after partial hepatectomy revealed that it is expressed in cells that have the capacity to proliferate and differentiate. These include hepatocytes, kidney mesenchymal cells, intestinal ileum, and colon epithelia as well as embryonic brain telencephalon neurons. Accordingly, transfection of NARC-1 in primary cultures of embryonic day 13.5 telencephalon cells led to enhanced recruitment of undifferentiated neural progenitor cells into the neuronal lineage, suggesting that NARC-1 is implicated in the differentiation of cortical neurons.

Insulin suppression of VLDL apo B secretion is not mediated by the LDL receptor

Insulin inhibits hepatic very low density lipoprotein (VLDL) apo B secretion in rats. Current studies test whether the insulin effect is LDL receptor-mediated by examining the effect of insulin on VLDL apo B secretion in hepatocytes derived from Ldlr-/- and control mice. Primary hepatocytes were incubated overnight with media containing 14C-leucine and either 0.1nM (basal) or 200nM insulin. Afterwards, secreted VLDL B100 and B48 were quantitated. Insulin reduced 14C-labeled B100 and B48 comparably in control and Ldlr-/- hepatocytes with a 62+/-12% vs. 59+/-12% decrease in B100, and a 56+/-11% vs. 61+/-9% decrease in B48. Results indicate: (1) mouse hepatocytes respond to insulin by reducing VLDL apo B output; (2) both VLDL B100 and B48 secretion are suppressed; and (3) insulin inhibition of VLDL apo B secretion is retained in Ldlr-/- hepatocytes.

Hepatic very low density lipoprotein-ApoB overproduction is associated with attenuated hepatic insulin signaling and overexpression of protein-tyrosine phosphatase 1B in a fructose-fed hamster model of insulin resistance

A fructose-fed hamster model of insulin resistance was previously documented to exhibit marked hepatic very low density lipoprotein (VLDL) overproduction. Here, we investigated whether VLDL overproduction was associated with down-regulation of hepatic insulin signaling and insulin resistance. Hepatocytes isolated from fructose-fed hamsters exhibited significantly reduced tyrosine phosphorylation of the insulin receptor and insulin receptor substrates 1 and 2. Phosphatidylinositol 3-kinase activity as well as insulin-stimulated Akt-Ser473 and Akt-Thr308 phosphorylation were also significantly reduced with fructose feeding. Interestingly, the protein mass and activity of protein-tyrosine phosphatase-1B (PTP-1B) were significantly higher in fructose-fed hamster hepatocytes. Chronic ex vivo exposure of control hamster hepatocytes to high insulin also appeared to attenuate insulin signaling and increase PTP-1B. Elevation in PTP-1B coincided with marked suppression of ER-60, a cysteine protease postulated to play a role in intracellular apoB degradation, and an increase in the synthesis and secretion of apoB. Sodium orthovanadate, a general phosphatase inhibitor, partially restored insulin receptor phosphorylation and significantly reduced apoB secretion. In summary, we hypothesize that fructose feeding induces hepatic insulin resistance at least in part via an increase in expression of PTP-1B. Induction of hepatic insulin resistance may then contribute to reduced apoB degradation and enhanced VLDL particle assembly and secretion.

Changes in serum levels of apolipoprotein A-1 as an indicator of protein metabolism after hepatectomy

The clinical significance of serum apolipoprotein A-1 levels as an indicator of hepatic protein synthesis after hepatectomy was investigated. A total of 50 patients who had undergone hepatectomy at our department from 1997 to 1999 were selected for this study. The serum levels of apolipoprotein A-1, indocyanine green dye retention at 15 minutes, lectin-cholesterol acyltransferase, prealbumin, and high-density lipoprotein cholesterol were measured in these patients preoperatively and on postoperative days 7 and 14. The type of hepatic resection conducted was partial resection in 13 cases, subsegmentectomy in 13 cases, segmentectomy in five cases, and bisegmentectomy in 19 cases. All the patients tolerated the operation, and none of the cases had any severe complications, such as liver failure. In most cases, the serum apolipoprotein A-1 levels decreased on postoperative day 7 and recovered by day 14. There were no significant differences in the changes in apolipoprotein A-1 levels between patients with the individual types of operative procedures. The serum apolipoprotein A-1 levels showed significant correlations with the serum high-density lipoprotein cholesterol, lectin-cholesterol acyltransferase and prealbumin levels on postoperative days 7 and 14; however, there was no significant correlation with the indocyanine green retention test. When the cases were divided into three groups according to the serum level of apolipoprotein A-1 on postoperative day 7 (group A: over 81 mg/dl, group B: 61-80 mg/dl, group C: under 60 mg/dl), the serum indocyanine green retention, prealbumin, lectin-cholesterol acyltransferase and high-density lipoprotein cholesterol levels in group C were significantly lower than those in group A on postoperative day 7. On the basis of these results, it is suggested that the pattern of changes in the serum apolipoprotein A-1 levels may be a good indicator of the hepatic protein synthetic ability during the perioperative period after hepatectomy.

Mechanisms of hepatic very low-density lipoprotein overproduction in insulin resistance

An important complication of insulin-resistant states, such as obesity and type 2 diabetes, is an atherogenic dyslipidemia profile characterized by hypertriglyceridemia, low plasma high-density lipoproteins (HDL) cholesterol and a small, dense low-density lipoprotein (LDL) particle profile. The physiological basis of this metabolic dyslipidemia appears to be hepatic overproduction of apoB-containing very low-density lipoprotein (VLDL) particles. This has focused attention on the mechanisms that regulate VLDL secretion in insulin-resistant states. Recent studies in animal models of insulin resistance, particularly the fructose-fed hamster, have enhanced our understanding of these mechanisms, and certain key factors have recently been identified that play important roles in hepatic insulin resistance and dysregulation of the VLDL secretory process. This review focuses on these recent developments as well as on the hypothesis that an interaction between enhanced flux of free fatty acids from peripheral tissues to liver, chronic up-regulation of de novo lipogenesis by hyperinsulinemia and attenuated insulin signaling in the liver may be critical to the VLDL overproduction state observed in insulin resistance. It should be noted that the focus of this review is on molecular mechanisms of the hypertriglyceridemic state associated with insulin resistance and not that observed in association with insulin deficiency (e.g., in streptozotocin-treated animals), which appears to have a different etiology and is related to a catabolic defect rather than secretory overproduction of triglyceride-rich lipoproteins.

Glucose-stimulated insulin secretion suppresses hepatic triglyceride-rich lipoprotein and apoB production

The current study assessed in vivo the effect of insulin on triglyceride-rich lipoprotein (TRL) production by rat liver. Hepatic triglyceride and apolipoprotein B (apoB) production were measured in anesthetized, fasted rats injected intravenously with Triton WR-1339 (400 mg/kg). After intravascular catabolism was blocked by detergent treatment, glucose (500 mg/kg) was injected to elicit insulin secretion, and serum triglyceride and apoB accumulation were monitored over the next 3 h. In glucose-injected rats, triglyceride secretion averaged 22.5 +/- 2.1 microg.ml(-1).min(-1), which was significantly less by 30% than that observed in saline-injected rats, which averaged 32.1 +/- 1.4 microg.ml(-1).min(-1). ApoB secretion was also significantly reduced by 66% in glucose-injected rats. ApoB immunoblotting indicated that both B100 and B48 production were significantly reduced after glucose injection. Results support the conclusion that insulin acts in vivo to suppress hepatic very low density lipoprotein (VLDL) triglyceride and apoB secretion and strengthen the concept of a regulatory role for insulin in VLDL metabolism postprandially.

Insulin-treated Zucker diabetic fatty rats retain the hypertriglyceridemia associated with obesity

Lipoprotein and apolipoprotein changes were evaluated in 10-week-old Zucker diabetic fatty (ZDF) male rats following 12 weeks of insulin treatment, which normalized blood glucose and maintained weight gaining characteristic of nondiabetic Zucker fatty rats. Compared with untreated ZDF rats (saline-injected), insulin treatment resulted in increased very-low-density lipoprotein (VLDL; d < 1.006 g/mL) and decreased alpha lipoprotein on agarose gel electrophoresis. These findings were consistent with an observed increase in VLDL triglyceride and cholesterol, and decreased high-density lipoprotein (HDL) cholesterol with insulin treatment in isolated lipoproteins. B100 levels were unchanged by insulin treatment, but B48 levels were significantly increased in the VLDL fraction. Insulin treatment depressed apolipoprotein (apo) A-I levels in HDL, but had little effect on total apo E, apo A-IV, or apo C, although apo C was redistributed to the VLDL fraction. These results suggest that insulin treatment of ZDF rats normalizes hyperglycemia and prevents age-related changes in lipoprotein parameters associated with development of insulinopenic diabetes. Insulin therapy in ZDF rats thereby sustains the hyperlipidemic lipoprotein pattern associated with hyperinsulinemia and obesity.

Enhanced prenyltransferase activity and Rab content in rat liver regeneration

Rabs are small GTP-binding proteins with a regulatory role in intracellular vesicular traffic. The modulation of their levels and activity in different physiological situations is poorly understood. During the first cell cycle of rat liver regeneration we observed a differential regulation of some Rabs, with a progressive increase of those involved in exocytosis and a progressive decrease of one involved in endocytosis. This could be related with the need of exposing growth factor receptors and prolonging the transduction of their signal in preparation for mitosis. Moreover, we observed an increased activity of protein prenyltransferases, the enzymes responsible for the prenylation of several proteins involved in crucial processes of proliferation, without a corresponding increase in the amount of prenyltransferase protein.

Phosphoinositide 3-kinase activity is necessary for insulin-dependent inhibition of apolipoprotein B secretion by rat hepatocytes and localizes to the endoplasmic reticulum

Insulin inhibits apolipoprotein B (apoB) secretion by primary rat hepatocytes through activation of phosphoinositide 3-kinase (PI 3-K). Current studies demonstrate that the PI 3-K inhibitor wortmannin inhibits both basal and insulin-stimulated PI 3-K activities. Wortmannin and LY 294002, two structurally distinct PI 3-K inhibitors, prevent insulin-dependent inhibition of apoB secretion in a dose-dependent manner. To link PI 3-K activation to insulin action on apoB, we investigated whether insulin induced localization of activated PI 3-K to the endoplasmic reticulum (ER), where apoB biogenesis is initiated. Insulin action results in a significant redistribution of PI 3-K to a low density microsome (LDM) fraction containing apoB protein and apoB mRNA. Insulin stimulates a significant increase in PI 3-K activity associated with insulin receptor substrate-1 as well as an increase in insulin receptor substrate-1/PI 3-K mass in LDM. Subfractionation of LDM on sucrose density gradients shows that insulin significantly increases the amount of PI 3-K present in an ER fraction containing apoB. Insulin stimulates PI 3-K activity in smooth and rough microsomes isolated from rat hepatocytes, the latter of which contain rough ER as demonstrated by electron microscopy. Studies indicate that 1) PI 3-K activity is necessary for insulin-dependent inhibition of apoB secretion by rat hepatocytes; 2) insulin action leads to the activation and localization of PI 3-K in an ER fraction containing apoB; and 3) insulin stimulates PI 3-K activity in the rough ER.

Effects of fatty acids on apolipoprotein B secretion by McArdle RH-7777 rat hepatoma cells

The effect of oleic acid (OA), stearic acid (SA) and elaidic acid (EA) on cellular and secreted apolipoprotein (apo) B was examined in McArdle RH-7777 (McArdle) hepatoma cells and in primary rat hepatocytes. ApoB secretion by McArdle cells was significantly inhibited by 20% in 8 h incubations in medium containing EA and SA and by 50% in medium containing OA. In contrast, apo B secretion and cellular apo B of primary rat hepatocytes was relatively unaffected by incubations in medium containing fatty acids. Both B100 and B48 secretion in McArdle wild type and B48 in apo B mRNA editing enzyme catalytic polypeptide transfectants expressing B48 were inhibited to a similar extent indicating an effect of OA on both apo B species. The effect of OA occurred without changes in cellular apo B or in apo B mRNA abundance suggesting a post-transcriptional mechanism. Time course studies indicate that the suppressive effect of OA requires 4 h of incubation suggesting the depletion of a limiting factor important in apoB secretion. By increasing the proportion of palmitic acid to OA in the medium, apoB secretion by McArdle cells was progressively restored to control levels implicating an unique role for newly synthesized saturated fatty acid.

Insulin-mediated inhibition of apolipoprotein B secretion requires an intracellular trafficking event and phosphatidylinositol 3-kinase activation: studies with brefeldin A and wortmannin in primary cultures of rat hepatocytes

Insulin inhibition of the secretion of apolipoprotein B (apo B) was studied in primary cultures of rat hepatocytes by using brefeldin A (BFA), an inhibitor of protein transport from the endoplasmic reticulum (ER) to the Golgi apparatus, and by using the phosphatidylinositol 3-kinase (PI 3-K) inhibitor wortmannin. Incubation of hepatocytes with BFA (10 micrograms/ml) for 1 h inhibited the subsequent secretion of apo B, albumin and transferrin for up to 3 h. BFA treatment resulted in the time-dependent accumulation in cells of [14C]leucine-labelled proteins and apo B. Under conditions where insulin decreased total apo B (cell plus secreted), BFA blocked the insulin-dependent effect. These results suggest that export of apo B from the ER is a prerequisite for the observed insulin effect. Treatment of hepatocytes with wortmannin for 20 min abolished insulin inhibition of apo B secretion, suggesting that the insulin effect on the apo B pathway involves activation of PI 3-K. Enzyme inhibitor studies indicate that chymostatin and (+)-(2S,3S)-3-[(S)-methyl-1-(3-methylbutylcarbamoyl)-butylcarba moyl]-2- oxiranecarboxylate (E-64-c) partially block insulin effects on apo B compared with leupeptin, which had no discernible effect. The cell-permeable derivative of E-64-c, EST, and N-Ac-Leu-Leu-norleucinal (ALLN) were most effective in blocking insulin effects on apo B. These results suggest that insulin action on apo B in primary rat hepatocytes involves (1) vesicular movement of apo B from the ER; (2) activation of PI 3-K and (3) a cellular protease that is either a cysteine- or calcium-activated neutral protease.

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)