Phosphoinositide 3-Kinase Activity Is Necessary for Insulin-dependent Inhibition of Apolipoprotein B Secretion by Rat Hepatocytes and Localizes to the Endoplasmic Reticulum

Phung, Thuy L.; Roncone, A.; Jensen, Kasper; Sparks, Charles E.; Sparks, Janet D.

doi:10.1074/jbc.272.49.30693

Cited by 114 publications

(108 citation statements)

References 68 publications

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“…The interface between PI3K-dependent signaling and apoB assembly and secretion is not known in detail, although in some studies, insulin-mediated signaling, via PI3K and Akt, regulates MTP expression and, hence, VLDL particle number and size (18,29,31,32). Although MTP is the target for some forms of PI3K-mediated regulation of VLDL assembly (32), MTP expression was unaffected by ABCA1 deficiency in this study (Fig.…”

Section: Discussioncontrasting

confidence: 39%

“…The current studies demonstrate that VLDL from HSKO mice is more buoyant and of larger diameter than that of WT mice, which is also consistent with ABCA1/PI3K exerting effects during the second step of VLDL assembly. Because PI3K is present in the endoplasmic reticulum and Golgi, is involved in intracellular vesicular trafficking, and is increased with insulin stimulation (29,35,36), we hypothesize that the lack of ABCA1 in hepatocytes and the resultant attenuation of PI3K activation results in slower VLDL transit through the secretory pathway, allowing additional time for second step VLDL particle expansion. This hypothesis is supported by the observations of defective lipid and vesicular trafficking from the Golgi to the plasma membrane in fibroblasts from TD subjects (37)(38)(39)(40).…”

Section: Discussionmentioning

confidence: 99%

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Targeted Deletion of Hepatocyte ABCA1 Leads to Very Low Density Lipoprotein Triglyceride Overproduction and Low Density Lipoprotein Hypercatabolism

Chung

Timmins

Duong

et al. 2010

Journal of Biological Chemistry

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Loss of ABCA1 activity in Tangier disease (TD) is associated with abnormal apoB lipoprotein (Lp) metabolism in addition to the complete absence of high density lipoprotein (HDL). We used hepatocyte-specific ABCA1 knock-out (HSKO) mice to test the hypothesis that hepatic ABCA1 plays dual roles in regulating Lp metabolism and nascent HDL formation. HSKO mice recapitulated the TD lipid phenotype with postprandial hypertriglyceridemia, markedly decreased LDL, and near absence of HDL. Triglyceride (TG) secretion was 2-fold higher in HSKO compared with wild type mice, primarily due to secretion of larger TG-enriched VLDL secondary to reduced hepatic phosphatidylinositol 3-kinase signaling. HSKO mice also displayed delayed clearance of postprandial TG and reduced post-heparin plasma lipolytic activity. In addition, hepatic LDLr expression and plasma LDL catabolism were increased 2-fold in HSKO compared with wild type mice. Last, adenoviral repletion of hepatic ABCA1 in HSKO mice normalized plasma VLDL TG and hepatic phosphatidylinositol 3-kinase signaling, with a partial recovery of HDL cholesterol levels, providing evidence that hepatic ABCA1 is involved in the reciprocal regulation of apoB Lp production and HDL formation. These findings suggest that altered apoB Lp metabolism in TD subjects may result from hepatic VLDL TG overproduction and increased hepatic LDLr expression and highlight hepatic ABCA1 as an important regulatory factor for apoB-containing Lp metabolism. ABCA1 (ATP-binding cassette transporter A1) is indispensable in the initial steps of high density lipoprotein (HDL)2 formation and the process of reverse cholesterol transport from peripheral tissues to the liver. ABCA1 is expressed in many cells; however, hepatocytes make the single most important contribution to plasma HDL concentration (1-3). Mutations in ABCA1 in humans cause Tangier disease (TD), an autosomal recessive disorder characterized by severe HDL deficiency, rapid plasma clearance of HDL and apoA-I, sterol deposition in tissues, and premature coronary atherosclerosis (4 -7). In addition to HDL deficiency, TD subjects have significantly elevated plasma TG and a 50% reduction in LDL cholesterol concentrations (4,8). The TG phenotype in TD disease is complicated, with most, but not all, TD subjects displaying elevated fasting or postprandial TG elevations (9). Clee et al. (8) reported an inverse relationship between dysfunctional ABCA1 alleles and plasma TG concentrations. In addition, data from case reports of 59 Tangier patients show variable TG concentrations, with mean, median, minimum, and maximum concentrations of 210, 175, 40, and 580 mg/dl, respectively (4). The underlying mechanisms for the increased plasma TG and decreased LDL concentrations in TD subjects have not been established. In one study, apoA-II enrichment of VLDL of TD subjects was proposed to result in reduced reactivity of VLDL with lipoprotein lipase (LPL) (9, 10). Another study suggested that ABCA1-dependent cholesterol efflux decreases VLDL secretion from murine hep...

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Section: Discussioncontrasting

confidence: 39%

Section: Discussionmentioning

confidence: 99%

Targeted Deletion of Hepatocyte ABCA1 Leads to Very Low Density Lipoprotein Triglyceride Overproduction and Low Density Lipoprotein Hypercatabolism

Chung

Timmins

Duong

et al. 2010

Journal of Biological Chemistry

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“…It is possible that the signal responsible for targeting apoB-containing lipoproteins in cells incubated with ⍀-3 fatty acids is established before the particles exit from the ER and serves to trigger the appropriate trafficking of the doomed substrate to post-ER degradation. Along these lines, it has been suggested that insulin-stimulated apoB degradation in rat primary hepatocytes involves PI3K translocation to the ER membrane and the sorting of apoB to a post-ER degradative pathway (51).…”

Section: Discussionmentioning

confidence: 99%

“…We have recently observed that brefeldin A also protects apoB from acute insulin-stimulated degradation in rat primary hepatocytes (50), a process that depends on PI3K activation (50,51). Therefore, to determine if similar signaling is also involved in the effects of ⍀-3 fatty acids, we treated rat primary hepatocytes for 5 h, in the absence or presence of the PI3K inhibitor wortmannin (1 M), with BSA alone or BSA complexed with DHA or OA.…”

Section: Relationship Between ⍀-3 Fatty Acid-induced Degradationmentioning

confidence: 99%

The Triple Threat to Nascent Apolipoprotein B

Fisher¹,

Pan²,

Chen³

et al. 2001

Journal of Biological Chemistry

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176

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We previously showed that ⍀-3 fatty acids reduce secretion of apolipoprotein B (apoB) from cultured hepatocytes by stimulating post-translational degradation. In this report, we now characterize this process, particularly in regard to the two known processes that degrade newly synthesized apoB, endoplasmic reticulum (ER)-associated degradation and re-uptake from the cell surface. First, we found that ⍀-3-induced degradation preferentially reduces the secretion of large, assembled apoBlipoprotein particles, and apoB polypeptide length is not a determinant. Second, based on several experimental approaches, ER-associated degradation is not involved. Third, re-uptake, the only process known to destroy fully assembled nascent lipoproteins, was clearly active in primary hepatocytes, but ⍀-3-induced degradation of apoB continued even when re-uptake was blocked. Cell fractionation showed that ⍀-3 fatty acids induced a striking loss of apoB 100 from the Golgi, while sparing apoB 100 in the ER, indicating a post-ER process. To determine the signaling involved, we used wortmannin, a phosphatidylinositol 3-kinase (PI3K) inhibitor, which blocked most, if not all, of the ⍀-3 fatty acid effect. Therefore, nascent apoB is subject to ER-associated degradation, re-uptake, and a third distinct degradative pathway that appears to target lipoproteins after considerable assembly and involves a post-ER compartment and PI3K signaling. Physiologic, pathophysiologic, and pharmacologic regulation of net apoB secretion may involve alterations in any of these three degradative steps.Apolipoprotein B (apoB), 1 the major protein of atherogenic lipoproteins, is synthesized primarily by hepatic and intestinal cells. Most studies have focused on apoB metabolism in the liver, given the greater contribution to the plasma apoB pool made by that organ and the availability of relatively convenient primary and transformed hepatic cell models. The apoB message level and translational rate in hepatic cells are largely constitutive, and so secretory control is achieved primarily through co-and post-translational degradation of the protein (e.g. see Refs. 2-4 for recent reviews). Two specific mechanisms for the destruction of newly synthesized apoB in hepatic cells have been characterized. The first is endoplasmic reticulum-associated degradation (ERAD). Newly synthesized apoB in the endoplasmic reticulum (ER) is initially complexed with small amounts of lipid that are thought to be shuttled by the microsomal triglyceride transfer protein (MTP) (5). During severe lipid deprivation (6, 7) or MTP deficiency (8, 9), this initial lipidation fails, and the apoB becomes ubiquitinylated, which targets it for degradation by proteasomes (10 -14).The second mechanism for degradation of newly synthesized apoB is the re-uptake pathway. Re-uptake can occur after fully assembled apoB-containing particles have been exported across the plasma membrane but before they have diffused away from the vicinity of the cell by traversing the unstirred water layer that is adjacent...

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“…The EGF-induced phosphorylation of IRSs was accompanied by activation of PtdIns 3-kinase via binding of p85, the regulatory subunit of the enzyme, to the phosphorylated IRSs. In view of the pivotal role of PtdIns 3-kinase in a variety of intracellular signalling cascades [11,19], IRS-1 and IRS-2 must be important members in EGFR-initiated signalling pathways in cultured hepatocytes.…”

Section: O N C L U S I O N Smentioning

confidence: 99%

Involvement of insulin receptor substrates in epidermal growth factor induced activation of phosphatidylinositol 3‐kinase in rat hepatocyte primary culture

Fujioka

2001

European Journal of Biochemistry

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Short-term incubation of adult rat hepatocytes with epidermal growth factor (EGF) caused tyrosine phosphorylation of insulin receptor substrate (IRS)-1 and IRS-2 when the cells had been submitted to primary culture from 1±18 h. Tyrosine-phosphorylated IRS-1 and IRS-2 bound to the regulatory subunit (p85) of phosphatidylinositol (PtdIns) 3-kinase, thereby activating the enzymic activity. Tyrosine phosphorylation of the IRSs and activation of PtdIns 3-kinase in 3 h cultured hepatocytes both proceeded similarly to the same actions of insulin; the activation was rapid and transient, with peak values at 15±30 s and with similar EC 50 s in the nm range in both cases. A possible involvement of insulin receptors in these insulin-like actions of EGF was excluded by the following three lines of evidence. Insulin caused tyrosine phosphorylation of the insulin receptor b-subunit but EGF did not. In contrast, the EGF receptor was phosphorylated by EGF, but the insulin receptor was not. The actions of EGF, but not those of insulin, were inhibited by AG1478, a selective inhibitor of EGF receptor tyrosine kinase. Cultured hepatocytes exposed to insulin or insulin-like growth factor-I (IGF-I) for a short period responded to the subsequent addition of EGF, whereas EGF-treated cells responded to insulin. The cells, however, displayed receptor desensitization under the same conditions, that is, no response was observed upon repeated addition of the same agonist, EGF, insulin or IGF-I. Thus, the EGF receptor-initiated signalling was mediated by PtdIns 3-kinase associated with tyrosine-phosphorylated IRSs in short-term cultured rat hepatocytes.Keywords: signal transduction; tyrosine kinase receptor; tyrosine phosphorylation.Membrane receptors having intrinsic protein tyrosine kinase activity are known to be responsible for promoting proliferation of hepatocytes [1] as well as many other types of cells [2]. If kept in a serum-free medium, DNA synthesis in hepatocytes occurs with the addition of growth factors, among which epidermal growth factor (EGF) is effective in vivo and in culture [3,4] as a result of its binding to typical tyrosine kinase related receptors. The EGF receptor (EGFR) is the prototype of the type I ErbB subfamily, which includes three additional members, ErbB-2/Neu, ErbB3 and ErbB4 [5]. All members of the ErbB subfamily have a domain that serves as a docking site for specific SH-2 domains, which are present in signalling proteins such as the p85 regulatory subunit of phosphatidylinositol (PtdIns) 3-kinase. EGF has been shown to activate PtdIns 3-kinase, although it does not bind directly to the EGFR but is instead activated via association with p120 cbl [6,7] or Gab1 [8], which are phosphorylated on tyrosine residues by the growth factor. PtdIns 3-kinase may also interact with heterodimers of other members of the EGFR family, such as ErbB3, in response to EGF [9,10].PtdIns 3-kinase plays important roles in a wide variety of biological responses to hormones, growth factors and cytokines [11]. The typical responses s...

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Phosphoinositide 3-Kinase Activity Is Necessary for Insulin-dependent Inhibition of Apolipoprotein B Secretion by Rat Hepatocytes and Localizes to the Endoplasmic Reticulum

Cited by 114 publications

References 68 publications

Targeted Deletion of Hepatocyte ABCA1 Leads to Very Low Density Lipoprotein Triglyceride Overproduction and Low Density Lipoprotein Hypercatabolism

Targeted Deletion of Hepatocyte ABCA1 Leads to Very Low Density Lipoprotein Triglyceride Overproduction and Low Density Lipoprotein Hypercatabolism

The Triple Threat to Nascent Apolipoprotein B

Involvement of insulin receptor substrates in epidermal growth factor induced activation of phosphatidylinositol 3‐kinase in rat hepatocyte primary culture

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