Dataset Information

Hepatic sortilin regulates both apolipoprotein B secretion and LDL catabolism.

ABSTRACT: Genome-wide association studies (GWAS) have identified a genetic variant at a locus on chromosome 1p13 that is associated with reduced risk of myocardial infarction, reduced plasma levels of LDL cholesterol (LDL-C), and markedly increased expression of the gene sortilin-1 (SORT1) in liver. Sortilin is a lysosomal sorting protein that binds ligands both in the Golgi apparatus and at the plasma membrane and traffics them to the lysosome. We previously reported that increased hepatic sortilin expression in mice reduced plasma LDL-C levels. Here we show that increased hepatic sortilin not only reduced hepatic apolipoprotein B (APOB) secretion, but also increased LDL catabolism, and that both effects were dependent on intact lysosomal targeting. Loss-of-function studies demonstrated that sortilin serves as a bona fide receptor for LDL in vivo in mice. Our data are consistent with a model in which increased hepatic sortilin binds intracellular APOB-containing particles in the Golgi apparatus as well as extracellular LDL at the plasma membrane and traffics them to the lysosome for degradation. We thus provide functional evidence that genetically increased hepatic sortilin expression both reduces hepatic APOB secretion and increases LDL catabolism, providing dual mechanisms for the very strong association between increased hepatic sortilin expression and reduced plasma LDL-C levels in humans.

SUBMITTER: Strong A

PROVIDER: S-EPMC3408750 | biostudies-literature | 2012 Aug

REPOSITORIES: biostudies-literature

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Hepatic sortilin regulates both apolipoprotein B secretion and LDL catabolism.

Strong Alanna A Ding Qiurong Q Edmondson Andrew C AC Millar John S JS Sachs Katherine V KV Li Xiaoyu X Kumaravel Arthi A Wang Margaret Ye MY Ai Ding D Guo Liang L Alexander Eric T ET Nguyen David D Lund-Katz Sissel S Phillips Michael C MC Morales Carlos R CR Tall Alan R AR Kathiresan Sekar S Fisher Edward A EA Musunuru Kiran K Rader Daniel J DJ

The Journal of clinical investigation 20120702 8

Genome-wide association studies (GWAS) have identified a genetic variant at a locus on chromosome 1p13 that is associated with reduced risk of myocardial infarction, reduced plasma levels of LDL cholesterol (LDL-C), and markedly increased expression of the gene sortilin-1 (SORT1) in liver. Sortilin is a lysosomal sorting protein that binds ligands both in the Golgi apparatus and at the plasma membrane and traffics them to the lysosome. We previously reported that increased hepatic sortilin expre ...[more]

PMID: 22751103

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Project description:Hepatic apolipoprotein B (apoB) lipoprotein production is metabolically regulated via the phosphoinositide 3-kinase cascade; however, the role of the key negative regulator of this pathway, the tumor suppressor phosphatase with tensin homology (PTEN), is unknown. Here, we demonstrate that hepatic protein levels of apoB100 and microsomal triglyceride transfer protein (MTP) are significantly down-regulated (73% and 36%, respectively) in the liver of PTEN liver-specific knockout (KO) mice, and this is accompanied by increased triglyceride (TG) accumulation and lipogenic gene expression, and reduced hepatic apoB secretion in freshly isolated hepatocytes. MTP protein mass and lipid transfer activity were also significantly reduced in liver of PTEN KO mice. Overexpression of the dominant negative mutant PTEN C/S124 (adenovirus expressing PTEN C/S mutant [AdPTENC/S]) possessing constitutive phospoinositide 3-kinase activity in HepG2 cells led to significant reductions in both secreted apoB100 and cellular MTP mass (76% and 34%, respectively), and increased messenger RNA (mRNA) levels of sterol regulatory element binding protein 1c (SREBP-1c), fatty acid synthase (FAS), and acetyl-CoA carboxylase (ACC). Reduced apoB100 secretion induced by AdPTENC/S was associated with increased degradation of newly-synthesized cellular apoB100, in a lactacystin-sensitive manner, suggesting enhanced proteasomal degradation. AdPTENC/S also reduced apoB-lipoprotein production in McA-RH7777 and primary hamster hepatocytes. Our findings suggest a link between PTEN expression and hepatic production of apoB-containing lipoproteins. We postulate that perturbations in PTEN not only may influence hepatic insulin signaling and hepatic lipogenesis, but also may alter hepatic apoB-lipoprotein production and the MTP stability. On loss of PTEN activity, increased lipid substrate availability in the face of reduced hepatic lipoprotein production capacity can rapidly lead to hepatosteatosis and fatty liver.

Project description:RATIONALE:Genome-wide association studies identified single-nucleotide polymorphisms near the SORT1 locus strongly associated with decreased plasma LDL-C (low-density lipoprotein cholesterol) levels and protection from atherosclerotic cardiovascular disease and myocardial infarction. The minor allele of the causal SORT1 single-nucleotide polymorphism locus creates a putative C/EBP? (CCAAT/enhancer-binding protein ?)-binding site in the SORT1 promoter, thereby increasing in homozygotes sortilin expression by 12-fold in liver, which is rich in this transcription factor. Our previous studies in mice have showed reductions in plasma LDL-C and its principal protein component, apoB (apolipoprotein B) with increased SORT1 expression, and in vitro studies suggested that sortilin promoted the presecretory lysosomal degradation of apoB associated with the LDL precursor, VLDL (very-low-density lipoprotein). OBJECTIVE:To determine directly that SORT1 overexpression results in apoB degradation and to identify the mechanisms by which this reduces apoB and VLDL secretion by the liver, thereby contributing to understanding the clinical phenotype of lower LDL-C levels. METHODS AND RESULTS:Pulse-chase studies directly established that SORT1 overexpression results in apoB degradation. As noted above, previous work implicated a role for lysosomes in this degradation. Through in vitro and in vivo studies, we now demonstrate that the sortilin-mediated route of apoB to lysosomes is unconventional and intersects with autophagy. Increased expression of sortilin diverts more apoB away from secretion, with both proteins trafficking to the endosomal compartment in vesicles that fuse with autophagosomes to form amphisomes. The amphisomes then merge with lysosomes. Furthermore, we show that sortilin itself is a regulator of autophagy and that its activity is scaled to the level of apoB synthesis. CONCLUSIONS:These results strongly suggest that an unconventional lysosomal targeting process dependent on autophagy degrades apoB that was diverted from the secretory pathway by sortilin and provides a mechanism contributing to the reduced LDL-C found in individuals with SORT1 overexpression.

Dataset Information

Hepatic sortilin regulates both apolipoprotein B secretion and LDL catabolism.

Publications

Hepatic sortilin regulates both apolipoprotein B secretion and LDL catabolism.

Similar Datasets

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