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- Perman, Jeanna, 1981
(author)
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Mechanisms for and consequences of cellular lipid accumulation - Role of the Very Low Density Lipoprotein (VLDL) receptor
- 2011
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Doctoral thesis (other academic/artistic)abstract
- Lipid accumulation in non adipose tissue is associated with various cases of tissue dysfunction and tissue failure. Reduced availability of oxygen is known to cause intracellular lipid accumulation in cardiomyocytes as well as in hearts. Cardiac lipid accumulation has been shown to cause impaired cardiac function but it is not fully clear how the lipids accumulate in the hypoxic myocardium. We have studied a model of hypoxic/ischemic myocardium using HL-1 cardiomyocytes incubated in hypoxic condition as well as an in vivo model where mice were subjected to a myocardial infarction causing cardiac ischemia. We found that the Very low density lipoprotein receptor (VLDLr), a member of the low density lipoprotein receptor (LDLr) family suggested to be able to mediate uptake of lipids, was significantly upregulated in response to hypoxia and that this upregulation was mediated through hypoxic activation of transcription factor Hif-1α. The VLDLr induced an increase in intracellular triglycerides which were mediated not primarily through increased uptake of fatty acids but from an increased uptake of extracellular triglyceride-rich lipoproteins. The uptake of lipoproteins was rapid in response to hypoxia. The increase in intracellular lipids caused an accumulation of cardiotoxic ceramides in the cardiomyocytes which induced myocardial ER-stress. ER-stress initially induces a cardioprotective response but prolonged ER-stress cause apoptosis which was increased when the VLDLr was expressed. Ablation of the VLDLr reduced the ER-stress. The mice lacking VLDLr expression showed a reduced infarct size which could be dependent on a reduced amount of toxic ceramides and apoptosis. We could also show that it was possible to block the harmful actions of the VLDLr by using VLDLr specific antibodies. Treatment with these antibodies reduced the lipid accumulation, ER-stress and apoptosis otherwise following a myocardial infarction. The hypoxic VLDLr expression is not restricted to species or tissue. We could see that the VLDLr was increased in human ischemic myocardium compared to non-ischemic biopsies. We could also see that the VLDLr expression was increased in human clear-cell renal carcinoma where in this case the increased VLDLr expression was not due to hypoxia but on constitutive Hif-1α activation. Like in the myocardium the VLDLr caused an accumulation of intracellular triglyceride in the cancer, which already contained great amounts of cholesterol esters. These results indicate that the VLDLr is an important mediator of post-ischemic intramyocardial lipid accumulation and that the blocking of this lipid uptake improves survival.
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| 2. |
- Perman, Jeanna, 1981, et al.
(author)
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The VLDL receptor promotes lipotoxicity and increases mortality in mice following an acute myocardial infarction.
- 2011
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In: The Journal of clinical investigation. - : American Society for Clinical Investigation. - 1558-8238 .- 0021-9738. ; 121:7, s. 2625-40
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Journal article (peer-reviewed)abstract
- Impaired cardiac function is associated with myocardial triglyceride accumulation, but it is not clear how the lipids accumulate or whether this accumulation is detrimental. Here we show that hypoxia/ischemia-induced accumulation of lipids in HL-1 cardiomyocytes and mouse hearts is dependent on expression of the VLDL receptor (VLDLR). Hypoxia-induced VLDLR expression in HL-1 cells was dependent on HIF-1α through its interaction with a hypoxia-responsive element in the Vldlr promoter, and VLDLR promoted the endocytosis of lipoproteins. Furthermore, VLDLR expression was higher in ischemic compared with nonischemic left ventricles from human hearts and was correlated with the total lipid droplet area in the cardiomyocytes. Importantly, Vldlr-/- mice showed improved survival and decreased infarct area following an induced myocardial infarction. ER stress, which leads to apoptosis, is known to be involved in ischemic heart disease. We found that ischemia-induced ER stress and apoptosis in mouse hearts were reduced in Vldlr-/- mice and in mice treated with antibodies specific for VLDLR. These findings suggest that VLDLR-induced lipid accumulation in the ischemic heart worsens survival by increasing ER stress and apoptosis.
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| 3. |
- Pirazzi, Carlo, et al.
(author)
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Patatin-like phospholipase domain-containing 3 (PNPLA3) I148M (rs738409) affects hepatic VLDL secretion in humans and in vitro
- 2012
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In: Journal of Hepatology. - : Elsevier BV. - 0168-8278 .- 1600-0641. ; 57:6, s. 1276-1282
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Journal article (peer-reviewed)abstract
- Background & Aims: The robust association between non-alcoholic fatty liver disease (NAFLD) and the genetic variant I148M (rs738409) in PNPLA3 has been widely replicated. The aim of this study was to investigate the effect of the PNPLA3 I148M mutation on: (1) hepatic secretion of very low density lipoproteins (VLDL) in humans; and (2) secretion of apolipoprotein B (apoB) from McA-RH 7777 cells, which secrete VLDL-sized apoB-containing lipoproteins. Methods: VLDL kinetics was analyzed after a bolus infusion of stable isotopes in 55 overweight/obese men genotyped for the PNPLA3 I148M variant. Intracellular lipid content, apoB secretion and glycerolipid metabolism were studied in McA-RH 7777 cells overexpressing the human 1481 wild type or 148M mutant PNPLA3 protein. Results: In humans, carriers of the PNPLA3 148M allele had increased liver fat compared to 1481 homozygotes, and kinetic analysis showed a relatively lower secretion of the large, triglyceride-rich VLDL (VLDL1) in 148M carriers vs. 1481 homozygotes for the same amount of liver fat. McA-RH 7777 cells overexpressing the 148M mutant protein showed a higher intracellular triglyceride content with a lower apoB secretion and fatty acid efflux, compared to cells overexpressing the 1481 wild type protein. The responses with 148M matched those observed in cells expressing the empty vector, indicating that the mutation results in loss of function. Conclusions: We have shown that PNPLA3 affects the secretion of apoB-containing lipoproteins both in humans and in vitro and that the 148M protein is a loss-of-function mutation. We propose that PNPLA3 148M promotes intracellular lipid accumulation in the liver by reducing the lipidation of VLDL.
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