Ubiquitin-dependent and -independent proteasomal degradation of apoB associated with endoplasmic reticulum and Golgi apparatus, respectively, in HepG2 cells

Monosialyl Ganglioside GM3 Decreases Apolipoprotein B-100 Secretion in Liver Cells

Some sialic acid-containing glycolipids are known to regulate development of atherosclerosis with accumulated plasma apolipoprotein B-100 (Apo-B)-containing lipoproteins, because Apo-B as an atherogenic apolipoprotein is assembled mainly in VLDL and LDL. Previously, we have elucidated that disialyl GD3 promotes the microsomal triglyceride transfer protein (MTP) gene expression and secretion of triglyceride (TG)-assembled ApoB, claiming the GD3 role in ApoB lipoprotein secretion in liver cells. In the synthetic pathway of gangliosides, GD3 is synthesized by addition of a sialic acid residue to GM3. Thus, there should be some regulatory links between GM3 and GD3. In this study, exogenous and endogenous monosialyl GM3 has been examined how GM3 plays a role in ApoB secretion in Chang liver cells in a view point of MTP and ApoB degradation in the same cells. The level of GM3 ganglioside in the GM3 synthase gene-transfected cells was increased in the cell extract, but not in the medium. In addition, GM3 synthase gene-transfected cells showed a diminished secretion of TG-enriched ApoB with a lower content of TG in the medium. Exogenous GM3 treatment for 24 h exerted a dose dependent inhibitory effect on ApoB secretion together with TG, while a liver-specific albumin was unchanged, indicating that GM3 effect is limited to ApoB secretion. GM3 decreased the mRNA level of MTP gene, too. ApoB protein assembly dysregulated by GM3 indicates the impaired ApoB secretion is caused by a proteasome-dependent pathway. Treatment with small interfering RNAs (siRNAs) decreased ApoB secretion, but GM3-specific antibody did not. These results indicate that plasma membrane associated GM3 inhibits ApoB secretion, lowers development of atherosclerosis by decreasing the secretion of TG-enriched ApoB containing lipoproteins, suggesting that GM3 is an inhibitor of ApoB and TG secretion in liver cells. J. Cell. Biochem. 118: 2168-2181, 2017. © 2017 Wiley Periodicals, Inc.

Complex role of autophagy in regulation of hepatic lipid and lipoprotein metabolism

Discovering new therapeutic interventions to treat lipid and lipoprotein disorders is of great interest and the discovery of autophagy as a regulator of lipid metabolism has opened up new avenues for targeting modulators of this pathway. Autophagy is a degradative process that targets cellular components to the lysosome and recent studies have indicated a role for autophagy in regulating hepatic lipid metabolism (known as lipophagy) as well as lipoprotein assembly. Autophagy directly targets apolipoprotein B-100 (apoB100), the structural protein component of very low-density lipoproteins (VLDLs), and further targets lipid droplets (LDs), the cellular storage for neutral lipids. Autophagy thus plays a complex and dual role in VLDL particle assembly by regulating apoB100 degradation as well as aiding the maturation of VLDL particles by hydrolyzing lipid from LDs. The purpose of this article is to review our current understanding of molecular and cellular mechanisms mediating authophagic control of hepatic lipid biogenesis and VLDL production as well as dysregulation in insulin resistance and dyslipidemia.

Identification of protein disulfide isomerase 1 as a key isomerase for disulfide bond formation in apolipoprotein B100

Pdi1 knockdown decreases apoB100 synthesis, reduces MTP activity and apoB100 lipidation, and impairs the oxidative folding of apoB100, which causes defective VLDL secretion. PDI1 promotes formation of disulfide bonds in apoB100 and serves as its disulfide isomerase.

Apolipoprotein (apo) B is an obligatory component of very low density lipoprotein (VLDL), and its cotranslational and posttranslational modifications are important in VLDL synthesis, secretion, and hepatic lipid homeostasis. ApoB100 contains 25 cysteine residues and eight disulfide bonds. Although these disulfide bonds were suggested to be important in maintaining apoB100 function, neither the specific oxidoreductase involved nor the direct role of these disulfide bonds in apoB100-lipidation is known. Here we used RNA knockdown to evaluate both MTP-dependent and -independent roles of PDI1 in apoB100 synthesis and lipidation in McA-RH7777 cells. Pdi1 knockdown did not elicit any discernible detrimental effect under normal, unstressed conditions. However, it decreased apoB100 synthesis with attenuated MTP activity, delayed apoB100 oxidative folding, and reduced apoB100 lipidation, leading to defective VLDL secretion. The oxidative folding–impaired apoB100 was secreted mainly associated with LDL instead of VLDL particles from PDI1-deficient cells, a phenotype that was fully rescued by overexpression of wild-type but not a catalytically inactive PDI1 that fully restored MTP activity. Further, we demonstrate that PDI1 directly interacts with apoB100 via its redox-active CXXC motifs and assists in the oxidative folding of apoB100. Taken together, these findings reveal an unsuspected, yet key role for PDI1 in oxidative folding of apoB100 and VLDL assembly.

Cloning of apoB intrabodies: specific knockdown of apoB in HepG2 cells

Apolipoprotein (apo) B is essential for the assembly and secretion of triglyceride-rich lipoproteins made by the liver. As the sole protein component in LDL, apoB is an important determinant of atherosclerosis susceptibility and a potential pharmaceutical target. Single-chain antibodies (sFvs) are the smallest fragment of an IgG molecule capable of maintaining the antigen binding specificity of the parental antibody. In the present study, we describe the cloning and construction of two intracellular antibodies (intrabodies) to human apoB. We targeted these intrabodies to the endoplasmic reticulum for the purpose of retaining nascent apoB within the ER, thereby preventing its secretion. Expression of the 1D1 intrabody in the apoB-secreting human hepatoma cell line HepG2 resulted in marked reduction of apoB secretion. This study demonstrates the utility of an intrabody to specifically block the secretion of a protein determinant of plasma LDL as a therapeutic strategy for the treatment of hyperlipidemia.

Ubiquitin-dependent and -independent proteasomal degradation of hepatitis B virus X protein

The hepatitis B virus X protein (HBX) plays key regulatory roles in viral replication and the development of hepatocellular carcinoma. HBX is an unstable protein; its instability is attributed to rapid degradation through the ubiquitin-proteasome pathway. Here, we show that the middle and carboxyl-terminal domains of HBX, independently fused to GFP, render the recombinant proteins susceptible to proteasomal degradation, while the amino-terminal domain has little effect on the ubiquitination or stability of HBX. Mutation of any single or combination of up to five of six lysine residues, all located in the middle and carboxyl-terminal domain, did not prevent HBX from being ubiquitinated, ruling out any specific lysine as the sole site of ubiquitination. Surprisingly, HBX in which all six lysines were mutated and showed no evidence of ubiquitination, was still susceptible to proteasomal degradation. These results suggest that both ubiquitin-dependent and -independent proteasomal degradation processes are operative in HBX turnover.

Obesity and atherogenic dyslipidemia

Obesity is associated with an increased risk of coronary heart disease, in part due to its strong association with atherogenic dyslipidemia, characterized by high triglycerides and low high-density lipoprotein (HDL) cholesterol. There has been substantial research effort focused on the mechanisms of the link between obesity and atherogenic dyslipidemia, both in the absence and presence of insulin resistance. After a brief overview of the epidemiology of atherogenic dyslipidemia, this article details the known molecular mechanisms of adipocyte function and its relationship to apoB-containing lipoprotein assembly and metabolism, both in the healthy as well as in the obese states. We also discuss the pathophysiology of low HDL cholesterol in obesity and the implications for cardiovascular disease risk.

Knockdown of apolipoprotein B, an atherogenic apolipoprotein, in HepG2 cells by lentivirus-mediated siRNA

ApoB is an important determinant of atherosclerosis susceptibility and a potential pharmaceutical target for lowering atherogenic lipoproteins. In the present study, we used a lentiviral vector to express short hairpin RNAs for inhibition of apoB production in HepG2 cells. We first demonstrated that lentivirus could efficiently deliver transgene into HepG2 cells by using GFP lentivirus. We then made three lentiviral siApoB constructs, two of which were highly efficient for silencing apoB expression in HepG2 cells. We showed that siApoB lentivirus specifically knocked down apoB but had no effects on other proteins such as apoAI and albumin. Consequently, the secretion of apoB was reduced markedly. The silencing effect of siApoB lentivirus appeared to be permanent. Knocking down apoB did not alter the expression of cytoplasmic stress proteins (HSP70 and HSP90) and their ER homologues (GRP78 and GRP94). Furthermore, neither IKKalpha and JNK nor phosphorylated IKK and JNK were increased in long-term apoB-deficient hepatocytes as compared to the control cells. Consistent with these findings, apoB-deficient hepatocytes responded to insulin to a similar extent as the control cells as determined by measuring insulin-induced phosphorylation of IRS and ERK. Our studies indicate that lentiviral siRNAs provide an excellent approach for delivering siRNA into HepG2 cells and may be used for gene therapy for hyperlipidemia.

Protection against fatty liver but normal adipogenesis in mice lacking adipose differentiation-related protein

Adipose differentiation-related protein (ADFP; also known as ADRP or adipophilin), is a lipid droplet (LD) protein found in most cells and tissues. ADFP expression is strongly induced in cells with increased lipid load. We have inactivated the Adfp gene in mice to better understand its role in lipid accumulation. The Adfp-deficient mice have unaltered adipose differentiation or lipolysis in vitro or in vivo. Importantly, they display a 60% reduction in hepatic triglyceride (TG) and are resistant to diet-induced fatty liver. To determine the mechanism for the reduced hepatic TG content, we measured hepatic lipogenesis, very-low-density lipoprotein (VLDL) secretion, and lipid uptake and utilization, all of which parameters were shown to be similar between mutant and wild-type mice. The finding of similar VLDL output in the presence of a reduction in total TG in the Adfp-deficient liver is explained by the retention of TG in the microsomes where VLDL is assembled. Given that lipid droplets are thought to form from the outer leaflet of the microsomal membrane, the reduction of TG in the cytosol with concomitant accumulation of TG in the microsome of Adfp-/- cells suggests that ADFP may facilitate the formation of new LDs. In the absence of ADFP, impairment of LD formation is associated with the accumulation of microsomal TG but a reduction in TG in other subcellular compartments.

Proteasome inhibition induces differential heat shock protein response but not unfolded protein response in HepG2 cells

Liver, a central organ responsible for the metabolism of carbohydrates, proteins, and lipoproteins, is exposed to various kinds of physiological, pathological, and environmental stresses. We hypothesized that blockage of proteasome degradation pathway induces heat shock protein (HSP) response and unfolded protein response in the liver cells. In this study, we have characterized cellular responses to proteasome inhibition in HepG2 cells, a well-differentiated human hepatoma cells. We found that proteasome inhibition induced differential response among cytosolic HSPs, that is, increased expression of HSP70, but no change in HSP40, HSC70, and HSP90. However, proteasome inhibition did not induce typical unfolded protein response as indicated by absence of stimulation of GRP78 and GRP94 proteins. Upon proteasome inhibition, inclusion bodies were accumulated, and ubiquitin-conjugated proteins appeared in insoluble fraction, together with HSP40, HSP70, HSC70, and HSP90. After proteasome inhibition, misfolded proteins were increased in the cytosol and in the ER compartment as evaluated by examining ubiquitin-conjugated proteins. However, essentially all ER-associated ubiquitin-conjugated proteins were located on the surface of the ER, which explains why proteasome inhibition does not induce unfolded protein response. In conclusion, proteasome inhibition induces differential HSP response, but not unfolded protein response in HepG2 cells. Our study also suggests that HSPs play important roles in directing proteasomal degradation and protein aggregate formation.

Inhibition of the synthesis of apolipoprotein B-containing lipoproteins

Increased serum concentrations of low density lipoproteins represent a major cardiovascular risk factor. Low-density lipoproteins are derived from very low density lipoproteins secreted by the liver. Apolipoprotein (apo)B that constitutes the essential structural protein of these lipoproteins exists in two forms, the full length form apoB-100 and the carboxy-terminal truncated apoB-48. The generation of apoB-48 is due to editing of the apoB mRNA which generates a premature stop translation codon. The editing of apoB mRNA is an important regulatory event because apoB-48-containing lipoproteins cannot be converted into the atherogenic low density lipoproteins. The apoB gene is constitutively expressed in liver and intestine, and the rate of apoB secretion is regulated post-transcriptionally. The translocation of apoB into the endoplasmic reticulum is complicated by the hydrophobicity of the nascent polypeptide. The assembly and secretion of apoB-containing lipoproteins within the endoplasmic reticulum is strictly dependent on the microsomal tricylceride transfer protein which shuttles triglycerides onto the nascent lipoprotein particle. The overall synthesis of apoB lipoproteins is regulated by proteosomal and nonproteosomal degradation and is dependent on triglyceride availability. Noninsulin dependent diabetes mellitus, obesity and the metabolic syndrome are characterized by an increased hepatic synthesis of apoB-containing lipoproteins. Interventions aimed to reduce the hepatic secretion of apoB-containing lipoproteins are therefore of great clinical importance. Lead targets in these pathways are discussed.

Lipid Disorders and Mutations in the APOB Gene

Background: Plasma lipoproteins are important determinants of atherosclerosis. Apolipoprotein (apo) B is a large, amphipathic glycoprotein that plays a central role in human lipoprotein metabolism. Two forms of apoB are produced from the APOB gene by a unique posttranscriptional editing process: apoB-48, which is required for chylomicron production in the small intestine, and apoB-100, required for VLDL production in the liver. In addition to being the essential structural component of VLDL, apoB-100 is the ligand for LDL-receptor-mediated endocytosis of LDL particles.

Content: The study of monogenic dyslipidemias has revealed important aspects of metabolic pathways. In this review, we discuss the regulation of apoB metabolism and examine how APOB gene defects can lead to both hypo- and hypercholesterolemia. The key clinical, metabolic, and genetic features of familial hypobetalipoproteinemia and familial ligand-defective apoB-100 are described.

Summary: Missense mutations in the LDL-receptor-binding domain of apoB cause familial ligand-defective apoB-100, characterized by hypercholesterolemia and premature coronary artery disease. Other mutations in APOB can cause familial hypobetalipoproteinemia, characterized by hypocholesterolemia and resistance to atherosclerosis. These naturally occurring mutations reveal key domains in apoB and demonstrate how monogenic dyslipidemias can provide insight into biologically important mechanisms.