Apolipoprotein O expression in mouse liver enhances hepatic lipid accumulation by impairing mitochondrial function.

Apolipoprotein O (ApoO) was recently observed in the cellular mitochondrial inner membrane, which plays a role in mitochondrial function and is associated with myocardiopathy. Empirical information on the physiological functions of apoO is therefore limited. In this study, we aimed to elucidate the effect of apoO on hepatic fatty acid metabolism. An adenoviral vector expressing hApoO was constructed and introduced into chow diet and high-fat diet induced mice and the L02 human hepatoma cell line. High levels of hApoO mRNA and protein were detected in the liver, and the expression of lipid metabolism genes was significantly altered compared with negative controls. The liver function indices (serum ALT and AST) were clearly elevated, and the ultrastructure of cellular mitochondria was distinctly altered in the liver after apoO overexpression. Further, mitochondrial membrane potential decreased with hApoO treatment in L02 cells. These results establish a link between apoO and lipid accumulation and could suggest a new pathway for regulating non-alcoholic fatty liver disease progression.

Microarray analysis provides new insights into the function of apolipoprotein O in HepG2 cell line.

BACKGROUND: Apolipoprotein O (apoO) is a new member of the apolipoprotein family. However, data on its physiological functions are limited and inconsistent. Using a microarray expression analysis, this study explored the function of apoO in liver cells. METHODS: HepG2 cells were treated either with oleic acid or tumor necrosis factor-α for 24 h. mRNA and protein expression of apoO were assessed by quantitative real-time PCR (qRT-PCR) and Western blot respectively. An efficient lentiviral siRNA vector targeting the human apoO gene was designed and constructed. The gene expression profile of HepG2 human hepatocellular carcinoma cells transfected with the apoO silencing vector was investigated using a whole-genome oligonucleotide microarray. The expression levels of some altered genes were validated using qRT-PCR. RESULTS: ApoO expression in HepG2 cells was dramatically affected by lipid and inflammatory stimuli. A total of 282 differentially expressed genes in apoO-silenced HepG2 cells were identified by microarray analysis. These genes included those participating in fatty acid metabolism, such as ACSL4, RGS16, CROT and CYP4F11, and genes participating in the inflammatory response, such as NFKBIZ, TNFSF15, USP2, IL-17, CCL23, NOTCH2, APH-1B and N2N. The gene Uncoupling protein 2 (UCP2), which is involved in both these metabolic pathways, demonstrated significant changes in mRNA level after transfection. CONCLUSIONS: It is likely that apoO participates in fatty acid metabolism and the inflammatory response in HepG2 cells, and UCP2 may act as a mediator between lipid metabolism and inflammation in apoO-silenced HepG2 cells.

Plasma apolipoprotein O level increased in the patients with acute coronary syndrome.

Apolipoprotein (apo) O is a novel apolipoprotein that is present predominantly in high density lipoprotein (HDL). However, overexpression of apoO does not impact on plasma HDL levels or functionality in human apoA-I transgenic mice. Thus, the physiological function of apoO is not yet known. In the present study, we investigated relationships between plasma apoO levels and high-sensitive C-reactive protein (hs-CRP) levels, as well as other lipid parameters in healthy subjects (n = 111) and patients with established acute coronary syndrome (ACS) (n = 50). ApoO was measured by the sandwich dot-blot technique with recombinant apoO as a protein standard. Mean apoO level in healthy subjects was 2.21 ± 0.83 µg/ml whereas it was 4.94 ± 1.59 µg/ml in ACS patients. There were significant differences in plasma level of apoO between two groups (P < 0.001). In univariate analysis, apoO correlated significantly with lg(hsCRP) (r = 0.48, P < 0.001) in ACS patients. Notably, no significant correlation between apoO and other lipid parameters was observed. Logistic regression analysis showed that plasma apoO level was an independent predictor of ACS (OR = 5.61, 95% CI 2.16-14.60, P < 0.001). In conclusion, apoO increased in ACS patients, and may be regarded as an independent inflammatory predictor of ACS patients.

Apolipoprotein A5 internalized by human adipocytes modulates cellular triglyceride content.

Apolipoprotein A5 (apoA5), an important determinant of plasma triglyceride (TG) levels, has been recently reported to modulate TG metabolism in hepatocytes. In this study, we investigated whether apoA5 can be internalized by adipocytes and regulate cellular TG storage. Human preadipocytes, derived from subcutaneous adipose tissue of patients undergoing abdominal surgery, were differentiated into mature adipocytes. Pulse-chase experiments revealed that apoA5 was internalized into human adipocytes, and ∼70% of the apoA5 internalized during the pulse remained intracellular within a 24-h chase, while 30% was degraded. Preincubation with heparin and the receptor-associated protein, both of which prevented the apoA5 interaction with members of the low-density lipoprotein receptor gene family, markedly reduced the uptake of apoA5 by 61% and 52%, respectively, which were subsequently confirmed by Western blot analysis. Using confocal microscopy, we demonstrated that labeled apoA5 surrounded lipid droplets in human adipocytes and colocalized with the known lipid droplet protein perilipin. Importantly, treatment of adipocytes with apoA5 significantly decreased cellular TG storage. In conclusion, apoA5 can be internalized by human adipocytes and may act as a novel regulator to control TG storage in human adipocytes.

Apolipoprotein A5 regulates intracellular triglyceride metabolism in adipocytes.

It has previously been demonstrated that apolipoprotein A5 (apoA5) can be internalized by human adipocytes and significantly decreases intracellular triglyceride content. In the present study, endocytosis of apoA5 by adipocytes under different conditions, and the underlying mechanism by which apoA5 regulates cellular triglyceride storage, was investigated. The results revealed that the apoA5 protein was detected in human subcutaneous abdominal adipose tissues. In addition, the uptake of apoA5 was attenuated in human obese adipose tissues and in cultured adipocytes with hypertrophy or insulin resistance. Low­density lipoprotein receptor protein 1 (LRP1) knockdown in adipocytes resulted in a decrease in internalized apoA5 content, suggesting that LRP1 serves a role in apoA5 uptake. Treatment of adipocytes with apoA5 decreased the expression of the lipid droplet­associated proteins such as cidec and perilipin. ApoA5­treated adipocytes demonstrated an increase in lipolysis activity and expression of uncoupling protein 1, which is the molecular effector of thermogenesis in brown adipocytes. These results suggested that decreased triglyceride accumulation in adipocytes induced by apoA5 may be associated with enhanced lipolysis and energy expenditure, which may result from reduced expression of cidec and perilipin. In conclusion, the present study demonstrated a novel role of apoA5 in regulating the intracellular triglyceride metabolism of adipocytes. The results of the present study suggested that apoA5 may serve as a potential therapeutic target for the treatment of obesity and its related disorders.

Intracellular role of exchangeable apolipoproteins in energy homeostasis, obesity and non-alcoholic fatty liver disease.

Exchangeable apolipoproteins play an important role in systemic lipid metabolism, especially for lipoproteins with which they are associated. Recently, emerging evidence has suggested that exchangeable apolipoproteins, such as apolipoprotein A4 (apoA4), apolipoprotein A5 (apoA5), apolipoprotein C3 (apoC3) and apolipoprotein E (apoE), also exert important effects on intracellular lipid homeostasis. There is a close link between lipid metabolism in adipose tissue and liver because the latter behaves as the metabolic sensor of dysfunctional adipose tissue and is a main target of lipotoxicity. Given that the energy balance between these two major lipogenic organs is intimately involved in the pathogenesis of obesity and non-alcoholic fatty liver disease (NAFLD), we here review recent findings concerning the intracellular function of exchangeable apolipoproteins in triglyceride metabolism in adipocytes and hepatocytes. These apolipoproteins may act as mediators of crosstalk between adipose tissue and liver, thus influencing development of obesity and hepatosteatosis. This review provides new insights into the physiological role of exchangeable apolipoproteins and identifies latent targets for therapeutic intervention of obesity and its related disorders.