Apolipoprotein O modulates cholesterol metabolism via NRF2/CYB5R3 independent of LDL receptor.

Apolipoprotein O (APOO) plays a critical intracellular role in regulating lipid metabolism. Here, we investigated the roles of APOO in metabolism and atherogenesis in mice. Hepatic APOO expression was increased in response to hyperlipidemia but was inhibited after simvastatin treatment. Using a novel APOO global knockout (Apoo-/-) model, it was found that APOO depletion aggravated diet-induced obesity and elevated plasma cholesterol levels. Upon crossing with low-density lipoprotein receptor (LDLR) and apolipoprotein E (APOE) knockout hyperlipidemic mouse models, Apoo-/- Apoe-/- and Apoo-/- Ldlr-/- mice exhibited elevated plasma cholesterol levels, with more severe atherosclerotic lesions than littermate controls. This indicated the effects of APOO on cholesterol metabolism independent of LDLR and APOE. Moreover, APOO deficiency reduced cholesterol excretion through bile and feces while decreasing phospholipid unsaturation by inhibiting NRF2 and CYB5R3. Restoration of CYB5R3 expression in vivo by adeno-associated virus (AAV) injection reversed the reduced degree of phospholipid unsaturation while decreasing blood cholesterol levels. This represents the first in vivo experimental validation of the role of APOO in plasma cholesterol metabolism independent of LDLR and elucidates a previously unrecognized cholesterol metabolism pathway involving NRF2/CYB5R3. APOO may be a metabolic regulator of total-body cholesterol homeostasis and a target for atherosclerosis management. Apolipoprotein O (APOO) regulates plasma cholesterol levels and atherosclerosis through a pathway involving CYB5R3 that regulates biliary and fecal cholesterol excretion, independently of the LDL receptor. In addition, down-regulation of APOO may lead to impaired mitochondrial function, which in turn aggravates diet-induced obesity and fat accumulation.

Metformin-induced PCSK9 inhibition further decreases LDL-C following statin treatment in patients with coronary artery disease and without diabetes.

In vitro investigations have established metformin's capacity to downregulate PCSK9 expression, suggesting a potential beneficial effect on atherogenic lipoprotein particles when combined with metformin therapy. Our objective was to assess whether metformin could mitigate statin-induced adverse effects on PCSK9, thereby improving lipid profiles in patients with coronary artery disease (CAD) but without diabetes. Employing an open-label, placebo-controlled, randomized trial, we randomized patients with CAD but without diabetes into CLA (Cholesterol-Lowering Agents alone: atorvastatin+/-ezetimibe, n=38) and Met+CLA groups (metformin plus CLA, n=33) at a 1:1 ratio. The primary endpoint was the therapeutic impact of one-month metformin combination treatment on LDL-C and PCSK9 levels. Baseline LDL-C and PCSK9 levels were 76.18 mg·dL-1 and 80.54 ng·mL-1, respectively. After one month, metformin significantly reduced LDL-C (-20.81%, P<0.001), enabling 72% of patients to attain guideline-recommended LDL-C goals. Noteworthy reductions in PCSK9 levels (-15.03%, P<0.001) were observed. Moreover, Met+CLA markedly reduced LDL particle number more than CLA alone (-10.65% vs 1.45%, P=0.009), primarily due to diminished small-dense LDL particle count. Mechanistically, our study demonstrated metformin's inhibition of statin-induced PCSK9 expression in human hepatocellular cells. In summary, a one-month metformin combination regimen reduced LDL-C levels in patients with CAD but without diabetes by inhibiting PCSK9 expression.

Macrophage-specific deletion of MIC26 (APOO) mitigates advanced atherosclerosis by increasing efferocytosis.

BACKGROUND AND AIMS: Recent studies have suggested that MIC26 (apolipoprotein O, APOO), a novel mitochondrial inner membrane protein, is involved in inflammation. Thus, the role of macrophage MIC26 in acute inflammation and chronic inflammatory disease atherosclerosis was investigated. METHODS: Macrophage-specific MIC26 knockout mice (MIC26LysM) were generated by crossing Apooflox/flox and LysMcre+/- mice. An endotoxemia mouse model was generated to explore the effects of macrophage MIC26 deficiency on acute inflammation, while an atherosclerosis mouse model was constructed by crossing MIC26LysM mice with Apoe-/- mice and challenged with a Western diet. Atherosclerotic plaques, primary macrophage function, and mitochondrial structure and function were analyzed. RESULTS: MIC26 knockout did not affect the median survival time and post-injection serum interleukin 1ß concentrations in mice with endotoxemia. Mice with MIC26 deficiency in an Apoe-/- background had smaller atherosclerotic lesions and necrotic core than the control group. In vitro studies found that the loss of MIC26 did not affect macrophage polarization, apoptosis, or lipid handling capacity, but increased efferocytosis (the ability to clear apoptotic cells). An in situ efferocytosis assay of plaques also showed that the ratio of macrophage-associated apoptotic cells to free apoptotic cells was higher in the MIC26-deficient group than in the control group, indicating increased efferocytosis. In addition, an in vivo thymus efferocytosis assay indicated that MIC26 deletion promoted efferocytosis. Mechanistically, the loss of MIC26 resulted in an abnormal mitochondrial inner membrane structure, increased mitochondrial fission, and decreased mitochondrial membrane potential. Loss of MIC26 reduced mitochondria optic atrophy type 1 (OPA1) protein, and OPA1 silencing in macrophages promoted efferocytosis. Overexpression of OPA1 abolished the increase in efferocytosis produced by MIC26 deficiency. CONCLUSIONS: Macrophage MIC26 deletion alleviated advanced atherosclerosis and necrotic core expansion by promoting efferocytosis. This mechanism may be related to the increased mitochondrial fission caused by reduced mitochondrial OPA1.

VDR alleviates endothelial cell injury in arteriovenous fistula through inhibition of P66Shc-mediated mitochondrial ROS.

To investigate the effects and mechanism of Vitamin D receptor (VDR) signaling on arteriovenous fistula (AVF) endothelial cell injury. Venous tissues of AVF stenosis patients were collected and analyzed, vascular morphology, reactive oxygen species (ROS), and the expression of VDR, P66Shc, fibronectin (FN), collagen-1 (Col-1) were detected. In addition, human umbilical vein endothelial cells (HUVECs) was used in in vitro studies. HUVECs was incubated with transforming growth factor-beta (TGF-ß, 50 ng/ml). Aditionally, paricalcitol, VDR overexpression plasmid and Pin1 inhibitor Juglone were used to investigate the regulatory mechanism of VDR in mitochondrial ROS. The parameters of ROS (e.g. MitoSox) and the expression of FN, Col-1 were tested. Moreover, the mitochondrial translocation of P66Shc was analyzed. The expression of VDR was obviously decreased in the venous tissues of AVF stenosis patients. On the contrary, the expression of P66Shc, P-P66Shc, FN, Col-1 and 8-OHdG were increased significantly in the venous tissues of AVF stenosis patients (P < 0.05). In line with this, the level of mitochondrial ROS and the expression of P66Shc, P-P66Shc, FN, Col-1 increased obviously in HUVECs cells under TGF-ß condition. Both VDR over-expression plasmid and Pin1 inhibitor Juglone could alleviate TGF-ß induced endothelial injury. Mechanistically, VDR overexpression plasmid and Juglone could inhibit the expression of Pin1, and then restrain P66Shc mitochondrial translocation, eventually reduce the level of mitochondrial ROS. Our research indicated that activation of VDR could alleviate venous endothelial cell dysfunction through inhibiting Pin1-mediated mitochondrial translocation of P66Shc and consequently reducing mitochondrial ROS. It suggested that VDR signaling might be an effective target for AVF stenosis treatment.

Loss of APOO (MIC26) aggravates obesity-related whitening of brown adipose tissue via PPARα-mediated functional interplay between mitochondria and peroxisomes.

BACKGROUND: Mitochondrial dysfunction and aberrant structure in adipose tissue occur in obesity and obesity-linked brown adipose tissue (BAT) whitening; however, whether this aberrant architecture contributes to or is the result of obesity is unknown. Apolipoprotein O (APOO) is a constitutive protein of the mitochondrial cristae organizing system complex. This study aimed to characterize the physiological consequences of APOO deficiency in vivo. METHODS: APOO expression was analyzed in different human and murine adipose depots, and mice lacking APOO in adipocytes (ApooACKO) are developed to examine the metabolic consequences of adipocyte-specific APOO ablation in vitro and in vivo. RESULTS: Results showed that APOO expression is reduced in BAT from both diet-induced and leptin-deficient obese mice. APOO-knockout mice showed increased adiposity, BAT dysfunction and whitening, reduced non-shivering thermogenesis, and blunted responses to cold stimuli. APOO deficiency disrupted mitochondrial structure in brown adipocytes and impaired oxidative phosphorylation, thereby inducing a shift from oxidative to glycolytic metabolism, increasing lipogenic enzyme levels and BAT whitening. APOO inactivation inhibited thermogenesis in BAT by reducing mitochondrial long-chain fatty acid oxidation. It also disturbed peroxisomal biogenesis and very long-chain fatty acid oxidation via peroxisome proliferator-activated receptor α. CONCLUSIONS: Altogether, APOO deficiency in adipocytes aggravates BAT whitening and diet-induced obesity; thus, APOO could be a therapeutic target for obesity.

Inflamed adipose tissue: A culprit underlying obesity and heart failure with preserved ejection fraction.

The incidence of heart failure with preserved ejection fraction is increasing in patients with obesity, diabetes, hypertension, and in the aging population. However, there is a lack of adequate clinical treatment. Patients with obesity-related heart failure with preserved ejection fraction display unique pathophysiological and phenotypic characteristics, suggesting that obesity could be one of its specific phenotypes. There has been an increasing recognition that overnutrition in obesity causes adipose tissue expansion and local and systemic inflammation, which consequently exacerbates cardiac remodeling and leads to the development of obese heart failure with preserved ejection fraction. Furthermore, overnutrition leads to cellular metabolic reprogramming and activates inflammatory signaling cascades in various cardiac cells, thereby promoting maladaptive cardiac remodeling. Growing evidence indicates that the innate immune response pathway from the NLRP3 inflammasome, to interleukin-1 to interleukin-6, is involved in the generation of obesity-related systemic inflammation and heart failure with preserved ejection fraction. This review established the existence of obese heart failure with preserved ejection fraction based on structural and functional changes, elaborated the inflammation mechanisms of obese heart failure with preserved ejection fraction, proposed that NLRP3 inflammasome activation may play an important role in adiposity-induced inflammation, and summarized the potential therapeutic approaches.