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Apolipoprotein M (ApoM) plays an important role in the occurrence and development of various diseases. The ApoM gene is located in a highly conserved region at chromosome 21, p21. 31. Many genes in this region are involved in immune responses and are very close to tumor necrosis factor α and lymphotoxin genes. Therefore, ApoM may also be involved in the immune system and participate in inflammation of the body.
ApoM is predominantly expressed in HDL in plasma, and only a small fraction is present in triacylglycerol-rich lipoproteins and low-density lipoprotein (LDL). The expression of ApoM is highly tissue-specific and is mainly expressed in normal human hepatocytes, renal tubular endothelial cells, and small intestine tissues. ApoM may have special functions related to liver lipid and lipoprotein metabolism in vivo. The abnormal expression of ApoM is closely related to the occurrence and development of some tumor diseases.
ApoM and Atherosclerosis
Huang's research clarifies the role of ApoM in various diseases. Esterified cholesterol in mature HDL particles can be engulfed by hepatocytes by pinocytosis, and triglycerides and cholesterol esters can be exchanged indirectly to LDL. The particle density of APOM is close to HDL2 and HDL3. In vitro experiments have confirmed that HDL (APOM+) is more effective as an antioxidant than HDL (ApoM-) and promotes cholesterol efflux from THP-1 cells. In addition, LDL (APOM +) is more resistant to oxidation than LDL (ApoM-). APOM can affect the conversion of HDL particles by lecithin cholesterol acyltransferase (LCAT). APOM has been shown to convert pre-β-HDL into mature HDL particles, participate in cholesterol efflux, and has anti-atherosclerotic effects.
Figure 1. Involvement of ApoM in a several diseases. (Huang., et al. 2015)
HDL promotes the reverse transport of cholesterol and has an anti-atherosclerotic effect. ApoM is an important component of HDL, and its changes can affect the progression of atherosclerosis. The most important manifestation is that ApoM can significantly affect the formation of β-HDL and the reverse transport of cholesterol in HDL. At present, at least three factors are known [forkhead box a2 (Foxa2), liver X receptor (LXR) and liver receptor homolog-1 (LRH -1)] has a regulatory effect on the expression of ApoM.
Gao et al. found that various pro-inflammatory cytokines and anti-inflammatory cytokines are involved in the pathogenesis of atherosclerosis, such as TNF-α, IL-1, IL-12, INF-γ, TGF-β, platelet-activating factor (PAF), and the like. These inflammatory factors are involved in the regulation of ApoM expression. Among them, TNF-α can significantly down-regulate APOM mRNA levels in HepG2 cells.
ApoM and Cancer
Hepatitis B virus (HBV) infection is a clear cause of liver cancer, and its acute infection significantly increases the risk of cirrhosis and hepatocellular carcinoma. HBV infection can upregulate ApoM expression and may in turn inhibit HBV replication. The study found that plasma ApoM levels were significantly elevated in patients with liver cancer.
The expression levels of ApoM, LRH-1 and HNF-1α messenger RNA in liver cancer tissues were significantly higher than those in adjacent tissues. Compared with normal tissues adjacent to liver cancer, the ability of liver cancer tissues to produce ApoM is significantly reduced. However, plasma ApoM is elevated in liver cancer tissues, which may be caused by abnormal liver function caused by ApoM participating in immune inflammatory response.
The study found that a small amount of ApoM is present in bone marrow cells and peripheral blood cells, so it is also possible that non-hepatic ApoM supplements ApoM in peripheral blood. The expression of ApoM was positively correlated with the degree of differentiation of liver cancer. Hu et al. found that overexpression of human microRNA-573 liver cancer cells reduced ApoM expression. Based on the above studies, ApoM may be a new marker to more fully assess and detect liver function in patients with hepatocellular carcinoma and may provide new treatment ideas for the cure of liver cancer.
Serum total cholesterol, LDL, and HDL levels were lower in patients with gastric cancer. Gastric cancer is a malignant tumor with strong LDL metabolism. The worse the differentiation of gastric cancer, the more significant the decrease in ApoM synthesis, and the more significant the decrease in serum HDL and LDL levels. Studies have shown that serum HDL-C levels in patients with gastrointestinal malignancies are significantly lower than the normal group, the lower the preoperative HDL-C, the higher the risk of surgery, the worse the prognosis. And HDL-C is positively correlated with ApoM, so ApoM may be a factor influencing the prognosis of postoperative patients with gastric cancer.
ApoM is also present in LDL, and LDL is negatively correlated with ApoM expression. Therefore, it is speculated that when the gastric cancer tissue cells have higher serum total cholesterol requirements and the LDL metabolism is more vigorous, these may cause ApoM expression levels to be down-regulated. The level of ApoM messenger RNA in gastric cancer tissues was significantly lower than that in adjacent tissues, and the level of ApoM in plasma was significantly higher than that in normal subjects. Because of the poor sensitivity and specificity of traditional gastric cancer biomarkers, ApoM may be a new auxiliary indicator for monitoring gastric cancer if the relationship between gastric cancer and ApoM expression can be clarified.