Biosynthesis of fatty acid
In this experiment, compared with the normal group, the contents of α-linolenic acid, docosahexaenoic acid, arachidonic acid, octadecanoic acid, docosahexaenoic acid and stearic acid in the plasma of rats in the model group were significantly changed. After GGQLD was adminstrated, compared with the model group, the contents of the above components in the plasma of the rats in the administration group all returned to the level of the normal group.
Reported studies have shown that polyunsaturated fatty acids (PUFAs) are important components of cell membrane phospholipids. The synthesis of PUFAs is based on the catalysis, dehydrogenation and prolongation of stearic acid through a series of enzymes[26]. In mammalian livers, eicosapentaenoic acid is catalyzed by Δ5 and Δ7 lengthening enzymes for two times to form 24-carbon pentaenoic acid, which is catalyzed by Δ6 desaturase to form 24-carbon hexaenoic acid, which is then transferred to peroxidase body through endoplasmic reticulum for a beta oxidation to form docosahexaenoic acid. Omega 3 fatty acids such as docosahexaenoic acid can reduce the levels of TC, TG and LDL-C, which are beneficial to cardiovascular health[27].
The mechanism of lipid-lowering effect of GGQLD is related to the biosynthesis of fatty acids in vivo, ω-3 polyunsaturated fatty acids mainly include α-linolenic acid (ALA), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which have the functions of protecting cardiovascular system, regulating blood lipid, anti-inflammation, anti-allergy, anti-tumor and improving immune regulation. The effects of ω-3 polyunsaturated fatty acids on blood lipids were different in different studies, but the effects on TG were similar. ω-3 polyunsaturated fatty acids have a direct inhibitory effect on the synthesis of TG, which may be related to the increase of beta oxidation and the clearance of TG-rich lipoproteins. ALA, as a maternal body, can be used as a beneficial supplement in diet. It can significantly reduce the TC, TG, LDL-C and body weight of rats induced by high-fat diet, increase the level of HDL-C, and prevent and treat hyperlipidemia[28]. Studies[29] have shown that ALA may play a role in reducing body weight and blood lipids by accelerating lipid oxidation, affecting the activities of key metabolic enzymes and reducing visceral fat accumulation. After high-fat diet, the circulatory alpha-linolenic acid in rats decreased significantly, which increased the risk of cardiovascular disease. After GGQLD intervention, the level of alpha-linolenic acid was significantly reversed, and the concentration of TC and TG was decreased by regulating the metabolism of alpha-linolenic acid in rats.
Arachidonic acid metabolism
Arachidonic acid (AA) metabolism is the core of inflammatory metabolic network. The two main metabolic pathways are cyclooxygenase (COX) pathway and perlipoxygenase (LOX) pathway. Protaglandins (PGs), thromboxans (TXs), leukotrienes (LTs) and lipid peroxides produced by COX pathway are potential targets for anti-inflammatory research[30].
AA metabolic abnormalities are associated with dyslipidemia and coronary heart disease. Thromboxane A2 and prostaglandin I2, metabolites of AA, promote platelet aggregation and agglutination respectively. Their balance maintains smooth blood circulation and protects endothelial cells from damage. When blood lipid is abnormal, AA synthesizes thromboxane A2 more quickly, which decreases the ability of blood vessel wall to synthesize prostaglandin I2. It was found that the ratio of plasma lipid peroxide and thromboxane A2/prostaglandin I2 increased in hyperlipidemic rats[31]. Studies have shown that thromboxane A2 and prostaglandin I2 are associated with TC and TG. Puerarin can promote vascular endothelial growth factor-like effect and inhibit the increase of thromboxane A2/prostaglandin I2 ratio induced by high-fat diet[32].
In this study, after GGQLD administration, the AA content in plasma of rats in the administration group decreased to the normal group as compared with that in the model group, and its lipid-lowering effect may be one of the mechanisms affecting arachidonic acid metabolism.
Glycerophospholipid metabolism
Lysophosphatidylcholine is a kind of phosphatidylcholine which contains a fatty acid chain[33]. It mainly refers to lysophosphatidylcholine (LPC), also known as hemolytic lecithin, followed by lysophosphatidylcholine ethanolamine (LPE), which participates in glycerol phospholipid metabolism. LPC is formed by the hydrolysis of phosphatidylcholine by phospholipase A2 or by the hydrolysis of lecithin-cholesterol acyltransferase (LCAT), producing fatty acids such as arachidonic acid, which is closely related to inflammation. LPC plays a role in lipid signaling by acting on LPC receptors, which are members of the G protein-coupled receptor family and participate in many kinds of cell-to-cell signaling. LPC is closely related to metabolic diseases such as dyslipidemia, diabetes mellitus and cardiovascular diseases[34].
Studies have shown that LPC is the core component of oxidized low density lipoprotein, which can change endothelial cell permeability and damage endothelial cells[35]. Many studies have reported a significant increase in plasma LPC in obesity or T2DM[36–38]. Barber[37] and other studies showed that the plasma LPC level significantly decreased after high-fat induction in rats. Regression analysis confirmed that part of LPC was related to IR and that diet and obesity were the main factors affecting blood LPC. LPC is not only involved in cell proliferation, tumor cell invasion and inflammation, but also in glucose metabolism. Yea et al. [39] showed that LPC (16:0) and LPC (14:0) could stimulate the glucose uptake of 3T3-L1 adipocytes and significantly reduce the blood sugar level of T2DM mice. In this study, after GGQLD administration, the plasma lysophosphatidylcholine level of rats in the administration group was reversed from that of the model group to that of the normal group, possibly through regulating the metabolism of glycerophospholipids to play its hypoglycemic and lipid-lowering role. Its specific mechanism of action and the diverse biological functions of lysophosphatidylcholine deserve further study.
Tryptophan metabolism
Indoleacetaldehyde (IAALD) belongs to indole derivatives and participates in tryptophan metabolism. Tryptophan (TRP) in humans and animals is mainly brought in by diet, mainly metabolized by kynurenine (KYN), and small amounts of which were metabolized through 5-hydroxytryptamine and indole-retaining pathway.
In this study, IAALD production was significantly reduced in the plasma of model group rats, suggesting that KYN pathway metabolized by indole 2, 3-dioxygenase (IDO) or tryptophan 2, 3-dioxygenase (TDO) increased, which is related to inflammation and cardiovascular disease. IDO was positively correlated with age, body mass index and negatively correlated with HDL-C. Studies[40] have shown that blood TRP, KYN and KYN/TRP ratios are associated with obesity.
The abnormal blood lipid and obesity of organisms make it in a chronic low-grade inflammatory state. Proinflammatory cytokines induce the increase of IDO expression, enhance the decomposition of TRP, and increase the production of the toxic metabolites in the KYN pathway. Recently, a prospective study[41] analyzed the correlation between tryptophan metabolism and T2DM, indicating that TRP and its metabolites increased significantly in the early stage of T2DM and decreased in the complete state of T2DM, predicting IR and assessing the risk of T2DM.
In addition, as an essential amino acid, the metabolism of TRP is closely related to intestinal microflora. In the intestine, intestinal microorganisms can convert dietary tryptophan into indole and indole derivatives, including indole propionic acid, indole acetic acid, indole acetaldehyde, etc. The changes of indole derivatives in vivo after high-fat diet suggest the imbalance of intestinal microflora. The main components of GGQLD have the anti-inflammatory effect, they can increase the contents of TRP metabolites to a certain extent. GGQLD may play a role in regulating TRP metabolism and intestinal homeostasis by inhibiting the activation of IDO.
Metabolism of arachidonic acid ethanolamine
Endogenous cannabinoid system (ECS) includes two cannabinoid receptors: CB1 and CB2. Two endogenous ligands, arachidonic ethanolamine (AEA) and arachidonic glycerol (2-AG), are also known as endocannabinoids. AEA mainly binds to CB1 receptor and acts as a partial agonist. The content level of AEA in blood can reflect the change of ECS in vivo to a certain degree. ECS participates in the regulation of food intake, lipid metabolism and energy metabolism, and plays a direct role in adipogenesis. In the pathological model induced by high-fat diet, ECS is over-activated, the expression of endogenous cannabinoid is increased, the biosynthesis of fatty acids and triglycerides in liver is induced, and the differentiation and maturation of preadipocytes are promoted. Adipocytes inhibit fat breakdown and promote obesity. The results showed that the expression of CB1 in peripheral tissues increased in obese groups and animal models induced by high fat diet, which eventually led to the disorder of glucose and lipid metabolism[42]. The level of TC and TG could be effectively reduced by giving CB1 antagonists to diet-fattening mice [6]. In this experiment, after 5 weeks of intervention with Gegen Qinlian Decoction, the appetite of rats decreased, the weight gain began to slow down, TC, TG and FPG decreased significantly, GGQLD could reduce the level of AEA in plasma of rats with dyslipidemia to a certain extent. It may inhibit appetite by inhibiting the expression of cannabinoid receptors in the central and peripheral nervous system, and accelerating the metabolism of peripheral fat, so as to improve the disorder of glycolipid metabolism.
Acyl carnitines
L-carnitine lipid is the key substance of lipid metabolism. Fatty acids combine with L-carnitine to form aliphatic carnitine, which passes through the mitochondrial membrane and enters the mitochondrial matrix under the mediation of aliphatic carnitine transferase, and then oxidizes and decomposes to release energy. L-carnitine can accelerate fat metabolism, improve heart function, reduce TC, TG, increase HDL-C and reduce body weight. If the fatty acid is not effectively oxidized by beta, it will cause accumulation of fatty acid, easily produce lipid toxicity and promote the development of inflammation. The increase of free fatty acids in rats with dyslipidemia and obesity requires more L-carnitine for effective beta oxidation. The decrease of L-carnitine may not be enough to compensate for the increase of free fatty acids by beta oxidation, leading to lipid metabolism disorder. Incomplete oxidation-derived acyl carnitine is significantly associated with metabolic diseases, such as long-chain acyl carnitine C18 in diet-induced obese rats. In this study, the plasma levels of L-octyl carnitine and long-chain acyl carnitine stearyl carnitine (C18:0) in the model group were significantly higher than those in the normal group, which was consistent with the changes of acyl carnitine metabolic profiles in patients with pre-T2DM[43]. Studies have shown that the increase of long-chain acyl carnitine in circulation can interfere with insulin signal transduction in cell membranes, which is related to the occurrence and development of insulin resistance, but the exact mechanism of action has not yet been elucidated[44]. Gegen Qinlian Decoction can regulate the level of acyl carnitine to a certain extent, which may promote the beta oxidation of fatty acids through many ways. Meanwhile, the specific contribution of acyl carnitine of various chain lengths to its biological activity deserves further study.
Sphingolipid and bile acid metabolism
In this experiment, the contents of sphingolipids and bile acids in plasma of rats in each group were significantly changed. Sphingolipids are an important component of cell membranes and participate in important physiological processes such as cell growth, differentiation and apoptosis[45]. In obese individuals, sphingolipids accumulate abnormally in tissues and cells, and circulatory levels increase abnormally. Increased sphingolipids can induce insulin resistance by interfering with signal transduction of insulin signaling pathway and promoting cell apoptosis. Bile acids are steroid acids mainly found in mammalian bile. Bile acid can regulate the digestion and absorption of intestinal and liver lipids, and its transformation is closely related to intestinal flora[46]. Bile acid involves key enzymes that balance cholesterol in vivo. Bile acid participates in glycolipid and energy metabolism[47] through different pathways. In this experiment, compared with the normal group, the increase of sphingolipid content in plasma and the decrease of bile acid content in the model group suggest that the obstacle of energy consumption, the accumulation of energy accelerate fat synthesis, and promote abnormal blood lipid and weight gain. Studying the changes of sphingolipid metabolism and bile acid levels in organisms has been widely applied to the study of metabolic diseases such as dyslipidemia, obesity and T2DM, and provides new ideas for preventing obesity and metabolic diseases related to obesity.