Hypertension is a multifactorial and complex disease that increases the risk of cardiovascular disease, and its causes and molecular mechanisms are not fully understood. In general, biomarkers can be widely used for risk stratification and treatment response assessment of cardiovascular disease. The pathophysiological mechanisms of hypertension require precise and accurate biomarkers, which are essential for effective management and monitoring of the disease course of hypertension. In this study, we designed cardiopulmonary exercise test, integrated the blood pressure and ultrasound-related indexes, blood biochemical indexes and metabolic compounds of the sample population at resting and different exercise states, and used logistic regression analysis and principal component analysis to construct a cluster model to screen biomarkers that judge hypertension, so as to reveal the real changes in patients' bodies. It was found that hypertension was accompanied by oxidative stress, lipid metabolism, glycolysis, aerobic oxidation, pentose phosphate pathway, inflammatory reaction and amino acid metabolism, and the related indicators were summarized in Fig. 6.
The preliminary statistics of the data by T-test showed that 37 indicators (P < 0.05) had statistically significant differences, among which 6 indicators (international normalized ratio, NADPH/NADP+(plasma), Palmitelaidic acid, NADPH/NADP+(erythrocyte), L-histidine and Interleukin-8) in the hypertension group were lower than those in the normal group. International normalized ratio (INR) for both groups was within the normal range (0.8 to 1.2). The low NADPH/NADP + in the hypertensive group indicates that oxidative stress was related to the occurrence of hypertension[23]. The low levels of histidine, palmitic acid and interleukin-8 were all attributed to hypertensive patients taking medication. Histidine forms natural aldehydes in liver tissues under the action of ethylamine oxidase, which leads to the failure of tissues and organs to absorb the drug effect. Patients with hypertension would try to eat less food rich in histidine during medication. The accumulation of palmitelaidic acid, the main saturated fatty acid in the blood, causes cardiomyocyte lipid toxicity by inducing oxidative stress and persistent endoplasmic reticulum stress, eventually inducing inflammatory response, cell hypertrophy, cell dysfunction and even cell death[24]. Theoretically, palmitic acid and interleukin-8 in hypertension group were both higher than those in normal group, but the results were opposite. It is speculated that hypertension drugs taken by patients for a long time have the effects of anti-inflammatory and reducing free fatty acids in the body (Felodipine/Valsartan and Amlodipine) [25] [26].
In the resting state, the binary logistic regression between groups showed that the effects of 11 indexes were large. interventricular septum and posterior wall thickness thickening are common symptoms in patients with hypertension[27] [28]. In hypertrophic myocardium, glucose utilization and glycolysis process are enhanced, while aerobic oxidation process was relatively weakened, namely "glycolysis-uncoupling of glucose oxidation"[29] [30], which led to the upregulation of D-xylulose, D-fructose and oxyadipic acid. At the same time, the hypertrophic ventricle increases the load of the heart, leading to heart failure, thus causing oxidative stress and inflammation. The decrease of oxygen utilization rate directly reduces the synthesis of ATP, and ribose, as the raw material of nucleotides, is accumulated[31]. In the long-term environment of oxygen deficiency, the erythrocyte would increase and aggregate compensatively[32, 33]. Leukocytes are the most important inflammatory cells mediating inflammatory response, and the leukocytes involved in inflammatory response are mainly neutrophils and lymphocytes[34]. The inflammatory response causes by hypertension leads to the impact of blood pressure on the vascular wall, which could damage the vascular endothelium, lead to coagulation process, smooth muscle cell proliferation, tube wall thickening, stiffness and lumen narrowing, and increase of apolipoprotein E, thus promoting the formation of vascular atherosclerosis[34, 35]. Glycosylated hemoglobin (HbA1c) is the most important regulatory marker in diabetes care, but there is still evidence showing a significant positive correlation between HbA1c and hypertension[36].
The risk factors of hypertension are positively correlated with hypertension. Increased left atrial diameter and high portal vein velocity are common symptoms associated with hypertension[37, 38]. L-Alpha-aminobutyric acid and Isocitric acid have not been considered risk markers for hypertension in the past. But Isocitric acid is involved in the tricarboxylic acid cycle. Aminobutyric acid has anti-diabetes, anti-hypertension, liver and kidney protection, sleep promotion and other activities[39, 40]. NADPH/NADP+(plasma) was a protective factor of hypertension, that is, oxidative stress was associated with the occurrence of hypertension. The 29 indicators screened by logistic regression analysis were further constructed by principal component analysis, and the study showed that the first 8 principal components could be used as the criteria for determining hypertension. The scatter plot of the first two principal component scores showed a good classification result of the model, which could clearly distinguish the hypertensive group from the normal group on the horizontal axis. Principal component analysis showed that NADPH/NADP+(plasma), international normalized ratio, NADPH/NADP+(erythrocyte), and palmitelaidic acid were key factors in determining hypertensive disease. The accumulation of major saturated fatty acids(palmitelaidic acid) in the blood may cause cardiomyocyte lipid toxicity by inducing oxidative stress and persistent endoplasmic reticulum stress. At the same time, NADPH/NADP + in plasma and erythrocyte indicate that indicators related to oxidative stress are the key to determine patients with hypertension.
With exercise status as the dependent variable, multiple univariate logistic regression analysis was carried out on the indicators of the hypertension group and the normal group respectively under resting and different exercise status. The study mainly focused on the intersection of the three groups (Group1, Group2 and Group3) of indicators and their unique indicators. Group2 and Group3 had 17 indicators in common, that is, these indicators were regulated in the human body in different exercise states. Pyruvic acid, lactic acid and heart rate are common research indicators during exercise[41, 42]. The metabolic characteristics of athletes in the high-load training stage are as follows: aerobic oxidative metabolism plays the biggest role, glycolysis occupies a large proportion, lactic acid accumulation, active amino acid metabolism, enhanced catabolism of most amino acids, and high oxidative stress level[43]. After exercise, the metabolism of organic acids in the body is obviously enhanced, and lactic acid, fumaric acid, succinic acid, malic acid and aconitic acid all change significantly. The metabolic processes involved include glycolysis and tricarboxylic acid cycle[43, 44]. Maleic acid has not been reported, but both maleic acid and fumaric acid are isomers of butene dioic acid. The changes in lipids are particularly significant during exercise[10, 45]. During exercise, fatty acid oxidation and degradation of triacylglycerol occurred, and oleic acid, palmitoleic acid, methylmalonic acid were involved in the process(https://hmdb.ca/metabolites). N-acetylneuraminic acid is found in high levels in the brain, adrenal glands and heart. It has not been reported in exercise studies, but it can regulate innate immunity, antiviral, anti-tumor, inhibit leukocyte adhesion and anti-inflammatory. Various derivatives of 3,4-dihydroxyhydrocinnamic acid (caffeic acid) can act as antioxidants in living organisms, helping to reduce the pathogenic effects of free radicals and oxidative compounds and prevent the oxidative stress caused by diseases[46].
The study suggested that 19 indicators unique to Group2 could not be regulated properly during exercise in the hypertensive group. The Human Metabolome Database(https://hmdb.ca/) was used to investigate pathways involved in these compounds. Fatty acids (myristelaidic acid, octanoic acid, 5-dodecenoic acid, 10(Z)-heptadecenoic acid, formic acid and petroselinic acid) oxidize during exercise. Octanoylcarnitine is involved in mitochondrial beta-oxidation of short chain Saturated fatty acids. Carbohydrates (glucose-6-phosphate, D-gluconolactone) participate in the pentose phosphate pathway. Citric acid participates in the tricarboxylic acid cycle. Pyroglutamic acid and sarcosine are involved in glutathione metabolism. 5-aminolevulinic acid is involved in glycine and serine metabolism and porphyrin metabolism. 2-hydroxybutyric acid is involved in glutamic acid metabolism and butanoate metabolism. Ketoleucine and 3-methyl-2-oxovaleric acid participate in the biosynthesis and degradation of valine, leucine and isoleucine. Adenosine monophosphate is involved in metabolism of various fatty acids and amino acids. Sarcosine is involved in glycine and serine metabolism and methionine metabolism.
The four indicators unique to Group3 were renin activity, CD3+, nicotinic acid and N-acetylserine. The renin-angiotensin-aldosterone system (RAAS) plays an important role in regulating blood pressure and maintaining electrolyte balance and internal environment stability. In pathological conditions, RAAS can increase blood pressure by constricting blood vessels, improving sympathetic nerve activity, and increasing water and sodium retention[5]. Nicotinic acid is an effective high-density lipoprotein raising drug involved in nicotinate and nicotinamide metabolism. Studies have shown that nicotinic acid is a new strategy to prevent atherosclerosis. In addition to its beneficial effect on high-density lipoprotein, niacin can also dilate peripheral blood vessels[47] [48]. In the pathogenesis of hypertension, endothelial dysfunction and immune system activation accompany the development of systemic inflammation and the production of inflammatory cytokines, and CD3 + are upregulated[49]. N-acetylserine has been a potential biomarker for severe sarcopenia and has not been reported in biomarker studies for hypertension[50].
In this study, the indexes screened by univariate logistic regression analysis with significant differences in different exercise states between the hypertension group and the normal group were further analyzed by principal component analysis. The variance contribution rate of PC1 was 21.91% (normal group) and 34.90% (hypertension group), respectively. The key indicators in the normal group were blood pressure and ultrasound related indicators, while the key indicators in the hypertension group were lactic acid, glycolysis, aerobic oxidation and lipid-related metabolites. Lactic acid becomes a metabolite with obvious changes during exercise in hypertensive patients. It has also been reported in other reference[51] [52]. Lactic acid is involved in pyruvate metabolism, gluconeogenesis and Warburg effect. Lactic acid accumulation can cause liver damage and nerve damage. Various derivatives of caffeic acid can protect against disease-induced oxidative stress[46]. Succinic acid and pyruvic acid participate in glycolysis-uncoupling of glucose oxidation. Methylmalonic acid, a malonic acid derivative, is an important intermediate in lipid metabolism, suggesting that the disease group used more fat for energy supply during exercise.
In this study, the cardiopulmonary exercise experiment was designed, and the indicators related to the risk of hypertension were screened by logistic regression analysis, and the correlation between this factor and hypertension was examined. The indicators were eventually retained in the cluster model constructed by principal component analysis to reduce the risk of misjudgment of hypertension. However, there are some limitations in this study. Due to the insufficient sample size of the included studies, large-scale randomized trials cannot be conducted to verify the classification effect of this biomarker. In the future, in order to further confirm the predictive value of this biomarker for the occurrence of hypertension, it is necessary to further expand the study sample. Moreover, various mathematical models such as K-Nearest Neighbor classification algorithm[53], Support Vector Machine[54] and Radial Basis Function[55] are used to confirm this.