Substantial evidence from previous studies, either in-vitro or clinical trials, have revealed TCM’s curative effects on hypertension and potential for drug discovery [12,14]. In this study, we performed LC-MS/MS for identification of the chemical constituents existing in the TCM formulation consisting of Polygonum multiflorum, Rehmannia glutinosa, Senna obtusifolia and Crataegus, to determine the bioactive compounds responsible for mitigating hypertension. From the numerous compounds detected (Supplementary Table 1), only the bioactive compounds which can contribute therapeutic effects [30] were selected, based on their OB and DL values. The OB and DL values of constituents are pivotal keys for successful development of therapeutic drugs with good oral absorption and permeability across the blood-brain barrier [31,32], in line with their molecular physicochemical properties [33]. While most network pharmacological analysis selections are based on OB≥30%, in actual fact there are no universal criteria to define high and low bioavailable compounds [34]. Thus, in this study, instead of selecting bioactive compounds with OB≥30%, we chose OB≥50% (as shown in Table 1) as the arbitrary classification threshold, with the hope that the selected bioactive compounds with higher OB would result in higher absorption in the human system [31]. Our selection was based on the observation that a lot of drugs failed in clinical trials due to poor absorption in the human system.
The filtering process revealed that 11 compounds fulfilled the selection criteria (Table 1), with some found in duplicate within the four medicinal plants. A total of 124 genes were predicted as the target genes for these 11 compounds, as illustrated in Fig. 1. While the SwissTargetPrediction Database estimates predictions of protein targets for druglike molecules using similarity principles efficiently, as has been experimentally observed previously, it does not however provide calculations for large peptides [35]. From our results (shown in Fig. 2), one prime example is 2-methoxy-N-[2-(methylthio)phenyl] benzamide, which was shown to possess most of the predicted targets, which might indicate that it has been experimentally observed to bind to or homologs thereof. It is an established fact that the targets for this bioactive compound, involving hypertension, is through regulation of blood circulation, such as ADORA1, ADORA2B, HMOX1, KCNJ5, NOS2 (illustrated in Fig. 1 and Fig. 4(a)) [36–39].
Apart from the target compounds, hypertension disease targets were also determined through the OMIM and GeneCards databases. In addition to the hypertension disease gene targets, we also searched the databases for hypertension-related diseases such as cardiovascular disease or categories of diseases possessing both keywords, which possess strong inter correlations, as shown in previous reports [3,4]. Subsequently, the targets for the bioactive compounds were overlapped with the targets from diseases to construct the PPI network and visualized using Cytoscape Version 3.8 for better illustration, as depicted in Fig. 3, with confidence level of 0.7. Our findings revealed that two main clusters were formed through this network, which consisted only of the target compounds (red node) and target diseases (blue node), in the bottom left and bottom right of the network, respectively. It showed that the target compounds were involved in neuroactive ligand receptor interaction while the target disease clusters were involved in HCM pathways. This indicates that the TCM decoction might trigger the central neural pathways, one of the multiple converging pathways leading to hypertension [40,41], through neuroactive ligand receptor interaction, to ameliorate hypertension.
The enrichment analysis for GO in biological processes and KEGG pathways demonstrated the involvement of this TCM decoction in numerous biological processes and pathways in the human system biology, as summarized in Fig. 4. As shown in Fig. 4 (a), the biological processes of high significance involve the regulation of biological quality, glutamate receptor signaling pathway, response to stress, regulation of localization and trans-synaptic signaling, among others. Among these biological processes, we found that regulation of heart contraction and regulation of blood circulation, which are strongly associated with hypertension [42,43], are also biological processes with p-value of 6.58x10-19 and 2.84x10-21, respectively. This is concomitant with our KEGG pathway enrichment analysis (Figure 3 (b)) which revealed the high significance of HCM, DCM and cardiac muscle contraction, with p-values of 2.12x10-17, 1.89x10-14 and 5.93 x10-08, respectively. All of these pathways are inter-linked, either directly or indirectly [42]. Additionally, other top pathways found are neuroactive ligand-receptor interaction, glutamatergic synapse, pathways in cancer and adrenergic signaling in cardiomyocytes, among others.
Our PPI network associated with enrichment analysis also suggests that this TCM decoction resolves hypertension conditions through neuroactive ligand receptor interaction [40]. We thus constructed a network to visualize all the hypertension-related pathways by overlapping the compound targets and disease targets, as shown in Fig. 5. No compound targets related to hypertension were however detected which indicates that the synergies of the decoction might instead be involved in pathways indirectly linked to hypertension, to improve hypertension conditions in the human system biology. For example, one of the compound targets, angiotensin converting enzyme (ACE), plays a role in elevating hypertension via constriction of the blood vessels in the pathway of the renin angiotensin system [44]. This indicates that this TCM decoction might be contributing to hypertension amelioration through alternative pathways instead of affecting hypertension itself directly, as most of the targets involved neuroactive ligand receptor interaction, one of the pathways leading to hypertension [40] and HCM, a disease caused by hypertension[45,46].