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Emodin is a principle ingredient isolated from rhubarb rhizome, which is commonly used for constipation or pain-related diseases in traditional Chinese medicine (TCM) practice. The transient receptor potential vanilloid 1 ion channel proteins (TRPV1) are abundantly expressed in the peripheral sensory neurons and are assumed to act as a kind of nociceptor involved in the perception of pain and development of hyperalgesia. The aim of this study was to further unravel the analgesic mechanisms of rhubarb through investigating the effects of its main constitutive ingredient emodin on the expression of TRPV1 mRNA as well as on its calcium- mediating functions in vitro. The primary DRG neurons with a high purity and viability were obtained, and the TRPV1 mRNA expression levels were examined by using real-time RT-PCR and the elevated amplitudes of intracellular [Ca²⁺]i in the DRG neurons evoked by TRPV1 agonist capsaicin were examined by confocal microscopy. The results showed that emodin could significantly down-regulate both the mRNA expression of TRPV1 and the capsaicin-evoked intracellular fluorescent intensity in the DRG neurons under both 37°C and 39°C in vitro. Concomitantly, all of the changes induced by emodin could not be blocked by pretreatment of the primary neurons with capsazepine, an antagonist of TRPV1. In conclusion, we established that the mRNA expression level of TRPV1 and its calcium-mediating function in naive DRG neurons could be down-regulated by emodin through perhaps the non-TRPV1 channel pathways, and this might be the molecular mechanisms for rhubarb to inhibit hyperalgesia induced by inflammatory stimuli.

Dietary capsaicin (CAP), the main irritant component in pepper, can reduce the incidence of diabetes, while metformin (MET) is a first-line oral hypoglycemic drug. The purpose of this study was to investigate whether CAP on the hypoglycemic effect of MET is pertinent to gut microbiota. The glucose and insulin tolerance of diabetic rats were monitored. The glycolipid metabolism was analyzed by detecting blood biochemical parameters. Liver pathological changes were observed by Hematoxylin eosin (HE) staining. The inflammatory cytokines and intestinal tight junction proteins were detected by RT-qPCR and Western blot. 16S rRNA sequencing was employed to analyze gut microbiota profiles. The results showed that CAP and MET co-treatment could significantly reduce fasting blood glucose, improve glucose tolerance, lessen liver injury and inflammatory infiltration, down-regulate inflammatory cytokines and up-regulate intestinal tight junction proteins in diabetic rats by comparing it with MET monotherapy. Moreover, CAP and MET co-treatment altered gut microbiota profiles by regulating microbials’ abundances such as Akkermansia. In conclusion, CAP showed the significant hypoglycemic effect of MET and remodulated gut microbiota profiles in diabetic rats.

Metastasis of osteosarcoma is an important adverse factor affecting patients’ survival, and cancer stemness is the crucial cause of distant metastasis. Capsaicin, the main component of pepper, has been proven in our previous work to inhibit osteosarcoma proliferation and enhance its drug sensitivity to cisplatin at low concentrations. This study aims to further explore the anti-osteosarcoma effect of capsaicin at low concentrations (100$\mu$M, 24h) on stemness and metastasis. The stemness of human osteosarcoma (HOS) cells was decreased significantly by capsaicin treatment. Additionally, the capsaicin treatment’s inhibition of cancer stem cells (CSCs) was dose-dependent on both sphere formation and sphere size. Meanwhile, capsaicin inhibited invasion and migration, which might be associated with 25 metastasis-related genes. SOX2 and EZH2 were the most two relevant stemness factors for capsaicin’s dose-dependent inhibition of osteosarcoma. The mRNAsi score of HOS stemness inhibited by capsaicin was strongly correlated with most metastasis-related genes of osteosarcoma. Capsaicin downregulated six metastasis-promoting genes and up-regulated three metastasis-inhibiting genes, which significantly affected the overall survival and/or disease-free survival of patients. In addition, the CSC re-adhesion scratch assay demonstrated that capsaicin inhibited the migration ability of osteosarcoma by inhibiting its stemness. Overall, capsaicin exerts a significant inhibitory effect on the stemness expression and metastatic ability of osteosarcoma. Moreover, it can inhibit the migratory ability of osteosarcoma by suppressing its stemness via downregulating SOX2 and EZH2. Therefore, capsaicin is expected to be a potential drug against osteosarcoma metastasis due to its ability to inhibit cancer stemness.

Structure and energies of capsaicin and its probable transients formed in oxidation processes (single electron transfer and hydrogen atom transfer) have been investigated using theoretical calculations. Molecular geometries and energies of truncated and complete capsaicin structures have been optimized using density functional theory (DFT) with Becke three-parameter Lee-Yang-Parr (B3LYP) functional and 6–31$++$G(d) basis set. The stable geometries have been confirmed by vibrational analysis. The calculations suggest that single-electron transfer takes place at phenolic O-atom in the first step followed by delocalization of positive charge over the whole molecule. Further, the first step of hydrogen atom abstraction should take place at phenolic group due to lowest dissociation energy but post-optimization bond dissociation energy is least for benzylic group in the side chain as compared to other transients. Effect of water as a solvent on the energies has also been studied using self-consistent reaction field calculation. Similar results are obtained for truncated and complete capsaicin structures. The present study also includes Mulliken spin, charge, vibrational frequencies and assignments of frequencies of the transients. The present study provides explanation for the observation of phenoxyl radical in fast kinetic studies using pulse radiolysis study and, formation of breakdown and dimeric products in other studies.

The new coronavirus (COVID-19) is a viral disease that was classified as a pandemic situation on a global scale in early 2020. Severe Acute Respiratory Syndrome (SARS-CoV)-2 has the enzyme Mpro, until then, best characterized as an important biological target for intracellular viral replication. To investigate the interactions between curcumins and other compounds derived from cinnamic acid with the SARS-CoV-2 Mpro protease as well as to infer their physicochemical and drug-like properties, four natural curcumins and eight related compounds were selected for in silico screening, of molecular docking with the biological target Mpro, to suggest a therapeutic method associated with antiSARS-CoV-2 drugs, such as anakinra, azithromycin, baricitinib, hydroxychloroquine and remdesivir. All curcumins and related compounds can act as synergistic inhibitors of the main viral protein in SARS-CoV-2. The curcumins and other compound ligands showed similar interactions with the enzyme comparable to the control ligands. The ligands capsaicin, dehydrozingerol, dibenzoylmethane and isoeugenol stand out, due to their strong interactions. Among the compounds tested in this study, capsaicin, an alkaloid that is obtained from the fruits of plants of the genus Capsicum, showed significant activity in terms of its potential to inhibit SARS-CoV-2 viral replication.