Inhibitory effect of caffeic acid on advanced glycation end product-induced renal fibrosis in vitro: A potential therapeutic target

Potential inhibitory effect of Auricularia auricula polysaccharide on advanced glycation end-products (AGEs)

In this study, a novel polysaccharide, AAP-2S, was extracted from Auricularia auricula, and the anti-glycosylation effect of AAP-2S and its underlying mechanisms were investigated using an in vitro BSA-fructose model and a cellular model. The results demonstrated the inhibiting formation of advanced glycation end products (AGEs) in vitro by AAP-2S. Concurrently, it attenuated oxidative damage to proteins in the model, preserved protein sulfhydryl groups from oxidation, reduced protein carbonylation, prevented structural alterations in proteins, and decreased the formation of β-crosslinked structures. Furthermore, AAP-2S demonstrated metal-chelating capabilities. GC-MS/MS-based metabolomics were employed to analyze changes in metabolic profiles induced by AAP-2S in a CML-induced HK-2 cell model. Mechanistic investigations revealed that AAP-2S could mitigate glycosylation and ameliorate cell fibrosis by modulating the RAGE/TGF-β/NOX4 pathway. This study provides a foundational framework for further exploration of Auricularia auricular polysaccharide as a natural anti-AGEs agent, paving the way for its potential development and application as a food additive.

Epithelial-mesenchymal plasticity in kidney fibrosis

Epithelial-mesenchymal transition (EMT) is an important biological process contributing to kidney fibrosis and chronic kidney disease. This process is characterized by decreased epithelial phenotypes/markers and increased mesenchymal phenotypes/markers. Tubular epithelial cells (TECs) are commonly susceptible to EMT by various stimuli, for example, transforming growth factor-β (TGF-β), cellular communication network factor 2, angiotensin-II, fibroblast growth factor-2, oncostatin M, matrix metalloproteinase-2, tissue plasminogen activator (t-PA), plasmin, interleukin-1β, and reactive oxygen species. Similarly, glomerular podocytes can undergo EMT via these stimuli and by high glucose condition in diabetic kidney disease. EMT of TECs and podocytes leads to tubulointerstitial fibrosis and glomerulosclerosis, respectively. Signaling pathways involved in EMT-mediated kidney fibrosis are diverse and complex. TGF-β1/Smad and Wnt/β-catenin pathways are the major venues triggering EMT in TECs and podocytes. These two pathways thus serve as the major therapeutic targets against EMT-mediated kidney fibrosis. To date, a number of EMT inhibitors have been identified and characterized. As expected, the majority of these EMT inhibitors affect TGF-β1/Smad and Wnt/β-catenin pathways. In addition to kidney fibrosis, these EMT-targeted antifibrotic inhibitors are expected to be effective for treatment against fibrosis in other organs/tissues.

Advanced glycation end products promote the progression of chronic kidney diseases by targeting calpain 6

Advanced glycation end products (AGEs) are produced by glycosylation or oxidation of proteins and lipids and are tightly involved in the chronic kidney disease (CKD) process. Calpain 6 (CAPN6) is a non-classical calpain that has been reported to be overexpressed in CKD. This study aimed to explore the effects of AGEs in CKD progress and their correlation with CAPN6. AGEs production was measured using ELISA. The CCK-8 assay was used to test cell proliferation. mRNA and protein levels were tested using qRT-PCR and western blot. The progress of glycolysis was tested by calculating the ATP and ECAR content in HK-2 cells. The expression of AGEs and CAPN6 was significantly increased in patients with CKD3, CKD4, and CKD5. AGEs treatment inhibited cell proliferation and glycolysis and accelerated apoptosis. Additionally, CAPN6 knockdown effectively reversed the effects of AGEs in HK-2 cells. In addition, overexpressed CAPN6 played similar role to AGEs, which suppressed cell proliferation and glycolysis and facilitated apoptosis. Moreover, the administration of 2-DG, a glycolysis inhibitor, counteracted the effects of CAPN6 silencing in HK-2 cells. Mechanistically, CAPN6 interacts with NF-κB and PDTC reduced CAPN6 expression in HK-2 cells. This investigation revealed that AGEs facilitate CKD development in vitro by modulating the expression of CAPN6.

A comprehensive insight into the effects of caffeic acid (CA) on pepsin: Multi-spectroscopy and MD simulations methods

The interaction between caffeic acid (CA) and pepsin was investigated using multi-spectroscopy approaches and molecular dynamic simulations (MDS). The effects of CA on the structure, stability, and activity of pepsin were studied. Fluorescence emission spectra and UV-vis absorption peaks all represented the static quenching mechanism of pepsin by CA. Moreover, the fluorescence spectra displayed that the interaction of CA exposed the tryptophan chromophores of pepsin to a more hydrophilic micro-environment. Consistent with the simulation results, thermodynamic parameters revealed that CA was bound to pepsin with a high binding affinity. The Van der Waals force and Hydrogen bond interaction were the dominant driving forces during the binding process. The circular dichroism (CD) spectroscopy analysis showed that the CA binding to pepsin decreased the contents of α-Helix and Random Coil but increased the content of β-sheet in the pepsin structure. Accordingly, MD simulations confirmed all the experimental results. As a result, CA is considered an inhibitor with adverse effects on pepsin activity.

Autophagy and its therapeutic potential in diabetic nephropathy

Diabetic nephropathy (DN), the leading cause of end-stage renal disease, is the most significant microvascular complication of diabetes and poses a severe public health concern due to a lack of effective clinical treatments. Autophagy is a lysosomal process that degrades damaged proteins and organelles to preserve cellular homeostasis. Emerging studies have shown that disorder in autophagy results in the accumulation of damaged proteins and organelles in diabetic renal cells and promotes the development of DN. Autophagy is regulated by nutrient-sensing pathways including AMPK, mTOR, and Sirt1, and several intracellular stress signaling pathways such as oxidative stress and endoplasmic reticulum stress. An abnormal nutritional status and excess cellular stresses caused by diabetes-related metabolic disorders disturb the autophagic flux, leading to cellular dysfunction and DN. Here, we summarized the role of autophagy in DN focusing on signaling pathways to modulate autophagy and therapeutic interferences of autophagy in DN.

Effect of transferrin glycation induced by high glucose on HK-2 cells in vitro

BACKGROUND AND OBJECTIVE

Glycation is a common post-transcriptional modification of proteins. Previous studies have shown that advanced glycation end modified transferrin (AGE-Tf) levels in diabetic rat kidney tissues were increased; however, its role in diabetic nephropathy remains unclear. In this study, differences in glycation degree and Tf sites induced by differing high glucose concentrations in vitro and the effect on total iron binding capacity (TIBC) were observed. Moreover, the effect of AGE-Tf on human renal tubular epithelial cells (HK-2) was investigated.

METHODS

In vitro Tf was incubated with increasing glucose concentrations (0 mM, 5.6 mM, 11.1 mM, 33.3 mM, 100 mM, 500 mM, and 1,000 mM) for AGE-Tf. Differences in AGE-Tf glycation degree and TIBC level were analyzed via colorimetric method. The AGE-Tf glycation sites were identified with LC-MS/MS. HK-2 cells were treated with AGE-Tf prepared with different glucose concentrations (33.3 mM and 500 mM) in vitro. The effects of AGE-Tf on HK-2 cell viability, proliferation, oxidative stress index, and Tf receptor expression levels were then observed.

RESULTS

With increasing glucose concentrations (100 mM, 500 mM, and 1,000 mM) in vitro, Tf glycation degree was significantly increased. The TIBC levels of AGE-Tf were decreased significantly with increasing glucose concentrations (33.3 mM, 100 mM, 500 mM, and 1,000 mM). Four glycated modification sites in Tf and 17 glycated modification sites were detected in AGE-Tf (500 mM) by LC-MS/MS. The structural types of AGEs were CML, G-H1, FL-1H2O, FL, and MG-H1. No significant differences were found in the survival rate of HK-2 cells among the AGE-Tf (500 mM), AGE-Tf (33.3 mM), and Tf groups (all p > 0.05). The apoptosis rate of HK-2 cells in the AGE-Tf (500 mM) group was significantly higher than that in the AGE-Tf (33.3 mM) group. Additionally, both of them were significantly higher than that in the Tf group (both p < 0.05). The MDA levels of HK-2 cells in the AGE-Tf (500 mM) and AGE-Tf (33.3 mM) groups were higher than that in the Tf group, but not significantly (both p > 0.05). The T-AOC level of HK-2 in the AGE-Tf (500 mM) group was significantly lower than that in the AGE-Tf (33.3 mM) and Tf groups (both p < 0.001). The GSH level of HK-2 cells in the AGE-Tf (500 mM) group was significantly lower than that in the Tf group (p < 0.05). The expression level of TfR in the AGE-Tf (500 mM) group was also significantly lower than that in the Tf group (p < 0.05).

CONCLUSION

The degree and sites of Tf glycation were increased in vitro secondary to high-glucose exposure; however, the binding ability of Tf to iron decreased gradually. After HK-2 was stimulated by AGE-Tf in vitro, the apoptosis of cells was increased, antioxidant capacity was decreased, and TfR expression levels were downregulated.

Inhibition of Advanced Glycation End Product Formation in Rat Tail Tendons by Polydatin and p-Coumaric acid: an In Vitro Study

Advanced glycation end products (AGEs) formed through non-enzymatic glycosylation between a protein and sugar molecule are highly harmful to the human body. In hyperglycemic patients, AGE formation is more due to high glucose circulating in the blood, causing inter and intra molecular cross-linking of collagen leading to reduction of collagen elasticity. This cross-linked collagen develops resistance to matrix metalloproteinases leading to impaired collagen turnover. The aim of this work is to determine the anti-glycation effects of polydatin and p-coumaric acid in preventing collagen cross-linking by incubating rat tail tendons (RTTs) as collagen source in high glucose concentration (50 mM) for a week. The RTTs were then characterized for tensile strength, cross-linking efficiency, circular dichroism spectrometry, collagen, glucose, and aldehyde contents. Electrophoresis was carried out to evaluate the level of cross-linking in collagen and the results confirmed the ability of the drugs in preventing complex intermolecular cross-link formation induced by non-enzymatic glycosylation. CD data showed alteration in the secondary structure of collagen where AGE formation had occurred. More collagen was extracted by pepsin from RTTs treated with glucose alone (6.88 mg/10 mg tendon) when compared with drug-treated groups (4.25, 2.56 mg/10 mg tendon for polydatin and p-coumaric acid, respectively). Tensile strength (20.66% and 18.95%), cross-linking percentage (32.5% and 29.84%), and glucose content (2.3 and 1.8 mg/100 mg) of drug-treated groups were similar to the positive control (19.07%, 30.13%, and 2.61 mg/100 mg) thus proving the anti-glycation potential of the drugs. Hence, both polydatin and p-coumaric acid could play a pivotal role in preventing AGE formation.