Endothelial dysfunction is a hallmark of several pathological conditions, including cancer, cardiovascular disease and inflammatory disorders. In these conditions, perturbed TCA cycle and subsequent succinate accumulation have been reported. The role of succinate as a regulator of immunological responses and inflammation is increasingly being recognized. Nevertheless, how endothelial cell function and phenotype are altered by elevated intracellular succinate has not been addressed yet. Thus, we employed numerous in vitro functional assays using primary HUVECs and diethyl succinate (DES), a cell membrane-permeable succinate analogue. An MTS assay 1 h post stimulation with DES suggested reduced metabolic activity in HUVECs. Concurrently, elevated production of ROS, including mitochondrial superoxide, and a reduction in mitochondrial membrane potential were observed. These findings were corroborated by Seahorse mito-stress testing, which revealed that DES acutely lowered the OCR, maximal respiration and ATP production. Given the link between mitochondrial stress and apoptosis, we examined important survival signalling pathways. DES transiently reduced ERK1/2 phosphorylation, a response that was followed by a skewed pro-apoptotic shift in the BAX to BCL2L1 gene expression ratio, which coincided with upregulating VEGF gene expression. This indicated an induction of mixed pro-apoptotic and pro-survival signals in the cell. However, the BAX/BCL-XL protein ratio was unchanged, suggesting that the cells did not commit themselves to apoptosis. An MTS assay, caspase 3/7 activity assay and annexin V/propidium iodide staining confirmed this finding. By contrast, stimulation with DES induced acute endothelial barrier permeability, forming intercellular gaps, altering cell size and associated actin filaments without affecting cell count. Notably, during overnight DES exposure gradual recovery of the endothelial barrier and cell sprouting was observed, alongside mitochondrial membrane potential restoration, albeit with sustained ROS production. COX-2 inhibition and EP4 receptor blockade hindered barrier restoration, implicating a role of COX-2/PGE₂/EP4 signalling in this process. Interestingly, ascorbic acid pre-treatment prevented DES-induced acute barrier disruption independently from ROS modulation. In conclusion, succinate acts as a significant regulator of endothelial mitochondrial function and barrier integrity, a response that is counterbalanced by upregulated VEGF and prostaglandin production by the endothelial cells.

This paper highlights the role of the succinate/SUCNR1 axis in pulmonary fibrosis. Receptor activation leads to profibrotic changes in IPF patient-derived fibroblasts. This finding could also be replicated in a mouse model of pulmonary fibrosis.

This study reveals that the IL-23/IL-17A axis drives neutrophil activation and GRβ upregulation in COPD through TYK2/JAK1-mediated signaling. Inhibition of TYK2/JAK1 with brepocitinib reduced neutrophilic inflammation and restored the GRα/GRβ balance in neutrophils, identifying TYK2/JAK1 as promising therapeutic targets for severe COPD.

Idiopathic pulmonary fibrosis (IPF) is a progressive disease, characterized by extracellular matrix (ECM) deposition leading to loss of compliance, compromising alveolar integrity and gas exchange. Transcriptomic and metabolic profiling of fibrotic lung tissue point towards dysregulation of metabolic pathways. Levels of tricarboxylic acid cycle intermediates including succinate are altered in IPF. In this study, we aim to understand the role of succinate, and its receptor SUCNR1 in IPF. Methods: SUCNR1 expression in human and mouse lung and in fibroblasts was investigated using western blots, qPCR, and FISH. In vitro assays with IPF patient derived fibroblasts were used to evaluate the effect of succinate treatment on the expression of fibrotic markers. In vivo studies with the bleomycin mouse model of PF were used to evaluate the effect of succinate on collagen accumulation and weight. Results: Several cell types in the lung express SUCNR1-mRNA including ATII cells, smooth muscle cells, fibroblasts, and macrophages. In IPF patient derived fibroblasts, succinate treatment increased expression of markers associated with fibrosis such as alpha smooth muscle actin (1.4-fold change; p=0.004) and collagen (2.6-fold change; p=0.017). In vivo, succinate treatment significantly increased collagen accumulation (1.5-fold change; p=0.02) in bleomycin treated mice compared to mice treated with saline. Moreover, succinate treatment exaggerated weight loss in the bleomycin treated mice. Conclusion: Succinate exerted pro-fibrotic effects in vitro and in vivo. Deciphering the mechanisms involved will be key to investigating SUCNR1 as a potential therapeutic target.