Acute effect of lactic acid on tumor-endothelial cell metabolic coupling in the tumor microenvironment

The present study aimed to systematically analyze alterations in the expression of mitochondrial-associated proteins in human bladder cancer T24 cells co-cultured with tumor-associated human umbilical vein endothelial cells (HUVECs), and to investigate the characteristics of bladder cancer cell energy metabolism. The present study used the following techniques: A co-culture system of T24 cells and HUVECs was constructed using a microfluidic chip as a 3D co-culture system; the concentration of lactic acid in the medium of the cells was determined using an automatic microplate reader; a qualitative analysis of mitochondria-associated protein expression was performed by immunofluorescent staining; and a quantitative analysis of mitochondrial-associated protein expression was conducted using western blotting. The present results revealed that between the control groups (monoculture of T24 cells or HUVECs), the mitochondrial-associated protein fluorescence intensity was increased in the HUVECs compared with the T24 cells. The fluorescence intensity of mitochondrial-associated proteins in the HUVEC control group was increased compared with the HUVECs in the experimental co-culture group. In the T24 cells, the protein fluorescence intensity was increased in the experimental co-culture group compared with the control group. In addition, the expression of mitochondria-associated proteins was increased in HUVECs compared with T24 cells in the control groups, while T24 cells in the experimental co-culture group had an increased expression compared with HUVECs in the experimental group (P<0.05). For T24 cells, the expression of mitochondrial-associated proteins was increased in the experimental group compared with the control group, and contrasting results were observed for the HUVECs (P<0.05). Determination of lactic acid concentration demonstrated that lactic acid concentration was highest in the experimental co-culture group, followed by the T24 control group and the HUVEC control group. In conclusion, the present study demonstrated that energy metabolism of the bladder tumor cells does not parallel the 'Warburg effect', since even under sufficient oxygen conditions the tumor cells still undergo glycolysis. Additionally, bladder tumor cells have an efficient oxidative phosphorylation process, wherein tumor cells promote glycolysis in adjacent interstitial cells, thereby causing increased formation of nutritional precursors. These high-energy metabolites are transferred to adjacent tumor cells in a specified direction and enter the Krebs Cycle. Ultimately, oxidative phosphorylation increases, and sufficient ATP is produced.