The pivotal role of high glucose-induced overexpression of PKCβ in the appearance of glucagon-like peptide-1 resistance in endothelial cells

Recently, it has been demonstrated that Glucagon-like peptide-1 (GLP-1) has a protective effect on endothelial cells. Our hypothesis is that this GLP-1 protective effect is partly lost when the cells are exposed to sustained high glucose concentrations. Human umbilical vein endothelial cells (HUVECs) were cultured for 21 days in normal glucose (5 mmol/L, NG) or high glucose (25 mmol/L glucose, HG). GLP-1 (7-37) and Ruboxistaurin were added at 50 and 500 nM, respectively, alone or in combination, 1 h before cell harvesting. Analysis of GLP-1 receptor protein levels, as well as of the gene expression of different ER stress-related genes, proliferation markers, antioxidant cell response-related genes, and PKA subunits, was performed. ROS production was also measured in HUVECs exposed to mentioned treatments. GLP-1 receptor expression was reduced in HUVECs exposed to chronic high glucose concentrations but was partially restored by a chemical PKCβ-specific inhibitor. GLP-1, added as an acute treatment in endothelial cells, had the capacity to induce the expression of Nrf2-detoxifying enzyme targets, to increase transcription levels of scavenger genes, to attenuate the expression of high glucose-induced PKA subunits, ER stress and also the apoptotic phenotype of HUVECs; these effects occured only when high glucose-induced PKCβ overexpression was reduced by Ruboxistaurin. In a similar manner, ROS production induced by high glucose was reduced by GLP-1 in the presence of PKCβ inhibitor. This study suggests that an increase in PKCβ, induced by high glucose, could have a role in endothelial GLP-1 resistance, reducing GLP-1 receptor levels and disrupting the GLP-1 canonical pathway.