INS and Endoplasmic Reticulum Stress: To examine whether ER stress impairs insulin signaling in endothelium, HUVECs were incubated with vehicle (0.02% DMSO) and tunicamycin at the dose of 1, 5, 10, and 20 μg/mL dissolved in 0.02% DMSO solution for 1 h, tunicamycin dose-dependently increased the protein expression of GRP78, an ER stress marker (Figure 1A).