Insulin can block apoptosis by decreasing oxidative stress via phosphatidylinositol 3-kinase- and extracellular signal-regulated protein kinase-dependent signaling pathways in HepG2 cells

Objective: Insulin has well-known activities in controlling energy metabolism, cellular proliferation and biosynthesis of functional molecules to maintain a biological homeostasis. Recently, several studies have suggested that insulin may protect cells from apoptosis in different cell lines; however, little is known about the nature of its anti-apoptotic activity. In many clinical disorders, including type 2 diabetes mellitus, oxidative stress and the production of reactive oxygen species (ROS) is increased. With these facts as a background, we examined here whether insulin protects HepG2 cells from apoptosis by decreasing oxidative stress and, if so, which signaling steps are involved in this process. Methods: Intracellular DNA content, the degree of nuclear condensation or poly(ADP-ribose) polymerase hydrolysis was measured to verify the occurrence of apoptotic events. Caspase-3 activity and ROS accumulation within cells were also measured. Western blot analysis was performed to identify signaling molecules activated in response to insulin. Results: Serum starvation resulted in a marked accumulation of ROS, activation of caspase-3, and subsequent apoptotic cell death which were, in turn, markedly blocked by the addition of insulin. The anti-apoptotic activity of insulin was sensitive to blockade of two different signaling steps, activations of phosphatidylinositol 3-kinase (PI3 kinase) and extracellular signal-regulated protein kinase (ERK). Conclusion: Insulin exerts an anti-apoptotic activity by suppressing the excessive accumulation of ROS within cells through signaling pathways including stimulation of PI3 kinase and ERK in HepG2 cells.