Insulin Regulates AKT/GSK-3β Signalling, Tau Phosphorylation, and Redox Homeostasis in SH-SY5Y Neuroblastoma Cells

Insulin (Ins) regulates multiple intracellular signalling pathways involved in cell survival, oxidative stress responses, and tau phosphorylation. Dysregulation of these pathways has been implicated in neurodegenerative disorders, including Alzheimer's disease (AD). The present study evaluated the effects of insulin on protein kinase B/glycogen synthase kinase-3 beta (AKT/GSK-3β) signalling, tau phosphorylation, and oxidative stress-related markers in SH-SY5Y neuroblastoma cells. Cell metabolic activity was assessed using the (diphenyltetrazolium bromide) MTT assay, while cell number and viability were evaluated by Trypan Blue exclusion, necrosis by lactate dehydrogenase (LDH) release, and apoptosis by Caspase-3 activity. Western blot analysis was performed to evaluate the expression of phosphorylated AKT (p-AKT), phosphorylated GSK-3β (p-GSK-3β Ser9), phosphorylated TAU (pTAU), nuclear factor erythroid 2-related factor 2 (NRF2), manganese superoxide dismutase (Mn-SOD), and copper/zinc superoxide dismutase (Cu/Zn-SOD). Lipid peroxidation was determined by measuring malondialdehyde (MDA) levels using a colorimetric/fluorometric assay. Insulin treatment increased MTT reduction (31.25%) and cell metabolic activity (119.15%) while reducing LDH release (19.2%) and Caspase-3 activity (31.26%). In addition, insulin significantly increased p-AKT (34.2%) and p-GSK-3β (Ser9) (19.9%) levels. A reduction in pTAU levels (53.39%) was also observed following insulin treatment. Furthermore, insulin increased NRF2 expression (18.77%), Cu/Zn-SOD (37.29%), and Mn-SOD (50.16%) and reduced MDA levels (13.95%). These findings indicate that insulin modulates signalling pathways associated with tau phosphorylation and cellular redox regulation in SH-SY5Y cells. Insulin treatment was associated with increased AKT and GSK-3β phosphorylation, reduced tau phosphorylation, and changes in oxidative stress-related markers in SH-SY5Y neuroblastoma cells. These findings support a role for insulin in the modulation of molecular pathways implicated in cellular stress responses and tau regulation. Further studies using differentiated neuronal models and disease-relevant conditions are required to determine the relevance of these observations to neurodegenerative disorders.