Insulin Has Multiple Antiamyloidogenic Effects on Human Neuronal Cells

Abstract

Alzheimer's disease is increased in diabetic patients. A defective insulin activity on the brain has been hypothesized to contribute to the neuronal cell dysregulation leading to AD, but the mechanism is not clear. We analyzed the effect of insulin on several molecular steps of amyloid precursor protein (APP) processing and β-amyloid (Aβ) intracellular accumulation in a panel of human neuronal cells and in human embryonic kidney 293 cells overexpressing APP-695. The data indicate that insulin, via its own receptor and the phosphatidylinositol-3-kinase/AKT pathway, influences APP phosphorylation at different sites. This rapid-onset, dose-dependent effect lasts many hours and mainly concerns dephosphorylation at the APP-T668 site. This effect of insulin was confirmed also in a human cortical neuronal cell line and in rat primary neurons. Cell fractionation and immunofluorescence studies indicated that insulin-induced APP-T668 dephosphorylation prevents the translocation of the APP intracellular domain fragment into the nucleus. As a consequence, insulin increases the transcription of antiamyloidogenic proteins such as the insulin-degrading enzyme, involved in Aβ degradation, and α-secretase. In contrast, the transcripts of pro-amyloidogenic proteins such as APP, β-secretase, and glycogen synthase kinase (Gsk)-3β are decreased. Moreover, cell exposure to insulin favors the nonamyloidogenic, α-secretase-dependent APP-processing pathway and reduces Aβ40 and Aβ42 intracellular accumulation, promoting their release in the extracellular compartment. The latter effects of insulin are independent of both Gsk-3β phosphorylation and APP-T668 dephosphorylation, as indicated by experiments with Gsk-3β inhibitors and with cells transfected with the nonphosphorylatable mutated APP-T668A analog. In human neuronal cells, therefore, insulin may prevent Aβ formation and accumulation by multiple mechanisms, both Gsk-3β dependent and independent.

Response to Comment on Vandal et al. Insulin Reverses the High-Fat Diet–Induced Increase in Brain Aβ and Improves Memory in an Animal Model of Alzheimer Disease. Diabetes 2014;63:4291–4301

“…Ghanim and Dandona for their comments (1) on our article (2). Their results showing that insulin infusion leads to decreased expression of amyloid precursor protein (APP) and presenilins in human monocytes (3) are in line with reports in cell culture (4) and support the use of insulin in Alzheimer disease (AD) (5). In triple transgenic (3xTg-AD) mice fed with a high-fat diet for 9 months, we have recently shown that an acute insulin injection decreases cortical soluble Ab40 and Ab42 back to the level of animals fed with a control diet only 5 min after the injection.…”

Aβ-Induced Insulin Resistance and the Effects of Insulin on the Cholesterol Synthesis Pathway and Aβ Secretion in Neural Cells

“…The release of Ab into the extracellular compartment can lead to its degradation by membraneassociated and secreted insulin-degrading enzymes [59] or Ab clearance. Along with increasing Ab secretion into the medium, insulin favors a-secretase activity with increased neurotrophic action, inhibits the translocation of the AbPP intracellular domain into the nucleus, and phosphorylates GSK3b, inhibiting its enzymatic activity for reduced tau pathology [60]. Studies have also shown that insulin inhibits the nuclear activity of the AbPP intracellular domain, reduces the transcription of genes that encode pro-amyloidogenic enzymes such as b-secretase and GSK3b, and increases the transcription of anti-amyloidogenic enzymes such as a-secretase and insulin-degrading enzyme [60,61].…”

“…Along with increasing Ab secretion into the medium, insulin favors a-secretase activity with increased neurotrophic action, inhibits the translocation of the AbPP intracellular domain into the nucleus, and phosphorylates GSK3b, inhibiting its enzymatic activity for reduced tau pathology [60]. Studies have also shown that insulin inhibits the nuclear activity of the AbPP intracellular domain, reduces the transcription of genes that encode pro-amyloidogenic enzymes such as b-secretase and GSK3b, and increases the transcription of anti-amyloidogenic enzymes such as a-secretase and insulin-degrading enzyme [60,61]. Our results together with those of others suggest that enhancing insulin signaling can relieve insulin resistance, have positive effects on the suppressed cholesterol metabolism of neural cells, and promote Ab release from the cells.…”

Inflammation, Defective Insulin Signaling, and Mitochondrial Dysfunction as Common Molecular Denominators Connecting Type 2 Diabetes to Alzheimer Disease

“…Remarkably, insulin protects against synapse loss induced by AbOs (51). Insulin further favors nonamyloidogenic processing of the amyloid precursor protein in vitro (97) and reduces tau hyperphosphorylation in a rat model of T2D (98). Exendin-4 was recently found to block AbO-induced impairment in insulin signaling in hippocampal cultured neurons (37).…”