N-terminal tripeptide of insulin-like growth factor-I protects against β-amyloid-induced somatostatin depletion by calcium and glycogen synthase kinase 3β modulation

The protective effects of insulin-like growth factor I on the somatostatin (SRIF) system in the temporal cortex after β-amyloid (Aβ) injury may be mediated through its N-terminal tripeptide glycine-proline-glutamate (GPE). GPE is cleaved to cyclo[Pro-Gly] (cPG), a metabolite suggested to mediate in neuroprotective actions. We evaluated the effects of GPE and cPG in the temporal cortex of Aβ25-35-treated rats on SRIF and SRIF receptor protein and mRNA levels, adenylyl cyclase activity, cell death, Aβ25-35 accumulation, cytosolic calcium levels ([Ca²⁺]c) and the intracellular signaling mechanisms involved. GPE and cPG did not change Aβ25-35 levels, but GPE partially restored SRIF and SRIF receptor 2 protein content and mRNA levels and protected against cell death after Aβ25-35 insult, which was coincident with Akt activation and glycogen synthase kinase 3β inhibition. In addition, GPE displaced glutamate from NMDA receptors and blocked the glutamate induced rise in cytosolic calcium in isolated rat neurons and moderately increased Ca²⁺ influx per se. Our findings suggest that GPE, but not its metabolite, mimics insulin-like growth factor I effects on the SRIF system through a mechanism independent of Aβ clearance that involves modulation of calcium and glycogen synthase kinase 3β signaling.