Exendin-4 induces a novel extended effect of ischemic tolerance via crosstalk with IGF-1R.

Glucagon-like peptide-1 (GLP-1) receptor (GLP-1R) agonist exendin-4 (Ex-4), a drug that has been used in the clinical treatment of type 2 diabetes mellitus, also confers a neuroprotective effect against stroke. Although GLP-1 analogs were reported to induce sustained insulin secretion and glucose tolerance improved after cessation of treatment, no study has revealed whether Ex-4 exerts sustained neuroprotection against stroke and the underlying mechanism after treatment cessation. In this study, mice were pretreated with Ex-4 for 7 days, and middle cerebral artery occlusion (MCAO) was performed on different days after cessation of Ex-4 treatment. Ex-4 ameliorated neurological dysfunction and reduced the infarct volume induced by MCAO. These protective effects lasted for 6 days after the cessation of Ex-4 treatment and were associated with sustained upregulation of PI3K, AKT, mTOR, and HIF-1α levels, as well as HIF-1α downstream genes. Knockdown of GLP-1R or HIF-1α in the brain by short hairpin RNA abolished Ex-4 treatment-mediated neuroprotection. In normal mice, Ex-4 treatment led to instant upregulation of p-PI3K, p-AKT, p-mTOR, and HIF-1α expression levels, which quickly returned to normal after cessation of Ex-4 treatment, while the expression levels of insulin growth factor-1 receptor (IGF-1R) remained high for 6 days after Ex-4 cessation. Additionally, Ex-4 did not directly induce IGF-1 production, which was only induced by MCAO. Ex-4 induces extended cerebral ischemic tolerance. This neuroprotective effect is associated with activation of GLP-1R and upregulation of IGF-1R in the brain, and the latter then activates the PI3K/AKT/mTOR/HIF-1 signaling pathway via binding to IGF-1 secreted from the ischemic brain.

Glucagon-like peptide-1 cleavage product GLP-1(9-36) reduces neuroinflammation from stroke via the activation of insulin-like growth factor 1 receptor in astrocytes.

Glucagon-like peptide-1 (GLP-1) is an endogenous gut hormone and a key regulator in maintaining glucose homeostasis by stimulating insulin secretion. Its natural cleavage product GLP-1 (9-36), which was formerly considered a "bio-inactive" metabolite mainly due to its low affinity for GLP-1 receptor, possesses unique properties such as cardiovascular protection. Little is known about the effects and mechanisms of GLP-1 (9-36) in cerebral ischemia and reperfusion injury. Here, we report that systemic application of GLP-1 (9-36) in adult mice facilitated functional recovery and reduced infarct volume, astrogliosis, and neuronal apoptosis following middle cerebral artery occlusion and reperfusion. Interestingly, these effects were still observed in GLP-1 receptor knockout (Glp-1rKO) mice but were partially reversed in insulin-like growth factor 1 (IGF-1) receptor knockdown (Igf-1rKD) mice. Primary astrocytes were cultured and subjected to oxygen-glucose deprivation/reoxygenation (OGD/R), and enzyme-linked immunosorbent assay indicated that GLP-1 (9-36) pretreatment reduces tumor necrosis factor-α, interleukin (IL)-1ß, and IL-6 levels. This effect was not diminished in Glp-1rKO astrocytes but was reversed in Igf-1rKO astrocytes, emphasizing that the anti-inflammatory effect of GLP-1 (9-36) in astrocytes is independent of GLP-1 receptor signaling and is instead mediated by IGF-1 receptor. Immunoprecipitation experiments showed that GLP-1 (9-36) directly interacts with IGF-1 receptor in astrocytes. Western blot data indicated that GLP-1 (9-36) activates IGF-1 receptor and downstream PI3K-AKT pathway in astrocytes upon OGD/R injury, which was abrogated by preincubation with IGF-1 receptor autophosphorylation inhibitor picropodophyllin. Thus, our findings suggest that GLP-1 (9-36) improved stroke outcome by reducing inflammation in astrocytes via interaction with IGF-1 receptor.

Activation of Paraventricular Melatonin Receptor 2 Mediates Melatonin-Conferred Cardioprotection Against Myocardial Ischemia/Reperfusion Injury.

Previous studies have shown that melatonin (Mel) can effectively ameliorate myocardial ischemia/reperfusion (MI/R) injury, but the mechanism is yet to be fully elucidated. Mel receptors are expressed in the paraventricular nucleus (PVN), which is also involved in regulating cardiac sympathetic nerve activity. The aim of this study was to examine whether Mel receptors in the PVN are involved in the protective effects of Mel against MI/R injury. The results of quantitative polymerase chain reaction, western blot, and immunofluorescence assays indicated that Mel receptor 2 (MT2) expression in the PVN was upregulated after MI/R. Intraperitoneal administration of Mel significantly improved post-MI/R cardiac function and reduced the infarct size, whereas shRNA silencing of MT2 in the PVN partially blocked this effect. Intraperitoneal administration of Mel reduced sympathetic nerve overexcitation caused by MI/R, whereas shRNA silencing of MT2 in the PVN partially diminished this effect. Furthermore, enzyme-linked immunosorbent assay and western blot results indicated that intraperitoneal administration of Mel lowered the levels of inflammatory cytokines in the PVN after MI/R injury, whereas the application of sh-MT2 in the PVN reduced this effect of Mel. Mel significantly reduced the levels of NF-κB after astrocyte oxygen and glucose deprivation/reoxygenation injury, and this effect was offset when MT2 was silenced. The above experimental results suggest that MT2 in the PVN partially mediated the protective effects of Mel against MI/R injury, and its underlying mechanisms may be related to postactivation amelioration of PVN inflammation and reduction of cardiac sympathetic nerve overexcitation.