Inhibitors of GSK-3 prevent corticosterone from inducing COX-1 expression in cardiomyocytes

Differential Regulation of Bcl-xL Gene Expression by Corticosterone, Progesterone, and Retinoic Acid

Corticosterone (CT), progesterone (PG), and retinoic acid (RA) are capable of inhibiting Doxorubicin (Dox) from inducing apoptosis in rat cardiomyocytes. Mechanistically, CT, PG, and RA induce increases of Bcl-xL protein and mRNA, and activate a 3.2 kb bcl-x gene promoter. CT and RA, but not PG, induced the activity of a 0.9 kb bcl-x promoter, containing sequences for AP-1 and NF-kB binding. RA, but not CT or PG, induced NF-kB activation. CT, but not PG or RA, induced AP-1 activation, and induction of the 0.9 kb bcl-x reporter by CT was inhibited by dominant negative c-Jun TAM-67 or removal of AP-1 binding site. Therefore, although CT, PG, and RA all induce Bcl-xL mRNA and protein, three independent mechanisms are in operation: while CT induces Bcl-xL via AP-1 transcription factor, and RA induces NF-kB activation and bcl-x promoter activity, PG induces Bcl-xL via a mechanism independent of NF-kB or AP-1.

Herpes Simplex Virus type-1 infection induces synaptic dysfunction in cultured cortical neurons via GSK-3 activation and intraneuronal amyloid-β protein accumulation

Increasing evidence suggests that recurrent Herpes Simplex Virus type 1 (HSV-1) infection spreading to the CNS is a risk factor for Alzheimer’s Disease (AD) but the underlying mechanisms have not been fully elucidated yet. Here we demonstrate that in cultured mouse cortical neurons HSV-1 induced Ca²⁺-dependent activation of glycogen synthase kinase (GSK)-3. This event was critical for the HSV-1-dependent phosphorylation of amyloid precursor protein (APP) at Thr668 and the following intraneuronal accumulation of amyloid-β protein (Aβ). HSV-1-infected neurons also exhibited: i) significantly reduced expression of the presynaptic proteins synapsin-1 and synaptophysin; ii) depressed synaptic transmission. These effects depended on GSK-3 activation and intraneuronal accumulation of Aβ. In fact, either the selective GSK-3 inhibitor, SB216763, or a specific antibody recognizing Aβ (4G8) significantly counteracted the effects induced by HSV-1 at the synaptic level. Moreover, in neurons derived from APP KO mice and infected with HSV-1 Aβ accumulation was not found and synaptic protein expression was only slightly reduced when compared to wild-type infected neurons. These data further support our contention that HSV-1 infections spreading to the CNS may contribute to AD phenotype.

GSK3β, but not GSK3α, inhibits the neuronal differentiation of neural progenitor cells as a downstream target of mammalian target of rapamycin complex1

Glycogen synthase kinase 3 (GSK3) acts as an important regulator during the proliferation and differentiation of neural progenitor cells (NPCs), but the roles of the isoforms of this molecule (GSK3α and GSK3β) have not been clearly defined. In this study, we investigated the functions of GSK3α and GSK3β in the context of neuronal differentiation of murine NPCs. Treatment of primary NPCs with a GSK3 inhibitor (SB216763) resulted in an increase in the percentage of TuJ1-positive immature neurons, suggesting an inhibitory role of GSK3 in embryonic neurogenesis. Downregulation of GSK3β expression increased the percentage of TuJ1-positive cells, while knock-down of GSK3α seemed to have no effect. When primary NPCs were engineered to stably express either isoform of GSK3 using retroviral vectors, GSK3β, but not GSK3α, inhibited neuronal differentiation and helped the cells to maintain the characteristics of NPCs. Mutant GSK3β (Y216F) failed to suppress neuronal differentiation, indicating that the kinase activity of GSK3β is important for this regulatory function. Similar results were obtained in vivo when a retroviral vector expressing GSK3β was delivered to E9.5 mouse brains using the ultrasound image-guided gene delivery technique. In addition, SB216763 was found to block the rapamycin-mediated inhibition of neuronal differentiation of NPCs. Taken together, our results demonstrate that GSK3β, but not GSK3α, negatively controls the neuronal differentiation of progenitor cells and that GSK3β may act downstream of the mammalian target of rapamycin complex1 signaling pathway.

Dexamethasone induces transcriptional activation of Bcl-xL gene and inhibits cardiac injury by myocardial ischemia

Psychological or physical stress causes an elevation of glucocorticoids in the circulating system. Glucocorticoids regulate a variety of physiological functions, from energy metabolism and biochemical homeostasis to immune response. Synthetic steroids are among the most prescribed drugs for immune suppression and chemotherapy. While glucocorticoids are best known for inducing apoptosis in a number of cell types, we have found that corticosteroids at stress relevant levels protect cardiomyocytes from apoptosis. Current study addresses whether glucocorticoids inhibit cardiac injury in vivo. Adult male C57BL6 mice were administered with dexamethasone (20mg/kg, i.p.) or vehicle control 20 h prior to left anterior descending coronary artery occlusion surgery. Myocardial infarction was measured by triphenyl tetrazoliumchloride staining in tissue slices and by levels of cardiac Troponin (cTn I) in the blood. Treatment of dexamethasone markedly reduced infarct size (19.6 ± 4.3%, vs. 29.2 ± 4.9%, p<0.01) and cTn I level in the blood (3.83 ± 0.66 ng/ml vs. 5.62 ± 0.37 ng/ml, p<0.01). In studying the mechanism of such protection, we found that dexamethasone induces the expression of Bcl-xL gene in the myocardium. With cardiomyocytes in culture, glucocorticoids increased transcription of Bcl-xL gene as evidenced by Bcl-xL mRNA increase and promoter activation. The glucocorticoid receptor antagonist mifepristone prevented dexamethasone from inducing cardiac protection or Bcl-xL expression. Our data suggest that activation of glucocorticoid receptor can prevent cardiac injury through transcriptional activation of Bcl-xL gene.