Neuroprotective effects of sodium orthovanadate after hypoxic-ischemic brain injury in neonatal rats

Dexamethasone protects neonatal hypoxic-ischemic brain injury via L-PGDS-dependent PGD2-DP1-pERK signaling pathway

BACKGROUND AND PURPOSE

Glucocorticoids pretreatment confers protection against neonatal hypoxic-ischemic (HI) brain injury. However, the molecular mechanism remains poorly elucidated. We tested the hypothesis that glucocorticoids protect against HI brain injury in neonatal rat by stimulation of lipocalin-type prostaglandin D synthase (L-PGDS)-induced prostaglandin D2 (PGD2)-DP1-pERK mediated signaling pathway.

METHODS

Dexamethasone and inhibitors were administered via intracerebroventricular (i.c.v) injections into 10-day-old rat brains. Levels of L-PGD2, D prostanoid (DP1) receptor, pERK1/2 and PGD2 were determined by Western immunoblotting and ELISA, respectively. Brain injury was evaluated 48 hours after conduction of HI in 10-day-old rat pups.

RESULTS

Dexamethasone pretreatment significantly upregulated L-PGDS expression and the biosynthesis of PGD2. Dexamethasone also selectively increased isoform pERK-44 level in the neonatal rat brains. Inhibitors of L-PGDS (SeCl4), DP1 (MK-0524) and MAPK (PD98059) abrogated dexamethasone-induced increases in pERK-44 level, respectively. Of importance, these inhibitors also blocked dexamethasone-mediated neuroprotective effects against HI brain injury in neonatal rat brains.

CONCLUSION

Interaction of glucocorticoids-GR signaling and L-PGDS-PGD2-DP1-pERK mediated pathway underlies the neuroprotective effects of dexamethasone pretreatment in neonatal HI brain injury.

Neuroprotective effect of memantine combined with topiramate in hypoxic-ischemic brain injury

Glutamate receptor-mediated neurotoxicity is a major mechanism contributing to hypoxic-ischemic brain injury (HIBI). Memantine is a safe non-competitive NMDA receptor blocker characterized by its low affinity and fast unblocking kinetics. Topiramate is an AMPA/KA receptor blocker and use-dependent sodium channel blocker with several other neuroprotective actions and little neurotoxicity. We hypothesized that the coadministration of memantine and topiramate would be highly effective to attenuate HIBI in neonatal rats. Seven-day-old Sprague-Dawley rat pups were subjected to right common carotid artery ligation and hypoxia for 2 h, and then were randomly and blindly assigned to one of four groups: vehicle, memantine, topiramate and combination group. Brain injury was evaluated by gross damage and weight deficit of the right hemisphere at 22d after hypoxic-ischemia (HI) and by neurofunctional assessment (foot-fault test) at 21d post-HI. Acute neuronal injury was also evaluated by microscopic damage grading at 72 h post-HI. Results showed the combination of memantine and topiramate improved both pathological outcome and performance significantly. The drug-induced apoptotic neurodegeneration was assessed by TUNEL staining at 48 h post-HI and the result showed no elevated apoptosis in all observed areas. The result of the experiment indicates the combination therapy is safe and highly effective to reduce brain damage after HIBI.

AKT1 mediates bypass of the G1/S checkpoint after genotoxic stress in normal human cells

Certain forms of hexavalent chromium [Cr(VI)] are human carcinogens. Our recent work has shown that a broad range protein tyrosine phosphatase (PTP) inhibitor, sodium orthovanadate (SOV), abrogated both Cr(VI)-induced growth arrest and clonogenic lethality. Notably, SOV enhanced Cr(VI) mutation frequency, ostensibly through forced survival of genetically damaged cells. In the present study, co-treatment with this PTP inhibitor bypassed the Cr(VI)-induced G(1)/S checkpoint arrest in diploid human lung fibroblasts (HLF). Moreover, the PTP inhibitor abrogated the Cr(VI)-induced decrease in the expression of key effectors of the G(1)/S checkpoint [Cyclin D1, phospho Ser 807/811 Rb (pRB), p27]. Cr(VI)-induced G(1) arrest was associated with the cytoplasmic appearance of pRb and the nuclear localization of p27, both of which were reversed by the PTP inhibitor. The PTP inhibitor's reversal of G(1)/S checkpoint effector localization after Cr exposure was found to be Akt1-dependent, as this was abrogated by transfection with either akt1 siRNA or an Akt1-kinase dead plasmid. Furthermore, Akt1 activation alone was sufficient to induce G(1)/S checkpoint bypass and to prevent Cr(VI)-induced changes in pRb and p27 localization. In conclusion, this work establishes Akt1 activation to be both sufficient to bypass the Cr(VI)-induced G(1)/S checkpoint, as well as necessary for the observed PTP inhibitor effects on key mediators of the G(1)/S transition. The potential for Akt to bypass G(1)/S checkpoint arrest in the face of genotoxic damage could increase genomic instability, which is a hallmark of neoplastic progression.

Enhanced clonogenic survival induced by protein tyrosine phosphatase (PTP) inhibition after Cr(VI) exposure is mediated by c-Raf and Ras activity

Our recent studies showed that maintenance of protein tyrosine phosphorylation by PTP inhibition enhanced cell growth, clonogenic survival, and mutagenesis after a single low-level Cr(VI) exposure, thereby suggesting that tyrosine phosphorylation-dependent signaling may govern inappropriate survival in human lung fibroblasts (HLFs). Our goal is to identify specific phospho-tyrosine regulator(s)/ downstream effectors involved in enhanced survival after Cr(VI) exposure and PTP inhibition. Phosphotyrosine profiling array showed that PTP inhibition following Cr(VI) exposure increased tyrosine phosphorylation of specific proteins, such as FGR and ABL, which are upstream regulators of both Erk and Akt pathways. To explore the roles of these pathways in the PTP-induced increase in clonogenic survival after Cr(VI) exposure, we examined the effect of combined Akt1 and Erk1/2 knockdown via siRNA technology. Akt1 and/or Erk1/2 silencing had no effect on the PTP inhibitor-induced increase in survival following Cr(VI) exposure, suggesting the presence of non-Akt/non-Erk-mediated survival signaling. Interestingly, geldanamycin, an HSP90 inhibitor and non-specific Raf inhibitor, abrogated the PTP inhibitor-mediated increase in survival following Cr(VI) exposure and abolished the expression/activity of c-Raf and activity of Mek. These findings prompted us to explore upstream regulators of Erk, i.e., Ras, c-Raf and Mek for their potential roles in clonogenic survival. GW5074, a specific c-Raf kinase inhibitor did not alter the effect of the PTP inhibitor but decreased Cr(VI)-mediated clonogenic lethality, potentially though Mek hyperactivation. A genetic approach with a c/a Mek1 mutant also showed that Mek activity was not directly associated with the PTP inhibitor effect. Finally, a genetic approach with d/n or c/a Ras and c-Raf mutants, showed that Ras and c-Raf activities play a substantive role in enhancing clonogenic survival by PTP inhibition following Cr(VI) insult. In conclusion, these studies highlight a novel pro-survival mechanism for clonogenic survival in the face of genotoxic stress in the presence of PTP inhibition via an Erk/Mek-independent and Ras/c-Raf-dependent regulation in normal human lung fibroblasts.

Bypass of hexavalent chromium-induced growth arrest by a protein tyrosine phosphatase inhibitor: enhanced survival and mutagenesis

Although the consequences of genotoxic injury include cell cycle arrest and apoptosis, cell survival responses after genotoxic injury can produce intrinsic death-resistance and contribute to the development of a transformed phenotype. Protein tyrosine phosphatases (PTPs) are integral components of key survival pathways, and are responsible for their inactivation, while PTP inhibition is often associated with enhanced cell proliferation. Our aim was to elucidate signaling events that modulate cell survival after genotoxin exposure. Diploid human lung fibroblasts (HLF) were treated with Cr(VI) (as Na(2)CrO(4)), the soluble oxyanionic dissolution product of certain particulate chromates, which are well-documented human respiratory carcinogens. In vitro soluble Cr(VI) induces a wide spectrum of DNA damage, in both the presence and absence of a broad-range PTP inhibitor, sodium orthovanadate (SOV). Notably, SOV abrogated Cr(VI)-induced clonogenic lethality. The enhanced survival of Cr(VI)-exposed cells after SOV treatment was predominantly due to a bypass of cell cycle arrest, as there was no effect of the PTP inhibitor on Cr-induced apoptosis. Moreover, the SOV effect was not due to decreased Cr uptake as evidenced by unchanged Cr-DNA adduct burden. Additionally, the bypass of Cr-induced growth arrest by SOV was accompanied by a decrease in Cr(VI)-induced expression of cell cycle inhibiting genes, and an increase in Cr(VI)-induced expression of cell cycle promoting genes. Importantly, SOV resulted in an increase in forward mutations at the HPRT locus, supporting the hypothesis that PTP inhibition in the presence of certain types of DNA damage may lead to increased genomic instability, via bypass of cell cycle checkpoints.