In the present study, we explored the new molecular pathway underlying HBO protection in the model of MCAO-induced HT. The benefits of HBO on HT, observed in our study, are in agreement with previous studies.
9,
10 The novelty and important finding of the study is as follows:
the protective effects of HBO are, at least partly, mediated through ATP/NAD+/Sirt1 pathway.
Early restoration of energy can interrupt the initial responses and delay or even disrupt the spiral of abnormal changes that culminate in cell death after ischemic stroke. HBO is able to increase the level of ATP and relieve the energy stress in experimental stroke,
27–29 but the mechanisms underlying HBO-induced protection have not been fully elucidated yet. In our study, we used the MCAO model, a well-established model of ischemic stroke. We demonstrated that hyperglycemia exaggerated the decrease of the ATP and NAD+ level, HT, and neurological deficits in MCAO rats. HBO treatment reduced the infarction volume, as well as hemorrhagic volume, and eventually improved the neurological deficits. Additionally,
HBO treatment increased the level of ATP and the activity of NAMPT, promoting the production of NAD+. We further demonstrated for the first time that
HBO activated ATP/NAD+/Sirt1 pathway and, consequently, reduced HT and apoptosis.
NAD+ played an important role in energy balance and cell survival in ischemic stroke. After brain ischemia, NAD+ fuels Sirt1 and enables it to regulate transcription factors involved in pathways linked to apoptosis and inflammation.
13 HBO has been reported to increase Sirt1 protein and mRNA expression in animal models of stroke.
14 We found that HBO treatment significantly increased the level of ATP and NAD+, ameliorated HT, and improved neurological functions in hyperglycemic MCAO rats.
Administration of NAD+ mimicked the effects of HBO. More important, using pharmacological manipulations,
this study showed for the first time that HBO positively modulated the activity of NAMPT and upregulated NAD+ levels by increasing the level of ATP, thereby, controlled Sirt1 expression, leading to deacetylation of NF-κB and p53. Our results were consistent with the previous studies that inhibition of NAMPT with FK866
13 or silence Sirt1 with Sirt1 siRNA
20 deteriorated the outcome after stroke. We demonstrated a novel ATP/NAD+/Sirt1 signaling pathway of HBO treatment for HT after stroke.
Previous studies have demonstrated that HBO increase the oxygen delivery to ischemic tissue and is able to restore energy status of the tissue.27,40,41 We think, therefore, that both direct delivery of oxygen and the activation of ATP/NAD+/Sirt1 pathway underlay HBO-induced BBB protection.
As an NAD+-dependent deacetylase, Sirt1 specifically promotes the transcription of a set of genes related to cell survival, energy metabolism, and inflammation. Sirt1 deficiency or knockdown attenuated the neuroprotection of NAD+ in ischemic stroke.
13,
42 Sirt1 mediated deacetylation of p53, reducing its proapoptotic effects, and it also deacetylated NF-κB, reducing its proinflammatory effects.
43 p53 mediates apoptosis through several pathways and results in activation of caspase-3, the executor of cell death.
44 NF-κB is the major transcription factor that transcribes proinflammatory mediators in the nervous system, including MMP-9. In ischemic stroke, MMP-9 is released from neurons and reactive astrocytes to degrade the components of BBB.
45–47 We showed that in hyperglycemic MCAO rats, Sirt1 was highly colocalized with NAMPT and positively regulated by ATP and NAD+. We also observed that HBO treatment decreased the level of acetylated NF-κB and acetylated p53 in hyperglycemic MCAO rats, and knockdown Sirt1 by Sirt1 siRNA abolished the effects of HBO. Finally, we proved that the beneficial effects were abolished by Sirt1 siRNA.
Taken together, we demonstrated that Sirt1 is a key mediator of HBO protective effects.
We think, therefore, that
the beneficial effects of HBO on neurological functions can be explained, at least partly, by the activation of ATP/NAD+/Sirt1 pathway and subsequent preservation of BBB in hyperglycemic MCAO rats.
