Salidroside Inhibits Inflammation Through PI3K/Akt/HIF Signaling After Focal Cerebral Ischemia in Rats

Salidroside is being investigated for its therapeutic potential in stroke because it is neuroprotective over an extended therapeutic window of time. In the present study, we investigated the mechanisms underlying the anti-inflammatory effects of salidroside (50 mg/kg intraperitoneally) in rats, given 1 h after reperfusion of a middle cerebral artery that had been occluded for 2 h. After 24 h, we found that salidroside increased the neuronal nuclear protein NeuN and reduced the marker of microglia and macrophages CD11b in the peri-infarct area of the brain. Salidroside also decreased IL-6, IL-1 beta, TNF-alpha, CD14, CD44, and iNOs mRNAs. At the same time, salidroside increased the ratio of phosphorylated protein kinase B (p-Akt) to total Akt. The phosphoinositide 3-kinase (PI3K) inhibitor LY294002 prevented this increase in p-Akt and reversed the inhibitory effects of salidroside on CD11b and inflammatory mediators. Salidroside also elevated the protein levels of hypoxia-inducible factor (HIF) subunits HIF1 alpha, HIF2 alpha, HIF3 alpha, and of erythropoietin (EPO). The stimulatory effects of salidroside on HIF alpha subunits were blocked by LY294002. Moreover, YC-1, a HIF inhibitor, abolished salidroside-mediated increase of HIF1 alpha and prevented the inhibitory effects of salidroside on CD11b and inflammatory mediators. Taken together, our results provide evidence for the first time that all three HIF alpha subunits and EPO can be regulated by PI3K/Akt in cerebral tissue, and that salidroside entrains this signaling pathway to induce production of HIF alpha subunits and EPO, one or more of which mediate the anti-inflammatory effects of salidroside after cerebral IRI.