Peroxisome proliferator‐activated receptorsγ (PPARγ) differently modulate the interleukin‐6 expression in the peri‐infarct cortical tissue in the acute and delayed phases of cerebral ischaemia

Abstract

Interleukin-6 (IL-6) exerts neuroprotective effects after cerebral ischaemia but can also exacerbate inflammation and induce neuronal death. The current study investigates the role of cerebral peroxisome proliferator-activated receptor(s) gamma (PPARgamma) in the regulation of IL-6 expression in the peri-infarct cortical tissue in rats exposed to focal cerebral ischaemia. Pioglitazone, a high-affinity PPARgamma ligand, was infused intracerebroventricularly (i.c.v.) via osmotic minipumps over a 5-day period before, during and 24 h or 48 h after middle cerebral artery occlusion (MCAO) for 90 min followed by reperfusion. The expression of PPARgamma and IL-6 in cortical tissue adjacent to the ischaemic core was studied 24 h and 48 h after MCAO. Pioglitazone augmented the ischaemia-induced upregulation of PPARgamma at both time points. Cerebral ischaemia substantially increased IL-6 expression in the peri-infarct cortical tissue. Twenty-four hours after MCAO, the majority of microglial cells/macrophages showed an intense IL-6 immunoreactivity. IL-6 was also localized in neurons, but the distribution of neurons positively stained for IL-6 at the border of the infarct was very heterogeneous. Pioglitazone effectively decreased the number of IL-6-immunoreactive cells and IL-6 protein levels at 24 h but not at 48 h after MCAO. Pioglitazone treatment reduced the infarct size and improved neurological functions. The present study demonstrates that cerebral PPARgamma suppresses the expression of IL-6 in ischaemic brain tissue during the initial phase of ischaemic stroke, in which the overproduction of IL-6 may aggravate neuronal damage, but not at later time points, when IL-6 promotes neuroprotection and inhibits neuronal death.