Focal cerebral ischemia is associated with a robust inflammation that contributes to the progression of ischemic neuronal damage. The mechanisms, which modulate the inflammation after focal ischemia, are not well-understood. Suppressor of cytokine signaling (SOCS) family of proteins controls the inflammation in the peripheral organs. Binding of pro-inflammatory cytokine IL-6 (formed in excess after ischemia) to its receptors induces transphosphorylation of the receptor-associated Janus kinases (JAKs). Phosphorylated JAKs in turn phosphorylate the down-stream STAT family of transcription factors, which dimerize and translocate into nucleus. Binding of phosphorylated STAT to DNA stimulates cytokine gene expression to generate more interleukins. This cycle leads to sustained inflammation unless controlled. Phosphorylated STAT also stimulates SOCS gene expression. SOCS proteins act as intracellular negative feedback regulators to inhibit JAK-STAT phosphorylation and thereby dampen the cytokine signal transduction. In the adult brain, SOCS proteins are expressed at a very low level, but can be induced rapidly. Our preliminary data showed an upregulation of SOCS-3 and STAT-3 expression after focal ischemia. We hypothesize that SOCS-3 induction is an endogenous neuroprotective event to control post-ischemic inflammation and neuronal damage. We also hypothesize that in the ischemic brain, SOCS-3 actions are mediated by STAT-3. Using antisense knockdown and adenovirus-induced overexpression of individual proteins, we will analyze the mechanism of action of the 3 control points of SOCS-3 pathway (IL-6, STAT-3 and SOCS-3) in modulating post-ischemic inflammation and neuronal damage. We will study (a) if SOCS-3 knockdown increases and overexpression decreases interleukin levels and STAT-3 phosphorylation after ischemia; (b) if IL-6 knockdown (starting point of the cascade) prevents STAT-3 phosphorylation and SOCS-3 expression after ischemia, and (c) if STAT-3 knockdown/overexpression modulates post-ischemic SOCS-3 induction. Using GeneChip in combination with antisense and adenovirus, we will analyze the ischemia-induced gene expression changes modulated by SOCS-3. The ultimate goal is to define the role of SOCS-3/STAT-3 in post-ischemic cerebral inflammation.
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