Mechanisms Underlying Oxidized Low-Density Lipoprotein-Induced Matrix Metalloproteinase-9 Expression in Rat Brain Astrocytes

Oxidized low-density lipoprotein (oxLDL) is a risk factor in many inflammatory diseases such as atherosclerosis. Several lines of evidence indicate that oxLDL stimulates multiple signaling pathways, and induces expression of several pro-inflammatory mediators such as prostaglandins and interleukin-6 (IL-6), adhesion molecules, and hydrolytic enzymes like inducible nitric oxide synthase (iNOS), phospholipase A2 and matrix metalloproteinases (MMPs) in various cell types. MMPs, a large family of zinc-dependent, neutral proteinases, are major components of the enzyme cascade responsible for degradation of extracellular matrix (ECM) and basement membrane proteins that participate in cell migration, differentiation, inflammation, and regeneration of peripheral nerves and axons after injury. All of these factors are implicated to facilitate inflammatory responses and the development of atherosclerotic plaques in the lesion. Ischemic injury to brain such as cerebral infarction is associated with disruption of the blood-brain barrier (BBB), raising the possibility of exposing the central nervous system (CNS) to plasma lipoproteins and increased oxidative stress. Moreover, increasing evidences have shown that oxLDL serves as a deteriorating cause in neurodegenerative disease as well as a risk factor of atherosclerosis. In CNS, astrocytes are the major cell type and are responsible for the maintenance of the integrity of the BBB. After brain injury, the exposure to oxLDL may induce the expression of various pro-inflammatory proteins such as MMPs in astrocytes. MMP-9 (gelatinase B) has been demonstrated to be markedly up-regulated in many inflammatory states in CNS. Several pro-inflammatory cytokines like TNF-脈, IL-1芻, and bradykinin are reported to induce MMP-9 expression. The elevated expression of MMP-9 may be regulated during brain inflammation and injury and implicated in tissue remodeling and inflammatory responses. Although oxLDL is reported to activate various signaling pathways including MAPKs, and PI3K/Akt cascade, and transcription factors such as AP-1 and NF-豈B, the relationship between the activation of these signals and expression of MMP-9 is not completely understood. Therefore, we hypothesize that oxLDL may affect the pathophysiology of astrocytes including proliferation, apoptosis, and gene expression. Our preliminary data showed that oxLDL induced some inflammatory genes expression such as MMP-9 in a time- and dose-dependent manner by Western blot and RT-PCR analyses. OxLDL stimulated several signaling molecules, including p42/p44 MAPK, Akt, non-receptor tyrosine kinases (e.g. c-Src and PYK2), and PDGF receptor phosphorylation in a time- and concentration-dependent manner, as determined by Western blot analysis. Moreover, we further indicated that oxLDL stimulated transcription factors activation such as AP-1 (c-Fos and c-Jun) and NF-豈B by Western blot and immunofluorescent staining. In future study, we will investigate the mechanisms underlying oxLDL-induced MMP-9 expression, and which signaling molecules may be involved in the regulatory pathway in astrocytes. These results will provide new insights into the mechanisms of the action of inflammatory mediators, supporting the hypothesis that oxLDL might contribute to promote inflammatory responses involved in the development of CNS diseases and injury. Increased understanding of signal transduction mechanisms underlying these genes regulation will create opportunities for the development of anti-inflammation therapeutic strategies.

Project ID:PC9605-0015
External Project ID:NSC96-2320-B182-009