Docosahexaenoic acid up-regulates both PI3K/AKT-dependent FABP7-PPARγ interaction and MKP3 that enhance GFAP in developing rat brain astrocytes.

The astrocyte marker, glial fibrillary acidic protein (GFAP), has essential functions in the brain, but may trigger astroglial scarring when expressed in excess. Docosahexaenoic acid (DHA) is an n-3 fatty acid that is protective during brain development. However, the effect of DHA on GFAP levels of developing brain remains unexplored. Here, we detected that treating developing rats with DHA-enriched fish-oil caused dose-dependent GFAP augmentation. We investigated the mechanism promoting GFAP, hypothesizing the participation of fatty acid-binding protein-7 (FABP7), known to bind DHA. We identified that DHA stimulated FABP7 expression in astrocytes, and FABP7-silencing suppressed DHA-induced GFAP, indicating FABP7-mediated GFAP increase. Further investigation proved FABP7 expression to be phosphatidylinositide 3-kinases (PI3K)/AKT and nuclear receptor peroxisome proliferator-activated receptor-gamma (PPARγ)-dependent. We found that PI3K/AKT activated PPARγ that triggered FABP7 expression via PPARγ-responsive elements within its gene. Towards identifying FABP7-downstream pathways, we considered our previous report that demonstrated cyclin-dependent kinase-5 (CDK5)-PPARγ-protein-protein complex to suppress GFAP. We found that the DHA-induced FABP7 underwent protein-protein interaction with PPARγ, which impeded CDK5-PPARγ formation. Hence, it appeared that enhanced FABP7-PPARγ in lieu of CDK5-PPARγ resulted in increased GFAP. PI3K/AKT not only stimulated formation of FABP7-PPARγ protein-protein complex, but also up-regulated a FABP7-independent MAP-kinase-phosphatase-3 pathway that inactivated CDK5 and hence attenuated CDK5-PPARγ. Overall, our data reveal that via the proximal PI3K/AKT, DHA induces FABP7-PPARγ, through genomic and non-genomic mechanisms, and MAP-kinase-phosphatase-3 that converged at attenuated CDK5-PPARγ and therefore, enhanced GFAP. Accordingly, our study demonstrates a DHA-mediated astroglial hyperactivation, pointing toward a probable injurious role of DHA in brain development.
(© 2016 International Society for Neurochemistry.)