Microglial activation plays an important role in the regulation of neuronal function and contributes to the development of neurodegeneration in Alzheimer's disease (AD). Activation of nuclear peroxisome proliferator-activated receptor gamma (PPAR gamma) by an endogenous agonist, 15-deoxy-Delta(12,14)-prostaglandin J2 (15d-PGJ2), has been shown to be beneficial in many diseases with aberrant immune responses. Here, we report that co-treatment with 15d-PGJ2 and its synergistic partner, 9-cis-retinoic acid (RA), may modulate, but not abolish, microglial immune response activated by beta-amyloid (A beta) and interferon gamma (IFN gamma). The co-treatment of RA and 15d-PGJ2 inhibited A beta/IFN gamma-activated immune response in primary microglia, as evidenced by suppressed expression of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2); and the effect was not affected by treatment with a PPAR gamma antagonist, GW9662. Data suggest that PPAR gamma activation may not contribute to the anti-inflammatory properties of the co-treatment. The co-treatment promoted microglial A beta clearance in cultures; and the effect can be prevented by blocking PPAR gamma activation using GW9662. The effects of the co-treatment on A beta clearance may be PPAR gamma-dependent. Intriguingly, secretion of microglial pro-nerve growth factor (pro-NGF) was inhibited by A beta/IFN gamma treatment in a dose-dependent manner, suggesting that secretion of microglial pro-NGF may not contribute to the A beta/IFN gamma-activated microglial immune response. Taken together, the co-treatment may be beneficial for AD therapy; however, our data suggest that multiple mechanisms may underlie the beneficial effects of the co-treatment and are not limited to PPAR gamma activation only.
