Neuroprotective effects of PPARγ agonists against oxidative insults in HT-22 cells

Peroxisome proliferator-activated receptors (PPARs) are involved in regulating many metabolic and inflammatory processes. The present study explores the role of PPAR ligands in protecting neuronal cultures from toxic insults. For that purpose, we used WY14643 [4-chloro-6-(2,3-xylidino)-2-pyrimidinylthio acetic acid] as a PPARα agonist, L-165041 and L-783483 as PPARβ ligands, and 15-deoxy-Δ12,14-PGJ2 (15d-PGJ2), troglitazone, and ciglitazone for PPARγ. Experiments were performed using HT-22, an immortalized mouse hippocampal cell line, and SK-N-SH, a human neuroblastoma cell line. Cell viability against glutamate, hydrogen peroxide (H2O2), and serum deprivation insults was determined using a calcein acetoxymethyl (AM) assay. Of the compounds tested, only 15d-PGJ2 and troglitazone showed a dose-dependent neuroprotection from glutamate and H2O2 insults in HT-22 cells. None of the PPAR agonists was protective in SK-N-SH cells. A minimum of 4–6 h preincubation with 15d-PGJ2 was required to achieve significant neuroprotection. On the other hand, troglitazone was protective even when administered simultaneously with glutamate, or for up to 8 h postglutamate insult. To investigate whether the neuroprotective effects are mediated through PPARγ, we first determined through Western blotting that HT-22 and SK-N-SH cells express PPARγ. However, the neuroprotective effects of those compounds are unlikely to be mediated through the PPARγ for two reasons: (1) various concentrations of another PPARγ agonist (ciglitazone) were not neuroprotective; (2) by itself, PPAR exhibits a low affinity for DNA, and high-affinity binding requires heterodimerization with RXR, the 9-cis-retinoic acid receptor; administering 9-cis-retinoic acid in conjunction with 15d-PGJ2 did not alter the neuroprotective effects of the latter. Our results demonstrate neuroprotective effects of 15d-PGJ2 and troglitazone that are likely independent of PPARγ.