Fenofibric Acid Protects Rod Precursor and Müller Cells from Oxidative Stress and Hypoxia in a PPAR-alpha-Dependent Mechanism

Abstract

Purpose: PPARα is a master regulator of glucose and lipid homeostasis, and is thought to play a protective role in diabetic retinopathy (DR) and other neurodegenerative disorders. Clinical trials indicated that the PPARα agonist fenofibrate significantly reduced diabetic retinopathy in human patients, but its physiological and molecular mechanisms of action remain unknown. The purpose of this study is to investigate the mechanism for fenofibrate and PPARα’s neuroprotective effects.

Methods: The bioactive fenofibrate metabolite fenofibric acid (FA) was used in vitro to determine its protective effect and molecular mechanisms of action. Oxidative stress and hypoxia were induced using 4-Hydroxynonenal and cobalt chloride, respectively, in rat rod precursor and human Müller cells. FA’s protective effect was evaluated using trypan blue dye exclusion. FA’s dependence upon PPARα was determined using primary cells isolated from PPARα knockout and wild-type mice. FA’s effect upon Ik-Bα and NOX4 signaling was assessed using western blotting and RT-qPCR, and adenovirus expressing PPARα was utilized to determine whether these effects were a consequence of PPARα activation.

Results: Our results showed that FA protected rod precursor and Müller cells from oxidative stress and hypoxia. Furthermore, we identified that FA did not protect primary cells isolated from PPARα knockout mice, indicating that its cytoprotective effect is PPARα-dependent. Moreover, both FA and PPARα over-expression up-regulated IκBα, which is known to repress NFκβ signaling and thus down-regulate NOX4, a primary source of ROS in both hypoxia and oxidative stress. Further, both FA and PPARα over-expression down-regulated NOX4 in hypoxia, suggesting that the effect is relevant in a hypoxic environment, and is a direct consequence of PPARα activation.

Conclusions: Taken together, our data have demonstrated for the first time that FA/PPARα have a neuroprotective effect in the retina, and moreover established that the protective effect is PPARα-dependent. Further, because both FA and PPARα over-expression activate the IκBα/NFκβ axis and subsequently down-regulate NOX4, this event may also be PPARα-specific. We have also identified that FA and PPARα affect IκBα down-regulation of NOX4, and we hypothesize that this may be responsible, at least in part, for its protective effect.

Keywords: 499 diabetic retinopathy • 699 retinal glia • 615 neuroprotection