Abstract
Purpose: :
Inflammation and oxidative stress wreak havoc in retina; RPE cells are particularly susceptible. As such, discovery of novel mechanisms for protecting these cells is critically important. Fumaric acid esters have been used widely for treatment of psoriasis. Given the robust antioxidant and anti-inflammatory effects elicited by these compounds, of late, increased interest has been given to determining the mechanism(s) underlying their beneficial effects, and to evaluating their efficacy in the treatment of other diseases in which oxidant- and/or inflammation-induced cellular damage play a major role. Here, we asked whether monomethylfumarate (MMF), the major bioactive metabolite in fumarate-based drugs, is of benefit in retina. We evaluated also the effects of MMF on activity of the cystine-glutamate exchanger (system xc-), the transport system majorly responsible for providing cells with cysteine, the rate-limiting amino acid in glutathione (GSH) synthesis.
Methods: :
C57BL/6 mice (age 6 wks) received MMF via intravitreal injection. 24 h later, mice were sacrificed, eyes enucleated and GSH levels measured. The effects of MMF on system xc- transport activity was assayed in ARPE-19 and primary mouse RPE cells via radiolabeled uptake assay. In the search for possible mechanism(s) underlying MMF-induced effects, we further assayed: (1) transport of MMF via the Na+-coupled monocarboxylate transporter SLC5A8 using the X. laevis oocyte heterologous expression system, (2) GPR109A-MMF interaction, (3) the effects of MMF on system xc- in Slc5a8- or Gpr109a-deficient primary RPE, (4) the effect of MMF on HDAC activity, and (5) the effects of MMF on hypoxia-inducible factor 1 α (Hif-1α) expression.
Results: :
MMF increases retinal GSH levels in vivo. This may be due in part to its ability to very effectively stimulate system xc- transport activity both in retinal cells. MMF is a transportable substrate for SLC5A8, and a ligand for GPR109A; however, the ability of MMF to stimulate system xc- activity persisted even in the absence of these two genes. MMF treatment was associated with increased Hif-1α mRNA and protein expression.
Conclusions: :
MMF effectively increases GSH levels in mouse retina; MMF-induced stimulation of system xc- activity and increased Hif-1α expression are likely involved. These data suggest that MMF may be a potentially useful therapeutic for treatment of retinal diseases like age-related macular degeneration in which oxidative stress, inflammation and resultant damage to RPE play major causative roles.
Keywords: retinal pigment epithelium • antioxidants • retina