Toll-like receptor 3 activation in retinal pigment epithelium cells - Mitogen-activated protein kinase pathways of cell death and vascular endothelial growth factor secretion.

PURPOSE: Toll-like receptor 3 (TLR3) is a receptor of the innate immune system, recognizing double-stranded RNA. TLR3 can lead to cytokine release or apoptosis and has recently been associated with the development of geographical atrophy via cytotoxic effects on the retinal pigment epithelium (RPE). The current study was conducted to elucidate the underlying pathways of TLR3 effects in the RPE.
METHODS: TLR3 activation via polyinosinic acid/polycytidylic acid (Poly I:C) was investigated in primary porcine RPE cells, focussing on cell death and vascular endothelial growth factor (VEGF) secretion. Primary cells were stimulated with different concentrations of Poly I:C. Cell death was investigated in trypan blue exclusion assay and cell death detection ELISA. VEGF and IFN-ß secretion were also detected in ELISA. As Mitogen-activated protein kinases (MAPK) play an important part in TLR3-mediated signal transduction, we investigated the influence of JNK, ERK1/2 and p38 on cell death and VEGF secretion, using commercially available inhibitors.
RESULTS: Activation of TLR3 by Poly I:C induced concentration-dependent cell death, partly mediated by JNK. ERK1/2 was activated and exerted some protection. Furthermore, higher concentrations of Poly I:C increased VEGF secretion after 4 and 24 hr, which was independent of MAPK.
CONCLUSION: The induction of cell death in RPE cells by TLR3 activation confirms possible involvement of TLR3 activation in GA. As cell death is partly mediated by JNK, more studies should be conducted investigating the role of JNK in RPE cell death to evaluate whether its inhibition might be a new therapeutic opportunity for the treatment of geographical atrophy. Additionally, effects on VEGF secretion can be found.