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Jian Liu, David Alexander Copland, Hsi An Amy Chiu, Lindsay B Nicholson, Andrew D Dick; Impaired autophagy in retinal pigment epithelial cells induces inflammatory responses in macrophages. Invest. Ophthalmol. Vis. Sci. 2014;55(13):66. doi: https://doi.org/.
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Age-related macular degeneration (AMD) is an insidious degenerative disease leading to a spectrum from cell loss of retinal pigment epithelium (RPE) and photoreceptors, to tissue atrophy and angiogenesis. Progression of disease is accompanied by the inflammasome activation and unchecked immune response. The underlying causes are polygenic and environmental. Here we investigated whether RPE cells, rendered dysfunctional through impaired autophagy or under oxidative stress, were capable of conditioning the macrophage response and amplifying pro-inflammatory responses, which may have the potential to accelerate AMD.
Wortmannin, a known autophagy inhibitor was used to impair autophagy in murine RPE cell culture or in mouse retina. Retenone was used to induce RPE autophagy via interference of mitochondrial DNA (mtDNA). Oxidative stress damage was induced in RPE cells by hydrogen peroxide. Retinal cell apoptosis was detected by TUNEL staining. In vitro RPE cytotoxicity was assessed by lactate dehydrogenase (LDH) release. An in vitro phagocytosis assay was applied to elucidate the role of dysfunctional RPE cells in modulating inflammatory cytokine and nitric oxide (NO) production from bone marrow derived macrophages (BMMΦ).
Wortmannin inhibited autophagy in the retina, leading to cell death. In vitro, failure of autophagy in RPE cells caused by wortmannin resulted in dose and time-dependent cytotoxicity after treatment with retenone. Importantly, dysfunction of RPE cells caused by impaired autophagy or oxidative stress did not induce their production of pro-inflammatory cytokines IL-1β, IL-6 and NO. In contrast, in vitro phagocytosis experiments demonstrate that BMMΦ engulfed these impaired RPE cells and as a result produced dramatically increased levels of NO, IL-1β and IL-6. On the other hand, BMMΦ engulfing heat-induced necrotic RPE cells or macrophages co-cultured with normal RPE did not induce inflammatory responses, where NO and cytokine production was unchanged.
Our results demonstrate that the initiation of inflammation by macrophages is conditioned by the state of dysfunctional RPE. The data indicate a possible means by which impaired RPE autophagy may elicit immune responses via myeloid cells, shedding light on the early stages of AMD pathogenesis.
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