Reductive carboxylation is a major metabolic pathway in the retinal pigment epithelium

Significance

In the vertebrate eye, a monolayer of cells, called the retinal pigment epithelium (RPE), is between the choroidal blood supply and the retina. The RPE provides metabolic support for the retina, including delivery of glucose and other nutrients. Here, we show that reductive carboxylation of α-ketoglutarate, a type of metabolism that supports growth and survival of cancer cells, is a prominent feature of RPE cells. We show that extreme oxidative stress can overwhelm the reductive carboxylation pathway. However, we also found that the RPE can be protected from extreme oxidative stress by supplementation with an NAD ⁺ precursor or α-ketoglutarate.

Abstract

The retinal pigment epithelium (RPE) is a monolayer of pigmented cells that requires an active metabolism to maintain outer retinal homeostasis and compensate for oxidative stress. Using ¹³C metabolic flux analysis in human RPE cells, we found that RPE has an exceptionally high capacity for reductive carboxylation, a metabolic pathway that has recently garnered significant interest because of its role in cancer cell survival. The capacity for reductive carboxylation in RPE exceeds that of all other cells tested, including retina, neural tissue, glial cells, and a cancer cell line. Loss of reductive carboxylation disrupts redox balance and increases RPE sensitivity to oxidative damage, suggesting that deficiencies of reductive carboxylation may contribute to RPE cell death. Supporting reductive carboxylation by supplementation with an NAD ⁺ precursor or its substrate α-ketoglutarate or treatment with a poly(ADP ribose) polymerase inhibitor protects reductive carboxylation and RPE viability from excessive oxidative stress. The ability of these treatments to rescue RPE could be the basis for an effective strategy to treat blinding diseases caused by RPE dysfunction.