RPE utilizes proline to enhance nutrient availability to the retina

Abstract

Purpose : We reported that the retinal pigment epithelium (RPE) preferentially metabolizes proline to support the retina. RPE could also use lactate released by the retina, which spares glucose for the retina. In this study, we aim to understand the relationship of proline with lactate and glucose in RPE metabolism, and the metabolic communications between RPE and the retina using ¹⁵N labeled proline.

Methods : To study how RPE uses glucose, lactate and proline, we incubated mature human RPE cells with 5.5 mM glucose, 5 mM ¹³C lactate or 5 mM proline in DMEM. After incubation for 24 and 48 hours, we compared the metabolites in the remaining medium. We also incubated freshly isolated mouse retina and RPE/choroid with ¹⁵N labeled proline and other amino acids. To study the proline utilization in vivo, we retro-orbitally injected 150 mg/kg ¹⁵N proline in mice and collected retina and RPE at 15, 30 and 60 min to trace ¹⁵N proline and its derived metabolites.

Results : Lactate supplementation increases ~40% more glucose remaining in the medium compared to glucose alone group at 24 hours. The addition of proline further increases the amount of leftover glucose (~63%). The RPE consumes similar amounts of proline and lactate when they are provided in the same concentration. Unlike lactate, proline supplementation significantly increases non-essential amino acids (NEAAs) such as aspartate and glutamate in the medium. Incubation of RPE/choroid with ¹⁵N proline confirms that ¹⁵N is rapidly incorporated into NEAAs to release into the medium. Retina could not utilize ¹⁵N proline directly but consumes ¹⁵N proline derived NEAAs from the medium. Tracing in vivo shows the concentrations of ¹⁵N proline derived NEAAs peak at 30 min and quickly drop afterwards in RPE. However, in the retina, the concentrations of NEAAs were gradually elevated or maintained after 30 min, supporting that RPE uses proline to generate NEAAs for the retina.

Conclusions : Proline enhances glucose sparing by the RPE in the presence of lactate. In addition, RPE uses proline to synthesize NEAAs, thereby increasing the nutrients available to the retina.

This is a 2021 ARVO Annual Meeting abstract.