Serine and 1-carbon metabolism are required for HIF-mediated protection against retinopathy of prematurity

We determined which metabolic pathways are activated by hypoxia-inducible factor 1-mediated (HIF-1-mediated) protection against oxygen-induced retinopathy (OIR) in newborn mice, the experimental correlate to retinopathy of prematurity, a leading cause of infant blindness. HIF-1 coordinates the change from oxidative to glycolytic metabolism and mediates flux through serine and 1-carbon metabolism (1CM) in hypoxic and cancer cells. We used untargeted metabolite profiling in vivo to demonstrate that hypoxia mimesis activates serine/1CM. Both [13C6] glucose labeling of metabolites in ex vivo retinal explants as well as in vivo [13C3] serine labeling of metabolites followed in liver lysates strongly suggest that retinal serine is primarily derived from hepatic glycolytic carbon and not from retinal glycolytic carbon in newborn pups. In HIF-1α2lox/2lox albumin-Cre-knockout mice, reduced or near-0 levels of serine/glycine further demonstrate the hepatic origin of retinal serine. Furthermore, inhibition of 1CM by methotrexate blocked HIF-mediated protection against OIR. This demonstrated that 1CM participates in protection induced by HIF-1 stabilization. The urea cycle also dominated pathway enrichment analyses of plasma samples. The dependence of retinal serine on hepatic HIF-1 and the upregulation of the urea cycle emphasize the importance of the liver to remote protection of the retina.

本研究明确了缺氧诱导因子1（hypoxia-inducible factor 1, HIF-1）介导的保护作用可激活新生小鼠体内对抗氧诱导视网膜病变（oxygen-induced retinopathy, OIR）的相关代谢通路；氧诱导视网膜病变是早产儿视网膜病变（retinopathy of prematurity, ROP）的实验对应模型，而早产儿视网膜病变是引发婴儿失明的首要病因。HIF-1可调控氧化代谢向糖酵解代谢的转变，并介导缺氧细胞与癌细胞内丝氨酸及一碳代谢（1-carbon metabolism, 1CM）的通量变化。本研究通过体内非靶向代谢组学（untargeted metabolite profiling）分析证实，缺氧模拟处理可激活丝氨酸/一碳代谢通路。通过离体视网膜外植体的[13C6]葡萄糖代谢物标记实验，以及体内[13C3]丝氨酸标记实验后对肝裂解物的检测结果，均可有力表明新生幼鼠的视网膜丝氨酸主要源自肝脏糖酵解碳源，而非视网膜自身的糖酵解碳源。在白蛋白-Cre介导的HIF-1α条件性敲除（HIF-1α2lox/2lox albumin-Cre-knockout）小鼠中，其体内丝氨酸/甘氨酸水平显著降低甚至接近零，进一步证实了视网膜丝氨酸的肝脏来源。此外，通过甲氨蝶呤（methotrexate）抑制一碳代谢，可阻断HIF-1介导的抗氧诱导视网膜病变保护作用。这一结果证实，一碳代谢参与了HIF-1稳定化所诱导的保护作用。血浆样本的通路富集分析（pathway enrichment analyses）结果同样以尿素循环（urea cycle）为核心富集通路。视网膜丝氨酸对肝脏HIF-1的依赖性，以及尿素循环的上调，均凸显了肝脏在视网膜远端保护作用中的重要性。