The Role of Heme Metabolism in Physio-Pathological Angiogenesis

Heme is an essential iron-containing porphyrin in endothelial cells (EC). Beyond its role as a cofactor in heme-containing proteins, it regulates cells signalling and endothelial energetic metabolism. Accordingly, heme levels are finely controlled at multiple levels, including its synthesis and interlinked metabolic pathways. Heme is synthesized through a series of enzymatic reaction starting from aminolevulinic acid synthase 1 (ALAS1) which catalyses the condensation of succinyl- CoA and glycine to produce 5-aminolevulinic acid (ALA). Then, multiple porphyrin intermediates are formed until iron incorporation into protoporphyrin IX (PPIX) produces de novo protoheme. In this context, heme synthesis is tightly regulated to ensure adequate heme availability while preventing the accumulation of “toxic” porphyrin intermediates. Moreover, the choline importer Feline Leukemia Virus Subgroup C Receptor 1a (FLVCR1a) has been identified as a major controller of endothelial heme synthesis and dynamics in EC. Nevertheless, the role of de novo heme synthesis and heme homeostasis during angiogenesis remains poorly understood. Here, we show that genes involved in heme metabolism are upregulated during angiogenesis. To interfere with endothelial heme and/or porphyrin homeostasis multiple pharmacological approaches were exploited in EC. First, administration of hemopexin (HX), a heme export facilitator, promoted angiogenesis by stimulating endogenous heme biosynthesis. Administration of 5-aminolevulinic acid (ALA), the first intermediate of the heme synthesis pathway, caused porphyrin “overdrive” and impaired EC function, affecting both physiological and pathological retinal angiogenesis. Finally, FLVCR1a targeting in EC led to reduced phosphatidylcholine levels, dysregulated heme metabolism, and impaired cholesterol synthesis and homeostasis, partly dependent on heme modulation. Moreover, rewired lipid membrane composition due to FLVCR1a targeting was associated with increased Notch signalling and impaired retinal and aberrant tumor angiogenesis. Collectively, these findings demonstrate the essential requirement of heme synthesis during angiogenesis, uncovering the detrimental role of endogenous porphyrins in EC. Moreover, these data uncover a previously unrecognized metabolic interplay between heme and lipid metabolism that could be exploited to hamper angiogenesis. In this view, ALA treatment and the genetic targeting of FLVCR1a effectively hampered pathological retinal and tumor angiogenesis, respectively. Finally, targeting heme metabolism might represent a promising approach to treat human diseases characterized by aberrant neovascularization