Control of intracellular heme levels: heme transporters and heme oxygenases

Abstract

Heme serves as a co-factor in proteins involved in fundamental biological processes including oxidative metabolism, oxygen storage and transport, signal transduction and drug metabolism. In addition, heme is important for systemic iron homeostasis in mammals. Heme has important regulatory roles in cell biology, yet excessive levels of intracellular heme are toxic; thus, mechanisms have evolved to control the acquisition, synthesis, catabolism and expulsion of cellular heme. Recently, a number of transporters of heme and heme synthesis intermediates have been described. Here we review aspects of heme metabolism and discuss our current understanding of heme transporters, with emphasis on the function of the cell-surface heme exporter, FLVCR. Knockdown of Flvcr in mice leads to both defective erythropoiesis and disturbed systemic iron homeostasis, underscoring the critical role of heme transporters in mammalian physiology. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Fig. 1. The heme biosynthesis pathway and putative mitochondrial transporters

The first step of heme synthesis occurs in the mitochondrial matrix with the condensation of succinyl-CoA and glycine by ALAS to generate ALA. ALA may be exported in exchange for glycine by an IM transporter SLC25A38. The mechanism of ALA export across the OM into the cytoplasm is unknown. ALA is converted to CPgenIII by four enzymatic reactions. CPgenIII is then transported back into the mitochondrial intermembrane space (IMS), possibly via ABCB6, where it is converted to PPgenIX by CPO. The conversion of PPgenIX to PPIX by PPO, its transport into the matrix and the addition of Fe⁺² by ferrochelatase (FECH) to generate heme are coupled processes, with transport of PPIX likely occurring via a proposed direct channeling through a complex of PPO-FECH (likely part of a larger complex with the mitochondrial iron importer, Mitoferrin, ABCB7 and ABCB10). The transfer of heme from the matrix to the IMS, for respiratory chain cytochrome assembly, may be mediated by ABCB10; the transporter responsible for its transfer across the outer mitochondrial membrane remains unidentified. See text for details.

Fig. 2. A generalized depiction of known and putative cell-surface and organelle-associated heme transporters

FLVCR is a cell-surface heme exporter. ABCG2 is a potential cell-surface heme exporter; HCP1 a putative heme importer. CD163 receptor-mediated endocytosis of the Hp:Hb complex suggests the existence of an endosomal/lysosomal heme exporter; HCP1 or HRG1 may fulfill these role(s). The transporters that mediate import of heme into peroxisomes and lysosomes for assembly of catalases and peroxidases are unknown (it is also unclear at present where final assembly of hemoproteins occurs). Heme is transferred into the nucleus in association with BVR, while cytoplasmic heme appears to be chaperoned by proteins such as glutathione S-transferase B and HBP23. See text for further details. Putative transporters of mitochondrial heme and heme biosynthesis intermediates are shown in detail in Fig. 1.

Fig. 3. FLVCR membrane topology

FLVCR is predicted to contain 12-transmembrane domains, with both N- and C-termini within the cell. It has YXXØ and di-leucine (LL) motifs in the N-terminal region that may allow proper membrane domain targeting and recycling. His and Tyr residues in the extracellular loops are predicted to be important for heme binding/transfer, whereas Tyr residues in the large intracellular loop may serve as targets of kinases and phosphatases. The 4-aa PDZ domain–binding motif at the C-terminal region is important for its interaction with the PDZ domain of the nonreceptor tyrosine phosphatase PTPN3.