Identification of a mammalian mitochondrial porphyrin transporter

Abstract

The movement of anionic porphyrins (for example, haem) across intracellular membranes is crucial to many biological processes, but their mitochondrial translocation and coordination with haem biosynthesis is not understood. Transport of porphyrins into isolated mitochondria is energy-dependent, as expected for the movement of anions into a negatively charged environment. ATP-binding cassette transporters actively facilitate the transmembrane movement of substances. We found that the mitochondrial ATP-binding cassette transporter ABCB6 is upregulated (messenger RNA and protein in human and mouse cells) by elevation of cellular porphyrins and postulated that ABCB6 has a function in porphyrin transport. We also predicted that ABCB6 is functionally linked to haem biosynthesis, because its mRNA is found in both human bone marrow and CD71+ early erythroid cells (by database searching), and because our results show that ABCB6 is highly expressed in human fetal liver, and Abcb6 in mouse embryonic liver. Here we demonstrate that ABCB6 is uniquely located in the outer mitochondrial membrane and is required for mitochondrial porphyrin uptake. After ABCB6 is upregulated in response to increased intracellular porphyrin, mitochondrial porphyrin uptake activates de novo porphyrin biosynthesis. This process is blocked when the Abcb6 gene is silenced. Our results challenge previous assumptions about the intracellular movement of porphyrins and the factors controlling haem biosynthesis.

Functional Coupling of ATP-binding Cassette Transporter Abcb6 to Cytochrome P450 Expression and Activity in Liver

“…This observation is relatively consistent with recent reports of growth and proliferation defects associated with loss of Abcb6 function in humans and in zebrafish (22). Although Abcb6 is capable of transporting heme precursors into the mitochondria (17,18) and plays a role in regulating porphyrin synthesis in vitro (17,26), Abcb6 deficiency in vivo did not have a significant impact on hepatic porphyrin or heme levels. This suggests that in vivo Abcb6 might not be important for basal porphyrin synthesis.…”

“…1f). These results suggest that although Abcb6 is capable of transporting heme precursors into the mitochondria (18,27) and plays a role in regulating porphyrin synthesis in vitro (17,18,26), Abcb6 deficiency in vivo does not have a significant impact on basal hepatic porphyrin or heme levels. These findings are consistent with recent reports that demonstrate a lack of functional association between Abcb6 and basal porphyrin synthesis in erythroid cells in vivo (18).…”

“…Abcb6 has been suggested to play a role in porphyrin synthesis (17). Thus, the significance of Abcb6 expression to hepatic heme synthesis was evaluated in Abcb6 Ϫ/Ϫ mice.…”

“…ABCB6 was initially characterized as a mitochondrial transporter capable of regulating porphyrin synthesis (17). However, recent studies suggest that loss of Abcb6 expression in mice does not affect basal porphyrin synthesis (18).…”

Regulation of heme biosynthesis and transport in metazoa

“…The enzyme receiving CPgenIII, coproporphyrinogen oxidase (CPOX), is anchored on the mitochondrial matrix with the active site in the intermembrane space [62]. ABCB6 was proposed to be responsible for transporting CPgenIII across the mitochondrial outer membrane [63]. However, a more recent study revealed that ABCB6 specifies the blood group Langereis and is not required for erythropoiesis [64].…”