Mitochondrial Heme Export Through FLVCR1b Controls Erythroid Differentiation.

Abstract 2090

Feline Leukemia Virus subgroup C Receptor 1 (FLVCR1) is a cell membrane heme exporter that contributes to maintain the balance between heme level and globin synthesis in erythroid precursors. Consistently, FLVCR1-null mice died in utero due to a failure of erythropoiesis¹. We previously reported the identification of a mitochondrial isoform of FLVCR1, named FLVCR1b, that was able to support fetal murine erythroid differentiation in the absence of the cell membrane isoform (herein called FLVCR1a)².

The aim of this work was to investigate the role of FLVCR1b during erythroid differentiation. To this end, overexpression and silencing experiments were performed. FLVCR1b overexpression promotes in vitro erythroid differentiation of K562 cells, as indicated by the increased transcription of globin mRNA and a higher percentage of benzidine positive cells, compared to control cells. On the contrary, FLVCR1a-overexpressing K562 cells were not able to differentiate. Interestingly, shRNA-mediated down-regulation of FLVCR1a led to the elevation of the percentage of benzidine positive cells compared to controls that further increased upon the stimulation of erythroid differentiation using sodium butyrate. A decrease of the percentage of benzidine positive cells was observed in K562 cells lacking both FLVCR1 isoforms compared to FLVCR1a-deficient cells. Moreover these cells were not able to differentiate upon stimulation with sodium butyrate. These results suggest a fundamental role of FLVCR1b during erythroid differentiation in vitro, supporting previous data obtained in the mouse model².

To understand the molecular mechanisms in which FLVCR1b is involved, we investigate whether FLVCR1b could affect heme export from mitochondria. The overexpression of FLVCR1b in HeLa cells led to increased intracellular heme content together with a strong induction of the heme degrading enzyme heme oxygenase 1 (HO-1) mRNA. Inhibition of the heme biosynthetic pathway using succinylacetone, completely prevented intracellular accumulation of heme observed when FLVCR1b was overexpressed. So, increased heme biosynthesis rate is responsible for the elevation of heme content. Accordingly, HeLa cells overexpressing FLVCR1b showed an alteration of heme biosynthesis enzymes and transporters. On the contrary, the specific loss of FLVCR1b using siRNA causes heme accumulation in mitochondria and a subsequent block of heme biosynthesis. These data are consistent with a role of FLVCR1b as a mitochondrial heme exporter. Similar results were also obtained in K562 cells thus suggesting that loss of FLVCR1b reduces the availability of heme for haemoglobin synthesis, a process essential during erythroid differentiation.

To assess whether mitochondrial heme accumulation due to the loss of FLVCR1b affect mitochondrial functionality, the mitochondrial Ca²⁺ response after agonist stimulation was monitored as a highly sensitive readout of mitochondrial state. It is well known that mitochondrial alterations cause defects in Ca²⁺ uptake by the organelle. The silencing of FLVCR1a and FVLCR1b in HeLa cells caused a significant reduction of Ca²⁺spike in the mitochondrial matrix evoked by agonist stimulation. These data suggested that when FLVCR1b is lost heme accumulates in mitochondria resulting in the alterations of mitochondrial functionality.

All together these data indicate that the impairment of erythroid differentiation observed in the absence of FLVCR1b is due to the block of heme export from mitochondria and a consequent impairment of mitochondrial functionality, which is essential for cell survival. These results, linking heme biosynthesis pathway to mitochondrial Ca²⁺signaling, will have broad implications in cellular metabolism.