Why Do Erythroid Cells Need to Export Heme?

Abstract SCI-24

Red cells are anucleated sacs involved in gas exchange and most importantly oxygen delivery. Over 90% of their protein content is hemoglobin. We inadvertently determined that the feline leukemia virus C receptor (FLVCR), a cytoplasmic membrane transporter that is required for proerythroblast survival, exports heme (1). Mice in which Flvcr is ablated die in utero from erythroid failure and adult mice in which Flvcr is deleted develop a severe macrocytic anemia (2). These observations led to an important question that will be addressed in this session: Why should developing erythroid cells need to export heme when its adequate supply in mature red cells is so critical? Although FLVCR is ubiquitously expressed in different human tissues by northern analyses and RT-PCR, western blot data suggest it is most abundant in liver, duodenum, brain, placenta, spleen, marrow, and uterus, which are sites of high heme synthesis or flux, and thus are cells that might need protection from heme toxicities. FLVCR’s impact on erythropoiesis is cell-autonomous – normal mice transplanted with Flvcr-deleted marrow develop severe macrocytic anemia and Flvcr-deleted mice transplanted with wild-type (WT) marrow cells regain normal erythropoiesis. Erythropoiesis is also rescued when mice are transplanted with Flvcr-deleted marrow transduced with a retroviral vector encoding huFLVCR. These observations led to the hypothesis that at the time during erythroid differentiation when heme synthesis initiates and before globin synthesis (which is transcriptionally (via Bach1) and translationally (via HRI regulated by heme) is significantly robust, cells must export heme so that heme synthesis does not exceed use. Erythroid cells may be especially vulnerable to heme toxicity as ALA-S2, the first and rate-limiting step of heme synthesis (in contrast to ALA-S1, the isoform of nonerythroid cells) is not inhibited by heme. Importantly, although FLVCR clearly exports heme from cells, we still do not have definitive proof that failure to export heme from proerythroblasts causes macrocytic anemia. Genetic studies to directly test this are under way and results will be presented. Data from competitive reconstitution studies with Flvcr-deleted and WT marrow will also be presented. When the cells are transplanted into irradiated WT mice at ratios from 50:50 to 95:5, the recipient’s red cells, as expected, are exclusively WT in phenotype. Few Ter119 + Flvcr-deleted cells are seen and CD71+ Ter119 low (proerythroblasts) have increased apoptosis. Of note, the T cell differentiation of Flvcr-deleted cells is also blocked at a specific stage (CD4, CD8 double positive cells in thymus) and interestingly, platelet production is increased. In preliminary studies, it appears that the enhanced platelet production occurs at the level of megakaryocytes (increased ploidy) and does not result from biased decision-making of a common BFU-meg progenitor. Given the emerging links between metabolism and cell cycle and recognizing that heme synthesis begins with succinyl CoA, a Krebs cycle intermediate, it seems likely that understanding FLVCR biology and heme trafficking will provide novel insights into these lineage and stage-specific decisions.