UCP2 Modulates Cell Proliferation through the MAPK/ERK Pathway during Erythropoiesis and Has No Effect on Heme Biosynthesis

Abstract

UCP2, an inner membrane mitochondrial protein, has been implicated in bioenergetics and Reactive Oxygen Species (ROS) modulation. UCP2 has been previously hypothesized to function as a facilitator of heme synthesis and iron metabolism by reducing ROS production. While UCP2 has been found to be induced by GATA1 during erythroid differentiation its role in erythropoiesis in vivo or in vitro has not been reported thus far. Here we report on the study of UCP2 role in erythropoiesis and the hematologic phenotype of UCP2 deficient mouse. In vivo we found that UCP2 protein peaks at early stages of erythroid maturation when cells are not fully committed in heme synthesis and then becomes undetectable at the reticulocyte stage. Iron incorporation into heme was unaltered in erythroid cells from UCP2 deficient mice. While heme synthesis was not influenced by UCP2 deficiency, mice lacking UCP2 had a delayed recovery from chemically induced hemolytic anemia. Analysis of the erythroid lineage from bone marrow and fetal liver revealed that in the UCP2 deficient mice the R3 (CD71high/Ter119high) population was reduced by 24%. The count of BFU-E and CFU-E colonies, scored in an erythroid colony assay, was unaffected, indicating an equivalent number of early erythroid progenitor cells in both UCP2 deficient and control cells. Ex-vivo differentiation assay revealed that UCP2 deficient c-kit+ progenitor cells expansion was overall reduced by 14% with population analysis determining that the main effect is at the R3 stage. No increased rate of apoptosis was found indicating that expansion rather than cell death is being compromised. Reduced expansion of c-kit+ cells was accompanied by 30% reduction in the phosphorylated form of ERK, a ROS dependent cytosolic regulator of cell proliferation. Analysis of ROS in UCP2 null erythroid progenitors revealed altered distribution of ROS resulting in 14% decrease in cytosolic and 32% increase in mitochondrial ROS. Restoration of the cytosolic oxidative state of erythroid progenitor cells by the pro-oxidant Paraquat reversed the effect of UCP2 deficiency on cell proliferation in in vitro differentiation assays. Together, these results indicate that UCP2 is a regulator of erythropoiesis and suggests that inhibition of UCP2 function may contribute to the development of anemia.