Investigation of the Role of Heme Oxygenase 1 During Erythroid Differentiation.

Abstract 1997

Poster Board I-1019

Heme is a complex of iron with protoporphyrin IX that is essential for the function of all aerobic cells. However, if left unguarded, non-protein-bound heme promotes free radical formation, resulting in cell damage and tissue injury. The highest amounts of organismal heme (75-80%) are present in circulating red blood cells (RBC) whose precursors synthesize heme with rates that are at least 1 order of magnitude higher than those in the liver (on the per cell basis), which is the second most active heme producer in the body. The only physiological mechanism of heme degradation is by heme oxygenases (HO1 and HO2) that catalyze the rate-limiting step in the oxidative degradation of heme and are, therefore, involved in the control of cellular heme levels. Red blood cells contain the majority of heme destined for catabolism; this process takes place in splenic and hepatic macrophages following erythrophagocytosis of senescent RBC. Although the heme-inducible HO isoform, HO1, has been extensively studied in hepatocytes and many other non-erythroid cells, virtually nothing is known about the expression of HO1 in developing RBC. Similarly, it is unknown whether HO1 plays any role in erythroid cell development under physiological or pathophysiological conditions. In this study we have shown that HO1 protein is expressed in uninduced murine erythroleukemic (MEL) cells and that its levels, somewhat surprisingly, do not decrease during DMSO-induced erythroid differentiation. Moreover, we demonstrated that heme significantly induces HO1 in both uninduced and induced MEL cells. Additionally, we investigated the effect of sodium arsenite (NaAsO₂), HO1 inducer, on heme and iron metabolism in MEL cells induced to erythroid differentiation. MEL cells treated with NaAsO₂ displayed a significant reduction in globin expression and increased ferritin levels. Moreover, NaAsO₂treatment decreased levels of transferrin receptor in cell membranes. These effects triggered by NaAsO₂ could be prevented by the addiction of tin-protophorphyrin (SnPP), HO1 activity inhibitor. Using a siRNA specifically targeting HO1, we observed an increase in globin expression together with a small decrease in the expressin of ferritin in DMSO-induced MEL cells. These results suggest that an as yet unknown mechanism exists to protect heme against endogenous HO1 action during erythroid differentiation. In summary, our results showing that NaAsO₂-induced HO1 in erythroid cells cause a defect in erythroid differentiation suggest that HO1 could play a role in some pathophysiological conditions such as thalassemias.