application) Iron is an important co-factor for many biological processes in all cells. In many cases iron is used as an electron donor or acceptor to catalyze enzymatic reactions. As part of heme, iron is employed by cytochromes, hemoglobin (Hgb), and many other enzymes. Nevertheless, iron is potentially toxic and can damage tissues by reacting with H2O2 to form free radicals that can directly damage the major biomolecules, including DNA, protein, and lipids. Hence, careful control of cellular iron levels is important. Dysregulation of iron-heme homeostasis is linked to many categories of human diseases, including anemias and porphyrias, liver diseases, inflammatory conditions, and neurodegenerative diseases. Accordingly, understanding iron homeostasis has important clinical implications. Simply understood, cellular iron homeostasis can be divided into iron influx and iron efflux. Many of the molecular details of iron influx have been clarified, but cellular iron efflux pathways are poorly understood. Here we propose to elucidate the molecular basis for cellular iron efflux. Recent studies of heme oxygenase-1 (H01) provide a clue to the molecular basis for cellular iron efflux. Thus, mice with a genetic deficiency of H01 have abnormally high tissue iron levels and low serum iron levels suggesting a role for HO1 in iron efflux. We have found that HO1 expression and activity are linked to cellular iron efflux and that facilitation of iron efflux by HO1 can protect cells from stress-induced death. In addition, we have recently identified a new ATP-dependent iron transporter that may be functionally linked to H01 activity and mediate the efflux of iron from cells. To understand the molecular basis of iron efflux from cells, we propose to ascertain whether heme oxygenase-2 (H02) and/or heme oxygenase-3 (HO3), like HO1, have a role in iron efflux from cells. We also propose to characterize an ATP-dependent iron transporter that may be functionally linked to HO1, HO2, and/or HO3 mediating cellular iron efflux. We will molecularly clone a cDNA encoding the ATP-dependent iron transporter.
