Effect of hypoxia on the binding and subcellular distribution of iron regulatory proteins

Iron regulatory proteins 1 and 2 (IRP1, IRP2) are key determinants of uptake and storage of iron by the liver, and are responsive to oxidative stress and hypoxia potentially at the level of both protein concentration and mRNA-binding activity. We examined the effect of hypoxia (1% O-2) on IRP1 and IRP2 levels (Western blots) and mRNA-binding activity (gel shift assays) in human hepatoma HepG2 cells, and compared them with HEK 293 cells, a renal cell line known to respond to hypoxia. Total IRP binding to an iron responsive element (IRE) mRNA probe was increased several fold by hypoxia in HEK 293 cells, maximally at 4-8 h. An earlier and more modest increase (1.5- to 2-fold, peaking at 2 h and then declining) was seen in HepG2 cells. In both cell lines, IRP1 made a greater contribution to IRE-binding activity than IRP2. IRP1 protein levels were increased slightly by hypoxia in HEK 293 but not in HepG2 cells. IRP1 was distributed between cytosolic and membrane-bound fractions, and in both cells hypoxia increased both the amount and IRE-binding activity of the membrane-associated IRP1 fraction. Further density gradient fractionation of HepG2 membranes revealed that hypoxia caused an increase in total membrane IRP1, with a shift in the membrane-bound fraction from Golgi to an endoplasmic reticulum (ER)-enriched fraction. Translocation of IRP to the ER has previously been shown to stabilize transferrin receptor mRNA, thus increasing iron availability to the cell. Iron depletion with deferoxamine also caused an increase in ER-associated IRP1. Phorbol ester caused serine phosphorylation of IRP1 and increased its association with the ER. The calcium ionophore ion-omycin likewise increased ER-associated TRP1, without affecting total IRE-binding activity. We conclude that IRP1 is translocated to the ER by multiple signals in HepG2 cells, including hypoxia, thereby facilitating its role in regulation of hepatic gene expression.