Our research is aimed at elucidating the mechanisms by which mammalian iron homeostasis is maintained in response to specific physiological and pathological situations. Iron is crucial to cell viability because it is a component of proteins that function in a number of physiological processes including respiration and cell division. However, excess iron can be toxic because it participated in the production of potentially lethal oxidizing agents. Mammals use a number of specific proteins to promote the specific and safe transport, uptake and storage of iron.
Cellular iron homeostasis is modulated through changes in synthesis of proteins involved in the uptake (transferrin receptor, TfR), storage (H- and L-ferritin), and utilization (erythroid 5-aminolevulinate synthase, eALAS) of this essential mineral. Synthesis of these proteins is regulated by cytosolic RNA binding proteins, the iron regulatory proteins (IRPs). Under low iron conditions IRPs bind stem-loop structures (IREs) in TfR, ferritin, eALAS and other mRNAs thereby regulating the translation or stability of the affected mRNA. In this manner iron, and other factors that regulate IRP activity, alter the uptake and metabolic fate of iron. IRPs are considered to be central regulators of iron metabolism.
Our studies fall into three areas. First, a major thrust of our research involves understanding how hormones and growth factors modulate iron metabolism through activation of signaling cascades that affect the phosphorylation state of IRP1 or IRP2. We have shown that the activity of both IRPs is regulated by changes in their phosphorylation status through the action of protein kinase C (PKC) or other protein kinases. We are examining how phosphorylation affects a number of aspects of IRP function. For example, with IRP1 we are investigating how phosphorylation affects the assembly or disassembly of its Fe-S cluster in the protein. The presence of absence of the Fe-S cluster is the primary mechanism for regulating whether or not IRP1 binds to mRNAs. We believe that phosphorylation enhances the susceptibility of the Fe-S cluster of IRP1 to physiological destabilizing agents such as superoxide anion and nitric oxide. Taken together, it is apparent that phosphorylation of IRP1 provides a mechanism through which extracellular agents modulate cellular iron metabolism by altering the set-point at which IRP1 responds to iron.
Second, there are up to 8 mRNA targets for IRPs and we are investigating how differences in RNA and protein structure allow IRPs to selectively affect the utilization of these mRNAs. Included in this approach are studies aimed at determining the physiological effects of IRP-mediated changes in mitochondrial aconitase abundance.
Third, we are developing transgenic animal models for studying the functions of IRP in modulating the expression of specific IRE-containing mRNAs including L-ferritin and mitochondrial aconitase.