ELIZABETH A. LEIBOLD, Ph.D.

LAB WEBSITE

B.S.
M.S.
Ph.D. Massachusetts Institute of Technology
Post-doctoral Associate, MIT

RESEARCH:

Our research is focused on understanding how cells respond to environmental and physiological stresses. Stresses such as toxic metals, reactive free radicals, and hypoxia (low oxygen) are deleterious to cells. To protect against stress, cells employ specialized defense mechanisms that allow them to survive during stress conditions. One stress that we have been studying is iron. Although iron is essential for growth and survival of most organisms, in excess free iron can be toxic. This toxicity is due to the ability of free iron to catalyze the formation of reactive oxygen species that damage lipids, proteins and DNA. Oxidative stress has been implicated in many diseases, including cancer and neurodegenerative diseases, as well as in aging. To prevent iron-catalyzed oxidative damage, cellular iron levels are carefully maintained by regulating the uptake and the storage of iron.

Cellular iron levels are monitored by iron sensor proteins known as iron-regulatory proteins 1 and 2 (IRP1 and 2). IRPs are cytosolic RNA-binding proteins that regulate the translation or stability of mRNAs encoding proteins involved in iron and energy homeostasis. IRPs bind to specific stem-loop structures, known as iron-responsive elements (IREs), that are located in the 5’- or 3’- untranslated regions of specific mRNAs, including the iron-storage protein ferritin, and the iron-uptake protein transferrin receptor. The binding of IRPs to 5’ IREs represses translation, whereas binding of IRPs to 3’ IREs stabilizes mRNA. Iron regulates the activity of IRP1: When iron is scarce, IRP1 binds to IREs, regulating translation or stability of IRE-mRNAs, whereas when iron is abundant, IRP1 forms an [4Fe-4S] cluster and is converted from an RNA-binding form to an aconitase. Aconitase is an enzyme that catalyzes the interconversion of citrate and isocitrate. IRP2 is also regulated by iron, however, unlike IRP1, IRP2 is regulated by iron-mediated proteolysis. By altering IRP1 and IRP2 expression by iron, IRE-mRNAs are coordinately regulated, leading to the maintenance of iron and energy homeostasis. In addition to iron, IRP1 and IRP2 activities are regulated by reactive oxygen species that are altered during oxidative stress and hypoxia.

How do IRP1 and IRP2 sense iron? Although IRP1 and IRP2 both regulate IRE-mRNAs, these proteins differ structurally, and iron regulates their activities by different mechanisms. In iron-replete cells, an [4Fe-4S] cluster assembles in IRP1 converting its from an RNA-binding form into an aconitase form. We are determining how the [4Fe-4S] cluster is assembled and to determine the function of the aconitase form of the protein. Unlike IRP1, IRP2 is degraded by the proteasome by a process involving iron-catalyzed oxidation. We are investigating the mechanism by which iron targets IRP2 degradation and identifying components involved in IRP2 degradation. We are using biochemical approaches to carry out these studies as well as using genetics models systems such as Caenorhabditis elegans and mice. Another question is why cells express two IRP1s each regulated by iron but by different mechanisms. One possibility is that IRP1 and IRP2 bind specific IRE-mRNAs whose expression is required during iron or oxidative stress. We are using a functional genomics screening approach to isolate novel IRP1 and IRP2 IRE-mRNAs.

Another area of interest is to determine the physiological consequences of alterations in IRP activity during hypoxia (low oxygen). Hypoxia is important for normal tissue physiology as well as being a component of several pathophysiological conditions, including heart and cerebrovascular diseases and tumor growth. Because iron and oxygen are intimately related, we are interested in determining the mechanisms regulating IRPs during hypoxia, and importantly to determine whether the regulation of IRPs is beneficial or detrimental for survival during hypoxic stress.

SELECT PUBLICATIONS:

Hanson, E. S. and Leibold, E. A. (1998) Regulation of iron regulatory protein 1 during hypoxia and hypoxia/reoxygenation. J. Biol. Chem. 273:7588 -7593.

Hanson, E.S. and Leibold, E.A. (1999) Regulation of iron regulatory proteins by reactive oxygen and nitrogen species. Gene Expression 7: 367-376, 1999.

Schneider, B. D. and Leibold, E.A. (2000) Regulation of mammalian iron metabolism. Current Opinion in Clinical Nutrition and Metabolic Care. 3:267-273.

Hanson, E. S., Foot, L.M. and Leibold, E.A. (1999) Hypoxia post-translationally activates iron regulatory protein 2. J. Biol. Chem. 274:5047-5052.

Leibold, E.A., Gahring, L. and Rogers, S. and (2001) Immunolocalization of iron regulatory proteins 1 and 2 in murine brain. Histochemistry and Cell Biology, 115:195-203.