McGahan, Christine, PhD
Research Professor in Pharmacology and Department Head
Title: Lens Metabolism

PhD: 1980, Pharmacology, Mount Sinai School of Medicine, City University of New York. Mentor, Dr. Peter Bentley
Post-doctoral: 1981-2, Harkness Eye Institute, Columbia University College of Physicians and Surgeons. Mentor, Dr. Laszlo Bito

E-mail: chris_mcgahan@ncsu.edu

Research Areas:

Regulation of Fe metabolism in the lens, the involvement of Fe in cataract formation, mechanisms underlying post-surgical regrowth of lens tissue, uveitis.

Current Research:

Cataracts are a major health problem worldwide. In the United States and in other developed countries this is primarily an economic issue since surgery is readily available and is such an effective treatment. However, this is a multi-billion dollar expense, making up 12% of the entire Medicare Budget. In addition, there are some significant side-effects to cataract surgery. In developing countries, cataracts are the leading cause of blindness.

Age-related cataractogenesis (the most common form) has a distinctive oxidative component. There are reduced levels of antioxidant compounds and increased disulfide bond formation resulting in protein cross-linking in cataractous lenses. Since oxidative damage results almost entirely from iron (Fe)-catalyzed free radical reactions, it is essential to understand the mechanisms underlying Fe metabolism in the lens. This is the central focus of our laboratory.

1. Regulation of Fe metabolism in the lens

Regulation of ferritin synthesis and protection against oxidative damage
Ferritin is an Fe storage protein found in all cells. Each ferritin molecule is capable of storing 4,500 atoms of Fe. It is well known that ferritin synthesis is regulated both at the transcriptional and more acutely at the translational level. The ferritin molecule is a multimeric protein consisting of 24 subunits. There are two different subunits called heavy (H) or light (L). The ratio of these subunits within this molecule is tissue specific implying some functional significance. Very little is currently known about how these ratios are regulated.

We have been concentrating on how the redox state of the cell affects translational regulation of ferritin synthesis and Fe storage in cultured lens epithelial cells. The lens exists in an unusual environment. It is an avascular tissue and nutrients are supplied by the aqueous humor which bathes the lens. The aqueous humor contains high concentrations of ascorbic acid (up to 20xs that of plasma) and relatively high concentrations of hydrogen peroxide. In addition, the lens contains extremely high concentrations of the important intracellular reducing agent, glutathione.

We have found that ascorbic acid greatly increases ferritin synthesis and concentration in lens cells. This may afford a protective effect by increasing the storage capacity for Fe thereby limiting its availability for catalyzing damaging free radical reactions. This may also provide a mechanism for the observed effectiveness of ascorbic acid in reducing cataract formation in humans on vitamin supplementation. We are continuing to define the mechanisms by which ascorbic acid and other reducing and oxidizing agents alter ferritin synthesis in these cells.

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- Alterations in H:L chain ratio in canine lens epithelial cells

In order to answer questions related to the effect of alterations in H:L chain ratios on Fe metabolism and the susceptibility of the lens to oxidative stress, we needed to devise a system by which we could carefully and separately control H and L chain expression. In order to do this we cloned and sequenced both H and L chain ferritin cDNA. We then determined that they assembled into holoferritin. An effective transfection protocol was devised and we were able to cause overexpression of H or L chain ferritin in our primary cultures of canine lens epithelial cells. This resulted in an alteration in Fe metabolism within the cells and also altered the cellular response to UV irradiation (details are presented in the IOVS, 2001 paper listed below). We intend to continue to pursue this overexpression model, detailing the effects of varying the H:L ratios more precisely.

- Hereditary Hyperferritinemia Cataract Syndrome (HHCS)

Since the original finding in 1995 of a human family with an autosomally dominantly inherited defect in regulation of ferritin synthesis which results in early bilateral cataract formation, many other families have been found. In this mutation, the normal negative regulation of L-chain synthesis is no longer active and L-chain synthesis proceeds without regulation resulting in accumulation of large amounts of ferritin in tissues and serum. However, cataract formation is the only overt pathology seen. Although the mechanism by which this dysregulation causes lenticular opacification is not known, it is likely to be either an accumulation of L-chain ferritin generally disrupting lens clarity, or it is possible that the altered ratio of H:L chain ferritin in the lens affects Fe storage and/or utilization. An intriguing finding was that the predominantly L-chain ferritin obtained from the lens of one of these patients contained little or no Fe. We are currently working to define the mechanism underlying this disorder since we have a system in which L-chain can be overexpressed in an uncontrolled manner, similar to that found in this mutation.

- Pathways for Fe uptake and disposition in lens cells

We have extensively studied uptake of Fe from transferrin as well as from low molecular weight sources and have found that there are source dependent pools of Fe within the cells (detailed in the Experimental Eye Research paper, 1998, listed below). We have developed a means of tracking the movement of 59Fe within the cells including its deposition into ferritin and into a low molecular weight pool. We continue to study the effects of oxidants and antioxidants on this movement (details are found in the Biochimica et Biophysica Acta paper, 2000, listed below).

Since the labile pool of Fe is most likely the source available for catalysis of oxidative damage. We are also developing a system for the measurement of a labile Fe pool in the cytoplasm utilizing a fluorescent dye, calcein. Addition of ascorbic acid has dramatic effects on the size of this pool. We are also examining the effects of overexpression of H or L chain on the dynamics of this pool.

2. Environmental effects on ferritin synthesis and Fe disposition within lens cells

We have found that low doses of UV light causes a dramatic increase in ferritin synthesis in lens cells. Higher doses are toxic. Intriguingly, UV light and ascorbic acid have additive effects on the synthesis of this important antioxidant protein. We are also examining the effects of UV light on Fe metabolism within these cells.

3. Fe dependent control of redox potential of the cell

We have some preliminary evidence that the Fe content of the cell may exert some control over the redox potential of the cell perhaps by altering NADPH production. This part of the project is just taking off.

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Collaborations:

Dr. Lloyd Fleisher
We have collaborated with Dr. Fleisher on his studies of the influence of cytokines on the uveitic response. Please see his website for more information.

Dr. Mike Davidson
We have collaborated with Dr. Davidson using his model for after-cataract formation in order to determine the mechanisms underlying this pathology in order to develop a means for preventing its occurrence. Please see his website for more information.

Representative Publications:

Goralska, M., J. Harned, L.N. Fleisher and M.C. McGahan. 1998. The effect of ascorbic acid and ferric ammonium citrate on iron uptake and storage in lens epithelial cells. Exp. Eye Res. 66:687-697.

Colitz, C.M.H., M.G. Davidson and M.C. McGahan. 1999. Telomerase activity in normal and cataractous lens epithelial cells. Exp. Eye Res. 69:641-649.

Colitz, C.M.H., D. Malarkey, M.J. Dykstra, M.C. McGahan, M.G. Davidson. 2000. Histologic and immunohistochemical characterization of lens capsular plaques in dogs with cataracts. Am. J. Veterinary Res. 61:139-143.

Fleisher, L.N., M.C. McGahan, and J.B. Ferrell. 2000. Rabbit pigmented ciliary epithelium produces interleukin-6 in response to inflammatory cytokines. Exp. Eye Res. 70: 271-279.

Goralska, M., B. Holley and M.C. McGahan. 2000. The effects of TEMPOL on ferritin synthesis and Fe metabolism. Biochim. Biophys. Acta. 1497: 51-60.

Davidson, M.G., D.K.Morgan, R.L. Malakof, I.M. Wormstone, J. Allen, M.C. McGahan. 2000. Canine lens capsular sac explants as a model for posterior capsular opacification. Graefe's Arch.Clin. Exp. Ophthalmol. 238:708-714.

Goralska, M., B.L. Holley, M.C. McGahan. 2001. Overexpression of H and L ferritin subunits in lens epithelial cells alters Fe metabolism and cellular response to UVB irradiation. Invest. Ophthalmol. Vis. Sci. 42:1721-1727.

Goralska, M., R. Dackor, B.L. Holley, M.C. McGahan. 2003. Alpha lipoic acid changes Fe uptake and storage in lens epithelial cells. Exp. Eye Res. 76(2):241-8.

Harned, J., A.M. Grimes and M.C. McGahan. 2003. The effect of UVB irradiation on ferritin subunit synthesis, ferritin assembly and Fe metabolism in cultured canine lens epithelial cells. Photochem. Photobiol. 77:440-445.

Goralska, M., B.L. Holley and M.C.McGahan. 2003. Identification of a mechanism by which lens epithelial cells limit accumulation of overexpressed H-chain ferritin. J. Biol. Chem. 278:42920-42926.

McGahan, M.C., Harned, J., Mukunnemkeril, M. Goralska, M., Fleisher, L. and Ferrell, J.B.2005. Iron alters glutamate secretion by regulating cytosolic aconitase activity. Am. J. Physiol.288: C1117-1124.

Goralska, M., S. Nagar, L.N. Fleisher and M.C. McGahan. 2005. Differential degradation of ferritin H- and L- chains: accumulation of L-ferritin in aging lens epithelial cells. Invest. Ophthalmol. Vis. Sci. 46: 3521-3529. Cover article.

Harned J, Fleisher LN, McGahan MC. 2006. Lens epithelial cells synthesize and secrete ceruloplasmin: Effects of ceruloplasmin and transferin on iron efflux and intracellular iron dynamics. Experimental Eye Research 83: 721-727.

Harned, J., Fleisher, L.N., McGahan, M.C. 2006. Lens epithelial cells synthesize and secrete ceruloplasmin: effects of ceruloplasmin and transferrin on iron efflux and intracellular iron dynamics. Exp. Eye Res. 83:721-727.

Goralska, M., Fleisher, L.N., McGahan, M.C. September 2007. Ferritin H- and L-Chains in Fiber Cell Canine and Human Lenses of Different Ages. IOVS, Vol. 48, No.9

Lall, M., J. Ferrell, S. Nagar, L.N. Fleisher, M.C. McGahan. 2008. Iron regulates L-cystinge uptake and glutathione levels in lens epithelial and retinal pigment epithelial cells by its effect on cytosolic aconitase. Invest. Opthalmol. Vis. Sci. 49:310-319 (Cover article)


Links:

McGahan Lab Personnel

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