Growth Factors in Aqueous Humor

Monday, October 20, 2014

Dear Editor: The aqueous humor plays an important function in the health of the anterior segment of the eye, serving to both nourish the eye and maintain the structural integrity of the anterior chamber. Although aqueous humor production and outflow has been extensively studied, little has been done to investigate its nourishing effects. Prior studies have only quantified electrolytes and basic molecules in the aqueous humor of normal humans. Although the protein concentration in the aqueous humor is only 0.2 mg/ml, proteins play an important role in the health of the anterior segment. More specifically, the interactions of different growth factors regulate most living cells in the eye along with the rest of the human body.1 With the advent of multiplex-bead testing, such as Luminex (Austin, TX), it is now possible to simultaneously test multiple factors on a small volume sample such as aqueous humor tap.2,3 Quantification of the normal levels of growth factors is important when trying to determine growth factor imbalances. With this knowledge, the aqueous humor can be tested to determine its role in anterior segment diseases in the future.

Aqueous humor was collected (100–200 _l) from 25 healthy and 25 diabetic patients undergoing cataract extraction. Patients in this study were treated with concordance to Health Insurance Portability and Accountability Act standards. The study was approved by the Texas Tech University Health Sciences Center institutional review board. The samples were then processed using Luminex multiplex-bead technology and tested for platelet-derived growth factor, brain-derived neurotrophic factor, glial cell-line derived neurotrophic factor, transforming growth factor _, epidermal growth factor, fibroblast growth factor, vascular endothelial growth factor, hepatocyte growth factor, tumor necrosis factor _, interferon _, interferon _, and interleukin 1, 2, 4, 5, 6, 8, and 10.

When tested for inflammatory markers, most of the samples showed undetectable levels of inflammation in both the control and the diabetic group. Growth factor testing did not show any statistically significant difference between the control group and the diabetic group. The results taken collectively did not reveal any obvious difference between the control and diabetic groups (P_0.47). None of the test for differences in individual growth factor was significant.

The advent of multiplex-bead technology provides a new tool in protein analysis. Recently, a few studies have used this technology to investigate intraocular disease states like Funding et al2 for inflammatory factors in failed penetrating grafts and Sato et al3 for 27 different factors in the vitreous of retinopathy of prematurity patients. Similarly, we had applied the same technology to investigate growth factors in diabetic eyes; however, there are some inherent shortcomings to this study due to the small sample size, sample dilutions, and the large number of factors tested. Despite these limitations, this study does provide some trends in the factors tested. As expected, inflammatory markers are not present in most eyes undergoing routine cataract surgery. Trends in growth factor levels in this study may be verified with a larger sampling size.4

This study provides the first quantification of a broad spectrum of growth factors and cytokines in the aqueous humor in patients undergoing cataract surgery in both diabetic and nondiabetic eyes. This information and technique will be useful for other future studies analyzing these aqueous components in various ocular disease states. Future studies utilizing multiplexbead technology will further elucidate the pathogenesis of ocular disease and allow more targeted therapies.

WILLIAM YANG, MD
Springfield, Illinois
JAY C. BRADLEY, MD
TED W. REID, PHD
DAVID L. MCCARTNEY, MD
Lubbock, Texas

References

1. Klenkler B, Sheardown H. Growth factors in the anterior segment: role in tissue maintenance, wound healing and ocular pathology. Exp Eye Res 2004;79:677– 88.

2. Funding M, Hansen TK, Gjedsted J, Ehlers N. Simultaneous quantification of 17 immune mediators in aqueous humour from patients with corneal rejection. Acta Ophthalmol Scand 2006;84:759–65.

3. Sato T, Kusaka S, Shimojo H, Fujikado T. Simultaneous analyses of vitreous levels of 27 cytokines in eyes with retinopathy of prematurity. Ophthalmology 2009;116: 2165–9.

4. Wilson SE, Bourne WM, Maguire LJ, et al. Aqueous humor composition in Fuchs’ dystrophy. Invest Ophthalmol Vis Sci 1989;30:449 –53.



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