Zinc is the second most abundant trace element in the human body after iron, and is present in high concentration in the human eye.
An international study has determined that, in line with other studies on zinc levels and age-related macular degeneration (AMD), elevated zinc may have an effect on AMD progression, but that the key to zinc's benefits may center on determining and controlling available zinc rather than increasing zinc supplementation.
Investigators, led by Po-Jung Pao, with the Department of Ocular Biology and Therapeutics and the Department of Structural and Molecular Biology at University College London, found new study results suggest that controlling levels of extracellular, biologically active, or "free" zinc could be key to harnessing zinc's benefits for AMD patients.
Zinc is the "second most abundant trace element in the human body" after iron and is present in high concentration in the human eye, Pao and colleagues noted. Ocular zinc is localized to the RPE/choroid complex, where a deficiency or overload of the element can lead to a “variety of problems."
Because zinc is often tightly bound to proteins, there is only a small amount of zinc ions that remain biologically available outside of those protein bonds.
Utilizing cultured primary human RPE cells in a zinc enriched environment, Pao and colleagues attempted to determine whether free zinc could "accelerate RPE differentiation, alter gene expression, and modify secretions of AMD-specific proteins" that reduce risk of AMD development and progression, investigators prosed. The RPE cells were differentiated into several zinc concentrations: 0, 75, 100, 125, 150, or 200 µm of zinc sulfate, for a 28-day period.
Twice weekly the researchers measured trans-epithelial resistance, toxicity, cell viability, pigmentation, coverage of pigment at different zinc concentrations, and free zinc levels. At the end of the 28-day period, the researchers performed an immunohistochemical analysis of the cells, and analyzed secreted proteins through mass spectrometry.
Pao and colleagues found a measurable difference in the maturation of RPE cells that developed in the presence of added zinc. The difference was most evident in an increase in transepithelial resistance (TER) in RPE cells exposed to 100 µm (2.7 nm free zinc)and 125 µm (3.4 nM free zinc) of added zinc, although Pao noted that TER development at 150 µm zinc (4.0 nM free zinc) and above showed no change in TER in comparison to RPE cells cultured in no additional zinc.
Study data also showed that pigmentation of RPE cells, a sign of healthy RPE cells, was more widespread in RPE cells in zinc-rich environments. Data also showed RPE cells in higher zinc environments secreted more proteins, such as apolipoprotein E (APOE), compliment factor H (CFH) and high temperature requirement serine protease A1 (HTRA1) which Pao wrote are genetically associated with AMD.
The research data revealed zinc supplements may be benefiting patients with AMD via a direct effect on RPE cells, and through affecting RPE cells zinc may influence photoreceptors and choroidal micro vessels in AMD eyes. Pao believed that changes in TER may also offer benefits related to the outer blood-retinal barrier. Zinc's effects on pigmentation of the RPE were also significant.
"Increased availability of zinc appears to be directly involved in the development and probably maintenance of a mature and healthy RPE," investigators wrote.
Pao and colleagues suggested the availability and concentration of free zinc that is contributing to RPE health, and that "the challenge for AMD research now is to determine the 'normal' free zinc concentration" in healthy eyes and help regulate zinc to optimal healthy levels in AMD eyes.
The study, "The effects of zinc supplementation on primary human retinal pigment epithelium,” was published online in Journal of Trace Elements in Medicine and Biology.