The effect of K(+) on caspase activity of corneal epithelial cells exposed to UVB

UV Protection in the Cornea: Failure and Rescue

Simple Summary

The sun is a deadly laser, and its damaging rays harm exposed tissues such as our skin and eyes. The skin’s protection and repair mechanisms are well understood and utilized in therapeutic approaches while the eye lacks such complete understanding of its defenses and therefore often lacks therapeutic support in most cases. The aim here was to document the similarities and differences between the two tissues as well as understand where current research stands on ocular, particularly corneal, ultraviolet protection. The objective is to identify what mechanisms may be best suited for future investigation and valuable therapeutic approaches.

Abstract

Ultraviolet (UV) irradiation induces DNA lesions in all directly exposed tissues. In the human body, two tissues are chronically exposed to UV: the skin and the cornea. The most frequent UV-induced DNA lesions are cyclobutane pyrimidine dimers (CPDs) that can lead to apoptosis or induce tumorigenesis. Lacking the protective pigmentation of the skin, the transparent cornea is particularly dependent on nucleotide excision repair (NER) to remove UV-induced DNA lesions. The DNA damage response also triggers intracellular autophagy mechanisms to remove damaged material in the cornea; these mechanisms are poorly understood despite their noted involvement in UV-related diseases. Therapeutic solutions involving xenogenic DNA-repair enzymes such as T4 endonuclease V or photolyases exist and are widely distributed for dermatological use. The corneal field lacks a similar set of tools to address DNA-lesions in photovulnerable patients, such as those with genetic disorders or recently transplanted tissue.

Ions, the Movement of Water and the Apoptotic Volume Decrease

The movement of water across the cell membrane is a natural biological process that occurs during growth, cell division, and cell death. Many cells are known to regulate changes in their cell volume through inherent compensatory regulatory mechanisms. Cells can sense an increase or decrease in their cell volume, and compensate through mechanisms known as a regulatory volume increase (RVI) or decrease (RVD) response, respectively. The transport of sodium, potassium along with other ions and osmolytes allows the movement of water in and out of the cell. These compensatory volume regulatory mechanisms maintain a cell at near constant volume. A hallmark of the physiological cell death process known as apoptosis is the loss of cell volume or cell shrinkage. This loss of cell volume is in stark contrast to what occurs during the accidental cell death process known as necrosis. During necrosis, cells swell or gain water, eventually resulting in cell lysis. Thus, whether a cell gains or loses water after injury is a defining feature of the specific mode of cell death. Cell shrinkage or the loss of cell volume during apoptosis has been termed apoptotic volume decrease or AVD. Over the years, this distinguishing feature of apoptosis has been largely ignored and thought to be a passive occurrence or simply a consequence of the cell death process. However, studies on AVD have defined an underlying movement of ions that result in not only the loss of cell volume, but also the activation and execution of the apoptotic process. This review explores the role ions play in controlling not only the movement of water, but the regulation of apoptosis. We will focus on what is known about specific ion channels and transporters identified to be involved in AVD, and how the movement of ions and water change the intracellular environment leading to stages of cell shrinkage and associated apoptotic characteristics. Finally, we will discuss these concepts as they apply to different cell types such as neurons, cardiomyocytes, and corneal epithelial cells.

Damaging Effects of Ultraviolet Radiation on the Cornea

The cornea sits at the anterior aspect of the eye and, like the skin, is highly exposed to ultraviolet radiation (UVR). The cornea blocks a significant proportion of UVB from reaching the posterior structures of the eye. However, UVA can penetrate the full thickness of the cornea, even reaching the anterior portion of the lens. Epidemiological data indicate that UVR is a contributing factor for a multitude of diseases of the cornea including pterygium, photokeratitis, climatic droplet keratopathy and ocular surface squamous neoplasia (OSSN), although the pathogenic mechanisms of each require further elucidation. UVR is a well-known genotoxic agent, and its effects have been well characterized in organs such as the skin. However, we are only beginning to identify its effects on the cornea, such as the UVR signature C → T and CC → TT transversions identified by sequencing and increased proliferative and shedding rates in response to UVR exposure. Alarmingly, a single low-dose exposure of UVR to the cornea is sufficient to elicit genetic, molecular and cellular changes, supporting the consideration of using protective measures, such as wearing sunglasses when outdoors. The aim of this review was to describe the adverse effects of UVR on the cornea.

Ultraviolet Irradiation-Induced Volume Alteration of Corneal Epithelial Cells

Purpose

The purpose of the study is to understand how extracellular stresses, such as ultraviolet (UV) irradiation, affect corneal epithelial cells. Cell volume changes, damage to corneal epithelial integrity, and cellular responses were assessed after exposure to UVC stresses.

Methods

Primary human and rabbit corneal epithelial cells were exposed to UVC light in culture conditions. Ultraviolet C irradiation–induced changes in cell size and volume were measured by real-time microscopy and self-quenching of the fluorescent dye calcein, respectively. The effects of UVC irradiation on Src and focal adhesion kinase (FAK) phosphorylation and FAK-dependent integrin signaling were detected by ELISA, immunoblotting, and immunostaining.

Results

Ultraviolet C irradiation induced both size and volume shifts in human and rabbit corneal epithelial cells. Ultraviolet C irradiation-induced decrease of cell volume elicited activation of Src and FAK, characterized by increased phosphorylations of Src^Y416, FAK^Y397, and FAK^Y925. In addition, immunostaining studies showed UVC irradiation–induced increases in phosphorylation of FAK and formation of integrin β5 clustering. Application of Kv channel blockers, including 4-aminopyridine (4-AP), α-DTX, and depressing substance-1 (BDS-1), effectively suppressed UVC irradiation–induced cell volume changes, and subsequently inhibited UVC irradiation–induced phosphorylation of Src/FAK, and formation of integrin β5 clustering, suggesting UVC irradiation–induced volume changes and Src/FAK activation. Hyperosmotic pressure–induced volume decreases were measured in comparison with effects of UVC irradiation on volume and Src/FAK activation. However, Kv channel blocker, 4-AP, had no effect on hyperosmotic pressure–induced responses.

Conclusions

The present study demonstrates that UVC irradiation–induced decreases in cell volume lead to Src/FAK activation due to a rapid loss of K ions through membrane Kv channels.