The Beta 2 Adrenergic Receptor Antagonist Timolol Improves Healing of Combined Burn and Radiation Wounds

Abstract

In a scenario involving a nuclear detonation during war or a terrorist attack, acute radiation exposure combined with thermal and blast effects results in severe skin injury. Although the cutaneous injury in such a scenario may not be lethal, it may lead to inflammation, delayed wound healing and loss of the skin barrier, resulting in an increased risk of infection. In this study, we tested the potential use of timolol, a beta-adrenergic receptor antagonist, to improve epidermal wound closure after combined burn and radiation injury using an ex vivo human skin culture model. Daily application of 10 μ M timolol after combined injury (burn and 10 Gy ex vivo irradiation) increased wound epithelialization by 5-20%. In addition, exposure to 10 Gy significantly suppressed epidermal keratinocyte proliferation by 46% at 48 h postirradiation. Similar to what has been observed in a thermal burn injury, the enzyme phenylethanolamine N-methyltransferase (PNMT), which generates epinephrine, was elevated in the combined thermal burn and radiation wounds. This likely resulted in elevated tissue levels of this catecholamine, which has been shown to delay healing. Thus, with the addition of timolol to the wound to block the binding of locally generated epinephrine to the beta-adrenergic receptor, healing is improved. This work suggests that by antagonizing local epinephrine action within the wound, a beta-adrenergic receptor antagonist such as timolol may be a useful adjunctive treatment to improve healing in the combined burn and radiation injury.

FIG. 1

Timolol promotes re-epithelialization in the combined burn/radiation wounds. Sections of human skin that received combined burn and radiation injury were incubated for 10 days, fixed, then H&E stained for histological analysis. Scale bars = 100 µm. Panels A and B: Partially re-epithelialized wounds. Arrows indicate the re-epithelialization. Panels C and D: Fully re-epithelialized wounds. Panel E: Positive control of fully re-epithelialized incisional wound. Panel F: Untreated human skin to present the epidermis. Yellow line indicates newly re-epithelialized dermal surface, underlying the injured and necrotic separated stratum corneum. The epidermis layer does not regenerate to full thickness under the culture conditions. Panel G: Boxplots of percentage healing in six treatment groups: burn; burn + timolol; burn + irradiation; burn + irradiation + timolol; irradiation + burn; irradiation + burn + timolol. Timolol-treated groups are shown as blue boxes to the right of their untreated controls. The horizontal line in the middle of each box represents the median, the lower and upper edges of the box represent the first and third quartiles, respectively, and the whiskers represent the smallest and largest nonoutlying observations, where an outlying observation (indicated with a circle) is defined here as any observation lying more than 1.5 interquartile ranges from the box.

FIG. 2

Radiation inhibits keratinocyte growth, and PNMT is elevated in the combined burn/radiation wounds. Panel A: Exposure (10 Gy) inhibited the growth of cultured keratinocytes at 48 h postirradiation compared to control. Immunohistochemistry was used to detect PNMT in the injured skin, and levels of intensity quantified in skin samples at day 10 (panel B). PNMT levels were increased in the burn, combined burn/ radiation and timolol-treated burn/radiation wounds compared to controls (*P < 0.05). Panels C–F: PNMT staining was observed in the epidermis (control, burn, burn/radiation wound and timolol-treated burn/radiation wound, respectively); wound sites are indicated by the arrows. The nonspecific staining of stratum corneum was not included in the quantification. Scale bar = 200 µm. Panel G: Schematic showing the regulation of beta 2 adrenergic receptors and wound healing.