Epidermaltrans-urocanic acid and the UV-A-induced photoaging of the skin

Abstract

The premature photoaging of the skin is mediated by the sensitization of reactive oxygen species after absorption of ultraviolet radiation by endogenous chromophores. Yet identification of UV-A-absorbing chromophores in the skin that quantitatively account for the action spectra of the physiological responses of photoaging has remained elusive. This paper reports that the in vitro action spectrum for singlet oxygen generation after excitation of trans-urocanic acid mimics the in vivo UV-A action spectrum for the photosagging of mouse skin. The data presented provide evidence suggesting that the UV-A excitation of trans-urocanic acid initiates chemical processes that result in the photoaging of skin.Most of the visible signs of aging result from chronic exposure of the skin to ultraviolet radiation (1-2). Unlike chronologically aged skin that results from a general atrophy and a gradual decline in the production of the dermal matrix (3), UV-A (320-400 nm) photoaged skin is characterized by a gross increase in the elastic fibers (elastin, fibrillin, and desmosine) of the skin replacing the collagenated dermal matrix (elastosis) (4-5), an increase in glycosaminoglycans (4-5), collagen cross-linking, epidermal thickening (4-6), and an increase in the number of dermal cysts (7). The deep lines, leathered appearance, and the sagging of the skin surface typically associated with ''old age'' are thought to result from UV-induced photodamage to the skin and to occur over the course of a lifetime (3). Although obvious differences between photoaged and chronologically aged skin exist, the anatomic basis of the visible signs of photoaging is not understood fully (4, 8). Absorption of UV-A must induce photobiologic effects within the skin that lead to the visible and histological differences of photoaged skin, and, although the mechanisms by which UV-A-induced photodamage occur have not been completely determined, reactive oxygen species are postulated to play a role (9). The natural shift toward a more prooxidant state in chronologically aged skin then could be exacerbated by absorption of UV-A radiation by endogenous chromophores like NADH͞NADPH (10-14), tryptophan (15), and riboflavin (9), which then sensitize the formation of reactive oxygen species. Solar radiation has been shown both to reduce the antioxidant population in the skin (16) and to sensitize the production of reactive oxygen species such as singlet oxygen, hydrogen peroxide, and the superoxide anion (9-19), increasing the potential for reactions like the oxidation of lipids and proteins (17) that influence the degree of cross-linking between collagen and other proteins (9) within the skin.The first step toward identifying the chromophore(s) responsible for physiological changes, such as those seen in photoaged skin, is to match the action spectrum for the physiological change with the absorption spectrum of the chromophore (20). Once such a comparison is made, the pathways that lead to the physiological change can be unraveled. For example, the epid...

Intrinsic and extrinsic factors in skin ageing: a review

“…Hanson [56] Lysosyme deposition on elastic fibres Increased Modest Gilchrest [20] Collagen production Decrease in amounts of mature collagen…”

“…UV light also reduced the quantity of epidermal Langerhans cells, while it induced proliferation of suppressor T cells, facilitating tumour induction [21,43]. Although plasma concentration of retinol increases with age, within the epidermis, vitamin A is destroyed by sun exposure [56].…”

Potential Use of Spin Traps to Control ROS in Antipollution Cosmetics—A Review

“…After UV-A irradiation absorption, endogenous photosensitizes such as flavins [45], NADH/ NADPH [46], urocanic acid [47], and some sterols [48] present in tissue are converted to their long-lived triplet state, which in turn can transfer energy to oxygen molecules to form an energetically excited and highly reactive singlet oxygen. Additionally, a novel class of UV-A photo-sensitizers comprised of skin biomolecules based on 3-hydroxypyridine derivatives such as enzymatic collagen cross-links, B6 vitamin, and glycation end products in chronologically aged skin are capable of skin photooxidative damage [49].…”

Solar radiation and human health

“…Growth, differentiation and Figure 13. Action spectra for elastosis in hairless mice skin according to Bissett et al [227], Wulf et al [228] and Kligman and Sayre [229], for skin sagging in hairless mice skin according to Bissett et al [227] and for singlet oxygen generation after excitation of trans-urocanic in vitro (Hanson and Simon) [226]. Figure 14.…”

“…Thus, UVA-absorbing chromophores are likely to be involved. UCA, porphyrins and riboflavin are candidates, and photosensitization processes involving oxygen may participate [226].…”