9-Hydroxyoctadecadienoic acid plays a crucial role in human skin photoaging.

Human skin is regularly exposed to ultraviolet (UV) rays from sunlight, leading to photoaging, which differs from intrinsic aging. Although the acute effects of UV exposure have been extensively studied, limited research has addressed the long-term consequences of chronic UV exposure. This study aimed to investigate the underlying causes of chronic photoaging. A questionnaire-based assessment of sunlight exposure was conducted among volunteers in their 20s and 50s, and the stratum corneum of their skin was analyzed for bioactive lipid content. Volunteers were categorized into low and high UV exposure groups based on the questionnaire scores. The analysis results revealed a significant increase in 9-hydroxyoctadecadienoic acid (9-HODE) levels in the skin of individuals in their 50s with high UV exposure. However, UV exposure did not affect 9-HODE levels in the skin of individuals in their 20s. In vitro experiments further indicated that 9-HODE contributes to chronic inflammation, pigmentary changes, and extracellular matrix alterations during photoaging. Specifically, 9-HODE stimulated cytokine production [interleukin-6 (IL6), IL8, and granulocyte-macrophage colony-stimulating factor (GM-CSF)] and reduced dickkopf-1 (DKK1) production in keratinocytes. In fibroblasts, 9-HODE stimulated matrix metalloproteinase-1 (MMP1) and MMP3 production while reducing collagen I (COL1) production. The expression of G2A, the receptor for 9-HODE, was also confirmed in fibroblasts, suggesting that 9-HODE exerts its effects via G2A, as observed in keratinocytes. Overall, these findings indicate that 9-HODE is a mediator of chronic photoaging and highlight its potential significance in photoaging prevention.

Skin lightness affects ultraviolet A-induced oxidative stress: Evaluation using ultraweak photon emission measurement.

The human skin is usually exposed to ultraviolet A (UVA) in the sunlight and experiences oxidative stress associated with skin disorders and aging. Although oxidative stress caused by UVA exposure is assumed to be dependent on skin colour, few studies have demonstrated this dependency. We investigated the effects of skin colour on UVA-induced oxidative stress using ultraweak photon emission (UPE) generated from the skin during oxidation processes. The UPE intensities of skin samples were detected using a photomultiplier tube every second without any labelling. We irradiated skin tissue of different colours with UVA and measured UPE over time. UVA-induced UPE could be detected from immediately after irradiation to 2 h after irradiation, indicating persistent oxidative stress. Skin lightness (L*) positively correlates with UPE intensity. Lighter-coloured skin exhibited more UVA-induced UPE, indicating higher oxidative stress. Additionally, oxidative stress persisted significantly more in lighter skin compared with darker skin. Skin tissues exhibited pigment darkening after UVA irradiation. Our results suggest that skin lightness affects oxidative stress induced by UV irradiation. Our study demonstrated the relationship between skin lightness and UVA-induced oxidative stress for the first time and offers new photodermatological insights into the human skin.

Blue light-induced lipid oxidation and the antioxidant property of hypotaurine: evaluation via measuring ultraweak photon emission.

The effects of blue light on human body have attracted attention. The human skin in contact with the outside environment is often exposed to blue light, and the effects of this exposure remain to be fully determined. Therefore, in this study, we investigated the effect of blue light, at the intensity typically found in sunlight, on lipids in the skin from an oxidation perspective. Peroxide value (POV) and ultraweak photon emission (UPE) measurements were conducted to evaluate lipid oxidation. Our results confirmed that blue light irradiation induced lipid oxidation, similar to ultraviolet A (UVA) irradiation. Also, the effects of various reagents on the blue light-induced UPE were evaluated; however, the results differed from those of the DPPH radical-scavenging ability. We speculated that this is due to the difference in the evaluation principle; nevertheless, among reagents, hypotaurine not only showed a high antioxidant effect but was also more effective against blue light-induced oxidation than UVA. Based on the difference in the antioxidant effect of the lipid sample in this study, the oxidation reaction induced by blue light may be different from the UVA-induced reaction. Our study provides new insights into the effects of blue light on lipids in the human skin, thereby promoting research regarding photooxidation.

Ultraviolet A irradiation induces ultraweak photon emission with characteristic spectral patterns from biomolecules present in human skin.

Oxidative stress is associated with photoaging of the skin as well as with skin cancer, and is therefore, critical to monitor. Ultraweak photon emission (UPE) is extremely weak light generated during the oxidative process in the living body and has been used as a non-invasive and label-free marker for the evaluation of oxidative stress. However, the mechanism of UPE generation is not clear. Therefore, we aimed to elucidate the molecular mechanism underlying UPE generation by analyzing the spectra of UPE generated from biomolecules in the skin during ultraviolet A (UVA) exposure. The spectra of UVA-induced UPE generated from linoleic acid, linolenic acid, elastin, phospholipids, and 5,6-dihydroxyindole-2-carboxylic acid were measured, and the spectrum of human skin tissue was also obtained. The spectral patterns varied for the different biomolecules and the peaks were distinct from those of the skin tissue. These results suggested that the UPE generated from skin tissue is a collection of light emitted by biomolecules. Moreover, we proposed that UPE is generated through a photosensitization reaction and energy transfer. The identified characteristic spectral patterns of UPE can be useful to elucidate UVA-induced oxidative stress in the skin, with implications for prevention and treatment of photoaging and skin diseases.

Oxidative stress in human facial skin observed by ultraweak photon emission imaging and its correlation with biophysical properties of skin.

Oxidative stress is associated with skin ageing and disease in humans. However, it is difficult to evaluate the effects of oxidative stress on the skin in vivo using conventional invasive methods. In this study, we performed two-dimensional imaging of ultra-weak photon emission (UPE) generated by excited species in oxidative reaction to determine regional variations in oxidative stress in human facial skin and analysed the relationship between UPE intensity and biophysical properties in vivo. UPE imaging of the facial skin of volunteers revealed regional variations in oxidative stress. The nose, its surrounding regions, and the area between eyebrows showed higher UPE intensity than other facial regions, indicating high oxidative stress in these regions. In contrast, only the region surrounding the eyes showed age-related alterations in UPE intensity; moreover, wrinkle score in these regions was correlated with UPE intensity. These results suggest that oxidative stress in the skin induces wrinkle formation. UPE intensity was correlated with porphyrin score in the skin; however, no correlation was observed between UPE intensity and skin colour parameters. This study provides insights into the treatment of facial skin areas vulnerable to ageing and helps improve our understanding of topical skin diseases related to oxidative stress.

Imaging of ultraweak photon emission for evaluating the oxidative stress of human skin.

Ultraweak photon emission (UPE) is generally observed in living organisms and often designated as biophoton emission. UPE is detectable from human skin, and its intensity increases by external stress such as ultraviolet (UV) irradiation. Presently, UPE measurement is used to evaluate oxidation status. The fact that the electronically excited species responsible for UPE are formed by reactive oxygen species (ROS)-induced lipid peroxidation and protein and nucleic acid oxidation is well known. The human skin undergoes oxidative stress by UV irradiation, resulting in various skin complications; therefore, it is essential to know the oxidation status of the skin. In this study, we assessed the characteristics of UV-induced UPE in the skin by the imaging and spectroscopy systems. Two-dimensional images obtained by a highly sensitive imaging system using a cooled charge-coupled device (CCD) camera revealed that UPE intensity increases with the amount of UV and is suppressed by antioxidants. Additionally, it is indicated that UPE is generated not only from the epidermis but also from the dermis. The spectra of UPE induced by UVA or UVB showed similar peaks in the visible light region. Furthermore, we confirmed the efficiency of sunscreen by the imaging technique. UPE measurement is a useful method to evaluate UV-induced oxidation in the human skin, and UPE imaging is an effective method to visually evaluate oxidative stress in the human skin.