Handelin protects human skin keratinocytes against ultraviolet B-induced photodamage via autophagy activation by regulating the AMPK-mTOR signaling pathway

Targeting non-classical autophagy-dependent ferroptosis and the subsequent HMGB1/TfR1 feedback loop accounts for alleviating solar dermatitis by senkyunolide I

Given the substantial risks associated with ultraviolet B (UVB) radiation-induced solar dermatitis, enhancing current strategies to combat UVB regarding skin diseases is imperative. The cross-talk between ferroptosis and inflammation has been proven to be an essential factor in UVB-induced solar dermatitis, whereas detailed process of how their interaction contributes to this remains unclear. Therefore, further investigation of ferroptosis-mediated processes and identification of corresponding inhibitory approaches hold promise for repairing skin damage. Senkyunolide I (Sen I), a bioactive component mainly extracted from the traditional Chinese medicinal plants, Ligusticum chuanxiong Hort. and Angelica sinensis (Oliv.) Diels, has demonstrated efficacy in combating oxidative stress and inflammation. In this study, we utilized UVB-irradiated HaCaT cells as an in vitro model and C57BL/6J mice as an in vivo model of solar dermatitis. Our findings revealed the pivotal roles of autophagy and ferroptosis in inducing skin inflammation, particularly emphasizing the activation of ferroptosis through macroautophagy. Surprisingly, this mechanism operated independently of ferritinophagy, a classical autophagy-driven ferroptosis pathway. Instead, our results highlighted Transferrin Receptor 1 (TfR1), tightly controlled by autophagy, as a crucial mediator of ferroptosis execution and amplifier of subsequent lethal signals. Furthermore, extracellular High Mobility Group Box 1 protein (HMGB1), released following UVB-induced ferroptotic cells from activated autophagic flux, initiated a feedback loop with TfR1, propagating ferroptosis to neighboring cells and exacerbating damage. Remarkably, Sen I administration showed a significant protective effect against UVB damage in both in vitro and in vivo models by interrupting this cascade. Consequently, we have illuminated a novel therapeutic pathway post-UVB exposure and identified Sen I as a potent natural molecule that safeguarded against UVB-induced solar dermatitis by suppressing the autophagy-ferroptosis-HMGB1-TfR1 axis, highlighting a new frontier in photoprotection.

Natural autophagy activators: A promising strategy for combating photoaging

BACKGROUND

Photodamage to the skin stands out as one of the most widespread epidermal challenges globally. Prolonged exposure to sunlight containing ultraviolet radiation (UVR) instigates stress, thereby compromising the skin's functionality and culminating in photoaging. Recent investigations have shed light on the importance of autophagy in shielding the skin from photodamage. Despite the acknowledgment of numerous phytochemicals possessing photoprotective attributes, their potential to induce autophagy remains relatively unexplored.

PURPOSE

Diminished autophagy activity in photoaged skin underscores the potential benefits of restoring autophagy through natural compounds to enhance photoprotection. Consequently, this study aims to highlight the role of natural compounds in safeguarding against photodamage and to assess their potential to induce autophagy via an in-silico approach.

METHODS

A thorough search of the literature was done using several databases, including PUBMED, Science Direct, and Google Scholar, to gather relevant studies. Several keywords such as Phytochemical, Photoprotection, mTOR, Ultraviolet Radiation, Reactive oxygen species, Photoaging, and Autophagy were utilized to ensure thorough exploration. To assess the autophagy potential of phytochemicals through virtual screening, computational methodologies such as molecular docking were employed, utilizing tools like AutoDock Vina. Receptor preparation for docking was facilitated using MGLTools.

RESULTS

The initiation of structural and functional deterioration in the skin due to ultraviolet radiation (UVR) or sunlight-induced reactive oxygen species/reactive nitrogen species (ROS/RNS) involves the modulation of various pathways. Natural compounds like phenolics, flavonoids, flavones, and anthocyanins, among others, possess chromophores capable of absorbing light, thereby offering photoprotection by modulating these pathways. In our molecular docking study, these phytochemicals have shown binding affinity with mTOR, a negative regulator of autophagy, indicating their potential as autophagy modulators.

CONCLUSION

This integrated review underscores the photoprotective characteristics of natural compounds, while the in-silico analysis reveals their potential to modulate autophagy, which could significantly contribute to their anti-photoaging properties. The findings of this study hold promise for the advancement of cosmeceuticals and therapeutics containing natural compounds aimed at addressing photoaging and various skin-related diseases. By leveraging their dual benefits of photoprotection and autophagy modulation, these natural compounds offer a multifaceted approach to combatting skin aging and related conditions.

Regulatory Mechanisms of Natural Active Ingredients and Compounds on Keratinocytes and Fibroblasts in Mitigating Skin Photoaging

Background

The mechanism underlying skin photoaging remains elusive because of the intricate cellular and molecular changes that contribute to this phenomenon, which have yet to be elucidated. In photoaging, the roles of keratinocytes and fibroblasts are vital for maintaining skin structure and elasticity. But these cells can get photo-induced damage during photoaging, causing skin morphological changes. Recently, the function of natural active ingredients in treating and preventing photoaging has drawn more attention, with researches often focusing on keratinocytes and fibroblasts.

Methods

We searched for studies published from 2007 to January 2024 in the Web of Science, PubMed, and ScienceDirect databases through the following keywords: natural plant, natural plant products or phytochemicals, traditional Chinese Medicine or Chinese herbal, plant extracts, solar skin aging, skin photoaging, and skin wrinkling. This review conducted the accordance of Preferred Reporting Items for Systematic Reviews and Meta-Analysis guidelines.

Results

In total, 87 researches were included in this review (Figure 1). In keratinocytes, natural compounds may primarily regulate signal pathways such as the NF-κB, MAPK, PI3K/AKT, and Nrf2/ARE pathways, reducing inflammation and cellular damage, thus slowing skin photoaging. Additionally, in fibroblasts, natural active ingredients primarily promote the TGF-β pathway, inhibit MMPs activity, and enhance collagen synthesis while potentially modulating the mTOR pathway, thereby protecting the dermal collagen network and reducing wrinkle formation. Several trials showed that natural compounds that regulate keratinocytes and fibroblasts responses have significant and safe therapeutic effects.

Conclusion

The demand for natural product-based ingredients in sunscreen formulations is rising. Natural compounds show promising anti-photoaging effects by targeting cellular pathways in keratinocytes and fibroblasts, providing potential therapeutic strategies. However, comprehensive clinical studies are needed to verify their efficacy and safety in mitigating photoaging, which should use advanced pharmacological methods to uncover the complex anti-photoaging mechanisms of natural compounds.

The skin photoprotective effect of trilinolein: Induction of cellular autophagy via the AMPK-mTOR signaling pathway

Trilinolein (TL) is an active substance contained in traditional Chinese herbs; modern studies have shown that trilinolein has anti-inflammatory and antioxidant effects on the body. This study delves into the photoprotective effect of trilinolein on UVB-irradiated Human Skin Fibroblast (HSF) cells and the underlying mechanisms. Our findings reveal that trilinolein had a photoprotective effect on HSF cells: trilinolein enhanced cellular autophagy, restored UVB-inhibited cell proliferative viability, and curbing UVB-induced reactive oxygen species (ROS) and apoptosis. Intriguingly, after inhibition of TL-induced autophagy via wortmannin, diminished trilinolein's photoprotective effects. Meanwhile, trilinolein was shown to modulate the AMPK-mTOR signaling pathway, thus enhance cellular autophagy in HSF cells, and this tendency was suppressed after the administration of compound C (AMPK inhibitor). In a mouse model of skin photodamage, trilinolein significantly mitigated photodamage extent through morphological and histopathological analyses. This study illuminates trilinolein could inhibit the photodamaging effects of UVB irradiation by regulating cellular autophagy through the AMPK-mTOR signaling pathway, suggesting its promising application in combating UV-induced skin disorders.