Author Ruixuan Geng, Seong-Gook Kang, and Tao Tong
Skin photoaging is mainly induced by ultraviolet (UV) irradiation and its manifestations include dry skin, coarse wrinkle, irregular pigmentation, and loss of skin elasticity, Dietary supplementation of nutraceuticals with therapeutic and preventive effects against skin photoaging, cellular and molecular mechanisms of UV-induced skin photoaging, Matrix metalloproteinases, transforming growth factors, skin adipose tissue, inflammation, oxidative stress, nuclear and mitochondrial DNA, telomeres, microRNA, advanced glycation end products, the hypothalamicpituitaryadrenal axis, and transient receptor potential cation channel V are key regulators that drive the photoaging-associated changes in skin, skin photoaging, dietary components to alleviate skin photoaging include the maintenance of skin moisture and extracellular matrix content, regulation of specific signaling pathways involved in the synthesis and degradation of the extracellular matrix, and antioxidant capacity, ingestion of food-derived functional components could be an attractive strategy to prevent skin photoaging damage, Skin aging includes natural aging, heat aging, and photoaging, photoaging is the most crucial factor causing skin aging damage. Skin photoaging is caused by long-term exposure to ultraviolet (UV) and manifests as rough, dry, and sagging skin, deeper skin wrinkles, excessive skin pigmentation, or angiotelectasis, even leading to various benign or malignant tumors, such as solar keratosis, squamous cell carcinoma, and malignant melanoma , UV activates or inhibits various signal pathways in the dermis and epidermis, leading to a decrease in the content of the extracellular matrix (ECM) and causing uneven structure or even skin collapse, methods to prevent or treat skin photoaging mainly include physical means of photoprotection (sunglasses, window films, clothing, etc.), topical treatment of active ingredients, and medical cosmetology,recently, there has been a growing awareness of the role of nutrition in skin health and specific dietary components have emerged as an effective alternative strategy to prevent and alleviate the symptoms of photoaging, dietary supplementation of functional components can protect skin from photoaging damage. Phytochemicals , functional proteins [9] and peptides , functional sugars, functional oils , probiotics , vitamins , and minerals are well-known to improve the photoaging-associated morphological abnormalities and functional decline, skin is in direct contact with the external environment and has the functions of feeling external stimuli, regulating body temperature, excreting skin metabolites, and protecting the body from physical, mechanical, and chemical damage and invasion by pathogenic microorganisms, skin consists of three parts: stratified epidermis, dermis, and subcutaneous tissue , epidermis is composed of keratinocytes (9095% of skin cells), Langerhans cells (2%), melanocytes (3%), and Merkel cells (0.5%), epidermis contains the stratum corneum, hyaline layer, granular layer, and germinal layer, from the shallowest to the deepest, stratum corneum is the key to maintaining optimal skin hydration, cells in the germinal layer continue to proliferate and migrate to the upper layer to supply the constantly shedding stratum corneum, germinal layer contains melanocytes that can produce melanin and the content of melanin is one of the factors that determines skin color, dermis is made up of connective tissue and also contains appendages including sweat glands, sebaceous glands, blood vessels, and nerves, dermis is divided into the papillary layer and reticular layer, and there is no obvious boundary between the two layers. The thickness of the dermis is around 0.070.12 mm; the dermis of the palms and soles is thicker (~1.4 mm); the eyelids and tympanic membrane are thinner (~0.05 mm), papillary layer is connected to the germinal layer of the epidermis, and the reticular layer is combined with the subcutaneous tissue, main cell types in the dermis is fibroblasts, which play a vital role in skin aging ,fibroblasts synthesize and secrete ECM, including collagen, hyaluronic acid (HA), and elastin, collagen is the most important ECM in the dermis of the skin, loss of collagen will directly lead to skin sagging, aging, and decreased elasticity, HA is synthesized at the plasma membrane by HA synthases 13 and is known to play a key role in wound healing and tissue repair processes due to its ability to maintain a humid environment, in the dermal fibroblasts, HA synthase 2 seems to be the major isoform, elastin is the main component of elastic fibers in matrix tissue and provides resilience and elasticity to tissues and organ, reticular layer contains collagen fibers, elastic fibers, and reticular fibers, which interweave into a net to create elasticity and toughness in the skin, existing evidence suggests that damage to macromolecules present in the dermal ECM is indeed associated with skin aging, subcutaneous tissue is composed of loose connective tissue and fat lobules, and it connects the dermis with the fascia, aponeurosis, or periosteum, It can buffer mechanical pressure, store energy, and maintain body temperature, moderate amount of UV radiation can kill microorganisms, regulate the nerves, endocrine, digestion, breathing, immune system, and promote the synthesis of vitamin D. However, exposure to chronic low-dose or instant high-dose UV radiation causes harm to the eyes, skin, and immune system and is associated with the clinical hallmarks of skin aging, photoaging is mainly caused by UVA and UVB. UVA has a strong ability to induce free radicals and lipid peroxidation in cells and undermines collagen fibers and elastic fibers in the dermal tissue, impact of UVA can reach the deep layer of the dermis due to its greater penetration ability, UVA has no direct effect on DNA damage, it can generate reactive oxygen species (ROS), leading to DNA oxidative damage indirectly, UVB chiefly leads to lesions in the epidermis and superficial dermis and could be absorbed by proteins and DNA in cells, causing cell damage and mutation, 80% of facial skin aging is caused by exposure to UV , macroscopic characteristics of skin photoaging include wrinkle formation, rough texture, pigmentation, and loss of skin elasticity, histological and ultrastructural studies have shown epidermal hyperplasia, damage, and disorder of collagen fibers, and a large accumulation of abnormal elastic substances in connective tissue in photoaged skin ,these effects are less pronounced in the epidermis owing to high turnover, in contrast, the dermal region is more susceptible to photodamage, which results in loss of skin resilience, mechanisms of photoaging, Matrix Metalloproteinases, TGF (Transforming Growth Factor)-?, TGF-? is a type of cytokine that regulates cell growth and differentiation and performs a central role in the inflammatory response, tissue repair, embryonic development, and the immune response, Reduction of Skin Adipose Tissue, Inflammation and Immune Disorders, Oxidative Stress, ROS, such as superoxide ions, H2O2, and hydroxyl radicals, are chemically active and oxidize unsaturated fatty acids from cell phospholipid molecules to malondialdehyde, directly hurting functional macromolecules such as biofilms and proteins , UV radiation induces the production of ROS, which breaks the dynamic balance between oxidation and antioxidant systems in the skin and reduces its ability to remove ROS, Nuclear DNA and mtDNA Damage, UV radiation can directly or indirectly hurt DNA, Mitochondrion is the main organelle involved in skin photoaging, close connection between mitochondria and skin health, the strategy of focusing on mitochondria as a therapeutic target to boost skin health has attracted the attention of clinicians and estheticians. A great quantity of bioactive compounds have been confirmed to ameliorate mitochondrial function and have positive effects on aging and diseased skin, Telomere Shortening, Telomeres are critical structures at the end of eukaryotic chromosomes made up of numerous copies of G-rich repeats. Without telomeres, chromosomes will fuse and genetic instability will occur , MicroRNA (miRNA), regulates the senescence of human skin fibroblasts, Accumulation of Advanced Glycation End Products (AGEs), prominent feature of aging at the molecular level is the gradual accumulation of proteins that have undergone non-enzymatic modification, one of the commonest of which is glycation, Gut Microbes, gut and skin, densely vascularized and richly innervated organs with crucial immune and neuroendocrine roles, are uniquely related in purpose and function, as our primary interface with the external environment, both organs are essential to the maintenance of physiologic homeostasis, gut microbiome is the major regulator of the gutskin axis, oral probiotics may counteract UV damage and relieve inflammatory dermatoses by regulating immune-related signal pathways, gut dysbiosis has been observed in conditions such as atopic dermatitis, mechanisms by which intestinal microbiota exert their influence on skin homeostasis appear to be related to the modulatory effect of gut microbes on systemic immunity, Activation of HypothalamicPituitaryAdrenal (HPA) Axis, HPA axis is associated with the activation of a wide range of responses involving the endocrine, nervous, and immune systems, collectively known as the stress responses, Skin has neuroendocrine capabilities that also encompass all elements of the HPA axis, Transient Receptor Potential Cation Channel V (TRPV), The TRP ion channel is a type of channel protein widely distributed in the peripheral and central nervous system. TRP allows cations to pass through the cell membrane non-selectively and is responsible for various sensory responses, including heat, cold, pain, stress, vision, and taste, Efficacy and Mechanisms of Dietary Components in Mitigating Skin Photoaging: Animal and Human Evidence, oral supplementation with aloe vera polysaccharides improve skin moisture and elasticity and relieve wrinkles and roughness in eye skin
