## 4 Discussion  
As the demand for anti-skin aging treatments increases, functional products for skin, such as whiteners, anti-wrinkle treatments, moisturizers, and UV protectors, are being developed. The skin functional product industry, including cosmetics and dietary supplements, is already one of the highest-valued market areas, and is continuing to grow. In particular, the market for natural-compound-based products is growing more rapidly than the overall market [5]. Unripe apple is known to be rich in polyphenols and an excellent source of natural antioxidants [17]. However, 20-30% of unripe apples are discarded every year, which causes economic and environmental problems in Korea [18]. Therefore, various methods to create value using the remaining unripe apples are required. In this study, we tried to explore the effects of UAE in preventing UVB-irradiated skin photoaging. Chronic exposure to UVB leads to ROS generation, inflammation, and increased cytokines and chemokines, which result in a deterioration in the barrier function of skin and the deepening of wrinkles through the degradation of ECM components, eventually resulting in the premature aging of the skin [19]. Growing evidence suggests the strategies for the prevention of skin photoaging are blocking UV from penetrating the skin layer, inhibiting ROS generation and inflammatory mediators, and inhibiting the gene activations of the MMPs [20]. Cutaneous moisture is another crucial factor in skin health and is connected to the stratum comeum, which functions as a barrier to skin water. Healthy skin retains more than 10% skin moisture. However, sunlight and UV irradiation cause skin moisture to decline below normal levels, thereby leading to dried and rough-textured skin, desquamation, and itching [21]. Given the consistency of the detrimental effects of UVB on the skin, the current study revealed that UVB induced severe wrinkle formation in the dorsal back skin of SKH1-hr hairless mice. Moreover, there was a significant increase in both wrinkle length and depth of wrinkles, as well as loss of the skin water contents compared to the unexposed UVB intact control. Histopathological analysis also showed a thickened epidermal layer and an increased formation of microfolds due to UVB irradiation. These manifestations of photoaging due to UVB, such as wrinkle formation, epidermal thickening, and loss of skin water, were mitigated by UAE. The alterations that occur during photoaging are known to be caused by an imbalance between the synthesis and degradation of components, such as collagen, elastin, and glycosaminoglycan, constituting the ECM in the dermal layer of the skin [22]. Indeed, wrinkle formation during the photoaging process is closely linked to the degradation of ECM components due to the activation of MMPs [20; 23]. Fibrillary collagen is cleaved by MMP1 or MMP13 and further degraded by MMP9, which leads to the accumulation of decomposed collagen fragments in skin tissue. Recent evidence indicates that the direct inhibition or gene regulation of MMPs can be a promising therapeutic target for photoaging and photocarcinogenesis [20; 24]. The present results showed that UAE inhibited the accumulation of decomposed collagen fibers in MT staining and recovered skin COL1 contents, the most abundant fibrous protein in the ECM, against UVB irradiation by inducing _COL1A1_ and _COL1A2_ mRNA expressions. Moreover, UAE inhibited the UVB-induced mRNA expressions of _MMP1_, _MMP9_, and _MMP13_. These results are in line with a recent study on MMP1 inhibition and type I procollagen-inducing effects of polyphenols in unripe apples on UVB-irradiated human skin fibroblasts [13]. Therefore, the anti-wrinkle formation effect of UAE may be due to the suppression of COL1 degradation through MMP gene regulation. A dramatic alteration observed during skin aging is the reduction in hyaluronic acid, responsible for binding and retaining water molecules, which causes the depletion of skin water [25].