## 4 Discussion  
Skin photoaging and photooxidative stress refer to the degeneration or damage of skin tissue structure and function caused by long-term overexposure to ultraviolet rays. Long-term exposure to UVA (320-420 nm) can cause damage to the dermal cells of the skin and degradation of the extracellular matrix, resulting in skin folds and relaxation [18]. UVA is the main external factor causing oxidative stress damage to the skin. Increased levels of reactive oxygen species (ROS) in response to UVA stimulation lead to lipid peroxidation, damage to organelles and DNA, and ultimately cell aging or death [19; 20]. The transcription factor NF-E2-related factor 2 (Nrf2) translocates from the cytoplasm to the nucleus by dissociating from Kelch-like ECH-associated protein 1 (Keap1) and binds to the ARE to activate downstream antioxidant enzyme gene expression [21; 22]. Therefore, Nrf2 is a master regulator of antioxidant defense and a proposed \"master regulator\" of the aging process [23]. The extracellular matrix (ECM) is a three-dimensional network of collagen and elastin fibers surrounded by a matrix such as hyaluronic acid (HA) to maintain skin plumpness and elasticity. In contrast, matrix metalloproteinases (MMPs) represent a family of extracellular zinc-dependent enzymes whose primary function is to degrade the ECM. An imbalance between them leads to loss of skin structure, resulting in crepey, dry and inelastic aging skin [24]. Exposure to UVA radiation induces matrix metalloproteinase activity to accelerate the degradation of extracellular matrix collagen and elastin fibers [25]. Therefore, the inhibition of MMPs effectively protects the dermal matrix structure, avoids tissue damage, and delays wrinkle formation. In addition, exposure to UV radiation also blocks transforming growth factor-\(\beta\) (TGF-\(\beta\))/Smad signaling. It reduces type I procollagen synthesis [26], a major pathway for repairing UVA damage. Previous studies have found that cell-free filtrates, extracellular polysaccharides and extracellular polypeptides of _B. amyloliquefaciens_ have strong antioxidant activity [11; 14; 27; 28]. However, no study has been conducted to prove whether its lysate is functional. Our preliminary experiments found that BAL1 obtained by ultrasonic cell crusher contains many small molecular proteins and polypeptides predicted to have antioxidant properties. Through in vitro antioxidant experiments, it was found that BAL1 has strong antioxidant capacity, which is shown in DPPH-, HO-, and ABTS-+ scavenging ability. Moreover, BAL1 has very low or no toxicity to CCC-ESF-1 up to 600 \(\mu\)g/mL and exhibits exceptionally significant protection against cell death due to UVA damage only above 150 \(\mu\)g/mL. To maintain physiological redox homeostasis, endogenous antioxidant enzymes (CAT), SOD, and GSH-px scavene excess reactive oxygen species from the cells [29]. As shown in Figure 3, 150 \(\mu\)g/mL of BAL1 significantly reduced the intracellular ROS levels, as reflected by ROS's significantly weaker green fluorescence than the UVA model group. Moreover, the total antioxidant capacity measured by CCC-ESF-1 cell lysate was significantly increased compared with the UVA model group. Its effect was close to that of ascorbic acid, which was attributed to the increased intracellular antioxidant enzyme activity and mRNA expression level of BAL1. The Nrf2 signaling pathway regulated the increased antioxidant enzyme activity and mRNA expression levels, and the binding of Keep1 to Nrf2 could reduce the transduction of the Nrf2 signaling pathway [30]. As shown in Figure 5, the experimental results confirmed that BAL1 promoted the nuclear displacement of intracellular Nrf2. The content and expression level of intracellular Keep1 were significantly reduced compared with the UVA model group. The nuclear displacement ratio (N/C) of Nrf2 increased from 0.365 to 1.849, thus activating the expression of antioxidant enzyme target genes of Nrf2.