Anti-Photoaging Effects of a Polysaccharide from Kappaphycus alvarezii In Vitro and In Vivo

is a rich source of polysaccharides, but their high molecular weight limits skin permeability and bioavailability. To address this, we employed a free-radical degradation method to produce a low-molecular-weight polysaccharide, KP-90. Evaluation in UVB-irradiated HaCaT cells and UVA-irradiated human dermal fibroblasts demonstrated that KP-90 significantly enhanced cell viability and mitigated oxidative stress by suppressing reactive oxygen species and malondialdehyde, while restoring antioxidant enzymes (SOD, CAT and GSH-Px). Furthermore, KP-90 downregulated matrix metalloproteinases (MMP-1, -3, -9) and pro-inflammatory cytokines (TNF-α, IL-6, IL-1β), thereby reducing extracellular matrix degradation and inflammation. These in vitro findings were corroborated in a UVB/UVA-irradiated nude mice model, where KP-90 alleviated epidermal hyperplasia, increased collagen I and hyaluronic acid synthesis, and improved visible signs such as wrinkles and skin laxity. These findings identify KP-90 against skin photoaging and provide a strategic approach for valorization underexploited marine biomass.