Dendrobine activates the SIRT3-SOD2 axis to protect keratinocytes from UVB-induced oxidative damage

Ultraviolet B (UVB) irradiation is a primary environmental factor leading to skin oxidative damage and photoaging, primarily by disrupting mitochondrial redox homeostasis in keratinocytes. Dendrobine (DDB), a bioactive sesquiterpene alkaloid derived from the edible and medicinal plant Dendrobium nobile, exhibits diverse pharmacological properties. However, its role and mechanism in protecting against UVB-induced skin damage remain unclear.

HaCaT human keratinocytes were pretreated with DDB (5-20 μM) prior to UVB irradiation (160 mJ/cm²). Cell viability was assessed using MTT and crystal violet staining assays. Oxidative stress markers, including reactive oxygen species (ROS), 4-hydroxy-2-nonenal (HNE), protein carbonyls, and glutathione (GSH/GSSG) ratio, were measured. Apoptosis was evaluated by flow cytometry, and expression of apoptosis-related proteins (Bax, Bcl-2, cleaved caspase-3, and cleaved caspase-9) and mitochondrial regulators (SIRT3 and SOD2) was analyzed by western blotting. SIRT3 and SOD2 enzymatic activities were determined using commercial kits. Surface plasmon resonance (SPR) and molecular docking were employed to assess direct binding between DDB and SIRT3. Pharmacological inhibition with 3-TYP was used to establish SIRT3 dependency.

DDB significantly rescued HaCaT cells from UVB-induced viability loss in a concentration-dependent manner. DDB attenuated UVB-induced oxidative stress, as evidenced by reduced ROS, HNE, and protein carbonyl levels, and restored the GSH/GSSG ratio. DDB suppressed apoptosis by decreasing Bax/Bcl-2 ratio and cleaved caspase-3/9 levels. Mechanistically, DDB restored UVB-inhibited enzymatic activities of SIRT3 and SOD2 and upregulated their protein expression. SPR analysis revealed that DDB directly binds to SIRT3 with micromolar affinity (K_D = 7.603 × 10^-6 M). Molecular docking predicted that DDB binds within a distinct pocket of SIRT3 without competing with its substrate peptide. Critically, pharmacological inhibition of SIRT3 with 3-TYP abolished DDB's protective effects against oxidative damage.

This study demonstrates that DDB protects keratinocytes from UVB-induced oxidative damage and apoptosis by directly activating the SIRT3-SOD2 axis, thereby restoring mitochondrial redox homeostasis. These findings establish DDB as a promising natural candidate for functional foods or pharmaceuticals aimed at preventing skin photoaging and preserving mitochondrial function.