Azelaic acid alleviates UVB-induced photoaging in keratinocytes by restoring Smad-dependent TGF-β signaling

Background: Skin photoaging, primarily caused by chronic ultraviolet B (UVB) exposure, is characterized by cellular senescence and extracellular matrix (ECM) degradation. Suppression of transforming growth factor-β (TGF-β)/Smad signaling plays a critical role in UVB-induced collagen loss and impaired skin repair. Azelaic acid (AzA), a clinically established dermatological agent with antioxidant and anti-inflammatory properties, has been reported to maintain skin homeostasis; however, its mechanistic role in protecting against UVB-induced photoaging remains unclear. Objective: This study aimed to investigate whether AzA alleviates UVB-induced photoaging in keratinocytes and to determine whether its protective effects are mediated through the restoration of Smad-dependent TGF-β signaling. Methods: An in vitro UVB-induced photoaging model was established using HaCaT keratinocytes. Following UVB irradiation, cells were treated with AzA. Cell viability and cytotoxicity were evaluated using CCK-8 and LDH assays, respectively. Cellular senescence was assessed by senescence-associated β-galactosidase staining. ECM-related markers were measured by ELISA. TGF-β/Smad signaling activity was analyzed by Western blotting, quantitative real-time PCR, and immunofluorescence. The selective TGF-β/Smad inhibitor SB431542 was used to verify pathway dependence. A keratinocyte-fibroblast conditioned medium model was also employed to assess the paracrine regulation of fibroblast ECM metabolism. Results: AzA significantly attenuated UVB-induced cellular senescence and partially restored cell viability in HaCaT cells. It significantly increased pro-collagen I levels and reduced MMP-1 levels, indicating the restoration of ECM-related protein balance. Mechanistically, AzA reversed UVB-induced suppression of TGF-β/Smad signaling by enhancing Smad2/3 phosphorylation, decreasing Smad7 expression, promoting the nuclear translocation of phosphorylated Smad2/3, and restoring Smad-dependent transcription. Pharmacological inhibition of TGF-β/Smad signaling largely abrogated these protective effects. Conditioned medium from AzA-treated keratinocytes also improved ECM-related responses in fibroblasts in a Smad-dependent manner. Conclusion: AzA alleviates UVB-induced photoaging by restoring Smad-dependent TGF-β signaling, preserving ECM homeostasis, and modulating epidermal-dermal crosstalk in vitro. These findings provide mechanistic support for the potential application of AzA in anti-photoaging strategies.