Efficacy and Mechanism of Silymarin Extract for Deep Second-Degree Burns in Rats

To investigate the therapeutic efficacy of silymarin extract on deep second-degree burns in vivo and in vitro and elucidate the underlying molecular mechanisms.

A total of 126 Sprague-Dawley rats were randomized into 7 groups using a random number table method (n=18), including normal control (NC), model, vehicle control (Veh), positive control (Mepore Moist Burn Gream, 25 mg/cm²), and low-, medium-, and high-dose silymarin extract groups. All treatments were applied topically twice daily during 1st week and once daily for the subsequent 14 days. Deep second-degree burn model was established using a controlled-temperature scald device in all groups except the NC group. Therapeutic efficacy was evaluated via wound closure dynamics and tissue water content (edema). Histopathological changes, including collagen deposition and structural integrity, were examined using HE and Masson's trichrome stainings. Inflammatory and oxidative microenvironment were characterized by quantifying the levels of tumor necrosis factor (TNF)-α, interleukin (IL)-1, malondialdehyde (MDA), superoxide dismutase (SOD), and glutathione (GSH) using commercial assay kits. Activation of the nuclear factor erythroid 2-related factor 2 (Nrf2) signaling axis [Nrf2, Kelch-like ECH-associated protein 1 (Keap1), heme oxygenase (HO)-1, and NAD(P)H:quinoneoxidoreductase 1 (NQO1)] was assessed using immunohistochemistry and Western blot, respectively. To corroborate the in vivo findings, HaCaT keratinocytes were treated with Nrf2 inhibitor ML385 to validate pathway dependency.

Silymarin extract treatment significantly accelerated wound closure and attenuated tissue edema in the scalded areas (P<0.05 or P<0.01). Histopathological assessment revealed that silymarin extract augmented collagen deposition and remodeling, promoted reepithelialization and regeneration of skin appendages, mitigated inflammatory cell infiltration, and facilitated necrotic tissue resolution (P<0.01). Mechanistically, silymarin extract upregulated the Nrf2 signaling pathway by promoting Nrf2 dissociation from Keap1. This activation potentiated antioxidant defenses (HO-1, NQO1, SOD, and GSH) and suppressed MDA (P<0.05 or P<0.01). Concurrently, silymarin extract significantly downregulated pro-inflammatory cytokines IL-1 and TNF-α (P<0.05 or P<0.01). These protective effects were abrogated in vitro by the specific Nrf2 inhibitor ML385.

Silymarin extract accelerates the healing of deep second-degree burns and mitigates oxidative stress-induced tissue injury by activating Nrf2 signaling axis, suggesting its potential as an effective therapeutic agent.