Silk protein sericin: a promising therapy for Achilles tendinopathy-evidence from an experimental rat model

This study investigated the efficacy of sericin in treating experimental Achilles tendinopathy (AT) in rats via the transforming growth factor-beta (TGF-β)/mothers against decapentaplegic (Smad) pathway compared with diclofenac sodium (DS).

An AT model was induced in rats using collagenase enzyme type I and divided into 5 groups: C (control), AT (diseased control), ATS (AT treated with sericin), ATN (AT treated with DS), and ATSN (AT treated with sericin and DS). Sericin injection was given on the 3^rd and 6^th days by intratendinous injection (0.8 g/kg/mL), and DS was administered for 14 days by oral gavage (1.1 mg/kg/day). Serum concentrations of total oxidant-antioxidant status (TOS-TAS), TGF-β1, decorin, Smad2, and connective tissue growth factor (CTGF) were measured. Histopathologic and immunohistochemical (IHC) studies were conducted on Achilles tendon samples.

The TOS, oxidative stress index (OSI), TGF-β1, Smad2, CTGF, and decorin serum concentrations were significantly higher in AT than in C and significantly lower in ATS than in AT (P<0.05). Histopathological examination revealed that irregular fibers, degeneration, and round cell nuclei were significantly elevated in AT. Spindle-shaped fibers were similar to those in C, and degeneration was reduced in ATS. TGF-β1 and Smad2/3 expression was increased, and collagen type I alpha-1 (Col1A1) expression was decreased in AT vs. C (P=0.001). In the ATS, TGF-β1 and Smad2/3 expression decreased, and Col1A1 expression increased. The Bonar score significantly increased in the AT group (P =0.001) and significantly decreased in the ATS group (P =0.027).

Sericin shows potential efficacy in reducing oxidative stress and modulating the TGF-β/Smad pathway in experimental AT models in rats. It may be a promising therapeutic agent for AT, warranting further clinical studies for validation. Key Points • This study revealed that sericin mitigates AT-induced damage through the TGF-β/Smad pathway in an AT rat model. • ELISA and IHC investigations corroborated the effectiveness of sericin via the pivotal TGF-β/Smad pathway in tissue repair. • Evidence indicates that sericin enhances collagen synthesis,shapes tendon fiber structure, and diminishes histopathological degeneration. • Sericin's antioxidant properties were reaffirmed in its AT treatment application.