TGF-β1 Conjugated to Gold Nanoparticles Results in Protein Conformational Changes and Attenuates the Biological Function

Gold nanoparticles (AuNPs) are widely used as carriers or therapeutic agents due to their great biocompatibility and unique physical properties. Transforming growth factor-beta 1 (TGF-1), a member of the cysteine-knot structural superfamily, plays a pivotal role in many diseases and is known as an immunosuppressive agent that attenuates immune response resulting in tumor growth. The results reported herein reflect strong interactions between TGF-1 and the surface of AuNPs when incubated with serum-containing medium, and demonstrate a time- and dose-dependent pattern. Compared with other serum proteins that can also bind to the AuNP surface, AuNP-TGF1 conjugate is a thermodynamically favored compound. Epithelial cells undergo epithelial-mesenchymal transition (EMT) upon treatment with TGF-1; however, treatment with AuNPs reverses this effect, as detected by cell morphology and expression levels of EMT markers. TGF-1 is found to bind to AuNPs through S-Au bonds by X-ray photoelectron spectroscopy. Fourier transform infrared spectroscopy is employed to analyze the conformational changes of TGF-1 on the surface of AuNPs. The results indicate that TGF-1 undergoes significant conformational changes at both secondary and tertiary structural levels after conjugation to the AuNP surface, which results in the deactivation of TGF-1 protein. An in vivo experiment also shows that addition of AuNPs attenuates the growth of TGF-1-secreting murine bladder tumor 2 cells in syngeneic C3H/HeN mice, but not in immunocompromised NOD-SCID mice, and this is associated with an increase in the number of tumor-infiltrating CD4+ and CD8+ T lymphocytes and a decrease in the number of intrasplenic Foxp3(+) lymphocytes. The findings demonstrate that AuNPs may be a promising agent for modulating tumor immunity through inhibiting immunosuppressive TGF-1 signaling.