The synthesis of broccoli sprout extract-loaded silk fibroin nanoparticles as efficient drug delivery vehicles: development and characterization

Targeted drug delivery of biological molecules using the development of biocompatible, non-toxic and biodegradable nanocarriers can be a promising method for cancer therapy. In this study, silk fibroin protein nanoparticles (SFPNPs) were synthesized as a targeted delivery system for sulforaphane-rich broccoli sprout extract (BSE). The BSE-loaded SFPNPs were conjugated with polyethylene glycol and folic acid, and then their physicochemical properties were characterized via UV-Vis, XRD, FTIR, DLS, FE-SEM and EDX analyses. In vitro, the release profile, antioxidant and anticancer activities of NPs were also studied. The FE-SEM and DLS analyses indicated stable NPs with an average size of 88.5 nm and high zeta potential (-32 mV). The sulforaphane release profile from NPs was pH-dependent, with the maximum release value (70%) observed in simulated intestinal fluid (pH = 7.4). Encapsulation of BSE also decreased the release rate of sulforaphane from the capsules compared to free BSE. In vitro cytotoxicity of BSE and NPs on breast cancer cell lines (MCF-7) was concentration-dependent, and the IC50 for BSE and NPs were 54 and 210 μg ml-1, respectively. Moreover, the NPs demonstrated no appreciable cytotoxicity in normal mouse fibroblast (L929) cell lines. These results indicated that biocompatible NPs synthesized as controlled and long-term targeted drug delivery systems can be a potential candidate for breast cancer therapy.

A novel silk fibroin protein-based fusion system for enhancing the expression of nanobodies in Escherichia coli

Nanobodies show a great potential in biomedical and biotechnology applications. Bacterial expression is the most widely used expression system for nanobody production. However, the yield of nanobodies is relatively low compared to that of eukaryotic systems. In this study, the repetitive amino acid sequence motifs (GAGAGS) found in silk fibroin protein (SFP) were developed as a novel fusion tag (SF-tag) to enhance the expression of nanobodies in Escherichia coli. SF-tags of 1 to 5 hexapeptide units were fused to the C-terminus of 4G8, a nanobody against human epididymis protein 4 (HE4). The protein yield of 4G8 variants was increased by the extension of hexapeptide units and achieved a 2.5 ~ 7.1-fold increase compared with that of untagged 4G8 (protein yield of 4G8-5C = 0.307 mg/g vs that of untagged 4G8 = 0.043 mg/g). Moreover, the fusion of SF-tags not only had no significant effect on the affinity of 4G8, but also showed a slight increase in the thermal stability of SF-tag-fused 4G8 variants. The fusion of SF-tags increased the transcription of 4G8 by 2.3 ~ 7.0-fold, indicating SF-tags enhanced the protein expression at the transcriptional level. To verify the applicability of the SF-tags for other nanobody expression, we further investigated the protein expression of two other anti-HE4 nanobodies 1G8 and 3A3 upon fusion with the SF-tags. Results indicated that the SF-tags enhanced the protein expression up to 5.2-fold and 5.7-fold for 1G8 and 3A3, respectively. For the first time, this study reported a novel and versatile fusion tag system based on the SFP for improving nanobody expression in Escherichia coli, which may enhance its potential for wider applications.Key points• A silk fibroin protein-based fusion tag (SF-tag) was developed to enhance the expression of nanobodies in Escherichia coli.• The SF-tag enhanced the nanobody expression at the transcriptional level.• The fusion of SF-tag had no significant effect on the affinity of nanobodies and could slightly increase the thermal stability of nanobodies.

Silk microgels formed by proteolytic enzyme activity

The proteolytic enzyme α-chymotrypsin selectively cleaves the amorphous regions of silk fibroin protein (SFP) and allows the crystalline regions to self-assemble into silk microgels (SMGs) at physiological temperature. These microgels consist of lamellar crystals in the micrometer scale, in contrast to the nanometer-scaled crystals in native silkworm fibers. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and zeta potential results demonstrated that α-chymotrypsin utilized only the non-amorphous domains or segments of the heavy chain of SFP to form negatively charged SMGs. The SMGs were characterized in terms of size, charge, structure, morphology, crystallinity, swelling kinetics, water content and thermal properties. The results suggest that the present technique of preparing SMGs by α-chymotrypsin is simple and efficient, and that the prepared SMGs have useful features for studies related to biomaterial and pharmaceutical needs. This process is also an easy way to obtain the amorphous peptide chains for further study.