# Sericin for Tissue Engineering
## 2 Sericin as a Degumming Product
### Cellular Response to Sercin and Influence of the Degumming Method on Sercin Biological Activity
While sericin alone could not reduce lipopolysaccharide-induced serum TNF\(\alpha\) expression, naringenin-loaded sericin reduced this effect [77]. The amelioration of TNF\(\alpha\) levels in the naringenin-loaded sericin group was stronger than that observed in the naringenin group [77]. Sericin exhibited a protective effect similar to that of metformin in the kidneys of rats with diabetic nephropathy through the suppression of the transforming growth factor (TGF)-\(\beta\)1/Smad3 pathway [78]. Sericin may also exert antioxidant activities, which highly vary depending on the species that produced it [31]. Sericin extracted from Antheraea mylitta cocoon was applied to AH927 cells and was found to reduce the oxidative stress induced by hydrogen peroxide [79]. Sericin inhibited lipid peroxidation and tyrosinase activity [44] and accelerated the attachment of human dermal fibroblasts in culture [80]. Sericin from B. mori stimulated cellular migration via the c-Jun mediated pathway [81] and increased the expression of osteocalcin and alkaline phosphatase in osteoblasts cultured on sericin-coated titanium surface [82]. The thermal stability of sericin varies depending on the chemicals used for its extraction [63]. Further, the biological activity of sericin also varies owing to differences in its molecular weight and amino acid composition, depending on the extraction method [29,32]. The biological response to sericin is highly dependent on the extraction method used. Sericin extracted by urea is highly toxic to cells, but heat-degraded sericin is relatively non-toxic [64]. In terms of anti-tyrosinase activity, sericin extracted using urea exhibits higher activity than alkali-degraded sericin [63]. Tyrosines are copper-containing enzymes that catalyze the conversion of monophenol to ortho-quinone [83]. Sericin  
Figure 6: cDNA microarray analysis. The application of soluble proteins from a silk mat changed the expression levels of genes after 2 h and 8 h. Layer 1 was obtained from the innermost layer of silkworm cocoon, and Layer 4 from the outermost layer (Reproduced from a published article [74] with permission). plays the role of an antioxidant and inhibits tyrosinase activity [84]. It may be useful for the treatment and prevention of reactive oxygen species (ROS)-related diseases, including cancer, neurodegenerative diseases, diabetes, and cardiovascular dysfunction [85; 86]. The comparison of the antioxidant activities of sericin proteins extracted by different methods showed that sericin extracted by the alkali method had the best activity, followed by that prepared by autoclaving and the conventional method [29]. During degumming, sericin undergoes fragmentation, and hence, has different molecular weights. The molecular weight of sericin has a marginal effect on cellular toxicity [1]. However, the crystallization and mechanical properties of sericin improve with an increase in its molecular weight [1]. Therefore, the in vivo response to sericin may also differ based on its molecular weight. The various biological activities associated with each degumming method are summarized in Table 2.  
Conflicting results from cellular experiments using sericin may be attributed to the following reasons. First, many papers did not describe sufficiently the extraction method of sericin that was employed. Considering the biological effect of the extraction method, the discrepancy in the biological response to sericin may be attributed to the differences in the extraction methods. Second, the biological response to sericin may be different depending on the species it was obtained from. Most studies tested sericin proteins from B. mori or A. myllita; their species-dependent activities have already been well demonstrated [31]. Third, the biological response to sericin may be also different depending on sericin molecular weight; however, there have been few papers about this.