# Sericin for Tissue Engineering
## 2 Sericin as a Degumming Product
### Cellular Response to Sercin and Influence of the Degumming Method on Sercin Biological Activity  
Sercin is a fundamental insect protein produced by the silkworm. There is no doubt that it is a foreign protein and hence immunogenic to humans. While some studies stated sericin to be a biocompatible and non-immunogenic protein, this should be accepted as a relative statement in comparison with highly immunogenic foreign proteins. Even human proteins may be immunogenic upon denaturation. As mentioned, different degumming processes produce different yield ratios and protein structures. Considering that protein structure is the main determinant of its biological response, the degumming process may influence sericin activity. The effects of the degumming process on the macrophage response and \(\beta\)-sheet content of silk fibroin have been documented [65]. As the silkworm cocoon protects the larva from the outer environment, sericin is assumed to have antibacterial properties. However, a recent paper proved that sericin lacks antibacterial properties [66]. Sercin may be an allergen for some people [67]. Immunogenic responses to sericin have been studied in RAW264.7 cells. Insoluble fibroin particles, but not soluble sericin, significantly increased tumor necrosis factor-\(\alpha\) (TNF\(\alpha\)) levels [27]. When sericin was administered to mouse monocytes and alveolar macrophages, the levels of TNF\(\alpha\) and interleukin-1\(\beta\) (IL1\(\beta\)) increased in a dose-dependent manner [68]. Interestingly, these elevated levels of TNF\(\alpha\) and IL1\(\beta\), following sericin administration, did not result in delayed wound healing in animal models [68]. Similarly, sericin increased the TNF\(\alpha\) level in RAW264.7 cells, but a silk mat with sericin enhanced new bone formation in the rat calvarial defect model [69]. Sercin also activated the nuclear factor kappa B (NF-kB) pathway, which is implicated in the inflammation of retinal pigment epithelial cells [70]. Sercin increased vascular endothelial growth factor (VEGF) in RAW264.7 cells through elevation of hypoxia inducible factor (HIF) expression [71]. The administration of sericin increased VEGF expression in human umbilical vein cells [72]. A silk mat produced by cutting and peeling off B. mori cocoons was tested for bone regeneration in the extraction socket of humans [73]. The soluble proteins (mainly sericin) released from these silk mats increased the expression of the genes associated with angiogenesis such as VEGF and inflammation (IL1\(\beta\) and TNF\(\alpha\)) (Figure 6) [74]. Sercin was shown to elevate the expression levels of IL1\(\beta\), TNF\(\alpha\), and VEGF [68; 69; 71]. When sericin was fractionated according to its molecular weight, the fraction with higher molecular weight increased TNF\(\alpha\) expression in RAW264.7 cells more than that with lower molecular weight [75]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sample** & **Helix** & **Sheet** & **Turn** & **Others** \\ \hline Hot water extracts & 3.40\% & 34.40\% & 14.20\% & 48.00\% \\ \hline \multirow{4}{*}{30\% EtOH extracts} & 40 \({}^{\circ}\)C & 2.30\% & 39.50\% & 13.90\% & 44.30\% \\ \cline{2-6}  & 60 \({}^{\circ}\)C & 4.30\% & 37.10\% & 14.10\% & 44.50\% \\ \cline{2-6}  & 80 \({}^{\circ}\)C & 6.90\% & 33.90\% & 14.90\% & 44.30\% \\ \hline \multirow{4}{*}{70\% EtOH extracts} & 40 \({}^{\circ}\)C & 0.60\% & 41.90\% & 13.40\% & 44.10\% \\ \cline{2-6}  & 60 \({}^{\circ}\)C & 0.60\% & 41.40\% & 13.60\% & 44.40\% \\ \cline{1-1} \cline{2-6}  & 80 \({}^{\circ}\)C & 1.70\% & 40.10\% & 13.80\% & 44.40\% \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Calculated conformations of various cocoon extracts prepared by using different solvents at different temperatures. Unlike most publications, some studies have reported the anti-inflammatory potential of sericin. Topical application of sericin (80 ug/mL) could significantly lower cyclooxygenase (COX)-2 levels as compared to water application in a carrageenan-induced inflammation model [76].