*3.3. Anti-Inflammatory Activity of Peptides from Sea Cucumbers*

Marine bioactive peptides are short amino acid sequences, generally 2–20 amino acids in length, that are biologically inactive within their respective precursor proteins until released by enzymic hydrolysis. Hydrolysates derived from marine sources have attracted considerable interest within the scientific community due to their diverse biological activities and applications in clinical treatment [48]. Protein constitutes more than 70% of the sea cucumber body and is an effective source of food-borne bioactive peptides. The enzymatic hydrolysates extracted from *Apostichopus japonicus* and *Acaudina leucoprocta* exhibited potent anti-inflammatory activity in a diet-induced hyperuricemic renal inflammation mouse model. The hydrolysates downregulated the proinflammatory cytokines (TNFα, IL-1β, and IL-6) and upregulated the anti-inflammatory cytokines TGFβ and IL-10 by modification of the TLR4/myeloid differentiation primary response 88(MyD88)/NF-κB signaling pathway. The amino acid sequences of peptides found in hydrolysates of *Apostichopus japonicus* and *Acaudina leucoprocta* have been characterized by MALDI-TOF/TOF-MS. GPS-GRP (Gly-Pro-Ser-Gly-Arg-Pro) and GPAGPR (Gly-Pro-Ala-Gly-Pro-Arg) were identified as the two major anti-inflammatory peptides from *Apostichopus japonicus*, while PQGETGA (Pro-Gln-Gly-Glu-Thr-Gly-Ala) and GFDGPEGPR (Gly-Phe-Asp-Gly-Pro-Glu-Gly-Pro-Arg) were detected with the highest abundance in *Acaudina leucoprocta* [49]. Zhang et al. [50] also reported two peptides (Gly-Lys (Figure 3: (**5**)) and Ala-Pro-Arg (Figure 3: (**6**))) from *Apostichopus japonicus* that showed marked anti-inflammatory activity in a CuSO4-induced zebrafish inflammation model. Moreover, a molecular docking analysis revealed that both peptides have a high affinity to bind and inhibit angiotensin-I converting enzyme (ACE-1), a therapeutic target in the treatment of inflammatory conditions [50]. Low molecular weight sea cucumber peptides (SCP, rich in aspartic acid, glycine, proline, and glutamic acid) isolated from *Stichopus japonicus*, a sea cucumber widely distributed along the coasts of China and Japan, displayed potent anti-inflammatory activity in LPS-stimulated RAW264.7 murine macrophages by the inhibition of NF-κB and activation of MAPK in macrophages [51]. Moreover, the anti-inflammatory activity of another SCP (rich in glycine, glutamic acid, and proline) isolated from *Stichopus japonicus* was also demonstrated in vivo, where it significantly inhibited serum proinflammatory cytokines and downregulated the overexpression of TLR4 and NF-κB in gastrocnemius muscles of rats [52]. The hydrolysate bioactive fraction, isolated from the sea cucumber species *Holothuria forskali*, reduced the vascular cell adhesion molecule (VCAM)-1 and IL-6 expression levels in endothelial cells and intercellular adhesion molecule (ICAM)-1 expression in subcutaneous adipose tissue and was shown to inhibit ACE-1 enzyme activity in an in vitro assay [53]. Recently, Jo et al. [54] isolated sea cucumber extracellular matrices (body wall collagen) from *Stichopus japonicus*, which possessed potent anti-inflammatory activity in a TNFα and IL-1β-induced osteoarthritis in vitro model. Moreover, the major yolk protein isolated from *Stichopus japonicus* attenuated experimental DSS-induced colitis by preventing tissue damage, promoting the expression of anti-inflammatory cytokines, and increasing the levels of short-chain fatty acids [55]. Similarly, sea cucumber (*Stichopus japonicus*) enzymatic hydrolysates have been shown to alleviate the inflammatory response via the downregulation of RANKL (receptor activator of NF-kB) and thereby inhibiting the NF-kB pathway in ovariectomized rats [56].

**Figure 3.** Structures of anti-inflammatory peptides derived from sea cucumbers (structures (**5**) and (**6**) re-used with permission from reference [50], John Wiley and Sons, 2021).
