Inflammation is the response of the immune system to harmful stimuli such as tissue injury, infection or toxic chemicals, which has the aim of eliminating irritants or pathogenic microorganisms and enhancing tissue repair. Uncontrolled long-lasting acute inflammation can gradually progress to chronic, causing a variety of chronic inflammatory diseases that are usually treated with anti-inflammatory drugs, but most of them are inadequate to control chronic responses and are also associated with adverse side effects. Thus, many efforts are being directed to develop alternative and more selective anti-inflammatory therapies from natural products. One main field of interest is the obtaining of bioactive peptides exhibiting anti-inflammatory activity from sustainable protein sources like edible insects or agroindustry and fishing by-products. This work highlighted the structure–activity relationship of anti-inflammatory peptides. Small peptides with molecular weight under 1 kDa and amino acid chain length between 2 to 20 residues are generally the most active because of the higher probability to be absorbed in the intestine and penetrate into cells when compared with the larger size peptides. The presence of hydrophobic (Val, Ile, Pro) and positively charged (His, Arg, Lys) amino acids is another common occurrence for anti-inflammatory peptides. Interestingly, a high percentage (77%) of these bioactive peptides can be found in alternative sustainable protein sources such as Tenebrio molitor or sunflower, apart from its original protein source. However, not all of these peptides with anti-inflammatory potential in vitro achieve good scores by the in silico bioactivity predictors studied. Therefore, it is essential to implement current bioinformatics tools, in order to complement in vitro experiments with prior prediction of potential bioactive peptides.
Inflammation is a physiological immune response triggered by noxious stimuli, and acts by restoring tissue homeostasis during infection or injury. This defense mechanism involves changes in vascular permeability, recruitment and accumulation of immune cells and release control of pro-inflammatory mediators at sites of immune reaction. Acute inflammation is the first line of defense, occurring immediately after inflammatory response induction, that temporarily (from minutes to a couple of days) triggers cellular and molecular events and interactions to prevent progression of damage or infection efficiently. Nonetheless, acute inflammatory response must be silenced over time to prevent loss of the immune function and further tissue deterioration, otherwise excessive and unregulated production of inflammatory mediators can lead to the development of chronic inflammation, a persistent inflammatory response. Furthermore, abnormal activation of inflammation-related enzymes, including inducible nitric oxide synthase (iNOS), cyclooxygenase-2 (COX-2), lipoxygenase (LOX) and phospholipase A 2 (PLA 2), play an important part in the development of inflammatory disorders.
Chronic inflammation is associated with increased risk of chronic diseases and disorders such as asthma, inflammatory bowel disease (IBD), cancer, cardiovascular disease, obesity and type-2 diabetes. Indeed, chronic inflammatory diseases dominate present-day morbidity and mortality worldwide with more than 50% of all deaths. Although the etiologies of chronic inflammatory diseases differ, the pathways that lead to pathological abnormalities are common. As a result, these inflammatory pathways can be explored as prospective targets for developing therapeutic treatments.
Regardless of initial stimuli's nature and location, the inflammatory cascade always involves the same steps: (1) noxious stimuli are recognized by cell surface receptors known as pattern-recognition receptors (PRRs); (2) inflammatory signaling pathways are triggered; (3) inflammatory mediators and signaling molecules are produced and released; and (4) blood vessels are dilated allowing inflammatory cells to accumulate in the inflamed tissue. Pattern-recognition receptors (PRRs), such as toll-like receptors (TLRs), are found in both immune and non-immune cells. PRRs can recognize conserved motifs of molecules expressed by bacterial structures called pathogen-associated molecular patterns (PAMPs), such as lipopolysaccharide (LPS) which is the major component of Gram-negative bacteria cell walls, or endogenous signals activated during tissue and cell injury named alarmins or danger-associated molecular patterns (DAMPS). Inflammatory mediators comprise cytokines of the interleukin-family (ILs) such as IL-6, IL-1β and IL-10, interferons (IFNs) like interferon-γ (INF-γ), tumor necrosis factors (TNFs) like tumor necrosis factor-α (TNF-α) and chemokines such IL-8, and they mediate inflammation through interaction with diverse cellular components or receptors such as IL-1 receptor (IL-1R), IL-6 receptor (IL-6R), and the TNF receptor (TNFR) among others. Once recognition of stimuli occurs, receptor activation triggers common signaling pathways including the nuclear factor kappa-B (NF-κB) and mitogen-activated protein kinase (MAPK). Regarding the NF-κB pathway, inactive protein complex NF-κB bounds to IκB protein; when PRRs recognize noxious stimuli, IκB protein degradation is induced. This releases NF-κB subunits p50 and p65 which translocate to the nucleus regulating genes involved in the inflammatory response. The NF-κB pathway regulate the production of pro-inflammatory cytokines (IL-1β, IL-6, IL-8, and TNF-α), anti-inflammatory cytokines (IL-10), expression of iNOS, which produces nitric oxide (NO), and COX-2, which is a key enzyme in the biosynthesis of inflammatory mediators such as prostaglandins and leukotrienes. Besides, it regulates the expression of adhesion molecules including vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1). In the case of MAPK pathway, it consists of a family of serine/threonine protein kinases, divided in three subfamilies: extracellular signal-regulated kinases (ERKs); C-Jun N-terminal kinases (JNKs); and p38 MAPKs; which upregulate or downregulate inflammation-related genes through protein phosphorylation. Particularly, the MAPK pathway stimulates enzyme phospholipase A2 (PLA 2) activity together with translocation of NF-κB subunits to nucleus.
While a great number of commercially anti-inflammatory drugs exist, these pharmacological therapies are often related with adverse side effects due to prolonged consumption. Non-steroidal anti-inflammatory drugs (NSAIDs), like ibuprofen, naproxen or aspirin, are some of the most frequently conventional drugs prescribed to combat chronic Inflammation. NSAIDs acts by inhibiting the activity of cyclooxygenase enzymes involved in the synthesis of pro-inflammatory mediators and they are associated with severe toxicity and hypertension and are contraindicated to older patients with cardiovascular, renal or hepatic complications. Therefore, natural compounds exhibiting anti-inflammatory activity, which can be included in functional foods or within nutraceutical formulations, are a potentially better alternative to synthetic drugs for the prevention and treatment of inflammatory diseases. For instance, the variety of immunomodulatory properties attributed to bioactive peptides has focused the attention on them for the treatment of inflammatory diseases. These peptides are small fragments of proteins that usually contain 2–20 amino acid residues per molecule and they achieve their biological activity once they are released from their parental protein. The release of bioactive peptides is accomplished by enzymatic or chemical hydrolysis, in vivo or in vitro simulated gastro-intestinal digestion or bacterial fermentation of the parental proteins. Although there are many studies that have observed the anti-inflammatory activity of single purified or synthesized peptides, others have evaluated the activity of whole protein hydrolysates composed of a mixture of diverse bioactive peptides. Once crossed the gastrointestinal barrier and survive enzyme degradation, active peptides are absorbed in the human body and perform a wide range of functions, including modulating physiological systems and inflammatory response due to the modulation of MAPK and NF-κB inflammatory signaling pathways through the downregulation of pro-inflammatory mediators and upregulation of anti-inflammatory mediators expression. The biological and functional properties of peptides are normally determined by their amino acid sequence, relative proportion of specific amino acids and the type of residues present at C- and N-terminals, as well as by their hydrophobicity, molecular weight/length and net charge. However, limited information about the relationship between structure and anti-inflammatory activity of bioactive peptides is available, which makes it difficult to understand the specific molecular mechanisms involved in their action.
Animal proteins from eggs, milk (casein and whey) and meat have been widely used for extracting bioactive peptides, but the production of these conventional animal-based protein sources is not feasible to meet the growing global protein demands in a sustainable manner. Plant protein has also been commonly used as a source of bioactive peptides, being legumes such as soybean among the most used crops. Nonetheless, the use of soybeans may be inconvenient because of the amount of antinutritional components that can cause negative effects. Moreover, soybean cultivation is typically associated with environmentally unfriendly and unsustainable practices.
The difficulties with conventional protein sources from animals and plants have prompted a search for other alternative or unconventional protein sources, such as those derived from edible insects and by-products from the food or agriculture industry. Edible insects can be produced with less negative impact on the environment than existing livestock. From the wide array of edible insects, yellow mealworms known as Tenebrio Molitor have been reported to provide a source of high-quality protein (53% protein in a dry basis) containing high amounts of essential amino acids. A recent study show that insect-derived protein can be rapidly digested and effectively absorbed, with no different from a high-quality dairy protein source. Alternative plant-based proteins are gaining increasing interest and a wide range of flour or defatted flours from different sources (especially legumes, cereals and oilseeds) are already in the market. By-products of the extraction of sunflower (Helianthus annuus L.) oil are considered an interesting raw material for biopeptides production due to their high content of protein. For instance, defatted sunflower meal (principal by-product) contains about 28–42% protein (dry basis). Other interesting crops, due to their favorable composition and availability, are Lupine family such as Lupinus albus or Lupinus angustifolius which have a high protein content (about 35%–40% in a dry basis) containing a high proportion of essential amino acids. Nutritional characteristics along with low water requirements and high productivity make chickpeas (Cicer arietinum) a great alternative to conventional protein sources. Chickpeas are an inexpensive source rich in protein (approximately 20% dry mass) with excellent balance of essential amino acid composition, high bioavailability, and low level of antinutritional factors. It is important to mention that pea protein, that is usually used as feed with a low added value, also contains a high content of protein (about 18% protein in a dry basis) and an adequate content of essential amino acids.
