Recent progress in preventive effect of collagen peptides on photoaging skin and action mechanism

Chongyang Li, Yu Fu,*, Hongjie Dai, Qiang Wang, Ruihang Gao, Yuhao Zhang,*

a Key Laboratory of Luminescent and Real-Time Analytical Chemistry, Ministry of Education, College of Food Science, Southwest University, Chongqing 400715, China

b Biological Science Research Center, Southwest University, Chongqing 400715, China

c Institute of Food Science and Technology, Chinese Academy of Agriculture Sciences/Key Laboratory of Agro-Products Processing,Ministry of Agriculture and Rural Affairs, Beijing 100193, China

d School of Food and Biological Engineering, Jiangsu University, Zhenjiang 212013, China

Keywords:

Collagen peptides

Photoprotection

MAPK

TGF-β-Smad

Action mechanism

A B S T R A C T

Ultraviolet (UV)-induced photoaging skin has become an urgent issue. The functional foods and cosmetics aiming to improve skin photoaging are developing rapidly, and the demand is gradually increasing year by year. Collagen peptides have been proven to display diverse physiological activities, such as excellent moisture retention activity, hygroscopicity, tyrosinase inhibitory activity and antioxidant activity, which indicates that they have great potential in amelioration of UV-induced photoaging. The main objective of this article is to recap the main mechanisms to improve photoaging skin by collagen peptides and their physiological activities in photo-protection. Furthermore, the extraction and structural characteristics of collagen peptides are overviewed. More importantly, some clinical trials on the beneficial effect on skin of collagen peptides are also discussed. In addition, prospects and challenges of collagen peptides are emphatically elucidated in this review. This article implies that collagen peptides have great potential as an effective ingredient in food and cosmetics industry with a wide application prospect.

1. Introduction

Collagen is the most abundant structural protein found in all animal skin and bones, accounting for about 30% of the total protein content, with a tight triple helix structure consisting of three identical or different peptide chains. Collagen sequences are highly conserved and consist of (Gly-X-Y)nrepeating motif, in which X is usually proline (Pro) and Y is usually 4-hydroxyproline(Hyp) [1]. At present, 29 different kinds of collagen have been identified from different biological tissues, while sequences,structures and functional properties of peptides vary considerably in terms of different types of collagen [2,3]. The most common and abundant one is type I collagen [4].

Skin, one of the largest organs, consists of epidermis, dermis and subcutis. Under internal aging factors and external environments,skin is subjected to degeneration (loss of physiological function,reduction of structural integrity, etc.) and represents signs of aging [5].Environmental factors that can accelerate skin aging comprise ultraviolet (UV) rays, tobacco, alcohol, diet and so on, among which the principal factor is UV radiation [6].

Long-term UV irradiation on skin will change the structure of skin, including shortening and thickening of collagen fibers,damaging of elastic fiber, loss of type I collagen and the changed proportion of collagen types in dermis. Thus, it may lead to reduction of skin elasticity, moisture and softness and appearance of brown spots and wrinkles, which is known as photoaging [7]. Photoaging is one of the main causes of skin aging. Theoretically, supplement of type I collagen can attenuate skin photodamage. However,the structure of collagen is relatively stable, which has a special triple helix structure, and its molecular weight (MW) is more than 300 kDa [8]. Therefore, absorption and utilization of collagen directly are inefficient. Under the treatment of acid, alkali, heat and enzyme,collagen can be degraded. The generated peptides with low MW have a higher possibility to be digested and absorbed by human body,thus nutritional and functional properties are improved, compared with intact collagen [3]. In the past years, an increasing number of studies have demonstrated the preventive effect of low MW collagen peptides on photoaging skin. In this article, the protective effects and molecular mechanisms of collagen peptides on photoaging skin are reviewed, which can provide a theoretical basis for application of collagen peptides in food and cosmetics industry.

2. Preparation and structural characteristics of collagen peptides

At present, enzymatic methods are commonly used for preparation of bioactive peptides. The reported proteases are divided into animal proteases (e.g. trypsin) [9], plant proteases (e.g. papain) [10]and microbial proteases (e.g. Alcalase) [11], among which Alcalase is the most commonly used due to its excellent capacity of hydrolysis.In order to increase the degree and hydrolysis yield of collagen,different combinations of proteases are widely used to hydrolyze collagen [12]. The summary of enzymatic hydrolysis of collagen and their bioactivities of collagen peptides are shown in Table 1.The enzymatically extracted collagen peptides possess biological activities, such as moisturizing, moisture absorption, inhibition on tyrosinase activity, antioxidation and potential anti-photoaging,and the beneficial effects are closely related to MW, amino acid composition and sequence of peptides.

Table 1Examples of preparation of collagen peptides by enzymatic hydrolysis.

The structure of collagen hydrolysates is characterized by MW,amino acid composition and sequence, which is also one of the main factors affecting efficacy of photoaging [21,22]. It has been reported that collagen peptides with anti-photoaging ability should have a MW of less than 3 kDa [23]. A recent study has revealed the effects of gelatin (SG) and its hydrolysates (SGH) extracted from salmon skin on photoaging skin. The results showed that SG and SGH improved photoaging skin by enhancing antioxidant capacity and immune regulation system of mice, among which the lower MW components(SGH, MW: 873 Da) were more potent [24]. Jelly fish collagen (JC)and collagen hydrolysate (JCH) showed the same pattern that JCH with the lower MW (MW: ＜ 5 kDa) had higher protective activity on photoaged mice because it was easier to reach plasma and target organs [25].

In addition, the anti-photoaging activity of collagen peptides is closely related to their antioxidant capacity, which is attributed to amino acid composition and sequence. Collagen peptides with antioxidant activity usually possess a high proportion of hydrophobic amino acids. Besides, it was well documented that all the antioxidant peptides possess Pro, His, Tyr, Trp, Met and Cys in the structure [26]. Moreover, the purified antioxidant peptides usually contain hydrophobic amino acids at the C- and N-terminal,such as Ala, Phe, Leu, Pro, etc. [27]. Three antioxidant peptides were isolated from large yellow croaker scales, and their sequences were identified as Gln-Arg-Pro-Pro-Glu-Pro-Arg, Glu-Lys-Val-Trp-Lys-Tyr-Cys-Asp and Val-Gly-Leu-Pro-Gly-Leu-Ser-Gly-Pro-Val-Gly,respectively [28]. The antioxidant activity followed a sequence order of Gly-Phe-Pro-Ser-Gly ＞ Gly-Pro-Ala-Gly-Pro-Ala-Gly ＞ Gly-Phe-Arg-Gly-Thr-Ile-Gly-Leu-Val-Gly for free radicals assays [13].Sun et al. [29]sequenced the tilapia skin collagen peptide with the highest free radical scavenging capacity, and its sequence was Leu-Ser-Gly-Tyr-Gly-Pro. Mendis et al. [30]isolated fish skin collagen peptide with strong antioxidant property and identified its sequence as His-Gly-Pro-Leu-Gly-Pro-Leu. These may confirm the foregoing theories.

3. Mechanism of skin photoaging

Long-term UV radiation provokes a series of changes through cell signaling pathways, leading to degradation of extracellular matrix(ECM) and occurrence of photoaging response. The main histological features of photoaged skin are changes in the composition of ECM,gradual decrease in collagen synthesis and increase in collagen degradation, leading to reduced content of collagen. Transforming growth factor β (TGF-β)/Smad is a main regulatory pathway for the decrease of collagen synthesis [31], and an imbalanced expression of matrix metalloproteinases (MMPs)/tissue inhibitors of matrix metalloproteinases (TIMPs) in dermal fibroblasts is the main reason for the increased ECM degradation [32].

TGF-β/Smad is an important signaling pathway regulating synthesis of type I collagen in dermis. TGF-β is an important cytokine that regulates the synthesis of ECM including collagen, elastin and fibronectin, which has two types of receptors, including type I TGF-β(TβRI) and type II TGF-β (TβRII) [33]. TGF-β, cell receptors and Smad proteins constitute TGF-β/Smad signaling pathway. TGF-β first binds to its receptor TβRII, resulting in the phosphorylation of transduced receptor TβRI, which can activate Smad2 and Smad3 through a series of signal transduction. After activation, Smad2 and Smad3 form a complex with Smad4, which is transferred to nucleus and binds to procollagen transcription promoter, thus promoting collagen synthesis [34]. Smad7 can interact with phosphorylated TβRI to interfere with the activation of Smad2 and Smad3, thus blocking TGF-β signal transduction [35]. Exposure to UV can not only downregulateTβRIImRNA expression and inhibit the phosphorylation of Smad2, but also up-regulate Smad7 expression, thus interfering with TGF-β/Smad signaling pathway and inhibiting the synthesis of type I collagen [36].

MMPs are a group of zinc-containing endopeptidases [37]. At present, 26 MMPs including MMP-1, MMP-2, MMP-3 and MMP-9 have been found, which can be divided into 6 categories according to their structure and specificity, including gelatinase, collagenase,matrix lysin, stromelysins, membrane matrix metalloproteinase(MT-MMP) and others [38,39]. MMPs are involved in degradation process of ECM. Different kinds of MMPs degrade different kinds of ECM proteins, among which MMP-1 can degrade type I and type III collagen, which is the most crucial MMPs for degrading collagen fibers. TIMPs are endogenous restrainer of MMPs that can down-regulate the activity of MMPs and reduce the specific binding of MMPs to substrate [40]. The expression of MMPs/TIMPs in dermal fibroblasts is regulated by several signaling pathways, most of which focus on mitogen-activated protein kinases (MAPK). Under the stimulation of UV, the generated free radicals stimulate the phosphorylation of extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK) and p38 MAPK through MAPK signaling pathway. Phosphorylation of ERK induces activation of c-Fos, while phosphorylation of JNK induces activation of c-Jun, leading to activation of activator protein 1 (AP-1), and finally induces expression of MMPs, degradation of collagen and skin photoaging [15,41].

TGF-β/Smad and MAPK signaling pathways affect collagen metabolism and participate in the photoaging process. Furthermore,nuclear factor κB (NF-κB), phosphatase and tensin homolog deleted on chromosome 10 (PTEN)/protein kinase B (PKB), hedgehog (Hh)and Wnt signaling pathways may also involve in the metabolism of collagen. Several recent studies have reported that UV can induce the activation of NF-κB, PTEN/PKB, Hh and Wnt pathways, thereby causing photoaging [42-46]. NF-κB and Hh signaling pathways participate in the expression of inflammatory factors and MAPK activation in UV-induced photoaging [42,47]. Moreover, UVB radiation can induce phosphorylation of PTEN and PKB in fibroblasts, upregulate the activity of AP-1, and increase the expression of MMP-1 and MMP-3 [43]. As for Wnt signaling pathway, UV irradiation can down-regulate the expression of Wnt3a, which has a positive correlation with photoaging related protein TGF-β2, thus decreasing secretion of collagen in fibroblasts [46].

To sum up, UV can induce skin photoaging probably through multiple signaling pathways, such as TGF-β/Smad, MAPK, NF-κB,PTEN/PKB and Wnt pathways (Fig. 1). Therefore, collagen peptides may ameliorate photoaging skin by mediating the related signaling pathways.

Fig. 1 Schematic diagram of skin photoaging. UV controls the synthesis and degradation of collagen by regulating multiple signaling pathways, causing skin photoaging.

4. Collagen peptides protect against the loss of moisturizing factor

Natural moisturizing factor (NMF) is the hydrolytic derivative of filaggrin protein [48]. NMF components are high-efficiency humectants such as urea, uric acid and pyrrolidone carboxylic acid, which can absorb water from the external environment [49].Therefore, NMF can maintain the basic stability of water content in skin dermis. Meanwhile, the ion interaction between hydrated NMF and keratin fibers can reduce the molecular force between fibers,thus increasing the elasticity of cuticle. The decreased content of NMF is affected by a variety of factors. The most important reason is UV radiation, which can impair the hydrolysis offilaggrin protein,ultimately leading to reduction of NMF content and decrease of skin hydration ability [50]. Collagen peptides contain a large number of NMF, such as Ser, Asp, Hyl, Hyp, etc. [51]. Moreover, there are many hydrophilic groups, such as hydroxyl and carboxyl in collagen peptides, which can enhance the ability of skin to bind and retain water and effectively improve water content of photoaging skin [52].

Hou et al. [53]prepared two collagen peptides with different MWs (Collagen peptide 1 (CP1): 2 kDa ＜ MW ＜ 6 kDa; CP2: MW ＜2 kDa) by enzymatic hydrolysis of cod skin, and evaluated their hygroscopicity and moisturizing property under different relative humidity conditions. Results showed that two collagen peptides had excellent water absorption and retention property, and CP2 exhibited better performance probably due to the lower MW. Animal experiments revealed that UVB repeated irradiation (10-30 mJ/cm2,3 times/week for 6 weeks) could increase transepidermal water loss (TEWL) and decrease water content in cuticle, while intake of collagen peptides could significantly reduce TEWL and increase water content, which may be related to the increased hyaluronic acid content in mice skin [54]. The active components of collagen peptides may stimulate fibroblasts, induce hyaluronic acid synthesis, and lead to increase of hyaluronic acid content in dermis [55]. Hyaluronic acid contains a large number of hydrophilic factors, which can absorb and maintain high moisture, thus improving water content and elasticity of skin [56]. The hygroscopic and moisturizing properties of collagen peptides are related to their MW. Generally, the lower MW is, the better hygroscopic and moisturizing properties are. The underlying reason may be that the peptides with lower MW have more polar residues, forming hydrogen bonds with water and improving moisture absorption and retention properties [57]. Collagen peptides from different sources, such as star fish body wall [58], sea cucumber wall [59],and giant croaker skin [60], show good moisture retention and hygroscopicity, and have great potential for improving moisture content of photoaging skin.

5. Protection against excessive production of melanin

The color of human skin is related to the content, type and distribution of melanin, which is a substance produced by melanocytes and belongs to stress mechanism. Melanin is able to absorb UV rays. When exposed to sunlight, melanocytes produce melanin, which darkens skin and absorbs excessive UV rays to prevent them from penetrating into human body, thus playing a protective role. However, under the stimulation of UV rays, melanin produces reactive oxygen species (ROS), which can cause mutations in cells, thus having a deleterious effect on the body [61]. Besides,when there is excessive UV ray, melanin will overproduce, causing a number of skin disorders, such as freckles, darkening of skin, age spots, and even skin cancer. Tyrosinase is the key enzyme of melanin formation, which can gradually transform tyrosine into melanin.Tyrosine residues in collagen peptides can combine with the active part of tyrosinase concomitant with a decreased tyrosinase activity.Tyrosinase cannot catalyze the conversion of tyrosine to melanin, and the production of melanin is reduced. Thus, collagen peptides can exhibit a photoprotective effect. The strength of tyrosinase inhibitory activity of peptide depends on its source, amino acid composition and MW, etc. Asp or Glu is not conducive to the binding of peptide and tyrosinase, presence of Arg is necessary for tyrosinase inhibitory activity of peptides, and hydrophobic aliphatic amino acid residues,such as Ala, Leu, Val, Phe and Met, are also important for tyrosinase inhibitory activity [62,63]. Collagen peptides are rich in hydrophobic amino acids and Chen et al. [64]reported that the hydrophobic amino acids content of collagen extracted from tilapia scales was 401 residues/1 000 amino acid residues. Niu et al. [65]prepared cod skin collagen peptides with hydrophobic amino acid content approximately 50% . This indicates that collagen peptides have potential applications in tyrosinase inhibition. From this perspective,a wide array of collagen peptides that possess tyrosinase inhibitory activity have been found from various collagenous sources. In recent years, collagen peptides that possess tyrosinase inhibitory activity have been identified from jelly fish [66], pacific cod skin [67], squid fish [16], porcine skin [68]. The inhibitory effect of different collagen peptides on tyrosinase activity is summarized in Table 2.

Table 2Summary on tyrosinase activity and melanin content of B16 melanoma cells inhibited by different sources of collagen peptides.

To date the exact mechanism that collagen peptides inhibit tyrosinase activity and subsequently reduce melanin content has not been clarified. Melanin production involves several signaling pathways, including multiple molecules and gene regulation, among which cyclic adenosine monophosphate (cAMP) is one of the important regulatory pathways that can up-regulate microphthalmiaassociated transcription factor (MITF) by activating protein kinase A to achieve melanin synthesis [72]. Hou et al. [67]proposed that the inhibition of collagen peptides on melanin synthesis may have a direct connection with the decreased cAMP expression. Several recent studies have shown that MAPK pathway can also affect the synthesis of melanin [73], while collagen peptides play a certain regulatory role in MAPK pathway [15,22]. Whether collagen peptides can regulate melanin synthesis through MAPK pathway has not been elucidated,which may be one of the major molecular mechanisms to explain the inhibition of melanin synthesis by collagen peptides.

6. Scavenging activity of photo-induced free radicals

Free radicals can attack proteins, nucleotide, lipids and other biological macromolecules, thus causing cell or tissue damage [74].Moreover, free radicals have a strong oxidative ability, which can induce oxidation reaction in organisms, leading to skin photoaging.Collagen peptides with antioxidant activity can scavenge excessive free radicals in human body and thus impede aging and improve aging skin.

Abdelhedi et al. [75]have shown that black-barred halfbeak skin gelatin hydrolysates scavenged 1,1-diphenyl-2-picrylhydrazyl (DPPH)radical by 43.39% at the concentration of 5 mg/mL. Nakchum et al. [16]reported that squid skin collagen hydrolysates with MW of 3-10 kDa exhibited excellent free radical scavenging activity and the IC50values for scavenging DPPH, hydroxyl and 2,2’-azino-bis-3-ethylbenzthiazoline-6-sulphonic acid (ABTS) radicals were 417.43,149.94 and 1 306 μg/mL, respectively. According to Chen et al. [76],antioxidant activity of peptides depended on the composition and sequence of amino acids, and His played an important role in antioxidant activity. Chi et al. [77]and Li et al. [78]claimed that MW was the main factor for determining antioxidant capacity of collagen hydrolysates. Antioxidant activity of collagen peptides is negatively related to the logarithm of MW, which may be due to the fact that peptides with low MW can expose more electronic donors, bind to free radicals and interrupt free radical chain reaction. Therefore, small peptides are more readily to pass through the intestinal barrier. However,it has been shown that peptides with MW of 1 400 Da have stronger antioxidant activity than those with MW of 900 Da and 200 Da [79].Collagen peptides from different sources, such as jumbo squid skin [23], skipjack tuna bone [80], blue fin leatherjacket heads [81],cuttle fish skin [82], andPinctada fucatamuscle [83], exhibit excellent antioxidant activity. Table 3 summarizesin vitroantioxidant activity of some collagen peptides derived from different collagenous sources.The presence of hydrophobic amino acids in collagen peptides is of great significance for antioxidant activity, and sulfur-containing amino acids can also improve the antioxidant activity of collagen peptides [84,85]. In addition, antioxidant activity of collagen peptides is related to the position and composition of amino acids.Hydrophobic amino acids at the terminal of collagen peptides show more potent antioxidant activity [86], and Gly-Tyr and Gly-Pro significantly contribute to the antioxidant activity of collagen peptides [87,88]. Therefore, it can be concluded that antioxidant activity of collagen peptides is related to their MW, amino acid composition and sequence.

Table 3In vitro antioxidant activity of collagen peptide derived from different sources.

Numerous studies have shown that collagen peptides can resist the damage of free radicals and improve the photoaging skin by enhancing the activity of antioxidant enzymes, scavenging the excessive free radicals in the body and reducing the oxidative stress. Superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GSH-Px)are capable of scavenging free radicals.In vivostudies have demonstrated that collagen peptides can significantly enhance the activities of antioxidant enzymes, such as SOD, CAT and GSH-Px,reduce the content of malondialdehyde (MDA) in serum and skin tissue homogenate, and effectively improve aging skin of mice [53,91].Wang et al. [92]reported the improvement effect of collagen peptides after oral intake on the skin of aging mice, and results showed that tilapia collagen peptides could significantly increase the activity of antioxidant enzymes (SOD and GSH-Px), but not in a dose-effect manner. Tao et al. [93]found that collagen peptides extracted from cartilage could reduce MDA content, improve the activities of SOD,CAT, and GSH-Px, enhance the endogenous antioxidant defense system, and scavenge excessive free radicals. Zhang et al. [94]reported that collagen peptides derived from silver carp skin could reduce oxidative stress in the body, increase the activities of SOD and CAT in serum and tissues, and significantly reduce the content of MDA. In addition, their antioxidant activity was more potent than those of tea polyphenols and casein peptides.

7. Morphology-based improvement of photoaging mice skin treated with collagen peptides

Healthy normal skin tissue structure is intact, the thickness of dermis is uniform and moderate, and collagen fibers are orderly arranged and evenly distributed. After being exposed to UV rays,skin tissue structure can change, the degenerated and non-functional collagen fibers replace normal fibers, and the arrangement of dermis fibers is disordered and coarser with serious clustering, severe keratinization, and epidermis hyperplasia and thickening. After the treatment of photoaging mice with collagen peptides (167 or 333 mg/kg/day), Hematoxylin-Eosin (H&E) staining was carried out to observe tissue changes on the skin of mice. It was observed thatcollagen peptides could improve the state of photoaging skin and dermal fiber tissue arrangement was more orderly and dense, which had a better ameliorating effect on the skin tissue disorder caused by photoaging (Fig. 2) [95].

Fig. 2 HE staining to evaluate the pathological changes of skin of photoaging mice treated with cat fish tripeptide peptide (CTP) from Pyun et al. [95].(A) Control, (B) UVB control, (C) CTP-167 (167 mg/kg/day CTP-treated group), (D) CTP-333 (333 mg/kg/day CTP-treated group). Graph adapted with permission from (Pyun, H. B. 2012. Effects of collagen tripeptide supplement on photoaging and epidermal skin barrier in UVB-exposed hairless mice.Preventive Nutrition and Food Science. 17: 245-253). Copyright (2012) The Korean Society of Food Science and Nutrition.

Normal skin is composed of 80% type I collagen and 15% type III collagen. In photoaging skin, collagen fibers become shorter and thicker, and type I collagen is lost. The ratio of type III to type I collagen is significantly increased, and proportion of collagen is altered [96]. The density of collagen and elastin in dermis is decreased, and structure and elasticity of skin are reduced, resulting in the thinner and harder skin. Collagen peptides can increase the content of collagen in skin, reduce the loss of type I collagen, balance the proportion of type III and type I collagen, and decrease the thickness of dermis, which help to ameliorate UV-induced skin damage [97].Hou et al. [53]stained the skin of mice with Sirius red and found that two collagen peptides repaired the endogenous collagen and elastin fiber, maintained the proportion of type III and type I collagen, thus played a protective role in photoaging skin, but lower MW peptides were more active. This is similar to the conclusion of Song et al. [21],who found that MW of collagen peptides affected skin repair of photoaging mice. Compared with high MW peptides (MW ＞ 1 kDa),oral administration of low MW peptides (0.5 kDa ＜ MW ＜ 1 kDa)had a better repair effect on the skin, while gelatin (120 kDa ＜MW ＜ 220 kDa) had no significant improvement effect on the skin. This may be related to the higher antioxidant performance of low MW peptides. Generally, low MW peptides can expose more electron donors, possess higher free radical scavenging ability,thus slow down the damage of free radicals on body skin and change the synthesis of ECM through signaling pathways (such as TGF-β/Smad, MAPK) [22,98]. In addition, low MW peptides are more easily absorbed and exhibit higher bioactivity, which contributes to repair of photoaging skin.

8. Absorption mechanism of collagen peptides and signaling pathway responsible for inhibiting photo-oxidation

Oral administration of collagen peptides has been proven to be safe. López-Morales et al. [99]revealed that collagen peptides(2.5 and 25 μg/mL) were non-toxic and harmless to Caco-2 and HepG2 cells. The daily administration of 2.25% -18% (m/m) of marine collagen peptides lasted for 24 months in rats, and there were no obvious adverse effects and health risks at this dose [100]. Collagen peptides extracted fromLates calcariferskin were orally administered to rats at the dose of 2 and 5 g/kg body weight (bw) for 2 weeks.No acute toxicity was found in rats, and the lethal dose 50 (LD50) of collagen peptides was greater than 5 g/kg bw [101]. Volunteers orally ingested 1 g of low MW collagen peptides daily for 12 weeks. The results showed that the subjects had no unhealthy symptoms during this period [102]. It has been shown that collagen peptides are safe for consumption.

In order to speculate on the action mechanism after oral administration, clarifying the bioavailability of collagen makes sense. It has reported that after oral ingestion of gelatin hydrolysates(183–385 mg/kg bw), the content of Hyp-containing peptides in plasma increased and reached a peak level after 0.5-2 h [103,104].Besides, the relative and absolute bioavailability of gelatin hydrolysate was more than 70% at the dose of 4 000 mg/kg bw [105].Oesser et al. [106]reported that mice were administrated with 10 mg of14C labeled gelatin hydrolysate/g bw and it was found that more than 90% of the gelatin hydrolysates were absorbed within 6 h after oral administration. At 12 h, the radioactivity in skin reached its peak. These indicated that collagen can be quickly digested and absorbed after oral administration. In addition to free amino acids, collagen could also be absorbed in the form of peptides [107], and the proportion of Asp and Glu absorbed in peptide form is more than 80% [105].The absorbed peptides can reach dermis of skin. After tracking the labeled collagen peptides, it could be observed that the radioactive substance stayed in skin for up to 14 days [108], and the concentration of Pro-Hyp was the highest [109]. Pro-Hyp could stimulate the growth offibroblasts in skin, affect the migration offibroblasts, and enhance the synthesis of hyaluronic acid [110,111], which may be due to the effective peptide fraction generated by oral ingestion of collagen peptides, while Yazaki et al. [109]indicated that oral intake of collagen peptides containing high concentration of tripeptides caused the incorporation of Pro-Hyp-Gly, Gly-Pro-Hyp and Pro-Hyp into the skin, thus Pro-Hyp may not be the only functional peptide component. These facts indicate that collagen peptides can be effectively absorbed and distributed in the skin, which can provide an effective effect on photoaging skin. However, the exact structurefunction relationship of collagen peptides is still unclear and remains to be further investigated.

The mechanism of collagen peptides to improve photoaging skin has been explored. It was found that collagen peptides can change the synthesis of ECM via regulating TGF-β/Smad, MAPK, NF-κB and other signaling pathways. On the one hand, collagen peptides may enhance antioxidant capacity, scavenge ROS, and then regulate related pathways, thereby attenuating photoaging. The cod skin collagen hydrolysate (CH) could down-regulate the activation of p-ERK, p-p38 and p-JNK, reduce the expression of MMPs, and attenuate photoaging effects in mice through MAPK pathway. In addition, CH also regulated TGF-β/Smad signaling pathway and inhibited the activation of NF-κB pathway to reduce photodamage [22,98]. A pentapeptide YGDEY isolated from tilapia skin gelatin hydrolysates suppressed the phosphorylation of p38 and JNK, inhibited the expression of p-p65 and p-IκB and down-regulated the expression of MMPs through MAPK and NF-κB pathways, thus playing a role in photo-protection [112].On the other hand, it was reported that collagen peptides could also bind to the active sites of MMP-1 and MMP-9, thereby inhibiting their activities. Abalone peptide could inhibit activities of MMP-1 and MMP-9 by forming hydrogen bonds with their active sites, among them N-terminal Ala, C-terminal Gly, and Pro at the third position of N-terminal made an important effort [113]. A peptide YGDEY derived from tilapia skin formed hydrogen bonds with MMP-1 and MMP-9 at Leu235 and Asp131 sites with a distance of 2.14 Å and 2.01 Å, respectively, thus reducing the activity of MMP-1 and MMP-9 [112]. LSGYGP derived from gelatin hydrolysate isolated from tilapia skin combined with Tyr240 and Thr241 of MMP-1 and Glu402 of MMP-9 to form hydrogen bonds, thereby exhibiting its inhibitory activity. Moreover, LSGYGP had a hydrophobic amino acid (Pro) at the carboxy terminal and the aromatic group in Tyr, which may accommodate with the S1’ pocket of MMP-1 and MMP-9 and enhance the ability of inhibitory MMPs [114]. The mechanism of collagen peptides ameliorating photoaging skin is summarized in Table 4 and Fig. 3.

Table 4Summary on the photo-protective signaling pathway of collagen peptides.

Fig. 3 The protective mechanism of collagen peptides on UVB-induced photoaging skin.

At present, as far as the current studies are concerned, there are several reports on the mechanism of collagen peptides for improving photoaging skin, and they mainly focused on TGF-β/Smad, MAPK,NF-κB signaling pathways. However, the exact mechanism of photoaging is complex and may simultaneously involve several pathways. In addition to the above pathways, PTEN/PKB, Hh and Wnt pathways may also be involved in photoaging. The influence of peptides on these pathways needs to be further investigated.Furthermore, the specific structure-function relationship and action mechanisms are less examined and further exploration is necessary for skin photo-protection.

9. Clinical trials on the beneficial effect on skin of collagen peptides

As previously mentioned, a growing number of preclinical experiments have proven that collagen peptides can improve photoaging skin through signaling pathways such as MAPK,NF-κB and TGF-β/Smad. However, animal experiments have certain limitations due to the differences in physiological functions between mice and human. In order to better understand the safety of oral collagen peptides and their beneficial effects on skin, it is necessary to conduct the controlled clinical trials. At present, a number of double-blind,placebo-controlled clinical trials have proved that collagen dipeptides(Pro-Hyp and Hyp-Gly) [116], specific porcine type I collagen hydrolysate [117]and sutchi catfish’s skin low-molecular-weight collagen peptides [102]exert positive impacts on human skin health, and they are safe without adverse reaction during the trial. Czajka et al. [118]found that after 90 days of oral nutritional supplements containing 8% fish collagen peptides, the skin elasticity of volunteers was significantly increased, the thickness of dermis was increased, and photo-damagerelated inflammation indices were reduced. Genovese et al. [119],adopting a randomized, double-blind, placebo-controlled method to determine the skin parameters of volunteers after taking the nutrients containing type I collagen hydrolysates or placebo for 90 days, found that oral nutritional supplements could promote the synthesis of collagen, improve skin photoaging parameters and have the potential photoaging protection for skin. Schwartz et al. [120]revealed that the subjects who took a dietary supplement containing chicken sternal cartilage collagen hydrolysates (1 g/day, 12 weeks) experienced the reduced skin dryness and increased collagen content, which can be used to counteract the natural photoaging process. However, there are still several clinical trials on collagen peptides to improve photoaging skin, and most of them are mainly nutritional supplements containing collagen peptides. The specific role of collagen peptides in improving photodamage skin is unclear and remains to be investigated. In addition, the mechanism of collagen peptides to improve the condition of photoaging skin is rarely studied in clinical trials. Therefore, a controlled, long-term human study is necessary to verify the antiphotoaging effect of collagen peptide and to better understand its potential mechanisms.

10. Prospects and challenges

Over the years, much attention has been paid to collagen peptides due to their exclusive bioactivity. This review indicates that collagen peptides can be used as a potential protective ingredient against photodamage, which indicates that there will be great potential for application of collagen peptides to meet this growing demand.Collagen mainly originates from animals, including terrestrial and aquatic animals. Nevertheless, there are potential risks in terrestrial animals owing to zoonoses, such as mad cow disease and foot-and-mouth disease. Hence, marine-derived collagen has become a hot topic in recent years. Marine-derived collagen can be extracted from marine animal by-products, which is of practical significance to reduce the waste of raw materials and protect the environment.However, collagen derived from marine organisms may have certain differences from mammal owing to their low content of imino acids(Pro and Hyp), which may affect digestibility and bioactivity of collagen peptides. However, the photoprotective effect of collagen peptides varies markedly with collagen source, types and extraction methods. Wang et al. [121]reported that compared with collagen peptides derived from porcine, bovine, and tilapia skin, hen-derived collagen peptides were more excellent in terms of photoprotection in UVA-induced photoaging human dermal fibroblasts with a positive effect on inhibition of MMP-1 and MMP-9. Offengenden et al. [122]reported that chicken collagen peptides obtained by Protease M and Alcalase is superior to those hydrolyzed by Protease M and Protex 50FP, Protease M and Protex 51FP, and Protex 50FP and Alcalase on the stimulating synthesis of type I collagen in human dermal fibroblasts. These can be attributed to the differences in utilization form and structure and amount of peptides in human blood after oral administration of collagen peptides from different sources and enzymatic extraction, thereby affecting their bioactivity. Therefore,the structure-function relationship of collagen peptides and the antiphotoaging activity will become one of the future research focuses on this field. Besides, further exploration is also necessary to obtain more specific information on the exact components to take effect after oral ingestion of mixed collagen peptides.

Last but not least, although much progress has been made in exploration of the specific anti-photoaging mechanism of collagen peptides, there are still many questions to be answered. At present,anti-photoaging studies of collagen peptides mainly focus on TGF-β/Smad, MAPK and NF-κB signaling pathways; however, the molecular mechanism is not comprehensively elucidated. Finally, the photoprotective effect of collagen peptides is still under investigation,and further studies on their clinical efficacies are warranted.

Conflict of interest statement

There are no conflicts to declare.

Acknowledgments

This work was financially supported by National Key R&D Program of China (No. 2016YFD0400200), National Natural Science Foundation of China (No. 31972102, 31671881, and 31901683), Chongqing Research Program of Basic Research and Frontier Technology (No. cstc2018jcyjA0939), Chongqing Technology Innovation and Application Demonstration Project (No.cstc2018jscx-msybX0204), Fundamental Research Funds for the Central Universities (No. XDJK2019B028), Innovation Program for Chongqing’s Overseas Returnees (cx2019072), and Fundamental Research Funds for the Central Universities, China (SWU 019009).