Bioactive Molecules from Forest Resources

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1		Tatjana Stevanovic	--	3128	2023-09-01 08:47:32	\|
2	Reference format revised.	Lindsay Dong	Meta information modification	3128	2023-09-01 08:49:42	\| \|
3	Reference format revised.	Lindsay Dong	Meta information modification	3128	2023-09-01 08:50:03	\|

This entry is adapted from the peer-reviewed paper 10.3390/molecules28052045

Forest trees are the world’s most important renewable natural resources in terms of their dominance among other biomasses and the diversity of molecules that they produce. Forest tree extractives include terpenes and polyphenols, widely recognized for their biological activity. These molecules are found in forest by-products, such as bark, buds, leaves, and knots, commonly ignored in forestry decisions. The present entry focuses on in vitro experimental bioactivity from the phytochemicals of Myrianthus arboreus, Acer rubrum, and Picea mariana forest resources and by-products with potential for further nutraceutical, cosmeceutical, and pharmaceutical development.

antioxidant extractive forest nutraceutical polyphenol terpenes

1. Introduction

Forests are considered one of the most important natural resources globally, not only for their undeniable ecological importance but also for their wealth of bioactive molecules. Wood-based materials include solid wood products, engineering composites, paper, and fiber products. Forest exploitation generates large amounts of by-products (leaves, twigs, knots, buds, bark, acorns, etc.) considered an important resource for obtaining antioxidant and bioactive molecules ^[1].

The demand for such molecules of natural origin for the food, cosmetic, and pharmaceutical industries has increased in recent years due to the perception of their lower toxicity and reduced healthcare costs regarding synthetic products ^[2]^[3]. Among the woody species, the differences in the chemical structures of the three main components that form the cell walls (cellulose, hemicelluloses, and lignins) are relatively small, especially when compared with the great diversity of the components extraneous to the cell walls, commonly designated as extractives, as they can be conveniently solubilized by organic solvents and/or water.

There are many types of chemical substances belonging to the forest tree extractives, including terpenoids and polyphenols, widely recognized for their antioxidant activity. Heartwood formation, one of the distinctive metabolic events of woody plants, involves the deposition of biologically important polyphenols, such as stilbenes, flavonoids, proanthocyanidins, lignans, and neolignans. These extractable “woody polyphenols” have a structural analogy with lignins, due to their common phenylpropanoid biosynthetic pathway ^[1]. As ubiquitous plant constituents, polyphenols are important for the human diet, being present in vegetables and fruits, as well as in medicinal herbaceous plants used in folk medicine. The potential application of these molecules as natural antioxidants is related to their chemical structure, which interferes with different phases of oxidative reactions in the organism. The number and position of the hydroxyl groups and the presence of aromatic rings allow them to react with oxygen species, also chelating ferric and other cations by transferring hydrogen atoms or by donating an electron to the radicals.

One of the most remarkable cases illustrating the presence of bioactive phenols in tree residues is that of Salix spp. The story of Salix bark constituents as analgesic agents goes deeply into the times of ancient Egyptians, as it was revealed by a discovery, in the mid-19th century, of two ancient papyrus scrolls dated 1500. B.C, one of which described the use of willow barks ^[4]. These discoveries inspired the identification of salicin, an important glucoside of salicylic acid, as the component of Salix bark by the very end of the 19th century.

Recognized representatives of bioactive polyphenols in foods include resveratrol in red wine, epigallocatechin gallate in green tea, chlorogenic acid in coffee, anthocyanins in colored fruits, and procyanidins in grape seed, which have been important research objects for food science and nutrition ^[5]. In forest trees, the preponderance of such phenolic compounds is particularly remarkable in bark and knot wood ^[6]^[7].

In addition to polyphenols, mono-, di-, sesqui-, and triterpenoids have been long known and utilized as forest-tree-derived resins and balsams. Terpenoids protect the tree from the invasion of pathogens and herbivores by inducing multiple defense mechanisms. Although the general composition of terpenes is characteristic of each species, it can differ between two individual trees. Drought, temperature fluctuations, air, and soil pollution, or the attack of pathogens can cause a reorganization of biosynthesis and emission of terpenes from trees ^[8].

The antioxidant or prooxidant activity of a particular terpene depends on its doses and structural characteristics—at high concentrations, they can act as prooxidants, whereas at low concentration, they can function as antioxidants ^[9]. For the monoterpenes group, the hydrocarbon-type, containing a methylene group in its structure, and the oxygenated type, showing phenolic structures, have the highest antioxidant activity, while in the case of sesquiterpenes, allylic alcohol types are the most active as antioxidants ^[10]. The phenolic O-H included in some types of diterpenes, such as phenolic derivatives of abietane-type resin acids, seem to be important in the antioxidant activity of these compounds, mainly in their sequestering properties. Dienone–phenol rearranged triterpenes have the highest antioxidant activity among different structural types of triterpenes. Among tetraterpenes, carotenoids act as very efficient antioxidants, which is attributable to the conjugated double bonds of their chemical structure ^[10].

In forest trees, terpenoids are stored mainly in the resin of the heartwood and sapwood of conifers, as well as in the sapwood of hardwoods ^[11]. They are also found in the needles of conifers and leaves of hardwoods. In conifers, the terpenoid group consists mostly of monoterpenes (e.g., α-,β-pinene), sesquiterpenes (e.g., β-caryophyllene), and diterpene resin acids (e.g., abietic acid derivatives), whereas triterpenes (e.g., betulin and lupeol) and sterols (e.g., β-sitosterol) dominate in hardwoods ^[12]^[13]. One of the most well-known and commercially successful derivative of diterpenoids isolated from forest resources is paclitaxel, which has been identified, for the first time, from the bark of Taxus brevifolia ^[14]. Paclitaxel is also found in Taxus canadensis ^[15] and is used as a chemotherapeutic agent for treating various metastatic cancers, such as ovarian and breast cancers. Other triterpenoids found in forest trees are betulin and betulinic acid present in the outer bark of birch. In contrast to betulinic acid, betulin is present in significantly larger quantities in the birch outer bark (10–35%) ^[16].

2. Bioactive Molecules from Promising Forest Resources of Myrianthus arboreus, Acer rubrum, and Picea mariana

2.1. Myrianthus arboreus

Myrianthus arboreus P. Beauv. (Cecropiaceae) is a tree that grows in tropical areas of the West African rainforest. It is an edible indigenous woody plant important for the rural economy of the region since, in addition to its direct dietary benefits, it is used as timber, firewood, and traditional medicine, also having sociocultural and religious roles ^[17]. Several bioactive compounds, notably bioactive phenols and terpenes have been identified in extracts from forest by-products of this tree (Table 1).

Table 1. Bioactive molecules identified in extracts from forest by-products of M. arboreous.

Molecule Name	Classification	Extract Type	Plant Tissue	Bioactivity	Ref.
Epicatechin	Flavonoid	Ethyl acetate fraction (EtOAc) of 70% ethanolic extract.	Stem bark	All compounds, except euscaphic acid, inhibited the in vitro action of α-amylase. Euscaphic acid stimulated the glucose uptake in C2C12 cells. Epigallocatechin, dulcisflavan, tormentic acid, and arjunolic acid showed hypoglycaemic and anti-hyperlipidaemic activities in treptozotocin (STZ)-induced diabetic rats. Dulcisflavan was considered the most active compound and an appropriate substrate for further drug development.	^[18]
Epigallocatechin	Flavonoid
Dulcisflavan	Flavonoid
Euscaphic acid	ursane-type triterpenoids
Tormentic acid	ursane-type triterpenoids
Arjunolic acid	oleanane-type pentacyclic triterpenoid
3β-O-trans-feruloyl-2α,19α-dihydroxyurs-12-en-28-oic acid (H1)	ursene-type pentacyclic triterpene	Ethyl acetate fraction (EtOAc) of 95% ethanolic extract.	Root bark	All compounds decreased in vitro the activity of hepatocellular glucose-6-phosphatase (G6Pase) and activated glycogen synthase via the phosphorylation of glycogen synthase kinase-3. The compound (H3) and isoorientin were determined to be the most potent in modulating glucose homeostasis in liver cells.	^[19]^[20]
2α-acetoxy-3β-O-trans-feruloyl-19α-hydroxyurs-12-en-28-oic acid (H3)	ursene-type pentacyclic triterpene
ursolic acid	pentacyclic triterpene
isoorientin	C-glycosylflavone
orientin	C-glycosylflavone
3,4-dihydroxybenzaldehyde	phenolic aldehyde
3β,6β-dihydroxyolean-12-en-29-oic acid (myrianthinic acid)	pentacyclic triterpene	Ethyl acetate fraction (EtOAc) of methanolic extract.	Stem bark	-	^[21]^[22]
2β/3β, 24-trihydroxy-olean-12-en-28-oic acid (arboreic acid)	oleanane-type triterpenoid	Ethyl acetate fraction (EtOAc) of methanolic extract.	Stem bark	-	^[21]^[22]
protocatechuic acid	phenolic acid	70% methanolic extract (MAL)	Leaves	MAL administration significantly reduced body weight gain, basal glycemia, and insulin resistance in mice receiving a high-fat diet (HFD). MAL significantly downregulated the mRNA expression of IL-6, IL-1β, and TNF-α, known as obesity-associated inflammatory markers. MAL improved the altered expression of adipokines (leptin and adiponectin) in obese mice.	^[23]
methylumbelliferone fucopyranoside	hydroxycoumarin
tectoridin	glycosyloxyisoflavone
vanillic acid	phenolic acid
medicagenic acid	triterpenoid
brahmic acid	pentacyclic triterpenoid
arjunolic acid	oleanane-type pentacyclic triterpenoid

M. arboreous root bark and stem bark are used by local communities to treat diabetes and its complications ^[13]. Diabetes mellitus is a metabolic syndrome characterized by hyperglycemia accompanied by alterations of fat, protein, and carbohydrate metabolism that results from defects in insulin secretion, reduced insulin action, or both ^[10]. During diabetes, high amounts of free radicals are generated from glucose oxidation and nonenzymatic glycation of proteins as well as an impairment of antioxidant enzymes, contributing to oxidative stress and the development of diabetic co-morbidities, such as cataracts, nephropathy, encephalopathy, and cardiovascular diseases ^[10].

The ethyl acetate fraction (EAc) from the ethanolic extract (EtOH) of the stem bark of M. arboreus acts as an antioxidant in vitro, also stimulating glucose uptake in C2C12 myotubes and 3T3-L1 adipocytes by inhibiting alpha-glucosidase and alpha-amylase activity ^[24]. This extract also provoked a significant decrease in body weight, total protein, HDL-cholesterol, plasma glucose, LDL-cholesterol, triglycerides, serum urea, and serum creatinine in streptozotocin-induced diabetic rats ^[25].

2.2. Acer rubrum

Red maple (Acer rubrum) is a native maple of Northeastern American forests. Both the red and sugar maple (Acer saccharum) species are widely regarded for the quality of their wood and their sap, which can be used to produce maple syrup. Other tissues of the red maple species, such as buds, flowers, leaves, twigs, and stem/branch barks, are employed as folk medicine by the First Nation of Canada and Native Americans to treat/prevent various diseases, including skin ailments, diabetes, and inflammation ^[26].

Red maple bark extract (RMBE) functions as an antioxidant due to its ability to scavenge DPPH (EC₅₀ DPPH = 5.02 µg/mL). Additionally, it has a high content of phenols (455 mg GAE/g) ^[27]. Indeed, when it was compared with an aqueous ethanol extract from propolis, a bee product used in the food industry, antioxidant values appeared of a similar order of magnitude. This comparable tendency held for cranberry pomace extract and cranberry fruit extracts, respectively, at 343.2 mg/g DPPH·(EC_50DPPH) and 3690–10,050 µmol TE/g (ORAC).

Anhydro-glucitol-core gallotannins (ACGs) represent a specific class of hydrolyzable tannins characteristic of red maple, particularly ginnalin A (P10), ginnalin 3,6 (P11), and ginnalin C (P9) (Figure 1). Among twelve hydrolyzable gallotannins, some ACGs have been identified in RMBE ^[28]. Using the DPPH spiking test, three of the identified compounds (ginnalin A, ginnalin C, and gallic acid) were tested to demonstrate their antioxidant activity. The latter revealed that at 0.5 mg/mL, ginnalin A, a major phenolic compound of RMBE, exhibited a higher peak area (PA) reduction (87.9%) than those of ginnalin C (75.8%) and gallic acid (75.9%). These results confirmed the ability of these compounds to scavenge oxidant radicals, such as DPPH ^[28].

Figure 1. Main molecules identified in promising extracts from bark of black spruce (Picea mariana) (a) and red maple (Acer rubrum) (b).

ACGs were evaluated for their effect on neutrophil viability using flow cytometry evolution. Neutrophils were chosen, as they are key mediators in chronic and acute inflammation ^[29]. At 100 μM, the three compounds significantly increased the rate of the late apoptotic cells. Ginnalin A and Ginnalin 3,6 had additive effects on neutrophil apoptosis while Ginnalin C is antagonistic to apoptosis induced by ginnalin A and Ginnalin 3,6. Overall, these results suggested that these isolated phenolic compounds can affect neutrophil functions, such as their programmed cell death ^[30]^[31].

Extracts from leaves, stems/twigs, bark, and sapwoods of A. rubrum were evaluated for their antiproliferative effects against human colon tumorigenic and non-tumorigenic cells ^[32] All extracts showed a greater ability to inhibit the growth of the colon cancer cells compared with normal cells in vitro, and this ability increased with Ginnalin A levels. The A. rubrum leaf extract was considered to be the most active antiproliferative extract ^[32].

2.3. Picea mariana

Black spruce (Picea mariana [Mill.] B.S.P.) is considered the most important commercial and reforestation species in eastern Canada. Its wood is highly appreciated for furniture, pulpwood, and lumber production, thereby generating by-products such as bark, leaves, and twigs with poor utilization. In recent years, some investigations have focused on the valorization of black spruce (BS) bark following multiple strategies, including chemical characterization and the study of the extract’s bioactivity to develop new natural products useful to counteract oxidative stress, aging, and psoriasis (Figure 2).

Figure 2. Strategy to valorize the Picea mariana bark extracts. The valorization of the bark extract from P. mariana involves several stages, starting from the chemical characterization and the identification of bioactive polyphenols, notably trans-resveratrol. The optimization of the extraction is crucial for guaranteeing reproducible yields of bioactive compounds. The most suitable parameters allowing polyphenolic extraction were determined to be 80 °C and a ratio of bark/water of 50 mg/mL especially for obtaining low-molecular-weight polyphenols. The toxicological studies are important for determining the safe concentrations for extracts both in vitro and in vivo. Extracts demonstrated to have potential activity as antiaging, antioxidant, and antipsoriatic agents.

A further in vitro study demonstrated that the ethyl acetate soluble fraction (BS-EAcf) from the aforementioned crude aqueous extract inhibited the production of cytokines, chemokines, adhesion molecule ICAM-1, nitric oxide, prostaglandin E2, elafin, and VEGF produced by psoriatic keratinocytes under TNF-α activation through down-regulating the NF-κB pathway and without causing keratinocyte toxicity (Figure 3) ^[33]. BS-EAcf can inhibit immune pathways associated with TNF-α-induced acute and chronic inflammatory responses by psoriatic keratinocytes, suggesting that the molecules contained in the aqueous extract from Picea mariana bark had a therapeutic potential to treat psoriasis.

Figure 3. Mechanism of action of Picea mariana bark extract as a potential antipsoriatic treatment.

The TNF-α is an inflammatory molecule crucial for psoriasis pathophysiology, as shown by the significant antipsoriatic effect of biological molecules (infliximab, etanercept, and adalimumab), which block its action. The inflammation induced by TNF occurs through activation of the NF-κB pathway, after the binding of this cytokine to their receptors. The Picea mariana extract inhibits the IκBα degradation and migration of p50/p65 subunit to the psoriatic keratinocyte nucleus, thereby avoiding the transcription for many proteins involved in the psoriasis pathogenesis, such as IL-6, IL-8, CX3CL1/fractalkine, iNOS, elafin, VEGF, and ICAM-1.

In addition to their antioxidant and antipsoriatic activity, extracts from this tree have been studied with regard to their in vitro antidiabetic potential ^[34]. For this, three different organs (needle, bark, and cone) of P. mariana were collected at different geographical locations, and their 80% ethanolic extracts were prepared ^[34]. All extracts protected PC-12-AC cells from glucose-induced toxicity. The total phenol content was higher in the cone extracts and lower in the needle extracts, which correlated with the antioxidant capacity, whereas this activity was not connected with the protective effect on glucose-induced toxicity ^[34]. The chemical characterization confirmed the presence in the extracts of terpenes and phenols, such as abietic acid, dehydroabietic acid, oxodehydroabietic acid, kaempferol, resveratrol, and taxifolin ^[34].

3. Challenges and Opportunities for Forest Extract Valorization

Forestry sectors worldwide have focused on the development and production of wood-based products on an industrial scale due to the importance of the timber industry and the existence of already established markets.

Extracts from forest exploitation by-products, such as bark, needles, buds, leaves, knots, etc., given their richness in antioxidant and bioactive compounds, particularly phenols and terpenes, have a potential for developing new value-added cosmeceutical, nutraceutical, or pharmaceutical products. This is the case from promising extracts of forest species, such as M. arboreus, A. rubrum, and P. mariana analyzed above.

However, there are several potential problems in the application of these extracts in terms of (a) the management of by-products in the context of current forestry practices; (b) the low solubility, storage instability, and lack of selectivity towards a particular ROS of forest antioxidants; and (c) the lack of integration between the forestry sector and the cosmetic, alimentary, chemical, and food industries. Consequently, one of the big questions remaining is how to integrate the use of forestry extracts from by-products in a holistic process considering the current forestry practices.

Previous research using the bark of P. mariana attempted to answer this question. The bark is a primary by-product of wood processing often discarded as a forestry residue, burned for energy production, or even transformed into advanced carbon material. Intending to design innovative products using this residue, an integrative process using the P. mariana bark through the simultaneous incorporation of two different types of extraction (hydrodistillation and hot water extraction) was designed ^[35]. This method produced three natural extracts: the essential oil and hydrosol capturing the BS fragrance and the hot water extract enriched with antioxidant polyphenols (trans-isorhapontin, trans-resveratrol, trans-piceide, and trans-astringin). The remaining bark residue after retrieval of valuable chemicals still preserved its calorific value, which was almost identical to that of the non-extracted bark (approximately 20 MJ/Kg), indicating that BS bark would still be available for combustion, which is a common use in the context of existing practices ^[35]. This green process uses water as the only solvent, which can be recycled by cohobation (re-injection of condensate water into the still during hydrodistillation). An interesting aspect is that it can be extrapolated to the management of barks from other forest species, offering eco-responsible solutions to add value to these forest wastes through a biorefinery approach compatible with existing forestry procedures (Figure 4).

Figure 4. Proposed integrative process to obtain valuable extracts from Picea mariana bark in the context of existing forestry practices.

In recent years, it has been suggested that the inefficiency of natural antioxidants could be due to their low solubility, permeability, storage instability, first-pass metabolism, or gastrointestinal degradation ^[36]. Novel antioxidant delivery systems using forest extracts have been developed to counteract the low solubility, storage instability, and astringency of these extracts. The encapsulation of Quercus resinosa leaves infusion of submicron to nanometer size by spray-drying retained the good antioxidant capacity of the original infusion ^[37]. A multifunctional yogurt enriched with nanocapsules of Quercus crassifolia bark extract together with omega 6 and 3 ^[36] was also developed to mask the astringent taste of phenolics while conserving their antioxidant activity ^[38].

The sustainable use of forest resources can be a challenging task. One of the risks associated with the commercial use of extractives is related to the over-exploitation of forests by communities, avoiding sustainable harvesting practices, particularly in countries with deficient forest regulation ^[39]. An example of this is the Himalayan ecosystems considered an important center of biodiversity, where the over-exploitation of forest resources, including the rampant removal of medicinal species coupled with the rapid threat of illegal trade, has resulted in widespread species extinction ^[40].

The need for more research in the area of chemical, biomedical, and forestry sciences is imperative and will require a synergistic collaborative framework between the wood industry with academia, the chemical, food, cosmetic and pharmaceutical industries. Research on business practices, strategic management, and marketing considering sustainability approaches, may help to address challenges, thereby stimulating the integration among the involved actors. Forests remain the most abundant sources of bioactive molecules on Earth, with innovative products based on them yet to be discovered and developed.

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Subjects: Forestry

Contributors MDPI registered users' name will be linked to their SciProfiles pages. To register with us, please refer to https://encyclopedia.pub/register :

Martha-Estrella García-Pérez

Pierre-Betu Kasangana

Tatjana Stevanovic

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Update Date: 01 Sep 2023

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