Previous research has reported important results that showed the therapeutic importance of medicinal plants. There is a growing need to identify bioactive compounds with anti-inflammatory, antimicrobial, and wound healing activities which could be a future drug candidate (15). As a result, these medicinal plants will have immense importance in limb care (lymphoedema).
Cytotoxicity assays were conducted for the three methanol extracts in the in vitro models, and all concentrations used were found to be safe to the cell lines used in these experiments. However, all plant extracts showed varying level of toxicity to Vero cells which might be attributable to their differences in phytochemical constituents(16). According to our results, A. aspera and A. indica had the lowest toxicity to Vero cell lines while L. inermis was more toxic which may be associated with the levels of biomolecules such as alkaloids and saponins in the extracts(17).
The anti-inflammatory activity of the plant extracts was determined using two parameters, cyclooxygenase, and 15-lipoxygenase inhibition assay. Both inhibition of LOX and COX are important in the management of chronic inflammatory conditions. Metabolism of arachidonic acid by COX enzymes leads to the secretion of prostaglandins and thromboxane that mediate pain and inflammation-related edema. The LOX pathway utilizes arachidonic acid to produce leukotriene, including the leukocyte chemoattractant LTB4 (18). All methanol extracts of A. aspera, A. indica and L. inermis were shown to inhibit both cyclooxygenase and 15-lypoxygenase enzymes.
All the methanol extracts tested were effective in inhibiting COX-2, a key enzyme in catalyzing the secretion of prostaglandins, thromboxane, and levuloglandins. Prostaglandins are known to have an effect against almost all known physiological and pathological processes through reversible interactions with G protein-coupled membrane receptors. Levuloglandins are a new category of products that seem to function through irreversible covalent bonding with a variety of proteins(19). L. inermis and A. indica were effective in inhibiting COX-1. Similarly, Jacob et al (2015) report that L. inermis showed dual inhibition of both LOX and COX enzymes, which is important in the reduction of chronic inflammatory conditions(20) and agrees with the current finding. In addition, ethanol and methanol extracts of L. inermis have been shown to inhibit inflammation in the carrageenan-induced rat paw edema model(21, 22). In our previous work, alkaloids, terpenoids, saponins, anthraquinones, tannins, phenols, steroids and flavonoids of plant origin were the major bioactive secondary metabolites isolated from the three methanol extracts which have been described to have significant anti-inflammatory activity (23). Flavonoids, present in both A. indica and L. inermis, have been reported to have a dual inhibitory activity of cyclooxygenase and lipoxygenase, known to inhibit the biosynthesis of prostaglandins(23).
In another study, Attiq et al (2018) reported that alkaloids and terpenoids which are present in L. inermis and A. aspera have been shown to inhibit COX-1 and COX-2-mediated PGE2 secretion in in vitro models. Further, alkaloids and terpenoids have been shown to inhibit PG2 and COX-2 through inhibition of cellular nuclear factor Kappa B (NF-kB) activity. In a similar study, terpenoids and flavonoids were reported to have the ability to reduce the secretion of pro-inflammatory cytokines such as IL-1beta, IL-6 and TNF-alpha (24). A. aspera was more selective to COX-2 than COX-1 with a higher value, i.e., 1.33, which was comparable to indomethacin. Selective COX-2 inhibitors such as Coxibs are preferred to decrease the incidence of gastrointestinal hemorrhage and ulceration upon long-term intake(25). The anti-inflammatory compounds with better COX-2 selectivity index have been shown to have low side effects (26).
Many inflammatory diseases are due to the lipoxygenase enzyme families such as 5-LOX, 8-LOX, 12-LOX, and 15-LOX enzymes. The isomeric enzyme 15-LOX is a major enzyme implicated in the synthesis of leukotrienes from arachidonic acids. Bio-active leukotrienes act as promoters for numerous pro-inflammatory and allergic reactions, therefore inhibition of leukotriene synthesis by 15-LOX is considered to be one of the treatment approaches to regulate inflammation (27).
The anti-LOX enzyme activity of the plant extracts was measured by the inhibition of linoleic acid which is a substrate for soybean lipoxygenase (15-LOX) enzyme. According to previously reported findings, methanol extract of L. inermis and A. aspera have been shown to have moderate activity (66-74.8%) inhibition) in the inhibition of 15-LOX (12). In another study, leaves of methanol extracts of A. aspera showed 70% inhibition of lipoxygenase enzyme, which agrees with the current finding (28). Mzindle (2017) reported that the aqueous and methanol extracts of A. aspera were shown to control the release of various mediators in both the early and late stages of inflammation, and were observed to have wound healing potential (29). In addition, methanol extract of A. aspera has shown significant activity in Carrageenan-induced paw edema in the rat model (30, 31). There is a direct relationship between anti-LOX activity and presence of plant secondary metabolites such as phenols and flavonoids in plant extracts. Furthermore, different studies have implicated oxygen free radicals in blocking the process of arachnoid acid metabolism by inhibiting lipoxygenase (LOX) enzyme activity (32).
Chaibi et al (2017) described the anti-inflammatory activity of methanol extract of L. inermis, which showed the methanol extract to be superior in inhibiting the LOX family, 5-LOX with IC50 value of 51 ± 0.23 mg/L(33). Schumacher et al (2011) reported that the methanol extract of A. indica inhibited the TNF-a-activated NF-kB pathway at 240 µg/ml, which indicated the anti-inflammatory activity of the crude extract (34). Plants with antioxidant properties can also have anti-inflammatory activities, because lipoxygenase is reported to be sensitive to antioxidants due to its inhibition of substrate (lipid hydrogen peroxide) formation that required for lipoxygenase catalysis (35).
Wound healing is a complex and ongoing process which includes homeostasis, re-epithelization, granulation, tissue reformation, and remodeling of the extracellular matrix. Even though wound healing can take place spontaneously without assistance, external factors such as wound infection have focused attention on wound healing (36). Many medicinal plants have been claimed to be useful for wound healing in Ethiopian traditional medicine, and some of the traditional medicines in use have been used for the management of dermatological disorders (11).
All methanol extracts exhibited dose dependent DPPH antioxidant activity and their free radical scavenging activity was correlated to the content of flavonoids, phenols and terpenoids (37). Methanol extract of leaves of A. aspera was effective in scavenging DPPH free radicals, which is in agreement with those previously reported findings(36, 38–40). Further, Fikru et al (2012) reported the considerable antioxidant and antimicrobial activity of methanol extract of A. aspera, and its wound healing properties. The wound healing activity could be due to its role in promoting fibroblast adhesion or reducing xenobiotic-induced leukocyte hyperactivity and inflammatory damage (41). Similarly, L. inermis was shown to have high antioxidant and wound healing potential, in agreement with previously reported findings (42–44). In another study, Alzohairy (2016) reported significant antioxidant activity of A. indica, and revealed that azadirachtin and nimbolide were the main compounds with radical scavenging activity and reductive potential (45).
Oxidative stress and free radicals have been implicated in impaired wound healing (46). Different reactive oxygen species and their degradation product are generated during the healing of cutaneous wounds, causing oxidation of biomolecules and ability to damage numerous molecules in the cell membrane (47, 48). In addition, high level reactive oxygen species have the capacity to inflict peroxidation of membrane lipids, aggression of tissue membranes and proteins, or harm to DNA and enzymes via oxidation of low-density lipoproteins (LDL) (47). Thus, decrease of antioxidant ability results in redox imbalance, which is a major cause of nonhealing wounds (49).
A previously established in vitro cell proliferation assay method was used to measure percent human epidermal keratinocytes in a monolayer cell model. The methanol extracts of leaves of L. inermis, A. aspera and A. indica demonstrated moderate activity in stimulating proliferation of human epidermal keratinocytes. The concentration that showed optimal stimulation was between 2.5mg/ml and 5 mg/ml. Plant secondary metabolites, triterpenes, are known to stimulate cell proliferation, which positively influences the wound healing effect of the methanol extracts of leaves of L. inermis, A. aspera, and A. indica (50).
Fikru et al (2012) reported that 5% and 10% ointment-based methanol extracts of A. aspera stimulated high DNA and protein content of granulation tissue in animal models, implying cellular multiplication and suggesting an increase in the synthesis of collagen, a predominant tissue in wound healing (51). In another study reported by Rekik et al (2019), L. inermis oil promoted wound healing via cell proliferation in animal models (43). Furthermore, the methanol extract of L. inermis also has antibacterial and anti-oxidant activity (52), which could contribute to wound healing. In another animal study, stem bark of A. indica was shown to increase the tissue DNA content of plant extract-treated wounds, indicating cell proliferation. In addition, there was considerable increase in the protein and hydroxyproline content of plant extract-treated wound tissues, which is an indication of fibroblast cells and epithelial cells migration, and synthesis of extracellular matrix in A. indica extract-treated mice (53).