Herbal Arsenal against Skin Ailments: A Review Supported by In Silico Molecular Docking Studies

Maintaining healthy skin is important for a healthy body. At present, skin diseases are numerous, representing a major health problem affecting all ages from neonates to the elderly worldwide. Many people may develop diseases that affect the skin, including cancer, herpes, and cellulitis. Long-term conventional treatment creates complicated disorders in vital organs of the body. It also imposes socioeconomic burdens on patients. Natural treatment is cheap and claimed to be safe. The use of plants is as old as mankind. Many medicinal plants and their parts are frequently used to treat these diseases, and they are also suitable raw materials for the production of new synthetic agents. A review of some plant families, viz., Fabaceae, Asteraceae, Lamiaceae, etc., used in the treatment of skin diseases is provided with their most common compounds and in silico studies that summarize the recent data that have been collected in this area.


Introduction
Molecular docking is an in silico procedure that is able to predict the mechanism of binding of a suggested ligand to its macromolecular target during the formation of a stable complex. Therefore, docking has become of great importance for the illustration of molecular interactions of natural compounds with different receptors [1][2][3].
The skin, the largest organ of the human body, functions as a physical barrier and an exterior interface of the body with the outer environment. The skin prevents the body from the invasion of external pathogens, as well as mechanical, thermal, and physical injuries from any substance that can be hazardous to humans. Just like any other organ and system of the body, this system is also very complex. The skin, with its derivatives such as nails, sweat glands, oil glands, and hair, makes up the integumentary system [4]. It is an incredible organ that protects the whole body. It consists of three main layers, including the epidermis (outermost layer), which consists of three types of cells, i.e., squamous cells, basal cells, and melanocytes; the second layer of the skin, the dermis, which contains blood and lymph vessels, hair follicles, etc.; and the subcutaneous fat layer. The focus on skin health is because everyone wants clearer, healthier, younger, and fresher skin, as skin-related complications can cause problems related to mental health, as well as low self-esteem [5].
Herbal medicine can be traced back to ancient civilizations. It entails the use of plants for medicinal purposes to cure illnesses and improve overall health [6]. Although herbal plants are low in toxicity and readily available, they play an important role in not only pharmacological research and drug production but also as plant components, being used specifically as therapeutic agents for drug synthesis [7]. The most widely used plant parts in the preparation of traditional medicines are the leaves (62%), either alone or in combination with other plant parts [6,7].
Skin disease refers to problems with the surface layer of the skin. Skin disorders have a serious impact on well-being and are difficult to manage due to their persistence [8]. Several microorganisms trigger skin ailments, including boils, scratching ringworm, skin diseases, leprosy, injury, skin infections, eczema, skin allergy inflammation, scabies, and psoriasis [9].
Scabies, a parasitic infection, has always been the most prevalent skin disorder, but, in some areas, it is entirely absent [10]. Sarcoptes scabiei is the mite that causes scabies. Infection with the scabies worm causes a rash of vesicles, nodules, and papules. The majority of this is due to host hypersensitivity, but the direct impact of worm invasion also plays a significant role [11].
A rash is a red, inflamed patch of skin or a set of discrete spots. Irritation, inflammation and allergies, fundamental conditions, and structural issues may all contribute to these symptoms. Acne, eczema, psoriasis, hives, etc., are causes of rashes [4].
Atopic eczema, a chronic condition that affects people who are genetically organized to overreact towards environmental stimuli, has become an inflammatory disease. It is often seen in people with asthma, allergic rhinitis, and atopy symptoms. Eczema is a common skin problem in children. Severe skin dryness and inflammation, scaly patches, redness, and lichenified plaque with abrasions are the most common dermatitis symptoms [12].
Acne is a contagious disease and one of the most common in humans. Acne leads to seborrhea, papules, comedowns, blackheads, nodules, and scars [13]. Acne is most often found on the face, chest area, and back of people who have a large number of oil glands [14].
Psoriasis is an inflammatory skin problem that causes keratinocytes, excessive proliferation resulting in scaly patches, extreme inflammation, and erythema [15].
The uncontrolled development of cells present in the skin is known as skin cancer. It occurs due to unfixed DNA damage to skin cells, most commonly due to UV from sunlight, causing mutations and even genetic abnormalities. This causes skin cells to grow rapidly, resulting in the formation of malignant tumors [16].
A burn is considered tissue damage due to fire, chemicals, or radiation. Burn wounds are classified as superficial, partial thickness, or full thickness. Swelling, epithelization, wound contraction, and granulation are all part of the healing process after a burn wound [17].
The current review presents the effect of different medicinal plants and FDA-approved formulas on the management of various skin disorders. A molecular docking study was conducted for major components of these medicinal plants on the active sites of granzyme B and human leukocyte elastase (HLE) enzymes, aiming to identify the potential compounds or class of compounds that may be responsible for the ameliorative effects on different skin ailments.

Medicinal Plants and Skin Disorders
Medicinal plants reported for the management of skin disorders (Table 1) are classified below according to their uses.

Some Reported Mechanism of Action
The use of herbal medicine is becoming popular worldwide. Herbal medicines are preferred over synthetic medicines, as they produce fewer side effects [186][187][188][189]. Additionally, phytochemicals can treat skin ailments by different mechanisms and by displaying various biological activities such as antioxidant, anti-inflammatory, and antiallergic [190][191][192]. Each plant has its own bioactivity, which depends upon the chemical nature and potency of the constituents present in it [193,194]. Some components reduce skin inflammation by inhibiting NF-κB, for example, Zingiber officinale. The squeezed extract of this in rats and mice elevates TNF-α in peritoneal cells, and its long-term use can increase the level of serum corticosterone and thus reduce proinflammatory markers [195]. Drugs such as Rosmarisum officinalis also help in the improvement of abnormal skin conditions. It constitutes rosmarinic acid, which can disturb the system activation inhibition of the C3b attachment. It also acts on the inhibition and reduction of proinflammatory mediators such as TNF-α and IL-1 [196]. Oenothera biennis constitutes β-sitosterol, which modulates NO, TNF-α, IL-1β, and TXB2, leading to the suppression of COX-2 gene expression, hence causing anti-inflammatory action [197].

FDA-Approved Formulas
The Food and Drug Administration (FDA), as well as in vitro and in vivo study results, has approved bacterial cellulose (BC) and plant cellulose (PC) products to be incorporated into the biomedical field and their applications due to their biocompatibility with human cells and potential activity in wound healing and in the therapeutics field [198].
Moreover, honey, a natural product, is rich in several phenolic compounds, sugars, and enzymes that possess antioxidant, anticarcinogenic, anti-inflammatory, and antimicrobial activity. The main role of honey in the development of the wound healing process appeared to be via the acceleration of dermal repair and epithelialization, angiogenesis promotion, immune response promotion, and the reduction in healing-related infections with pathogenic microorganisms. The FDA approved many formulas containing honey as the main ingredient, among which is L-Mesitran ® (manufactured by Triticum Company-UK) Ointment, which consists of 48% medical-grade honey, lanolin, cod liver oil, sunflower oil, calendula, aloe vera, zinc oxide, and vitamins C and E. Additionally, Revamil Gel ® (manufactured by Maximed Pharrma-Lebanon) was FDA approved, containing 100% medical-grade honey, together with Therahoney ® Gel (manufactured by Medline Industries Inc.-USA), containing 100% Manuka honey [199].

Phytoconstituents of Medicinal Plants
Many phytochemical constituents have shown potential bioactivities, to which the biological activities of medicinal plant extracts can be attributed. Table 2 summarizes some of them in the context of treating skin disorders. 2.

Methodology of Molecular Docking Studies
Based on the aforementioned, human granzyme B in complex with 2-acetamido-2deoxy-beta-D-glucopyranose [219] was downloaded from PDB (Code: 1IAU), while the crystal structure of highly glycosylated human leukocyte elastase in complex with a thiazolidinedione inhibitor (5- pentan-2-yl]carbamoyl]phenyl]methyl]-2-oxidanylidene-1,3-thiazol-1-ium-4-olate) [220] was also downloaded from PDB (Code: 6F5M). Both enzymes were cleaned for missing amino acids or gaps in their sequences. Hydrogens were added, water molecules were removed if present, and simulation for forcefield CHARMm and partial charge MMFF was applied. A heavy atom was built, and fixation of atom constraints was applied before enzyme minimization. The receptor was identified, and the binding site was highlighted from the complexed ligand, which was later cut off for the comparative docking study. The structures of the selected active constituents were downloaded from PubChem with the .svd extension and opened in the program. A simulation for all selected 23 active constituents was applied with the CHARMm forcefield and partial charge MMFF, and ligand preparation was carried out. The 23 resulting compounds, together with the reference ligand, were allowed to dock against both enzymes using the C-docker protocol.

Results and Discussion of Computational Studies
Molecular docking is of great importance for illustrating the molecular interactions of natural compounds with different receptors [221]. Although each docking program operates slightly differently, they share common features that involve ligand and receptor, sampling, and scoring. Thus, a molecular docking study was performed using the selected software Discovery Studio 4.1 [222][223][224]. Twenty-three interesting phytoconstituents of the previously detailed plants were selected for in silico docking trials to explore their activity and possible mechanism of binding against two essential enzymes human granzyme B and human leukocyte elastase, where the inhibition of either or both of those enzymes could aid in the treatment of various skin diseases.
The 2D interaction energy of the 23 active constituents compared to the reference ligand 2-acetamido-2-deoxy-beta-D-glucopyranose, together with their C-docker interaction energy, is displayed in Table 3. The ligand displayed -27.55 Kcal/mol, saponin showed -28.10 Kcal/mol, and the rest of the constituents showed -21.42 to -1.05 Kcal/mol. Both S-methyl-L-cysteine and N-acetyl cysteine were unsuccessful in the inhibition of granzyme B. The reference ligand performed its inhibitory action via four H-bonds with essential amino acids in the granzyme B sequence (Ala 93, Asn 98, Tyr 175, and Asp 176) and via van der Waals forces with six other amino acids (Asn 95, Ser 100, Asn 101, Ser 177, Thr 178, and Ile 179). Saponin was the only constituent better than the inhibitor, displaying better interaction energy and binding mode comparable to the ligand, as shown in Figure 1 The results of the docking study against human leukocyte elastase are presented in Table 4. It is shown that the reference complexed thiazolidinedione inhibitor displayed Cdocker interaction energy equivalent to −33.57 Kcal/mol, while both constituents saponin and amaranthine displayed −48.               Granzyme B is a serine protease found in the granules of natural killer (NK) cells and cytotoxic T cells. It is involved in inducing inflammation by cytokine release stimulation and also involved in remodeling of the extracellular matrix. Elevated levels of granzyme B are also implicated in various autoimmune diseases, several skin diseases, and type 1 diabetes [225].
On the other hand, human leukocyte elastase (HLE) is a serine proteinase involved in inflammation and tissue degradation. HLE inhibitors are believed to treat a number of diseases, such as emphysema and cystic fibrosis [220]. Natural products can have enzyme inhibitory potential for the management of different disorders [226]. According to the in silico study results, cyclamin, a saponin, is suggested to be a successful constituent for treating most underlying skin diseases owing to its chemical structure that possesses aliphatic rings, richness in oxygen atoms, and the ability to bind effectively with key amino acids of the binding sites of both granzyme B and HLE.

Conclusions
Herbs have great potential to treat various kinds of skin problems. Compared to various allopathic drugs, they have a comparatively low cost and can be of great benefit to many patients, especially poor people. Herbs are rich sources of active ingredients and can be a safer and cost-effective method for the management of skin ailments, ranging from rashes to skin cancer. FDA-approved formulas containing natural sources such as honey and biological cellulose are available and aid greatly in the treatment of skin diseases. Different mechanisms are displayed by such phytochemicals, such as inhibition of multiple inflammatory mediators, ranging from NF-κ, TNF-α, IL-1, TXB2, to COX-2. Their mechanism of action was elucidated via molecular modeling studies that were performed on the active sites of two essential proteins: granzyme B, which is a serine protease found in the granules of natural killer cells (NK cells) and cytotoxic T cells; and human leukocyte elastase (HLE), which is a serine proteinase involved in inflammation and tissue degradation. Molecular docking studies have confirmed that phytoconstituents of natural origin have potential beneficial effects on various skin disorders, especially those containing saponin. Owing to the aliphatic chains and structure rich in oxygen atoms, cyclamin saponin was able to display a comparable and stable complex with both enzymes. C-docker interaction energy expressed by saponin was −28.

Conflicts of Interest:
The authors have no conflicts of interest to declare.