Therapeutic Potential of Ranunculus Species (Ranunculaceae): A Literature Review on Traditional Medicinal Herbs

The genus Ranunculus includes approximately 600 species and is distributed worldwide. To date, several researchers have investigated the chemical and biological activities of Ranunculus species, and my research team has found them to have antimalarial effects. This review is based on the available information on the traditional uses and pharmacological studies of Ranunculus species. The present paper covers online literature, particularly from 2010 to 2021, and books on the ethnopharmacology and botany of Ranunculus species. Previous studies on the biological activity of crude or purified compounds from Ranunculus species, including R. sceleratus Linn., R. japonicus Thunb., R. muricatus Linn., R. ternatus Thunb., R. arvensis Linn., R. diffusus DC., R. sardous Crantz, R. ficaria Linn., R. hyperboreus Rotlb., and R. pedatus Waldst. & Kit., have provided new insights into their activities, such as antibacterial and antiprotozoal effects as well as antioxidant, immunomodulatory, and anticarcinogenic properties. In addition, the anti-inflammatory and analgesic effects of plants used in traditional medicine applications have been confirmed. Therefore, there is a need for more diverse studies on the chemical and pharmacological activities of highly purified molecules from Ranunculus species extracts to understand the mechanisms underlying their activities and identify novel drug candidates.


Introduction
The genus Ranunculus includes approximately 600 species globally. Recent taxonomic reports suggest that this genus has a monophyletic origin and is divided into two subgenera and seventeen sections [1]. Owing to its wide distribution, the genus has high genetic diversity. Several Ranunculus species have been used in folk medicine to treat various diseases or symptoms, such as jaundice, nebula, edema, malaria, asthma, pain, gout, rheumatism, inflammatory skin disorders, cancer, and hypertension. In addition, researchers have reported that Ranunculus extracts possess antioxidant, anti-inflammatory, antimutagenic, antimalarial, antibacterial, antitumoral, cardioprotective, and wound-healing properties [2][3][4][5][6][7].
Over the last decade, various studies have investigated the chemical components and pharmacological activities of Ranunculus species [8,9]. However, no recent review has been published detailing the aspects of the plants that have been investigated, including their biology, traditional uses, phytoconstituents, therapeutic activities, and clinical applications, since a previous review article was reported in 2012 [10]. Thus, this article aims to provide an up-to-date survey of the advances in and prospects of the research on the phytochemicals and pharmacological potential of Ranunculus species.

Search Strategy
This review article is based on the information available on the phytochemical, toxicological, and pharmacological studies on the traditional uses of Ranunculus species. The present paper covers online literature (Google Scholar, PubMed, ScienceDirect, Scopus, SpringerLink, and Web of Science), particularly from 2010 to 2021, and books on the ethnopharmacology and botany of Ranunculus species. The following words were used as key search terms: ("Ranunculus" OR "Ranunculus species") AND ("herbal medicine" OR "herb medicine" OR "ethnopharmacological effects" OR "ethnopharmacological activity" OR "phytomedicine" OR "treatment" OR "drug". The range of the article publication year for the search (from 2010 to 2021) was selected because the previous review by Aslam et al. covered almost all literature data published by 2012 [10].

Taxonomy, Distribution, and Morphology
Ranunculaceae Juss., or the buttercup family, has a worldwide distribution, representing a large group comprising more than 2500 species belonging to 59 genera. Its family members live under a wide range of ecological conditions, especially in the Northern Hemisphere [1,11]. Among the family, Ranunculus, comprising 600 species, is distributed across all continents [11]. Ranunculus species are highly genetically diverse; therefore, their classification is challenging. As a result, generic delimitation and infrageneric classification of these species are still under consideration.
Initially, Ranunculus species were classified based on the descriptions of their achenes (e.g., the shape of their body and beak, pericarp structure, and indumentum), flowers (e.g., the number of sepals and honey-leaves, gloss and color of the petals, and shape of the nectaries), roots (e.g., whether they were uniform or dimorphic with fibrous and tuberous roots) [12], and fruit anatomy [13]. Later, Tamura classified the genera into seven subgenera based on the reassessment of the achene structure: Pallasiantha, Coptidium, Ficaria, Batrachium, Crymodes, Gampsoceras, and Ranunculus [1,11]. In this classification, the subgenera of Ranunculus were further subdivided into 20 sections [11].
Subsequently, DNA markers were utilized to delineate the phylogenetic relationships within Ranunculaceae [14][15][16][17][18][19][20][21][22][23]. The sequences of the internal transcribed spacer region of nuclear ribosomal DNA are mostly used as DNA barcode markers for phylogenetic studies at the generic/subgeneric level [24,25]. In combination with data from the chloroplast genome and other external data, this nuclear marker also offers insights into the reticulate patterns caused by hybridization [26,27]. Moreover, a complete study of the taxonomy of the genus using both DNA markers and morphological data suggested the separation of 226 species into two subgenera and 17 sections [20].

Phytochemical Investigations of Ranunculus Species
Ranunculus sceleratus Linn., commonly known as the celery-leaved buttercup, is a flowering plant species distributed over the Northern Hemisphere. The main constituents of R. sceleratus L. are flavonoids, steroids such as pyrogallol tannins, and the glycoside ranunculin [28]. Ranunculin is hydrolyzed after the leaves of R. sceleratus L. are dried or crushed and generates protoanemonin associated with the toxic properties of buttercups. Because of its instability, protoanemonin dimerizes to produce anemonin, a nonirritant form [29,30]. In addition, the 70% ethanolic extracts from the aerial parts of R. sceleratus L. have been found to be abundant in myristic acid [31], and sapigenin 4 -O-alpha-rhamnopyranoside, apigenin 7-O-beta-glucopyranosyl-4 -O-alpha-rhamnopyranoside, tricin 7-O-beta-glucopyranoside, tricin, and isoscopoletin have been identified as R. sceleratus-derived compounds in the extract [32].
In Ranunculus species, several bioactive compounds and Ranunculus-specific constituents have been identified, such as ranunculosides, muricazine, and muracatanes. Although many other species related to Ranunculus have also been studied to evaluate their pharmacological activities, the novel bioactive compounds found in Ranunculus species with high pharmacological effects show nutraceutical and pharmaceutical potential. Pharmacological properties and molecular formula of Ranunculus species compounds reported in articles published from 2010 to 2021 are summarized in Table 1.
All parts of Ranunculus sceleratus Linn. are poisonous when fresh; however, the plant is used in folk medicine to treat various diseases after heating or drying [7]. In recent decades, ethnopharmacological effects have been experimentally proven by several studies ( Table 2). The two ranunculins, protoanemonin and anemonin, have shown fungicidal, antimicrobial, antimutanenic, and antipyretic properties [29,30,51], and have been used for ethnopharmacological purposes in many countries [52,53]. Sharif et al. performed an in vivo study to evaluate the effects of hypertension treatment using normotensive and fructose-induced hypertensive rats, in which the aqueous fraction produced the most interesting effects. Furthermore, mechanistic studies with various pharmacological antagonists have demonstrated that the hypotensive response induced by R. sceleratus L. is caused by the involvement of a muscarinic receptor, angiotensin-converting enzyme inhibition, ganglionic block, and nitric oxide release [54]. In addition, the 70% ethanolic extracts from the aerial parts of R. sceleratus L. revealed that abundant myristic acid in the extract inhibited nitrite concentration in LPS-stimulated RAW 264.7 macrophage cell line [31]. Moreover, R. sceleratus-derived compounds, sapigenin 4 -O-alpha-rhamnopyranoside, apigenin 7-O-beta-glucopyranosyl-4 -O-alpha-rhamnopyranoside, tricin 7-O-beta-glucopyranoside, tricin, and isoscopoletin, showed inhibitory activity against the hepatitis B virus [32]. In addition to the treatment effect of R. sceleratus extract, fresh R. sceleratus for TianJiu therapy, which involves adding Chinese medicinal herbal paste on designated acupoints, showed good therapeutic effect on intrahepatic cholestasis in rats, although the fresh form of R. sceleratus L. is known as an irritant [55]. Specific mechanisms by which the extract induces irritant or nonirritant responses have not been revealed. To eluciate this phenomenon, a methanolic extract of R. sceleratus L. was used to demonstrate the mechanism of both irritant and non-irritant properties induced by the extract in topical inflammation. When arachidonic acid elicited the inflammatory process, the effect of the extract was generally proinflammatory or neutral. However, if the response was caused by the application of an irritant, such as etradecanoylphorbol acetate, the extract mainly resulted in anti-inflammatory effects. This effect was mentioned as a counter-irritant, and the extract itself could be an irritant in physiological conditions but could also counteract the action of previously applied irritants [7].

Ranunculus ficaria Linn.
Ranunculus ficaria Linn. is an herbal astringent commonly used to treat hemorrhoids internally or externally [67]. Various methods have been applied for ethnopharmacological use. Infusion or decoction of the leaves and roots of R. ficaria was known to have trophic and anti-inflammatory effects in varicose veins, hemorrhoids, and skin disorders in Romania. The macerate and tinctures obtained from this plant are used to treat hemorrhoids by stimulating blood circulation as a traditional medication [67]. The compositions found in R. ficaria could inhibit nitrite accumulation, and thus may be useful for preventing inflammatory diseases mediated by the excessive production of nitric oxide, according to an in vitro macrophage study. However, a previous report suggested that clinicians should consider using lesser celandine (pilewort, R. ficaria) as a causative agent owing to its hepatotoxicity [35,68].

Ranunculus diffusus DC.
The phytomedicinal effects of Ranunculus diffusus DC. have recently been reported. The methanol extract of R. diffusus showed photoaging protective effects on ultraviolet B radiation-induced skin by inhibiting the p38-AP-1 signal cascade. In addition, the extract exerted anti-inflammatory effects without toxicity by suppressing Src and Syk, which are targets of NF-κB signaling [63,64].

Ranunculus hyperboreus Rotlb.
Ranunculus hyperboreus Rotlb. is a subarctic and subalpine plant that lives in extreme environmental conditions. R. hyperboreus extract induces anti-inflammatory activity by regulating the gene expression and protein levels of inflammation-related enzymes, such as iNOS and COX-2, and proinflammatory cytokines, such as TNF-α, IL-1β, and IL-6 [65].

Ranunculus pedatus Waldst. & Kit.
The wound healing activity of Ranunculus pedatus Waldst. & Kitt. was evaluated using its methanolic extract and was found to exert significant effects on wound healing with robust anti-inflammatory activity in both incision and excision wound animal models [66].

Conclusions
The chemical and biological activities of Ranunculus species have been investigated using plant extracts. Contemporary research on the biological activity of the extracts of the species mentioned above has uncovered many activities, including antibacterial, antiviral, and antiprotozoal effects, as well as antioxidant and anticarcinogenic properties. In addition, these studies have demonstrated that herbal extracts exert hepatoprotective, hypoglycemic, and thyroid regulatory effects. Moreover, the anti-inflammatory and analgesic effects of the plants, known from the application of traditional medicine, have been confirmed. Furthermore, the molecules isolated from Ranunculus species showed promising pharmacological activity. Therefore, it is expected that effective purified molecules could be discovered from Ranunculus species to develop novel drugs through intensive research.

Conflicts of Interest:
The author declares no conflict of interest.