New Findings from Natural Substances

Heliotropium, an Ethnomedicinal Plant: Past and Present Uses

César Donoso-Fierro¹, José Becerra¹, ², Claudio Rojas-Leyton¹, Claudia Pérez¹, ², *

¹ Laboratorio de Química de Productos Naturales, Departamento de Botánica, Facultad de Ciencias Naturales y Oceanográficas, Universidad de Concepción, Concepción, Chile

² Universidad de Concepción, Unidad de Desarrollo Tecnológico, UDT, Biobío 4191996, Chile

Abstract

The genus Heliotropium is formed of herbaceous plants belonging to the family Boraginaceae. In Chile and around the world, many Heliotropium species are commonly used in traditional and complementary medicine to treat various diseases. Members of this genus are also recognized for unique biosynthesized phytochemicals, mainly terpenoids, phenolics and alkaloids. Due to important phyto-constituents, as well as their therapeutic potential, many Heliotropium species have been subjected to chemical, biological and pharmacological investigations. This review details the many ethnomedicinal uses for Heliotropium, with an emphasis on Chilean species, and analyzes their scientific validation based on the chemical constituents and pharmacological properties of Heliotropium reported in academic publications. In addition, we discuss the critical conclusions, as well as some suggestions for future phytochemical and biological studies with Heliotropium species.

Keywords: Boraginaceae, Ethnomedicine, Heliotropium, Pyrrolizidine alkaloids.

* Corresponding author Claudia Pérez: Laboratorio de Química de Productos Naturales, Departamento de Botánica, Facultad de Ciencias Naturales y Oceanográficas, Universidad de Concepción, Concepción, Chile; E-mail: [email protected]

INTRODUCTION

Heliotropes are herbaceous plants that belong to the Boraginaceae family, and the subfamily Heliotropiaceae. Its name is derived from the Greek helios-, sun and -tropium, turning; thus, the name heliotrope evokes the ancient belief that inflorescence is oriented toward the sun [1].

The genus Heliotropium is made up of about 300 spp [2, 3]. Some heliotropes are popular garden plants and others are considered weeds [4]. They can be found in tropical, subtropical and temperate zones across all the continents [2, 5]. Neotropical representatives cover only 50 to 60 species. Members of both

sections, i.e. Heliotropium sect. Heliothamnus I.M. Johnst. (Andean, approximately 11 spp.) and H. sect. Cochranea (Miers) Kuntze (Atacama Desert, 17 spp.) are long-lived shrubs between 0.4-4.0 meters tall. The roughly 25 remaining species are quite divergent in their vegetative morphology. These species include small annual sub-bushes with succulent leaves (H. amplexicaule Vahl) (e.g., H. paronychioides A. DC.), halophytic, clump-forming plants with massive roots (H. curassa- vicum L.), perennial herbs with root-tuber (H. microstachyum, Ruiz & Pav.), and decumbent perennial grasses with extensive shoots, abundant rooting and creeping (H. veronicifolium, Griseb.) [2].

The genus Heliotropium L. Sect. Cochranea (Miers) Kuntze is composed of four main lineages, three of which have their center of diversity in South America. According to its geographic distribution, it has been suggested that its origin could be related to the Andean uprising and the formation of arid environments in South America [6]. In Chile, H. stenophyllum (Hook & Arn) is distributed from the Atacama Region to the Metropolitan Region of Santiago, inhabiting arid to semi-arid zones, from the coastal zone to the interior valleys. It is a very bushy shrub, with rigid dark green branches and linear leaves covered with hair on their underside, alternates, sessile, and amphistomatic. Flowers are small (5 mm long), and disposed in terminal scorpiod monchasia (boragoids [7]). Calyx lobes are linear lanceolate, partially fused. The corolla is infundibuliform, exceeding the calyx, white with a yellow throat. A protuberant nectar disk at the base surrounds the ovary. The style is elongated. The gynoecium is glabrous. The stigma is elongated into a conical structure with a basal receptive area, which is typical of Heliotropiaceae. It can reach up to two meters in height (Fig. 1).

Fig. (1))

Helitropium stenophyllum (Palito negro) (a) Flowers, (b) shrub.

There are varied ethnomedicinal uses for the different Heliotropium species. In general, they are used to treat skin diseases, poisonous bites, venereal diseases [8], renal, hepatic, and osteoarticular disorders [9], as antimalarial agents [10, 11], and for treating cancer [12]. Certain species are characterized by living in arid zones and being exposed to extreme environmental factors. For these reasons, they produce resinous exudates that cover the leaves and stem, secreted by special glands called trichomes. The resin is principally composed of terpenoids, lipids, waxes and flavonoids, which act as a protective barrier against high temperatures, UV radiation, water scarcity, herbivores and pathogens [4, 13]. Previous studies have attributed this resistance to the antioxidant, antimicrobial, antiviral and antifungal properties of the components of this resin [13], which are rich in secondary metabolites, with potential therapeutic uses due to their relevant biological activity. In this context, anticarcinogenic activity has been registered in compounds extracted from diverse plants around the world [12, 14-16]. For example, H. indicum is rich in pyrrolizidine alkaloids (PAs) [4], which are of great pharmacological, biological and chemotaxonomic interest, and have been isolated from a wide variety of plants, especially from the Boraginaceae family [17]. Within this group of anticarcinogenic alkaloids, the most important is N-oxide [18], as well as acetylindicine, indicinine, heleurine, heliotrine, supinine, supinidine and lindelofidine. This review examines the different ethnomedicinal uses of the genus, with an emphasis on Chilean species, and analyzes the scientific validation of said uses based on its chemical constituents and pharmacological properties as reported in academic studies.

Ethnomedicinal use of Heliotropium Species

Throughout human history, plants have played an important role in the health traditions of essentially every culture around the world, featuring in a wide variety of treatments, many of which may be incomprehensible by modern Western societies. However, these healthcare alternatives can be as valid today as they were 5,000 years ago [19]. In this regard, the World Health Organization (WHO) has estimated that more than 80% of the global population depends on medicinal plants to meet their primary healthcare needs, and that much of Traditional, Complementary and Alternative Medicine (T/CAM) involves the use of plant extracts or their active ingredients. As such, medicinal plants are a valuable natural resource contributing to disease treatment and prevention through T/CAM, especially in developing countries (up to 80% in African countries, 71% in Chile, and 40% in Colombia and China) [20-22].

It is estimated that there are around 298,000 species of plants on the planet, of which 215,644 have been cataloged and 20% probably have medicinal potential that is important for human health [23]. Additionally, it is estimated that close to 60% of pharmaceutical drugs worldwide come from the vegetable kingdom [24]. However, there has been a significant loss of traditional knowledge regarding medicinal plant use in many developing countries, conserved mainly by oral transmission from one generation to the next. Furthermore, the availability of such plants has been reduced by deforestation, conversion of forests to agricultural fields and monoculture tree plantations, market influences, globalization, uprooting of native lands [25, 26] and lack of access to traditional gathering environments [27-31].

Heliotropium indicum and Other Species used in Traditional Medicine

Various Heliotropium species are still used in traditional medicine around the world to treat a wide range of diseases (Table 1). Among the best-known representatives for their ethnomedicinal and medicinal uses is Heliotropium indicum (Linn.) [4]. In fact, in India, it is used to treat skin diseases, poisonous bites, stomach pain and nerve disorders [32]. In some African countries, it is used to treat malaria, abdominal pain, and dermatitis [10]. In the Philippines and Senegal, it is used orally as a diuretic to treat kidney stones [33, 34]. In Jamaica, the decoction of the entire plant is taken orally to treat sore throat, fever, ulcers, and venereal diseases. Externally, it is used in the vaginal cavity to induce abortion in pregnant women and is administered rectally to treat local pains in the rectum [8]. In South America, H. indicum is used as an antimalarial and biocide for control of the malaria vector in some parts of the low jungle in the Peruvian Amazon [11], and it is also used to treat cancer [12]. A study on the ethnomedicinal uses of native plants used by the traditional communities of Pernambuco, Brazil, reports this species in the treatment of influenza, headache, edema, inflammation, and vision problems [35].

Table 1 Ethnomedicinal uses of Heliotropium species around the world.

* Information on ethnomedicinal uses of H. stenophyllum was obtained through interviews conducted by Dr. Donoso with; Virginia Bonilla & Claudio Galleguillos Phytotherapists, and Juana Puelles, Rosalia Aguirre, Jimena Adaos, Healers considered mentors of medicinal habits of their respective communities, in the Coquimbo Region, Chile.

H. curussauicum var. argentinum (specimen belonging to the Pharmacobotanical Herbarium, National Institute of Pharmacology of Argentina) has a broad therapeutic spectrum in traditional medicine, including gout, rheumatism, and neuralgias, arteriosclerosis, renal, hepatic, osteoarticular and cardiocirculatory diseases. Muscular strains, phlebitis and varicose veins are also treated [9].

In Chile, the Ministry of Health (MINSAL), following the recommendations of the World Health Organization (WHO), seeks to promote the use of medicinal plants registered and certified by the Regulation of the National System of Control of Pharmaceutical Products through the publication of a Traditional Herbal Medicine Registration Scheme (THMRS), which includes a total of 103 recognized medicinal plants [36] (MINSAL, 2010). However, nowadays the THMRS does not include species of the genus Heliotropium, mainly due to the lack of awareness of its application in traditional medicine and the consequent need for chemical, pharmacological and clinical studies. Although the genus Heliotropium and its medicinal uses are little known among the general Chilean population, H. stenophyllum is recognized and used by traditional healers of the Coquimbo Region in the treatment of diverse diseases, including stomach affections, respiratory and kidneys diseases, as a hepatic stimulant, even for the treatment of tumors, among others (Table 1). In addition, there are few reports in the scientific literature on their ethnomedicinal use, although there are some reports of the use of leaves as vaginal cleansers (Río Hurtado, Region IV, Chile) [3].

The classical knowledge-based approach is the ethnopharmacological approach, where the traditional medicinal use of plants constitutes the basis for the selection of the test material and the pharmacological assay. However, in Chile, this rich ethnomedicinal knowledge of Heliotropium has remained unexplored hitherto.

Phytochemicals Constituents and Biological Activity of Heliotropium Species

Plants are a potential source of health-beneficial compounds, where the discovery and development of new drugs are based on studying and analyzing different plant species. Given the large diversity of plants, there is a great deal of structural diversity studied, and it is represented by many bioactive compounds of pharmacological interest reported to date [37].

Heliotropium indicum is one of the most known and studied cosmopolitan plants belonging to the genus Heliotropium. Hence, many secondary metabolites have been isolated and identified from this species. Among these, pirrolizidine alkaloids (PAs) stand out, which include indicine 1, 12-O-acetyl indicine 2 [38], heliotridine 3, lasiocarpine 4 [39], indicine-N-oxide 5 [40, 41], retronecine 6, trachelan-thamide 7 [42], heliotrine 8 [12], lasiocarpine-N-oxide 9, echinatine 10, heleurine 11, supinine 12 [4], helindicine 13 and lycopsamine 14 [17] (Fig. 2).

Fig. (2))

Alkaloids in Heliotropium indicum.

Several extracts and molecules isolated from H. indicum have a broad biological activity spectrum. The essential oil extracted from the aerial parts has antibacterial activity against Mycobacterium tuberculosis H37Ra with a MIC of 20.8 μg / mL [43]. The methanol extract is active against Klebsiella spp., Proteus mirabilis, Pseudomonas aeruginosa, Staphylococcus aureus, Bacillus subtilis and Escherichia coli [44]. The aqueous extract from the leaves has antifungal activity against Fusarium oxyparum [17, 44] and histo-gastroprotective effects on rats’ stomach body walls [45]. Applying ethanol extract of H. indicum leaf at 5% and 10% on excision wounds in induced diabetic rats, exhibits wound healing properties [46]. Different extracts obtained from H. indicum possess significant antiproliferative and antitumor activity. The ethanolic extract from the whole plant has antiproliferative activity on SKBR3 human breast cancer cell line with IC50 34.5 μg/mL [47]. Previous studies have highlighted the antitumor activity of N-oxide in phase I study in patients with advanced cancer [48] and for the treatment of refractory acute leukemia [49].

In Chile, phytochemical studies of the Boraginaceae Family began in the 1990s with the study of the antioxidant and antifeeding capacity of the most abundant aromatic phenolic constituents for H. stenophyllum [50].

Phenolic Compounds from the Helitropium Genera in Chile

Phenolic compounds are synthesized and accumulated in practically all plant tissues [51], and their distribution is non-uniform as these compounds are present at tissular, cellular and subcellular levels. Insoluble phenolic compounds are found in cell walls, while soluble compounds are present within plant cell vacuoles [52]. The importance of polyphenols is that their antioxidant action lets them protect cellular components against oxidative damage, thus limiting the risk of several diseases associated with oxidative stress [53]. These compounds act as reducing agents (free radical scavengers), hydrogen donors, singlet oxygen quenchers and metal chelators [54]. Flavonoids are a broad class of polyphenolic compounds that comprise an abundant and widespread type of secondary metabolites reported in Heliotropium genera.

The first chemical study of the resinous exudate identified diverse groups of phenolic compounds with antioxidant properties: galangin 15, pinocembrin 16, naringenin 17 and 2-geranyl-4-hydroxyphenyl acetate 18 [51]. Other antioxidant compounds identified for the species are 4'-acetoxy-5-hydroxy-7- methoxy-flavanone 19, 5,3',4'-trihydroxy-7-methoxyflavanone 20; 5,7-dihydroxy-3-methoxyflavone 21; 5,4'-dihydroxy-7-methoxyflavanone 22; 3,7,4'-trihydroxy- 5,3'-dimethoxyflavone 23 [55] (Fig. 3).

Fig. (3))

Phenolics in H. stenophyllum.

The most significant compound is galangin 15, with anti-inflammatory properties that induce apoptosis and decrease mitochondrial membrane potential (MMT) in gastric carcinogenic cell lines (SGC-7901). Specifically, the apoptosis occurred via a mitochondrial pathway involving caspase-8/Bid/Bax activation [56]. Galangin 15 also elevated the expression of GSH-synthesizing enzymes in human keratinocytes, which have a prominent role in cellular defense, especially against oxidative stress [57]. In vivo studies showed that it reduces the volume and weight of laryngeal tumors in nude mice. It also has a potential role in suppressing human laryngeal cancer by inhibiting cell proliferation, activating apoptosis, and autophagy [58]. Galangin, when combined with TNF-Related Apoptosis Inducing Ligand (TRAIL), inhibits the progression of human breast cancer, both in vitro and in vivo, which leads to apoptosis of carcinogenic cells through the activation of caspase-3, associated with the phosphorilation of AMP-activated protein kinase (AMPK) [59]. This flavonoid also inhibits the proliferation of osteosarcoma cells. On the other hand, it significantly increases osteoblast differentiation marker expression (type I collagen, alkaline phosphatase, osteocalcin and osteopontin) and increases alkaline phosphatase activity in human osteosarcoma cells [60].

Pinocembrin 16 is isolated from a wide variety of plants, mainly from the heartwood of Pinus, Eucalyptus, Populus, Euphorbia, Sparattosperma leucanthu and Zingiber officinale (ginger). Its wide range of pharmacological activities has been well documented, including antimicrobial, anti-inflammatory, antioxidant, anticarcinogenic and antiallergenic activities [61, 62]. In vivo immunopharmacological studies of mouse airways have been reported to inhibit hyperreactivity, eosinophilic inflammation, goblet cell hyperplasia, and mucus production in allergic mice induced by ovalbumin (OVA) [63]. Preclinical studies have showed their ability to reduce reactive oxygen species, modulate mitochondrial activity, protect the blood-brain barrier, and regulate apoptosis) [64].

In the same way, naringenin (4', 5, 7-trihydroxyflavanone) 17 is one of the most abundant flavonoids in citrus fruits and tomatoes. It has antioxidant, anti-inflammatory and antitumoral activity [65-67]. In a recent study, pre-administration of 17 significantly reduces the severity of induced colitis, decreasing pro-inflammatory mediators (inducible NO synthase (iNOS)), intercellular adhesion molecule-1 (ICAM-1), monocyte chemoattractant protein-1 (MCP-1), cyclo-oxygenase-2 (Cox2), TNF-α, and IL-6 (mRNA) in the colon mucosa [68]. Photo-aging studies obtained from mice showed that 17 significantly inhibits UVB-induced wrinkle formation, trans-epidermal water loss, and matrix metallopeptidase 13 (MMP-13) expression [69]. Naringenin significantly reduces the levels of Transforming Growth Factor (TGF-β1) on cells and plasma from bleomycin-induced pulmonary fibrosis, also preventing lung metastases [70]. Carcinogenic cell proliferation increases TGF-β1 secretion from breast cancer cells and inhibits pulmonary metastasis by inhibiting the activation of protein kinase C (PKC) [71]. Naringenin induces cell death in epidermoid carcinoma cells, by means of reactive oxygen species (ROS) generation, mitochondrial depolarization, nuclear condensation, DNA fragmentation, cell cycle arrest in G0 / G1 phase and activation of caspase-3 [72].

The structure−activity relationships of flavonoids have been determined in many antioxidant activities assays, yet results vary depending on the protocol [73]. The flavonoids 16, 17, 20, 21, 3-acetoxy-5,7-dihydroxyflavanone (pinobanksin-3- acetate) 30, 5,7,3'-trihydroxy-4'-methoxyflavanone (hesperetin) 31, 5,7,4'-trihydroxy-3-3'-dimethoxyflavone (3-O-methylisorbamnetin) 32, and 5,4'-dihydroxy-3,7,3'-trimethoxyflavone (pachypodol) 33 were identified in the resinous exudate of Heliotropium sinuatum [74]. Subsequent studies have evidenced very high antioxidant capacity due to its phenolic content, namely flavonoids and a new aryl phenol 4-(3', 5'-dihydroxynonadecyl phenol) [75]. In this study, compound 20 had the greatest contribution to the antioxidant activity of the resin. The antioxidant behavior of flavonoids such as flavone, flavanol and isoflavone isolated from H. sinuatum, together with the related activity-structure relationships, show that the antioxidant activity depends on both the substitution of hydroxyl groups in the flavonoid skeleton and the presence of an unsaturation at the C2-C3 bond in conjunction with oxo function in C4 [76].

Heliotropium glutinosum Phil. (Heliotropiceae) is a resinous bush that grows at a height of 2000 m near Chañaral, Chile. Phytochemical examinations of H. glutinosum have identified a wide range of bioactive compounds. The secondary metabolites isolated and identified from the resinous exudates were a new aromatic geranyl derivative: 4-methoxy-3-[(2)-7'-methyl-3'- hydroxymethyl-2', 6'-octadienyl] phenol, and three flavonoids: 19, 26 and 5,3'-dihydroxy-7,4'- dimethoxyflavanone 36. In the analysis of antioxidant activity, dimethoxyflav- anone possessed the antioxidant capacity, as observed for other species of Heliotropium [77].

The flavonoids 7, 3'-dimethyleriodictyol 24, ayanin 25, sakuranetin 26, kumatakenin 27 and rhamnocitrin 28 were isolated from the resinous exudates of H. chenopodiaceum var. ericoideum [78]. The same study isolated the flavonoids 15 and 21 in combination with unknown aromatic geranyl derivatives from the species H. filifolium. Given the aforementioned properties of H. stenophyllum [55], the study confirmed the chemical proximity of the two species, unlike H. chenopodiaceum var. ericoideum, which did not present geranyl derivatives. Further studies into the species identified a new compound named filifolinol 29, from the resinous exudate of H. filifolium [79]. This compound has a new rearranged product of the cyclization of the oxidized side chain of an aromatic geranyl compound, which relates it biogenetically to H. stenophyllum [55].

Flavonoids such as ayanin 25 have also drawn attention for their biological activity. Ayanin proved to be a potent inhibitor of BCRP (breast cancer resistance protein), only slightly less than Ko143, the most potent inhibitor of ABCG2 gen known to date [80]. The ayanin isolated from Siegesbeckia pubescens has been used in Eastern medicine to treat arthritis, strokes, skin rashes, edema and itchy eczema. In vitro trials in murine hippocampal cell line (HT22) evidenced neuroprotective and anti-neuroinflammatory activity [81]. The administration of ayanin in mice with atopic dermatitis produced a preventive effect and general improvement. The flavonoid inhibits the release of histamine, the synthesis of the IL-4 ligand in basophils. Epidemiological studies report a significant decrease in asthma incidence among populations which consume this flavonoid regularly [82].

Sakuranetin 26 is a flavone with many defensive functions in plants. Studies attribute antifungal [83-85], antibacterial, cytotoxic [86] and anti-inflammatory properties [87]. Sakuranetin isolated from Baccharis retusa DC. (Asteraceae) performs anthelminthic activity against Leishmania (L.) amazonensis, Leishmania (V.) braziliensis, Leishmania (L.) major and Leishmania (L.) chagasi with IC50 between 43-52 μg/mL, and against T. Trypomastigotes cruzi (IC50 = 20.17 μg/mL) [88]. In an in vivo trial where sakuranetin (20 mg/kg) isolated from B. retusa (Asteraceae) was administered to an experimental murine model (BALB/c) of asthma, anti-inflammatory and antioxidant effects were observed, preventing changes in vascular and distal