Phytochemical and Pharmacological Properties of Capparis spinosa as a Medicinal Plant

Over the past decades, there has been increasing attention on polyphenol-rich foods including fruits and vegetables on human health. Polyphenols have been shown to possess some potential beneficial effects on human health and they are widely found in foods consumed by populations worldwide. Capparis spinosa (C. spinosa) is an important source of different secondary metabolites of interest to humankind. The traditional therapeutic applications of C. spinosa have been reported in Ancient Romans. Numerous bioactive phytochemical constituents have been isolated and identified from different parts (aerial parts, roots and seeds) of C. spinosa which are responsible alone or in combination for its various pharmacological activities. Therefore, this paper is a review of publications on the phytochemical and pharmacological properties of C. spinosa. There is insufficient evidence to suggest that C. spinosa or its extracts are able to improve the biomarkers of cardiovascular disease and diabetes. However, these studies used different parts of C. spinosa plant, methods of preparation and types of solvents, which cause the evaluation of activity of C. spinosa difficult and involve quite heterogeneous data. There is also evidence, although limited, to suggest benefits of C. spinosa in improving human health. Therefore, the relationship between C. spinosa and improved human health outcomes requires further study.


Introduction
Medicinal plants have been used since ancient times as therapeutic agents for the management of health and treatment of diseases because they possess health-promoting effects and contain bioactive components [1]. According to the World Health Organization (WHO) [2], 80% of the world's populations rely mainly on traditional medicine. In China, 30-50% of the overall medicinal consumption is estimated from the preparations of traditional medicine [3]. Approximately 90% of the population in Germany reported that they have used natural remedies for certain health purposes [2]. Therefore, there is increasing use and popularity of traditional medicine in both the developing and industrialized countries [4], demonstrating that the global market for traditional medicine continues to be strong. The international market for herbal medicines has hit over $60 billion yearly and it continues to increase gradually [4]. Therefore, medicinal plants such as Capparis spinosa (C. spinosa) continue to play a major role in healthcare systems [4].
C. spinosa is one of the most important economical species in the Capparidaceae family which has a wide range of diversity (i.e., about 40-50 genera and 700-900 species) [5]. Capparidaceae has been known to be closely related to the family of the Brassicaceae (Cruciferae) that is rich in glucosinolates and flavonoids [5]. C. spinosa is also known as Caper, wild watermelon (in China) [6], Cappero (in Italy),

Taxonomic Revision of Genus Capparis
There are 813 plant name records that matched Capparis in the plant database 'The Plant List' (at http://www.theplantlist.org). When C. spinosa was searched in the same database, 22 plant name records were found. There have been 250 different species in the genus of Capparis which are recognized morphologically. However, the use of morphological markers of the Capparis species has its limitations in defining the subspecies and varieties because of the free hybridization of different Capparis species and the presence of their intermediate forms [14]. Therefore, the classification of Capparis remains to be very ambiguous and controversial [15]. Aichi-Yousfi et al. [16] demonstrated that the use of amplified fragment length polymorphism (AFLP) analysis was efficient to make definitive discrimination among the genetic diversity and relationship between various species of Capparis. The authors used three primer combinations of AFLP markers to genotype 213 Capparis accessions belonging to six Tunisian Capparis species [16].
1.3. Cultivation of C. spinosa C. spinosa which is an aromatic plant is usually cultivated in tropical and subtropical regions [10]. The most common propagation of C. spinosa is vegetative cuttings [10]. It can flourish under dry hot conditions in either well-drained or poor soils. In addition, C. spinosa is salt tolerant and resistant to drought [17]. Although C. spinosa can be grown in a wide range of environmental conditions, it is generally grown on sandy loam soils with low alkalinity [18]. It grows and flowers from May to October covering the summer drought [19]. Since it has deep, extensive root systems and can be grown in harsh environments, it has been recommended for the prevention of land degradation and soil erosion control [17].

Traditional Uses of C. spinosa
Different parts of C. spinosa including fruits and roots have been used as a traditional herbal remedy since ancient times for its beneficial effects on human diseases [20]. Ancient Egypt and Arab consumed the roots of C. spinosa to treat liver and kidney diseases; Ancient Romans used C. spinosa for the treatment of paralysis; Moroccans used C. spinosa to treat diabetes [8]. In the Northern areas of Pakistan, the root barks of C. spinosa have been used to treat splenomegaly, mental disorders and tubercular glands [21]. In China, C. spinosa has been used in traditional Uighur Medicine for the treatment of rheumatoid arthritis and gout [6]. In Iran, C. spinosa is used to treat hemorrhoids and gout [22]. Table 1 shows the main traditional uses of C. spinosa used to ease symptoms and treat diseases.
Since C. spinosa is rich in flavonoids, many studies have identified and quantified flavonoids found in C. spinosa. Flavonoid compounds such as rutin and quercertin are detected in C. spinosa [39]. Flavonoids have received considerable attention for their positive effects on health because of their anti-oxidative property [40]. Flavonoids are hydroxylated phenolic compounds and are found in plants [39]. Flavonoids have a 15-carbon skeleton, which consists of two benzene rings and a heterocyclic pyrane ring [41]. Flavonoids can be divided into several classes based on their structures such as flavones, flavonols, flavanones, isoflavonoids and others [41]. The consumption of flavonoids is suggested to reduce the risk of cardiovascular disease (CVD) and promote human health [41]. Rutin strengthens capillaries and inhibit platelet clump formation in the blood vessels due to its high radical scavenging and antioxidant properties [42]. In addition, rutin reduces low density lipoprotein (LDL) cholesterol level, which is associated with improve CVD risk biomarkers [43]. Quercetin has been linked with a reduced risk of CVD because of its anti-hypertensive and anti-platelet aggregating properties [44].
Afsharypuor et al. [31] reported that leaves and ripe fruits of C. spinosa contained the glucosinolates degradation products which were methyl, isopropyl and set-butyl isothiocyanates. Glucosinolates are secondary plant metabolites that are of pharmacological interest because they may have a role in the prevention of diseases and reducing the risk of carcinogenesis. Similarly, Çaliş et al. [25] also reported that indole-3 acetonitrile glycosides was detected in fruits of C. spinosa.
Some studies also investigated the flavonoids in flower buds of C. spinosa [11,49]. Inocencio et al. [11] used HPLC coupled with a diode-array detector to isolate kaempferol 3-rhamnosyl-rutinoside, kaempferol 3-rutinoside and quercetin 3-rutinoside from the flower buds of C. spinosa. The authors reported that rutin (quercetin-3-O-rutinoside) was the most abundant compound followed by kaempferol 3-rutinoside [11]. Also, they found that 10 g of C. spinosa would provide approximately 65 mg of flavonoid glycosides [11]. The main glucosinolates in shoots and buds of C. spinosa was glucocapparin, which contributed 90% of the total glucosinolates [49]. It is suggested that the composition of glucosinolates may vary according to the bud size of C. spinosa.
Sterols such as stigmasterol, sitosterol, campesterol and avenasterol were also determined from the seed oils of C. spinosa [36]. The total amount of sterols in the seed oils of C. spinosa ranged from 4962 to 10,008 mg/kg. In addition, the seed oils of C. spinosa are also rich in tocopherols with γand δtocopherols as the major vitamin E active compounds [36]. The total amount of tocopherols in the seed oils of C. spinosa ranged from 249 to 1985 mg/100 g [36]. Citrostadienol, cycloartanol, gramisterol, hexadecanol, octadecanol, β-amyrin and tetracosanol were also determined from the seeds of C. spinosa [36].

Pharmacological Effects of C. spinosa
Although many studies using various parts of C. spinosa have reported diverse pharmacological activities including anti-diabetic and anti-hypertensive, there is still no conclusive information regarding the association between C. spinosa and its health benefits. This is because only a very few studies that involved human subjects examined the effect of C. spinosa consumption on human health. Similar to other plants [54,55], although there exists literature related to the health benefits of C. spinosa, studies often infer a causal correlation between a bioactive substance of C. spinosa, and the observe health outcomes. This approach is likely to be an oversimplification of the complex mechanisms in the body that will eventually lead to the observed health outcomes. Therefore, the conclusions of such studies on the effects of changes in the dietary intake should be interpreted with caution because the health outcomes may not be attributed to the action of a single bioactive substance [56]. Table 3 illustrates major pharmacological effect of C. spinosa.

Anti-Diabetic
In a randomised, double-blind, placebo-controlled clinical trial of 54 type 2 diabetic patients, Huseini et al. [59] reported that patients who took 1200 mg C. spinosa fruit extracts daily for 2 months had a significantly lower level of glycosylated hemoglobin and fasting blood glucose than the control group (p = 0.043 and 0.037, respectively). The findings of their study demonstrated an improvement in hypertriglyceridemia and hyperglycemia in diabetic patients [59]. In addition, no renal and hepatic adverse events were reported in the patients [59]. The possible mechanism is that C. spinosa decreases the rate of carbohydrate absorption and exert its postprandial hypoglycemic effect in the gastrointestinal tract [58]. Therefore, the consumption of C. spinosa may be beneficial and safe for controlling and treating blood glucose levels in diabetic patients.

Anti-Obesity
Eddouks et al. [57] demonstrated that there was a significant weight loss in diabetic rats fed with the aqueous extract of powdered fruits of C. spinosa (20 mg/kg) after 2 weeks (p < 0.01). Another study by Lemhadri et al. [58] reported that repeated oral administration of aqueous extracts of C. spinosa was associated with a significant loss of body weight in high fat diet-fed mice after 2 weeks (p < 0.01). Therefore, it is suggested that C. spinosa may be used for weight loss management. However, further studies are needed to confirm such findings.

Cholesterol-Lowering
C. spinosa has been reported to be associated with improved plasma lipid parameters. Huseini et al. [59] reported a significant decrease in the triglyceride level of type 2 diabetic patients who were supplemented with 1200 mg C. spinosa fruit extracts daily for 2 months (p = 0.029). The possible mechanism for the cholesterol-lowering effect of C. spinosa is that C. spinosa decreases the activity of 3-hydroxy-3-methyl-glutaryl coenzyme A reductase (HMG-CoA reductase), which plays an important role in the biosynthesis of cholesterol [69]. Therefore, C. spinosa may be useful for the treatment of fatty liver disease and metabolic syndrome because it plays an important role in the inhibition of gluconeogenesis in liver.

Anti-Hypertensive
C. spinosa has the potential to be used for the treatment of hypertension. In a study of spontaneously hypertensive rats, Ali et al. [61] reported that when the aqueous extract of powdered fruits of C. spinosa (150 mg/kg) was administered for 20 days, the systolic blood pressure was significantly decreased after 8 days (p < 0.05), 12 days (p < 0.01) and 16 days (p < 0.001). In addition, there was also a significant increase in the concentration of urinary sodium (p < 0.001), potassium (p < 0.001) and chloride (p < 0.001) after 20 days [61]. No change in heart rate was observed during the period. In addition, there was also no difference in the activity of plasma angiotensin-converting enzyme (ACE) and renin after 20 days [61]. It is suggested that C. spinosa decreases blood pressure by increasing the excretion of renal electrolytes and inhibiting the ACE activity. The inhibition of ACE activity is associated with a decrease in blood pressure [70]. Therefore, C. spinosa may play an important in the reduction of blood pressure.

Anti-Microbial
In a study investigating the antibacterial activity of C. spinosa, Boga et al. [33] reported that the growth rate of Deinococcus radiophilus (D. radiophilus) was significantly inhibited by the addition of aqueous extracts from roots of C. spinosa as compared to the control. However, no inhibition was shown on the growth rate of Escherichia coli (E. coli) by the addition of aqueous extracts from roots of C. spinosa [33].
Another study by Mahboubi and Mahboubi [22] evaluated the antimicrobial activity of aqueous, ethanolic, ethyl acetate and methanolic extracts of C. spinosa roots and fruits. The authors reported that the aqueous extracts of C. spinosa roots exhibited a higher antimicrobial activity against a wide range of microorganisms than that of fruit aqueous extracts of C. spinosa fruits [22]. The authors demonstrated that the aqueous extracts from roots of C. Gull et al. [62] reported that the methanolic extracts of C. spinosa stem barks and shoots had the greatest antibacterial activity against B. subtilis with growth inhibition zones of 26.8 mm and 24.6 mm, respectively. While the methanolic extracts of C. spinosa fruits had the highest growth inhibition zones (24.9 mm) against Pasteurella multocida (P. multocida) followed by B. subtilis (23.9 mm), E. coli (20.9 mm) and S. aureus (17.7 mm) [62]. On the other hand, the highest antibacterial activity of the methanolic extracts of C. spinosa flowers and roots was observed against E. coli with growth inhibition zones of 26.5 mm and 23.9 mm, respectively [62]. In addition, polysaccharides of C. spinosa leaves have been suggested to exhibit antimicrobial activity. Mazarei et al. [71] reported that polysaccharides of C. spinosa leaves had a higher antimicrobial activity against E. coli, S. dysenteriae and Salmonella typhi (S. typhi).
Mahasneh [72] reported that butanol extract of C. spinosa showed a moderate to good antifungal activity against C. albicans and A. flavus. In addition, ethanol extracts of C. spinosa exhibited weak cytotoxicity against Helicobactor pylori (H. pylori) isolates [63]. The infection of H. pylori is associated with several gastroduodenal diseases such as gastric cancer [73]. It is suggested that the method of preparing extracts and the type of solvent used may affect the microbial activity of C. spinosa.

Anti-Inflammatory
In an in vivo mouse model, El Azhary et al. [64] reported that Swiss albino mice treated with C. spinosa leaf extracts had a significantly reduced edema than control, demonstrating the anti-inflammatory activity of C. spinosa. In addition, the authors also found that C. spinosa had significantly decreased the dermis thickness and immune cell infiltration in the inflammatory site. Similarly, Moutia et al. [65] reported that C. spinosa leaf extract was shown to exhibit anti-inflammatory activity in vitro on human peripheral blood mononuclear cells (PBMC) obtained from healthy subjects. The authors also found that PBMC treated with the aqueous fraction of C. spinosa leaf extract had a significant increase in interleukin (IL)-4 gene expression (an anti-inflammatory cytokine) and a significant decrease in IL-17 gene expression (pro-inflammatory cytokine) [65]. Therefore, these studies suggested that C. spinosa leaf extracts exhibit anti-inflammatory activity by inhibiting the pro-inflammatory cytokines expression and immune cell infiltration [64,65].
C. spinosa root extracts were reported to relieve pain in complete Freund's adjuvant (CFA)-induced rheumatoid arthritis and mono-iodoacetate (MIA)-induced osteoarthritis male Sprague-Dawley rats [66]. It is suggested that the pain reliever effect of C. spinosa roots could be due to the presence of spermidine alkaloids which have anti-inflammatory effects [66]. Similarly, the lyophilised methanolic extracts from flowering buds of C. spinosa was also reported to exhibit anti-inflammatory effect by reducing the production of reactive oxygen species (ROS), nitric oxide (NO) and prostaglandins (PGE 2 ) induced by IL-1β on human chondrocytes [74]. These findings were consistent with another study by Feng et al. [75] who demonstrated that the fraction eluted by ethanol-water (50:50, v/v) from C. spinosa fruits was shown to exhibit the most significant anti-arthritic response in male Wistar rats, providing additional evidence that C. spinosa possesses anti-inflammatory effects.
Similarly, C. spinosa exhibited anti-inflammatory activity to suppress cytokine production of dendritic cells (DC), induced by lipopolysaccharide (LPS). In a study investigating the effect of C. spinosa fruit ethanol extracts on the maturation of mouse bone marrow-derived DC Hamuti et al. [67] reported that different types of C. spinosa extracts exhibited different effects on the maturation of DC. DC is the key regulator in many aspects in homeostasis of immune system [76] and inflammatory skin disorders [77]. These different types of C. spinosa extracts were labelled as C. spinosa extract (CSE) 2 water (W), CSE middle-layered isolated (M) W, CSE3W, CSE2 dimethyl sulfoxide (D), CSEMD and CSE3D according to their methods of preparation which were either dissolving in water or dimethyl sulfoxide (DMSO) at a concentration of 200 mg/mL. The authors reported that CSEMD and CSEMW dose-dependently increased the expression of cluster of differentiation (CD) 40, CD80 and CD86 [67]. CSE2W was shown to significantly increase the expression of CD40 but not CD80 [67]. Although CSE3D and CSE3W had significantly inhibited the secretions of pro-inflammatory cytokines including IL-1β, IL-12p40, IL-6 and tumor necrosis factor (TNF)-γ induced by LPS, CSE3D had a higher inhibitory activity than CSE3W [67].

Antihepatotoxic
A study by Gadgoli and Mishra [51] evaluated the effects of C. spinosa on the antihepatotoxic on rats against paracetamol and carbontetrachloride induced toxicity in vivo. In addition, the authors also investigated the effects of C. spinosa on galactosamine and thioacetamide induced toxicities in vitro [51]. In their study, methanol soluble fraction of the aqueous extract of aerial parts of C. spinosa was reported to exhibit significant reductions in serum glutamyl pyruvate transaminase (SGPT), serum glutamyl oxalacetate transaminase (SGOT), alkaline phosphatase and total bilirubin in paracetamol and carbontetrachloride induced hepatotoxicity in albino rats of Wistar strain [51]. In addition, the authors also reported that aerial parts of C. spinosa showed significant anti-hepatotoxic activity in galactosamine and thioacetamide induced hepatotoxicity in isolated rat hepatocytes [51].
Kazemian et al. [68] reported that both groups of diabetic rats receiving 0.2 g/kg and 0.4 g/kg of hydroalcoholic extract of C. spinosa had a significant reduction of alanine aminotransferase (ALT) and alkaline phosphatase (ALP) (p < 0.05 for both groups) after 4 weeks of treatment. Therefore, it is suggested that C. spinosa extract does not cause any toxic effect on the liver [68].

Other Pharmacological Properties
C. spinosa also exhibits potential protective effects against cognitive impairment. In a study investigating the effect of C. spinosa on the learning and memory damage induced by chronic administration of LPS (175 µg/kg) in young male Sprague-Dawley rats, Goel et al. [78] reported that the aqueous extract of C. spinosa buds had significantly reduced the neurodegeneration in the first region in the hippocampal circuit (CA1) region of hippocampus, suggesting that C. spinosa could be used to treat cognitive disorders.
In addition, another study by Mohebali et al. [79] reported that the expression of β-secretase enzyme (BACE)-1, presenilin protein (PSEN)-1 and PSEN-2 and amyloid precursor protein (APP) genes were significantly down regulated in amyloidogenic related genes in amyloid-beta (Aβ) peptide-injected Wister albino rats treated with hydroalcoholic extracts of C. spinosa leaves and fruits as compared to the control group. Another study by Turgut et al. [80] reported that the extracts of C. spinosa seeds significantly protected against the damage of DNA bands and attenuated cognitive impairment induced by D-galactose in male, Bagg albino, laboratory-bred strain of the House Mouse (BALB/c) mice. Chronic neurodegenerative diseases such as Alzheimer's disease are associated with the aggregation and misfolding of proteins. The accumulation of Aβ peptide is the key protein involved in the development of Alzheimer's disease and the accumulation of Aβ peptide can be caused by malfunction in βand γsecretase enzymes. BACE is encoded by BACE-1; γ-secretase enzyme is encoded by PSEN-1 and PSEN-2 [81].
C. spinosa also exhibits inhibitory effects on human immunodeficiency virus (HIV)-1 reverse transcriptase [82]. Lam and Ng [82] isolated a protein with an N-terminal amino acid sequence from the seeds of C. spinosa that inhibited HIV-1 reverse transcriptase. The protein exhibited some similarly to imidazoleglycerol phosphate synthase [82]. Lam and Ng [82] reported that the protein possessed HIV-1 reverse transcriptase inhibitory activity with the half maximal inhibitory concentration (IC 50 ) of 0.23 µM. It is suggested that the inhibition is due to the protein-protein interaction which causes homologous retroviral reverse transcriptase to be inhibited by HIV-protease [82].

Adverse Effects
Currently, the consumption of C. spinosa is not associated with any adverse effects according to the published literature, providing evidence that C. spinosa is safe to consume [83].

Conclusions
A literature review has highlighted that C. spinosa exhibits important pharmacological effects because C. spinosa is rich in many bioactive compounds including flavonoids. Therefore, given the few clinical studies cited, it is very risky to highlight the potential role of C. spinosa on treatment such as diabetes, hypertension and obesity. This suggests that there are many opportunities for the food and healthcare industry to explore the health benefits of C. spinosa because there is a potential growth market for C. spinosa. However, the majority of the studies that reported beneficial effects of C. spinosa on health are animal-based studies. Moreover, these studies used different parts of C. spinosa plant, types of solvents and methods of preparation, which cause the evaluation of activity of C. spinosa difficult. In addition, these studies involve quite heterogeneous data.
Therefore, future prospective research should screen for individual polyphenol constituents that possess health-promoting properties in C. spinosa. This is because a cause-effect relationship between C. spinosa and its health effects can only be established when the composition of C. spinosa is properly characterised and standardised. In addition, there is limited evidence of randomized controlled trials on humans to support such health beneficial effects of C. spinosa when compared to other plants such as walnuts that have high economic interests to the food industry. The underlying mechanism influencing human health by the consumption of C. spinosa still remains unclear. The effect of shortand long-term consumption of C. spinosa on human health therefore needs to be further evaluated.
Furthermore, the role of the gut microbiota in the degradation of polyphenolic compounds present in the plant has not been considered. This aspect should be put forward in the perspectives [84].