Introduction
Cancer is a worldwide public health problem that develops following an abnormal growth of potentially invasive cells, capable of infinite replication and metastasis [
1‐
6]. It maintains a poor prognosis, with the yearly incidence and death rate steadily increasing over time, discovering new therapeutic molecules and strategies an ever-present necessity [
7‐
12]. Opportunely, several biologically active natural molecules have been researched and applied for cancer treatment, including cucurbitacins and their derivatives which, are mainly organized in the groups: A, B, C, D, E, I, H, Q, R, and dihydrocucurbitacin B [
13]. From the chemical point of view, cucurbitacins are cucurbitane-type tetracyclic triterpenoid saponins with 30 carbons atoms on their basic skeleton (the detailed chemical data can be found in Figures S1, S2 and S3 and Table 1, and the synthesis of cucurbitacins in Figures S4 to S12, in the Additional file
1).
Cucurbitacin B isolated from
Trichosanthes cucumerina (snake gourd) is the greatest source of cucurbitacins, which ensued the attention of scientists because of its several anticancer mechanisms [
14,
15]. There are numerous health benefits displayed by the entire Cucurbitaceae family, each vegetable or fruit with a unique effect on human health [
16]. For example, the well-known vegetable
Cucumis sativus (cucumber) rich in cucurbitacins A, B, C, D, and E, has been reported to alleviate symptoms of indigestion and constipation, to help with skin problems and to promote hair growth [
17]. Cucurbits have been also to stimulate the central nervous system and to treat dizziness. Furthermore, their seeds have also been used in the treatment of depression since their content in
l-tryptophan increases serotonin levels, more commonly known as the “happy hormone” in the brain [
18]. In this updated and comprehensive review, the latest data on the anticancer activity of cucurbitacins, targets and molecular mechanisms of action, and perspectives in chemotherapeutic therapy by association with other agents are synthesized. In the light of the encouraging results obtained from preclinical pharmacological studies, new windows are opening for the generation of new ideas and strategies for cancer chemotherapy.
Cucurbitacins: origin, traditional uses and therapeutic potential
Among living organisms, some of them exhibit secondary metabolites unique to some species being considered chemotaxonomic markers because they reveal an evolutionary relationship [
21]. Nevertheless, some other metabolites occur in organisms without taxonomic similarity, which is related to specific roles in nature. Hence, environment-mediated stimuli probably lead to the emergence of metabolites in non-related species [
22]. Cucurbitacins belong to the latter classification, representing an intriguing convergence between plants, fungi and even some animals. These stimuli can be authenticated with an analogous ecological role—their intensive bitterness is toxic to a wide range of herbivores, microorganisms, insects, and parasites, thus protecting their hosts from predators [
23]. Cucurbitacins are more commonly present within the plant kingdom, with the vast majority being found in the Cucurbitaceae family, although they can also exist in several other taxonomically distant families. Nonetheless, some studies have also reported the presence of cucurbitacins in both the fungi and the animal kingdom. Therefore, establishing the distributions of secondary metabolites in this family is of utmost importance.
Cucurbitacins are mainly found in the Cucurbitaceae family, which consists of 965 plant species in 120 genera. Most of them are consumed in vegetables or in fruits, but they also show medicinal value, the reason why they have been used for traditional medicine in China [
24]. Cucurbitacins within this family are more commonly present in
Cucumis sativus (cucumber),
Cucurbita moschata (pumpkin),
Citrullus lanatus (watermelon),
Cucumis melo (melon),
Cucurbita pepo (squash),
Lagenaria siceraria (bottle gourd),
Luffa acutangula (ridge gourd), and
Bryonia dioica (red/white bryony). Furthermore, a special kind of cucurbitacins from the Cucurbitaceae family called momordicosides can be found in the
Momordica charantia (bitter melon or African cucumber). Cucurbitacins have diverse therapeutically important and attractive bioactivities, including in cancer chemoprevention/chemotherapy, and numerous preclinical studies have proven these properties (Table
1) [
25‐
27].
Table 1
Summarized data regarding the botanical sources and potential pharmacological effects of cucurbitacins
Cucumis sativus (Cucumber) | Anticancer potential against liver cancer HepG2 cell | |
Cucurbita moschata (Butternut Squash) | Anticancer potential ↓cells tumour growth ↑ribosome-inactivating proteins | |
Cucumis melo (Melon) | Analgesic, anti-inflammatory, antioxidant, antiulcer, anticancer, antimicrobial, diuretic, antidiabetic, hepatoprotective, immunomodulator | |
Citrullus lanatus (Watermelon) | Antidiabetes | |
Cucurbita pepo (Pumpkin) | Antioxidant, anti-inflammatory, antiviral, antimicrobial, analgesic Anti-carcinogenic, anti-proliferative Pro-apoptotic properties against tumor cells | |
Iberis amara (Wild candytuft) | Anti-inflammatory | |
Coutarea hexandra (Coutarea) | Treatment of malaria, cancer, inflammation, and diabetes | |
Begonia nantoensis | Cytotoxic effect against cancer cell lines | |
Kageneckia oblonga (Bollen) | Analgesic, antipyretic, anti-inflammatory | |
Rubus chingii (Chinese raspberry fruit) | Anti-ageing, anticancer, antioxidant, anti-inflammatory and antidiabetic | |
Russula lepida (Russula) | Antitumor activity | |
Dorid nudibranchs | Cytotoxicity against cancer cells | |
The extract from
Cucumis sativus flowers was isolated and tested for their anticancer potential against liver cancer HepG2 cell line. With a LD
50 of 103.7 µg/mL, this extract induced apoptosis in the HepG2 cell line [
28].
Hou et al. evaluated the anticancer potential of
Cucurbita moschata in an in vitro model, using K562 human leukaemia cells, B16 murine melanoma cells, and A549 lung adenocarcinoma cells. They confirmed that isolated compounds from this vegetable inhibit cell tumour growth by working like ribosome-inactivating proteins [
17,
29,
30].
Cucumis melo is well known for its beneficial pharmacological assets, such as analgesic, anti-inflammatory, antioxidant, antiulcer, anticancer, antimicrobial, diuretic, antidiabetic, hepatoprotective, and immunomodulator [
31]. Vella et al. have recently established the significance of seeds and peels from
Cucurmis melo, which until then were only considered waste. They concluded that the
Cucumis melo by-products are a source of polyphenols and tannins, responsible for antioxidant and anticarcinogenic properties, respectively. Hence, the valorization of this entire vegetable is extremely important because it can reduce waste, as well as its environmental impact and the economic costs associated with its disposal. Besides, it can also be a source of active compounds for cosmetics, and pharmaceutical products [
17,
32].
Citrullus lanatus has a potential use in the treatment of diabetes mellitus being investigated in vivo with obese and diabetic-induced rats. Watermelon extracts seems to play an important role in the prevention of diabetes’s complications, through attenuation of specific parameters in the kidneys and liver of diabetic animals [
33].
Cucurbita pepo has been used in traditional medicine in several countries, treating patients infected with worms and parasites. In Europe, it has helped in the treatment of prostate enlargements, as well as irritable bladders. Moreover, it has also been used due to its antioxidant, anti-carcinogenic, anti-inflammatory, antiviral, antimicrobial, and analgesic properties. The oil obtained by cold pressure from
Cucurbita pepo seeds is rich in antioxidant and antimicrobial components. Thus, it can be useful in some cosmetic formulations as it protects against dermatological wounds [
17,
34]. There have been some reports regarding the cytotoxicity activity of
Cucurbita pepo. Wang et al. detected a dose-dependent inhibitory effect between
Cucurbita pepo fruit extracts and HeLa and HepG2 cell growth [
35]. Martinez et al. observed in HL60 tumour cells, that it inhibits significantly the H
2O
2-induced damage and presents anti-proliferative and pro-apoptotic properties [
36]. Besides the Cucurbitaceae family, cucurbitacins are also present in other plant families and some fungi and animals. Below follows some of these examples.
One of the most extensive plant families distributed worldwide is Brassicaceae, which comprises almost 400 genera and 4000 species. Interestingly, this family used to be known as Cruciferae owing to their cruciform appearance and flower shape [
37]. A vast majority of the several Brassicaceae species are vegetables, which are commonly identified by their functional properties, like their phytochemical composition. The phytochemicals are categorised as micronutrients, macronutrients, and secondary metabolites. There are currently some reports linking a wide spectrum of bioactivity of these secondary metabolites to the prevention and treatment of several chronic diseases, such as obesity, type 2 diabetes, cardiovascular diseases, cancer, and osteoporosis. Furthermore, antioxidant activity, as well as antimicrobial capacity have also been detailed. This phenomenon is mainly on account of the synergistic effect of glucosinolates, polyphenols, and triterpenes (specific cucurbitacins), the main constituents in cruciferous plants [
38]. Khayyal et al. evaluated the anti-inflammatory and antioxidant properties of
Iberis amara extracts (rich in cucurbitacins) in rats, making use of both acute and chronic experimental models of inflammation. A dose-dependent reduction in inflammation was observed in both models, reflecting the extract's potent anti-inflammatory properties. Furthermore, other biological roles have also been assigned to
Iberis amara (due to the presence of cucurbitacins B, E, and I), such as antifeedant activity, protecting many brassicaceous species [
39,
40].
Picria fel-terrae, Neopicrorhiza scrophulariiflora and Gratiola officinalis belong to the family Scrophulariaceae, and have been used in traditional medicine, due to their cucurbitacins' content. [
14,
41,
42].
Coutarea hexandra, from the Rubiaceae family, has been used traditionally in the treatment of malaria, inflammation, and diabetes. Olmedo et al. performed, with cucurbitacins isolated from the ethanolic extract (80%) of this plant, sulphorhodamine B assay in the following cancer cell lines: breast (MCF-7), lung (H-460) and central nervous system (SF-268), observing moderate cytotoxicity [
15]. Wu et al. isolated some cucurbitacins from
Begonia nantoensis (Begoniaceae family) such as cucurbitacin B, dihydrocucurbitacin B, cucurbitacin E, dihydrocucurbitacin E, and cucurbitacin I and evaluated their cytotoxicity. These compounds presented strong cytotoxic effects against the following cancer cell lines: gastric (NUGC-3), nasopharyngeal (HONE-1), breast (MCF-7) and lung (A549) [
43].
Stems and leaves of
Kageneckia oblonga (Rosaceae family) containing cucurbitacin’s derivatives have analgesic, antipyretic, and anti-inflammatory activity [
44,
45]. East Asian countries commonly use
Rubus chingii s unripe fruits to treat several diseases, especially those related to kidney deficiencies. Pharmacological studies validated the anti-ageing, anticancer, antioxidant, anti-inflammatory and antidiabetic properties due to its content in cucurbitacins [
46].
Aquilaria agallocha (Thymelaeaceae family) is one of the largest producers of agarwood used throughout history in religious ceremonies, herbal medicines and perfumes, and whose composition is rich in cucurbitacins particularly E and I) [
47]. Cucurbitacins have also been isolated from a few genera of mushrooms, including
Hebeloma vinosophyllum,
Russula lepida, and
Leucopaxillus gentianeus. Several studies reported that cucurbitacins exert a protective role against mushroom predators, such as microorganisms and insects, due to their cytotoxicity. For example, the reason why the
Hebeloma vinosophyllum is a poisonous mushroom is believed to be due to the presence of hebevinosides I–XI, together with hydroxyhebevinogenin and methoxyhebevinogenin, some derivatives from cucurbitacins [
48,
49].
Russula lepida (Russulaceae family) has already been exploited throughout the years as both food and medicinal agents in China due to its content in seco-cucurbitacins—at least three seco-cucurbitacins were already isolated. Specifically, the extracts of
Russula lepida’s fruiting bodies exhibited antitumour activity [
50]. Furthermore, they also demonstrated a protein tyrosine phosphatase 1B (PTP1B) inhibitory activity—a compound manipulated as a negative regulator in the signal transduction via insulin and leptin pathways—without showing cytotoxicity [
51,
52].
The mushroom
Leucopaxillus gentianeus, to protect itself from external predators, first accumulates inactive fatty acid esters, such as 16-oleyl, 16-linoleyl, and 16-palmityl esters in its tissues. Then, after injury, some lipases cleave these esters, transforming them into a more active compound—the bioactive metabolite cucurbitacin B, warding off external attacks due to its toxicity [
23,
53]. The shell-less marine molluscs (
Dorid nudibranchs) are particularly susceptible to predators and as a chemical defence mechanism, they release terpenoid metabolites—particularly cucurbitacins. Interestingly, these cucurbitacins showed modest in vitro cytotoxicity against human ovarian carcinoma (HEY) and human glioblastoma/astrocytoma (U373) cell lines [
54].
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