Introduction

Cancer is one of the most life-threatening diseases of the present century, which consists of over 277 different types [1]. According to GLOBOCAN 2018, cancer has a very high rate of occurrence with a mortality of around 9.6 million per year globally [2–6]. It is now well-established that the alterations of various vital genes and proteins drive the transformation of normal cells to cancer phenotypes, which ultimately lead to cancer [1, 7–16]. Besides, many factors contribute to the pathogenesis of cancer, including elevated metabolic requirements that lead to the upregulation of enzymes required for the synthesis of fatty acids and modulation of several signaling pathways [17, 18]. These molecular alterations are caused by various factors such as an imbalanced diet, physical inactivity, pollution, and consumption of addictive substances such as tobacco and alcohol [4, 19]. The past couple of decades have evidenced substantial improvements in conventional methods for the treatment of cancer, such as surgery, radiation, and chemotherapy [20–23]. In addition, novel therapeutic modalities have emerged for the treatment of this disease such as immunotherapy, gene therapy, targeted therapy, personalized medicine, and nano vaccines [24–29].

Regardless of the advancement, the treatment approaches have not shown significant improvement in terms of survival and quality of life (QOL) of cancer patients due to several factors such as chemoresistance, radioresistance, adverse side effects of drugs, and cancer recurrence [30–34]. Further, the majority of the drugs used in conventional methods target a single protein or pathway, which induce several survival signals that restrict their efficacy. Therefore, the development of alternative therapy is required for improving the survival and QOL of cancer patients. This drives the urge to develop safe, efficacious, affordable, and multi-targeted agents for the management of this disease [4, 9, 35–38].

Accumulating pieces of evidence, since the rise of human civilization, suggest that medicinal plants have gained colossal importance in different traditional medicinal systems such as Ayurveda, Siddha, Unani, and Traditional Chinese Medicine (TCM) owing to their limitless properties in disease prevention and treatment [5, 39–42]. Studies have also advocated the immense biological properties of compounds isolated from these plants as potential candidates against various fatal diseases, including cancer [43–56]. Guggulsterone (GS) is one of those naturally derived multi–targeted compounds that have exhibited massive therapeutic potential against cancer. Therefore, the current review summarizes its prospects in the prevention and treatment of cancer.

GS is a sterol compound derived from the gum resins of the guggul plant Commiphora wightii (C. wightii) that belongs to the family of Burseraceae [57, 58]. C. wightii is commonly found in Somalia, Northeast Africa, Southern Arab countries, and countries of Southeast Asia such as India, Bangladesh, and Pakistan [59–61]. In India, C. wightii is mostly distributed in the States of Maharashtra, Gujarat, Rajasthan, and Karnataka; however, the States of Rajasthan and Gujarat form the commercial centers of this gum [59]. Before being justified with its present name as C. wightii by Bhandari, it was named as C. mukul or Blasmodendron mukul and then as C. roxburghii [62]. The oleogum resin of this plant is commonly known as guggul (in Hindi), guggulu (in Sanskrit), gukkulu and maishakshi (in Tamil), and Indian bdellium (in English) [59]. The therapeutic benefits of C. wightii have been reported in the treatment of various diseases including tumors, malignant sores, ulcers, obesity, and liver and intestinal problems; hence, it has been widely used in Ayurveda for thousands of years [57]. Besides, GS has also gained importance due to its chemopreventive and therapeutic properties against various cancers and their hallmarks [63, 64]. This plant metabolite was first recognized as an antagonist for the nuclear receptor, farnesoid X receptor (FXR) [57]. The later studies revealed that it could also target the receptors of mineralocorticoid, glucocorticoid, androgen and estrogen [60]. Subsequently, the underlying mechanism behind GS-mediated hypolipidemic effect was identified. GS was found to decrease FXR activity and increase bile salt export pump, a transporter that regulates bile efflux [60]. Further, this sterol was also found to modulate the expression of several proteins such as cyclin D1, c-Myc, matrix metalloproteinase (MMP)-9, cyclooxygenase 2 (COX-2), and vascular endothelial growth factor (VEGF); and anti-apoptotic proteins such as inhibitor of apoptosis protein 1 (IAP1), X-linked inhibitor of apoptosis protein (XIAP), B-cell lymphoma 2 (Bcl-2), Bcl-2-related protein A1 (Bfl-1/A1), cellular FLICE (FADD-like IL-1β-converting enzyme)-inhibitory protein (cFLIP), and survivin in various cancer models [64]. GS has also shown to suppress tumor necrosis factor (TNF)-α, IkappaB kinase (IKK), IkappaB alpha (IκBα), nuclear factor kappa B (NF-κB) activation, and NF-κB regulated gene expression [64, 65]. This compound was also reported to modulate other pathways like phosphoinositide 3-kinase (PI3K)-Akt, steroid receptor coactivator (Src)/focal adhesion kinase (FAK), Janus kinase (JAK)/signal transducers and activators of transcription (STAT) and their downstream molecules [66, 67].

As mentioned, GS acts as an antagonist for FXR. Studies have reported that FXR helps in stabilizing the metabolism of bile acids and cholesterol and it is involved in the development of different diseases such as cardiovascular diseases and cancer [57, 68, 69]. Numerous lines of evidence also suggest that interruption of cholesterol homeostasis is one of the major events in cancer development as the failure in the sustenance of cholesterol synthesis through feedback inhibition results in high recruitment of cholesterol and its precursors in cancer cells [70, 71]. Thus, GS, being an FXR antagonist, could be used as an effective regimen for the treatment of cancer. Therefore, through inhibition of FXR and modulation of several other genes and proteins, GS has shown promising effects in the prevention and treatment of different cancers.

Sources of GS

GS is the main component of guggulipid, which consists of a mixture of diterpenes, steroids, sterols, esters, and higher alcohols, extracted from the gum resin of the medicinal plant C. wightii [72–75]. The Z- and E-stereoisomers of GS represent the main active compound of the plant [76]. The plant gum resin also contains 0.4% essential oils that mainly include myrcene, long-chain aliphatic tetrols that are esterified at the primary-OH group with the ferulic acid [77].

Traditional uses of C. wightii

The importance of C. wightii has been mentioned in its ancient medicinal writings of Ayurveda. The Sushruta Samhita explains the use of this plant and its gum resin against various diseases [58]. These medicinal writings mention the oral consumption of guggul can be used to heal conditions such as internal tumors and malignant sores, intestinal worms, liver dysfunction, edema, and to treat inflammatory diseases, gynaecological diseases, and obesity [78, 79]. C. wightii is one of the common ancient medicinal plants, that is taken to improve heart condition, vascular health, wound healing, and to treat vitiligo in Ayurveda [80, 81]. Further, the gum resin of C. wightii finds its use in TCM for the treatment of arthritis, trauma, and other blood-related diseases [78]. The guggul is also used in Yunani medicine for treating nervous diseases, scrofulous infections, urinary disorders, and skin diseases. It is also locally applied as a paste in hemorrhoids, incipient abscesses, and ulcers [82].

Biological properties of C. wightii

It is already well established that the gum resin of C. wightii has been used traditionally for centuries against various chronic diseases like arthritis, obesity, diabetes and cardiovascular diseases (Figure 1) [83]. Recent studies have reported that the resin extracts from C. wightii lower the levels of low-density lipoprotein (LDL) and cholesterol in different experimental settings [58, 84, 85]. The C. wightii was shown to decrease LDL, very-low-density lipoprotein (VLDL), and cholesterol and increase high-density lipoprotein (HDL). It was also shown to reduce high fat-induced obesity in rat models [86]. Additionally, C. wightii was reported to improve oxidative stress, inflammation, edema, and necrosis in an ischemic rat model, thereby displaying its cardioprotective effects [80]. In another study, C. wightii was also shown to prevent lipid layer damage by inhibiting lipid peroxidation [87]. Further, this plant was shown to inhibit diabetes by decreasing the expression of aspartate aminotransferase, alanine aminotransferase, and oxidative markers, i.e. lipid peroxidation and protein oxidation; and inducing nuclear receptors such as peroxisome proliferator-activated receptor alpha (PPARα), peroxisome proliferator-activated receptor-gamma (PPARγ) and liver X receptor [88, 89]. Besides, another constituent, dehydroabietic acid, from Commiphora sp. was found to enhance the diabetic wound healing via reversing TNF-α-induced activation of forkhead box protein O1 (FOXO1) and the transforming growth factor β (TGF-β)1/Smad3 signaling [90].

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Figure 1. 

Biological activities of Commiphora wightii (Mark W. Skinner/www.discoverlife.org)

Further, guggulipid could act as an antinociceptive agent by reducing neural responses and hyperalgesic activities in an in vivo model of neuropathic pain [91]. Studies have also advocated the neuroprotective properties of C. wightii by modulating various markers of oxidative stress such as thiobarbituric acid reactive substances, nitric oxide (NO), TNF-α, glutathione (GSH), superoxide dismutase (SOD), and catalase levels [92], thus indicating its potential against neuroinflammation-related disorders [93]. The anticancer property of C. wightii has also been highlighted in several studies where it was found to inhibit cancer cell proliferation by inducing cell cycle arrest and apoptosis in prostate cancer (PC) cells [94, 95]. Similar anticancer activities were also reported by different compounds isolated from C. wightii in various cancers such as breast cancer, lung cancer, colon cancer, and melanoma [96]. Furthermore, C. wightii was shown to induce anti-bacterial properties by inhibiting the growth of gram-positive and gram-negative bacteria [97]. Additional findings also suggest the use of this plant and its extracts against gastric ulcer, skin injury [98], dementia [99], arthritic inflammation [100], and blood clots [101].

Chemical nature of GS

GS, also known as 4, 17(20)-pregnadiene-3, 16-dione (C₂₁H₂₈O₂), is the key component of the guggulu resin from C. wightii, which exists in E- and Z-isomers (Figure 2) and are represented as its cis- and trans-forms respectively [72, 77, 102, 103]. These E- and Z-GS are steroidal isomers and are inter-convertible in 3D space. They differ in the arrangement of CH3 molecule at C20 position and a distorted rotation of C-C double bond present at C17 and C20 positions is observed [104].

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Figure 2. 

Structures of the isomers of GS. (A) E-GS; (B) Z-GS

Molecular targets of GS

The multi-targeted compound, GS, is basically identified as an FXR inhibitor. FXR is a nuclear hormone receptor that regulates bile acid production and transport [79]. This protein might also plausibly regulate apoptosis in the cancer cells [105]. The activation of FXR is known to promote TGF-β-induced epithelial-mesenchymal transition (EMT), which is a hallmark of cancer [106]. FXR has also been identified as a marker of breast cancer-associated bone metastasis, and Z-GS has been found to efficiently inhibit FXR and its target associated bone proteins such as osteopontin, osteocalcin, and bone sialoprotein [107]. Besides FXR, GS is also known to alter the expression of various proteins in the cell and thus regulate different cellular processes such as cell growth, cell metabolism, cell survival, invasion, EMT and metastasis. Numerous preclinical studies on different cancer models have reported the chemoprotective effect of GS via modulation of multiple factors that are involved in cell growth and proliferation: COX-2, receptor activator of nuclear factor-κB ligand (RANKL); cell cycle proteins: c-Myc, cyclin D, cdc2, p21, p27; VEGF, MMP-9; proteins contributing to EMT: N-cadherin, TGF-β; molecules involved in apoptosis: caspases, survivin, Bcl-2, IAP1, XIAP, Bfl-1/A1, Bcl-2-associated X protein (Bax) and Bcl-2 homologous antagonist/killer (Bak); enzymes: IκBα kinase, Src homology 2 domain-containing protein tyrosine phosphatase 1 (SHP-1); markers involved in cell development: homeobox 2, caudal type homeobox 2 (CDX2) [65, 94, 106, 108–122]. Most of these targets constitute the key components of the major pathways regulated by GS in cancer cells such as the NF-κB pathway [123, 124]; the intrinsic mitochondrial apoptotic pathway [125, 126]; the JAK/STAT pathway [67] and the STAT3 pathway [123]. GS also regulates other pathways like the Akt pathway [66, 112]; the c-Jun N-terminal kinase (JNK) pathway [112, 127]; the extracellular signal-regulated kinase (ERK) pathway [127]; steroid Src/FAK signaling [67]; β-catenin signaling [128]; and the mitogen-activated protein kinase (MAPK) pathway [124]. Further, the proteomic profiling of GS treated cancer cells have revealed that GS-induced downregulation of several factors such as proteins contributing to cell growth: methionine adenosyltransferase 2A (MAT2A) and U1 small nuclear ribonucleoprotein A (SNRPA); proteins involved in cell growth and migration: F-box only protein 2 (FBXO2), high mobility group box 3 (HMGB3), and Ras-related protein Rab-21; proteins contributing to tumorigenesis: caveolin-1, importin karyopherin subunit alpha 2 (KPNA2), and protein arginine N-methyltransferase 5 (PRMT5); proteins involved in purine metabolism: purine nucleoside phosphorylase (PNP); proteins involved in DNA replication: DNA replication licensing factor minichromosome maintenance complex component 3 (MCM3), and replication protein A 70 kDa DNA-binding subunit (RPA1); heat shock proteins (HSP): heme oxygenase-2 (HO-2), Hsp70 and Hsp27 [126]. GS also induced upregulation of annexin A7, which is involved in exocytosis/tumor suppression and the cytoskeletal protein, filamin B that regulates cell shape and migration [126]. In concordance with other studies, this study also demonstrated that GS-mediated suppression of colorectal cancer (CRC) involved modulation of the prime cellular pathway, the TNF-α/NF-κB signaling pathway [126]. GS is also known to induce reactive oxygen species (ROS)-dependent apoptosis in cancer cells via modulation of JNK [129–131]. Furthermore, 14-3-3 zeta, which is involved in cancer recurrence and therapeutic resistance, has also been identified as a molecular target of GS [132, 133]. In addition, GS is also known to inhibit expression of P-glycoprotein (P-gp) in cancer cells, thereby chemosensitizing these cells to standard chemotherapeutic drugs [134–136]. For example, GS sensitized hepatocellular carcinoma (HCC) cells to doxorubicin (DOX) by modulation of COX-2/P-gp dependent pathway [137], MCF-7/DOX cells to DOX by reducing the levels of Bcl-2 and P-gp [138], gall bladder cancer cells to gemcitabine through suppression of NF-κB [139], and glioblastoma cells to sonic hedgehog inhibitor SANT-1 via inflection of Ras/NF-κB pathway [140]. Furthermore, the combination of GS with bexarotene decreased DOX resistance in breast cancer cells by stimulating the secretion of exosome-associated breast cancer resistance protein (BCRP) [141]. GS has also been reported to induce CCAAT/enhancer-binding protein homologous protein (CHOP)-dependent death receptor (DR)-5 expressions through modulation of ROS-dependent endoplasmic reticulum (ER)-stress, and thus sensitize liver cancer cells to TNF-related apoptosis inducing ligand (TRAIL)-induced apoptosis [142]. Thus, GS acts on a diverse range of molecular targets and prevent the development and progression of cancer (Figure 3).

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Figure 3. 

Modulation of molecular pathways by GS. GS modulates several signaling pathways and regulate the expression of various proteins involved in inflammation, apoptosis, cell cycle, angiogenesis, invasion, metastases, and chemoresistance

The tumor microenvironment (TME) comprises of cancer cells, cancer stem cells, immune cells, factors such as growth factors, cytokines, enzymes, chemotherapeutic drugs, the components of the extracellular matrix (ECM), fibroblasts, inflammatory cells, blood vessels, and signaling molecules [143–146]. These components regulate tumorigenesis and various associated processes, such as oxidative stress, EMT, metastasis and autophagy [147–153]. The TME is immunosuppressive; therefore, identification of new and effective compounds that would restore the immune response in tumor cells and inhibit the progression of tumors is critical for the prevention and treatment of cancer [154, 155]. The natural compound, GS, plays a potential role in remodeling of TME by regulating oxidative stress, autophagy, and expression of ECM proteins. For instance, GS was reported to protect PC12 cells from hydrogen peroxide-induced oxidative stress by decreasing the levels of extracellular lactate dehydrogenase, NO and ROS, and preventing the loss of mitochondrial membrane potential (ΔΨm) [156]. The antioxidant activity of GS was further evinced through GS-induced reduction of plasma trimethylamine-N-oxide expression and stimulation of Nrf2/HO-1 signaling that protects the cells from ROS-induced oxidative stress [157, 158]. GS, being an FXR antagonist, was also found to be involved in the regulation of autophagy [159, 160]. This compound has also been reportedly associated with FXR-mediated differentiation of bone marrow stromal cells into osteoblasts or adipocytes [161]. A particular study also evinced GS-induced modulation of TME components like TGFβ-induced EMT markers and COX-2 in inflammatory cells [162]. Furthermore, GS was reported to regulate the survival and stimulation of hepatic stellate cells through the regulation of NF-κB and apoptosis [114]. Thus, GS shows efficacy in the regulation of TME remodeling and exerts a significant anticancer effect.

Biological activities of GS

Congregate evidence suggests that GS has enormous potential in the prevention and treatment of various chronic diseases in humans owing to its multi-targeted properties. These include Alzheimer’s disease, arthritis, asthma, cancer, dermatitis, diabetes, gingivitis, inflammatory bowel disease, infectious diseases, intestinal metaplasia, otitis media, respiratory diseases, pancreatitis and psoriasis [79]. However, in the current review, we focus on its anti-cancer properties.

Effect of GS in different cancers

Surfeit number of preclinical evidences unveiled the cancer chemopreventive and therapeutic properties of GS against a wide range of cancers, including cancers of the brain, breast, colon, head and neck, liver, pancreas and prostate, and hematological malignancies like leukemia and lymphoma.

These studies proved that GS has enormous potential for both the prevention and treatment of different cancers (Table 1) and are briefly summarized below.

Pk and pharmacodynamics of GS

A limited number of studies have reported the Pk and pharmacodynamics of GS to date. In 1998, Verma et al. [220], performed HPLC of Z-GS, E-GS, drug-free rat serum, GS-spiked serum, serum from GS (50 mg/kg)-dosed rats (at 4 h), and serum from GS-dosed rats (at 24 h). The recovery percentage of Z-GS and E-GS from spiked serum samples was more than 90% at all concentrations of spiking (25, 50, 250, 2, 500 ng/mL) while the chromatogram analysis suggested that under in vivo conditions, GS is plausibly metabolized from Z- to E-form upon administration and is retained in the same form in the body. In another study, the effect of both oral and intravenous administration on various Pk parameters of Z-GS and E-GS were analysed; however, no statistically significant difference was observed. It was reported that the Pk parameters like terminal half-life, systemic clearance, area under the curve, and volume of distribution were 4.48 and 3.56 h, 1.76 and 2.24 L/h, 5.95 and 4.75 μgh/mL, and 11.36 and 10.76 L, respectively. These results demonstrated that the absorption of Z-GS was rapid from the gastrointestinal tract, leading to maximum concentration (C_max) in the serum 2 h after oral administration. Moreover, the bioavailability of orally administered Z-GS relative to the intravenous administration was found to be 42.9% [221]. In another study, Bhatta et al. [222], developed a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for the simultaneous determination of both Z- and E-GS in rabbit plasma. However, this method faced a limitation of the long run time of 20 min. Subsequently, in 2015, Chhonker et al. [223], developed and validated an extremely specific and sensitive LC-MS/MS method for the estimation of E- and Z-GS in rat plasma within a run time of 6 min. Using this method, the ADME properties of GS such as metabolic stability, pH-dependent stability, plasma protein binding, solubility and Pk of GS isomers administered orally in rats were evaluated. Moreover, E- and Z-GS were reported to be soluble up to 50 μM (1, 561 ng/mL) under physiological conditions. The findings also suggested that E- and Z-GS were stable in the gastrointestinal fluid and were not subjected to enzymatic degradation; their plasma protein binding was high and independent of the concentration of GS. Further, both the stereoisomers exhibited low bioavailability of GS owing to wide first pass metabolism through high clearance and short half-life in rats [223].

The efficacy, toxicity, and pharmacodynamics of a drug depend hugely on its metabolic fate. In 2012, Yang et al. [224], reported a basic metabolic profile of Z-GS. However, to gain an in-depth understanding of the Pk and metabolism of GS, in 2018, Chhonker et al. [225], conducted another metabolic investigation where GS was found to metabolize to produce nineteen metabolites in human liver microsomes, and S9 fractions and hydroxylation was identified as the prime metabolic pathway. Also, the binding efficiency of GS with human serum albumin ranged from low to moderate. Further, CYP profiling and inhibition studies revealed that GS is a substrate for various CYPs, majorly CYP3A4, and it inhibited CYP2C19 [225]. Recently, another study was performed to detect electrophilic reactive metabolites of GS isomers. It was hypothesized that these metabolites are responsible for the toxic reactions of GS. The results showed that hydroxylated metabolites of GS isomers formed adducts with the trapping agents, GSH and N-acetylcysteine [226]. Overall, these studies form the background for further structural modification of GS, enhancement in the stability, and designing of its analogs.

Discussion and conclusion

The increasing rate of cancer cases and deaths pose a huge concern worldwide [227–231]. The currently available drugs are expensive, induce severe adverse side-effects, and are not very effective [6, 38, 232]. Thus, alternative drugs are sought for the efficient management of cancer, and natural compounds have shown promising results [233–245]. Among these compounds, GS, the FXR antagonist, has exhibited immense potential as an anticancer agent against various cancer types. This multi-targeted agent has shown to affect different cellular processes such as inflammation, cell proliferation, survival, angiogenesis, invasion, metastasis, EMT, and apoptosis in cancer cells in various preclinical models. In this process, GS has been reported to inflect multiple pathways like NF-κB, STAT-3, ERK/MAPK, JAK/STAT, ROS/JNK and PI3K/Akt, and several genes and proteins such as COX-2, MMP-9, p38, PGE2, HIF-1α, VEGF, interleukins, cyclin D1, survivin, p21, p27, p53, PARP, Bid and caspases. This active metabolite of guggulipid has also been known to regulate cellular stress and ΔΨm in several in vitro models. Further, GS also inhibited tumor growth remarkably in in vivo models of cancer. Not only this, but GS could also induce sensitization of cancer cells to standard chemotherapeutic drugs like DOX and gemcitabine in various cancer models. It has also been reported to sensitize the cancer cells to TRAIL and enhance the efficacy of radiation in cancer models, thereby leading to cancer cell death. These studies show that GS is a potential therapeutic agent for the prevention and treatment of various cancers. However, the preclinical findings need to be validated in clinical settings with a detailed investigation of its safety, toxicity, and bioavailability. Studies have identified two prime stereoisoforms of GS, Z-GS and E-GS, along with 19 metabolites. GS is characterized by physiological solubility upto 50 μM, high plasma protein binding, and limited bioavailability. Hence, studies have been carried out to design analogs of this pharmacophore with enhanced bioavailability. Thus, modulation of the pharmacodynamics and Pk properties of this compound may lead to the development of analogs and derivatives of GS for better management of cancer.