3. Preclinical research on phytomedicines targeting key
regulators of anti-cancer drug resistance in CSCs
CSCs contribute to drug resistance by regulation of EMT; elevated
expression of ABC transporters, increase in aldehyde dehydrogenase
(ALDH) enzymes, resistance to DNA damage and cell death, slow cycling of
microRNAs and regulation of tumor microenvironment (Makena et
al., 2020) . Phytocompounds targeting either one of these key regulators
of anti-cancer drug resistance (as shown in Figure 3 ) can prove
to be useful in the elimination of CSCs and in improving the outcome of
cancer disease treatment.
Curcumin is a dietary polyphenol extracted from turmeric rhizomes
(Curcuma longa ). It has been explored for past many years and
considered to be a potential anti-cancer therapeutic agent. Recently,
reports have revealed that curcumin targets CSCs in breast, thyroid, and
brain cancers. It can act in different ways but converges to a final
outcome of reducing the tumor cells. In a study, curcumin was reported
to downregulate EMT (Vimentin, Fibronectin, β-catenin,) and stemness
(Sox-2, Nanog, and Oct-4) markers whereas in another study it reduced
the expression of ABC transporters in breast CSCs (Hu et al.,
2019; Zhou et al., 2015) . It dysregulated JAK/STAT3 signaling pathway
in papillary thyroid CSCs (Khan et al., 2020) . Also, recently,
in order to boost the stability and water solubility, thus improving the
drug’s permeability and antitumor activity, curcumin liposome was
constructed. As a result, its apoptotic effect on glioblastoma stem
cells was established (Y. Wang et al., 2017) .
Ovatodiolide is a macrocyclic diterpenoid isolated from Anisomeles
indica, whose effect against different cancers like glioblastoma,
nasopharyngeal carcinoma and oral cancer was studied in vitro andin vivo and its potential therapeutic properties were
established. It was found that ovatodiolide reduces stemness markers
(CD44, CD133, Sox2, Klf4, Nanog and Oct-4) and decreases expression of
EMT genes. It modulated JAK2/STAT3 signaling pathway by inhibiting
either JAK2 or STAT3; thereby dysregulating transcription of genes.
Also, it induced apoptosis of tumor cells. Furthermore, in vivostudies on oral carcinoma mouse xenografts were carried out and
promising results were obtained. Treatment of nude mice (previously
injected with SAS cells) with 3.6 mg/kg ovatodiolide depicted 2.2-folds
lesser tumor growth compared to the untreated mice (Lin et al.,
2018; S.-C. Liu et al., 2019; Su et al., 2019) .
Stem extract of Dendrobium venustum containing Lusianthridin
downregulated Src-STAT3-c-Myc pathway and suppressed CD133, ABCG2, and
ALDH1A1 stemness markers which induced apoptosis in lung CSCs(Bhummaphan et al., 2019) . Polygonum cuspidatum root
extract which mainly comprises of 2-Ethoxystypandrone showed inhibition
of STAT3 signaling in hepatocellular carcinoma (W. Li et al.,
2019) .
Most of the compounds that have been evaluated have shown to target cell
death/ apoptosis pathway, while some also contribute to cell cycle
arrest. Fruit extract of Alstonia scholaris induced apoptosis in
glioma stem cells, owing to the presence of two nor-monoterpenoid indole
alkaloids, Scholarisine Q(1) and R(2) (B. Wang et al., 2018) .
Similarly, bark extract of Walsura pinnata Hassk and rhizome
extract of Costus speciosus induced apoptosis in leukemia and
prostate cancer cells respectively (Elkady, 2019; Leong et al.,
2017) . Viola odorata, a plant possessing active components such
as saponin, salicylic acid derivatives, glycosides, alkaloids,
anthocyanidins and cyclotides was shown to induce apoptosis and reduce
migration and growth of breast CSC (Yousefnia et al., 2020) .Berberis is a plant possessing bioactive compound berberine,
which reportedly causes G0-G1 arrest. It was found to be effective in
reducing stemness and cell migration in neuroblastoma and prostate CSCs
respectively (El-Merahbi et al., 2014; Naveen et al., 2016) .
Targeting stemness markers and EMT genes thus presents with the hope to
decrease CSCs. Cinnamic acid was shown to decrease stemness in colon
CSCs (Soltanian et al., 2018) . Carnosol modulated EMT genes and
induced apoptosis in glioblastoma CSCs (Giacomelli et al.,
2017) . Likewise, N-butylidenephthalide, a bioactive component ofAngelica Sinensis induced apoptosis in human bladder cancer cells
and supressed tumor in BFTC-xenograft animal models (100 and 200 mg/kg
dose) (Chiu et al., 2017) .
Phenethyl isothiocyanate, a component of cruciferous vegetables like
broccoli and water cress promoted oxidative stress and downregulated
cancer stemness genes in cervical and colon carcinomas respectively. A
study reported that NOD-SCID mice injected with 10μM PEITC pre-treated
HeLa CSCs yielded lower tumor volume compared to the control group
(untreated HeLa CSCs). In another study, nude mice were treated with 20
mg/kg PEITC after EpCAM+ cell inoculation to determine whether PEITC
suppresses CSCs in vivo and reduction in tumor growth was
observed (Upadhyaya et al., 2019; Yun et al., 2017) .
Rhizome extract of Atractylodes macrocephala Koidz downregulated
AKT/mTOR pathway and brought about alteration in glucose metabolism and
stem-like behaviour in colon cancer cell line. Subsequently, 25 mg/kg
and 75 mg/kg Atractylenolide-1 inhibited colorectal tumor progression in
xenografted nude mice (K. Wang et al., 2020) . Similarly, total
flavonoids of Fructus Viticis modulated AKT/mTOR pathway and
stemness characteristics in lung CSCs (Cao et al., 2016) .
Allicin commonly found in garlic and it was evaluated as a promising
compound for treatment of melanoma cells (Jobani et al., 2018) .
Shikonin, a natural derivative of naphthoquinone, found in the root
tissues of the traditional Chinese medical (TCM) herb Lithospermum
erythrorhizon was proven to be effective against glioblastoma stem
cells (J. Liu et al., 2015) . An extract of herbal mixture (H3)
consisting of 3 oriental herbal plants (Meliae fructus ,Cinnamon bark and Sparganium rhizome ) was investigated for
anticancer activity in vitro and in vivo (200 mg/kg dose
of H3 used) and was found to be a promising therapeutic agent(Pak et al., 2016) .