Disease complexity has always impelled researchers to focus on experimentation, directed towards drug discovery and its targeted delivery, notably in cancer. Synthetic drugs recommended for cancer display colossal side-effects. In today’s date, plant-based medicines serve decent effectiveness over synthetic equivalents or derivatives with minimal side-effects. Many phytochemicals, mostly in the form of anticancer compounds, have already been successfully established as anticancer drugs, viz., Camptothecin, Paclitaxel, Podophyllotoxin, Vinblastine, and Vincristine [1]. The present study is aimed at deriving the mechanistic insights of a potential anticancer agent, Moscatilin, a bibenzyl derivative, by paralleling it with few other critical plant-derived drugs such as Resveratrol, a stilbenoid; Paclitaxel, a tetracyclic diterpenoid; Colchicine, an alkaloid; and Shikonin, a naphthoquinone derivative. Mitomycin-C (a clinical chemotherapeutic drug) was also used to compare the cytotoxic effect of Moscatilin.
“Moscatilin” is a bibenzyl derivative, primarily present in the orchid genus, Dendrobium. Moscatilin functions as a potential anticancer agent and the research efforts on this compound have been increasing of late [2]. It was found to induce significant cytotoxicity in the FaDu (human hypopharyngeal squamosa carcinoma) cell lines and numerous other cancer cell lines through several mechanisms notably, apoptosis through deoxyribonucleic acid (DNA) damage, c-Jun N-terminal kinase (JNK)/stress-activated protein kinase (SAPK) activation and tubulin depolymerization [3,4,5]. Moscatilin has also been seen causing cell cycle blockade in the Gap 2/Mitosis (G2/M) phase along with mitotic catastrophe [6]. The compound was observed hindering metastasis and migration by inhibiting Akt and Twist signaling pathways in breast cancer cells [7]. Besides, Moscatilin repressed tumor angiogenesis and growth in human umbilical vein endothelial cells (HUVEC), halting endothelial nitric oxide synthase (eNOS), Extracellular signal-regulated protein kinases (ERK1/2), and Akt pathways [8]. Its significance lies in causing cytotoxicity in neoplasms at non-toxic concentrations [9]. The exact mechanisms of antitumor activity of Moscatilin are yet to be understood. Additionally, its potency needs to be compared too with biodrugs of clinical relevance.
Resveratrol (3,5,4′-Trihydroxystilbene) is essentially a polyphenol and a natural nutraceutical phytoalexin derived naturally from fruits such as grapes blueberries, cranberries, and also peanuts. It is a structural analog of Moscatilin and presence of both of them has been reported from an ornamental orchid, Dendrobium ovatum (a threatened species, endemic to the Western Ghats, India) and hybrids of Dendrobium [10]. Antioxidative, anticancer, and anti-angiogenic properties of Resveratrol have been widely reported [11, 12]. Colchicine disrupts tubular dynamics interrupting cell cycle progression. The capping of microtubules with Colchicine induces steric clashes, subsequently resulting in microtubular disassembly. Microtubular disassembly eventuates “c-mitosis” — an artificially induced mitosis, under the influence of Colchicine, where the nuclear division gets aborted, causing the doubling of chromosome number [13]. Paclitaxel (derived from the bark of plant, Taxus brevifolia) disturbs the dynamic activity of the microtubule, resulting in microtubular stabilization, obstructing the cell cycle at the mitotic (M)-phase [14]. Shikonin is derived from the roots of Lithospermum erythrorhizon and its anticancer, anti-inflammatory, and anti-obesity effects have been well-described [15]. This compound has been identified as a potent blocker of Pyruvate kinase Muscle isozyme M2 (PKM2) [16]. Shikonin also shows more affinity towards PKM2 than any of its isoforms, viz., pyruvate kinase-M1 (PKM1), Pyruvate kinase liver type (PKL), and Pyruvate kinase red blood cell type (PKR) [17]. Mitomycin-C is a vesicant, and it induces severe tissue injury when slips out of the vein. It is reported to cause damage to nucleic acids — both DNA and ribonucleic acid (RNA), resulting in the shrinkage of tumor cells [18, 19]. This clinical drug has numerous side effects, such as mouth sores, reduced appetite, fatigue, hair loss, diarrhoea, and bladder inflammation. Mitomycin-C has numerous functions starting from antibiotic properties to immunosuppression. It has been proven that Mitomycin-C can be applied extensively in vascularized composite allotransplantation (VCA), a clinical procedure by which allograft rejection can be prevented by suppressing the immunological responses in T-cells [20].
In the current study, we have performed the molecular docking studies of all the six compounds (Moscatilin, Resveratrol, Paclitaxel, Colchicine, Shikonin, and Mitomycin-C) with two relevant proteins, viz., Anaphase-Promoting Complex subunit 10/Death of Cyclase 1 (APC10/DOC1) and PKM2. APC is an ubiquitin ligase (E3) complex, operating at the metaphase-to-anaphase transition of the cell cycle [21]. It favors the polyubiquitination of the enzyme “securin”, an anaphase inhibitor, enabling “separase” to digest the “cohesins” that hold the sister chromatids together. Moreover, it also promotes the degradation of cyclin B, which is an activating subunit of cyclin-dependent kinase 1 (cdk1). It also performs the task of polyubiquitination at specific lysine residue on a target protein and is responsible for inducing somatic mutations in Cancer [22], whereas Pyruvate kinase converts Phosphoenolpyruvate (PEP) to Pyruvate during Glycolysis. The Pyruvate thus generated gets converted into Acetyl-CoA and joins the Citric acid cycle. Pyruvate kinase exists as isoforms such as PKM1, PKM2, PKR, and PKL and they are expressed differentially in various tissues and cell types. PKM2 exists as a highly active tetrameric form and a low active dimeric form. The rate-limiting step of Glycolysis is governed by this dimeric form of PKM2, which shifts the glucose metabolism from the typical respiratory cascade to lactate metabolism in tumor cells [23]. PKM2 also manages the metabolic processes associated with cancer cells, and its high expression has been reported in various cancer types [24]. Blocking of this protein in cancerous cells is significant, as it forms the rate-determining step towards the end of the glycolytic reaction, generating a considerable amount of energy. Hence, resisting this step would promote the cancer cells to become nutrition deficient, eventually leading to their death. PKM2 is involved in both glycolytic and non-glycolytic pathways, apart from playing a vital role in tumor malignancy. Therefore, it is regarded as one of the remarkable therapeutic targets of the cancer disease.
Overall, the present study has two major goals; firstly, to ascertain the efficacy of Moscatilin on two target proteins APC 10/DOC1 (PDB ID: 1JHJ) and PKM2 (PDB ID: 1ZJH) and secondly, to compare the potential of Moscatilin with structurally related ligand Resveratrol and clinically used ligands such as Paclitaxel, Colchicine, Shikonin, and Mitomycin-C. The study hypothesized that blocking the catalytic sites of the above two proteins by Moscatilin might disclose novel avenues proving its pertinence in cancer therapy. Moreover, this research investigation also includes the characterization and comparison of adsorption, distribution, metabolism, excretion, and toxicity (ADMET) profiles of Moscatilin and the other five ligands of interest.
