Implication of Integrated in Silico Docking and Moma Simulation for the Development of Curcuminoids as Sphingosine Kinase 1 (SK1) Inhibitors for Cancer Treatment

Sphingosine kinase 1 (SphK1) enzyme catalyzes the phosphorylation of sphingosine into sphingosine-1-phosphate (S1P). S1P plays an important role in diverse biological processes. In numerous human cancers, overexpression of SphK1 led to the severity of disease, drug resistance and reduced patient survival. Furthermore, SphK1 increased activity was connected to play a crucial role in immunological responses such as asthma, rheumatoid arthritis and sepsis. Inhibitors of SphK1 enzyme helps in reducing the adverse effects caused by over expression of SpkH1. Hence, there is a growing demand for the discovery of new SphK1 inhibitors. Curcumin (5) a major constituent of turmeric rhizomes, displayed the inhibition activity of cancers by regulating multiple cellular signaling pathways. Henceforth, the present molecular docking study suggested that curcumin (5) could be valuable SphK1 inhibitor as a result of its binding affinity to the SphK1 active site similar to that of CTK8F1052 (4). Further exploration of curcumin (5) efficacy in inhibiting the SphK1 activity is necessary to support computational docking findings.


Introduction
Sphingosine kinase 1 (SphK1) is generally locolaized in cytosol and belong to a class of guanosine triphosphate (GTP. G)-binding protein (G-protein)-coupled receptor kinases (GRK). In humans, SphK1 is encoded by SPHK1 gene located in chromosome 17. SphK1 catalyzed the phosphorylation of sphingosine into sphingosine-1-phosphate (S1P). S1P was recognized as an important player in multiple biological developments, including cancer and inflammation [1][2][3][4][5]. Sphingosine kinase (SphK) catalyzed adenosine triphosphate (ATP)-dependent phosphorylation of sphingosine lipid followed by alteration of the last step in production of S1P. SphK subsists in two isoforms; SphK1 and SphK2. Different sphingolipid metabolites operated as supplementary lipid messengers which participated in miscellaneous cellular processes, comprising of cellular migration, proliferation, and apoptosis. SphKs were recognized as biomarkers in different types of cancer by promoting angiogenesis and tumorigenesis [6]. Amplified expression of 277 sphingosine kinase 1 (SphK) was concomitant with breast cancer progression including the resistance to drug therapies [7][8][9].
Furthermore, exogenous administration of S1P amplified the cytotoxic potential of chemotherapy drugs, cyclophosphamide (1), doxorubicin (2) and docetaxel (3) against breast cancer metastatic cell lines (Figure 1) [10]. Nevertheless, SphK1 was up-regulated in numerous human cancers where there was a correlation between SphK1 expression, severity of disease, drug resistance and reduced patient survival [11][12][13]. SphK1 activity was connected to play a crucial role in immunological responses such as asthma, rheumatoid arthritis and sepsis [14][15][16]. Subsequently, the application of SphK1 antagonists inhibited cell growth and induced apoptosis in different human cancer cell lines. In addition, SK1 antagonists exhibited radio-sensitizing effects on triple-negative breast cancer (TNBC) cell lines, increasing the anti-proliferative and pro-apoptotic effects that were induced by ionizing radiation [17]. SphK1 inhibition also stimulated apoptosis and reduced cell proliferation in both in vivo and in vitro TNBC models [18]. Lately, SphK1 was recognized as a critical player in the development of cancer, inflammation, autoimmune, neurological and cardiovascular disorders (CVDs).
Over expression of SphK1 was related to cancer prognosis and development. The SphK1 inhibitor, CTK8F1052 (SHI-II) (4) suppressed cell growth and induced apoptosis in human hepatoma HepG2 cells. CTK8F1052 (4)-initiated down-regulation of HepG2 cell proliferation was associated with Wnt signaling pathway which is network of proteins involved in embryonic development and cancer through Wnt5A-mediated β-catenin degradation [19].
Therefore, there is a prodigious interest in the development of new SphK1 inhibitors which could serve as potential players in the cancer treatment. Targeting sphingosine-1-phosphate production decrease with inhibition drugs is a practical approach to retune the cell signaling aberrations that resulted in endocrine resistance.
This approach could contribute to increased efficacy of current therapeutic practices by re-sensitizing cells to endocrine therapy (hormone therapy). Crosstalk between sphingolipid signaling pathways and steroid hormones is a feasible therapeutic target.
In case of solid tumor cancers, sphingosine kinase (SK) is a key enzyme metabolizing pro-apoptotic ceramide to pro-survival sphingosine-1-phosphate (S1P) which is a likely therapeutic target. Sphk1/2 selective inhibitor, CTK8F1052 (4) impeded breast cancer viability, clonogenic survival and proliferation and acted as novel estrogen receptor (ER) signaling inhibitors in breast carcinoma. Curcumin (curcumin-I. 5), demethoxycurcumin (curcumin-II) (6) and bisdemethoxycurcumin (curcumin-III) (7) are the major phytochemicals present in turmeric rhizomes that are responsible for wide range of health benefits including antiinflammatory, antioxidant and antitumor activities ( Figure 2).

Curcumin (5) inhibited animal and human cancers by regulating
multiple cellular signaling pathways [20][21][22]. Recently, turmeric and curcuminoids were engaged in clinical trials for the evaluation of their effectiveness in the treatment of multiple diseases. In addition, curcumin displayed synergism with cytotoxic drugs in the treatment of different cancers [23][24][25]. Henceforth, in silico approach was selected to evaluate the effectiveness of curcuminoids in inhibiting SphK1 thereby preventing the phosphorylation of sphingosine which was identified as one of the principles causes of chemo resistance to selected chemotherapeutic drugs .   Table 1: Examination of the "Lipinski's rule of five" parameters of CTK8F1052 (4), curcumin (5), demethoxycurcumin (6) and bisdemethoxycurcumin (7).

SphK1 and Ligands Docking
PyRx virtual screening software interlinked with AutoDock Vina was used for SphK1dockings with CTK6F1052 (4), curcumin
The enclosed lipid pocket of SpkH1 was subjugated by curcumin (5) where the alkyl chain of sphingosine was bound to the phenol ring situated at the end of the pocket. The phenol moiety of curcumin (5) pointed toward the opening in the cleft between N-terminal domain (residues 9-150 and 357-364, containing the C1-C3 domains),   Demethoxycurcumin (6) and bisdemethoxycurcumin (7) interactions with SpkH1 receptor were similar to that of curcumin (5) except the missing of hydrogen bonds between the two curcumonoids with ASP81 of SpkH1 suggesting that curcumin (5) could be a better inhibitor of SpkH1 compared to other two curcuminoids. The interactions of CTK8F1052 (4), curcumin (5), demethoxycurcumin (6) and bisdemethoxycurcumin (7)

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Copyrights @ Rao Gollapudi, et al. Open Acc J Oncol Med 282 interacting ligand and receptor residues are labelled and shown as surface in different colors (Figure 7). Collectively, the computational results from in silico docking of curcumin (5) and CTK8F1052 on SphK1 receptor suggested that curcumin (5) could be a potential SphK1 inhibitor with binding affinity comparable to CTK8F1052. Furthermore, these findings were verified through unbinding simulations of docked ligands/receptor complexes on MoMA-LigPath (a web server). Consequently, curcumin (5) was selected for further investigation to evaluate its effectiveness as a competent inhibitor of SphK1. A graphical representation of docked CTK8F1052 (4), curcumin (5), demethoxycurcumin (6) and bisdemethoxycurcumin (7) on SphK1 receptor is shown in (Figure 8).

Conclusion
SphK1 is a lipid kinase that catalyzes the conversion of sphingosine to sphingosine-1-phosphate (S1P). SphK1 performs a key role in lymphocyte trafficking, angiogenesis, and response to apoptotic stimuli. Furthermore, SphK1 is essential enzyme in altering the S1P levels in cells and hence transpires as an important regulator for multiple cellular functions making it a potential target in the drug discovery process. Our present in silico docking study suggested that CTK8F1052 (4), curcumin (5), demethoxycurcumin (6) and bisdemethoxycurcumin (7) displayed interactions with SphK1 through hydrogen bonds and van der Waals with binding energies (Kcal) of -9, -9.8, 9.7 and -9.5, respectively. The binding interactions between curcumin (5) and SphK1 are similar to that of CTK8F1052 (4) interactions with SphK1 for the inhibition of SphK1. Moreover, curcumin (5) displayed no toxicity up to 8 g per day in the treatment of multiple diseases [45]. These findings could be helpful in the development of targeted therapeutic strategy for cancer treatment. Henceforth, curcumin (5) could be a potential SpkH1 inhibitor as CTK8F1052 (4).