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Introduction
Osteosarcoma is the most common primary solid cancer of bone and is identified by the development of osteoid and/or immature bone by malignant mesenchymal cells. The incidence of osteosarcoma is 2-3 cases per million across the globe, annually. However, the incidence is higher in adolescents (around 15-19 years of age), attributed to 8-11 cases per million per year [ 1 ]. Men are 1.4 times more likely to develop osteosarcoma than females. Different treatment strategies have been suggested and tested against osteosarcoma [ 1 , 2 ]. Preoperative chemotherapy, surgical excision, pathologic mapping of the resected tumor, and postoperative chemotherapy based on the degree of necrosis of the tumor constitute the standard treatment provided for osteosarcoma patients [ 1 , 3 ]. The main chemotherapeutic agents for osteosarcoma include doxorubicin, adriamycin, cisplatin, methotrexate, cyclophosphamide, and epirubicin. Myelosuppression and gastrointestinal responses, such as anemia, nausea, and vomiting are the most frequent adverse effects of chemotherapy [ 4 ]. Cardiotoxicity is the primary adverse effect of anthracycline antibiotics (doxorubicin and epirubicin), and the risk of heart failure rises to 25-30% when adriamycin doses are higher than 550 mg/m 2 [ 5 ]. Cisplatin can also cause kidney damage, hearing loss, hypomagnesemia, and peripheral neuropathy as side effects [ 6 ].
Colorectal cancer (CRC) is the third most frequent cancer worldwide. More than 1·2 million patients are diagnosed with CRC every year, and more than 600,000 die from the disease [ 7 ]. Risk factors of CRC include improper diet, tobacco smoking, alcohol abuse, and hereditary cancer syndromes. Anticancer drugs like Cetuximab and Bevacizumab have been predominantly used for the treatment of CRC; however, they were reported to cause off-target toxicity [ 8 ]. Chemotherapy targets the actively proliferating cancer cells with a potential risk of harming the normal cells [ 9 , 10 ]. They include blood cells, mouth cells, digestive system cells, and hair follicle cells. Therefore, cancer patients are generally advised to be monitored for the rest of their lives for any potential long-term repercussions of their therapies [ 11 ].
Collectively, in spite of the progressive cancer treatments like chemotherapy, surgery, radiation therapy, etc., a rise in cancer cases every year worldwide poses a global burden. Therefore, novel anticancer medicines with high efficiency, increased specificity, lower toxicity, and easy accessibility are the current requirements.
Herbal medicines were considered to be an important part of disease treatment from ancient times. It has been reported that about 80% of the world population relies on herbal-based medicines, according to the World Health Organization [ 12 ]. Traditional medical systems such as Siddha, Ayurveda, etc. employ varied plants and plant-derived compounds to treat diseases including cancer for their biological properties [ 13 ]. Herbal medicines are generally believed to be safe without side effects, undemandingly available, affordable to rural people, and improve the quality of life [ 14 ].
One among the plant derivatives is farnesol, an acyclic sesquiterpene alcohol, predominantly found in essential oils of various plants such as citronella, lemon grass, tuberose, etc. [ 15 ]. It has been shown to have anti-inflammatory and anticancer properties. It was previously used to suppress allergic asthma, gliosis, and edema. Farnesol was found to alter the expression of several inflammatory mediators such cyclooxygenase-2, inducible nitric oxide synthase, tumor necrosis factor alpha, and interleukin-6 through modulating Ras protein and nuclear factor kappa-light-chain-enhancer of activated B cells activation. Farnesol was demonstrated to have anticancer properties by regulating the tumorigenic proteins and signal transduction cascades in different tumor cell lines in previous studies [ 15 ]. Moreover, it has the ability to suppress angiogenesis, cell growth, and survival of cancer cells while inducing apoptosis [ 16 ].
Hence, in this study, we have analyzed the cytotoxic effects of farnesol against human osteosarcoma and colorectal carcinoma cell lines. Saos-2 cell line is the commonly used cell line for osteosarcoma studies, which exhibits numerous osteoblastic features [ 17 ]. Similarly, the HCT-116 cell line is one of the common cell lines employed in studying the development and progression of colon cancer with respect to proliferation, migration, and invasion [ 18 ]. Therefore, the overall objectives of the study were to evaluate the cytotoxic potential of farnesol, to analyze the morphological changes induced by farnesol, and to elucidate the mechanism of farnesol-induced cell death in Saos-2 and HCT-116 cells.
Materials and methods
Chemicals
Farnesol was procured from Sigma-Aldrich (Burlington, United States). Cell culture essentials like Dulbecco’s modified eagle medium (DMEM) (minimum-low glucose), penicillin, streptomycin, trypsin-EDTA, fetal bovine serum (FBS) and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) were purchased from GIBCO BRL (Gaithersburg, United States). Other chemicals used in various assays were of analytical grade and purchased locally.
Cell culture
The Saos-2 osteosarcoma cell lines and HCT-116 colorectal carcinoma cell lines were procured from the National Centre for Cell Science (Pune, India) and cultured under 5% CO 2 and 37 o C in DMEM with 10% FBS, penicillin (100 units/mL), and streptomycin (100 μg/mL). Saos-2 and HCT-116 cells were treated with different concentrations of farnesol (10, 20, 40, 60, 80, and 100 µM/mL) and (20, 40, 60, 80, 100, and 120 µM/mL), respectively, and incubated for 24 hours.
MTT Assay
Saos-2 cells and HCT-116 cells were seeded in 96-well plates and treated with different concentrations of farnesol for 24 hours. After the treatment, MTT reagent was added and incubated for 4 hours at 37 o C. The purple formazan crystals were dissolved in dimethyl sulfoxide, and the optical density was measured at 570 nm. Based on the MTT assay, the concentration of farnesol for the subsequent experiments was fixed to be 60 µM/mL [ 19 ].
4,6-Diamidino-2-Phenylindole Staining
After treating the Saos-2 and HCT-116 cells with 60 µM/mL of farnesol, nuclear morphology was observed by 4,6-diamidino-2-phenylindole (DAPI) staining, according to Dmitrieva and Burg, 2008. Both control and farnesol-treated cells were fixed in 4% formaldehyde and stained with DAPI, followed by microscopic observation. The staining was performed in triplicates [ 20 ].
Acridine Orange/Ethidium Bromide Staining
The farnesol (60 µM/mL) treated HCT-116 cells and control cells were harvested, washed with PBS, and fixed in ethanol. Acridine orange/ethidium bromide (AO/EB) dye mixture was added to the cells. The cells after staining were viewed under a fluorescent microscope and images were captured. The staining was performed in triplicates [ 21 ].
Dichlorodihydrofluorescein Diacetate Staining
Farnesol-treated (60 µM/mL) Saos-2 cells and the untreated-control cells were stained with dichlorodihydrofluorescein diacetate (DCFH-DA) which was followed by an incubation at 37 o C for 30 minutes. The green fluorescence was measured at 485 ± 10 nm and 530 ± 10 nm [ 22 ]. The staining was performed in triplicates.
Statistical analysis
Data were reported as mean ± SD. Statistical analysis was performed using IBM SPSS Statistics for Windows, Version 16 (Released 2007; IBM Corp., Armonk, New York, United States). To determine the significance of the difference between the control and treatment groups, the data were subjected to Dunnett’s test and one-way ANOVA. A p-value <0.05 was considered significant.
Results
Following a 24-hour treatment with farnesol, the antiproliferative effect of farnesol on osteosarcoma cancer cells and CRC cells was assessed by MTT assay. It was observed that increasing concentrations of farnesol treatment resulted in decreased viability and proliferation of Saos-2 and HCT-116 cells. Maximum cytotoxicity was observed at 100 µM/mL of farnesol in Saos-2 cells and 120 µM/mL of farnesol in HCT-116 cells (Figures 1 , 2 ).
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Observation of farnesol-treated Saos-2 and HCT-116 cells under a phase contrast microscope revealed a decrease in the number of cells and altered morphology when compared to the control untreated cells (Figures 3 , 4 ).
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Using DAPI labeling, the apoptotic alterations caused by farnesol were examined. The Saos-2 and HCT-116 cells treated with farnesol had contracted and fragmented nuclei in contrast to the intact nuclear morphology of the control cells. An increase in the number of DAPI-positive cells after farnesol treatment compared to control demonstrated the cytotoxicity of farnesol (Figures 5 , 6 ).
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To further detect apoptosis in tumor cells, fluorescent imaging using AO/EB was carried out. The green cells indicate normal and viable cells, whereas the yellow and orange cells indicate early and late apoptosis, respectively, and red-colored cells indicate necrotic cells. The findings demonstrate that farnesol is cytotoxic to HCT-116 cells, as evidenced by an increase in the number of early and late apoptotic cells in the treatment group compared to the control group (Figure 7 ).
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To demonstrate the mechanism of farnesol-induced apoptosis, a DCFH-DA assay was performed. The results indicated that increased green fluorescence upon farnesol treatment represents elevated intracellular oxidative stress in Saos-2 cells due to the accumulation of reactive oxygen species (Figure 8 ).
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Discussion
To address the limitations of conventional anticancer medications and therapies, several novel anticancer drugs, plants, and plant compounds have been studied in recent decades. Among these, natural plant-based products have gained a great interest in developing anticancer drugs. The presence of components such as flavonoids, vinca alkaloids, diterpenes, sesquiterpenes, tannins, etc. in the plants is responsible for their anticancer activities [ 23 ]. Plant derivatives like curcumin (polyphenol), silibinin (flavonolignan), eugenol (alkaloid), Hederoside C (triterpene), and coleusin factor (diterpenoid) were reported to suppress osteosarcoma [ 24 ]. Similarly, halocynthiaxanthin (carotenoid), luteolin (flavonoid), curcumin (polyphenol), sesamol (phenolic compound), resveratrol (polyphenol), ginsenosides (triterpenoid saponin), etc. were effective in preventing the development and progression of colorectal cancer, according to earlier studies [ 25 ].
Likewise, farnesol (trans, trans-3,7,11-trimethyl-2,6,10-dodecatriene-1-ol) is one of the plant compounds, recognized for its medicinal properties. The antimicrobial, antifungal, antibiofilm, antitumor, and anticancer properties of farnesol have been investigated in several in vitro and in vivo studies previously [ 26 ]. Farnesol has been studied in several tumor cell lines, including the prostate, breast, lung, pancreas, cervical cancers, oral squamous cell carcinoma, meningioma, multiple myeloma, and T lymphoblastic leukemia cells [ 27 ]. Farnesol has been shown to suppress cancer cell proliferation, induce apoptosis, and prevent angiogenesis. In an in vitro study, farnesol was observed to suppress the proliferation and growth of Caco-2 (adenocarcinoma) and A549 (lung cancer) cell lines. Moreover, farnesol promoted the growth of normal human lung epithelial BEAS-2B cell line. This denotes that farnesol possesses cancer cell-specific targeting ability [ 27 ]. Similarly, our study also showed that farnesol was able to significantly reduce the viability of Saos-2 cells and HCT-116 cells in a dose-dependent manner. The reduction in the proliferation and survival of cancer cells upon farnesol treatment could be attributed to the cytotoxic property of farnesol against Saos-2 and HCT-116 cells.
The results of the MTT assay and microscopic analysis revealed that farnesol has a potent antiproliferative property. Further, to elucidate the mechanism of farnesol-induced cell death, DAPI, AO/EB, and DCFH-DA staining were performed. Apoptosis is one of the important mechanisms of cell death caused by anticancer agents [ 28 ]. It is a regulated cascade that inhibits the survival and growth of abnormally proliferating cancer cells. Apoptosis is generally classified into two types intrinsic and extrinsic apoptosis. The hallmarks of apoptosis are reported to be altered morphological structure, shrinkage of cells, disrupted cell membrane, condensed and fragmented nuclei, change in the oxidative milieu, and aberrated cellular signaling pathways. In a previous study, chitosan-encapsulated nickel oxide, tin dioxide, and farnesol nanoparticles induced nuclear changes in the breast cancer MDA-MB-231 line [ 29 ]. Consistently, DAPI staining in our study also showed fragmented and condensed nuclei in Saos-2 and HCT-116 cells, leading to apoptosis. To confirm farnesol-induced apoptosis, AO/EB staining was also performed. The principle behind AO/EB staining is that acridine orange enters both the live and dead cells, producing green fluorescence in the living cells. Whereas only dead cells with compromised cell membrane allows ethidium bromide to enter the cells producing a yellow to red fluorescence. Farnesol was demonstrated to induce early and late apoptosis in leukemia MOLT-4 cells and in HeLa cervical cancer cells indicated by increased yellow and orange fluorescence [ 9 , 30 ]. In accordance, the present study also exhibited the apoptotic property of farnesol against HCT-116 cells by promoting early and late apoptosis. However, in our study farnesol was observed to significantly promote early apoptosis than late apoptosis. Increased production of ROS creates oxidative stress in the cancer cells which ultimately results in apoptosis. Elevated levels of ROS induce the activation of the mitochondria-dependent pathway of apoptosis. Farnesol was previously reported to induce ROS production in leukemia MOLT-4 cells, which led to a decrease in cell viability [ 9 ]. Correspondingly, increased production of ROS in Saos-2 cells was evidenced by the DCFH-DA assay in the current study.
To summarize, farnesol exhibited cytotoxicity against Saos-2 and HCT-116 cells by inducing ROS-mediated apoptosis and it could be a potential anticancer drug in the near future with additional extensive studies.
Limitations
This research work elucidates the cytotoxic potential of farnesol against Saos-2 and HCT-116 cells. However, the specificity of farnesol on cancer cells without affecting the normal cells has not been studied currently. Analyzing apoptosis and cell cycle would provide additional insight. Also, the pharmacokinetics and pharmacodynamics aspects must be evaluated. Provided, the safety of this novel compound must be tested for an efficient anticancer treatment against osteosarcoma and CRC.
Conclusions
The findings indicate that farnesol possesses significant antiproliferative and apoptotic effects against Saos-2 cells and HCT-116 cells in a concentration-dependent manner. This study imparts a base for detailed in vivo studies of farnesol as a potent anticancer drug for osteosarcoma and CRC.
Author Contributions
Concept and design: Devaraj Ezhilarasan, Zakir Hussain Fathima Hinaz, Santhosh Pragya , Karthik Shree Harini
Acquisition, analysis, or interpretation of data: Devaraj Ezhilarasan, Zakir Hussain Fathima Hinaz, Santhosh Pragya , Karthik Shree Harini
Drafting of the manuscript: Devaraj Ezhilarasan, Zakir Hussain Fathima Hinaz, Santhosh Pragya , Karthik Shree Harini
Critical review of the manuscript for important intellectual content: Devaraj Ezhilarasan, Zakir Hussain Fathima Hinaz, Santhosh Pragya , Karthik Shree Harini
Supervision: Devaraj Ezhilarasan
Human Ethics
Consent was obtained or waived by all participants in this study
Animal Ethics
Animal subjects: All authors have confirmed that this study did not involve animal subjects or tissue.
The authors have declared that no competing interests exist.
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