The incidence of colon cancer has been increasing constantly because of the recent changes in life style which include consumption of foods with low/no vegetables and fruits, lack of physical activities/exercise, consumption of excess alcohol and exposure to hazardous chemical substances [
19,
20]. Although routine check-up and early detection has reduced the death rate, yet colon cancer claims a large number of lives every year globally. Therefore, there is an urgent need for identifying novel therapeutics or drug candidates which will specifically act on the cancer cells without affecting the normal cells [
21].
Here, we explored the anticancer activity of
Eclipta alba methanol extract EAME against various human cancer cells such as HCT-116, PC-3, MCF-7 and RCC-45. WI-38 cells line was chosen to evaluate the effect of the extract on normal cells. These cells were selected based on the availability as well as on their sensitivity. Initial screening of anticancer activity of EAME was done on HCT-116, HT-29, PC-3, MCF-7, RCC-45 and H-1299 (lung cancer cell lines). Based on their sensitivities to this extract HCT-116, PC-3, MCF-7 and RCC-45 cells were then chosen for further studies. HCT-116 was more sensitive compared to HT-29 (may be due to presence of more mutated oncogenes in HT-29 compared to HCT-116), we decided to test with HCT-116 colon cancer cells and so did not continue with HT-29. More over as revealed here, the EAME showed very specific toxicity towards colon cancer cells HCT-116 compared to the other cancer cells PC-3, MCF-7 and RCC-45 cells (Fig.
1). Notably, the toxicity of EAME to the colon cancer cells HCT-116 was more appreciated after identifying its relative nontoxicity on normal cells WI-38 based on all three types of assays used here (Figs.
1 &
2).
Cancer cells possess higher basal level of reactive oxygen species (ROS) because of its higher metabolic rate and other functions which is distinct from normal cells [
22]. Elevated level of ROS is essential for cancer cells to grow, proliferate as well as for metastasis. At the same time an excess over the required level of ROS may lead cancer cells to oxidative stress and possible death [
23]. Cancer cells in contrast to the normal cells were shown to carry limited antioxidant mechanism required to scavenge the excess ROS produced, and to prevent associated cellular damage [
23]. It is possible that anticancer activity of the extract studied here is due to the presence of compounds that altered the redox balance essential for the survival of the HCT-116 cells. This activity may be either inducing ROS level or inhibiting the ROS level in HCT-116 cells [
23]. Negligible toxicity of EAME towards the normal cell is probably due to the presence of strong antioxidant and anti-inflammatory mechanisms present in the normal cells [
24]. β-sitosterol, a phytosterol isolated from methanolic extract of
Eclipta alba was reported to possess antioxidant as well anticancer property, and induce apoptosis via caspase 8-dependent pathways in cancer cells [
25]. The antioxidant mechanism shown by the other compounds i.e., wedelolactone, oleanolic acid and eclalbasaponins present in the EAME might be responsible for the observed activity against HCT-116 cells and negligible toxicity towards the normal cells. Further investigations on these compounds will be necessary to understand exact mechanism of specificity towards cancer cells in contrast to the normal cells. The IC-50 values of HCT-116, MCF-7, PC-3 and RCC-45, were found to be 179 ± 0.81, 400 ± 1.01, 470 ± 1.04 and 498 ± 1.90 μg/ml, respectively suggesting relative HCT-116 cell specific effect of
Eclipta alba extract. The selectivity index (SI) (data not included) obtained from IC-50 values suggested the level of specificity towards the cells. The mechanism underling the cell specific toxicity induced by the extract may be the result of DNA damage as well. Although in general uncontrolled cellular growth due to change in multiple cellular pathways results in cellular transformation, yet different cancers are unique in their own way due to harboring their unique set of mutations [
26]. In fact, many cancers carry their individual markers. Colon cancer was shown to be the results of chromosomal instability resulting in the activation of proto-oncogene KRAS. In addition, colorectal cancer also carry mutation in multiple tumor suppressors such as loss of p53, adenomatous polyposis coli (APC) and the defect in the long arm of chromosome 18 due to loss of heterozygosity (LOH) [
27]. HCT-116 cells carry KRAS mutation/activation. In many cases breast cancer is caused by mutation in BRCA1 and BRCA2 genes [
28]. Therefore, relatively higher sensitivity of HCT-116 cells to this extract is the reflection of its unique genetic nature. Additional experimentation will be required to obtain a deeper insight into the cellular process targeted by the extract.
This study revealed that the extract efficiently blocked two essential properties required by cancer cells for their growth and proliferation, that is, the ability of cancer cells to grow and proliferate in forming colony and migrate for metastasis. The extract could block both of these essential properties of HCT-116 cells. In cancer cells metastasis is a complex biological process that includes invasion and migration of cancer cells; it is also a major cause of death in cancer patients. Currently, many independent groups are engaged to find suitable drug candidates to inhibit metastasis to control the growth of tumor [
29]. The results presented here provided dependable evidence that the methanolic extract of
Eclipta alba carries promising anti-colon cancer compounds which would be worth following-up in the future to understand and for possible therapeutic development.