Date Published: April 18, 2017
Publisher: Public Library of Science
Author(s): Laxman Nawale, Parul Dubey, Bhushan Chaudhari, Dhiman Sarkar, Asmita Prabhune, Yi-Hsien Hsieh.
Sophorolipids (SLs) are glycolipid biosurfactants that have been shown to display anticancer activity. In the present study, we report anti-proliferative studies on purified forms of novel SLs synthesized using cetyl alcohol as the substrate (referred as SLCA) and their anticancer mechanism in human cervical cancer cells. Antiproliferative effect of column purified SLCA fractions (A, B, C, D, E and F) was examined in panel of human cancer cell lines as well as primary cells. Among these fractions, SLCA B and C significantly inhibited the survival of HeLa and HCT 116 cells without affecting the viability of normal human umbilical vein endothelial cells (HUVEC). The two fractions were identified as cetyl alcohol sophorolipids with non-hydroxylated tail differing in the degree of acetylation on sophorose head group. At an IC50 concentration SLCA B (16.32 μg ml-1) and SLCA C (14.14 μg ml-1) blocked the cell cycle progression of HeLa cells at G1/S phase in time-dependent manner. Moreover, SLCA B and SLCA C induced apoptosis in HeLa cells through an increase in intracellular Ca2+ leading to depolarization of mitochondrial membrane potential and increase in the caspase-3, -8 and -9 activity. All these findings suggest that these SLCAs could be explored for their chemopreventive potential in cervical cancer.
Sophorolipids (SLs), belong to the class of glycolipid biosurfactants that are synthesized extracellularly by certain non-pathogenic yeasts. SLs initially gained attention because of the alkane utilizing ability of the yeasts. But later they exponentially attained recognition owing to possession of several properties such as emulsification, anti-microbial, anti-viral, anti-cancer that played role in various fields like detergent industry, cosmetics, pharmaceuticals etc. .
Sophorolipids (SLs) are glycolipid molecules that are considered to be of great interest in management of various ailments owing to their varied biological properties. SLs being amphiphilic in nature partially resemble the cell membrane lipids, thus they are able to span through the membrane and gain entry inside the cell. Additionally, the ability of SLs to selectively affect cancerous cells without harming normal cells may be considered beneficial in minimizing the side effects that are generally associated with current therapeutic regimens . SLs have shown to possess anticancer effect against various cancers such as pancreatic, liver and lung. Recently, in vitro and in vivo anticancer activity of di-acetylated lactonic SLs of varying chain length and degree of unsaturation has been reported in human cervical cancer . Moreover, we have recently reported the anti-cancer activity of crude cetyl alcohol and oleic acid SLs in breast and cervical cancer cell lines . The SLs described in the reported work comprise of a mixture of structurally different forms varying in degree of acetylation on sophorose, fatty acid chain length and internal esterification of end carboxylic acid group (in oleic acid SL). After confirming the presence of anti-cancer activity in the crude SLCAs it becomes necessary to investigate and report the activity for the purified forms (Refer S3 Fig). Prior presence of extensive literature on anti-proliferative property of oleic acid SL [2,4,5,19] prompted us to divert the investigation on unexplored purified forms of novel cetyl alcohol SLs synthesized microbially by Candida bombicola. Thus, the present study was extended by initially conducting cytotoxicity study of six different column purified fractions of cetyl alcohol sophorolipids (SLCA A, B, C, D, E, F) on panel of cancer cell lines and primary cells. The results revealed higher potency of SLCAs B and C against cervical cancer cells (HeLa) without affecting the viability of normal cells. The superior growth inhibition activity of both SLCA B and SLCA C in comparison to other SLCA could be attributed to difference in their structure with others. Using LC-MS, the fractions SLCA B and SLCA C were identified as the non-acetylated and mono-acetylated SL molecules respectively with sophorose head group conjugated to 16 carbon non-hydroxylated fatty tail (Refer S1 and S2 Figs). The methylated end in SLCA B and presence of additional acetyl group in SLCA C increases the hydrophobicity of the molecule. Similarly, studies on anti-cancer efficacy of lactonic sophorolipids against esophageal cancer cells also show that more hydrophobic diacetylated lactonic SL is more effective than the monoacetylated lactonic SL . Thus, in the case of cetyl alcohol SL we can hypothesize that SLCA B & C being more hydrophobic in comparison to other SLCA forms display superior growth inhibition activity.
In conclusion, this study indicates that two sophorolipids (SLCA B and SLCA C) have potential growth regulatory role in human cervical HeLa cancer cells via inducing apoptosis. SLCAs stimulate the activation of caspase-3, -8 and -9 with alteration of mitochondrial membrane potential. Among these two SLCAs, SLCA C was found to be more cytotoxic than SLCA B. Both SLCAs showed similar apoptotic effect in human cervical cancer cells in a time-dependent manner; arresting the cells at G1/S phase for 6, 12, 18 and 24 h. The underlying mechanism of apoptosis was result of depolarization of mitochondrial membrane and elevation in intracellular calcium level leading to activation of caspases. Thus, both SLCA B and SLCA C exhibited potential as chemopreventive anti-tumor agents that could improve common anticancer therapies.