Biosurfactants as Anticancer Agents: Glycolipids Affect Skin Cells in a Differential Manner Dependent on Chemical Structure

Abstract

Melanomas account for 80% of skin cancer deaths. Due to the strong relationship between melanomas and U.V. radiation, sunscreens have been recommended for use as a primary preventative measure. However, there is a need for targeted, less invasive treatment strategies. Glycolipids such as sophorolipids and rhamnolipids are microbially derived biosurfactants possessing bioactive properties such as antimicrobial, immunomodulatory and anticancer effects. This study aimed to ascertain the differing effects of glycolipids on skin cells. Highly purified and fully characterized preparations of sophorolipids and rhamnolipids were used to treat spontaneously transformed human keratinocyte (HaCaT) and the human malignant melanocyte (SK-MEL-28) cell lines. Cell viability and morphological analyses revealed that glycolipids have differential effects on the skin cells dependent on their chemical structure. Lactonic sophorolipids and mono-rhamnolipids were shown to have a significantly detrimental effect on melanoma cell viability compared to healthy human keratinocytes. These glycolipids were shown to induce cell death via necrosis. Additionally, sophorolipids were shown to significantly inhibit SK-MEL-28 cell migration. These findings suggest that glycolipids could be used as bioactive agents with selective inhibitory effects. As such, glycolipids could be a substitute for synthetically derived surfactants in sunscreens to provide additional benefit and have the potential as novel anti-skin-cancer therapies.

Keywords: anticancer; biosurfactant; glycolipid; melanoma; microbiology.

Figure 1

The effect on HaCaT and SK-MEL-28 cell viability when treated with (A) acidic sophorolipids at 0–100 μg mL⁻¹; (B) lactonic sophorolipids at 0–100 μg mL⁻¹; (C) mono-rhamnolipids at 0–100 μg mL⁻¹; (D) di-rhamnolipids at 0–100 μg mL⁻¹; (E) acidic sophorolipids at 0–500 μg mL⁻¹; (F) mono-rhamnolipids at 0–500 μg mL⁻¹; (G) SLES at 0–100 μg mL⁻¹. Data are the mean results of three independently conducted experiments, each with six replicates per treatment group. Error bars show standard error from the mean. Statistical significance was determined using a two-way ANOVA followed by Bonferroni pos hoc test, * = p ≤ 0.05.

Figure 2

Light microscopy images of HaCaT and SK-MEL-28 cells untreated (0 μg mL⁻¹) or following 24 h of treatment with a vehicle control (MeOH), acidic sophorolipid (ASL) (100 μg mL⁻¹ and 500 μg mL⁻¹), lactonic sophorolipid (LSL) (100 μg mL⁻¹), mono-rhamnolipid (MRL) (100 μg mL⁻¹ and 500 μg mL⁻¹), di-rhamnolipid (DRL) (100 μg mL⁻¹) and SLES at (100 μg mL⁻¹). Images were randomly selected from three independently conducted experiments with each treatment group imaged three times; scale bar is 100 μm.

Figure 3

Morphological assessment of the pattern of cell death induced by glycolipids using AO/PI staining following a 24 h treatment under the same conditions as reported in Figure 2. The vast number of HaCaT and SK-MEL-28 cells treated with vehicle control, acidic sophorolipids (100 μg mL⁻¹ and 500 μg mL⁻¹) and 100 μg mL⁻¹ of mono-rhamnolipids were morphologically viable (green stains with uncondensed nuclei) while 100 μg mL⁻¹ treatment concentrations of lactonic sophorolipids, di-rhamnolipids and 500 μg mL⁻¹ of mono-rhamnolipids resulted in significant reduction in the cell population, with the few adherent cells staining red/orange (necrotic cell death). Scale bar set at 100 μm.

Figure 4

Scratch assay showing the effect on cellular migration of each glycolipid preparation compared to untreated (0) and 1% (v/v) methanol vehicle control (V. ctrl) on HaCaT cells (A), SK-MEL-28 cells (B) and a direct comparison of both cell types treated with glycolipids (C). Data shown are the areas of cell-free zones measured in a scratch to the cellular monolayer following 24 h of treatment. Measurements were determined from analysis of randomly selected images using Image J software. Statistical significance was determined from three independent experiments via one-way ANOVA followed by Dunnett’s multiple comparison test, * = p ≤ 0.05 for the individual cell lines (A,B) and two-way ANOVA followed by Bonferroni’s multiple comparison test, * = p ≤ 0.05 for comparison of the two cell lines (C).