Solamargine triggers cellular necrosis selectively in different types of human melanoma cancer cells through extrinsic lysosomal mitochondrial death pathway

Previous reports showed that the Steroidal Glycoalkaloid Solamargine inhibited proliferation of non-melanoma skin cancer cells. However, Solamargine was not tested systematically on different types of melanoma cells and was not simultaneously tested on normal cells either. In this study we aimed to investigate the effect of Solamargine and the mechanism involved in inhibiting the growth of different types of melanoma cells.

Solamargine effect was tested on normal cells and on another three melanoma cell lines. Vertical growth phase metastatic and primary melanoma cell lines WM239 and WM115, respectively and the radial growth phase benign melanoma cells WM35 were used. The half inhibitory concentration IC₅₀ of Solamargine was determined using Alamarblue assay. The cellular and subcellular changes were assessed using light and Transmission Electron Microscope, respectively. The percentage of cells undergoing apoptosis and necrosis were measured using Flow cytometry. The different protein expression was detected and measured using western blotting. The efficacy of Solamargine was determined by performing the clonogenic assay. The data collected was analyzed statistically on the means of the triplicate of at least three independent repeated experiments using one-way ANOVA test for parametric data and Kruskal–Wallis for non-parametric data. Differences were considered significant when the P values were less than 0.05.

Hereby, we demonstrate that Solamargine rapidly, selectively and effectively inhibited the growth of metastatic and primary melanoma cells WM239 and WM115 respectively, with minimum effect on normal and benign WM35 cells. Solamargine caused cellular necrosis to the two malignant melanoma cell lines (WM115, WM239), by rapid induction of lysosomal membrane permeabilization as confirmed by cathepsin B upregulation which triggered the extrinsic mitochondrial death pathway represented by the release of cytochrome c and upregulation of TNFR1. Solamargine disrupted the intrinsic apoptosis pathway as revealed by the down regulation of hILP/XIAP, resulting in caspase-3 cleavage, upregulation of Bcl-xL, and Bcl2, and down regulation of Apaf-1 and Bax in WM115 and WM239 cells only. Solamargine showed high efficacy in vitro particularly against the vertical growth phase melanoma cells.

Our findings suggest that Solamargine is a promising anti-malignant melanoma drug which warrants further attention.