Solamargine, a steroidal alkaloid glycoside, induces oncosis in human K562 leukemia and squamous cell carcinoma KB cells

The aim of this study was to demonstrate that solamargine (SM) mediates its cytotoxicity via oncosis. The cytotoxicity of SM on cancer cells was examined by 3-(4,5-dimethylthiazol)-2 and 5-diphenyltetrazolium bromide assay. Cell membrane integrity was assessed by detecting the leakage of cytoplasmic content, the release of lactate dehydrogenase (LDH), and the uptake of propidium iodide (PI). We use whole-cell recording to measure the time courses of membrane blebbing and disruption in human K562 leukemia and squamous cell carcinoma KB cells. The effects of SM on cytoskeletal systems were detected by Western blotting and immunofluorescence. The cytotoxicity of SM did not correlate with the expression level of the multidrug resistance MDR1 and was triggered rapidly by 10 mu M SM (5 min caused 50% maximum cytotoxicity). Its rapid ability to make propidium iodide (PI) permeate into tumor cells, LDH release, and leakage of cytoplasmic content at the same rate (within minutes), suggests a killing mechanism that involves plasma membrane perturbation. SM induced membrane blebbing within 5 min of sustained application. Both chelating extracellular calcium with EGTA and clamping intracellular calcium concentrations with a high concentration of the cell-permeable chelator BAPTA-AM did not prevent blebbing. Polyethylene glycols having molecular weights a parts per thousand yen3,350 blocked membrane blebbing. SM also disrupted the cytoskeletal systems: degradation of microtubules and actin proteins. SM can rapidly initiate acute injury and bursting by damaging to cellular membrane and may offer a novel therapeutic strategy in treatment of cancer.