Gene expression analysis of renal cell carcinoma cell line A-498 after 3-hour treatment with englerin A.. Homo sapiens

Natural products possess enormous structural and chemical diversity and are a rich source of drugs or drug-like leads. Although the final products may not necessarily represent the active ingredients of the natural source, the majority of all drugs in the market have their origin in nature. Englerin A is guaiane sesquiterpene that was isolated from the Tanzanian plant Phyllanthus engleri and has attracted much attention because of its unique structure and cytotoxicity profile in cancer cells. Englerin A was documented to have potent cytotoxicity preferentially against renal cancer cells when tested against the NCI60 cell panel. Very recently, however, a study which screened englerin A against 524 cancer cell lines from the cancer cell line encyclopedia reported that englerin A was also cytotoxic to a small subset of cancer cells in addition to renal cancer cells.[19]. In addition to its unique structure and cytotoxicity profile, englerin A is distinctive because it can cause cell death by multiple death mechanisms including necrosis and apoptosis, with apoptosis occurring in the absence of caspase activation. The distinctive features of englerin A suggest that it has a unique mechanism of action and recent reports, though lacking consensus, support this notion. Englerin A has been shown to activate protein kinase C theta (PKCθ) and was proposed to affect cell viability by promoting glucose dependence while simultaneously starving cells of glucose. Englerin A has also been shown to increase cytosolic calcium levels which may play a role in the decreased phosphorylation and activity of the oncoprotein, EWS-FLI1, in Ewing’s sarcoma. Two other groups have reported that englerin A inhibits tumor cell growth by activating the transient receptor potential cation channel, subfamily C, member 4 (TRPC4) ion channel, while a third group reported that englerin A antagonized L-type calcium channels. However, in at least two of these studies, the concentration of englerin A that was required to modulate these calcium channels was much higher than that required to kill renal carcinoma cells and other cells sensitive to the cytotoxic effects of englerin A. For instance, the most recent study reported that englerin A displaced a radiolabeled tool compound bound to an L-type calcium channel in a concentration-dependent manner with a Ki of 5.7 M. In contrast, the EC50 of englerin A in reducing the viability or growth of most renal cancer cell lines in the NCI60 cell panel is less than 60 nM, suggesting that modulation of L-type or TRPC calcium channels is not required for englerin A to induce toxic effects in these cells. It is still not clear what mechanisms account for the cytotoxicity of englerin A at nanomolar levels in these cells. We anticipate that there may be other mechanisms and targets of englerin A as natural products generally have multiple targets. In the current study, we chose microarray as well as other technologies to explore the mechanism(s) of action of englerin A at a more global level. Overall design: Gene expression analysis was performed on A-498 cells treated with 100 nM englerin A during 3 hours. Four replicates of vehicle control and englerin A treated A-498 cells were used in the experiment.