This study further showed that tumors excised from the EA-treated mice revealed increased inhibitory phosphorylation of the insulin receptor substrate 1 (IRS1) and decreased activity of the Cytoskeletal Signaling inhibitor PI3/AKT pathway, in line with our in vitro results in A498 cells. Based on their in vitro results, the authors of this study concluded that EA bound and activated PKCθ to inhibit insulin signaling while, concurrently, activating HSF1, a known inducer of selleck chemicals glucose dependence,
thus, starving cells of glucose while promoting glucose addiction. However, because the in vitro binding studies with EA and PKCθ were indirect without any binding kinetic analyses, it is unclear if PKCθ is a primary target of EA. Furthermore, the experiments demonstrating inhibition of glucose uptake by EA were performed using EA at 10 μM, a concentration of EA approximately 200-fold higher than its IC50. It is well established that when cells are starved, the energy sensor, AMP-activated protein kinase, becomes activated by phosphorylation resulting in the induction of autophagy. If EA inhibits glucose uptake, it would be expected to result in a higher ADP/ATP and AMP/ATP ratio and consequent activation of AMPK. Our results, however, did not reveal activation of AMPK by EA at a concentration of 100 nM, a concentration that is highly cytotoxic to A498 cells. Hence, it is possible that the effects
of EA on glucose uptake may occur at micro molar concentrations that are much higher than required for cell death (nanomolar) and could represent off-target effects. Moreover, as a natural product, EA would be expected to have multiple VX-661 mw targets and most likely has targets in addition to PKCθ. Such targets may include those associated with the ER stress since it is well established that ER stress results in the induction of cell death and autophagy . An example
of agent that induces autophagy and cell death by inducing ER stress in RCC includes STF-62247 which targets VHL-deficient RCC . EA may target proteins within the Golgi complex analogous oxyclozanide to carminomycin I, a natural product with selective toxicity to VHL-deficient CC-RCC . In conclusion, EA induces cell death via multiple mechanisms and likely has multiple cellular targets. The identification of these targets and pathways affected by this unique agent will be invaluable in understanding the high RCC- selectivity of EA and allow development of highly effective chemotherapeutics for the treatment of metastatic RCC, a highly treatment resistant cancer. Acknowledgment We gratefully acknowledge Dr. Stoyan Dimitrov for his assistance with the flow cytometry studies. This work was supported by a fund from Academia Sinica (A. L. Yu) and, in part, by an NIH grant (CA 133002) awarded to Emmanuel Theodorakis. References 1. Nguyen MM, Gill IS, Ellison LM: The evolving presentation of renal carcinoma in the United States: trends from the Surveillance, Epidemiology, and End Results program.