Abstract 1255: Delta-tocopherol induced endoplasmic reticulum stress causes autophagic degradation of ER and cell death in bladder cancer models

Vitamin E has been the subject of numerous basic and clinical studies, which have demonstrated a range of results that necessitate the further analysis of its individual components. We demonstrate that of the major tocopherol components of Vitamin E, Delta tocopherol (δ-T) is significantly more effective than the more common Alpha- and Gamma- tocopherols at inducing bladder cancer cell death in vitro and reduces tumor growth and burden in vivo. As a major site of protein synthesis and folding the endoplasmic reticulum (ER) is highly sensitive to stress induced by increased unfolded protein load, which induces both pro-survival and ultimately pro-apoptotic responses via the different branches of the unfolded protein response (UPR). δ-T induces the UPR in human bladder cancer cells and ultimately results in cell death, inducing both apoptosis and autophagic cell death. Analysis of bladder cancer cells treated with δ-T revealed strong evidence of UPR activation in the form of upregulation and activation of ER stress sensors PERK and IRE1α, and the upregulation of folding chaperone GRP78 and transcription factors ATF4 and CHOP. These changes in turn induced downstream effects including Death Receptor 5 (DR5)-mediated apoptosis and visible induction of autophagy. In addition to the observation of autophagosome formation, induction of autophagy was further confirmed by upregulation of protein expression of components of the autophagic machinery LC3B and Atg family members. Electron Microscope imaging of treated cells revealed the presence of ER-whorls within the large autophagosomes which were themselves visible under conventional light microscopy, which has to our knowledge not been previously observed in cancer models. This observation is supported by the ER stress-based mechanism of action exhibited by δ-T on bladder cancer cells, which recapitulates the observations of the few previous reports of “ER-phagy” described in yeast models. ER stress induced expansion of the ER is believed to aid in the improvement of stressed cells' protein folding capacity, and selective degradation of the ER-whorls formed in this manner by a previously undescribed mechanism of autophagy was first described in yeast and remains unexplored in human cancer models. These findings may provide new insights into δ-T's mechanism of action and inform new approaches to the development of tocopherols as novel chemopreventive agents. Citation Format: Christopher A. Blair, Hanze Hu, Tim Huyn