Two molecular mechanisms of apoptosis resistance

Abstract

Many cancer cells develop resistance to apoptosis. It is important to understand how this phenotype develops, so that these cancers can be effectively treated. The results presented in this dissertation describe two molecular mechanisms of apoptosis resistance. A mouse keratinocyte model system consisting of the benign 308 parental cell line and two malignantly progressed variants (6M90 and 6R90 cells) were used to explore the relationship between reactive oxygen species (ROS) and apoptosis resistance. Previous work showed elevated basal levels of ROS in 6M90 and 6R90 cells. The results shown here demonstrate increased resistance to UV-induced apoptosis of the variants compared to the parental line. Pharmacological and genetic approaches were used to decrease the steady-state levels of ROS in the two malignant cell lines. This increased their sensitivity to apoptosis. ROS are implicated in the activation of the anti-apoptotic Akt kinase. 6M90 and 6R90 cells had higher levels of activated Akt. Modulation of ROS levels in the 6M90 and 6R90 cells decreased the levels of activated Akt. These studies provide a molecular mechanism to explain the chronically elevated ROS and apoptosis resistance seen in many tumors. Another mechanism by which tumor cells resist apoptosis is to upregulate the anti-apoptotic protein Bcl-2. A putative response element (PPRE) for the peroxisome proliferator activated nuclear receptor (PPAR) was found in the 3' UTR of bcl-2. Further experiments indicated that the gamma subtype of PPAR bound the putative PPRE and could activate transcription. In cells transfected with PPARgamma, increased levels of bcl-2 mRNA and Bcl-2 protein were seen as compared to empty vector-transfected cells. When treated with bile acids to induce apoptosis, the PPARgamma-transfected cells were twice as resistant as empty vector-transfected cells. These studies show, for the first time, that a sequence within the 3' end of the bcl-2 gene can regulate transcription of the gene through interactions with PPARgamma. These findings may be particularly relevant in colon cancer, where PPARgamma and Bcl-2 are often overexpressed.