Molecular mechanisms of tumor progression in mouse skin keratinocytes

Abstract

Our laboratory had previously developed an in vitro tumor progression model which demonstrated that elevation of Reactive Oxygen Species (ROS) played a functional role in the development and maintenance of malignantly transformed mouse skin keratinocytes. The elevation of ROS levels were attributed to the repression of the anti-oxidant enzyme catalase. We hypothesized that repression of catalase, a potential tumor suppressor gene, led to the elevation of hydrogen peroxide which functions as a secondary messenger to active mitogenic signaling in the malignant cells. The first goal of these studies was to further characterize the repression of catalase in vivo. Using tumor samples generated by the mouse skin chemical carcinogenesis protocol, we determined that papillomas expressed higher levels of catalase protein and message than carcinomas. These results recapitulated the observations we made in the in vitro tumor progression model. The next goal was to determine the mechanism(s) behind the repression of catalase. Nuclear run-on analysis showed that catalase repression in the malignantly transformed cells was dependent on transcription. Results from luciferase reporter assays indicated that malignant cells have lower catalase promoter activities than benign papilloma cells, in part through the Wilm's tumor suppressor (WT1) binding site within the proximal promoter region. We concluded that WT1 element acts as a transcriptional repressor of catalase in this tumor progression model. The second part of this dissertation is focused on the role of the Rac1 signaling in tumor progression. Rac1 has been shown to activate NADPH oxidase to produce superoxide, potentially contributing to the elevation of ROS. We found that conditional expression of a dominant negative Rac1 was able to decrease multiple markers of malignancy including: growth, migration and invasion potential. In addition, these phenotypic changes were accompanied by a decrease in mitogenic signaling. Furthermore, we showed that inhibition of Rac1 signaling could reduce tumor growth in vivo. However, we were unable to show any decrease in intracellular levels of ROS. Based on these results, we concluded that Rac1 signaling plays a key role in mouse skin tumor progression through a ROS independent manner.