Ras regulation of mitochondrial fission promotes tumor growth

Pancreatic cancer ranks 4th in the United States for cancer associated deaths. Despite recent advances in our understanding of cancer biology, pancreatic cancer patients have the poorest prognosis of all cancer types. The median survival rate for pancreatic cancer is 6 months and the 5-year survival rate of roughly 8%. Due to these high mortality statistics, there is an urgent need to better understand pancreatic cancer biology in order to discover novel pathways and targets that may be exploited for therapeutic benefit. Under this premise, this dissertation set out to understand the contribution of mitochondrial dynamics to pancreatic cancer growth.
One of the primary mutations in pancreatic cancer occurs in KRas, a master signaling protein which is responsible for controlling a variety of cellular processes. The best characterized pathway downstream of KRas is the MAPK pathway which promotes cell growth and proliferation. KRas mutations render the GTPase constitutively active which results in perpetual signaling through its downstream effector pathways including the MAPK pathway. Recently, studies have shown that in diseases and abnormal metabolic states such as Alzheimer’s disease and hyperglycemia, respectively, MAPK activation can cause changes in mitochondrial dynamics. Due to the ability of KRas to activate the MAPK pathway in pancreatic cancer, we examined whether the MAPK pathway causes changes to mitochondrial morphology in cancer and if tumor growth ensues as a result of these changes.
Thus, in chapter 2 we demonstrate that HRas signals through the MAPK pathway to phosphorylate dynamin related protein 1 (Drp1) which in turn causes mitochondrial fission in HEK cells and pancreatic cancer cell lines. Furthermore, this Ras-mediated Drp1 induced mitochondrial fission is necessary for tumor growth in a xenograft model. In chapter 3, we show that HRas can partially inhibit mitochondrial fusion as well, which acts to shift the mitochondrial morphology of mutant Ras cells further toward the fragmented state. In chapter 4, we utilize in vitro and in vivo models of pancreatic cancer to study the contribution of Drp1 to tumor growth in a more physiologically relevant system. We find that KRas-Drp1 signaling causes increased cell accumulation as well as glycolytic metabolism in MEF cells, partially through upregulation of the glycolytic enzyme hexokinase 2 (HK2). In vivo, we show that loss of Drp1 in a pancreatic cancer mouse model results in a 45-day survival advantage. Drp1 null tumor cells derived from these mice have undergone global metabolic reprogramming to maintain glycolytic flux and HK2 expression. Furthermore, these cells have compromised mitochondrial function and an increase in catabolism of lipids, which suggests that after Drp1 loss, these cells may attempt to compensate for lost mitochondrial function. Taken together, the data presented in this thesis support a model whereby KRas signals to promote Drp1 activation which in turn results in metabolic rewiring of tumor cells that support their tumorigenic properties.