HuR-dependent SOD2 protein synthesis is an early adaptation to anchorage independence

Abstract

The presence of malignant ascites is a common feature of advanced stage ovarian cancer. During metastasis, as cells detach from the tumor and extravasate into the peritoneal fluid, ovarian cancer cells must adapt to survive the loss of anchorage support and evade anoikis. An important pro-survival adaptation in this context is the ability of tumor cells to increase their antioxidant capacity and restore cellular redox balance. We previously showed that the mitochondrial superoxide dismutase SOD2 is necessary for ovarian cancer cell anoikis resistance, anchorage-independent survival and spheroid formation, and intraperitoneal spread in vivo. We now demonstrate that the upregulation of SOD2 protein expression is an early event initiated in response to anchorage independence and occurs at the post-transcriptional level. SOD2 protein synthesis is rapidly induced in the cytosol within 2 hours of matrix detachment. Polyribosome profiling demonstrates an increase in the number of ribosomes bound to SOD2 mRNA, indicating an increase in SOD2 translation in response to anchorage-independence. Mechanistically, we find that anchorage-independence induces cytosolic accumulation of the RNA binding protein HuR/ELAVL1, leads to HuR binding to SOD2 mRNA, and that the presence of HuR is necessary for the increase in SOD2 mRNA association with the heavy polyribosome fraction and SOD2 protein synthesis. Cellular detachment activates the stress-response protein kinase p38 MAPK, which is necessary for HuR-SOD2 mRNA binding and the rapid increase in SOD2 protein expression. Moreover, HuR is necessary for optimal cell survival during early stages of anchorage independence. These findings uncover a novel post-transcriptional stress response mechanism by which tumor cells are able to rapidly increase their mitochondrial antioxidant capacity to adapt to stress associated with anchorage-independence.