Hypoxia and the Mechanical Microenvironment

Abstract

Rac1b, a splice variant of the small GTPase Rac1, is associated with several pathways that lead to cancer progression. When Rac1b is localized to the membrane of the cell, it can associate with downstream effectors, ultimately leading to cancer development. Cancer cells can be subjected to hypoxia, a lack of oxygen. Hypoxia is common in breast tumors, selects more invasive cancer cells, and can lead to cancer drug resistance. Since both Rac1b and hypoxia have been implicated in cancer progression, this study sought to characterize the effect of hypoxia on Rac1b’s localization. By transfecting cells with a YFP-Rac1b plasmid and placing the cells in either a hypoxic or normoxic environment, the effect of hypoxia on Rac1b’s localization could be quantified. Our data show that hypoxia leads Rac1b to localize to the nucleus of the cell, an unanticipated discovery. This study then aimed to explore the mechanism by which Rac1b is localized to the nucleus. SmgGDS, a nucleocytoplasmic shuttling protein in the armadillo (ARM) family, is a guanine nucleotide exchange factor (GEF). SmgGDS can interact with multiple GTPases, including Rac1b, and is overexpressed in various types of cancer10. We hypothesize that SmgGDS, in hypoxic conditions, shuttles Rac1b from the plasma membrane to the nucleus. We tested this by co-transfecting cells with a YFP-Rac1b plasmid and either SmgGDS shRNA or SmgGDS cDNA. We then placed the cells in either a hypoxic or normoxic environment and quantified the localization of Rac1b. Our data show that SmgGDS is necessary for Rac1b to be translocated to the nucleus under hypoxic conditions. Knocking down SmgGDS inhibited Rac1b’s localization to the nucleus. Future work to further uncover this mechanism will have significant implications for understanding cancer progression and developing targeted cancer therapies.