Selenium is an essential micronutrient and naturally occurs in the form of selenite (SeIV). The SeIV compound, - (predominately HSeO3 at physiological pH) has been demonstrated to be an efficient agent in inflammation therapy and cancer prevention by many clinical and pre-clinical trials. SeIV function depends on a very narrow concentration range which separates essential, therapeutic, and toxic effects. However, how cellular SeIV levels are regulated is still unknown. One of the main reasons for this gap has been the lack of any knowledge of SeIV transport proteins in mammals. Recently we discovered that SLC39A8 (ZIP8), a member of the zinc (Zn) importer family, directly transports SeIV in the presence of Zn2+ and this uptake induces hypersensitivity to SeIV toxicity. This proposal will follow these findings by establishing the greater role of ZIP8 in SeIV transport and relevant cellular responses. That SeIV transport is critical to its therapeutic applications will be demonstrated by determining the effect of ZIP8 activity on NF-?B inhibition and selective cytotoxicity to prostate cancer cells. The role of ZIP8 and its transport mechanisms will be characterized via gain and loss of ZIP8 function in X. laevis oocytes, mammalian cell lines, and transgenic mouse lines. Three integrated aims are proposed: 1) characterize specific ZIP8 transport properties using X. laevis oocytes and mouse fetal fibroblast (MFF) expression systems;2) Determine correlation between ZIP8 transport function and SeIV cytotoxicity and the regulation of inhibitory SeIV/Zn2+ effects on NF-?B by ZIP8 in prostate cancer line DU145;3) Test the hypothesis that ZIP8 is the major SeIV transporter in vivo using the Slc39a8(neo/neo) hypomorphic mouse line and the ZIP8 overexpressing BTZIP8-3 mouse line. Results of these studies will provide the direct characterization of SeIV transport properties and elucidate the physiological role of ZIP8 uptake in regulating SeIV dose response in key concentration dependent functions. This will improve our fundamental understanding of SeIV transport and provide bases for improving current SeIV therapies.