XB-ART-52121Oncotarget. June 7, 2016; 7 (23): 35327-40.
Zinc- and bicarbonate-dependent ZIP8 transporter mediates selenite uptake.
Selenite (HSeO3-) is a monovalent anion of the essential trace element and micronutrient selenium (Se). In therapeutic concentrations, HSeO3- has been studied for treating certain cancers, serious inflammatory disorders, and septic shock. Little is known, however, about HSeO3- uptake into mammalian cells; until now, no mammalian HSeO3- uptake transporter has been identified. The ubiquitous mammalian ZIP8 divalent cation transporter (encoded by the SLC39A8 gene) is bicarbonate-dependent, moving endogenous substrates (Zn2+, Mn2+, Fe2+ or Co2+) and nonessential metals such as Cd2+ into the cell. Herein we studied HSeO3- uptake in: human and mouse cell cultures, shRNA-knockdown experiments, Xenopus oocytes, wild-type mice and two transgenic mouse lines having genetically altered ZIP8 expression, and mouse erythrocytes ex vivo. In mammalian cell culture, excess Zn2+ levels and/or ZIP8 over-expression can be associated with diminished viability in selenite-treated cells. Intraperitoneal HSeO3- causes the largest ZIP8-dependent increases in intracellular Se content in liver, followed by kidney, heart, lung and spleen. In every model system studied, HSeO3- uptake is tightly associated with ZIP8 protein levels and sufficient Zn2+ and HCO3- concentrations, suggesting that the ZIP8-mediated electroneutral complex transported contains three ions: Zn2+/(HCO3-)(HSeO3-). Transporters having three different ions in their transport complex are not without precedent. Although there might be other HSeO3- influx transporters as yet undiscovered, data herein suggest that mammalian ZIP8 plays a major role in HSeO3- uptake.
PubMed ID: 27166256
PMC ID: PMC5085232
Article link: Oncotarget.
Grant support: R15 ES022800 NIEHS NIH HHS , P30 ES006096 NIEHS NIH HHS , R01 ES020137 NIEHS NIH HHS , R01 ES010416 NIEHS NIH HHS , T32 ES016646 NIEHS NIH HHS
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|Figure 1. Intracellular Se content and toxicity; ZIP8 protein levels in human prostate cancer cellsA. effect of Zn2+ on Se content in selenite-treated DU145 cells; Zn2+ added concomitantly with HSeO3− for 30 min. B. effect of Zn2+ on selenite-mediated viability of DU145 cells following 12-h treatment, varying HSeO3− amounts. Untreated cultures were given a ratio value of 1.0., and other ratios of experimental regimens are expressed, relative to 1.0. C. effect of EDTA and Zn2+ on Se content in selenite-treated cells; EDTA (5 mM) + Zn2+ (200 μM) pretreatment was 5 min; at far right, Zn2+ was washed out prior to HSeO3− treatment for 30 min. D. Western blots of human ZIP8 in five prostate cancer lines (left lanes) and five prostate non-cancer cell lines (right lanes). GAPDH, glyceraldehyde 3-phosphate dehydrogenase, lane-loading control. E. cell viability of Zn2+-treated ZIP8-MEFs versus LUC-MEFs, as a function of selenite concentration; treatment (5 μM Zn2+ and 25 mM HCO3−) was 16 h. For panels B and E, “relative viability” of cells not treated with selenite is given a value of 1.0, and viability of all other experimental regimens are expressed relative to that control.|
|Figure 7. The two proposed models of selenite transportA. ZIP8-mediated HSeO3− uptake model, which is both zinc- and bicarbonate-dependent. Right, the extracellular HSeO3− conversion model. In the former, ZIP8-mediated HSeO3− uptake can be prevented with prior chelation of Zn2+ by EDTA. Intracellular HSeO3− reacts spontaneously with proteins containing coordinated thiol groups−−modifying signaling pathways dependent on zinc-finger proteins, or thiol proteins. Functional downstream effects on multiple targets are dependent on HSeO3− concentration and ZIP8 levels. HSeO3− reactions with zinc-finger proteins might release free Zn2+, which, in addition to Zn2+ transported by ZIP8, is proposed to elevate labile Zn2+ concentrations and participate in subsequent zinc-signaling functions. Concentration-dependent effects (bottom) range from anti-cancer and anti-inflammatory, to apoptosis and oxidative stress, to cytotoxicity. B. In the alternative model, HSeO3− uptake is proposed to be regulated by extracellular thiol reduction of selenite–in the extracellular milieu–followed by uptake of unknown reduced Se product(s) [22, 35].|