Much is known about the transport of arsenite and antimonite into microbes, but the identities of mammalian transport proteins are unknown. The Saccharomyces cerevisiae FPS1 gene encodes a membrane protein homologous to the bacterial aquaglyceroporin GlpF and to mammalian aquaglyceroporins AQP7 and AQP9. Fps1p mediates glycerol uptake and glycerol efflux in response to hypoosmotic shock. Fps1p has been shown to facilitate uptake of the metalloids arsenite and antimonite, and the Escherichia coli homolog, GlpF, facilitates the uptake and sensitivity to metalloid salts. In this study, the ability of mammalian aquaglyceroporins AQP7 and AQP9 to substitute for the yeast Fps1p was examined. The fps1⌬ strain of S. cerevisiae exhibits increased tolerance to arsenite and antimonite compared to a wild-type strain. Introduction of a plasmid containing AQP9 reverses the metalloid tolerance of the deletion strain. AQP7 was not expressed in yeast. The fps1⌬ cells exhibit reduced transport of 73 As(III) or 125 Sb(III), but uptake is enhanced by expression of AQP9. Xenopus laevis oocytes microinjected with either AQP7 or AQP9 cRNA exhibited increased transport of 73 As(III). These results suggest that AQP9 and AQP7 may be a major routes of arsenite uptake into mammalian cells, an observation potentially of large importance for understanding the action of arsenite as a human toxin and carcinogen, as well as its efficacy as a chemotherapeutic agent for acute promyelocytic leukemia.Fps1p ͉ GlpF ͉ acute promyelocytic leukemia

Aquaglyceroporins form the subset of the aquaporin water channel family that is permeable to glycerol and certain small, uncharged solutes. AQP9 has unusually broad solute permeability and is expressed in hepatocyte plasma membranes. Proteoliposomes reconstituted with expressed, purified rat AQP9 protein were compared with simple liposomes for solute permeability. At pH 7.5, AQP9 proteoliposomes exhibited Hg 2؉ -inhibitible glycerol and urea permeabilities that were increased 63-fold and 90-fold over background. ␤-Hydroxybutyrate permeability was not increased above background, and osmotic water permeability was only minimally elevated. During starvation, the liver takes up glycerol for gluconeogenesis. Expression of AQP9 in liver was induced up to 20-fold in rats fasted for 24 -96 h, and the AQP9 level gradually declined after refeeding. No changes in liver AQP9 levels were observed in rats fed ketogenic diets or high-protein diets, but AQP9 levels were elevated in livers of rats made diabetic by streptozotocin injection. When blood glucose levels of the diabetic rats were restored to normal by insulin treatments, the AQP9 levels returned to baseline. Confocal immunofluorescence revealed AQP9 immunostaining on the sinusoidal surfaces of hepatocyte plates throughout the livers of control rats. Denser immunostaining was observed in the same distribution in livers of fasted and streptozotocin-treated rats. We conclude that AQP9 serves as membrane channel in hepatocytes for glycerol and urea at physiological pH, but not for ␤-hydroxybutyrate. In addition, levels of AQP9 expression fluctuate depending on the nutritional status of the subject and the circulating insulin levels. T he AQP9 cDNA was first isolated during efforts to clone urea transporters by expression in oocytes (1). Expressed in testes, leukocytes, and brain, AQP9 is abundant in liver (1) where it resides in hepatocyte plasma membranes facing the sinusoids (2-4). The coding sequence of AQP9 is more closely related to AQP3 (5) and AQP7 (6), which are both permeated by glycerol and water. This subset of proteins, referred to as aquaglyceroporins, is functionally distinct from the water-selective homologs AQP1, AQP2, AQP4, and AQP5 (7). The original studies of Xenopus laevis oocytes expressing rat AQP9 reported permeability to a wide range of 14 C-or 3 H-labeled solutes including polyols, carbamides, purines, pyrimidines, nucleosides, and monocarboxylates (1). Glycerol and urea permeability have been confirmed with AQP9 oocytes (8), but studies of proteoliposomes reconstituted with purified AQP9 protein have not been reported.The physiological functions of AQP9 are uncertain. During prolonged fasting, glycerol released from adipocytes via AQP7 may be taken up by the liver via AQP9 for gluconeogenesis. Urea, a byproduct of amino acid deamination, and ␤-hydroxybutyrate, an alternative fuel, may be released from liver via AQP9. An elegant series of recent studies of adipocyte AQP7 and liver AQP9 mRNAs and promoters suggested that the genes are coordinately ...

The study of water transport began long before the molecular identification of water channels with studies of water-permeable tissues. The discovery of the first aquaporin, AQP1, occurred during experiments focused on the identity of the Rh blood group antigens. Since then the field has expanded dramatically to study aquaporins in all types of organisms. In mammals, some of the aquaporins transport only water. However, there are some family members that collectively transport a diverse set of solutes. The aquaporins can be regulated by factors that affect channel permeability or subcellular localization. An extensive set of studies examines the physiological role of many of the mammalian aquaporins. However, much is still to be discovered about the physiological role of this membrane protein family.

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