1.A.8.9.2
Aquaporin-9 (Aqp9) (permeable to glycerol, urea, polyols, carbamides, purines, pyrmidines, nucleosides, monocarboxylates, pentavalent methylated arsenicals and the arsenic chemotherapeutic drug, trisenox (McDermott et al., 2009).  It is poorly permeable to water and not permeable to β-hydroxybutyrate (Carbrey et al., 2003). (Regulated by CFTR and NHERF1 in response to cyclic AMP (Pietrement et al., 2008)) The 7 Å projection structure and a homology model revealed that pore-lining residues and the hydrophobic edge of the tripathic pore of GlpF (1.A.8.1.1) provide the basis for broad substrate specificity (Viadiu et al., 2007).  It is important for urea transport in mouse hepatocytes (Jelen et al. 2012).  Activation of the PPARα transcription factor results in reduction in the abundance of AQP9 in periportal hepatocytes, but its activation in the fed state directs glycerol into glycerolipid synthesis rather than into de novo synthesis of glucose (Lebeck et al. 2015).  Azacytidine up-regulates AQP9 and enhances arsenic trioxide (As₂O₃)-mediated cytotoxicity in acute myeloid leukemia (AML) (Chau et al. 2015).  Human Aqp9 transports hydrogen peroxide (HOOH) (Watanabe et al. 2016) and plays a role in certain types of cancer (Zheng et al. 2020). Human aquaporin 9 regulates Leydig cell steroidogenesis in diabetes (Kannan et al. 2022).