| Summary: | 碩士 === 國立臺灣大學 === 生物化學暨分子生物學研究所 === 106 === Vasopressin (AVP) is a peptide hormone that regulates water permeability of the kidney collecting ducts. In response to AVP within an hour, AVP mobilizes the water channel protein aquaporin-2 (AQP2) from intracellular storage vesicles to the apical membrane thereby increasing its water permeability. In response to AVP between hours and days, AVP increases overall AQP2 gene expression thereby contributing to water permeability regulation. These two mechanisms are seemingly unrelated. Here, we are proposing a hypothesis that could potentially relate these two processes via α-actinin 4. α-actinin 4 was previously identified in the collecting duct cell model (mpkCCD) in response to AVP stimulation. In one hand, α-actinin 4 being an actin-binding protein could participate in actin cytoskeleton dynamics, a process required for AQP2 trafficking. On the other hand, α-actinin 4 also acts as a transcriptional co-activator of a number of transcription factors including glucocorticoid receptor (GR) that has been implicated in AVP-induced AQP2 gene expression in the mpkCCD cells. It is plausible that AVP may induce α-actinin 4 interaction with GR to trigger AQP2 gene expression. In support of our hypothesis, we found that short hairpin RNA (shRNA)-mediated α-actinin 4 knockdown in the mpkCCD cells reduced AVP-induced AQP2 mRNA and protein levels. α-actinin 4 did not affect AVP-induced AQP2 mRNA stability but partially enhanced AVP-induced AQP2 transcription by activating AQP2 promoter. To investigate the molecule mechanism involved, we found that AVP induced α-actinin 4 nuclear translocation and also increased the amount of GR in the nuclei of the mpkCCD cells. GR knockdown in the mpkCCD cells showed decreased AVP-induced AQP2 mRNA and protein levels. We next found that GR enhanced AVP-induced AQP2 promoter activity by promoter-reporter assay. Above results suggest that GR plays a role in AVP-induced AQP2 expression. Another interesting finding was that GR also appears to regulate vasopressin V2 receptor mRNA levels. In summary, we conclude that α-actinin 4 acts as transcriptional co-activator of GR and regulates AVP-induced AQP2 transcription in the kidney collecting duct cells.
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