Transcriptional regulation and cellular trafficking of aquaporin-2 water channel

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We had examined the regulatory mechanism of aquaporin-2 (AQP-2) water channel in both in vitro and in vivo systems. (1) in vitro study : Various fragments of 5'-flanking region of murine AQP-2 gene up to -9.5 kb were cloned into a luciferase (Luc) reporter plasmid, and they were transiently transfected into MDCK or mIMCD3 cells. Hypertonicity-response enhancers were at least resided at two segments, namely tonicity-response enhancer (TonE)(-570〜-560bp) and unknown region between -6.1 and -4.3 kb of the 5'-flanking region of AQP-2 gene. The latter had the different structure and mechanism of response to hypertonicity from those of TonE. They regulated AQP-2 transcriptional regulation independently of arginine vasopressin (AVP). The region between -6.1 and -4.3 kb dominantly received hypertonicity signal, but its regulation further collaborated with TonE to activate the transcription of AQP-2 gene. On the contrary, TonE per se could be involved in hypotonicity-regulated AQP-2 transcripti … More on. Hypotonicity per se did not alter basal activity of Luc, but attenuated cAMP-induced Luc activity. This action was mediated through JNK. These findings indicate that tonicity-response enhancers are located at the 5'-flanking region of AQP-2, and regulate hyper- and hypotonicity-induced AQP-2 transcription. (2) in vivo study : We examined whether aging affects kidney expression of AQP-2 in glucocorticoid-deficient rats. Impaired water excretion was found in glucocorticoid-deficient rats, but its impairment was much serious in the aged rats compared with the young ones. Kidney AQP-2 expression was reduced in the aged rats. Plasma AVP was not sufficiently suppressed in the glucocorticoid-deficient t rats, and the expression of kidney AQP-2 mRNA and protein were rather upregulated in the aged rats with glucocorticoid deficiency. The present findings indicate that the upregulation of AQP-2 against aging plays a crucial role in persistent impairment in water excretion, dependent upon non-suppressible release of AVP, in aged rats with glucocorticoid deficiency. Less