AQP2-induced acceleration of renal cell proliferation involves the activation of a regulatory volume increase mechanism dependent on NHE2

Abstract

We have previously shown in renal cells that expression of the water channel Aquaporin 2 (AQP2) increases the rate of cell proliferation by shortening the transit time through the S and G2/M phases of the cell cycle. This acceleration is due, at least in part; to a down-regulation of regulatory volume decrease (RVD) mechanisms when volume needs to be increased in order to proceed into the S phase. We hypothesize that in order to augment cell volume, RVD mechanisms may be overtaken by regulatory volume increase mechanisms (RVI). In this study, we investigated if the isoform 2 of the Na+/H+ exchanger (NHE2), the main ion transporter involved in RVI responses, contributed to the AQP2-increased renal cell proliferation. Two cortical collecting duct cell lines were used: WT-RCCD1 (not expressing AQPs) and AQP2-RCCD1 (transfected with AQP2). We here demonstrate, for the first time, that AQP2-expressing cells present both higher intracellular pH and higher NHE2 protein expression than WT-RCCD1 cells. In AQP2-expresing cells NHE2 inhibition decreased cell proliferation and delayed cell cycle progression by slowing S and G2/M phases. We also observed that only in AQP2-expressing cells a NHE2-dependent RVI response was activated in the S phase. These observations suggest that the AQP2-increased proliferation involves the activation of a regulatory volume increase mechanism dependent on NHE2. Therefore, we propose that the accelerated proliferation of AQP2-expressing cells requires a coordinated modulation of the RVD/RVI activity that contributes to cell volume changes during cell cycle progression.