Adaptation to alkalosis induces cell cycle delay and apoptosis in cortical collecting duct cells: Role of aquaporin‐2

Abstract

Collecting ducts (CD) not only constitute the final site for regulating urine concentration by increasing apical membrane Aquaporin-2 (AQP2) expression, but are also essential for the control of acid-base status. The aim of this work was to examine, in renal cells, the effects of chronic alkalosis on cell growth/death as well as to define whether AQP2 expression plays any role during this adaptation. Two CD cell lines were used: WT- (not expressing AQPs) and AQP2-RCCD(1) (expressing apical AQP2). Our results showed that AQP2 expression per se accelerates cell proliferation by an increase in cell cycle progression. Chronic alkalosis induced, in both cells lines, a time-dependent reduction in cell growth. Even more, cell cycle movement, assessed by 5-bromodeoxyuridine pulse-chase and propidium iodide analyses, revealed a G2/M phase cell accumulation associated with longer S- and G2/M-transit times. This G2/M arrest is paralleled with changes consistent with apoptosis. All these effects appeared 24 h before and were always more pronounced in cells expressing AQP2. Moreover, in AQP2-expressing cells, part of the observed alkalosis cell growth decrease is explained by AQP2 protein down-regulation. We conclude that in CD cells alkalosis causes a reduction in cell growth by cell cycle delay that triggers apoptosis as an adaptive reaction to this environment stress. Since cell volume changes are prerequisite for the initiation of cell proliferation or apoptosis, we propose that AQP2 expression facilitates cell swelling or shrinkage leading to the activation of channels necessary to the control of these processes.

Aquaporin‐1 plays important role in proliferation by affecting cell cycle progression

“…A higher expression of cyclins might well explain this phenotype, and this contributes to understanding the greater proliferative capacity of these cells. Interestingly, adding to the findings of the few studies that have explored the role AQPs might play over the course of the cell cycle (Delporte et al, 1996;Yoneda et al, 2001;Rivarola et al, 2010;Di Giusto et al, 2012;Shimizu et al, 2014), we have learned that AQP expression normally rises while cells are in the G0/G1 phase (Delporte et al, 1996;Yoneda et al, 2001), probably by favoring an increase in cell volume. These proteins become important for cells to move into the S and G2/M phases and allow cell cycle progression.…”

“…A few authors have described the involvement of some AQPs in cell cycle progression and cell death by apoptosis. For instance, expression of AQP2 in collecting duct cells has been associated with the rapid regulatory volume decrease that allows these cells to enter into apoptosis (Flamenco et al, ), and was also involved in the acceleration of cell proliferation per se, by increasing the rate of cell cycle progression in these renal cells (Rivarola et al, ; Di Giusto et al, 2012). A more recent study confirmed that AQP5 expression in esophageal squamous cell carcinoma (ESCC) may affect cell proliferation by modulating the expression of genes involved in cell cycle progression.…”

Role of aquaporins in cell proliferation: What else beyond water permeability?

“…Later, it was shown that AQP2 expression itself speeds up the proliferation and cell cycle progression of renal collecting duct cells by decreasing the transit time through S and G2/M phases of the cell cycle, 21 probably by favoring an increase in cell volume. 20 A range of experimental approaches using drugs or specific culture conditions that allow manipulation of cell cycle progression have also contributed to our understanding of the role of AQPs in this process.…”

“…It has been proposed that AQP2 expression facilitates changes in cell volume and the activation of channels or transporters necessary for the control of cell growth and cell death, resulting in a more rapid AVD and more rapid achievement of adequate levels of ions necessary to activate the enzymatic apoptotic cascade. 21,124 In cortical neurons of the central nervous system, Jessica et al 125 analyzed the expression of different AQPs while cells undergo apoptosis induced by treatment with lactacystin, a specific proteasome inhibitor. [126][127][128] These authors found that AQP4 was highly downregulated, suggesting that this aquaporin, considered to be the main water channel in the brain, 129 does not play an important role in the loss of water during AVD.…”

“…Reviewing the literature, we find that this association has been suggested mainly in relation to a role for these proteins in cell volume changes, 3,5,8 or in the case of a subgroup of AQPs, called aquaglyceroporins, 11,12 to their permeability to solutes such as glycerol and hydrogen peroxide. [13][14][15][16][17][18] Additionally, various studies have demonstrated that AQP expression is modified over the course of the cell cycle [19][20][21][22][23] and, more recently, we have shown that overexpression of AQP1 and AQP3 alters the levels of essential checkpoint proteins (cyclins) relevant for cell cycle progression, in addition to modifying the levels of transcription factors, and cytokines. 13,24 In this review, we have sought to gather the most relevant evidence associating AQPs with cell proliferation and we will discuss whether the canonical function of water permeation of these proteins would be always essential for this process or the role in proliferation might be independent of this function.…”