Receptor-mediated control of regulatory volume decrease (RVD) and apoptotic volume decrease (AVD)

Abstract

A fundamental property of animal cells is the ability to regulate their own cell volume. Even under hypotonic stress imposed by either decreased extracellular or increased intracellular osmolarity, the cells can re-adjust their volume after transient osmotic swelling by a mechanism known as regulatory volume decrease (RVD). In most cell types, RVD is accomplished mainly by KCl efflux induced by parallel activation of K+ and Cl- channels. We have studied the molecular mechanism of RVD in a human epithelial cell line (Intestine 407). Osmotic swelling results in a significant increase in the cytosolic Ca2+ concentration and thereby activates intermediate-conductance Ca2+-dependent K+ (IK) channels. Osmotic swelling also induces ATP release from the cells to the extracellular compartment. Released ATP stimulates purinergic ATP (P2Y2) receptors, thereby inducing phospholipase C-mediated Ca2+ mobilization. Thus, RVD is facilitated by stimulation of P2Y2 receptors due to augmentation of IK channels. In contrast, stimulation of another G protein-coupled Ca2+-sensing receptor (CaR) enhances the activity of volume-sensitive outwardly rectifying Cl- channels, thereby facilitating RVD. Therefore, it is possible that Ca2+ efflux stimulated by swelling-induced and P2Y2 receptor-mediated intracellular Ca2+ mobilization activates the CaR, thereby secondarily upregulating the volume-regulatory Cl- conductance. On the other hand, the initial process towards apoptotic cell death is coupled to normotonic cell shrinkage, called apoptotic volume decrease (AVD). Stimulation of death receptors, such as TNF receptor and Fas, induces AVD and thereafter biochemical apoptotic events in human lymphoid (U937), human epithelial (HeLa), mouse neuroblastoma x rat glioma hybrid (NG108-15) and rat phaeochromocytoma (PC12) cells. In those cells exhibiting AVD, facilitation of RVD is always observed. Both AVD induction and RVD facilitation as well as succeeding apoptotic events can be abolished by prior treatment with a blocker of volume-regulatory K+ or Cl- channels, suggesting that AVD is caused by normotonic activation of ion channels that are normally involved in RVD under hypotonic conditions. Therefore, it is likely that G protein-coupled receptors involved in RVD regulation and death receptors triggering AVD may share common downstream signals which should give us key clues to the detailed mechanisms of volume regulation and survival of animal cells. In this Topical Review, we look at the physiological ionic mechanisms of cell volume regulation and cell death-associated volume changes from the facet of receptor-mediated cellular processes.

Figure 1. Schematic illustration of the ionic mechanisms for RVD and RVI under physiological conditions as well as for AVD and NVI under pathophysiological conditions

The pump-leak balance mechanism is depicted in the centre cell. Three different mechanisms for RVI and RVD are depicted in the upper left and upper right cells, respectively. The AVD- and NVI-inducing mechanisms are given in the lower left and lower right cells, respectively. Apoptotic and necrotic cell death may be triggered by persistent cell shrinkage and swelling, as depicted in the left and right cells, respectively. (See text for details.)

Figure 2. Effects of stimulation and inhibition of P2 purinergic receptors (A) and stimulation of CaR (B) on RVD observed in the presence (1 mm: A) and nominal absence (B) of extracellular Ca²⁺ in Intestine 407 cells

A hypotonic challenge (70 % osmolality) was applied at time zero. Cell volume was normalized to that before the hypotonic challenge. Data represent the mean ±s.e.m. (vertical bars) of 5-10 observations. (Data in A are adapted from Dezaki et al. 2000b.)

Figure 3. Schematic model of the molecular mechanism of RVD and its control mediated by G protein-coupled receptors, P2Y₂R and CaR, in Intestine 407 cells

Gq and Gs represent G proteins coupled to phospholipase C (PLC) and adenylate cyclase (AC), respectively. PMCA represents the plasmalemmal Ca²⁺ pump. (See text for details.)

Figure 4. Death receptor-mediated AVD induction and RVD facilitation

Induction of AVD (A) and facilitation of RVD (B) by 2 h treatment with 10 ng ml⁻¹ TNFα (plus 1 μg ml⁻¹ CHX) in PC12 cells (top) or 0.5 μg ml⁻¹ anti-Fas antibody in U937 cells (bottom), and prevention of AVD induction (A) by simultaneous treatment with a K⁺ channel blocker, Ba²⁺ (5 mm), in PC12 cells (top) or a Cl⁻ channel blocker, DIDS (0.5 mm), in U937 cells (bottom). Cell volume measurements were carried out by an electronic cell-sizing technique after washing out any drugs, and the data were normalized by those measured immediately before apoptotic (A) or hypotonic stimulation (B). Data represent the mean ±s.e.m. (vertical bars) of 10 observations. * P < 0.05 vs. corresponding control. (Data from PC12 cells were described in the text, although not shown, in Maeno et al. 2000.)