Pathophysiology and puzzles of the volume-sensitive outwardly rectifying anion channel

Abstract

Cell swelling activates or upregulates a number of anion channels. Of the volume-activated or -regulated anion channels (VAACs or VRACs), the volume-sensitive outwardly rectifying anion channel (VSOR) is most prominently activated and ubiquitously expressed. This channel is known to be involved in a variety of physiological processes including cell volume regulation, cell proliferation, differentiation and cell migration as well as cell turnover involving apoptosis. Recent studies have shown that VSOR activity is also involved in a number of pathophysiological processes including the acquisition of cisplatin resistance by cancer cells, ischaemia-reperfusion-induced death of cardiomyocytes and hippocampal neurons, glial necrosis under lactacidosis as well as neuronal necrosis under excitotoxicity. Moreover, VSOR serves as the pathway for glutamate release from astrocytes under ischaemic conditions and when stimulated by bradykinin, an initial mediator of inflammation. So far, many signalling molecules including the EGF receptor, PI3K, Src, PLCgamma and Rho/Rho kinase have been implicated in the regulation of VSOR activity. However, our pharmacological studies suggest that these signals are not essential components of the swelling-induced VSOR activation mechanism even though some of these signals may play permissive or modulatory roles. Molecular identification of VSOR is required to address the question of how cells sense volume expansion and activate VSOR.

Figure 1. Roles of VSOR activity in apoptotic and necrotic cell death

Top, roles of VSOR activity in the apoptotic volume decrease (AVD) in response to apoptotic stimuli, ischaemia–reperfusion and acquired cisplatin resistance of cancer cells. Bottom, roles of VSOR activity in the necrotic volume increase (NVI) in response to lactacidosis or excitotoxicity. For details, see the text. Abbreviations: STS, staurosporine; FasL, Fas ligand; TNFα, tumor necrosis factor α; ROS, reactive oxygen species; GluR, glutamate receptor; NHE, Na⁺/H⁺ exchanger; MCT, monocarboxylate transporter; RVI, regulatory volume increase; RVD, regulatory volume decrease.

Figure 2. Roles of VSOR activity in glutamate release from astrocytes in response to ischaemia- or hypotonicity-induced cell swelling and to bradykinin-induced ROS production

For details, see the text. Abbreviations: B₂R, B2 bradykinin receptor; ROS, reactive oxygen species.

Figure 3. Regulation of VSOR activity by signalling molecule cascades

To date, the following intracellular signalling cascades have been observed to be activated upon osmotic cell swelling and have been reported to be associated with activation or augmentation of VSOR activity: the Ras–Raf–MEK–ERK, PI3K–NOX–H₂O₂, Src–PLCγ–Ca²⁺ and Rho–ROK–MLCK pathways. The earliest reports for these signals are as follows: angiotensin II type 1 (AT₁R) (Browe & Baumgarten, 2004), epidermal growth factor receptor (EGFR) (Tilly et al. 1993), mitogen-activated protein kinase kinase (MEK) (Crepel et al. 1998), extracellular signal-regulated kinase (ERK) (Tilly et al. 1993), phosphatidylinositol 3-kinase (PI3K) (Feranchak et al. 1998), NADPH oxidase (NOX) and H₂O₂ (Varela et al. 2004), tyrosine kinase Src (Lepple-Wienhues et al. 1998), phospholipase Cγ (PLCγ) (Varela et al. 2007), Ca²⁺ (Basavappa et al. 1995), low molecular weight GTPase Rho (Tilly et al. 1996), Rho kinase (ROK) (Nilius et al. 1999), and myosin light chain kinase (MLCK) (Nilius et al. 2000).

Figure 4. Pharmacological characterization of VSOR whole-cell currents

Whole-cell recordings were performed in the absence (Control) or presence of the indicated agents, as described in the legend for Table 1, before (Isotonic: iso) and during hypotonic stimulation (Hypotonic: hypo). Arrowheads represent the zero current level. Each symbol in the I–V curves represents the mean value of instantaneous current density (measured 5 ms after the onset of the test pulse) ±s.e.m. (vertical bar).

Figure 5. Volume expansion sensitivity of VSOR activity

The relationship between relative cell surface area and VSOR current density is shown. Arrowhead indicates the threshold cell volume for VSOR activation in response to cell swelling. Filled and open arrows represent augmentation of the volume expansion sensitivity of VSOR by decreasing the threshold without changing the slope (broken line) and by increasing the slope without changing the threshold (dotted line). For details, see the text.