Signaling in cell proliferation, cell migration, and apoptosis is highly affected by osmotic stress and changes in cell volume, although the mechanisms underlying the significance of cell volume as a signal in cell growth and death are poorly understood. In this study, we used NIH-3T3 fibroblasts in a serum- and nutrient-free inorganic medium (300 mosM) to analyze the effects of osmotic stress on MAPK activity and PDGF receptor (PDGFR)-β-mediated signal transduction. We found that hypoosmolarity (cell swelling at 211 mosM) induced the phosphorylation and nuclear translocation of ERK1/2, most likely via a pathway independent of PDGFR-β and MEK1/2. Conversely, hyperosmolarity (cell shrinkage at 582 mosM) moved nuclear and phosphorylated ERK1/2 to the cytoplasm and induced the phosphorylation and nuclear translocation of p38 and phosphorylation of JNK1/2. In a series of parallel experiments, hypoosmolarity did not affect PDGF-BB-induced activation of PDGFR-β, whereas hyperosmolarity strongly inhibited ligand-dependent PDGFR-β activation as well as downstream mitogenic signal components of the receptor, including Akt and the MEK1/2-ERK1/2 pathway. Based on these results, we conclude that ligand-dependent activation of PDGFR-β and its downstream effectors Akt, MEK1/2, and ERK1/2 is strongly modulated (inhibited) by hyperosmotic cell shrinkage, whereas cell swelling does not seem to affect the activation of the receptor but rather to activate ERK1/2 via a different mechanism. It is thus likely that cell swelling via activation of ERK1/2 and cell shrinkage via activation of the p38 and JNK pathway and inhibition of the PDGFR signaling pathway may act as key players in the regulation of tissue homeostasis.
- mitogen-activated protein kinases
- platelet-derived growth factor receptor-β
- volume sensor
the mechanisms responsible for the maintenance of a constant cell volume under “resting” conditions and for the cell's capability of counteracting volume perturbation by volume recovery processes have been described in a wide variety of cells and have been studied in detail for many years (23, 26, 36). Increasing evidence now supports the conclusion that signaling in cell volume control cooperates with signaling in cell proliferation, cell migration, and apoptosis. However, we still know little as to the function of a change in cell volume as a direct signal in the regulation of these cellular parameters. As discussed by Hoffmann and Ussing (27), the term “volume regulation” may lead to the misconception that cells have one preferred volume, which is not the case since the “preferred” volume may highly depend on the differential and functional state of the cell, such as in secreting epithelia (21, 27) and in cells during growth and proliferation per se. Thus, cell proliferation is often stimulated by osmotic swelling and inhibited by osmotic shrinkage (2, 7, 14, 62), and cell shrinkage is an early event in and a hallmark of the apoptotic mode of programmed cell death (5, 18, 34, 43, 44, 50, 61), such that cell division depends on an increase in cell volume (see Refs. 36 and 62) and is delayed in shrunken cells (49). Indeed, it has been proposed that a change in cell volume acts as an immediate signal that impinges on the apoptotic death process (7, 13, 18, 39, 47, 48, 58, 73) as well as on progression through the cell cycle (36–38). Therefore, it is important to identify the level of cross-talk between signaling pathways in cell volume regulation, cell proliferation, and apoptosis to understand the potential role of changes in cell volume as a direct signal controlling cell growth and death in tissue homeostasis and developmental processes.
Growth control and apoptosis in mammalian cells are highly regulated by receptor tyrosine kinase receptors (RTKs), which, upon ligand-dependent activation, launch the activation of a series of downstream signal transduction pathways, including MAPKs and the cell survival-promoting protein kinase B (Akt) (33, 45). Generally, activation of p38 MAPK and JNK is associated with the inhibition of cell cycle progression and promotion of apoptosis, whereas activation of MEK1/2-ERK1/2 and phosphatidylinositol 3-kinase (PI3K)-Akt pathways inhibit apoptosis and stimulate proliferative responses (9, 12, 18, 20, 25, 67, 67, 70, 77). That osmotic stress activates MAPKs was first described in Saccharomyces cerevisiae (28) and later also in mammalian cells (67). With a few exceptions (see, e.g., Ref. 52), cell shrinkage by hyperosmotic stress is associated with the stimulation of p38 MAPK and JNK (3, 4, 15, 18–20, 31, 42, 46, 56, 61, 61, 69, 74, 75), whereas cell swelling by hypoosmotic stress stimulates ERK1/2 and Akt (31, 63, 73, 79) in concurrence with the conclusion that cell shrinkage is a hallmark of apoptosis and that cell swelling is associated with proliferative responses. Furthermore, changes in cell volume may impinge on the cross-talk between different MAPK cascades, such that p38 MAPK inhibits ERK1/2 following osmotic shrinkage in fibroblasts (18) and Ehrlich-Lettre ascites cells (55), and direct p38 MAPK-mediated inhibition of ERK1/2 has been demonstrated (78). MEK-independent ERK inhibition by p38 has also been demonstrated in heart cells (30).
A number of reports have also implicated a role of RTKs in osmosensing and volume control as upstream regulators of, e.g., MAPKs and the PI3K/Akt pathway. A prominent example is that of the EGF receptor (EGFR) family, including the EGFR in Swiss 3T3 fibroblasts and ErbB4 in cerebellar granule neurons, which are activated by cell swelling induced by hypoosmotic stress following the activation of PI3K/Akt and/or MEK1/2-ERK1/2 pathways (17, 41). Stimulation of the EGFR by cell swelling has also been reported in other cell types (41, 54). Conversely, cell shrinkage by hypertonic stress blocks EGFR-mediated activation of PI3K/Akt and MEK1/2-ERK1/2 pathways in kidney cells, although blockage of signaling is induced immediately downstream of Ras leaving ligand-dependent activation of the receptor unaffected by hypertonic stress (11). Probably these observations are also cell type specific, since hypertonic stress in hepatoma cells mediates the phosphorylation of EGFR, which is required for CD95-mediated initiation of apoptosis (59, 60). Indeed, it is important to realize that the activation or deactivation of MAPKs and other signal pathways during osmotic stress may also highly depend on culture conditions and the strength of the osmotic stress.
In the present study, we used NIH-3T3 fibroblasts to investigate the effect of osmotic stress on the activity of MAPKs and on signal transduction via the RTK PDGF receptor (PDGFR)-β, which plays an important role in cell proliferation, cell survival, cell migration, and angiogenesis in development and tissue homeostasis (46, 65). Significantly, experiments were performed in serum-depleted inorganic medium to eliminate the interference of signal molecules from serum, such as growth factors, that impinge on the activity of RTKs and downstream signal transduction pathways. Furthermore, the inorganic medium allowed us to directly evaluate the differences in the activation of signal pathways during cell swelling and cell shrinkage at which the differences are caused exclusively by differences in osmotic pressure. Thereby, the mechanisms of changes in cell volume can be studied simultaneously in one cell type to investigate the modulation of signal transduction pathways, including the possible causal relationship between MAPKs, PDGFR-β, and the Akt signaling pathway in an attempt to get closer to the understanding of cell volume as a signal in cell proliferation and apoptosis. A portion of these results has previously been presented in abstract form (24).
MATERIALS AND METHODS
Reagents and antibodies.
Unless otherwise stated, reagents were of analytical grade and obtained from Sigma (St. Louis, MO). Recombinant PDGF-BB was from Calbiochem and was used at 25 ng/ml. Primary antibodies [rabbit polyclonal anti-PDGFR-β and anti-phosphorylated (p-)PDGFR-β (p-Tyr857)] were obtained from Santa Cruz Biotechnology; rabbit polyclonal anti-p38 MAPK, anti-p-p38 MAPK (p-Thr180/p-Tyr182), anti-ERK1/2, anti-p-ERK1/2 (p-Thr202/p-Tyr204), anti-p-JNK (detecting both JNK1 and JNK2 phosphorylated at Thr183/Tyr185), and anti-p-MEK1/2 (Ser217/221) were from Cell Signaling Technology (Beverly, MA), as was the mouse monoclonal antibody against p-Akt (which detects Akt1, Akt2, and Akt3 phosphorylated at Ser473). Monoclonal mouse anti-β-actin was from Sigma. The secondary antibody for immunoflourescence microscopy analysis was Cy3-conjugated affinipure goat anti-rabbit IgG (Jackson ImmunoResearch). Secondary antibodies for Western blot analysis were alkaline phosphatase-conjugated goat anti-rabbit and goat anti-mouse IgG (Jackson ImmunoResearch).
Cell culture and saline solutions.
NIH-3T3 cells were maintained in RPMI-1640 medium (Sigma) supplemented with 10% FCS and 1% penicillin-streptomycin (Invitrogen, Carlsbad, CA) in a humidified incubator at 37°C and 5% CO2. Cells were passaged every 3–4 days and only passages 5–25 were used for experiments. Experiments were carried out in cultures with a confluency of 80–90% in the presence and absence of PDGF-BB. For experiments in Ringer solutions, the medium was removed, cells were washed fast twice with PBS, and isotonic, hypotonic, or hypertonic Ringer was then added (defined as time 0). The standard saline Ringer solution had an osmolarity of 300 mosM and contained (in mM) 143 NaCl, 5 KCl, 1 MgSO4, 1 Na2HPO4, 1 CaCl2, 3.3 MOPS, 3.3 TES, and 5 HEPES (pH 7.4). The standard hypertonic Ringer solution was prepared by either the addition of NaCl or sucrose to obtain a final osmolarity of 582 mosM. Sucrose is a mean to induce hypertonicity without changing the ionic strength of the medium. Hypotonic Ringer solution (211 mosM) was prepared by reducing the concentration of all components except MOPS, TES, and HEPES compared with the standard solution. Osmolarities of the saline solutions were regularly controlled using a freezing point osmometer. Unless otherwise indicated, experiments were carried out at 37°C.
Cells incubated in isotonic, hypertonic, and hypotonic Ringer solutions were continuously examined with phase-contrast microscopy on a Nikon Diaphot 300 microscope to monitor changes in cell volume. Images were taken with a Nikon D1 digital camera and processed using Adobe Photoshop version 6.0.
SDS-PAGE and Western blot analysis.
Cell lysates were prepared essentially as previously described (66). Briefly, cells grown to confluence in 78-cm2 culture dishes were washed with ice-cold PBS (136.89 mM NaCl, 2.68 mM KCl, 8.10 mM Na2HPO4, and 1.7 mM KH2HPO4), added to 200 μl of boiling lysis buffer (1% SDS and 10 mM Tris·HCl; pH 7.5), and transferred to Eppendorf tubes using a rubber policeman. Lysates were then homogenized through 27-gauge syringes and spinned down at 16,000 g to clear the homogenate. A 5-μl aliquot was removed for protein determination using the DC Protein Assay from Bio-Rad. Aliquots of ∼20 μg protein (equal amounts in each well for a given experiment, calculated by protein content determination and verified by Ponceau S staining) were resolved under reducing conditions using 10% NuPAGE Bis-Tris gels and NuPAGE MOPS-SDS running buffer (NP0002) as previously described (8) using Fermentas protein standards and a NOVEX XCell system from Invitrogen. Separated proteins were electrotransferred to nitrocellulose membranes using the XCell II Blot Module (NOVEX), followed by staining in 1% Ponceau S red solution. Nitrocellulose membranes were blocked in 5% milk in Tris-buffered saline-Tween 20 [TBST; 10 mM Tris·HCl (pH 7.5), 150 mM NaCl, and 0.1% Tween 20] plus 0.5% Na-azide for 2 h at room temperature and incubated overnight or for 2 h at room temperature with primary antibodies diluted in 5% nonfat dry milk and TBST. All antibodies were diluted to 1:200 except for anti-β-actin, which was used at 1:30,000. Membranes were washed four times for 5 min in TBST, incubated for 1 h with secondary antibodies diluted to 1:600 in 5% milk and TBST, and washed four times for 10 min in TBST prior to being developed with 5-bromo-4-chloro-3-indolyl phosphate/nitroblue tetrazolium solution (KLP, Gaithersburg, MD). Developed blots were scanned, and the band intensity was estimated from arbitrary densitometric values obtained using UN-SCAN-IT software. Blots were further processed for publication in Adobe Photoshop version 6.0.
Immunofluorescence microscopy analysis.
Immunofluorescence microscopy analysis was essentially performed as previously described (66). Primary antibodies were used at 1:50 dilution. Fluorescent secondary antibodies were diluted to 1:600, and 4′,6-diamidino-2-phenylindole (Molecular Probes) was used at 1:1,000 dilution. Cells were visualized on an Eclipse E600 microscope (Nikon) with EPI-FL3 filters and a MagnaFire cooled charge-coupled device camera (Optronics, Goleta, CA), and digital images were processed using Adobe Photoshop version 6.0.
Significance was evaluated using a double-sided Student's t-test, with P < 0.05 taken to indicate a statistically significant difference. All experiments were conducted between three and six times.
Effects of osmotic stress on activation of p38.
The activation of p38 during hypertonic stress (582 mosM) induced by the addition of either NaCl or sucrose as well as hypotonic stress (211 mosM) stress in Ringer solution was initially investigated with antibodies directed against p38 and p-p38 using Western blot analysis. The phosphorylation level of p38 increased slightly after 0.5 min of hypertonic stress induced with NaCl, and this level increased to about fivefold after 10 min of osmotic stress (Fig. 1, A and D). A similar increase in p-p38 was observed in the presence of sucrose (Fig. 1, B and D). In contrast, no increase in p-p38 was observed upon hypotonic (211 mosM) stress during 30 min (Fig. 1, C and D). During a more dramatic osmotic stress (150 mosM), an increase in p-p38 could, however, be observed (data not shown), probably as a general stress response. We then analyzed the cellular localization of p38 and p-p38 by immunofluorescence microscopy after 5 and 30 min of hypertonic (NaCl, 582 mosM) and hypotonic (211 mosM) stress. As demonstrated, p38 primarily localized to the cytoplasm and outside the nucleus in isotonic cells, whereas cells treated with NaCl showed a more prominent nuclear localization of p38 both after 5 (Fig. 1E) and 30 min (Fig. 1G). In hypotonic cells, p38 localized in a pattern similar to that of isotonic cells. We next examined the localization and level of p-p38, which in isotonic cells localized to the cytoplasm and slightly to the nucleus (Fig. 1, F and H). Upon hypertonic stress induced by NaCl, the level of p-p38 dramatically increased and predominantly localized to the nucleus in accordance with the increased activation of p38, as demonstrated by Western blot analysis (Fig. 1A). There was a clear difference with respect to the nuclear localization of p-p38 between 5-min hypertonically treated cells (Fig. 1F) and 30-min hypertonically treated cells (Fig. 1H), showing that the effect cannot be just an effect of the hypertonicity on the antibody but is a real and gradual effect on the nuclear localization caused by hypertonic treatment. In contrast, in hypotonic cells, p38 localized in a pattern similar to that of isotonic cells and similar to that of nonphosphorylated p38 both after 5 and 30 min.
Effects of osmotic stress on activation of ERK1/2.
Activation of ERK1/2 during hypertonic stress (582 mosM) induced by the addition of either NaCl or sucrose as well as hypotonic stress (211 mosM) stress was initially investigated with antibodies directed against ERK1/2 and p-ERK1/2 using Western blot analysis. The phosphorylation level of ERK1/2 was unaffected or slightly reduced upon hypertonic stress (Fig. 2, A, B, and D), whereas hypotonic cells showed a small but significant increase in ERK1/2 phosphorylation after 0.5 min of incubation, and this increase in phosphorylation of ERK1/2 in hypotonic cells appeared throughout the experiment (Fig. 2, C and D). Concomitantly, immunofluorescence microscopy analysis showed that the level and nuclear localization of ERK1/2 characteristically decreased upon hypertonic stress and increased in hypotonic cells (Fig. 2E). In a series of parallel experiments, supplementation of serum in full medium generally induced a high level of ERK1/2 phosphorylation, which was largely unaffected by hypotonic stress (data not shown).
Effects of osmotic stress on activation of MEK1/2.
To investigate the position of MEK1/2 as an upstream regulator of ERK1/2 during osmotic stress, we examined the level of p-MEK1/2 upon hypertonic (582 mosM) and hypotonic (211 mosM) stress. As shown in Fig. 3, there were no significant changes in the activation of MEK1/2 at either osmotic situation, indicating that ERK1/2 upon hypotonic treatment is activated independently of MEK1/2.
Effects of osmotic stress on activation of JNK1/2.
Previously, JNK1/2 has been suggested as a MAPK participating in osmotic stress responses (see Introduction). To investigate this in further detail, cells were subjected to hypertonic stress (582 mosM) induced by NaCl and hypotonic stress (211 mosM) with antibodies directed against p-JNK1/2 using Western blot analysis. Characteristically, hypotonic cells showed no increase in the phosphorylation of JNK1/2, whereas hypertonic stress significantly increased the phosphorylation level of JNK1/2 at 46 kDa after 30 min of incubation (Fig. 4).
Activation of PDGFR-β and its mitogenic downstream components is strongly inhibited in hypertonic cells.
To investigate the effect of hypertonicity on PDGFR-mediated signal transduction, cells were subjected to either isotonic or hypertonic sucrose-Ringer solution (582 mosM) in the presence and absence of PDGF-BB followed by Western blot analysis with antibodies directed against PDGFR-β and p-PDGFR-β as well as the phosphorylated forms of ERK1/2, MEK1/2, Akt, and p38. After 30 s of incubation, PDGF-BB clearly increased the level of p-PDGFR-β about fivefold in isotonic cultures, whereas hypertonic cells showed no increase in p-PDGFR-β (Fig. 5, A and D). The level of p-PDGFR-β in isotonic cells further increased ∼20-fold after 2 and 5 min of incubation, followed by a decrease in receptor phosphorylation after 10 and 30 min of incubation (Fig. 5, A–D), signifying the conventional step in RTK downregulation. In contrast, hypertonic cells showed a very moderate increase in receptor phosphorylation only after 10 and 30 min of incubation. Concomitantly, activation of the mitogenic ERK1/2-MEK1/2 pathway by PDGF-BB after 2, 5, and 10 min of incubation was dramatically reduced in hypertonic cells but increased after 30 min of PDGF-BB stimulation (Fig. 5, E and F) parallel to a moderate increase in phosphorylation of p38 after 10 min in hypertonic cells in the presence of PDGF-BB (Fig. 5, B, C, and G). In addition, PDGF-BB-mediated activation of mitogenic Akt was strongly reduced in hypertonic cells up to 30 min after the addition of the ligand, as shown in the Western blot of p-Akt in Fig. 6A and from the mean intensity of p-Akt after the addition of PDGF-BB in isotonic and hypertonic medium, respectively, in Fig. 6B. Thus, hypertonicity strongly inhibits and delays the early and major ligand-dependent activation of PDGFR-β and the mitogenic downstream components of the receptor in signal transduction.
PDGFR-β is not activated by cell swelling induced by hypotonic stress.
With the aim to examine the effect of hypotonic stress on the activation of PDGFR-β, we performed Western blot analysis on the level of receptor phosphorylation after incubation of cells in hypotonic (211 mosM) Ringer solution, where cells are clearly swollen compared with isotonic cells (Fig. 7A). As evaluated by Western blot analysis, the level of p-PDGFR-β was kept at a low level in both isotonic and hypotonic cells, in clear contrast to the level of p-ERK1/2, which increased about threefold after cells were incubated in hypotonic Ringer solution (Fig. 7, B and C). These results support the conclusion that PDGFR-β is not activated by hypotonic stress and that ERK1/2 in hypotonic cells is activated independently of both the receptor and MEK1/2. To investigate whether hypotonicity affects PDGFR-β activation with PDGF-BB, the intensity of PDGFR-β, ERK1/2, and p38 phosphorylaton was measured in hypotonic Ringer solution relative to the isotonic control as a function of time after the addition of PDGF-BB (25 ng/ml). There were no significant differences in any of the groups. Thus, hypotonicity (cell swelling) does not affect the affinity of PDGFR-β for the agonist.
It is well known that MAPKs and other members of protein kinases in signal transduction, such as the PI3K/Akt pathway, are major targets of osmotic stress in eukaryotic cells (25). However, we still know little as to the precise mechanisms that control either the activation or deactivation of signal transduction components largely because different cell types in cultures produce different results and because the culture conditions chosen to perform the experimentation may vary significantly from one research group to another. In this report, we used NIH-3T3 fibroblasts in inorganic Ringer solution to study changes in protein kinase activity in signal transduction exclusively by differences in osmotic pressure, i.e., independently of variations in nutrient composition and in background signaling originating from growth factors and other ligands present in serum.
Differential changes in activation of MAPKs upon osmotic stress.
In our study, we initially found that ERK1/2 and p38 act oppositely in terms of their activation upon osmotic stress in fibroblasts. Prominently, ERK1/2 is phosphorylated and targeted to the nucleus upon hypotonic Ringer solution (Fig. 2), indicating that cell swelling facilitates progression through the cell cycle, since nuclear translocation of ERK1/2 is a critical step to transduce cell growth (6). In hypertonic Ringer solution, phosphorylated ERK1/2 remained at a level comparable with that of isotonic controls, as evaluated by Western blot analysis. However, immunofluorescence microscopy revealed that hypertonic stress moves nuclear p-Erk1/2 to cytoplasmic puncta, supporting the conclusion that a change in cell volume is a critical step in growth signaling through ERK1/2. Importantly, these observations may be compromised in fibroblasts cultured in the presence of serum, which greatly stimulates the activation of ERK1/2; we have previously shown under these circumstances that cell shrinkage significantly decreases the phosphorylation of ERK1/2. Conversely, hypertonic stress strongly induced an early activation of p38 by phosphorylation and nuclear translocation of p-p38 (Fig. 1), whereas hypotonic stress had no effect on either the level of protein phosphorylation and cellular distribution of p-p38. Similar observations have been previously demonstrated for p38 and ERK1/2 in hypertonic growth medium in the presence of serum in fibroblasts (18), suggesting that cell shrinkage blocks the early activation of receptor-mediated signal transduction in cell survival and growth. Concomitantly, hypertonic stress induced the late activation of JNK, as evaluated by the level of phosphorylation of this MAPK using Western blot analysis (Fig. 4). With respect to the mechanism of p38 activation, studies in HEK-293 cells (76) and fibroblasts (18) have indicated that shrinkage activation of p38 involves Cdc42 and/or Rac, small GTP-binding proteins (G proteins of the Rho family), which have been directly shown to be activated by osmotic shrinkage (40). The physiological consequences of p38 and JNK activation by hyperosmolarity have mainly been studied in long-time experiments where p38 and JNK normally counteract cell proliferation and activate the apoptotic process (9, 18, 57, 64, 77). p38 is also involved in adaptive transcriptional regulation in response to osmotic stress (67), which correlates well with the present finding of a translocation to the nucleus of p-p38. Finally, p38 is so rapidly modulated after cell shrinkage that it can also play a role in the regulation of osmolyte transport during regulatory volume increase, which has also been shown to be the case in some cell types (56, 61, 69).
The mechanisms by which ERK1/2 in fibroblasts is activated upon hypotonic stress are presently unknown. One of the most common pathways leading to ERK1/2 activation is the signaling cascade initiated by growth factor receptors, such as PDGFR, via the small G protein Ras and MEK1/2. However, the hypososmotic activation of ERK1/2 seems not to be caused by activation of this pathway, since neither MEK1/2 (Fig. 3) nor PDGFR-β (Fig. 7) were activated by phosphorylation upon hypotonic stress in Ringer solution. In human embryonic intestinal epithelial cells, ERK1/2 is phosphorylated upon cell swelling via Ras-independent activation of leukotriene D4 (LTD4) (53), and we have previously demonstrated that LTD4 is released after cell swelling in Ehrlich ascites tumor cells and is responsible for the activation of the swelling-activated K+ channel (22, 29, 35, 71). Indeed, ERK1/2 is involved in acute volume regulation by activation of volume regulatory channels (16, 68), adding further complexity to the role of ERK1/2 during changes in cell volume upon osmotic stress.
In conclusion, we prepared a serum- and nutrient-free medium to demonstrate that cell swelling in hypotonic medium induces the phosphorylation and nuclear translocation of ERK1/2 via a pathway independent of signaling via MEK1/2. Conversely, cell shrinkage in hypertonic medium inhibited the nuclear localization of p-ERK1/2 and stimulated the phosphorylation of p38 and JNK1/2 as well as translocation of p38 to the nucleus.
Activation of PDGFR-β and its mitogenic downstream components is inhibited after shrinkage in hypertonic medium but unchanged in hypotonic medium.
To demonstrate the effect of hypertonic stress on ligand-dependent activation of RTK in cultures with a high signal-to-noise ratio, we investigated the activity of PDGFR-β and its signal transduction in fibroblasts in Ringer solution.
PDGF signaling controls a series of cellular responses in cell survival, apoptosis, proliferation, and migration during embryonic and postnatal development as well as in tissue homeostasis (1, 51, 72). Activation of its receptors by dimerization of PDGFR-α and/or PDGFR-β is followed by the phosphorylation of tyrosines on the cytosolic side of the receptors at specific docking sites for other proteins that induce distinct cascades of signal transduction such as PI3K in the Akt pathway and Grb2 in the Ras-Raf-MEK1/2-ERK1/2 pathway (72). In our analysis, we found that hypertonic stress effectively blocks the early and major PDGF-BB-mediated activation of PDGFR-β in fibroblasts and consequently ligand-dependent activation of MEK1/2-ERK1/2 and PI3K/Akt pathways (Fig. 5), supporting the conclusion that shrinkage blocks the activation of receptor-mediated signal transduction in cell survival and growth. It is presently unknown by which mechanisms hypertonic stress affects the activity of the receptor, but our data add further evidence to the role of RTKs in sensing osmotic stress and/or in the registration of changes in cell volume that impinge upon signaling in development and tissue homeostasis. Previously, members of the EGFR family have been shown to become activated upon cell swelling by hypotonic stress in fibroblasts (17, 54), keratinocytes (32), kidney cells, and cerebellar granule neurons (41, 54), in which an increase in cell volume activates EGFRs at a level comparable with that of its natural ligands. Since we were unable to detect any changes in PDGFR-β activity in fibroblasts upon cell swelling induced by hypotonic stress in either the absence (Fig. 7, B and C) or presence of PDGF-BB (Fig. 7D), we suggest that diverse types of RTKs may respond differently to changes in cell volume and/or osmotic stress, although it is currently unknown whether ligand-dependent activation of the EGFR is affected by hypertonic stress in these cells.
Hypertonic stress has a major impact on ligand-dependent activation of PDGFR-β, and it is likely that inhibition of its downstream components in signal transduction is brought about in combination with a direct inhibition of both PDGFR-β and mechanisms downstream of the receptor. Hypertonic stress has previously been shown to reduce Akt activity by inhibiting the specific phosphorylation of Akt and/or by promoting PP2A-mediated dephosphorylation of Akt (46), but certainly inhibition of ligand-dependent receptor autophosphorylation prevents the receptor docking of PI3K and activation of the PI3K/Akt pathway as well. The docking and activation of other receptor adaptor proteins in PDGFR-β-mediated signal transduction in fibroblasts, such as Grb2, PLC-γ, Src, and Shp2, may be affected in a similar manner in hypertonic cells, preventing receptor transduction. Another important observation is that hypertonic stress-induced phosphorylation of p38 is increased in PDGF-BB-stimulated cells and concomitantly with a decrease in the activation of Akt, MEK1/2, and ERK1/2, whereas p38 phosphorylation in isotonic cells is maintained at a low level in both the absence and presence of the ligand (Fig. 5). These observations indicate the presence of functional cross-talk between different MAPK pathways during osmotic stress in fibroblasts, in which inhibition of PDGFR-mediated signal transduction components, such as ERK1/2, is likely to be associated with an increase in p38 activity in fibroblasts.
The findings that signaling through RTKs are strongly affected by an increase in osmotic pressure open up a new realm of opportunities in investigating cellular signaling in development and tissue homeostasis. In this scenario, PDGFR-β may potentially function as part of a volume sensory unit involved after cell shrinkage. PDGFR-β-mediated signaling through the PI3K/Akt pathway is an important regulator of the balance between cell survival and cell death, by which Akt promotes cell survival by phosphorylating and inactivating a number of proteins involved in apoptosis (10). We have previously demonstrated that inhibition of Akt and MEK1/2-ERK1/2 by hypertonic stress in serum-stimulated fibroblasts is associated with cell death by apoptosis (18). It is therefore important to realize that an osmotic change in the extracellular milieu may act as a key player in the regulation of tissue homeostasis and in developmental processes, since deficiencies in PDGFR signaling lead to a plethora of human diseases and disorders such as neurodegenerative diseases and cancer (1, 51).
This work was supported by Danish Natural Sciences Research Foundation Grant 21-04-0535 (to E. K. Hoffmann and S. T. Christensen), Carlsberg Foundation Grant 0894-10 (to E. K. Hoffmann), Danish Cancer Society Grant DP05072 (to E. K. Hoffmann and S. T. Christensen), and Novo Nordic (to E. K. Hoffmann).
We are indebted to Birthe J. Hansen and Ecaterina Magnussen for excellent technical assistance.
↵* S. T. Christensen and E. K. Hoffmann contributed equally to this work.
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- Copyright © 2008 the American Physiological Society