Effects of osmotic stress on the activity of MAPKs and PDGFR-β-mediated signal transduction in NIH-3T3 fibroblasts

Contact Blot: Microfluidic Control and Measurement of Cell-Cell Contact State to Assess Contact-Inhibited ERK Signaling

Extracellular signal-regulated kinase (ERK) signaling is essential to regulated cell behaviors, including cell proliferation, differentiation, and apoptosis. The influence of cell-cell contacts on ERK signaling is central to epithelial cells, yet few studies have sought to understand the same in cancer cells, particularly with single-cell resolution. To acquire same-cell measurements of both phenotypic (cell-contact state) and targeted-protein profile (ERK phosphorylation), we prepend high-content, whole-cell imaging prior to endpoint cellular-resolution western blot analyses for each of hundreds of individual HeLa cancer cells cultured on that same chip, which we call contact Blot. By indexing the phosphorylation level of ERK in each cell or cell-cluster to the imaged cell-contact state, we compare ERK signaling between isolated and in-contact cells. We observe attenuated (~2×) ERK signaling in HeLa cells which are in-contact versus isolated. Attenuation is sustained when the HeLa cells are challenged with hyperosmotic stress. Our findings show the impact of cell-cell contacts on ERK activation with isolated and in-contact cells, while introducing a multi omics tool for control and scrutiny of cell-cell interactions.

Actin dynamics in protein homeostasis

Cell homeostasis is maintained in all organisms by the constant adjustment of cell constituents and organisation to account for environmental context. Fine-tuning of the optimal balance of proteins for the conditions, or protein homeostasis, is critical to maintaining cell homeostasis. Actin, a major constituent of the cytoskeleton, forms many different structures which are acutely sensitive to the cell environment. Furthermore, actin structures interact with and are critically important for the function and regulation of multiple factors involved with mRNA and protein production and degradation, and protein regulation. Altogether, actin is a key, if often overlooked, regulator of protein homeostasis across eukaryotes. In this review, we highlight these roles and how they are altered following cell stress, from mRNA transcription to protein degradation.

Importance of the Microenvironment and Mechanosensing in Adipose Tissue Biology

The expansion of adipose tissue is an adaptive mechanism that increases nutrient buffering capacity in response to an overall positive energy balance. Over the course of expansion, the adipose microenvironment undergoes continual remodeling to maintain its structural and functional integrity. However, in the long run, adipose tissue remodeling, typically characterized by adipocyte hypertrophy, immune cells infiltration, fibrosis and changes in vascular architecture, generates mechanical stress on adipose cells. This mechanical stimulus is then transduced into a biochemical signal that alters adipose function through mechanotransduction. In this review, we describe the physical changes occurring during adipose tissue remodeling, and how they regulate adipose cell physiology and promote obesity-associated dysfunction in adipose tissue.

Exogenous Melatonin Ameliorates the Negative Effect of Osmotic Stress in Human and Bovine Ovarian Stromal Cells

Ovarian tissue cryopreservation transplantation (OTCT) is the most flexible option to preserve fertility in women and children with cancer. However, OTCT is associated with follicle loss and an accompanying short lifespan of the grafts. Cryopreservation-induced damage could be due to cryoprotective agent (CPA) toxicity and osmotic shock. Therefore, one way to avoid this damage is to maintain the cell volume within osmotic tolerance limits (OTLs). Here, we aimed to determine, for the first time, the OTLs of ovarian stromal cells (OSCs) and their relationship with reactive oxygen species (ROS) and mitochondrial respiratory chain activity (MRCA) of OSCs. We evaluated the effect of an optimal dose of melatonin on OTLs, viability, MRCA, ROS and total antioxidant capacity (TAC) of both human and bovine OSCs in plated and suspended cells. The OTLs of OSCs were between 200 and 375 mOsm/kg in bovine and between 150 and 500 mOsm/kg in human. Melatonin expands OTLs of OSCs. Furthermore, melatonin significantly reduced ROS and improved TAC, MRCA and viability. Due to the narrow osmotic window of OSCs, it is important to optimize the current protocols of OTCT to maintain enough alive stromal cells, which are necessary for follicle development and graft longevity. The addition of melatonin is a promising strategy for improved cryopreservation media.

Tumorigenic mesenchymal clusters are less sensitive to moderate osmotic stresses due to low amounts of junctional E-cadherin

The migration of tumorigenic cells is a critical step for metastatic breast cancer progression. Although the role of the extracellular matrix in breast cancer cell migration has been extensively described, the effect of osmotic stress on the migration of tumor breast cohorts remains unclear. Most of our understanding on the effect of osmotic stresses on cell migration comes from studies at the level of the single cell in isolation and does not take cell-cell interactions into account. Here, we study the impact of moderate osmotic stress on the migration of cell clusters composed of either non-tumorigenic or tumorigenic cells. We observe a decrease in migration distance and speed for non-tumorigenic cells but not for tumorigenic ones. To explain these differences, we investigate how osmotic stress impacts the mechanical properties of cell clusters and affects their volumes. Our findings show that tumorigenic mesenchymal cells are less sensitive to osmotic stress than non-tumorigenic cells and suggest that this difference is associated with a lower expression of E-cadherin. Using EGTA treatments, we confirm that the establishment of cell-cell adhesive interactions is a key component of the behavior of cell clusters in response to osmotic stress. This study provides evidence on the low sensitivity of mesenchymal tumorigenic clusters to moderate osmotic stress and highlights the importance of cadherin-based junctions in the response to osmotic stress.

Exosomal Vimentin from Adipocyte Progenitors Protects Fibroblasts against Osmotic Stress and Inhibits Apoptosis to Enhance Wound Healing

Mechanical stress following injury regulates the quality and speed of wound healing. Improper mechanotransduction can lead to impaired wound healing and scar formation. Vimentin intermediate filaments control fibroblasts’ response to mechanical stress and lack of vimentin makes cells significantly vulnerable to environmental stress. We previously reported the involvement of exosomal vimentin in mediating wound healing. Here we performed in vitro and in vivo experiments to explore the effect of wide-type and vimentin knockout exosomes in accelerating wound healing under osmotic stress condition. Our results showed that osmotic stress increases the size and enhances the release of exosomes. Furthermore, our findings revealed that exosomal vimentin enhances wound healing by protecting fibroblasts against osmotic stress and inhibiting stress-induced apoptosis. These data suggest that exosomes could be considered either as a stress modifier to restore the osmotic balance or as a conveyer of stress to induce osmotic stress-driven conditions.

Molecular characterization of p38 MAPK and tissue-specific expression under cadmium stress in red swamp crayfish (Procambarus clarkii)

Keeping harmful pollutants out of some crucial tissues as much as possible is a key trait for the organism to survive in adverse conditions, such as in heavy-metal contaminated aquatic environments. In the current study, it was hypothesized that the p38 Mitogen-activated Protein Kinase (MAPK) controls the distribution of cadmium (Cd) in red swamp crayfish (Procambarus clarkii) by regulating the accumulation of Cd in different tissues under Cd-stressed conditions. To test this hypothesis, this study analyzed the p38 MAPK gene and compared the differential expression levels in the heart, antennal gland, gill, hepatopancreas, and muscles. Differences in expression levels of p38 MAPK gene between different tissues were conducted under controlled Cd exposure. This study found that the expression of p38 MAPK is tissue-specific in all tested samples under non-stressed condition. Under Cd-stressed condition and with the prolongation of Cd exposure time, the content of Cd in all examined tissues of P. clarkii has substantially increased compared to the control, although the Cd contents in the heart, antennal gland, and muscle remained relatively lower than those observed in the hepatopancreas. Consequently, the levels of p38 MAPK in the heart, antennal gland, and muscle were higher than the level in the hepatopancreas. These results indicate that p38 MAPK regulates the distribution and accumulation of Cd in different tissues of P. clarkii under Cd-stressed condition. Furthermore, the results suggested that the higher expression of p38 MAPK played a crucial role in keeping Cd out of the tissues in the Cd-rich aquatic environment to maintain the normal physiological function of the red swamp crayfish, a process necessary for survival and growth.

In Situ Single-Cell Western Blot on Adherent Cell Culture

Integrating 2D culture of adherent mammalian cells with single-cell western blotting (in situ scWB) uses microfluidic design to eliminate the requirement for trypsin release of cells to suspension, prior to single-cell isolation and protein analysis. To assay HeLa cells from an attached starting state, we culture adherent cells in fibronectin-functionalized microwells formed in a thin layer of polyacrylamide gel. To integrate the culture, lysis, and assay workflow, we introduce a one-step copolymerization process that creates protein-decorated microwells. After single-cell culture, we lyse each cell in the microwell and perform western blotting on each resultant lysate. We observe cell spreading after overnight microwell-based culture. scWB reports increased phosphorylation of MAP kinases (ERK1/2, p38) under hypertonic conditions. We validate the in situ scWB with slab-gel western blot, while revealing cell-to-cell heterogeneity in stress responses.

Mitogen-activated protein kinases as key players in osmotic stress signaling

BACKGROUND

Osmotic stress arises from the difference between intracellular and extracellular osmolality. It induces cell swelling or shrinkage as a consequence of water influx or efflux, which threatens cellular activities. Mitogen-activated protein kinases (MAPKs) play central roles in signaling pathways in osmotic stress responses, including the regulation of intracellular levels of inorganic ions and organic osmolytes.

SCOPE OF REVIEW

The present review summarizes the cellular osmotic stress response and the function and regulation of the vertebrate MAPK signaling pathways involved. We also describe recent findings regarding apoptosis signal-regulating kinase 3 (ASK3), a MAP3K member, to demonstrate its regulatory effects on signaling molecules beyond MAPKs.

MAJOR CONCLUSIONS

MAPKs are rapidly activated by osmotic stress and have diverse roles, such as cell volume regulation, gene expression, and cell survival/death. There is significant cell type specificity in the function and regulation of MAPKs. Based on its activity change during osmotic stress and its regulation of the WNK1-SPAK/OSR1 pathway, ASK3 is expected to play important roles in osmosensing mechanisms and cellular functions related to osmoregulation.

GENERAL SIGNIFICANCE

MAPKs are essential for various cellular responses to osmotic stress; thus, the identification of the upstream regulators of MAPK pathways will provide valuable clues regarding the cellular osmosensing mechanism, which remains elusive in mammals. The elucidation of in vivo MAPK functions is also important because osmotic stress in physiological and pathophysiological conditions often results from changes in the intracellular osmolality. These studies potentially contribute to the establishment of therapeutic strategies against diseases that accompany osmotic perturbation.

Regulation of p53 in NIH3T3 mouse fibroblasts following hyperosmotic stress

The aim of this project was to analyze the regulation of p53 expression in NIH3T3 fibroblasts under the influence of increasing hyperosmotic stress. Expression of p53 showed a biphasic response pattern in NIH3T3 cells under increasing osmotic stress (337 mOsm to 737 mOsm) with a maximum at 587 mOsm. Under isotonic conditions p53 expression increased after addition of the proteasome inhibitor MG132 indicating that cellular p53 levels in unperturbed cells is kept low by proteasomal degradation. However, under hypertonic conditions p53 synthesis as well as p53 degradation were significantly reduced and it is demonstrated that the increase in p53 expression observed when tonicity is increased from 337 to 587 mOsm reflects that degradation is more inhibited than synthesis, whereas the decrease in p53 expression at higher tonicities reflects that synthesis is more inhibited than degradation. The activity of the p53 regulating proteins p38 MAP kinase and the ubiquitin ligase MDM2 were studied as a function of increasing osmolarity. MDM2 protein expression was unchanged at all osmolarities, whereas MDM2 phosphorylation (Ser¹⁶⁶) increased at osmolarities up to 537 mOsm and remained constant at higher osmolarities. Phosphorylation of p38 increased at osmolarities up to 687 mOsm which correlated with an increased phosphorylation of p53 (Ser¹⁵) and the decreased p53 degradation. Caspase-3 activity increased gradually with hypertonicity and at 737 mOsm both Caspase-3 activity and annexin V binding are high even though p53 expression and activity are low, indicating that initiation of apoptosis under severe hypertonic conditions is not strictly controlled by p53.

mTOR ensures increased release and reduced uptake of the organic osmolyte taurine under hypoosmotic conditions in mouse fibroblasts

Mammalian target of rapamycin (mTOR) is a serine/threonine kinase that modulates translation in response to growth factors and alterations in nutrient availability following hypoxia and DNA damage. Here we demonstrate that mTOR activity in Ehrlich Lettré ascites (ELA) cells is transiently increased within minutes following osmotic cell swelling and that inhibition of phosphatidylinositol-3-phosphatase (PTEN) counteracts the upstream phosphatidylinositol kinase and potentiates mTOR activity. PTEN inhibition concomitantly potentiates swelling-induced taurine release via the volume-sensitive transporter for organic osmolytes and anion channels (VSOAC) and enhances swelling-induced inhibition of taurine uptake via the taurine-specific transporter (TauT). Chronic osmotic stress, i.e., exposure to hypotonic or hypertonic media for 24 h, reduces and increases mTOR activity in ELA cells, respectively. Using rapamycin, we demonstrate that mTOR inhibition is accompanied by reduction in TauT activity and increase in VSOAC activity in cells expressing high (NIH3T3 fibroblasts) or low (ELA) amounts of mTOR protein. The effect of mTOR inhibition on TauT activity reflects reduced TauT mRNA, TauT protein abundance, and an overall reduction in protein synthesis, whereas the effect on VSOAC is mimicked by catalase inhibition and correlates with reduced catalase mRNA abundance. Hence, mTOR activity favors loss of taurine following hypoosmotic cell swelling, i.e., release via VSOAC and uptake via TauT during acute hypotonic exposure is potentiated and reduced, respectively, by phosphorylation involving mTOR and/or the kinases upstream to mTOR. Decrease in TauT activity during chronic hypotonic exposure, on the other hand, involves reduction in expression/activity of TauT and enzymes in antioxidative defense.

Loss of Arp2/3 induces an NF-κB-dependent, nonautonomous effect on chemotactic signaling

A decrease in Arp2/3 levels results in an NF-κB–dependent increase in the expression of several secreted factors, resulting in nonautonomous effects on chemotaxis.

Arp2/3-branched actin is critical for cytoskeletal dynamics and cell migration. However, perturbations and diseases affecting this network have phenotypes that cannot be fully explained by cell-autonomous effects. In this paper, we report nonautonomous effects of Arp2/3 depletion. We show that, upon Arp2/3 depletion, the expression of numerous genes encoding secreted factors, including chemokines, growth factors, and matrix metalloproteases, was increased, a signature resembling the senescence-associated secretory phenotype. These factors affected epidermal growth factor chemotaxis in a nonautonomous way, resolving the recent contradictions about the role of Arp2/3 in chemotaxis. We demonstrate that these genes were activated by nuclear factor κB via a CCM2–MEKK3 pathway that has been implicated in hyperosmotic stress signaling. Consistent with this, Arp2/3-depleted cells showed misregulation of volume control and reduced actin in the submembranous cortex. The defects in osmotic signaling in the Arp2/3-depleted cells can be rescued by hypoosmotic treatment. Thus, perturbations of Arp2/3 have nonautonomous effects that should be considered when evaluating experimental manipulations and diseases affecting the Arp2/3-actin cytoskeleton.

Mechanical properties of growing melanocytic nevi and the progression to melanoma

Melanocytic nevi are benign proliferations that sometimes turn into malignant melanoma in a way that is still unclear from the biochemical and genetic point of view. Diagnostic and prognostic tools are then mostly based on dermoscopic examination and morphological analysis of histological tissues. To investigate the role of mechanics and geometry in the morpholgical dynamics of melanocytic nevi, we study a computation model for cell proliferation in a layered non-linear elastic tissue. Numerical simulations suggest that the morphology of the nevus is correlated to the initial location of the proliferating cell starting the growth process and to the mechanical properties of the tissue. Our results also support that melanocytes are subject to compressive stresses that fluctuate widely in the nevus and depend on the growth stage. Numerical simulations of cells in the epidermis releasing matrix metalloproteinases display an accelerated invasion of the dermis by destroying the basal membrane. Moreover, we suggest experimentally that osmotic stress and collagen inhibit growth in primary melanoma cells while the effect is much weaker in metastatic cells. Knowing that morphological features of nevi might also reflect geometry and mechanics rather than malignancy could be relevant for diagnostic purposes.

Ion channels and transporters in the development of drug resistance in cancer cells

Multi-drug resistance (MDR) to chemotherapy is the major challenge in the treatment of cancer. MDR can develop by numerous mechanisms including decreased drug uptake, increased drug efflux and the failure to undergo drug-induced apoptosis. Evasion of drug-induced apoptosis through modulation of ion transporters is the main focus of this paper and we demonstrate how pro-apoptotic ion channels are downregulated, while anti-apoptotic ion transporters are upregulated in MDR. We also discuss whether upregulation of ion transport proteins that are important for proliferation contribute to MDR. Finally, we discuss the possibility that the development of MDR involves sequential and localized upregulation of ion channels involved in proliferation and migration and a concomitant global and persistent downregulation of ion channels involved in apoptosis.

Role of ion transport in control of apoptotic cell death

Cell shrinkage is a hallmark and contributes to signaling of apoptosis. Apoptotic cell shrinkage requires ion transport across the cell membrane involving K(+) channels, Cl(-) or anion channels, Na(+)/H(+) exchange, Na(+),K(+),Cl(-) cotransport, and Na(+)/K(+)ATPase. Activation of K(+) channels fosters K(+) exit with decrease of cytosolic K(+) concentration, activation of anion channels triggers exit of Cl(-), organic osmolytes, and HCO3(-). Cellular loss of K(+) and organic osmolytes as well as cytosolic acidification favor apoptosis. Ca(2+) entry through Ca(2+)-permeable cation channels may result in apoptosis by affecting mitochondrial integrity, stimulating proteinases, inducing cell shrinkage due to activation of Ca(2+)-sensitive K(+) channels, and triggering cell-membrane scrambling. Signaling involved in the modification of cell-volume regulatory ion transport during apoptosis include mitogen-activated kinases p38, JNK, ERK1/2, MEKK1, MKK4, the small G proteins Cdc42, and/or Rac and the transcription factor p53. Osmosensing involves integrin receptors, focal adhesion kinases, and tyrosine kinase receptors. Hyperosmotic shock leads to vesicular acidification followed by activation of acid sphingomyelinase, ceramide formation, release of reactive oxygen species, activation of the tyrosine kinase Yes with subsequent stimulation of CD95 trafficking to the cell membrane. Apoptosis is counteracted by mechanisms involved in regulatory volume increase (RVI), by organic osmolytes, by focal adhesion kinase, and by heat-shock proteins. Clearly, our knowledge on the interplay between cell-volume regulatory mechanisms and suicidal cell death is still far from complete and substantial additional experimental effort is needed to elucidate the role of cell-volume regulatory mechanisms in suicidal cell death.

The apoptotic volume decrease is an upstream event of MAP kinase activation during Staurosporine-induced apoptosis in HeLa cells

Persistent cell shrinkage, called apoptotic volume decrease (AVD), is a pivotal event of apoptosis. Activation of the volume-sensitive outwardly rectifying Cl⁻ channel (VSOR) is involved in the AVD induction. On the other hand, activation of the MAP kinase (MAPK) cascade is also known to play a critical role in apoptosis. In the present study, we investigated the relationship between the AVD induction and the stress-responsive MAPK cascade activation during the apoptosis process induced by staurosporine (STS) in HeLa cells. STS was found to induce AVD within 2–5 min and phosphorylation of c-Jun N-terminal kinase (JNK) and p38 MAPK after over 20–30 min. VSOR blockers suppressed not only STS-induced AVD but also phosphorylation of JNK and p38 as well as activation of caspase-3/7. Moreover, a p38 inhibitor, SB203580, and a JNK inhibitor, SP600125, failed to affect STS-induced AVD, whereas these compounds reduced STS-induced activation of caspase-3/7. Also, treatment with ASK1-specific siRNA suppressed STS-induced caspase-3/7 activation without affecting the AVD induction. Furthermore, sustained osmotic cell shrinkage per se was found to trigger phosphorylation of JNK and p38, caspase activation, and cell death. Thus, it is suggested that activation of p38 and JNK is a downstream event of AVD for the STS-induced apoptosis of HeLa cells.

Effects of A-CREB, a dominant negative inhibitor of CREB, on the expression of c-fos and other immediate early genes in the rat SON during hyperosmotic stimulation in vivo

Intraperitoneal administration of hypertonic saline to the rat supraoptic nucleus (SON) increases the expression of several immediate early genes (IEG) and the vasopressin gene. These increases have usually been attributed to action of the cyclic-AMP Response Element Binding Protein (CREB). In this paper, we study the role of CREB in these events in vivo by delivering a potent dominant-negative form of CREB, known as A-CREB, to the rat SON through the use of an adeno-associated viral (AAV) vector. Preliminary experiments on HEK 293 cells in vitro showed that the A-CREB vector that we used completely eliminated CREB-induced c-fos expression. We stereotaxically injected this AAV-A-CREB into one SON and a control AAV into the contralateral SON of the same rat. Two weeks following these injections we injected hypertonic saline intraperitoneally into the rat. Using this paradigm, we could measure the relative effects of inhibiting CREB on the induced expression of c-fos, ngfi-a, ngfi-b, and vasopressin genes in the A-CREB AAV injected SON versus the control AAV injected SON in the same rat. We found only a small (20%) decrease of c-fos expression and a 30% decrease of ngfi-b expression in the presence of the A-CREB. There were no significant changes in expression found in the other IEGs nor in vasopressin that were produced by the A-CREB. This suggests that CREB may play only a minor role in the expression of IEGs and vasopressin in the osmotically activated SON in vivo.

Ion channels involved in cell volume regulation: effects on migration, proliferation, and programmed cell death in non adherent EAT cells and adherent ELA cells

This mini review outlines studies of cell volume regulation in two closely related mammalian cell lines: nonadherent Ehrlich ascites tumour cells (EATC) and adherent Ehrlich Lettre ascites (ELA) cells. Focus is on the regulatory volume decrease (RVD) that occurs after cell swelling, the volume regulatory ion channels involved, and the mechanisms (cellular signalling pathways) that regulate these channels. Finally, I shall also briefly review current investigations in these two cell lines that focuses on how changes in cell volume can regulate cell functions such as cell migration, proliferation, and programmed cell death.

Stress clamp experiments on multicellular tumor spheroids

The precise role of the microenvironment on tumor growth is poorly understood. Whereas the tumor is in constant competition with the surrounding tissue, little is known about the mechanics of this interaction. Using a novel experimental procedure, we study quantitatively the effect of an applied mechanical stress on the long-term growth of a spheroid cell aggregate. We observe that a stress of 10 kPa is sufficient to drastically reduce growth by inhibition of cell proliferation mainly in the core of the spheroid. We compare the results to a simple numerical model developed to describe the role of mechanics in cancer progression.

Permissive effect of EGFR-activated pathways on RVI and their anti-apoptotic effect in hypertonicity-exposed mIMCD3 cells

Hypertonicity is a stressful stimulus leading to cell shrinkage and apoptotic cell death. Apoptosis can be prevented if cells are able to activate the mechanism of RVI (regulatory volume increase). This study in mIMCD3 cells presents evidence of a permissive role of the EGFR (epidermal growth factor receptor) on RVI, achieved for the most part through the two main EGFR-triggered signalling chains, the MAPK (mitogen-activated protein kinase)/ERK (extracellular-signal-regulated kinase) and the PI3K (phosphoinositide 3-kinase)/Akt (also known as protein kinase B) pathways. Hyperosmotic solutions (450 mosM) made by addition of NaCl, increased EGFR phosphorylation, which is prevented by GM6001 and AG1478, blockers respectively, of MMPs (matrix metalloproteinases) and EGFR. Inhibition of EGFR, ERK (PD98059) or PI3K/Akt (wortmannin) phosphorylation reduced RVI by 60, 48 and 58% respectively. The NHE (Na+/H+ exchanger) seems to be the essential mediator of this effect since (i) NHE is the main contributor to RVI, (ii) EGFR, ERK and PI3K/Akt blockers added together with the NHE blocker zoniporide reduce RVI by non-additive effects and (iii) All the blockers significantly lowered the NHE rate in cells challenged by an NH4Cl pulse. Besides reducing RVI, the inhibition of MMP, EGFR and PI3K/Akt had a strong pro-apoptotic effect increasing cell death by 2–3.7-fold. This effect was significantly lower when RVI inhibition did not involve the EGFR-PI3K/Akt pathway. These results provide evidence that Akt and its permissive effect on RVI have a predominant influence on cell survival under hypertonic conditions in IMCD3 cells. This role of Akt operates under the influence of EGFR activation, promoted by MMP.

Osmosensory mechanisms in cellular and systemic volume regulation

Perturbations of cellular and systemic osmolarity severely challenge the function of all organisms and are consequently regulated very tightly. Here we outline current evidence on how cells sense volume perturbations, with particular focus on mechanisms relevant to the kidneys and to extracellular osmolarity and whole body volume homeostasis. There are a variety of molecular signals that respond to perturbations in cell volume and osmosensors or volume sensors responding to these signals. The early signals of volume perturbation include integrins, the cytoskeleton, receptor tyrosine kinases, and transient receptor potential channels. We also present current evidence on the localization and function of central and peripheral systemic osmosensors and conclude with a brief look at the still limited evidence on pathophysiological conditions associated with deranged sensing of cell volume.

Cell volume homeostatic mechanisms: effectors and signalling pathways

Cell volume homeostasis and its fine-tuning to the specific physiological context at any given moment are processes fundamental to normal cell function. The understanding of cell volume regulation owes much to August Krogh, yet has advanced greatly over the last decades. In this review, we outline the historical context of studies of cell volume regulation, focusing on the lineage started by Krogh, Bodil Schmidt-Nielsen, Hans-Henrik Ussing, and their students. The early work was focused on understanding the functional behaviour, kinetics and thermodynamics of the volume-regulatory ion transport mechanisms. Later work addressed the mechanisms through which cellular signalling pathways regulate the volume regulatory effectors or flux pathways. These studies were facilitated by the molecular identification of most of the relevant channels and transporters, and more recently also by the increased understanding of their structures. Finally, much current research in the field focuses on the most up- and downstream components of these paths: how cells sense changes in cell volume, and how cell volume changes in turn regulate cell function under physiological and pathophysiological conditions.

Osmotic stress alters chromatin condensation and nucleocytoplasmic transport

Osmotic stress is a potent regulator of biological function in many cell types, but its mechanism of action is only partially understood. In this study, we examined whether changes in extracellular osmolality can alter chromatin condensation and the rate of nucleocytoplasmic transport, as potential mechanisms by which osmotic stress can act. Transport of 10 kDa dextran was measured both within and between the nucleus and the cytoplasm using two different photobleaching methods. A mathematical model was developed to describe fluorescence recovery via nucleocytoplasmic transport. As osmolality increased, the diffusion coefficient of dextran decreased in the cytoplasm, but not the nucleus. Hyper-osmotic stress decreased nuclear size and increased nuclear lacunarity, indicating that while the nucleus was getting smaller, the pores and channels interdigitating the chromatin had expanded. The rate of nucleocytoplasmic transport was increased under hyper-osmotic stress but was insensitive to hypo-osmotic stress, consistent with the nonlinear osmotic properties of the nucleus. The mechanism of this osmotic sensitivity appears to be a change in the size and geometry of the nucleus, resulting in a shorter effective diffusion distance for the nucleus. These results may explain physical mechanisms by which osmotic stress can influence intracellular signaling pathways that rely on nucleocytoplasmic transport.

Inhibition of interferon-α-induced signaling by hyperosmolarity and hydrophobic bile acids

Apart from viral conditions, host factors such as elevated bile acid concentrations are determinants of successful interferon-α (IFN-α) treatment in patients with chronic hepatitis C or B. The present study demonstrates that hydrophobic bile acids inhibit Jak1- and Tyk2-phosphorylation, which lead to blockade of STAT1-mediated IFN-α-signaling in the sodium-taurocholate cotransporting peptide (NTCP)-transfected human hepatoma cell line HepG2, resulting in a decreased mRNA and protein expression of IFN-stimulated genes such as myxovirus resistance protein A (MxA) or dsRNA-activated protein kinase (PKR). In addition, hyperosmotic stress leads to an inhibition of IFN-α-induced Jak1- and Tyk2-phosphorylation, and STAT1/STAT2-phosphorylation and gene expression. This inhibitory effect of hydrophobic bile acids or hyperosmolarity is not due to caspase-mediated cleavage or lysosomal degradation of the cognate receptors or to the generation of oxidative stress, activation of p38- or Erk-mediated MAPK pathways or phosphatase activity. Preincubation with the organic osmolyte betaine blocked the inhibitory effect of bile acids or hyperosmolarity on MxA protein expression, but had no effect on transcript levels or activation of STAT1, suggesting that betaine mediates its effects on MxA expression at a translational or post-translational level. Our findings could provide a rationale for betaine use in cholestatic HBV/HCV patients undergoing interferon therapy.

Effect of varying osmotic conditions on the response of bovine nucleus pulposus cells to growth factors and the activation of the ERK and Akt pathways

Intervertebral disc and especially nucleus pulposus is characterized by low cellularity. Additionally, extreme variations in osmolality are observed in this tissue, as a result of its specific physicochemical environment, daily activities, or degeneration. In this study, we investigated the role of osmotic fluctuations in the proliferative response of nucleus pulposus cells to exogenous growth factors. In particular, we examined the effect of platelet-derived growth factor (PDGF) and insulin-like growth factor-I (IGF-I) on the proliferation of bovine nucleus pulposus cells and on the activation of the MEK/ERK and PI-3-K/Akt pathways under varying osmotic conditions, in an effort to understand the mechanisms regulating cell proliferation in the intact and the degenerated intervertebral disc. Exposure of cells to high osmolality restrained novel DNA synthesis induced by PDGF or IGF-I in a dose-dependent manner and reduced ERK and Akt activation stimulated by serum or isolated growth factors. Our findings indicate that hyperosmolality imposes a strict control in intervertebral disc cells' proliferation, while hypo-osmotic conditions prevailing in degenerated discs may offer a more permissive environment for cellular proliferation.

Inhibition of protein kinase Akt1 by apoptosis signal-regulating kinase-1 (ASK1) is involved in apoptotic inhibition of regulatory volume increase

Most animal cell types regulate their cell volume after an osmotic volume change. The regulatory volume increase (RVI) occurs through uptake of NaCl and osmotically obliged water after osmotic shrinkage. However, apoptotic cells undergo persistent cell shrinkage without showing signs of RVI. Persistence of the apoptotic volume decrease is a prerequisite to apoptosis induction. We previously demonstrated that volume regulation is inhibited in human epithelial HeLa cells stimulated with the apoptosis inducer. Here, we studied signaling mechanisms underlying the apoptotic inhibition of RVI in HeLa cells. Hypertonic stimulation was found to induce phosphorylation of a Ser/Thr protein kinase Akt (protein kinase B). Shrinkage-induced Akt activation was essential for RVI induction because RVI was suppressed by an Akt inhibitor, expression of a dominant negative form of Akt, or small interfering RNA-mediated knockdown of Akt1 (but not Akt2). Staurosporine, tumor necrosis factor-alpha, or a Fas ligand inhibited both RVI and hypertonicity-induced Akt activation in a manner sensitive to a scavenger for reactive oxygen species (ROS). Any of apoptosis inducers also induced phosphorylation of apoptosis signal-regulating kinase 1 (ASK1) in a ROS-dependent manner. Suppression of (ASK1) expression blocked the effects of apoptosis, in hypertonic conditions, on both RVI induction and Akt activation. Thus, it is concluded that in human epithelial cells, shrinkage-induced activation of Akt1 is involved in the RVI process and that apoptotic inhibition of RVI is caused by inhibition of Akt activation, which results from ROS-mediated activation of ASK1.

Curcumin protects against hyperosmoticity-induced IL-1beta elevation in human corneal epithelial cell via MAPK pathways

Increased tear osmolarity is an essential feature of dry eye disease. Curcumin, a natural polyphenol extracted from herb turmeric, has recently been reported to have anti-inflammatory effects. However, its anti-inflammatory effects have not been investigated in dry eye disease. It has been reported that elevated osmolarity achieved by adding sodium chloride to the culture medium of corneal epithelial cells increased the production of IL-1beta, a proinflammation cytokine. This in vitro dry eye model was used to test the anti-inflammatory effects of curcumin. In the present study, a 450 mOsM hyperosmotic medium was produced by adding sodium chloride to the culture medium to reach a final concentration of 90mM. Human corneal epithelial cells cultured in this hyperosmotic medium for 24h showed an increase of IL-1beta, IL-6 and TNF-alpha levels in the conditioned medium. IL-1beta was also upregulated at mRNA levels. Activation of p38 MAP kinase (p38), JNK MAP kinase (JNK) and NF-kappaB in cultured corneal epithelial cells were also induced by hyperosmotic conditions. Curcumin at concentrations of 1-30muM did not affect the cell viability of cultured corneal epithelial cells. Pretreatment of curcumin (5muM) completely abolished the increased production of IL-1beta induced by the hyperosmotic medium. Increased phosphorylation of p38 caused by high osmolarity was also completely abolished by curcumin, whereas the phosphorylation of JNK was only partially inhibited. SB 203580 (p38 inhibitor), but not SP 600125 (JNK inhibitor), completely suppressed hyperosmoticity-induced IL-1beta production, indicating that the inhibition of production of IL-1beta by curcumin may be achieved through the p38 signal pathway. Curcumin completely abolished a hyperosmoticity-induced increase of NF-kappaB p65. NF-kappaB inhibitor suppressed hyperosmoticity-induced IL-1beta production. p38 inhibitor suppressed hyperosmoticity-induced NF-kappaB activation, indicating that NF-kappaB activation was dependent on p38 activation. The present study suggests that curcumin might have therapeutic potential for treating dry eye disease.

Physiology of cell volume regulation in vertebrates

The ability to control cell volume is pivotal for cell function. Cell volume perturbation elicits a wide array of signaling events, leading to protective (e.g., cytoskeletal rearrangement) and adaptive (e.g., altered expression of osmolyte transporters and heat shock proteins) measures and, in most cases, activation of volume regulatory osmolyte transport. After acute swelling, cell volume is regulated by the process of regulatory volume decrease (RVD), which involves the activation of KCl cotransport and of channels mediating K(+), Cl(-), and taurine efflux. Conversely, after acute shrinkage, cell volume is regulated by the process of regulatory volume increase (RVI), which is mediated primarily by Na(+)/H(+) exchange, Na(+)-K(+)-2Cl(-) cotransport, and Na(+) channels. Here, we review in detail the current knowledge regarding the molecular identity of these transport pathways and their regulation by, e.g., membrane deformation, ionic strength, Ca(2+), protein kinases and phosphatases, cytoskeletal elements, GTP binding proteins, lipid mediators, and reactive oxygen species, upon changes in cell volume. We also discuss the nature of the upstream elements in volume sensing in vertebrate organisms. Importantly, cell volume impacts on a wide array of physiological processes, including transepithelial transport; cell migration, proliferation, and death; and changes in cell volume function as specific signals regulating these processes. A discussion of this issue concludes the review.

Hyperosmotic stress induces Rho/Rho kinase/LIM kinase-mediated cofilin phosphorylation in tubular cells: key role in the osmotically triggered F-actin response

Hyperosmotic stress induces cytoskeleton reorganization and a net increase in cellular F-actin, but the underlying mechanisms are incompletely understood. Whereas de novo F-actin polymerization likely contributes to the actin response, the role of F-actin severing is unknown. To address this problem, we investigated whether hyperosmolarity regulates cofilin, a key actin-severing protein, the activity of which is inhibited by phosphorylation. Since the small GTPases Rho and Rac are sensitive to cell volume changes and can regulate cofilin phosphorylation, we also asked whether they might link osmostress to cofilin. Here we show that hyperosmolarity induced rapid, sustained, and reversible phosphorylation of cofilin in kidney tubular (LLC-PK1 and Madin-Darby canine kidney) cells. Hyperosmolarity-provoked cofilin phosphorylation was mediated by the Rho/Rho kinase (ROCK)/LIM kinase (LIMK) but not the Rac/PAK/LIMK pathway, because 1) dominant negative (DN) Rho and DN-ROCK but not DN-Rac and DN-PAK inhibited cofilin phosphorylation; 2) constitutively active (CA) Rho and CA-ROCK but not CA-Rac and CA-PAK induced cofilin phosphorylation; 3) hyperosmolarity induced LIMK-2 phosphorylation, and 4) inhibition of ROCK by Y-27632 suppressed the hypertonicity-triggered LIMK-2 and cofilin phosphorylation.We thenexamined whether cofilin and its phosphorylation play a role in the hypertonicity-triggered F-actin changes. Downregulation of cofilin by small interfering RNA increased the resting F-actin level and eliminated any further rise upon hypertonic treatment. Inhibition of cofilin phosphorylation by Y-27632 prevented the hyperosmolarity-provoked F-actin increase. Taken together, cofilin is necessary for maintaining the osmotic responsiveness of the cytoskeleton in tubular cells, and the Rho/ROCK/LIMK-mediated cofilin phosphorylation is a key mechanism in the hyperosmotic stress-induced F-actin increase.

Cell volume regulation: physiology and pathophysiology

Cell volume perturbation initiates a wide array of intracellular signalling cascades, leading to protective and adaptive events and, in most cases, activation of volume-regulatory osmolyte transport, water loss, and hence restoration of cell volume and cellular function. Cell volume is challenged not only under physiological conditions, e.g. following accumulation of nutrients, during epithelial absorption/secretion processes, following hormonal/autocrine stimulation, and during induction of apoptosis, but also under pathophysiological conditions, e.g. hypoxia, ischaemia and hyponatremia/hypernatremia. On the other hand, it has recently become clear that an increase or reduction in cell volume can also serve as a specific signal in the regulation of physiological processes such as transepithelial transport, cell migration, proliferation and death. Although the mechanisms by which cell volume perturbations are sensed are still far from clear, significant progress has been made with respect to the nature of the sensors, transducers and effectors that convert a change in cell volume into a physiological response. In the present review, we summarize recent major developments in the field, and emphasize the relationship between cell volume regulation and organism physiology/pathophysiology.