Although cancer is a diverse set of diseases, cancer cells share a number of adaptive hallmarks. Dysregulated pH is emerging as a hallmark of cancer because cancers show a 'reversed' pH gradient with a constitutively increased intracellular pH that is higher than the extracellular pH. This gradient enables cancer progression by promoting proliferation, the evasion of apoptosis, metabolic adaptation, migration and invasion. Several new advances, including an increased understanding of pH sensors, have provided insight into the molecular basis for pH-dependent cell behaviours that are relevant to cancer cell biology. We highlight the central role of pH sensors in cancer cell adaptations and suggest how dysregulated pH could be exploited to develop cancer-specific therapeutics.

SUMMARY It is well understood that antigen-presenting cells (APC) within tumors typically do not maintain cytotoxic T cell (CTL) function, despite engaging them. Across multiple mouse tumor models and human tumor biopsies, we have delineated the intratumoral dendritic-cell (DC) populations as distinct from macrophage populations. Within these, CD103+ DCs are extremely sparse and yet remarkably capable CTL stimulators. These are uniquely dependent upon IRF8, Zbtb46 and Batf3 transcription factors and generated by GM-CSF and Flt3L cytokines. Regressing tumors have higher proportions of these cells, T-cell dependent immune clearance relies upon them, and abundance of their transcripts in human tumors correlates with clinical outcome. This cell type presents opportunities for prognostic and therapeutic approaches across multiple cancer types.

The NHE family of ion exchangers includes six isoforms (NHE1-NHE6) that function in an electroneutral exchange of intracellular H(+) for extracellular Na(+). This review focuses on the only ubiquitously expressed isoform, NHE1, which is localized at the plasma membrane where it plays a critical role in intracellular pH (pHi) and cell volume homeostasis. All NHE isoforms share a similar topology: an N-terminus of 12 transmembrane (TM) alpha-helices that collectively function in ion exchange, and a C-terminal cytoplasmic regulatory domain that modulates transport activity by the TM domain. Extracellular signals, mediated by diverse classes of cell-surface receptors, regulate NHE1 activity through distinct signaling networks that converge to directly modify the C-terminal regulatory domain. Modifications in the C-terminus, including phosphorylation and the binding of regulatory proteins, control transport activity by altering the affinity of the TM domain for intracellular H(+). Recently, it was determined that NHE1 also functions as a membrane anchor for the actin-based cytoskeleton, independently of its role in ion translocation. Through its effects on pHi homeostasis, cell volume, and the actin cortical network, NHE1 regulates a number of cell behaviors, including adhesion, shape determination, migration, and proliferation.

The association of actin filaments with the plasma membrane maintains cell shape and adhesion. Here, we show that the plasma membrane ion exchanger NHE1 acts as an anchor for actin filaments to control the integrity of the cortical cytoskeleton. This occurs through a previously unrecognized structural link between NHE1 and the actin binding proteins ezrin, radixin, and moesin (ERM). NHE1 and ERM proteins associate directly and colocalize in lamellipodia. Fibroblasts expressing NHE1 with mutations that disrupt ERM binding, but not ion translocation, have impaired organization of focal adhesions and actin stress fibers, and an irregular cell shape. We propose a structural role for NHE1 in regulating the cortical cytoskeleton that is independent of its function as an ion exchanger.

Directed cell movement is a multi-step process requiring an initial spatial polarization that is established by asymmetric stimulation of Rho GTPases, phosphoinositides (PIs), and actin polymerization. We report that the Na-H exchanger isoform 1 (NHE1), a ubiquitously expressed plasma membrane ion exchanger, is necessary for establishing polarity in migrating fibroblasts. In fibroblasts, NHE1 is predominantly localized in lamellipodia, where it functions as a plasma membrane anchor for actin filaments by its direct binding of ezrin/radixin/moesin (ERM) proteins. Migration in a wounding assay was impaired in fibroblasts expressing NHE1 with mutations that independently disrupt ERM binding and cytoskeletal anchoring or ion transport. Disrupting either function of NHE1 impaired polarity, as indicated by loss of directionality, mislocalization of the Golgi apparatus away from the orientation of the wound edge, and inhibition of PI signaling. Both functions of NHE1 were also required for remodeling of focal adhesions. Most notably, lack of ion transport inhibited de-adhesion, resulting in trailing edges that failed to retract. These findings indicate that by regulating asymmetric signals that establish polarity and by coordinating focal adhesion remodeling at the cell front and rear, cytoskeletal anchoring by NHE1 and its localized activity in lamellipodia act cooperatively to integrate cues for directed migration.

et al., 1983; Conti and Minelli, 1994). Since glia synthesize the glutamine used by neurons, this requires transfer of the amino acid between the two cell types (Ottersen et al., 1992). The role of glutamine in nitrogen metabolism and synaptic transmission thus involves

Dysregulated pH is a common characteristic of cancer cells, as they have an increased intracellular pH ( pH i ) and a decreased extracellular pH ( pH e ) compared with normal cells. Recent work has expanded our knowledge of how dysregulated pH dynamics influences cancer cell behaviors, including proliferation, metastasis, metabolic adaptation and tumorigenesis. Emerging data suggest that the dysregulated pH of cancers enables these specific cell behaviors by altering the structure and function of selective pH-sensitive proteins, termed pH sensors. Recent findings also show that, by blocking pH i increases, cancer cell behaviors can be attenuated. This suggests ion transporter inhibition as an effective therapeutic approach, either singly or in combination with targeted therapies. In this Cell Science at a Glance article and accompanying poster, we highlight the interconnected roles of dysregulated pH dynamics in cancer initiation, progression and adaptation.

Activity of the ubiquitously expressed Na؉ -H ؉ exchanger subtype NHE1 is stimulated upon activation of receptor tyrosine kinases and G protein-coupled receptors. The intracellular signaling pathways mediating receptor regulation of the exchanger, however, are poorly understood. Using transient expression of dominant interfering and constitutively active alleles in CCL39 fibroblasts, we determined that the GTPases Ha-Ras and G␣13 stimulate NHE1 through distinct signaling cascades. Exchange activity stimulated by constitutively active RasV12 occurs through a Raf1-and mitogen-activated protein kinase kinase/extracellular signal-regulated kinase kinase (MEK)-dependent mechanism. Constitutively active G␣13QL, recently shown to stimulate the Jun kinase cascade, activates NHE1 through a Cdc42-and MEK kinase (MEKK1)-dependent mechanism that is independent of Rac1. Constitutively active Rac1V12 does stimulate NHE1 through a MEKK1-dependent mechanism, but dominant interfering Rac1N17 does not inhibit G␣13QL-mediated or constitutively active Cdc42V12-mediated stimulation of the exchanger. Conversely, Cdc42N17 does not inhibit Rac1V12 activation of NHE1, suggesting that Rac1 and Cdc42 independently regulate a MEKK1-dependent activation of the exchanger. Rapid (<10 min) stimulation of NHE1 with a G␣13/G␣ z chimera also was inhibited by a kinase-inactive MEKK. G␣13QL, but not RasV12, also stimulates NHE1 through a RhoA-dependent pathway that is independent of MEKK, and microinjection of mutationally active G␣13 results in a Rho phenotype of increased stress fiber formation. These findings indicate a new target for Rho-like proteins: the regulation of H ؉ exchange and intracellular pH. Our findings also suggest that a MEKK cascade diverges to regulate effectors other than transcription factors.The ubiquitously expressed Na ϩ -H ϩ exchanger subtype NHE1 1 plays a major role in intracellular pH (pH i ) homeostasis and in cell volume regulation (1). NHE1 activity is stimulated by hormones, cytokines, and growth factors, resulting in an increase in pH i . Hyperosmotic shock (2) and cell adhesion (3) also activate NHE1. Increases in NHE1 activity are associated with increased cell proliferation (4, 5), differentiation (6, 7), and neoplastic transformation (8 -10). Receptor (11, 12), but not osmotic (2), activation of NHE1 is associated with increased phosphorylation of the exchanger on serine residues, suggesting kinase-dependent regulatory mechanisms. Although activation of protein kinase C stimulates NHE1, growth factors and vasoactive agents can stimulate the exchanger independently of this kinase. Mutational activation of three GTPases, Ha-Ras (8, 9), G␣ q (13-15), and G␣13 (13-15), stimulates NHE1 activity. Of these GTPases, only G␣ q activates the exchanger through a protein kinase C-dependent mechanism (14).2 The downstream signaling events mediating Ha-Ras and G␣13 stimulation of NHE1 have not been identified. Ha-Ras and G␣13 regulate two parallel MAP kinase signaling cascades, and these cascades include serine/threonine p...