Calcium-induced p56(Lck) phosphorylation in human T lymphocytes via calmodulin dependent kinase

Ca2+ Signaling and Src Functions in Tumor Cells

Signaling by calcium ion (Ca2+) plays a prominent role in cell physiology, and these mechanisms are frequently altered in tumor cells. In this review, we consider the interplay of Ca2+ signaling and the functions of the proto-oncogene non-receptor tyrosine kinase c-Src in tumor cells, and the viral oncogenic variant v-Src in transformed cells. Also, other members of the Src-family kinases are considered in this context. The role of Ca2+ in the cell is frequently mediated by Ca2+-binding proteins, where the Ca2+-sensor protein calmodulin (CaM) plays a prominent, essential role in many cellular signaling pathways. Thus, we cover the available information on the role and direct interaction of CaM with c-Src and v-Src in cancerous cells, the phosphorylation of CaM by v-Src/c-Src, and the actions of different CaM-regulated Ser/Thr-protein kinases and the CaM-dependent phosphatase calcineurin on v-Src/c-Src. Finally, we mention some clinical implications of these systems to identify mechanisms that could be targeted for the therapeutic treatment of human cancers.

Impact of lipid rafts on transient receptor potential channel activities

Members of the transient receptor potential (TRP) superfamily are cation channels that are expressed in nearly every mammalian cell type and respond as cellular sensors to various environmental stimuli. Light, pressure, osmolarity, temperature, and other stimuli can induce TRP calcium conductivity and correspondingly trigger many signaling processes in cells. Disruption of TRP channel activity, as a rule, harms cellular function. Despite numerous studies, the mechanisms of TRP channel regulation are not yet sufficiently clear, in part, because TRP channels are regulated by a broad set of ligands having diverse physical and chemical features. It is now known that some TRP members are located in membrane microdomains termed lipid rafts. Moreover, interaction between specific raft-associated lipids with channels may be a key regulation mechanism. This review examines recent findings related to the roles of lipid rafts in regulation of TRP channel activity. The mechanistic events of channel interactions with the main lipid raft constituent, cholesterol, are being clarified. Better understanding of mechanisms behind such interactions would help establish the key elements of TRP channel regulation and hence allow control of cellular responses to environmental stimuli.

Lipid raft integrity is required for human leukemia Jurkat T-cell migratory activity

Lipid rafts are membrane microdomains featuring high cholesterol, sphingolipid, and protein content. These microdomains recruit various receptors, ion channels, and signaling molecules for coordination of various cellular functions, including synaptic transmission, immune response, cytoskeletal organization, adhesion, and migration. Many of these processes also depend on Ca2+ intake. We have previously shown in Jurkat cells that activity of transient receptor potential vanilloid, type 6 (TRPV6) calcium channel, and TRPV6-mediated Ca2+ influx, depend on lipid raft integrity. In this study, using the transwell cell migration assay and time-lapse video microscopy with Jurkat cells, we found that lipid raft destruction was associated with: inhibited cell adhesion and migration; and decreased mean speed, maximum speed, and trajectory length. Using String Server, we constructed a Protein Interaction Network (PIN). The network indicated that TRPV6 proteins interact with the highest probability (0.9) with Src family kinase members (SFKs) involved in processes related to cell migration. Analysis of detergent-resistant membrane fractions and immunoelectron microscopy data confirmed an association in lipid rafts between TRPV6 and Lck kinase, an SFKs member. Destruction of lipid rafts led to uncoupling of TRPV6 clusters with Lck and their departure from the plasma membrane into the cytosol of the cells. Src family kinases are generally associated with their roles in tumor invasion and progression, epithelial-mesenchymal transitions, angiogenesis, and metastatic development. We suggest that a functional interaction between TRPV6 calcium channels and SFKs members in lipid rafts is one of necessary elements of migration and oncogenic signaling in leukemia cells.

Src-family tyrosine kinases and the Ca2+ signal

In this review, we shall describe the rich crosstalk between non-receptor Src-family kinases (SFKs) and the Ca²⁺ transient generated in activated cells by a variety of extracellular and intracellular stimuli, resulting in diverse signaling events. The exchange of information between SFKs and Ca²⁺ is reciprocal, as it flows in both directions. These kinases are main actors in pathways leading to the generation of the Ca²⁺ signal, and reciprocally, the Ca²⁺ signal modulates SFKs activity and functions. We will cover how SFKs participate in the generation of the cytosolic Ca²⁺ rise upon activation of a series of receptors and the mechanism of clearance of this Ca²⁺ signal. The role of SFKs modulating Ca²⁺-translocating channels participating in these events will be amply discussed. Finally, the role of the Ca²⁺ sensor protein calmodulin on the activity of c-Src, and potentially on other SFKs, will be outlined as well. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

Molecular pathways leading to oxidative stress-induced phosphorylation of Akt

Oxidative stress can activate a variety of intracellular signaling pathways. The authors previously reported the CaM-K inhibitor KN-93 inhibited hydrogen peroxide-induced phosphorylation of Akt on threonine 308 (T308). In this report they demonstrate that phosphorylation of T308 in response to hydrogen peroxide treatment is not inhibited by LY294002, suggesting that phosphorylation of this residue in response to oxidative stress is largely PI3K independent. In contrast, hydrogen peroxide-induced phosphorylation of Akt on serine 473 (S473) was downregulated by both PI3K and CaM-K inhibition, indicating that hydrogen peroxideinduced phosphorylation of Akt on S473 was largely dependent on both PI3K and a CaM-K activity. Further, it is reported that p56(Lck) had a substantial role in hydrogen peroxide-induced phosphorylation of S473, but only a minimal role in hydrogen peroxide-induced phosphorylation of T308. These data suggest that in response to hydrogen peroxide, two pathways are activated in Jurkat T lymphocytes that converge to result in the phosphorylation of Akt on S473 and T308. One pathway involves the CaM-Ks that may directly phosphorylate Akt on T308. In this pathway, neither the Src kinases nor PI3K are required. The other pathway mediated by hydrogen peroxide results in the phosphorylation of Akt on S473 and requires CaM-K, PI3K, and Src activity.

Expression of Fc Fragment Receptors of Immunoglobulin G (FcγRs) in Rat Hepatic Stellate Cells

Hepatic stellate cells (HSCs) are now considered the major cell type in the liver mediating the development of liver fibrosis. Recently it was demonstrated that HSCs express membrane proteins involved in antigen presentation. We further evaluate immunological properties of HSCs by examining the expression and function of the Fc fragment of immunoglobulin G (IgG) in HSCs. In this study, we document the presence of mRNAs for three FcγRs in HSCs. Ligand binding assay indicated the existence of FcγRs with different binding affinities on membranes of HSCs. We also documented that the abundance of the three Fcγ R mRNAs increased upon activation of HSCs in vitro. Moreover, an examination of the biological activities of IgG revealed that exposure to IgG significantly stimulated HSC differentiation and proliferation. Furthermore, we studied the intracellular signaling protein, LcK, in HSCs and regulation of Lck expression and phosphorylation by IgG. Although IgG did not regulate Lck abundance and phosphorylation in HSCs, highly phosphorylated Lck was present in these cells. In conclusion, we provided evidence that HSCs expresses receptors for the Fc fragment of IgG, and IgG regulates HSC differentiation and proliferation. Therefore, immunoglobulin G may play a role in HSC activation.

Linker for activation of T cells, zeta-associated protein-70, and Src homology 2 domain-containing leukocyte protein-76 are required for TCR-induced microtubule-organizing center polarization

Engagement of the T cell with Ag on an APC results in a series of immediate signaling events emanating from the stimulation of the TCR. These events include the induced phosphorylation of a number of cellular proteins with a subsequent increase in intracellular calcium and the restructuring of the microtubule and actin cytoskeleton within the T cell. This restructuring of the cytoskeleton culminates in the polarization of the T cell's secretory apparatus toward the engaging APC, allowing the T cell to direct secretion of cytokines toward the appropriate APC. This polarization can be monitored by analyzing the position of the microtubule-organizing center (MTOC), as it moves toward the interface of the T cell and APC. The requirements for MTOC polarization were examined at a single-cell level by studying the interaction of a Jurkat cell line expressing a fluorescently labeled MTOC with Staphylococcal enterotoxin superantigen-bound Raji B cell line, which served as the APC. We found that repolarization of the MTOC substantially followed fluxes in calcium. We also used immobilized anti-TCR mAb and Jurkat signaling mutants, defective in TCR-induced calcium increases, to determine whether signaling components that are necessary for a calcium response also play a role in MTOC polarization. We found that zeta-associated protein-70 as well as its substrate adaptor proteins linker for activation of T cells and Src homology 2 domain-containing leukocyte protein-76 are required for MTOC polarization. Moreover, our studies revealed that a calcium-dependent event not requiring calcineurin or calcium/calmodulin-dependent kinase is required for TCR-induced polarization of the MTOC.

Participation of the calcium/calmodulin-dependent kinases in hydrogen peroxide-induced Ikappa B phosphorylation in human T lymphocytes

NF-kappaB is an important transcription factor that has a role in a variety of responses such as inflammation, oncogenesis, apoptosis, and viral replication. Oxidative stress is well known to induce the activation of NF-kappaB. Cells can be exposed to either endogenously produced oxidants or oxidants produced by surrounding cells. In addition, ischemia reperfusion and certain cancer therapies such as chemotherapy and photodynamic therapy are thought to result in oxygen radical production. Because of the important role that NF-kappaB has in multiple responses, it is critical to determine the mechanisms by which oxidative stress induces NF-kappaB activity. We report that the calmodulin antagonist W-7 and the calcium/calmodulin-dependent (CaM) kinase inhibitors KN-93 and K252a, can block oxidative stress-induced IkappaB phosphorylation in Jurkat T lymphocytes. Furthermore, KN-93 but not KN-92 can block hydrogen peroxide-induced Akt and IKK phosphorylation. In addition, we found that expression of a kinase-dead CaM-KIV construct in two cell lines inhibits IkappaB phosphorylation or degradation and that expression of CaM-KIV augments hydrogen peroxide-induced IkappaB phosphorylation and degradation. Although the CaM kinases appear to be required for this response, increases in intracellular calcium do not appear to be required. These results identify the CaM kinases as potential targets that can be used to minimize NF-kappaB activation in response to oxidative stress.

cAMP potentiates H(2)O(2)-induced ERK1/2 phosphorylation without the requirement for MEK1/2 phosphorylation

In Jurkat T lymphocytes, hydrogen peroxide (H(2)O(2)) potentiates the phosphorylation level of extracellular signal-regulated kinase 1 and 2 (ERK1/2) caused by T cell receptor (TCR) stimulation with anti-CD3 and anti-CD28 or anti-CD3 alone. Submillimolar concentrations of H(2)O(2)-induced phosphorylation of ERK1/2 and MAP/ERK kinase 1 and 2 (MEK1/2) without antigenic stimulation. H(2)O(2) also induced the electrophoretic mobility shift of Lck from 56 to 60 kDa. The MEK inhibitor, PD98059 attenuated ERK1/2 and MEK1/2 phosphorylation, as well as the migration shift of Lck induced by H(2)O(2). The phospholipase C (PLC) inhibitor, U73122, and EGTA reduced the phosphorylation of both ERK1/2 and MEK1/2 induced by H(2)O(2). Interestingly, an increase of intracellular cAMP level with forskolin or 8-(4-chlorophenylthio)-cAMP augmented ERK1/2 phosphorylation by H(2)O(2), while inhibiting MEK1/2 phosphorylation by H(2)O(2). These results demonstrate an alternative pathway that results in augmentation of ERK1/2 phosphorylation without concomitant MEK1/2 phosphorylation in T cells.

Regulation of conjunctival goblet cell secretion by Ca(2+)and protein kinase C

Conjunctival goblet cells secrete mucus in response to cholinergic (muscarinic) agonists, but the underlying signaling pathways activated in this tissue are not well understood. Cholinergic agonists usually activate phospholipase C to produce inositol 1,4,5 trisphosphate and diacylglycerol. Inositol 1,4,5 trisphosphate increases the intracellular Ca(2+)concentration ([Ca2(+)](i)) while diacylglycerol activates protein kinase C (PKC). PKC and Ca(2+), either by itself or with calmodulin, activate cellular functions. Goblet cell glycoprotein secretion, our index of mucin secretion, was measured from pieces of rat conjunctiva with an enzyme-linked lectin assay using the lectin Ulex europaeus agglutinin I (UEA-I). UEA-I selectively recognizes high molecular weight glycoproteins secreted by the goblet cells. Increasing the [Ca(+)](i)with the Ca(2+)ionophore ionomycin stimulated glycoprotein secretion from conjunctival goblet cells. Cholinergic agonist-induced secretion was completely blocked by chelation of extracellular Ca(2+)and by the Ca(2+)/calmodulin-dependent protein kinase inhibitors KN93 and W7 as well as their inactive analogs KN92 and W5. Activation of classical and novel PKC isozymes by phorbol 12-myristate 13-acetate and phorbol 12,13-dibutyrate stimulated goblet cell glycoprotein secretion. When ionomycin and PMA were added simultaneously, secretion was additive. PKC isozymes were identified by Western blotting analyses with antibodies specific to nine of the 11 PKC isozymes (PKCgamma and zeta were not tested). All nine PKC isozymes were identified in the conjunctival epithelium. The cellular location of the PKC isozymes was determined by immunofluorescence microscopy. Goblet cells contained the classical PKC isozymes PKCalpha, -betaI and -betaII, the novel PKC isozymes PKCepsilon, -theta;, and - mu, and the atypical PKC isozyme PKCzeta. We were unable to determine if PKC activation is required for cholinergic-agonist induced secretion because the PKC inhibitors chelerythrine and staurosporine alone greatly increased secretion. We conclude that Ca(2+)plays a major role in cholinergic agonist-induced conjunctival goblet cell secretion, but this agonist appears not to use Ca(2+)/calmodulin-dependent protein kinases. We also conclude that activated PKC can stimulate goblet cell secretion and that seven different PKC isoforms are present in the goblet cells.

Kinases: positive and negative regulators of apoptosis

Cells sense and respond to extracellular factors via receptors on the cell surface that trigger intracellular signaling pathways. The signals received by the receptors on hematopoietic cells often determine if the cell proliferates, survives or undergoes apoptosis. Apoptosis can be induced by almost any cytotoxic stimuli. These stimuli may be an absence of signals arising from cellular receptors, stimulation of specific ligand receptors on the cell surface, chemotherapeutic agents, and ionizing radiation or oxygen radicals, as well as a number of other factors. Cellular kinases and phosphatases participate in signaling cascades that influence this process. We review the ability of the calmodulin-dependent-kinases, I-kappaB kinases, PI3-kinases, Jakkinases, PKC, PKA, and MAP kinase signaling pathways (Erk, Jnk, and p38), to influence the apoptotic process. In addition, we discuss the cross-talk that exists between signaling cascades that are pro-apoptotic and anti-apoptotic.

Calcium-induced ERK activation in human T lymphocytes occurs via p56(Lck) and CaM-kinase

We previously demonstrated that stimulation of human T-lymphocytes with calcium ionophores induced the phosphorylation and enzymatic activation of ERK2. We now report on the mechanism by which calcium-ionophore-induced activation of ERK1 and 2 occurs in these cells. The activation of ERK1 and 2 by increases in intracellular calcium was inhibited by calmidazolium suggesting the involvement of calmodulin in this response. To further elucidate the mechanism by which calcium-induced ERK activation occurs, we used the CaM-kinase inhibitor KN-93 and an inactive analog of KN-93 (KN-92). KN-93, but not KN-92, blocked ionomycin-induced activation of ERK1 and 2 in human T lymphocytes. We previously demonstrated that stimulation of T lymphocytes with ionomycin or A23187 resulted in a CaM-kinase-dependent shift in the mobility of p56(Lck). To determine if p56(Lck) was involved in calcium-induced ERK activation, we stimulated the p56(Lck) negative Jurkat cell derivatives, J.CaM1.6 and J.CaM1/Rep3, with ionomycin. In these p56(Lck) negative cell lines, activation of ERK1 and 2 in response to ionomycin was only minimally detected. When J.CaM1 cells were reconstituted with p56(Lck), ionomycin induced ERK1 and 2 activation. Treatment of Jurkat cells with PP2, an inhibitor of p56(Lck), inhibited calcium-induced, but not PMA-induced, ERK1 and 2 activation. Treatment of Jurkat cells with the MEK inhibitor PD98059 blocked ionomycin-induced ERK activation, but not the shift in the mobility of p56(Lck). Our data suggests that increases in intracellular calcium induce the activation of ERK1 and 2 in human T lymphocytes via sequential activation of CaM-kinase and phosphorylation of p56(Lck).