Protein kinase C isotype expression and regulation of lymphoid cell motility

PKCθ regulates T cell motility via ezrin-radixin-moesin localization to the uropod

Cell motility is a fundamental process crucial for function in many cell types, including T cells. T cell motility is critical for T cell-mediated immune responses, including initiation, activation, and effector function. While many extracellular receptors and cytoskeletal regulators have been shown to control T cell migration, relatively few signaling mediators have been identified that can modulate T cell motility. In this study, we find a previously unknown role for PKCθ in regulating T cell migration to lymph nodes. PKCθ localizes to the migrating T cell uropod and regulates localization of the MTOC, CD43 and ERM proteins to the uropod. Furthermore, PKCθ-deficient T cells are less responsive to chemokine induced migration and are defective in migration to lymph nodes. Our results reveal a novel role for PKCθ in regulating T cell migration and demonstrate that PKCθ signals downstream of CCR7 to regulate protein localization and uropod formation.

Chk1 inhibitor Gö6976 enhances the sensitivity of nasopharyngeal carcinoma cells to radiotherapy and chemotherapy in vitro and in vivo

Nasopharyngeal carcinoma (NPC) is a malignant tumor. This type of carcinoma has a low 5-year patient survival rate. Thus, there is a need for improved therapeutics. We determined that the Chk1 inhibitor Gö6976 enhanced the sensitivity of CNE1 and CNE2 cells to ionizing radiation (IR) or cisplatin by abrogating S and G(2)/M arrest and subsequently promoting apoptosis. Furthermore, Gö6976 appeared to sensitize NPC xenografts in nude mice to IR or cisplatin treatment. This is the first report to show that the Chk1 inhibitor Gö6976 sensitizes NPC cells to treatment using in vitro and in vivo models.

Phosphorylation of NG2 proteoglycan by protein kinase C-alpha regulates polarized membrane distribution and cell motility

Protein kinase C (PKC)-alpha phosphorylation of recombinant NG2 cytoplasmic domain and phorbol ester-induced PKC-dependent phosphorylation of full-length NG2 expressed in U251 cells are both blocked by mutation of Thr(2256), identifying this residue as a primary phosphorylation site. In untreated U251/NG2 cells, NG2 is present along with ezrin and alpha(3)beta(1) integrin in apical cell surface protrusions. Phorbol ester treatment causes redistribution of all three components to lamellipodia, accompanied by increased cell motility. U251 cells expressing NG2 with a valine substitution at position 2256 are resistant to phorbol ester treatment: NG2 remains in membrane protrusions and cell motility is unchanged. In contrast, NG2 with a glutamic acid substitution at position 2256 redistributes to lamellipodia even without phorbol ester treatment, rendering transfected U251 cells spontaneously motile. PKC-alpha-mediated NG2 phosphorylation at Thr(2256) is therefore a key step for initiating cell polarization and motility.

Structural basis for tetrapyrrole coordination by uroporphyrinogen decarboxylase

Uroporphyrinogen decarboxylase (URO-D), an essential enzyme that functions in the heme biosynthetic pathway, catalyzes decarboxylation of all four acetate groups of uroporphyrinogen to form coproporphyrinogen. Here we report crystal structures of URO-D in complex with the I and III isomer coproporphyrinogen products. Crystallization required use of a novel enzymatic approach to generate the highly oxygen-sensitive porphyrinogen substrate in situ. The tetrapyrrole product adopts a domed conformation that lies against a collar of conserved hydrophobic residues and allows formation of hydrogen bonding interactions between a carboxylate oxygen atom of the invariant Asp86 residue and the pyrrole NH groups. Structural and biochemical analyses of URO-D proteins mutated at Asp86 support the conclusion that this residue makes important contributions to binding and likely promotes catalysis by stabilizing a positive charge on a reaction intermediate. The central coordination geometry of Asp86 allows the initial substrates and the various partially decarboxylated intermediates to be bound with equivalent activating interactions, and thereby explains how all four of the substrate acetate groups can be decarboxylated at the same catalytic center.

Heat shock pretreatment influences the expression of PKC isoforms during sepsis

Protein Kinase C (PKC) plays a central role in signal transduction and participates in diverse biological and biochemical functions. PKC dysfunction leads to general immunosuppression that, in turn, increases host susceptibility to infection and sepsis. In our previous study, we demonstrated that the mortality of sepsis is significantly decreased in rats treated with heat shock. It was considered that the modulation of PKC content by previous heat shock might contribute to the resistance to a severe infection. In this study, we attempted to understand the change of various PKC isoforms in the lymphocytes during sepsis and to investigate the role of previous heat shock in influencing PKC expression. Cecal ligation and puncture (CLP) was used as the experimental sepsis model for its biphasic clinical manifestation. Heat shock protein and PKC isoforms were detected by immunochemical study. Ten PKC isoforms (alpha, beta, gamma, delta, epsilon, zeta, theta, iota, lambda, and mu) were detected from peripheral lymphocytes. Results showed that all the PKC isoforms have a declination tendency along with the progression of CLP-induced sepsis, and previous heat shock treatment could prevent the declination of PKC content, particularly the isoforms beta, gamma, and epsilon, during sepsis. We suggest that heat shock response may participate in maintenance of PKC expression and contribute to decrease the severity of systemic infection.

Crucial importance of PKC-β(I) in LFA-1–mediated locomotion of activated T cells

Crawling T cell locomotion in which activated lymphocyte function-associated antigen 1 (LFA-1) integrins participate is associated with translocation of the protein kinase C-beta (PKC-beta) isoenzyme to the microtubule cytoskeleton. In normal T cells and T lymphoma cell lines, this type of motility is accompanied by PKC-beta-sensitive cytoskeletal rearrangements and the formation of trailing cell extensions, which are supported by microtubules. Expression of PKC-beta(I) and enhanced green fluorescent protein (EGFP) in nonmotile PKC-beta-deficient T cells restored their locomotory behavior in response to a triggering stimulus delivered via LFA-1 and correlated directly with the degree of cell polarization. We have also shown that PKC-beta(I) is a component of the tubulin-enriched LFA-1-cytoskeletal complex assembled upon LFA-1 cross-linking. These observations may have physiological equivalents at advanced (post-integrin activation) stages of lymphocyte extravasation.

Porin is present in the plasma membrane where it is concentrated in caveolae and caveolae-related domains

Mitochondrial porin, or voltage-dependent anion channel, is a pore-forming protein first discovered in the outer mitochondrial membrane. Later investigations have provided indications for its presence also in other cellular membranes, including the plasma membrane, and in caveolae. This extra-mitochondrial localization is debated and no clear-cut conclusion has been reached up to now. In this work, we used biochemical and electrophysiological techniques to detect and characterize porin within isolated caveolae and caveolae-like domains (low density Triton-insoluble fractions). A new procedure was used to isolate porin from plasma membrane. The outer surface of cultured CEM cells was biotinylated by an impermeable reagent. Low density Triton-insoluble fractions were prepared from the labeled cells and used as starting material to purify a biotinylated protein with the same electrophoretic mobility and immunoreactivity of mitochondrial porin. In planar bilayers, the porin from these sources formed slightly anion-selective pores with properties indistinguishable from those of mitochondrial porin. This work thus provides a strong indication of the presence of porin in the plasma membrane, and specifically in caveolae and caveolae-like domains.

New perspectives on PKCtheta, a member of the novel subfamily of protein kinase C

Members of the protein kinase C (PKC) family of serine/threonine protein kinases have been implicated in numerous cellular responses in a large variety of cell types. Expression patterns of individual members and differences in their cofactor requirements and potential substrate specificity suggest that each isoenzyme may be involved in specific regulatory processes. The PKCtheta isoenzyme exhibits a relatively restricted expression pattern with high protein levels found predominantly in hematopoietic cells and skeletal muscle. PKCtheta was found to be expressed in T, but not B lymphocytes, and to colocalize with the T-cell antigen receptor (TCR) at the site of contact between the antigen-responding T cell and the antigen-presenting cell (APC). Colocalization of PKCtheta with the TCR was selective for this isoenzyme and occurred only upon antigen-mediated responses leading to T-cell activation and proliferation. PKCtheta was found to be involved in the regulation of transcriptional activation of early-activation genes, predominantly AP-1, and its cellular distribution and activation were found to be regulated by the 14-3-3 protein. Other findings indicated that PKCtheta can associate with the HIV negative factor (Nef) protein, suggesting that altered regulation of PKCtheta by Nef may contribute to the T-cell impairments that are characteristic of infection by HIV. PKCtheta is expressed at relatively high levels in skeletal muscle, where it is suggested to play a role in signal transduction in both the developing and mature neuromuscular junction. In addition, PKCtheta appears to be involved in the insulin-mediated response of intact skeletal muscle, as well as in experimentally induced insulin resistance of skeletal muscle. Further studies suggest that PKCtheta is expressed in endothelial cells and is involved in multiple processes essential for angiogenesis and wound healing, including the regulation of cell cycle progression, formation and maintenance of actin cytoskeleton, and formation of capillary tubes. Here, we review recent progress in the study of PKCtheta and discuss its potential role in various cellular responses.

Calcineurin preferentially synergizes with PKC-theta to activate JNK and IL-2 promoter in T lymphocytes

Costimulation of the T cell receptor (TCR) and CD28 is required for optimal interleukin-2 (IL-2) induction. These signals, which can be replaced by the pharmacological agents phorbol ester (PMA) and Ca2+ ionophore, synergistically activate the mitogen-activated protein kinase (MAPK) JNK. Cyclosporin A, an inhibitor of the Ca2+-dependent phosphatase calcineurin which blocks IL-2 induction, abrogates Ca2+-triggered synergistic JNK activation. As protein kinase C (PKC) downregulation inhibits PMA+ionophore-induced JNK activation, we examined whether a particular PKC isoform is preferentially involved in this response. We found that PKC-theta but neither PKC-alpha nor PKC-epsilon participates in JNK activation, whereas all three PKCs lead to ERK MAPK activation. PKC-theta specifically cooperates with calcineurin, and together their signals converge on (or upstream of) Rac leading to potent JNK activation. Similarly, calcineurin and PKC-theta specifically synergize to induce transcription of reporters driven by the c-jun and IL-2 promoters. PKC-theta and calcineurin are also partially responsible for the synergistic activation of JNK following TCR and CD28 ligation. Preferential cooperation between PKC-theta and calcineurin is observed in Jurkat T cells but not in HeLa cells. These results indicate that PKC isozymes have distinct biological functions and suggest that synergistic JNK activation is an important function for PKC-theta in T-cell activation.

Locomotion and chemotaxis of lymphocytes

The behaviour of locomotor T and B lymphocytes and the chemoattractants to which they respond in vitro are reviewed. Following activation, T cells respond by locomotion and chemotaxis to cytokine attractants including IL-15 and IL-2 and several chemokines. In activated B cells chemotaxis may be signalled through the antigen receptor. Conversely resting lymphocytes respond poorly to the above signals though their locomotion is activated by contact with high endothelial venular cells. These differences in locomotion between resting and activated lymphocytes, together with differences in adhesion, may explain why activated lymphocytes migrate preferentially into inflammatory sites while resting cells recirculate.