Activation of mitogen-activated protein kinase by protein kinase C isotypes alpha, beta I and gamma, but not epsilon

Intracellular modulation of signaling pathways by annexin A6 regulates terminal differentiation of chondrocytes

Annexin A6 (AnxA6) is highly expressed in hypertrophic and terminally differentiated growth plate chondrocytes. Rib chondrocytes isolated from newborn AnxA6-/- mice showed delayed terminal differentiation as indicated by reduced terminal differentiation markers, including alkaline phosphatase, matrix metalloproteases-13, osteocalcin, and runx2, and reduced mineralization. Lack of AnxA6 in chondrocytes led to a decreased intracellular Ca(2+) concentration and protein kinase C α (PKCα) activity, ultimately resulting in reduced extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase (MAPK) activities. The 45 C-terminal amino acids of AnxA6 (AnxA6(1-627)) were responsible for the direct binding of AnxA6 to PKCα. Consequently, transfection of AnxA6-/- chondrocytes with full-length AnxA6 rescued the reduced expression of terminal differentiation markers, whereas transfection of AnxA6-/- chondrocytes with AnxA6(1-627) did not or only partially rescued the decreased mRNA levels of terminal differentiation markers. In addition, lack of AnxA6 in matrix vesicles, which initiate the mineralization process in growth plate cartilage, resulted in reduced alkaline phosphatase activity and Ca(2+) and inorganic phosphate (P(i)) content and the inability to form hydroxyapatite-like crystals in vitro. Histological analysis of femoral, tibial, and rib growth plates from newborn mice revealed that the hypertrophic zone of growth plates from newborn AnxA6-/- mice was reduced in size. In addition, reduced mineralization was evident in the hypertrophic zone of AnxA6-/- growth plate cartilage, although apoptosis was not altered compared with wild type growth plates. In conclusion, AnxA6 via its stimulatory actions on PKCα and its role in mediating Ca(2+) flux across membranes regulates terminal differentiation and mineralization events of chondrocytes.

Inhibition of H-Ras and MAPK is compensated by PKC-dependent pathways in annexin A6 expressing cells

High-density lipoprotein (HDL)-induced activation of the Ras/MAPK pathway can be mediated by protein kinase C (PKC)-dependent and independent pathways. Although both pathways co-exist in cells, we showed that binding of HDL to scavenger receptor BI (SR-BI) in CHO cells activates Ras and MAPK in a PKC-independent manner. We have recently identified that HDL-induced activation of Ras and Raf-1 is reduced in annexin A6 expressing CHO cells (CHOanx6). In the present study we demonstrate that despite the loss of Ras and Raf-1 activity, HDL induces MAPK phosphorylation in CHOanx6 cells. Since annexin A6 is a PKCalpha-binding protein we therefore investigated the possible involvement of PKC in HDL-induced Ras and MAPK activation in CHOanx6 cells. Taken together our findings demonstrate that HDL-induced H-Ras and MAPK activation is PKC-dependent in cells expressing annexin A6 to compensate for the loss of PKC-independent activation of H-Ras and MAPK.

Differential signalling of purinoceptors in HeLa cells through the extracellular signal-regulated kinase and protein kinase C pathways

We have previously shown that HeLa cells express P2Y2 and P2Y6 receptors endogenously and determined the pathways by which the P2Y2 controls proliferation and Na+/K+ATPase activity. Our objective in this study was to investigate the hypothesis that P2Y6 also controls proliferation and Na+/K+ATPase activity; the pathways used in these actions were partially characterised. We found that P2Y6 activation controlled cell proliferation but not the activity of the Na+/K+ATPase. UDP activation of P2Y6 provoked: (a) an increase in free cytosolic calcium; (b) the activation of protein kinase C-alpha, -beta, -delta, -epsilon, and -zeta but not of PKC-iota and -eta; (c) the phosphorylation of the extracellular signal-regulated protein kinases 1 and 2 (ERK1/2); (d) the expression of c-Fos protein. The P2Y6 induced cell proliferation was blocked by the mitogen-activated protein kinase kinase (MAPKK) inhibitor PD098059, thereby indicating that the ERK pathway mediates the mitogenic signalling of P2Y6. PKC and phosphoinositide 3-kinase (PI3K) inhibitors were tested at two different time points of ERK1/2 phosphorylation (10 and 60 min). The results suggest that novel PKCs and PI3K initiate the response but both conventional and atypical PKCs are required for the maintenance of the UDP-induced phosphorylation of ERK1/2. The induction of c-Fos was greatly diminished by conventional or atypical PKC-zeta inhibition, suggesting that it may be due to PKC-alpha/beta and -zeta activity. These observations demonstrate that UDP acts as a proliferative agent in HeLa cells activating multiple signalling pathways involving conventional, novel, and atypical PKCs, PI3K, and ERK. Of these pathways, conventional and atypical PKCs appear responsible for the induction of c-Fos, while ERK is responsible for cell proliferation and depends upon both novel and atypical PKCs and PI3K activities.

Activator protein-1 activation in acute lung injury

The role of activator protein-1 (AP-1) in inflammation is primarily unknown. AP-1 was evaluated in nuclear extracts from alveolar macrophages and whole lung nuclear extracts during acute lung injury after deposition of IgG immune complexes. AP-1 activation occurred in macrophages and in whole lung extracts, but with distinctly different time courses. Low levels of constitutive AP-1 were observed in normal rat lung as determined by the electrophoretic mobility shift assay. Increased AP-1 was detected 2 hours after initiation of the inflammatory response in lung with a further increase by 4 hours, while AP-1 activation was found in alveolar macrophages 0.5 hour after onset of the inflammatory response. mRNAs and proteins for c-fos, c-jun, jun-B, and jun-D were all up-regulated in whole lung tissues and in alveolar macrophages during acute lung injury induced by IgG immune complex deposition. Depletion of lung macrophages sharply reduced AP-1 activation, as did anti-tumor necrosis factor-alpha, whereas complement depletion showed no effect on lung AP-1 activation. The data suggest that activation of AP-1 occurs in both alveolar macrophages and in the lung, and this activation process is macrophage- and tumor necrosis factor-alpha-dependent.

Suppression of urokinase expression and invasiveness by urinary trypsin inhibitor is mediated through inhibition of protein kinase C- and MEK/ERK/c-Jun-dependent signaling pathways

Urinary trypsin inhibitor (UTI), a Kunitz-type protease inhibitor, interacts with cells as a negative modulator of the invasive cells. Human ovarian cancer cell line, HRA, was treated with phorbol ester (PMA) to evaluate the effect on expression of urokinase-type plasminogen activator (uPA), since the action of uPA has been implicated in matrix degradation and cell motility. Preincubation of the cells with UTI reduced the ability of PMA to trigger the uPA expression at the gene level and at the protein level. UTI-induced down-regulation of PMA-stimulated uPA expression is irreversible and is independent of a cytotoxic effect. Down-regulation of uPA by UTI is mediated by its binding to the cells. We next asked whether the mechanism of inhibition of uPA expression by UTI was due to interference with the protein kinase C second messenger system. An assay for PKC activity demonstrated that UTI does not directly inhibit the catalytic activity of PKC and that PMA translocation of PKC from cytosol to membrane was inhibited by UTI, indicating that UTI inhibits the activation cascade of PKC. PMA could also activate a signaling pathway involving MEK1/ERK2/c-Jun-dependent uPA expression. When cells were preincubated with UTI, we could detect suppression of phosphorylation of these proteins. Like several types of PKC inhibitor, UTI inhibited PMA-stimulated invasiveness. We conclude that UTI markedly suppresses the cell motility possibly through negative regulation of PKC- and MEK/ERK/c-Jun-dependent mechanisms, and that these changes in behavior are correlated with a coordinated down-regulation of uPA which is likely to contribute to the cell invasion processes.

Protein kinase c-dependent pathway is critical for the production of pro-inflammatory cytokines (TNF-alpha, IL-1beta, IL-6)

The authors hypothesized that certain PKC isoforms play an important role in the induction of pro-inflammatory cytokine (TNF-alpha, IL-1beta, IL-6) synthesis. To test this hypothesis, the cytosol-to-membrane translocation of select PKC isoforms with tested cytokine production in human monocytes cultured in vitro was correlated. It is reported that in monocytes treated with phorbol ester (PMA), translocation of PKC isoforms alpha, betaII, delta and epsilon precede cytokine synthesis. Moreover, specific inhibition of PKC translocation that occurs in the presence of Calphostin C is reflected in downstream events: lack of MAP kinases phosphorylation, loss of DNA binding ability by AP-1 transcription factor, and the reduction of pro-inflammatory cytokine synthesis. Thus, the cytosol-to-membrane translocation of PKC isoforms alpha, betaII, delta and epsilon with the subsequent activation of: (1) MAP kinases; and (2) AP-1 transcription factor, may represent critical steps in the induction of signalling cascade leading to TNF-alpha, IL-1beta, IL-6 synthesis in human monocytes.

Molecular heterogeneity in medulloblastoma with implications for differing tumor biology

Medulloblastomas and related primitive neuroectodermal tumors are the second most common malignant tumors of childhood. In spite of improvements in cancer therapy, these tumors are still associated with significant morbidity and mortality. Although these tumors share similar histologic features, recent molecular studies suggest that they could represent a genetically mixed group of tumors. The genetic events that might play a role in the biology of these tumors also could allow a molecular subtyping of medulloblastomas. Such molecular subtyping of medulloblastomas could allow for the use of newer therapeutic techniques, such as gene therapy, for selective targeting of critical genetic events in subsets of medulloblastomas. It is becoming increasingly clear that in medulloblastomas, the morphologic similarity of "small blue" cells does not imply similar or shared molecular characteristics, with implications for differing tumor biology.

MAP kinase pathways

MAP kinases help to mediate diverse processes ranging from transcription of protooncogenes to programmed cell death. More than a dozen mammalian MAP kinase family members have been discovered and include, among others, the well studied ERKs and several stress-sensitive enzymes. MAP kinases lie within protein kinase cascades. Each cascade consists of no fewer than three enzymes that are activated in series. Cascades convey information to effectors, coordinates incoming information from other signaling pathways, amplify signals, and allow for a variety of response patterns. Subcellular localization of enzymes in the cascades is an important aspect of their mechanisms of action and contributes to cell-type and ligand-specific responses. Recent findings on these properties of MAP kinase cascades are the major focus of this review.

No role for Ca++ or protein kinase C in alpha-1A adrenergic receptor activation of mitogen-activated protein kinase pathways in transfected PC12 cells

We studied the role of Ca++ and protein kinase C (PKC) in alpha-1A adrenergic receptor (AR)-mediated activation of mitogen-activated protein kinase pathways in PC12 cells. In PC12 cells stably transfected with the human alpha-1A AR, norepinephrine (NE) strongly activated both extracellular signal regulated kinases (ERKs) and c-jun-NH2-terminal kinases (JNK). Ten nanomolar thapsigargin (TG) increased cytoplasmic Ca++ at least as much as NE but did not activate ERKs or JNK. Higher concentrations of TG caused a small activation of ERKs but not JNK. Emptying [Ca++]i stores by pretreatment with TG prevented the NE-stimulated increase in [Ca++]i but not ERK or JNK activation. The Ca++ chelator bis(2-aminophenoxy)ethane-N-N-N'-N'-tetraacetate (BAPTA) dose dependently abolished NE-stimulated Ca++ responses but not ERK or JNK activation. NE increased tyrosine phosphorylation of Pyk2, and this response was neither blocked by BAPTA nor mimicked by TG. The phorbol ester tumor promoting agent (TPA) caused a dose-dependent activation of ERKs that was potentiated by 10 nM TG. TPA caused only a small activation of JNK relative to that caused by NE, which was not affected by TG. The potent PKC inhibitor bisindolylmaleimide I dose dependently inhibited ERK and JNK activation by TPA, but not NE. ATP and UTP activated similar mitogen-activated protein kinase responses through endogenous P2Y2 receptors, and these responses were not blocked by BAPTA or bisindolylmaleimide I, suggesting that these results may be generalizable to other Gq/11-coupled receptors. The results suggest that Ca++ release and PKC activation are neither necessary nor sufficient for alpha-1A AR-mediated activation of mitogenic responses in PC12 cells.

Taxol selectively blocks microtubule dependent NF-kappaB activation by phorbol ester via inhibition of IkappaBalpha phosphorylation and degradation

Activation of the NF-kappa-B transcription factors has been shown to be directly influenced by changes in the microtubule cytoskeleton network. To better understand cytoskeletal regulation of NF-kappaB, experiments were performed to determine whether the microtubule (MT) stabilizing agent taxol could modulate NF-kappaB activation in the presence of different NF-kappa-B inducers. Pretreatment of murine NIH3T3 and human 293 cells with 5 microM taxol resulted in complete inhibition of phorbol, 12-myristate, 13-acetate (PMA) mediated NF-kappaB activation, detected as the loss of DNA binding and reduced NF-kappaB dependent reporter gene activity. Furthermore, in COS-7 and NIH3T3 cells, PMA-induced Ikappa-Balpha turnover was dramatically reduced in taxol treated cells, mediated via the inhibition of IkappaBalpha phosphorylation. However, taxol did not prevent TNF-alpha induced Ikappa-Balpha phosphorylation, degradation, or NF-kappaB activation, indicating that TNF-alpha acts through a microtubule-independent pathway. In vitro kinase assays with PMA stimulated cell extracts demonstrated that taxol reduced protein kinase C activity by 30%, thus implicating the loss of PKC activity as a possible regulatory target of taxol-mediated suppression of NF-kappa-B. Since PMA causes modulation of cytoarchitecture through PKC activation, microtubule integrity and cell morphology was analysed by indirect immunofluorescence. Both PMA and nocodazole, a MT depolymerizing agent, caused microtubule depolymerization, whereas TNF-alpha did not alter MT integrity; concomitant taxol treatment blocked both nocodazole and PMA induced depolymerization of MTs, as well as NF-kappaB induction, thus demonstrating a link between microtubule depolymerization and NF-kappaB activation. These observations illustrate a novel biological activity of taxol as a selective inhibitor of NF-kappa-B activity, suggesting a link between the state of microtubule integrity and gene regulation.

Changes in PKC isoforms in human alveolar macrophages compared with blood monocytes

Alveolar macrophages play an important role in host defense and in other types of inflammatory processes in the lung. These cells exhibit many alterations in function compared with their precursor cells, blood monocytes. To evaluate a potential mechanism for these differences in function, we evaluated expression of protein kinase C (PKC) isoforms. We found an increase in Ca2+-dependent PKC isoforms in monocytes compared with alveolar macrophages. We also found differential expression of the Ca2+-independent isoforms in alveolar macrophages compared with monocytes. One consequence of the activation of PKC can be increased expression of mitogen-activated protein (MAP) kinase pathways. Therefore, we also evaluated activation of the MAP kinase extracellular signal-regulated kinase (ERK) 2 by the phorbol ester phorbol 12-myristate 13-acetate (PMA). PMA activated ERK2 kinase in both alveolar macrophages and monocytes; however, monocytes consistently showed a significantly greater activation of ERK2 kinase by PMA compared with alveolar macrophages. Another known consequence of the activation of PKC and subsequent activation of ERK kinase is activation of the transcription factor activator protein-1 (AP-1). We evaluated the activation of AP-1 by PMA in both monocytes and macrophages. We found very little detectable activation of AP-1, as assessed in a gel shift assay, in alveolar macrophages, whereas monocytes showed a substantial activation of AP-1 by PMA. These studies show that the differential expression of PKC isoforms in alveolar macrophages and blood monocytes is associated with important functional alterations in the cells.

Differential translocation of protein kinase C isozymes by phorbol esters, EGF, and ANG II in rat liver WB cells

The protein kinase C (PKC) family represents an important group of enzymes whose activation is associated with their translocation from the cytosol to different cellular membranes. In this study, the spatial distribution of PKC-alpha, -delta and -epsilon in rat liver epithelial (WB) cells has been examined by Western blot analysis after subcellular fractionation. Cytosolic, membrane, nuclear, and cytoskeletal fractions were obtained from cells stimulated with phorbol 12-myristate 13-acetate (PMA), angiotensin II (ANG II), or epidermal growth factor (EGF). PMA caused most of the PKC-alpha, -delta and -epsilon initially present in the cytosol to be transported to the membrane and nuclear fractions. In contrast, both ANG II and EGF induced only a minor translocation of PKC-alpha to the membrane fraction but caused a statistically significant membrane-directed movement of PKC-delta and -epsilon. Translocation of PKC-delta and -epsilon to the nucleus induced by ANG II and EGF was transient and quantitatively smaller than that induced by PMA. PKC-delta and -epsilon were present in the cytoskeleton of resting cells, but although PMA, ANG II, and EGF caused some changes in their content, these were variable, suggesting that the cytoskeleton fraction was heterogeneous. PKC depletion inhibited ANG II-induced mitogenesis and the sustained activation of Raf-1 and extracellular regulated protein kinase (ERK). However, although PKC depletion inhibited EGF-induced mitogenesis, the maximum EGF-induced activation of the ERK pathway was only slightly retarded. We hypothesize that PKC-delta and -epsilon are involved in mitogenesis via both ERK-dependent and ERK-independent mechanisms. These results support the notion that specific PKC isozymes exert spatially defined effects by virtue of their directed translocation to distinct intracellular sites.

Stimulation of phosphorylation of mitogen-activated protein kinase by 1alpha,25-dihydroxyvitamin D3 in promyelocytic NB4 leukemia cells: a structure-function study

Recent studies have shown that 1alpha,25-dihydroxyvitamin D3 [1alpha,25-(OH)2D3] actions in cell growth and differentiation are mediated by both its nuclear receptor (VDRnuc) and its rapid membrane-related effects. In the present study, we investigated the effect of 1alpha,25-(OH)2D3 on p42mapk phosphorylation using human acute promyelocytic leukemia cells (NB4). 1Alpha,25-(OH)2D3 (10[-8] M) significantly increased p42mapk phosphorylation in a time- and dose-dependent manner, with the earliest response detectable at 30 sec. Because 1alpha,25-(OH)2D3 is a conformationally flexible molecule, we have used a series of conformationally locked (6-s-cis vs. 6-s-trans) analogs to evaluate which shape is optimal for activation. Four 6-s-cis-locked analogs (HF, JM, JN, and JP) and two 6-s-trans-locked analog (JB and JD) were studied. HF, JM, JN, and JP all increased p42mapk phosphorylation at 1 and 5 min (10[-8] M), but JB and JD had little effect. Analog HL [1beta,25-(OH)2D3], a specific antagonist for only the rapid effects of 1alpha,25-(OH)2D3, attenuated 1alpha,25-(OH)2D3-induced p42mapk phosphorylation 65-90%. To assess the potential involvement of the VDRnuc in mediating the analog's action, the relative abilities of the analogs to compete with [3H]1alpha,25-(OH)2D3 for binding in vitro to the VDRnuc of NB4 cells was measured. All 6-s-cis analogs bound poorly to VDRnuc (relative competitive index, 0.5-2%) compared with 1alpha,25-(OH)2D3 (relative competitive index, 100%). The present studies demonstrate for the first time that in NB4 cells 1alpha,25-(OH)2D3 rapidly activates the p42mapk pathway, and that this effect can be selectively mediated by analogs that can assume a 6-s-cis conformation.

PKC-epsilon is required for mechano-sensitive activation of ERK1/2 in endothelial cells

Mechano-sensitive regulation of endothelial cells (EC) function by shear stress is critical for flow-induced vasodilation and gene expression. Previous studies by our laboratory demonstrated that shear stress activates the 44- and 42-kDa extracellular signal-regulated kinases (ERK1/2) in EC in a time- and force-dependent manner. ERK1/2 activation was inhibited by protein kinase C (PKC) down-regulation with phorbol 12,13-dibutyrate (1 microM for 24 h) but not by calcium chelation with BAPTA-AM (acetoxymethyl ester of BAPTA) (75 microM for 30 min), suggesting that a novel PKC isoform (delta, epsilon, eta, theta) mediates shear stress-induced ERK1/2 activation. Western blotting with PKC isoform-specific antibodies demonstrated expression of PKC-alpha, -epsilon, and -zeta isoforms in EC. PKC-epsilon was specifically inhibited by transfection with antisense PKC-epsilon phosphorothioate oligonucleotides (1,000 nM for 6 h). Antisense treatment decreased PKC-epsilon protein levels by 80 +/- 13% after 72 h and completely inhibited shear stress-stimulated ERK1/2 activation. Scrambled PKC-epsilon oligonucleotides and antisense PKC-alpha and PKC-zeta oligonucleotides had no effect on ERK1/2 activity. PKC-epsilon appeared specific for mechano-sensitive ERK1/2 activation, as antisense PKC-epsilon oligonucleotides did not inhibit ERK1/2 activation by EGF or bradykinin but did inhibit ERK1/2 activation upon EC adhesion to fibronectin. These results define a pathway for shear stress-mediated ERK1/2 activation and establish a new function for PKC-epsilon as part of a mechano-sensitive signal transduction pathway in EC.