Protein kinase C activates the MEK-ERK pathway in a manner independent of Ras and dependent on Raf

The structural basis for cancer treatment decisions

Cancer treatment decisions rely on genetics, large data screens and clinical pharmacology. Here we point out that genetic analysis and treatment decisions may overlook critical elements in cancer development, progression and drug resistance. Two critical structural elements are missing in genetics-based decision-making: the mechanisms of oncogenic mutations and the cellular network which is rewired in cancer. These lay the foundation for the structural basis for cancer treatment decisions, which is rooted in the physical principles of the molecular conformational behavior of single molecules and their interactions. Improved tumor mutational analysis platforms and knowledge of the redundant pathways which can take over in cancer, may not only supplement known actionable findings, but forecast possible cancer progression and resistance. Such forward-looking can be powerful, endowing the oncologist with mechanistic insight and cancer prognosis, and consequently more informed treatment options. Examples include redundant pathways taking over after inhibition of EGFR constitutive activation, mutations in PIK3CA p110α and p85, and the non-hotspot AKT1 mutants conferring constitutive membrane localization.

Protein kinase Mζ is involved in the modulatory effect of fluoxetine on hippocampal neurogenesis in vitro

The efficacy of chronic selective serotonin reuptake inhibitors (SSRIs) on depression is paralleled by the recovery of deficits in hippocampal neurogenesis related to sustained stress and elevated glucocorticoids. Previous studies have shown that atypical protein kinase C (aPKC) is implicated in the regulation of neurogenesis and the antidepressant response. Whether the specific aPKC isoforms (PKCζ, PKMζ and PKCι) are involved in SSRI-induced hippocampal neurogenesis and the underlying mechanisms is unknown. The present study shows that PKMζ and PKCι but not PKCζ are expressed in rat embryonic hippocampal neural stem cells (NSCs), whereas PKMζ but not PKCι expression is increased by the SSRI fluoxetine both in the absence and presence of the glucocorticoid receptor agonist dexamethasone. PKMζ shRNA significantly decreased neuronal proliferation and neuron-oriented differentiation, increased NSC apoptosis, and blocked the stimulatory effect of fluoxetine on NSC neurogenesis. Fluoxetine significantly increased PKMζ expression in hippocampal NSCs in a 5-hydroxytryptamine-1A (5-HT1A) receptor-dependent manner in both the absence and presence of dexamethasone. The PKMζ peptide blocker ZIP and MEK inhibitor U0126 significantly inhibited the increase in extracellular signal-regulated kinase 1/2 and cyclic adenosine monophosphate response element binding protein phosphorylation in the mitogen-activated protein kinase (MAPK) pathway and hippocampal NSC neurogenesis in response to fluoxetine and the 5-HT1A receptor agonist 8-OH DPAT. Collectively, our results suggest that the SSRI fluoxetine increases hippocampal NSC neurogenesis via a PKMζ-mediated mechanism that links 5-HT1A receptor activation with the phosphorylation of the downstream MAPK signaling pathway.

Therapeutic potential of targeting the oncogenic SHP2 phosphatase

The Src homology 2 domain containing protein tyrosine phosphatase-2 (SHP2) is an oncogenic phosphatase associated with various kinds of leukemia and solid tumors. Thus, there is substantial interest in developing SHP2 inhibitors as potential anticancer and antileukemia agents. Using a structure-guided and fragment-based library approach, we identified a novel hydroxyindole carboxylic acid-based SHP2 inhibitor 11a-1, with an IC₅₀ value of 200 nM and greater than 5-fold selectivity against 20 mammalian PTPs. Structural and modeling studies reveal that the hydroxyindole carboxylic acid anchors the inhibitor to the SHP2 active site, while interactions of the oxalamide linker and the phenylthiophene tail with residues in the β₅–β₆ loop contribute to 11a-1’s binding potency and selectivity. Evidence suggests that 11a-1 specifically attenuates the SHP2-dependent signaling inside the cell. Moreover, 11a-1 blocks growth factor mediated Erk1/2 and Akt activation and exhibits excellent antiproliferative activity in lung cancer and breast cancer as well as leukemia cell lines.

Different molecular profiles are associated with breast cancer cell homing compared with colonisation of bone: evidence using a novel bone-seeking cell line

Advanced breast cancer is associated with the development of incurable bone metastasis. The two key processes involved, tumour cell homing to and subsequent colonisation of bone, remain to be clearly defined. Genetic studies have indicated that different genes facilitate homing and colonisation of secondary sites. To identify specific changes in gene and protein expression associated with bone-homing or colonisation, we have developed a novel bone-seeking clone of MDA-MB-231 breast cancer cells that exclusively forms tumours in long bones following i.v. injection in nude mice. Bone-homing cells were indistinguishable from parental cells in terms of growth rate in vitro and when grown subcutaneously in vivo. Only bone-homing ability differed between the lines; once established in bone, tumours from both lines displayed similar rates of progression and caused the same extent of lytic bone disease. By comparing the molecular profile of a panel of metastasis-associated genes, we have identified differential expression profiles associated with bone-homing or colonisation. Bone-homing cells had decreased expression of the cell adhesion molecule fibronectin and the migration and calcium signal binding protein S100A4, in addition to increased expression of interleukin 1B. Bone colonisation was associated with increased fibronectin and upregulation of molecules influencing signal transduction pathways and breakdown of extracellular matrix, including hRAS and matrix metalloproteinase 9. Our data support the hypothesis that during early stages of breast cancer bone metastasis, a specific set of genes are altered to facilitate bone-homing, and that disruption of these may be required for effective therapeutic targeting of this process.

Effect of cinnamon water extract on monocyte-to-macrophage differentiation and scavenger receptor activity

Background

Water soluble cinnamon extract has been shown to increase insulin sensitivity and modulate macrophage activation, a desirable trait for the management of obesity or atherosclerosis. Our present study investigated whether cinnamon water extract (CWE) may influence the differentiation of monocytes into macrophages and the activity of macrophage scavenger receptors, commonly observed in atherosclerotic lesions.

Methods

We investigated the effect of CWE on the expression of various surface markers and the uptake of acetylated low density lipoprotein (LDL) in phorbol-12-myristate-13-acetate (PMA)-stimulated THP-1 cells. The protein levels of PMA or macrophage-colony stimulating factor (M-CSF)-stimulated type 1 macrophage scavenger receptor (SRA) were analyzed. Finally, the role of extracellar signal-related kinase (ERK) 1/2 in SRA synthesis and the effect of CWE on PMA-stimulated ERK1/2 were determined.

Results

CWE inhibited the differentiation of monocyte by decreasing the expression of CD11b, CD36 and SRA and the uptake of acetyl LDL. CWE suppressed the upregulation of SRA by M-CSF and modulated ERK1/2 activity, which was required for PMA-induced SRA synthesis.

Conclusions

Our results demonstrate that CWE was able to interfere with monocyte differentiation and macrophage scavenger activity, indicating its potential in preventing the development of atherosclerotic lesions.

Ras palmitoylation is necessary for N-Ras activation and signal propagation in growth factor signalling

Ras GTPases undergo post-translational modifications that govern their subcellular trafficking and localization. In particular, palmitoylation of the Golgi tags N-Ras and H-Ras for exocytotic transport and residency at the PM (plasma membrane). Following depalmitoylation, PM-Ras redistributes to all subcellular membranes causing an accumulation of palmitate-free Ras at endomembranes, including the Golgi and endoplasmic reticulum. Palmitoylation is unanimously regarded as a critical modification at the crossroads of Ras activity and trafficking control, but its precise relevance to native wild-type Ras function in growth factor signalling is unknown. We show in the present study by use of palmitoylation-deficient N-Ras mutants and via the analysis of palmitate content of agonist-activated GTP-loaded N-Ras that only palmitoylated N-Ras becomes activated by agonists. In line with an essential role of palmitoylation in Ras activation, dominant-negative RasS17N loses its blocking potency if rendered devoid of palmitoylation. Live-cell Ras-GTP imaging shows that N-Ras activation proceeds only at the PM, consistent with activated N-Ras-GTP being palmitoylated. Finally, palmitoylation-deficient N-Ras does not sustain EGF (epidermal growth factor) or serum-elicited mitogenic signalling, confirming that palmitoylation is essential for signal transduction by N-Ras. These findings document that N-Ras activation proceeds at the PM and suggest that depalmitoylation, by removing Ras from the PM, may contribute to the shutdown of Ras signalling.

Brd2 inhibits adipogenesis via the ERK1/2 signaling pathway in 3T3-L1 adipocytes

Bromodomain-containing protein 2 (Brd2) is a nuclear serine/threonine kinase involved in transcriptional regulation. In 3T3-L1 adipocytes, Brd2 normally co-represses PPARγ (peroxisome proliferator-activated receptor gamma) and inhibits adipogenesis. Here, we show that Brd2 over-expression in preadipocytes inhibits their differentiation into adipocytes, while Brd2 knockdown promotes adipogenic differentiation in vitro and forces cells to undergo adipogenesis independent of the MDI (methyisobutylxanthane, dexamethasone and insulin) induction. In this study, the two key transcription factors for adipogenesis, PPARγ and C/EBPα (CCAAT/enhancer binding protein-α) were persistently expressed during the differentiation of preadipocytes to mature adipocytes in Brd2 knockdown 3T3-L1 cells, but their expression was inhibited in cells in which Brd2 was overexpressed. To investigate the role of Brd2 in signal transduction we examined the expression of several signaling molecules involved in the regulation of gene expression and cell differentiation by immunoblotting assay. Down-regulation of Brd2 expression in 3T3-L1 cells led to a decrease in extracellular signal-regulated kinase1/2 (ERK1/2) activity and, conversely, the up-regulation of Brd2 leads to increase in ERK1/2 phosphorylation. Nevertheless, changes in Brd2 expression do not affect the activities of JNK and p38 MAPK. In addition, the phosphorylation of Rafs is not affected by changes in Brd2 expression in 3T3-L1 cells. MEK inhibitor UO126 partly restores differentiation of 3T3-L1 cells that overexpress Brd2. In conclusion, these results indicate that Brd2 regulates ERK1/2 activity independently of Raf signaling in 3T3-L1 adipocytes.

Combined PKC and MEK inhibition in uveal melanoma with GNAQ and GNA11 mutations

Uveal melanoma (UM) is a genetically and biologically distinct type of melanoma, and once metastatic there is no effective treatment currently available. Eighty percent of UMs harbor mutations in the Gαq family members GNAQ and GNA11. Understanding the effector pathways downstream of these oncoproteins is important to identify opportunities for targeted therapy. We report consistent activation of the protein kinase C (PKC) and MAPK pathways as a consequence of GNAQ or GNA11 mutation. PKC inhibition with AEB071 or AHT956 suppressed PKC and MAPK signalling and induced G1 arrest selectively in melanoma cell lines carrying GNAQ or GNA11 mutations. In contrast, treatment with two different MEK inhibitors, PD0325901 and MEK162, inhibited the proliferation of melanoma cell lines irrespective of their mutation status, indicating that in the context of GNAQ or GNA11 mutation MAPK activation can be attributed to activated PKC. AEB071 significantly slowed the growth of tumors in an allograft model of GNAQ(Q209L)-transduced melanocytes, but did not induce tumor shrinkage. In vivo and in vitro studies showed that PKC inhibitors alone were unable to induce sustained suppression of MAP-kinase signaling. However, combinations of PKC and MEK inhibition, using either PD0325901or MEK162, led to sustained MAP-kinase pathway inhibition and showed a strong synergistic effect in halting proliferation and in inducing apoptosis in vitro. Furthermore, combining PKC and MEK inhibition was efficacious in vivo, causing marked tumor regression in a UM xenograft model. Our data identify PKC as a rational therapeutic target for melanoma patients with GNAQ or GNA11 mutations and demonstrate that combined MEK and PKC inhibition is synergistic, with superior efficacy compared to treatment with either approach alone.

Targeting SHP2 for EGFR inhibitor resistant non-small cell lung carcinoma

Targeted therapy with inhibitors of epidermal growth factor receptor (EGFR) has produced a noticeable benefit to non-small cell lung cancer (NSCLC) patients whose tumors carry activating mutations (e.g. L858R) in EGFR. Unfortunately, these patients develop drug resistance after treatment, due to acquired secondary gatekeeper mutations in EGFR (e.g. T790M). Given the critical role of SHP2 in growth factor receptor signaling, we sought to determine whether targeting SHP2 could have therapeutic value for EGFR inhibitor resistant NSCLC. We show that SHP2 is required for EGF-stimulated ERK1/2 phosphorylation and proliferation in EGFR inhibitor resistant NSCLC cell line H1975, which harbors the EGFR T790M/L858R double-mutant. We demonstrate that treatment of H1975 cells with II-B08, a specific SHP2 inhibitor, phenocopies the observed growth inhibition and reduced ERK1/2 activation seen in cells treated with SHP2 siRNA. Importantly, we also find that II-B08 exhibits marked anti-tumor activity in H1975 xenograft mice. Finally, we observe that combined inhibition of SHP2 and PI3K impairs both the ERK1/2 and PI3K/AKT signaling axes and produces significantly greater effects on repressing H1975 cell growth than inhibition of either protein individually. Collectively, these results suggest that targeting SHP2 may represent an effective strategy for treatment of EGFR inhibitor resistant NSCLCs.

Synergy between enzastaurin doxorubicin in inducing melanoma apoptosis

Melanoma is resistant to most standard chemotherapeutics. We analysed the combined effect of doxorubicin and enzastaurin on cell death of four melanoma cell lines, namely G361, SK-MEL3, A375 and SAN. Enzastaurin IC50 was calculated by measure of growth inhibition with MTS assay and corresponded to 2 μM; the half maximal cytotoxicity of doxorubicin was obtained at 3 μM dose. Evaluation of combination index showed synergism (CI > 1) or additive effect (CI = 1) with all melanoma cell lines, with enzastaurin doses ≥0.6 μM and doxorubicin doses ≥1 μM. Combination of the two drugs resulted in increase in caspase 3 and 8 activation, in comparison with activation by single agents. Caspase 8 activation was impaired by TNFR-1 blocking. Our results show doxorubicin-stimulated production of TNFα, whereas enzastaurin-stimulated TNFR-1 expression on plasma membrane. The effect on TNFR-1 appeared to be mediated by PKCζ inhibition. Taken together, our findings suggest that enzastaurin increases doxorubicin-induced apoptosis of melanoma by a mechanism involving, at least in part, activation of the TNF-α signal.

Mitogen-Activated Protein (MAP) Kinase Scaffolding Proteins: A Recount

The mitogen-activated protein kinase (MAPK) pathway is the canonical signaling pathway for many receptor tyrosine kinases, such as the Epidermal Growth Factor Receptor. Downstream of the receptors, this pathway involves the activation of a kinase cascade that culminates in a transcriptional response and affects processes, such as cell migration and adhesion. In addition, the strength and duration of the upstream signal also influence the mode of the cellular response that is switched on. Thus, the same components can in principle coordinate opposite responses, such as proliferation and differentiation. In recent years, it has become evident that MAPK signaling is regulated and fine-tuned by proteins that can bind to several MAPK signaling proteins simultaneously and, thereby, affect their function. These so-called MAPK scaffolding proteins are, thus, important coordinators of the signaling response in cells. In this review, we summarize the recent advances in the research on MAPK/extracellular signal-regulated kinase (ERK) pathway scaffolders. We will not only review the well-known members of the family, such as kinase suppressor of Ras (KSR), but also put a special focus on the function of the recently identified or less studied scaffolders, such as fibroblast growth factor receptor substrate 2, flotillin-1 and mitogen-activated protein kinase organizer 1.

Expression pattern of protein kinase Cδ during mouse embryogenesis

Background

The members of the protein kinase C (PKC) family consist of serine/threonine kinases classified according to their regulatory domain. Those that belong to the novel PKC subfamily, such as PKCδ, are dependent on diacylglycerol but not Calcium when considering their catalytic activity. Although several studies have shown the importance of PKCδ in different cellular events in health and disease, the overall in vivo distribution of this PKC isoform during development is still lacking. Through Lac Z and antibody staining procedures, we show here the in vivo expression of PKCδ during mouse embryogenesis.

Results

Ganglia were the domains with most prominent expression of PKCδ in most of the stages analysed, although PKCδ could also be detected in heart and somites at earlier stages, and cartilage primordium and skin among other sites in older embryos.

Conclusions

The strong expression of PKCδ in ganglia during murine development shown in this study suggests a significant role of this isoform as well as redundancy with other PKCs within the nervous system, since PKCδ deficient mice develop normally.

PIKE mediates EGFR proliferative signaling in squamous cell carcinoma cells

One of the key drivers for squamous cell carcinoma (SCC) proliferation is activation of the epidermal growth factor receptor (EGFR), a known proto-oncogene. However, the mechanism of EGFR-dependent SCC proliferation remains unclear. Our previous studies indicate that epidermal growth factor (EGF)-induced SCC cell proliferation requires the SH3 domain of phospholipase C-γ1 (PLC-γ1), but not its catalytic activity. The SH3 domain of PLC-γ1 is known to activate the short form of nuclear phosphatidylinositol 3-kinase enhancer (PIKE) that enhances the activity of nuclear class Ia phosphatidylinositol 3-kinase (PI3K) required for proliferation. However, PIKE has been described for more than a decade to be present exclusively in neuronal cells. In the present study, we found that PIKE was highly expressed in malignant human keratinocytes (SCC4 and SCC12B2) but had low expression in normal human keratinocytes. Immunohistochemical analysis showed strong nuclear staining of PIKE in human epidermal and tongue SCC specimens but little staining in the adjacent non-cancerous epithelium. Treatment of SCC4 cells with EGF-induced translocation of PLC-γ1 to the nucleus and binding of PLC-γ1 to the nuclear PIKE. Knockdown of PLC-γ1 or PIKE blocked EGF-induced activation of class Ia PI3K and protein kinase C-ζ and phosphorylation of nucleolin in the nucleus as well as EGF-induced SCC cell proliferation. However, inhibition of the catalytic activity of PLC-γ1 had little effect. These data suggest that PIKE has a critical role in EGF-induced SCC cell proliferation and may function as a proto-oncogene in SCC.

Specifying protein kinase C functions in melanoma

The protein kinase C (PKC) family of serine/threonine protein kinases is a heterogeneous group of enzymes receiving and integrating signals involved in both normal melanocyte biology and melanoma pathology. Alterations in PKC enzyme expression and activation contribute to the malignant phenotype of melanoma in both oncogenic and tumor suppressive roles. Delineating the diverse and often context-dependent functions of PKC enzymes in melanocyte/melanoma biology is key to capitalize on these kinases as drug targets. This review summarizes several of the diverse functions of PKC in melanocyte and melanoma biology with a focus on PKC enzyme regulation and function.

The expression and role of non-canonical (PKC) signaling in nucleus pulposus cell metabolism

Canonical Wnt/β-catenin (hereafter Wnt) signaling regulates the proliferation and differentiation of various cell types. However, the role of non-canonical signaling including protein kinase C (PKC) signaling has not been investigated in intervertebral disc (IVD) cells. The aim of this study was to elucidate whether the activation of PKC signaling act to modulate Wnt signaling in IVD cells. We performed several reporter assays, real-time reverse transcription polymerase chain reaction (RT-PCR), immunohistochemical and immunofluorescence analyses, and western blot analyses using rat nucleus pulposus (NP) cells. We also examined the cell proliferation and cell cycle distribution under phorbol 12-myristate 13-acetate (PMA) stimulation, a known activator of PKC signaling. We found that NP cells exhibited decreased β-catenin mRNA and protein levels upon stimulation with PMA. PMA treatment promoted proliferation and cell cycle progression in a time- and dose-dependent manner. In addition, activation of the PKC signaling also regulated the expression of aggrecan. Finally, activation by PMA induced the expression of several PKC isoforms in NP cells. It is concluded that activation of PKC signaling might lead to an increase in matrix synthesis and cell proliferation, thereby inhibiting IVD degeneration. Crosstalk in these signaling pathways plays an important role in the regulation of IVD homeostasis.

PKCδ regulates death receptor 5 expression induced by PS-341 through ATF4-ATF3/CHOP axis in human lung cancer cells

PS-341 (bortezomib), a proteasome inhibitor, has been approved for the treatment of multiple myeloma. Our previous work has shown that PS-341 induces death receptor 5 (DR5)-dependent apoptosis and enhances the TNF-related apoptosis-inducing ligand-induced apoptosis in human non-small cell lung cancer cells. However, the definite mechanism remains undefined. In the present study, we reveal that PKCδ and RSK2 mediate PS-341-induced DR5 upregulation, involving coactivation of endoplasmic reticulum (ER) stress. We discovered that PS-341 activated ER stress through elevating the expression of BiP, p-eIF2α, IRE1α, ATF4, ATF3, and CCAAT/enhancer-binding protein homologous protein (CHOP). Further study showed that DR5 upregulation was dependent on ATF4, ATF3, and CHOP expression. Silencing either one of the ATF4, ATF3, and CHOP expression decreased DR5 upregulation and subsequent apoptosis. We determined that ATF4 regulated ATF3 and CHOP expression. Thereafter, ATF3 and CHOP formed a complex and regulated DR5 expression. In addition, we discovered that the phosphorylation of extracellular signal-regulated kinase (ERK) 1/2 and RSK2 were elevated after PS-341 treatment and inhibition of their phosphorylation using MAP-ERK kinase 1/2 inhibitor decreased the DR5 level, indicating that ERK/RSK2 signaling is involved in DR5 upregulation. Furthermore, we detected the cleavage of PKCδ, and the blockage of PKCδ expression cut down DR5 upregulation and apoptosis. Importantly, knockdown of PKCδ expression decreased the induction of ER stress and the phosphorylation of ERK1/2 and RSK2, suggesting that PKCδ regulates DR5 expression through ERK/RSK2 signaling and ATF4-CHOP/ATF3 axis. Collectively, we show that PS-341 induces PKCδ-dependent DR5 expression through activation of ERK/RSK2 and ER stress signaling pathway.

Bryostatin-1 vs. TPPB: dose-dependent APP processing and PKC-α, -δ, and -ε isoform activation in SH-SY5Y neuronal cells

Activation of the α-secretase processing pathway of amyloid precursor protein (APP) is recognized as an important mechanism which diverts APP processing from production of beta-amyloid (Aβ) to non toxic sAPPα, decreasing Alzheimer’s disease (AD) plaque formation and AD-associated cognitive deficits. Two potent classes of PKC modulators can activate the α-secretase pathway, the benzo/indolactams and bryostatin/bryologues. While both modulate PKC-dependent APP processing, no direct comparisons of their relative pharmacological potencies have been accomplished which could assist in the development of AD therapies. In this study, we measured the activation of α-secretase APP processing and PKC-α, -δ, and -ε induced by the benzolactam-APP modulator TPPB and bryostatin-1 in the neuroblastoma cell line SH-SY5Y which expresses APP and α- and β-secretase processing mechanisms. Bryostatin-1 produced a more rapid, potent, and sustained activation of α-secretase APP processing than TPPB and selectively activated PKC-δ and PKC-ε. Although TPPB also activated α-secretase, its potency was approximately 10- to 100-fold lower, possibly reflecting lower PKC-δ and -ε activation. Because bryostatin-1 is a highly potent PKC-δ and -ε activator which activates α-secretase APP processing, further characterization of bryostatin-1/bryologues may help refine their use as important tools for the clinical management of AD.

Cross-talk between mitogenic Ras/MAPK and survival PI3K/Akt pathways: a fine balance

In the present paper, we describe multiple levels of cross-talk between the PI3K (phosphoinositide 3-kinase)/Akt and Ras/MAPK (mitogen-activated protein kinase) signalling pathways. Experimental data and computer simulations demonstrate that cross-talk is context-dependent and that both pathways can activate or inhibit each other. Positive influence of the PI3K pathway on the MAPK pathway is most effective at sufficiently low doses of growth factors, whereas negative influence of the MAPK pathway on the PI3K pathway is mostly pronounced at high doses of growth factors. Pathway cross-talk endows a cell with emerging capabilities for processing and decoding signals from multiple receptors activated by different combinations of extracellular cues.

ATP and noradrenaline activate CREB in astrocytes via noncanonical Ca(2+) and cyclic AMP independent pathways

In neurons, it is well established that CREB contributes to learning and memory by orchestrating the translation of experience into the activity-dependent (i.e., driven by neurotransmitters) transcription of plasticity-related genes. The activity-dependent CREB-triggered transcription requires the concerted action of cyclic AMP/protein kinase A and Ca(2+) /calcineurin via the CREB-regulated transcription co-activator (CRTC). It is not known, however, whether a comparable molecular sequence occurs in astrocytes, despite the unquestionable contribution of these cells to brain plasticity. Here we sought to determine whether and how ATP and noradrenaline cause CREB-dependent transcription in rat cortical astrocyte cultures. Both transmitters induced CREB phosphorylation (Western Blots), CREB-dependent transcription (CRE-luciferase reporter assays), and the transcription of Bdnf, a canonical regulator of synaptic plasticity (quantitative RT-PCR). We indentified a Ca(2+) and diacylglycerol-independent protein kinase C at the uppermost position of the cascade leading to CREB-dependent transcription. Notably, CREB-dependent transcription was partially dependent on ERK1/2 and CRTC, but independent of cyclic AMP/protein kinase A or Ca(2+) /calcineurin. We conclude that ATP and noradrenaline activate CREB-dependent transcription in cortical astrocytes via an atypical protein kinase C. It is of relevance that the signaling involved be starkly different to the one described in neurons since there is no convergence of Ca(2+) and cyclic AMP-dependent pathways on CRTC, which, moreover, exerts a modulatory rather than a central role. Our data thus point to the existence of an alternative, non-neuronal, glia-based role of CREB in plasticity.

Signaling through the neuropeptide GPCR PAC₁ induces neuritogenesis via a single linear cAMP- and ERK-dependent pathway using a novel cAMP sensor

Both cAMP and ERK are necessary for neuroendocrine cell neuritogenesis, and pituitary adenylate cyclase-activating polypeptide (PACAP) activates each. It is important to know whether cAMP and ERK are arranged in a novel, linear pathway or in two parallel pathways using known signaling mechanisms. Native cellular responses [cAMP elevation, ERK phosphorylation, cAMP responsive element binding (CREB) phosphorylation, and neuritogenesis] and promoter-reporter gene activation after treatment with forskolin, cAMP analogs, and PACAP were measured in Neuroscreen-1 (NS-1) cells, a PC12 variant enabling simultaneous morphological, molecular biological, and biochemical analysis. Forskolin (25 μM) and cAMP analogs (8-bromo-cAMP, dibutyryl-cAMP, and 8-chlorophenylthio-cAMP) stimulated ERK phosphorylation and neuritogenesis in NS-1 cells. Both ERK phosphorylation and neuritogenesis were MEK dependent (blocked by 10 μM U0126) and PKA independent (insensitive to 30 μM H-89 or 100 nM myristoylated protein kinase A inhibitor). CREB phosphorylation induced by PACAP was blocked by H-89. The exchange protein activated by cAMP (Epac)-selective 8-(4-chlorophenylthio)-2'-O-Me-cAMP (100-500 μM) activated Rap1 without affecting the other cAMP-dependent processes. Thus, PACAP-38 potently stimulated two distinct and independent cAMP pathways leading to CREB or ERK activation in NS-1 cells. Drug concentrations for appropriate effect were derived from control data for all compounds. In summary, a novel PKA- and Epac-independent signaling pathway: PACAP → adenylate cyclase → cAMP → ERK → neuritogenesis has been identified.

ERK acts in parallel to PKCδ to mediate the connexin43-dependent potentiation of Runx2 activity by FGF2 in MC3T3 osteoblasts

The gap junction protein, connexin43 (Cx43), plays an important role in skeletal biology. Previously, we have shown that Cx43 can enhance the signaling and transcriptional response to fibroblast growth factor 2 (FGF2) in osteoblasts by increasing protein kinase C-δ (PKCδ) activation to affect Runx2 activity. In the present study, we show by luciferase reporter assays that the ERK signaling cascade acts in parallel to PKCδ to modulate Runx2 activity downstream of the Cx43-dependent amplification of FGF2 signaling. The PKCδ-independent activation of ERK by FGF2 was confirmed by Western blotting, as was the Cx43-dependent enhancement of ERK activation. Consistent with our prior observations for PKCδ, flow cytometry analyses show that Cx43 overexpression enhances the percentage of phospho-ERK-positive cells in response to FGF2, supporting the notion that shared signals among gap junction-coupled cells result in the enhanced response to FGF2. Western blots and luciferase reporter assays performed on osteoblasts cultured under low-density and high-density conditions revealed that cell-cell contacts are required for Cx43 to amplify ERK activation and gene transcription. Similarly, inhibition of gap junctional communication with the channel blocker 18β-glycyrrhetinic acid attenuates the Cx43-dependent enhancement of Runx2-transcriptional activity. In total, these data underscore the importance of cell-cell communication and activation of the ERK and PKCδ pathways in the coordination of the osteoblast response to FGF2 among populations of osteoblasts.

Flotillin-1/reggie-2 protein plays dual role in activation of receptor-tyrosine kinase/mitogen-activated protein kinase signaling

Our previous work has shown that the membrane microdomain-associated flotillin proteins are potentially involved in epidermal growth factor (EGF) receptor signaling. Here we show that knockdown of flotillin-1/reggie-2 results in reduced EGF-induced phosphorylation of specific tyrosines in the EGF receptor (EGFR) and in inefficient activation of the downstream mitogen-activated protein (MAP) kinase and Akt signaling. Although flotillin-1 has been implicated in endocytosis, its depletion affects neither the endocytosis nor the ubiquitination of the EGFR. However, EGF-induced clustering of EGFR at the cell surface is altered in cells lacking flotillin-1. Furthermore, we show that flotillins form molecular complexes with EGFR in an EGF/EGFR kinase-independent manner. However, knockdown of flotillin-1 appears to affect the activation of the downstream MAP kinase signaling more directly. We here show that flotillin-1 forms a complex with CRAF, MEK1, ERK, and KSR1 (kinase suppressor of RAS) and that flotillin-1 knockdown leads to a direct inactivation of ERK1/2. Thus, flotillin-1 plays a direct role during both the early phase (activation of the receptor) and late (activation of MAP kinases) phase of growth factor signaling. Our results here unveil a novel role for flotillin-1 as a scaffolding factor in the regulation of classical MAP kinase signaling. Furthermore, our results imply that other receptor-tyrosine kinases may also rely on flotillin-1 upon activation, thus suggesting a general role for flotillin-1 as a novel factor in receptor-tyrosine kinase/MAP kinase signaling.

Formation of ternary complex of human biliverdin reductase-protein kinase Cδ-ERK2 protein is essential for ERK2-mediated activation of Elk1 protein, nuclear factor-κB, and inducible nitric-oxidase synthase (iNOS)

Growth factors, insulin, oxidative stress, and cytokines activate ERK1/2 by PKCδ and MEK1/2. Human biliverdin reductase (hBVR), a Ser/Thr/Tyr kinase and intracellular scaffold/bridge/anchor, is a nuclear transporter of MEK1/2-stimulated ERK1/2 (Lerner-Marmarosh, N., Miralem, T., Gibbs, P. E., and Maines, M. D. (2008) Proc. Natl. Acad. Sci. U.S.A. 105, 6870-6875). hBVR, PKCδ, and MEK1/2 overlap in their tissue expression profile and type of activators. Presently, we report on formation of an hBVR-PKCδ-ERK2 ternary complex that is essential for ERK2 signal transduction and activation of genes linked to cell proliferation and cancer. MEK1/2 and the protein phosphatase PP2A were also present in the complex. When cells were stimulated with insulin-like growth factor-1 (IGF-1), an increased interaction between hBVR and PKCδ was detected by FRET-fluorescence lifetime imaging microscopy. hBVR and ERK2 were phosphorylated by PKCδ; however, the PKC was not a substrate for either ERK2 or hBVR. IGF-1 and phorbol ester increased hBVR/PKCδ binding; hBVR was required for the activation of PKCδ and its interaction with ERK2. The C-terminal phenylalanine residues of PKCδ (Phe(660), Phe(663), and Phe(665)) were necessary for binding to ERK2 but not for hBVR binding. Formation of the hBVR-PKCδ-ERK2 complex required the hBVR docking site for ERK, FXFP (DEF, C-box) and D(δ)-box (ILXXLXL) motifs. The hBVR-based peptide KKRILHCLGLA inhibited PKC activation and PKCδ/ERK2 interaction. Phorbol ester- and TNF-α-dependent activation of the ERK-regulated transcription factors Elk1 and NF-κB and expression of the iNOS gene were suppressed by hBVR siRNA; those activities were rescued by hBVR. The findings reveal the direct input of hBVR in PKCδ/ERK signaling and identify hBVR-based peptide regulators of ERK-mediated gene activation.

Nuclear factor-kappaB (NF-kappaB) mediates a protective response in cancer cells treated with inhibitors of fatty acid synthase

The efficacy of drugs used to treat cancer can be significantly attenuated by adaptive responses of neoplastic cells to drug-induced stress. To determine how cancer cells respond to inhibition of the enzyme fatty acid synthase (FAS), we focused on NF-κB-mediated pathways, which can be activated by various cellular stresses. Treating lung cancer cells with C93, a pharmacological inhibitor of FAS, results in changes indicative of a rapid initiation of NF-κB signaling, including translocation of RelA/p65 NF-κB to the nucleus, activation of a transfected NF-κB-luciferase reporter, and increased expression of NF-κB-dependent transcripts, IL-6, IL-8, and COX-2. Verifying that these responses to C93 are specifically related to inhibition of FAS, we confirmed that levels of these same transcripts increase in response to siRNA targeting FAS. Inhibiting this NF-κB response (either by transfecting a mutant IκBα or treating with bortezomib) resulted in increased cell killing by C93, indicating that the NF-κB response is protective in this setting. Because inhibiting FAS leads to accumulation of intermediate metabolites of fatty acid biosynthesis, we then questioned whether protein kinase C (PKC) is involved in this response to metabolic stress. Immunofluorescence microscopy revealed that C93 treatment results in cellular translocation of PKCα and PKCβ isoforms and increased PKCα-dependent phosphorylation of the IκBα subunit of NF-κB. Furthermore, inhibiting PKC activity with RO-31-8220 or PKCα isoform-specific siRNA attenuates C93-induced IκBα phosphorylation and NF-κB activation and also potentiates C93-induced cell killing. These results suggest a link between PKC and NF-κB in protecting cancer cells from metabolic stress induced by inhibiting FAS.

Bradykinin activation of extracellular signal-regulated kinases in human trabecular meshwork cells

Bradykinin stimulation of B(2) kinin receptors has been shown to promote matrix metallo-proteinase (MMP) secretion from trabecular meshwork cells and to increase conventional outflow facility. Because acute secretion of MMPs can be dependent on the activity of extracellular signal-regulated MAP kinases (ERK1/2), experiments were performed to determine bradykinin effects on ERK1/2 in cultured human trabecular meshwork cells and the relationship of these effects to MMP-9 release. Treatment of cells with bradykinin produced a rapid 4-to 6-fold increase in ERK1/2 phosphorylation. Stimulation of ERK1/2 activity peaked within 2 min and then declined to control levels by 60 min. The response maximum occurred with 100nM bradykinin and the estimated EC₅₀ was 0.7nM. Treatment of cells with the B₂ kinin receptor agonist, Tyr⁸- bradykinin, also stimulated ERK1/2 phosphorylation while the B₁ agonist, Lys- [Des-Arg⁹]- bradykinin had no significant effect. In addition, activation of ERK1/2 by bradykinin or Tyr⁸- bradykinin was blocked by the selective B₂ receptor antagonist, Hoe-140. Inhibition of MAP kinase kinase (MEK) with U0126 also blocked bradykinin-induced ERK1/2 phosphorylation. Suppression of protein kinase C activity with the nonselective inhibitor, GF109203X, or by down-regulation with phorbol ester, diminished, but did not eliminate, bradykinin activation of ERK1/2. A similar decrease of ERK1/2 stimulation was observed when Src kinase was inhibited by 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2). Finally, blockade of bradykinin-induced ERK1/2 activation substantially reduced the peptide's action to stimulate MMP-9 release into the extracellular environment. The data demonstrate that bradykinin promotes ERK1/2 activation in human trabecular meshwork cells. The effect is mediated by B₂ kinin receptors, involves two different signaling pathways, and results in increased secretion of MMP-9.

Protein kinase Czeta mediates micro-opioid receptor-induced cross-desensitization of chemokine receptor CCR5

We have previously shown that the μ-opioid receptor (MOR) is capable of mediating cross-desensitization of several chemokine receptors including CCR5, but the biochemical mechanism of this process has not been fully elucidated. We have carried out a series of functional and biochemical studies and found that the mechanism of MOR-induced cross-desensitization of CCR5 involves the activation of PKCζ. Inhibition of PKCζ by its pseudosubstrate inhibitor, or its siRNA, or dominant negative mutants suppresses the cross-desensitization of CCR5. Our results further indicate that the activation of PKCζ is mediated through a pathway involving phosphoinositol-dependent kinase-1 (PDK1). In addition, activation of MOR elevates the phosphorylation level and kinase activity of PKCζ. The phosphorylation of PKCζ can be suppressed by a dominant negative mutant of PDK1. We observed that following MOR activation, the interaction between PKCζ and PDK1 is immediately increased based on the analysis of fluorescent resonance energy transfer in cells with the expression of PKCζ-YFP and PDK1-CFP. In addition, cells expressing PKCζ kinase motif mutants (Lys-281, Thr-410, Thr-560) fail to exhibit full MOR-induced desensitization of CCR5 activity. Taken together, we propose that upon DAMGO treatment, MOR activates PKCζ through a PDK1-dependent signaling pathway to induce CCR5 phosphorylation and desensitization. Because CCR5 is a highly proinflammatory receptor, and a critical coreceptor for HIV-1, these results may provide a novel approach for the development of specific therapeutic agents to treat patients with certain inflammatory diseases or AIDS.

PKCδ regulates Mdm2 independently of p53 in the apoptotic response to DNA damage

Apoptosis is the key process in which cells with defective genome can be eliminated. Dys-regulation of apoptosis causes accumulation of irreparable mutation arisen from DNA damage and is the underlying cause of carcinogenesis. PKCδ is a multifunctional kinase involved in signal transduction of genotoxic-induced apoptosis. Previous studies have demonstrated that PKCδ transactivates p53 in response to DNA damage. These findings led us to determine if Mdm2, a nuclear phospho-protein and negative regulator of p53, could also be a PKCδ-modulated substrate. We discovered that inhibition of PKCδ down-regulates Mdm2 protein expression regardless of p53 status. Given that Mdm2 mRNA change was detected in p53-proficient, but not deficient cells, PKCδ affected Mdm2 on the post-translational level. Interestingly, treatment of MG132 restored Mdm2 expression to the steady-state level. Further investigation showed that PKCδ inhibited Mdm2 ubiquitination in p53-deficient cells and loss of PKCδ resulted in an increase in Mdm2 proteosomal degradation. Moreover, P300/CBP-associated factor (PCAF), an ubiquitin ligase 3 for Mdm2, was observed to participate in Mdm2 ubiquitination by PKCδ inhibition and knock-down of PCAF rescued Mdm2 diminution. Finally, as shown for PKCδ, Mdm2 was also required to exert pro-apoptotic response caused by genotoxic agents in p53-null cells. In addition, overexpression of Mdm2 restored inhibitory effect of apoptosis in cells silenced for PKCδ. Taken together, we conclude that PKCδ regulates Mdm2 expression distinctively of p53 pathway by affecting Mdm2 ubiquitination and maintenance of Mdm2 expression by PKCδ is important to ensure normal genotoxic cell death response in human cancer cells.

PKC-1 acts with the ERK MAPK signaling pathway to regulate Caenorhabditis elegans mechanosensory response

In most animals, multiple genes encode protein kinase C (PKC) proteins. Pharmacological studies have revealed numerous roles for this protein family, yet the in vivo roles of specific PKC proteins and the functional targets of PKC activation are poorly understood. We find that in Caenorhabditis elegans, two PKC genes, pkc-1 and tpa-1, are required for mechanosensory response; the role of the nPKCε/η ortholog, pkc-1, was examined in detail. pkc-1 function is required for response to nose touch in adult C. elegans and pkc-1 likely acts in the interneurons that regulate locomotion which are direct synaptic targets of mechanosensory neurons. Previous studies have suggested numerous possible targets of pkc-1; our analysis indicates that pkc-1 may act via the extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK) pathway. We find that ERK/MAPK pathway function is required for mechanosensory response in C. elegans and that at least one component of this pathway, lin-45 Raf, acts in interneurons of the mechanosensory circuit. Genetic analysis indicates that lin-45 and pkc-1 act together to regulate nose touch response. Thus, these results functionally link two conserved signaling pathways in adult C. elegans neurons and define distinct roles for PKC genes in vivo.

ER-α36, a novel variant of ER-α, mediates estrogen-stimulated proliferation of endometrial carcinoma cells via the PKCδ/ERK pathway

BACKGROUND

Recently, a variant of ER-α, ER-α36 was identified and cloned. ER-α36 lacks intrinsic transcription activity and mainly mediates non-genomic estrogen signaling. The purpose of this study was to investigate the function and the underlying mechanisms of ER-α36 in growth regulation of endometrial Ishikawa cancer cells.

METHODS

The cellular localization of ER-α36 and ER-α66 were determined by immunofluorescence in the Ishikawa cells. Ishikawa endometrial cancer control cells transfected with an empty expression vector, Ishikawa cells with shRNA knockdown of ER-α36 (Ishikawa/RNAiER36) and Ishikawa cells with shRNA knockdown of ER-α66 (Ishikawa/RNAiER66) were treated with E2 and E2-conjugated to bovine serum albumin (E2-BSA, membrane impermeable) in the absence and presence of different kinase inhibitors HBDDE, bisindolylmaleimide, rottlerin, H89 and U0126. The phosphorylation levels of signaling molecules and cyclin D1/cdk4 expression were examined with Western blot analysis and cell growth was monitored with the MTT assay.

RESULTS

Immunofluorescence staining of Ishikawa cells demonstrated that ER-α36 was expressed mainly on the plasma membrane and in the cytoplasm, while ER-α66 was predominantly localized in the cell nucleus. Both E2 and E2-BSA rapidly activated PKCδ not PKCα in Ishikawa cells, which could be abrogated by ER-α36 shRNA expression. E2-and E2-BSA-induced ERK phosphorylation required ER-α36 and PKCδ. However, only E2 was able to induce Camp-dependent protein kinase A (PKA) phosphorylation. Furthermore, E2 enhances cyclin D1/cdk4 expression via ER-α36.

CONCLUSION

E2 activates the PKCδ/ERK pathway and enhances cyclin D1/cdk4 expression via the membrane-initiated signaling pathways mediated by ER-α36, suggesting a possible involvement of ER-α36 in E2-dependent growth-promoting effects in endometrial cancer cells.

Breviscapine protects against cardiac hypertrophy through blocking PKC-alpha-dependent signaling

Breviscapine is a mixture of flavonoid glycosides extracted from the Chinese herbs. Previous studies have shown that breviscapine possesses comprehensive pharmacological functions. However, very little is known about whether breviscapine have protective role on cardiac hypertrophy. The aim of the present study was to determine whether breviscapine attenuates cardiac hypertrophy induced by angiotensin II (Ang II) in cultured neonatal rat cardiac myocytes in vitro and pressure-overload-induced cardiac hypertrophy in mice in vivo. Our data demonstrated that breviscapine (2.5-15 microM) dose-dependently blocked cardiac hypertrophy induced by Ang II (1 microM) in vitro. The results further revealed that breviscapine (50 mg/kg/day) prevented cardiac hypertrophy induced by aortic banding as assessed by heart weight/body weight and lung weight/body weight ratios, echocardiographic parameters, and gene expression of hypertrophic markers. The inhibitory effect of breviscapine on cardiac hypertrophy is mediated by disrupting PKC-alpha-dependent ERK1/2 and PI3K/AKT signaling. Further studies showed that breviscapine inhibited inflammation by blocking NF-kappaB signaling, and attenuated fibrosis and collagen synthesis through abrogating Smad2/3 signaling. Therefore, these findings indicate that breviscapine, which is a potentially safe and inexpensive therapy for clinical use, has protective potential in targeting cardiac hypertrophy and fibrosis through suppression of PKC-alpha-dependent signaling.

RasGRP1 is essential for ras activation by the tumor promoter 12-O-tetradecanoylphorbol-13-acetate in epidermal keratinocytes

RasGRP1 is a guanine nucleotide exchange factor for Ras that binds with high affinity to diacylglycerol analogs like the phorbol esters. Recently, we demonstrated a role for RasGRP1 in skin carcinogenesis and suggested its participation in the action of tumor-promoting phorbol esters like 12-O-tetradecanoylphorbol-13-acetate (TPA) on Ras pathways in epidermal cells. Given the importance of Ras in carcinogenesis, we sought to discern whether RasGRP1 was a critical pathway in Ras activation, using a RasGRP1 knockout (KO) mouse model to examine the response of keratinocytes to TPA. In contrast to the effect seen in wild type keratinocytes, Ras(GTP) levels were barely detected in RasGRP1 KO cells even after 60 min of exposure to phorbol esters. The lack of response was rescued by enforced expression of RasGRP1. Furthermore, small hairpin RNA-induced silencing of RasGRP1 abrogated the effect of TPA on Ras. Analysis of Ras isoforms showed that both H-Ras and N-Ras depended on RasGRP1 for activation by TPA, whereas activation of K-Ras could not be detected. Although RasGRP1 was dispensable for ERK activation in response to TPA, JNK activation was reduced in the KO keratinocytes. Notably, TPA-induced phosphorylation of JNK2, but not JNK1, was reduced by RasGRP1 depletion. These data identify RasGRP1 as a critical molecule in the activation of Ras by TPA in primary mouse keratinocytes and suggest JNK2 as one of the relevant downstream targets. Given the role of TPA as a skin tumor promoter, our findings provide additional support for a role for RasGRP1 in skin carcinogenesis.

Key role of ERK pathway signaling in lupus

Systemic lupus erythematosus is a poorly understood autoimmune disease, characterized by autoantibodies to nuclear antigens and immune complex deposition in organs like the kidney. Current evidence indicates that a pathologic CD4+T cell subset, characterized by impaired extracellular signal-regulated kinase (ERK) pathway signaling, DNA hypomethylation, and consequent aberrant gene expression contributes to disease pathogenesis. Hydralazine is a lupus-inducing drug that also decreases T cell DNA methylation by inhibiting the ERK signaling pathway, replicating the defect found in lupus T cells. These observations suggest that defective ERK pathway signaling alters gene expression in T cells by inhibiting DNA methylation, contributing to lupus pathogenesis. The signaling defect in hydralazine-treated and lupus T cells has now been mapped to protein kinase C delta. Understanding the mechanism causing decreased ERK pathway signaling in lupus may shed light on mechanisms contributing to disease development in genetically predisposed people.

Glutamate induces neurotrophic factor production from microglia via protein kinase C pathway

Microglia are intrinsic immune cells in the central nervous system and play key roles in the pathogenesis of various central nervous system disorders. Microglia have been shown to attack damaged neurons by secreting a variety of neurotoxic factors including inflammatory cytokines, reactive oxygen species and glutamate. On the other hand, they can produce neurotrophic factors (NTFs) which support neuronal survival and growth. However, the precise mechanism that regulates microglial NTF production is not fully understood, and the relation between glutamate and NTFs remains unclear. In the present study, we show that glutamate significantly induces microglial NTF production by the activation of N-methyl-d-aspartate (NMDA) receptors, group III metabotropic glutamate receptors, and glutamate transporters. Activation of NMDA receptors and group III metabotropic glutamate receptors induces intracellular Ca(2+) release from the endoplasmic reticulum. Further, stimulation of glutamate transporters leads to influx of extracellular Ca(2+) in a Na(+)-dependent manner. This intracellular Ca(2+) elevation activates the protein kinase C pathway which induces microglial NTF expression and production. These results suggest that microglia play a neuroprotective role during the excitotoxic state in neurodegenerative diseases.

Up- or downregulation of tescalcin in HL-60 cells is associated with their differentiation to either granulocytic or macrophage-like lineage

Tescalcin is a 25-kDa EF-hand Ca(2+)-binding protein that is differentially expressed in several mammalian tissues. Previous studies demonstrated that expression of this protein is essential for differentiation of hematopoietic precursor cell lines and primary stem cells into megakaryocytes. Here we show that tescalcin is expressed in primary human granulocytes and is upregulated in human promyelocytic leukemia HL-60 cells that have been induced to differentiate along the granulocytic lineage. However, during induced macrophage-like differentiation of HL-60 cells the expression of tescalcin is downregulated. The decrease in expression is associated with a rapid drop in tescalcin mRNA level, whereas upregulation occurs via a post-transcriptional mechanism. Tescalcin is necessary for HL-60 differentiation into granulocytes as its knockdown by shRNA impairs the ability of HL-60 cells to acquire the characteristic phenotypes such as phagocytic activity and generation of reactive oxygen species measured by respiratory burst assay. Both up- and downregulation of tescalcin require activation of the MEK/ERK cascade. It appears that commitment of HL-60 cells toward granulocytic versus macrophage-like lineage correlates with expression of tescalcin and kinetics of ERK activation. In retinoic acid-induced granulocytic differentiation, the activation of ERK and upregulation of tescalcin occurs slowly (16-48 h). In contrast, in PMA-induced macrophage-like differentiation the activation of ERK is rapid (15-30 min) and tescalcin is downregulated. These studies indicate that tescalcin is one of the key gene products that is involved in switching differentiation program in some cell types.

P19 H-ras induces G1/S phase delay maintaining cells in a reversible quiescence state

Background

Three functional c-ras genes, known as c-H-ras, c-K-ras, and c-N-ras, have been largely studied in mammalian cells with important insights into normal and tumorigenic cellular signal transduction events. Two K-Ras mRNAs are obtained from the same pre-mRNA by alternative splicing. H-Ras pre-mRNA can also be alternatively spliced in the IDX and 4A terminal exons, yielding the p19 and p21 proteins, respectively. However, despite the Ras gene family's established role in tumorigenic cellular signal transduction events, little is known about p19 function. Previous results showed that p19 did not interact with two known p21 effectors, Raf1 and Rin1, but was shown to interact with RACK1, a scaffolding protein that promotes multi-protein complexes in different signaling pathways (Cancer Res 2003, 63 p5178). This observation suggests that p19 and p21 play differential and complementary roles in the cell.

Principal Findings

We found that p19 regulates telomerase activity through its interaction with p73α/β proteins. We also found that p19 overexpression induces G1/S phase delay; an observation that correlates with hypophosphorylation of both Akt and p70SK6. Similarly, we also observed that FOXO1 is upregulated when p19 is overexpressed. The three observations of (1) hypophosphorylation of Akt, (2) G1/S phase delay and (3) upregulation of FOXO1 lead us to conclude that p19 induces G1/S phase delay, thereby maintaining cells in a reversible quiescence state and preventing entry into apoptosis. We then assessed the effect of p19 RNAi on HeLa cell growth and found that p19 RNAi increases cell growth, thereby having the opposite effect of arrest of the G1/S phase or producing a cellular quiescence state.

Significance

Interestingly, p19 induces FOXO1 that in combination with the G1/S phase delay and hypophosphorylation of both Akt and p70SK6 leads to maintenance of a reversible cellular quiescence state, thereby preventing entry into apoptosis.

The role of beta(1)Pix/caveolin-1 interaction in endothelin signaling through Galpha subunits

Endothelin-1 (ET-1) is a potent mitogen that transmits signals through its cognate G protein-coupled receptors to stimulate extracellular signal-regulated kinase Erk1/2. Endothelin-1 receptors (ET-Rs) are known to interact with caveolin-1 and co-localize in caveolae which integrate different receptor and signaling proteins. We have recently shown that beta(1)Pix binds specifically to ET-Rs. Here, we show that beta(1)Pix binding to caveolin-1 is dependent on heterotrimeric G proteins activation state. beta(1)Pix interaction with different G proteins is increased in the presence of the G protein activator AMF. Moreover, extraction of cholesterol with methyl-beta-cyclodextrin disrupts the binding of beta(1)Pix to Galpha(q), Galpha(12) and phospho-Erk1/2 but not the binding of beta(1)Pix to G(beta1). The disruption of beta(1)Pix dimerization strongly reduced the binding of caveolin-1, Galpha(q) and Galpha(12). Constitutively active mutants of Galpha(q) and Galpha(12) increased Cdc42 activation when co-expressed with beta(1)Pix but not in the presence of beta(1)Pix dimerization deficient mutant beta(1)PixDelta (602-611). ET-1 stimulation increased the binding of phosphorylated Erk1/2 to beta(1)Pix but not to beta(1)PixDelta (602-611). RGS3 decreased ET-1-induced Cdc42 activation. These results strongly suggest that the activation of ET-Rs leads to the compartmentalization and the binding of Galpha(q) to beta(1)Pix in caveolae, where dimeric beta(1)Pix acts as platform to facilitate the binding and the activation of Erk1/2.

Activation of protein kinase C-alpha is essential for stimulation of cell proliferation by ceramide 1-phosphate

We previously demonstrated that ceramide-1-phosphate (C1P) stimulates fibroblast and macrophage proliferation, but the mechanisms involved in this action have only been partially described. Here we demonstrate that C1P induces translocation of protein kinase C-alpha (PKC-alpha) from the soluble to the membrane fraction of bone marrow-derived macrophages. Translocation of this enzyme was accompanied by its phosphorylation on Ser 657 residue. Activation of PKC-alpha was independent of prior stimulation of phosphatidylinositol-dependent or phosphatidylcholine-dependent phospholipase C activities, but required activation of sphingomyelin synthesis. Inhibition of PKC-alpha activation also blocked C1P-stimulated macrophage proliferation indicating that this enzyme is essential for the mitogenic effect of C1P.

Protein kinase C is inhibited by bisphosphonates in prostate cancer PC-3 cells

Bisphosphonates are expected to be effective at preventing tumor metastasis to bone tissue. Since protein kinase C (PKC) plays a crucial role in cancer progression, we examined the effect of bisphosphonates on PKC expression to clarify the mechanism behind the inhibition of the bone metastasis of prostate cancer by bisphosphonates. We found that pamidronate inhibits PKC protein expression and PKC activity in prostate cancer PC-3 cells. PKC protein expression was markedly reduced by treatment with 100 microM of pamidronate. The inhibitory effect of PKC expression by pamidronate was specific for PKCalpha and PKCzeta. Nitrogen-containing bisphosphonates are known to inhibit the mevalonate pathway, but the effect of pamidronate on PKC expression was not due to the inhibition of this pathway. Urokinase-type plasminogen activator (uPA) is one of the critical proteins in tumor metastasis and decreased in bisphosphonate-treated PC-3 cells. We also showed that uPA expression was suppressed by PKC inhibitors (calphostin C and staurosporine) and induced by a PKC activator (PMA) in PC-3 cells, suggesting that the inhibition of uPA by bisphosphonates is involved in PKC inhibition. This is the first finding that bisphosphonates suppress PKC expression in cancer cells. These results strongly suggest that one of the mechanisms behind the inhibitory effect of bisphosphonates on tumor bone metastasis is mediated by PKC inhibition.

Protein kinase C epsilon is involved in ionizing radiation induced bystander response in human cells

Our earlier study demonstrated the induction of PKC isoforms (betaII, PKC-alpha/beta, PKC-theta) by ionizing radiation induced bystander response in human cells. In this study, we extended our investigation to yet another important member of PKC family, PKC epsilon (PKCepsilon). PKCepsilon functions both as an anti-apoptotic and pro-apoptotic protein and it is the only PKC isozyme implicated in oncogenesis. Given the importance of PKCepsilon in oncogenesis, we wished to determine whether or not PKCepsilon is involved in bystander response. Gene expression array analysis demonstrated a 2-3-fold increase in PKCepsilon expression in the bystander human primary fibroblast cells that were co-cultured in double-sided Mylar dishes for 3h with human primary fibroblast cells irradiated with 5Gy of alpha-particles. The elevated PKCepsilon expression in bystander cells was verified by quantitative real time PCR. Suppression of PKCepsilon expression by small molecule inhibitor Bisindolylmaleimide IX (Ro 31-8220) considerably reduced the frequency of micronuclei (MN) induced both by 5Gy of gamma-rays (low LET) and alpha-particles (high LET) in bystander cells. Similar cytoprotective effects were observed in bystander cells after siRNA mediated silencing of PKCepsilon suggestive of its critical role in mediating some of the bystander effects (BE). Our novel study suggests the possibility that PKC signaling pathway may be a critical molecular target for suppression of ionizing radiation induced biological effects in bystander cells.

Increased erythropoiesis of beta-thalassaemia/Hb E proerythroblasts is mediated by high basal levels of ERK1/2 activation

Beta-thalassaemia is one of the most common inherited anaemias, arising from a partial or complete loss of beta-globin chain synthesis. In severe cases, marked bone marrow erythroid hyperplasia, believed to result from erythropoietin (EPO)-mediated feedback from the anaemic condition is common, however, as yet, no study has investigated EPO-mediated signal transduction in thalassaemic erythroid cells. Using proerythroblasts generated from peripheral blood circulating CD34+ haematopoietic progenitor cells, the activation of the mitogen-activated protein kinase/extracellular signal-regulated kinases (MAPK/ERKs) pathway was examined under conditions of steady state growth, cytokine deprivation and post-EPO stimulation. Levels of cellular cyclic adenosine monophosphate (cAMP) and Ca2+ were determined as was the degree of erythroid expansion. A significantly higher basal level of phosphorylation of ERK1/2 was observed in beta-thalassaemia/Hb E proerythroblasts as compared to normal controls, which was coupled with significantly higher levels of both cAMP and Ca2+. Modulation of either cAMP or Ca2+ or direct inhibition of MAPK/ERK kinase (MEK) reduced basal levels of ERK1/2 phosphorylation, as well as significantly reducing the level of erythroid expansion. These results suggest that, in contrast to current models, hyper proliferation of beta-thalassaemia/Hb E proerythroblasts is an intrinsic process driven by higher basal levels of ERK1/2 phosphorylation resulting from deregulation of levels of cAMP and Ca2+.

Sprouty2 interacts with protein kinase C delta and disrupts phosphorylation of protein kinase D1

The Sprouty (Spry) proteins act as inhibitors of the Ras/ERK pathway downstream of receptor tyrosine kinases. In this study, we report a novel interaction between protein kinase C delta (PKCdelta) and Spry2. Endogenous PKCdelta and Spry2 interact in cells upon basic fibroblast growth factor stimulation, indicating a physiological relevance for the interaction. This interaction appeared to require the full-length Spry2 protein and was conformation-dependent. Conformational constraints were released upon FGFR1 activation, allowing the interaction to occur. Although this interaction did not affect the phosphorylation of PKCdelta by another kinase, it reduced the phosphorylation of a PKCdelta substrate, protein kinase D1 (PKD1). Spry2 was found to interact more strongly with PKCdelta with increasing amounts of PKD1, which indicated that instead of competing with PKD1 for binding with PKCdelta, it was more likely to form a trimeric complex with both PKCdelta and PKD1. Formation of the complex was found to be dependent on an existing PKCdelta-PKD1 interaction. By disrupting the interaction between PKCdelta and PKD1, Spry2 was unable to associate with PKCdelta to form the trimeric complex. As a consequence of this trimeric complex, the existing interaction between PKCdelta and PKD1 was increased, and the transfer of phosphate groups from PKCdelta to PKD1 was at least partly blocked by Spry2. The action of Spry2 on PKCdelta resulted in the inhibition of both ERK phosphorylation and invasion of PC-3 cells via PKCdelta signaling. By disrupting the capacity of PKCdelta to phosphorylate its cognate substrates, Spry2 may serve to modulate PKCdelta signaling downstream of receptor tyrosine kinases and to regulate the physiological outcome.

Phorbol ester enhances KAI1 transcription by recruiting Tip60/Pontin complexes

Down-regulation of the KAI1 (CD82) metastasis suppressor is common in advanced human cancer, but underlying mechanism(s) regulating KAI1 expression are only now being elucidated. Recent data provide evidence that low levels of KAI1 mRNA in LNCaP cells are caused by binding of beta-catenin/Reptin complexes to a specific motif in the proximal promoter, which prevents binding of Tip60/Pontin activator complexes to the same motif, thus inhibiting transcription. Here, we explored a pathway by which phorbol 12-myristate 13-acetate (PMA) up-regulates KAI1 transcription in LNCaP prostate cancer cells. Pretreatment with specific inhibitors showed that induction of KAI1 by PMA uses classic isoforms of protein kinase C (cPKC), is independent of Ras and Raf, and requires activation of MEK1/2 and ERK1/2, but does not involve p38MAPK. Induction of KAI1 transcription by PMA was associated with enhanced overall acetylation of histones H3 and H4, but only acetylation of H3 was blocked by a PKC inhibitor. Chromatin immunoprecipitation showed that PMA induces recruitment of Tip60/Pontin activator complexes to NFkappaB-p50 motifs in the proximal promoter, and this was blocked by a PKC inhibitor. These changes were not associated with differences in overall levels of Tip60, Pontin, beta-catenin, or Reptin protein expression but with PMA-induced nuclear translocation of Tip60.

Epithelial sodium channel regulated by differential composition of a signaling complex

Hormonal control of transepithelial sodium (Na(+)) transport utilizes phosphatidylinositide 3'-kinase (PI3K) and Raf-MAPK/ERK kinase (MEK)-ERK-dependent signaling pathways, which impact numerous cell functions. How signals transmitted by these pathways are sorted and appropriately transmitted to alter Na(+) transport without altering other physiologic processes is not well understood. Here, we report the identification of a signaling complex that selectively modulates the cell surface expression of the epithelial sodium channel (ENaC), an ion channel that is essential for fluid and electrolyte balance in mammals. Raf-1 and the ubiquitin ligase, Nedd4-2, are constitutively-expressed inhibitory components of this ENaC regulatory complex, which interact with, and decrease the expression of, cell surface ENaC. The activities of Nedd4-2 and Raf-1 are inhibited cooperatively by the PI3K-dependent kinase serum- and glucocorticoid-induced kinase 1 (SGK1), and the Raf-1-interacting protein glucocorticoid-induced leucine zipper (GILZ1), which are aldosterone-stimulated components of the complex. Together, SGK1 and GILZ1 synergistically stimulate ENaC cell surface expression. Interestingly, GILZ1 and SGK1 do not have synergistic, and in fact have opposite, effects on an unrelated activity, FKHRL1-driven gene transcription. Together, these data suggest that GILZ1 and SGK1 provide a physical and functional link between the PI3K- and Raf-1-dependent signaling modules and represent a unique mechanism for specifically controlling Na(+) transport without inappropriately activating other cell functions.