RACK1 mediates activation of JNK by protein kinase C [corrected]

c-Jun N-terminal kinase is largely involved in the regulation of tricellular tight junctions via tricellulin in human pancreatic duct epithelial cells

Tricellulin (TRIC) is a tight junction protein at tricellular contacts where three epithelial cells meet, and it is required for the maintenance of the epithelial barrier. To investigate whether TRIC is regulated via a c-Jun N-terminal kinase (JNK) pathway, human pancreatic HPAC cells, highly expressed at tricellular contacts, were exposed to various stimuli such as the JNK activators anisomycin and 12-O-tetradecanoylphorbol 13-acetate (TPA), and the proinflammatory cytokines IL-1β, TNFα, and IL-1α. TRIC expression and the barrier function were moderated by treatment with the JNK activator anisomycin, and suppressed not only by inhibitors of JNK and PKC but also by siRNAs of TRIC. TRIC expression was induced by treatment with the PKC activator TPA and proinflammatory cytokines IL-1β, TNFα, and IL-1α, whereas the changes were inhibited by a JNK inhibitor. Furthermore, in normal human pancreatic duct epithelial cells using hTERT-transfected primary cultured cells, the responses of TRIC expression to the various stimuli were similar to those in HPAC cells. TRIC expression in tricellular tight junctions is strongly regulated together with the barrier function via the JNK transduction pathway. These findings suggest that JNK may be involved in the regulation of tricellular tight junctions including TRIC expression and the barrier function during normal remodeling of epithelial cells, and prevent disruption of the epithelial barrier in inflammation and other disorders in pancreatic duct epithelial cells.

Differential role of PKC isoforms in GnRH and phorbol 12-myristate 13-acetate activation of extracellular signal-regulated kinase and Jun N-terminal kinase

GnRH is the first key hormone of reproduction. The role of protein kinase C (PKC) isoforms in GnRH-stimulated MAPK [ERK and Jun N-terminal kinase (JNK)] was examined in the αT3-1 and LβT2 gonadotrope cells. Incubation of the cells with GnRH resulted in a protracted activation of ERK1/2 and a slower and more transient activation of JNK1/2. Gonadotropes express conventional PKCα and conventional PKCβII, novel PKCδ, novel PKCε, and novel PKCθ, and atypical PKC-ι/λ. The use of green fluorescent protein-PKC constructs revealed that GnRH induced rapid translocation of PKCα and PKCβII to the plasma membrane, followed by their redistribution to the cytosol. PKCδ and PKCε localized to the cytoplasm and Golgi, followed by the rapid redistribution by GnRH of PKCδ to the perinuclear zone and of PKCε to the plasma membrane. Interestingly, PKCα, PKCβII, and PKCε translocation to the plasma membrane was more pronounced and more prolonged in phorbol-12-myristate-13-acetate (PMA) than in GnRH-treated cells. The use of selective inhibitors and dominant-negative plasmids for the various PKCs has revealed that PKCβII, PKCδ, and PKCε mediate ERK2 activation by GnRH, whereas PKCα, PKCβII, PKCδ, and PKCε mediate ERK2 activation by PMA. Also, PKCα, PKCβII, PKCδ, and PKCε are involved in GnRH and PMA stimulation of JNK1 in a cell-context-dependent manner. We present preliminary evidence that persistent vs. transient redistribution of selected PKCs or redistribution of a given PKC to the perinuclear zone vs. the plasma membrane may dictate its selective role in ERK or JNK activation. Thus, we have described the contribution of selective PKCs to ERK and JNK activation by GnRH.

Asc1 supports cell-wall integrity near bud sites by a Pkc1 independent mechanism

Background

The yeast ribosomal protein Asc1 is a WD-protein family member. Its mammalian ortholog, RACK1 was initially discovered as a receptor for activated protein C kinase (PKC) that functions to maintain the active conformation of PKC and to support its movement to target sites. In the budding yeast though, a connection between Asc1p and the PKC signaling pathway has never been reported.

Methodology/Principal Findings

In the present study we found that asc1-deletion mutant (asc1Δ) presents some of the hallmarks of PKC signaling mutants. These include an increased sensitivity to staurosporine, a specific Pkc1p inhibitor, and susceptibility to cell-wall perturbing treatments such as hypotonic- and heat shock conditions and zymolase treatment. Microscopic analysis of asc1Δ cells revealed cell-wall invaginations near bud sites after exposure to hypotonic conditions, and the dynamic of cells' survival after this stress further supports the involvement of Asc1p in maintaining the cell-wall integrity during the mid-to late stages of bud formation. Genetic interactions between asc1 and pkc1 reveal synergistic sensitivities of a double-knock out mutant (asc1Δ/pkc1Δ) to cell-wall stress conditions, and high basal level of PKC signaling in asc1Δ. Furthermore, Asc1p has no effect on the cellular distribution or redistribution of Pkc1p at optimal or at cell-wall stress conditions.

Conclusions/Significance

Taken together, our data support the idea that unlike its mammalian orthologs, Asc1p acts remotely from Pkc1p, to regulate the integrity of the cell-wall. We speculate that its role is exerted through translation regulation of bud-site related mRNAs during cells' growth.

Mitogen-activated protein kinase p38 and MK2, MK3 and MK5: ménage à trois or ménage à quatre?

The mitogen-activated protein kinase (MAPK) signalling pathways play pivotal roles in cellular processes such as proliferation, apoptosis, gene regulation, differentiation, and cell motility. The typical mammalian MAPK pathways ERK1/2, JNK, p38(MAPK), and ERK5 operate through a concatenation of three successive phosphorylation events mediated by a MAPK kinase kinase, a MAPK kinase, and a MAPK. MAPKs phosphorylate substrates with distinct functions, including other protein kinases referred to as MAPK-activated protein kinases. One family of related MAPK-activated protein kinases includes MK2, MK3, and MK5. While it is generally accepted that MK2 and MK3 are bona fide substrates for p38(MAPK), the genuineness of MK5 as a p38(MAPK) substrate is disputed. This review summarizes the findings pro and contra an authentic p38(MAPK)-MK5 relationship, discusses possible explanations for these discrepancies, and proposes experiments that may help to unequivocally clarify whether MK5 is indeed a substrate for p38(MAPK).

Expression of receptor for activated C kinase 1 in healing skin and mucosal wounds

Postnatal (adult) mammalian wound healing results in the formation of scar, whereas fetal mammals are able to effect wound repair without scar. We have investigated the expression pattern of the receptor of activated C kinase 1 (RACK1), a pleiotropic G-protein-like molecule, in healing skin and mucosal wounds in a rabbit model after obtaining a full-length clone of the rabbit RACK1 cDNA. In both adult skin and mucosal wounds, RACK1 mRNA expression is decreased relative to unwounded controls. In contrast, in fetal skin wounds RACK1 expression is unaltered from fetal control. Fibroblasts derived from adult skin tissue express more RACK1 message than fetal skin fibroblasts. These observations suggest that RACK1 may play an important role in distinguishing scarless fetal wound healing from its scirrhous counterpart in adults.

A crucial role for RACK1 in the regulation of glucose-stimulated IRE1alpha activation in pancreatic beta cells

Autophosphorylation of inositol-requiring enzyme 1alpha (IRE1alpha) is required for its activation, which elicits the cellular unfolded protein response (UPR) and is functionally connected with insulin biosynthesis in pancreatic beta cells. We found that the scaffold protein receptor for activated C-kinase 1 (RACK1) interacted with IRE1alpha in a glucose-stimulated or endoplasmic reticulum (ER) stress-responsive manner in pancreatic beta cells and primary islets. RACK1 mediated the glucose-inducible assembly of a complex containing IRE1alpha, RACK1, and protein phosphatase 2A (PP2A) to promote dephosphorylation of IRE1alpha by PP2A, thereby inhibiting glucose-stimulated IRE1alpha activation and attenuating IRE1alpha-dependent increases in insulin production. Moreover, IRE1alpha activation was increased and RACK1 abundance was decreased in a mouse model of diabetes. Thus, our findings demonstrate that RACK1 functions as a key component in regulating the IRE1alpha signaling pathway in pancreatic beta cells.

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.

Functional alterations in protein kinase C beta II expression in melanoma

Protein kinase C (PKC) is a heterogeneous family of serine/threonine protein kinases that have different biological effects in normal and neoplastic melanocytes (MCs). To explore the mechanism behind their differential response to PKC activation, we analyzed the expression profile of all nine PKC isoforms in normal human MCs, HPV16 E6/E7 immortalized MCs, and a panel of melanoma cell lines. We found reduced PKCbeta and increased PKCzeta and PKCiota expression at both the protein and mRNA levels in immortalized MCs and melanoma lines. We focused on PKCbeta as it has been functionally linked to melanin production and oxidative stress response. Re-expression of PKCbeta in melanoma cells inhibited colony formation in soft agar, indicating that PKCbeta loss in melanoma is important for melanoma growth. PKCbetaII, but not PKCbetaI, was localized to the mitochondria, and inhibition of PKCbeta significantly reduced UV-induced reactive oxygen species (ROS) in MCs with high PKCbeta expression. Thus alterations in PKCbeta expression in melanoma contribute to their neoplastic phenotype, possibly by reducing oxidative stress, and may constitute a selective therapeutic target.

c-Jun regulates phosphoinositide-dependent kinase 1 transcription: implication for Akt and protein kinase C activities and melanoma tumorigenesis

Mutations in N-RAS and B-RAF, which commonly occur in melanomas, result in constitutive activation of the mitogen-activated protein kinase (MAPK)/extracellular signal-regulated protein kinase (ERK) signaling. Active ERK increases expression and activity of the c-Jun transcription factor, linking ERK and Jun N-terminal kinase (JNK) cascades. Here, we show that c-Jun regulates transcription of phosphoinositide-dependent kinase 1 (PDK1) with a concomitant impact on Akt and protein kinase C (PKC) activity and related substrates. Inhibition of c-Jun reduces PDK1 expression and attenuates Akt and PKC activity, which can be restored by exogenous PDK1. c-Jun regulation of PDK1 in melanoma contributes to growth rate and the ability to form tumors in mice. Correspondingly, increased levels of c-Jun in melanoma cell lines coincide with up-regulation of PDK1 and phosphorylation of PKC and Akt. The identification of c-Jun as a transcriptional regulator of PDK1 expression highlights key mechanisms underlying c-Jun oncogenic activity, and provides new insight into the nature of up-regulated Akt and PKC in melanoma.

Stress responses and conditioning effects in mesothelial cells exposed to peritoneal dialysis fluid

Renal replacement therapy by peritoneal dialysis is frequently complicated by technical failure. Peritoneal dialysis fluids (PDF) cause injury to the peritoneal mesothelial cell layer due to their cytotoxicity. As only isolated elements of the involved cellular processes have been studied before, we aimed at a global assessment of the mesothelial stress response to PDF. Following single or repeated exposure to PDF or control medium, proteomics and bioinformatics techniques were combined to study effects in mesothelial cells (MeT-5A). Protein expression was assessed by two-dimensional gel electrophoresis, and significantly altered spots were identified by MALDI-TOF MS and MS2 techniques. The lists of experimentally derived candidate proteins were expanded by a next neighbor approach and analyzed for significantly enriched biological processes. To address the problem of an unknown portion of false positive spots in 2DGE, only proteins showing significant p-values on both levels were further interpreted. Single PDF exposure resulted in reduction of biological processes in favor of reparative responses, including protein metabolism, modification and folding, with chaperones as a major subgroup. The observed biological processes triggered by this acute PDF exposure mainly contained functionally interwoven multitasking proteins contributing as well to cytoskeletal reorganization and defense mechanisms. Repeated PDF exposure resulted in attenuated protein regulation, reflecting inhibition of stress responses by high levels of preinduced chaperones. The identified proteins were less attributable to acute cellular injury but rather to specialized functions with a reduced number of involved multitasking proteins. This finding agrees well with the concept of conditioning effects and cytoprotection. In conclusion, this study describes the reprogrammed proteome of mesothelial cells during recovery from PDF exposure and adaption to repetitive stress. A broad stress response with a number of highly overlapping processes and multitasking proteins shifts toward a more specific response of only few less overlapping processes.

RACK-1 overexpression protects against goniothalamin-induced cell death

Goniothalamin, a styryllactone, has been shown to induce cytotoxicity via apoptosis in several tumor cell lines. In this study, we have examined the potential role of several genes, which were stably transfected into T-cell lines and which regulate apoptosis in different ways, on goniothalamin-induced cell death. Overexpression of full-length receptor for activated protein C-kinase 1 (RACK-1) and pc3n3, which up-regulates endogenous RACK-1, in both Jurkat and W7.2 T cells resulted in inhibition of goniothalamin-induced cell death as assessed by MTT and clonogenic assays. However, overexpression of rFau (antisense sequence to Finkel-Biskis-Reilly murine sarcoma virus-associated ubiquitously expressed gene) in W7.2 cells did not confer resistance to goniothalamin-induced cell death. Etoposide, a clinically used cytotoxic agent, was equipotent in causing cytotoxicity in all the stable transfectants. Assessment of DNA damage by Comet assay revealed goniothalamin-induced DNA strand breaks as early as 1 h in vector control but this effect was inhibited in RACK-1 and pc3n3 stably transfected W7.2 cells. This data demonstrate that RACK-1 plays a crucial role in regulating cell death signalling pathways induced by goniothalamin.

Phosphorylation of histone H3 by protein kinase C signaling plays a critical role in the regulation of the developmentally important TBX2 gene

The mechanism(s) regulating the expression of the TBX2 gene, a key regulator of development, is poorly understood and thus limits an understanding of its function(s). Here we demonstrate that 12-O-tetradecanoylphorbol-13-acetate (TPA) induces TBX2 expression in normal human fibroblasts in a protein kinase C (PKC)-dependent and MAPK-independent manner. Our data further reveal that TPA activates transcription of TBX2 through activating MSK1, which leads to an increase in phosphorylated histone H3 and the recruitment of Sp1 to the TBX2 gene. In addition, TPA was shown to activate MSK1 in a PKC-dependent and MAPK-independent manner. This study is the first to provide evidence that phosphorylation of histone H3 leads to the transcriptional activation of the TBX2 gene and to link MSK1 to PKC.

Role for RACK1 orthologue Cpc2 in the modulation of stress response in fission yeast

The receptor of activated C kinase (RACK1) is a protein highly conserved among eukaryotes. In mammalian cells, RACK1 functions as an adaptor to favor protein kinase C (PKC)-mediated phosphorylation and subsequent activation of c-Jun NH(2)-terminal kinase mitogen-activated protein kinase. Cpc2, the RACK1 orthologue in the fission yeast Schizosaccharomyces pombe, is involved in the control of G2/M transition and interacts with Pck2, a PKC-type protein member of the cell integrity Pmk1 mitogen-activated protein kinase (MAPK) pathway. Both RACK1 and Cpc2 are structural components of the 40S ribosomal subunit, and recent data suggest that they might be involved in the control of translation. In this work, we present data supporting that Cpc2 negatively regulates the cell integrity transduction pathway by favoring translation of the tyrosine-phosphatases Pyp1 and Pyp2 that deactivate Pmk1. In addition, Cpc2 positively regulates the synthesis of the stress-responsive transcription factor Atf1 and the cytoplasmic catalase, a detoxificant enzyme induced by treatment with hydrogen peroxide. These results provide for the first time strong evidence that the RACK1-type Cpc2 protein controls from the ribosome the extent of the activation of MAPK cascades, the cellular defense against oxidative stress, and the progression of the cell cycle by regulating positively the translation of specific gene products involved in key biological processes.

c-Jun N-terminal kinase 2 (JNK2) antagonizes the signaling of differentiation by JNK1 in human myeloid leukemia cells resistant to vitamin D

1,25-Dihydroxyvitamin D3 (1,25D) induces differentiation of myeloid leukemia cells, but resistant cells are also encountered. We studied the mechanistic basis for the resistance in a model system using enhancers of 1,25D, the antioxidant carnosic acid and a kinase inhibitor SB202190. Knock-down (KD) of JNK2p54 unexpectedly increased the intensity of differentiation induced by the 1,25D, carnosic acid and SB202190 (DCS) combination. This was associated with upregulation of activated JNK1p46, and the transcription factors regulated by the JNK pathway, c-Jun, ATF2 and JunB, as well as C/EBP beta. In contrast, KD of JNK1p46 reduced the intensity of DCS-induced differentiation, and partially abrogated activation of c-Jun/AP-1 transcription factors.

Receptor for activated C-kinase 1, a novel binding partner of adiponectin receptor 1

Adiponectin is an adipose tissue derived hormone with anti-diabetic and insulin-sensitizing properties. Two adiponectin receptors, AdipoR1 and AdipoR2, have recently been identified, yet the signaling pathways triggered through adiponectin receptors remain to be elucidated. Using a yeast two-hybrid screen, we identified an adaptor protein, receptor for activated protein kinase C1 (RACK1), as an interacting partner of human AdipoR1. RACK1 was confirmed to interact with AdipoR1 by co-immunoprecipitation and co-localization analysis in mammalian cells. The interaction was enhanced by adiponectin stimulation. In addition, the knockdown of RACK1 by RNA interference inhibited adiponectin-stimulated glucose uptake in HepG2 cells. These results suggest that RACK1 may act as a key bridging factor in adiponectin signaling transduction through interacting with AdipoR1.

Designing heart performance by gene transfer

The birth of molecular cardiology can be traced to the development and implementation of high-fidelity genetic approaches for manipulating the heart. Recombinant viral vector-based technology offers a highly effective approach to genetically engineer cardiac muscle in vitro and in vivo. This review highlights discoveries made in cardiac muscle physiology through the use of targeted viral-mediated genetic modification. Here the history of cardiac gene transfer technology and the strengths and limitations of viral and nonviral vectors for gene delivery are reviewed. A comprehensive account is given of the application of gene transfer technology for studying key cardiac muscle targets including Ca(2+) handling, the sarcomere, the cytoskeleton, and signaling molecules and their posttranslational modifications. The primary objective of this review is to provide a thorough analysis of gene transfer studies for understanding cardiac physiology in health and disease. By comparing results obtained from gene transfer with those obtained from transgenesis and biophysical and biochemical methodologies, this review provides a global view of cardiac structure-function with an eye towards future areas of research. The data presented here serve as a basis for discovery of new therapeutic targets for remediation of acquired and inherited cardiac diseases.

Formation of stress granules inhibits apoptosis by suppressing stress-responsive MAPK pathways

When confronted with environmental stress, cells either activate defence mechanisms to survive, or initiate apoptosis, depending on the type of stress. Certain types of stress, such as hypoxia, heatshock and arsenite (type 1 stress), induce cells to assemble cytoplasmic stress granules (SGs), a major adaptive defence mechanism. SGs are multimolecular aggregates of stalled translation pre-initiation complexes that prevent the accumulation of mis-folded proteins. Type 2 stress, which includes X-rays and genotoxic drugs, induce apoptosis through the stress-activated p38 and JNK MAPK (SAPK) pathways. A functional relationship between the SG and SAPK responses is unknown. Here, we report that SG formation negatively regulates the SAPK apoptotic response, and that the signalling scaffold protein RACK1 functions as a mediator between the two responses. RACK1 binds to the stress-responsive MTK1 MAPKKK and facilitates its activation by type 2 stress; however, under conditions of type 1 stress, RACK1 is sequestered into SGs. Thus, type 1 conditions suppress activation of the MTK1-SAPK pathway and apoptosis induced by type 2 stress. These findings may be relevant to the problem of hypoxia-induced resistance to cancer chemotherapy.

RACK1 functions in rice innate immunity by interacting with the Rac1 immune complex

A small GTPase, Rac1, plays a key role in rice (Oryza sativa) innate immunity as part of a complex of regulatory proteins. Here, we used affinity column chromatography to identify rice RACK1 (for Receptor for Activated C-Kinase 1) as an interactor with Rac1. RACK1 functions in various mammalian signaling pathways and is involved in hormone signaling and development in plants. Rice contains two RACK1 genes, RACK1A and RACK1B, and the RACK1A protein interacts with the GTP form of Rac1. Rac1 positively regulates RACK1A at both the transcriptional and posttranscriptional levels. RACK1A transcription was also induced by a fungal elicitor and by abscisic acid, jasmonate, and auxin. Analysis of transgenic rice plants and cell cultures indicates that RACK1A plays a role in the production of reactive oxygen species (ROS) and in resistance against rice blast infection. Overexpression of RACK1A enhances ROS production in rice seedlings. RACK1A was shown to interact with the N terminus of NADPH oxidase, RAR1, and SGT1, key regulators of plant disease resistance. These results suggest that RACK1A functions in rice innate immunity by interacting with multiple proteins in the Rac1 immune complex.

Transcription analysis in the MeLiM swine model identifies RACK1 as a potential marker of malignancy for human melanocytic proliferation

BACKGROUND

Metastatic melanoma is a severe disease. Few experimental animal models of metastatic melanoma exist. MeLiM minipigs exhibit spontaneous melanoma. Cutaneous and metastatic lesions are histologically similar to human's. However, most of them eventually spontaneously regress. Our purpose was to investigate whether the MeLiM model could reveal markers of malignancy in human melanocytic proliferations.

RESULTS

We compared the serial analysis of gene expression (SAGE) between normal pig skin melanocytes and melanoma cells from an early pulmonary metastasis of MeLiM minipigs. Tag identification revealed 55 regulated genes, including GNB2L1 which was found upregulated in the melanoma library. In situ hybridisation confirmed GNB2L1 overexpression in MeLiM melanocytic lesions. GNB2L1 encodes the adaptor protein RACK1, recently shown to influence melanoma cell lines tumorigenicity. We studied the expression of RACK1 by immunofluorescence and confocal microscopy in tissues specimens of normal skin, in cutaneous and metastatic melanoma developped in MeLiM minipigs and in human patients. In pig and human samples, the results were similar. RACK1 protein was not detected in normal epidermal melanocytes. By contrast, RACK1 signal was highly increased in the cytoplasm of all melanocytic cells of superficial spreading melanoma, recurrent dermal lesions and metastatic melanoma. RACK1 partially colocalised with activated PKCalphabeta. In pig metastases, additional nuclear RACK1 did not associate to BDNF expression. In human nevi, the RACK1 signal was low.

CONCLUSION

RACK1 overexpression detected in situ in human melanoma specimens characterized cutaneous and metastatic melanoma raising the possibility that RACK1 can be a potential marker of malignancy in human melanoma. The MeLiM strain provides a relevant model for exploring mechanisms of melanocytic malignant transformation in humans. This study may contribute to a better understanding of melanoma pathophysiology and to progress in diagnosis.

RACK1 and CIS mediate the degradation of BimEL in cancer cells

RACK1 is a 7-WD motif-containing protein with numerous downstream effectors regulating various cellular functions. Using a yeast two-hybrid screen, we identified dynein light chain 1 as a novel interacting partner of RACK1. Additionally, we demonstrated that RACK1 formed a complex with DLC1 and Bim, specifically BimEL, in the presence of apoptotic agents. Upon paclitaxel treatment, RACK1, DLC1, and CIS mediated the degradation of BimEL through the ElonginB/C-Cullin2-CIS ubiquitin-protein isopeptide ligase complex. We further showed that RACK1 conferred paclitaxel resistance to breast cancer cells in vitro and in vivo. Finally, we observed an inverse correlation between CIS and BimEL levels in both ovarian and breast cancer cell lines and specimens. Our study suggests a role of RACK1 in protecting cancer cells from apoptosis by regulating the degradation of BimEL, which together with CIS could play an important role of drug resistance in chemotherapy.

Preclinical studies of celastrol and acetyl isogambogic acid in melanoma

PURPOSE

Sensitize melanomas to apoptosis and inhibit their growth and metastatic potential by compounds that mimic the activities of activating transcription factor 2 (ATF2)-driven peptides.

EXPERIMENTAL DESIGN

Small-molecule chemical library consisting of 3,280 compounds was screened to identify compounds that elicit properties identified for ATF2 peptide, including (a) sensitization of melanoma cells to apoptosis, (b) inhibition of ATF2 transcriptional activity, (c) activation of c-Jun NH(2)-terminal kinase (JNK) and c-Jun transcriptional activity, and (d) inhibition of melanoma growth and metastasis in mouse models.

RESULTS

Two compounds, celastrol (CSL) and acetyl isogambogic acid, could, within a low micromolar range, efficiently elicit cell death in melanoma cells. Both compounds efficiently inhibit ATF2 transcriptional activities, activate JNK, and increase c-Jun transcriptional activities. Similar to the ATF2 peptide, both compounds require JNK activity for their ability to inhibit melanoma cell viability. Derivatives of CSL were identified as potent inducers of cell death in mouse and human melanomas. CSL and a derivative (CA19) could also efficiently inhibit growth of human and mouse melanoma tumors and reduce the number of lung metastases in syngeneic and xenograft mouse models.

CONCLUSIONS

These studies show for the first time the effect of CSL and acetyl isogambogic acid on melanoma. These compounds elicit activities that resemble the well-characterized ATF2 peptide and may therefore offer new approaches for the treatment of this tumor type.

Acetylcholinesterase-R increases germ cell apoptosis but enhances sperm motility

Changes in protein subdomains through alternative splicing often modify protein-protein interactions, altering biological processes. A relevant example is that of the stress-induced up-regulation of the acetylcholinesterase (AChE-R) splice variant, a common response in various tissues. In germ cells of male transgenic TgR mice, AChE-R excess associates with reduced sperm differentiation and sperm counts. To explore the mechanism(s) by which AChE-R up-regulation affects spermatogenesis, we identified AChE-R's protein partners through a yeast two-hybrid screen. In meiotic spermatocytes from TgR mice, we detected AChE-R interaction with the scaffold protein RACK1 and elevated apoptosis. This correlated with reduced scavenging by RACK1 of the pro-apoptotic TAp73, an outcome compatible with the increased apoptosis. In contrast, at later stages in sperm development, AChE-R's interaction with the glycolytic enzyme enolase-α elevates enolase activity. In transfected cells, enforced AChE-R excess increased glucose uptake and adenosine tri-phosphate (ATP) levels. Correspondingly, TgR sperm cells display elevated ATP levels, mitochondrial hyperactivity and increased motility. In human donors' sperm, we found direct association of sperm motility with AChE-R expression. Interchanging interactions with RACK1 and enolase-α may hence enable AChE-R to affect both sperm differentiation and function by participating in independent cellular pathways.

Understanding signaling cascades in melanoma

Understanding regulatory pathways involved in melanoma development and progression has advanced significantly in recent years. It is now appreciated that melanoma is the result of complex changes in multiple signaling pathways that affect growth control, metabolism, motility and the ability to escape cell death programs. Here we review the major signaling pathways currently known to be deregulated in melanoma with an implication to its development and progression. Among these pathways are Ras, B-Raf, MEK, PTEN, phosphatidylinositol-3 kinase (PI3Ks) and Akt which are constitutively activated in a significant number of melanoma tumors, in most cases due to genomic change. Other pathways discussed in this review include the [Janus kinase/signal transducer and activator of transcription (JAK/STAT), transforming growth factor-beta pathways which are also activated in melanoma, although the underlying mechanism is not yet clear. As a paradigm for remodeled signaling pathways, melanoma also offers a unique opportunity for targeted drug development.

Requirements for PKC-augmented JNK activation by MKK4/7

The c-Jun N-terminal kinases (JNKs) are activated in response to stress, DNA damage, and cytokines by MKK4 and MKK7. We recently demonstrated that PKC can augment the degree of JNK activation by phosphorylating JNK, which requires the adaptor protein RACK1. Here we report on the conditions required for PKC-dependent JNK activation. In vitro kinase assays reveal that PKC phosphorylation of JNK is not sufficient for its activation but rather augments JNK activation by canonical JNK upstream kinases MKK4 or MKK7 alone or in combination. Further, to enhance JNK activity, PKC phosphorylation of JNK should precede its phosphorylation by MKK4/7. Inhibition of PKC phosphorylation of JNK affects both early and late phases of JNK activation following UV-irradiation and reduces the apoptotic response mediated by JNK. These data provide important insight into the requirements for PKC activation of JNK signaling.

Prostaglandin E(2) induces breast cancer related aromatase promoters via activation of p38 and c-Jun NH(2)-terminal kinase in adipose fibroblasts

Aromatase is the key enzyme for estrogen biosynthesis. A distal promoter, PI.4, maintains baseline levels of aromatase in normal breast adipose tissue. In contrast, malignant breast epithelial cells secrete prostaglandin E(2) (PGE(2)), which stimulates aromatase expression via proximal promoters PI.3/PII in a cyclic AMP (cAMP)- and protein kinase C (PKC)-dependent manner in adjacent breast adipose fibroblasts (BAF), leading to increased local concentrations of estrogen. Although an effective treatment for breast cancer, aromatase inhibitors indiscriminately abolish estrogen synthesis in all tissues, causing major side effects. To identify drug targets to selectively block aromatase and estrogen production in breast cancer, we investigated PGE(2)-stimulated signaling pathways essential for aromatase induction downstream of cAMP and PKC in human BAFs. Here, we show that PGE(2) or its surrogate hormonal mixture dibutyryl cAMP (Bt(2)cAMP) + phorbol diacetate (PDA) stimulated the p38, c-jun NH(2)-terminal kinase (JNK)-1, and extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase pathways. Inhibition or small interfering RNA-mediated knockdown of p38 or JNK1, but not ERK, inhibited PGE(2)- or Bt(2)cAMP + PDA-induced aromatase activity and expression via PI.3/PII. Conversely, overexpression of wild-type p38alpha or JNK1 enhanced PGE(2)-stimulated aromatase expression via PII. PGE(2) or Bt(2)cAMP + PDA stimulated c-Jun and activating transcription factor-2 (ATF2) phosphorylation and binding to the PI.3/PII region. Specific activation of protein kinase A (PKA) or EPAC with cAMP analogues stimulated p38 and JNK1; however, only PKA-activating cAMP analogues induced aromatase expression. The PKC activator PDA effectively stimulated p38 and JNK1 phosphorylation but not aromatase expression. Taken together, PGE(2) activation of p38 and JNK1 via PKA and PKC is necessary for aromatase induction in BAFs, and p38 and JNK1 are potential new drug targets for tissue-specific ablation of aromatase expression in breast cancer.

Signaling pathways from membrane lipid rafts to JNK1 activation in reactive nitrogen species-induced non-apoptotic cell death

At present, the signaling pathways controlling reactive nitrogen species (RNS)-induced non-apoptotic cell death are relatively less understood. In this work, various RNS donors are found to induce caspase-independent non-apoptotic cell death in mouse embryonic fibroblasts (MEF). In search of the molecular mechanisms, we first established the role of c-Jun N-terminal kinase (JNK) in RNS-induced non-apoptotic cell death. RNS readily activate JNK, and the jnk1-/- MEF are resistant to RNS-induced cell death. Moreover, the reconstitution of JNK1 effectively restores the sensitivity to RNS. Next, we identified tumor necrosis factor receptor-associated factor 2 (TRAF2) and apoptosis signal-regulating kinase 1 (ASK1) as the essential upstream molecules for RNS-induced JNK activation and cell death. RNS fail to activate JNK and induce cell death in traf2-/- MEF; and reconstitution of TRAF2 effectively restores the responsiveness of traf2-/- MEF to RNS. Moreover, RNS-induced ASK1 activation is impaired in traf2-/- cells and overexpression of a mutant ASK1 protein suppresses RNS-induced cell death in wild-type MEF cells. Last, we explored the signaling events upstream of TRAF2 and found that translocation of TRAF2 and JNK1 onto membrane lipid rafts is required for RNS-mediated JNK1 activation and cell death. Taken together, data from our study reveal a novel signaling pathway regulating RNS-induced JNK1 activation and non-apoptotic cell death.

Metabolic stress signaling mediated by mixed-lineage kinases

Saturated free fatty acid (FFA) is a major source of metabolic stress that activates the c-Jun NH(2)-terminal kinase (JNK). This FFA-stimulated JNK pathway is relevant to hallmarks of metabolic syndrome, including insulin resistance. Here we used gene ablation studies in mice to demonstrate a central role for mixed-lineage protein kinases (MLK) in this signaling pathway. Saturated FFA causes protein kinase C (PKC)-dependent activation of MLK3 that subsequently causes increased JNK activity by a mechanism that requires the MAP kinase kinases MKK4 and MKK7. Loss of PKC, MLK3, MKK4, or MKK7 expression prevents FFA-stimulated JNK activation. Together, these data establish a signaling pathway that mediates effects of metabolic stress on insulin resistance.

Wiring the cell signaling circuitry by the NF-kappa B and JNK1 crosstalk and its applications in human diseases

Integration of the cell signaling circuitry determines the ultimate response of a cell to extracellular stimuli. The transcription factor nuclear factor-kappa B (NF-kappaB) and mitogen-activated protein kinase JNK1 are major players in the cell signaling circuitry, regulating numerous cellular events and being implicated in the process of many human diseases and certain types of cancer. The interplay between NF-kappaB and JNK1 provides a paradigm that shows how the crosstalk between different signaling pathways decides the function of the cell signaling circuitry. Understanding the wiring of the cell signaling circuitry may hold the key for cell signaling-based therapy of human diseases.

JNK/c-Jun signaling pathway mediates the fluoride-induced down-regulation of MMP-20 in vitro

Delayed removal of amelogenins, which are initially hydrolyzed by matrix metalloproteinase MMP-20, is a characteristic of enamel fluorosis. In this study, we investigated the regulation of MMP-20 and possible effects of fluoride on MMP-20 expression in human ameloblast lineage cells. Protein expression and signaling pathways of human ameloblast lineage cells, exposed to 10 muM fluoride, were compared to control cells without fluoride exposure. The role of activator protein-1 in MMP-20 regulation was analyzed by DNA-protein affinity precipitation and luciferase reporter gene assays. MMP-20 protein levels in human ameloblast lineage cells decreased in the presence of fluoride, while amelogenin and TIMP-2 were not altered. Fluoride also decreased the transcription of a luciferase reporter gene driven by the MMP-20 promoter. Down-regulation of MMP-20 by fluoride was related to suppression of JNK/c-Jun phosphorylation. In contrast, the JNK activator elevated the expression of MMP-20. Three c-Jun binding sites on the MMP-20 promoter were identified for the first time, and were occupied by c-Jun as MMP-20 was induced. Deletion of any one of AP-1 binding sites on the MMP-20 promoter significantly reduced the transcription of downstream luciferase reporter. These in vitro findings suggest that c-Jun is a key regulatory element for MMP-20 expression, and human ameloblast lineage cells can respond to fluoride by down-regulating MMP-20 transcription through the JNK/c-Jun signaling pathway.

T-Lymphokine–Activated Killer Cell–Originated Protein Kinase Functions as a Positive Regulator of c-Jun-NH2-Kinase 1 Signaling and H-Ras–Induced Cell Transformation

T-lymphokine-activated killer cell-originated protein kinase (TOPK) is overexpressed in highly proliferating tumors such as leukemias and myelomas, and seems to play a key role in tumorigenesis or metastasis. However, the precise role and regulatory mechanism explaining the effects of TOPK on tumor cells still remain elusive. Here, we reported that TOPK regulates UVB-induced c-Jun-NH2-kinase 1 (JNK1) activity, and is essential for H-Ras-induced activator protein-1 activity and cell transformation. We showed that TOPK associated with and phosphorylated JNK1 following UVB irradiation in vitro or in vivo. Moreover, UVB-induced JNK1 activity was greatly augmented in mouse epidermal JB6 Cl41 cells that stably expressed TOPK cDNA. On the other hand, JNK1 activity was markedly attenuated by stable expression of small interfering RNA against TOPK in malignant melanoma RPMI 7951 cells. Interestingly, TOPK interacted with JNK-interacting protein 1 and caused an elevation of JNK-interacting protein 1 scaffolding activity, thereby enhancing JNK1 activity. Furthermore, JNK1 was required for TOPK-mediated activator protein-1 transcriptional activity and transformed foci induced by UVB or H-Ras. Taken together, these findings showed that TOPK positively modulated UVB-induced JNK1 activity and played a pivotal role in JNK1-mediated cell transformation induced by H-Ras. These studies might also provide a novel molecular mechanism for the role of TOPK in UVB-mediated skin carcinogenesis.

WD-repeat-propeller-FYVE protein, ProF, binds VAMP2 and protein kinase Czeta

We have recently identified a protein, consisting of seven WD repeats, presumably forming a beta-propeller, and a domain identified in Fab1p, YOTB, VAC1p, and EEA1 (FYVE) domain, ProF. The FYVE domain targets the protein to vesicular membranes, while the WD repeats allow binding of the activated kinases Akt and protein kinase (PK)Czeta. Here, we describe the vesicle-associated membrane protein 2 (VAMP2) as interaction partner of ProF. The interaction is demonstrated with overexpressed and endogenous proteins in mammalian cells. ProF and VAMP2 partially colocalize on vesicular structures with PKCzeta and the proteins form a ternary complex. VAMP2 can be phosphorylated by activated PKCzeta in vitro and the presence of ProF increases the PKCzeta-dependent phosphorylation of VAMP2 in vitro. ProF is an adaptor protein that brings together a kinase with its substrate. VAMP2 is known to regulate docking and fusion of vesicles and to play a role in targeting vesicles to the plasma membrane. The complex may be involved in vesicle cycling in various secretory pathways.

Rewired ERK-JNK signaling pathways in melanoma

Constitutive activation of MEK-ERK signaling is often found in melanomas. Here, we identify a mechanism that links ERK with JNK signaling in human melanoma. Constitutively active ERK increases c-Jun transcription and stability, which are mediated by CREB and GSK3, respectively. Subsequently, c-Jun increases transcription of target genes, including RACK1, an adaptor protein that enables PKC to phosphorylate and enhance JNK activity, enforcing a feed-forward mechanism of the JNK-Jun pathway. Activated c-Jun is also responsible for elevated cyclin D1 expression, which is frequently overexpressed in human melanoma. Our data reveal that, in human melanoma, the rewired ERK signaling pathway upregulates JNK and activates the c-Jun oncogene and its downstream targets, including RACK1 and cyclin D1.

Protein kinase C-ERK1/2 signal pathway switches glucose depletion-induced necrosis to apoptosis by regulating superoxide dismutases and suppressing reactive oxygen species production in A549 lung cancer cells

Cells typically die by either apoptosis or necrosis. However, the consequences of apoptosis and necrosis are quite different for a whole organism. In the case of apoptosis, the cell content remains packed in the apoptotic bodies that are removed by macrophages, and thereby inflammation does not occur; during necrosis, the cell membrane is ruptured, and the cytosolic constituents are released into the extracellular space provoking inflammation. Recently, inflammation and necrosis have been suggested to promote tumor growth. We investigated the molecular mechanism underlying cell death in response to glucose depletion (GD), a common characteristic of the tumor microenvironment. GD induced necrosis through production of reactive oxygen species (ROS) in A549 lung carcinoma cells. Inhibition of ROS production by N-acetyl-L-cysteine and catalase prevented necrosis and switched the cell death mode to apoptosis that depends on mitochondrial death pathway involving caspase-9 and caspase-3 activation, indicating a critical role of ROS in determination of GD-induced cell death mode. We demonstrate that protein kinase C-dependent extracellular regulated kinase 1/2 (ERK1/2) activation also switched GD-induced necrosis to apoptosis through inhibition of ROS production possibly by inducing manganese superoxide dismutase (SOD) expression and by preventing GD-induced degradation of copper zinc SOD. Thus, these results suggest that GD-induced cell death mode is determined by the protein kinase C/ERK1/2 signal pathway that regulates MnSOD and CuZnSOD and that these antioxidants may exert their known tumor suppressive activities by inducing necrosis-to-apoptosis switch.

Isoform specificity of protein kinase Cs in synaptic plasticity

Protein kinase Cs (PKCs) are implicated in many forms of synaptic plasticity. However, the specific isoform(s) of PKC that underlie(s) these events are often not known. We have used Aplysia as a model system in order to investigate the isoform specificity of PKC actions due to the presence of fewer isoforms and a large number of documented physiological roles for PKC in synaptic plasticity in this system. In particular, we have shown that distinct isoforms mediate distinct types of synaptic plasticity induced by the same neurotransmitter: The novel calcium-independent PKC Apl II is required for actions mediated by serotonin (5-HT) alone, while the classical calcium-dependent PKC Apl I is required for actions mediated when 5-HT is coupled to activity. We will discuss the reasons for PKC isoform specificity, assess the tools used to uncover isoform specificity, and discuss the implications of isoform specificity for understanding the roles of PKC in regulating synaptic plasticity.

RACK1 regulates Ki-Ras-mediated signaling and morphological transformation of NIH 3T3 cells

Activating Ras mutations are involved in a significant fraction of human tumors. A suppressor screen using a retroviral mouse fibroblast cDNA library was performed to identify novel factors in Ras-mediated transformation. We identified a novel potent inhibitor of Ras-mediated morphological transformation encoded by a truncated version of the receptor for activated C-kinase (RACK1). The truncated protein, designated RACK1DeltaWD1, lacked the N-terminal 49 amino acids encoding the first of the 7 WD40 repeats in RACK1. RACK1DeltaWD1 expression restored contact inhibition, stress fiber formation and reduced ERK phosphorylation in Ki-Ras transformed NIH 3T3 cells. We demonstrate that truncated RACK1 is involved in complexes consisting of wild-type RACK1 and protein kinase C isoforms alpha, betaI and delta, compromising the transduction of an activated Ras signal to the Raf-MEK-ERK pathway. The cellular localization of RACK1DeltaWD1 differed from wtRACK1, indicating that signaling complexes containing the truncated version of RACK1 are incorrectly localized. Notably, 12-O-tetradecanoyl-13-phorbol acetate (TPA) mediated intracellular translocation of RACK1-interacting PKC alpha and delta was abrogated in RACK1DeltaWD1-expressing cells. Our data support a model where RACK1 acts as a key factor in Ki-Ras-mediated morphological transformation.

RACK1 competes with HSP90 for binding to HIF-1alpha and is required for O(2)-independent and HSP90 inhibitor-induced degradation of HIF-1alpha

Hypoxia-inducible factor 1 (HIF-1) regulates transcription in response to changes in O(2) concentration. O(2)-dependent degradation of the HIF-1alpha subunit is mediated by prolyl hydroxylase (PHD), the von Hippel-Lindau (VHL)/Elongin-C/Elongin-B E3 ubiquitin ligase complex, and the proteasome. Inhibition of heat-shock protein 90 (HSP90) leads to O(2)/PHD/VHL-independent degradation of HIF-1alpha. We have identified the receptor of activated protein kinase C (RACK1) as a HIF-1alpha-interacting protein that promotes PHD/VHL-independent proteasomal degradation of HIF-1alpha. RACK1 competes with HSP90 for binding to the PAS-A domain of HIF-1alpha in vitro and in human cells. HIF-1alpha degradation induced by the HSP90 inhibitor 17-allylaminogeldanamycin is abolished by RACK1 loss of function. RACK1 binds to Elongin-C and promotes ubiquitination of HIF-1alpha. Elongin-C-binding sites in RACK1 and VHL show significant sequence similarity. Thus, RACK1 is an essential component of an O(2)/PHD/VHL-independent mechanism for regulating HIF-1alpha stability through competition with HSP90 and recruitment of the Elongin-C/B ubiquitin ligase complex.

Identification of key functional domains in the C terminus of the K+-Cl- cotransporters

The K+-Cl- cotransporter (KCC) isoforms constitute a functionally heterogeneous group of ion carriers. Emerging evidence suggests that the C terminus (Ct) of these proteins is important in conveying isoform-specific traits and that it may harbor interacting sites for 4beta-phorbol 12-myristate 13-acetate (PMA)-induced effectors. In this study, we have generated KCC2-KCC4 chimeras to identify key functional domains in the Ct of these carriers and single point mutations to determine whether canonical protein kinase C sites underlie KCC2-specific behaviors. Functional characterization of wild-type (wt) and mutant carriers in Xenopus laevis oocytes showed for the first time that the KCCs do not exhibit similar sensitivities to changes in osmolality and that this distinguishing feature as well as differences in transport activity under both hypotonic and isotonic conditions are in part determined by the residue composition of the distal Ct. At the same time, several mutations in this domain and in the proximal Ct of the KCCs were found to generate allosteric-like effects, suggesting that the regions analyzed are important in defining conformational ensembles and that isoform-specific structural configurations could thus account for variant functional traits as well. Characterization of the other mutants in this work showed that KCC2 is not inhibited by PMA through phosphorylation of its canonical protein kinase C sites. Intriguingly, however, the substitutions N728S and S940A were seen to alter the PMA effect paradoxically, suggesting again that allosteric changes in the Ct are important determinants of transport activity and, furthermore, that the structural configuration of this domain can convey specific functional traits by defining the accessibility of cotransporter sites to regulatory intermediates such as PMA-induced effectors.

NF-κB Is Required for UV-Induced JNK Activation via Induction of PKCδ

Ultraviolet (UV) exerts its biological activities by activating downstream effectors, including NF-kappaB, JNK, and caspases. Activation of JNK is required for UV-induced apoptosis. It is unknown whether any crosstalk occurs between NF-kappaB and JNK in response to UV and, if so, how it affects UV killing. Here we report that NF-kappaB promotes UV-induced JNK activation, thereby contributing to UV-induced apoptosis. UV-induced JNK activation is impaired in RelA/NF-kappaB null murine embryonic fibroblasts. In resting cells, the preexisting nuclear RelA has already been recruited to PKCdelta promoter and is essential for its expression. UV-induced rapid and robust activation of JNK requires PKCdelta, which augments JNK phosphorylation-activation by its upstream kinases. The RelA/NF-kappaB-PKCdelta-JNK pathway is critical for UV-induced apoptosis, as it induces the immediate expression of the proapoptotic Fas ligand. Thus, our results demonstrate that RelA/NF-kappaB via PKCdelta positively regulates UV-induced JNK activation and provide a mechanism by which NF-kappaB promotes UV-induced apoptosis.

Current strategies in overcoming resistance of cancer cells to apoptosis melanoma as a model

Most anticancer agents mediate their effects through common pathways which induce apoptosis or in some cases necrosis of cancer cells. The apoptotic pathways are regulated by Bcl-2 family proteins, which include both pro- and anti-apoptotic members. Much is known about the interactions of these proteins involved in apoptosis and this information is being utilized in the development of new reagents that may be used to treat patients with cancers. The inhibitor of apoptosis family of proteins constitute a second group of proteins which inhibit the effector caspases. Reagents that inhibit their activity are also under development. Resistance of cancer cells to treatment can in many instances be attributed to activation of intracellular signal pathways involved in survival, such as the Ras-Raf-MEK-ERK1/2 or the P13K-Akt pathway. Again, much has been learned about the control of these pathways and their activation of resistance mechanisms. Inhibitors of such pathways are being evaluated in preclinical and clinical studies and are showing promise as a new class of anticancer agents. Much of the progress in future studies will likely depend on the ability to target these new treatments to particular subgroups of patients with tumor characteristics that make them responsive to the agents in question.