c-Jun amino-terminal kinase is regulated by Galpha12/Galpha13 and obligate for differentiation of P19 embryonal carcinoma cells by retinoic acid

GPCR-Gα13 Involvement in Mitochondrial Function, Oxidative Stress, and Prostate Cancer

Gα13 and Gα12, encoded by the GNA13 and GNA12 genes, respectively, are members of the G12 family of Gα proteins that, along with their associated Gβγ subunits, mediate signaling from specific G protein-coupled receptors (GPCRs). Advanced prostate cancers have increased expression of GPCRs such as CXC Motif Chemokine Receptor 4 (CXCR4), lysophosphatidic acid receptor (LPAR), and protease activated receptor 1 (PAR-1). These GPCRs signal through either the G12 family, or through Gα13 exclusively, often in addition to other G proteins. The effect of Gα13 can be distinct from that of Gα12, and the role of Gα13 in prostate cancer initiation and progression is largely unexplored. The oncogenic effect of Gα13 on cell migration and invasion in prostate cancer has been characterized, but little is known about other biological processes such as mitochondrial function and oxidative stress. Current knowledge on the link between Gα13 and oxidative stress is based on animal studies in which GPCR-Gα13 signaling decreased superoxide levels, and the overexpression of constitutively active Gα13 promoted antioxidant gene activation. In human samples, mitochondrial superoxide dismutase 2 (SOD2) correlates with prostate cancer risk and prognostic Gleason grade. However, overexpression of SOD2 in prostate cancer cells yielded conflicting results on cell growth and survival under basal versus oxidative stress conditions. Hence, it is necessary to explore the effect of Gα13 on prostate cancer tumorigenesis, as well as the effect of Gα13 on SOD2 in prostate cancer cell growth under oxidative stress conditions.

MAP3K4 kinase action and dual role in cancer

It is commonly known that the MAPK pathway is involved in translating environmental inputs, regulating downstream reactions, and maintaining the intrinsic dynamic balance. Numerous essential elements and regulatory processes are included in this pathway, which are essential to its functionality. Among these, MAP3K4, a member of the serine/threonine kinases family, plays vital roles throughout the organism's life cycle, including the regulation of apoptosis and autophagy. Moreover, MAP3K4 can interact with key partners like GADD45, which affects organism's growth and development. Notably, MAP3K4 functions as both a tumor promotor and suppressor, being activated by a variety of factors and triggering diverse downstream pathways that differently influence cancer progression. The aim of this study is to provide a brief overview of physiological functions of MAP3K4 and shed light on its contradictory roles in tumorigenesis.

Puromycin-resistant lentiviral control shRNA vector, pLKO.1 induces unexpected cellular differentiation of P19 embryonic stem cells

RNA silencing is used as a common method for investigating loss-of-function effects of genes of interest. In mammalian cells, RNA interference (RNAi) or RNA silencing can be achieved by transient siRNA (small or short interfering RNA) transfection or by stable shRNA (short hairpin RNA) systems. Various vectors are used for efficient delivery of shRNA. Lentiviral vectors offer an efficient delivery system for stable and long-term expression of the shRNA in mammalian cells. The widely used lentiviral pLKO.1 plasmid vector is very popular in RNAi studies. A large RNAi database, a TRC (the RNAi Consortium) library, was established based on the pLKO.1-TRC plasmid vector. This plasmid (also called pLKO.1-puro) has a puromycin-resistant gene for selection in mammalian cells along with designs for generating lentiviral particles as well for RNA silencing. While using the pLKO.1-puro TRC control shRNA plasmid for transfection in murine P19 embryonic stem (ES) cells, it was unexpectedly discovered that this plasmid vector induced robust endodermal differentiation. Since P19 ES cells are pluripotent and respond to external stimuli that have the potential to alter the phenotype and thus its stemness, other cell types used in RNA silencing studies do not display the obvious effect and therefore, may affect experiments in subtle ways that would go undetected. This study for the first time provides evidence that raises concern and warrants extreme caution while using the pLKO.1-puro control shRNA vector because of its unexpected non-specific effects on cellular integrity.

c-Jun Contributes to Transcriptional Control of GNA12 Expression in Prostate Cancer Cells

GNA12 is the α subunit of a heterotrimeric G protein that possesses oncogenic potential. Activated GNA12 also promotes prostate and breast cancer cell invasion in vitro and in vivo, and its expression is up-regulated in many tumors, particularly metastatic tissues. In this study, we explored the control of expression of GNA12 in prostate cancer cells. Initial studies on LnCAP (low metastatic potential, containing low levels of GNA12) and PC3 (high metastatic potential, containing high GNA12 levels) cells revealed that GNA12 mRNA levels correlated with protein levels, suggesting control at the transcriptional level. To identify potential factors controlling GNA12 transcription, we cloned the upstream 5′ regulatory region of the human GNA12 gene and examined its activity using reporter assays. Deletion analysis revealed the highest level of promoter activity in a 784 bp region, and subsequent in silico analysis indicated the presence of transcription factor binding sites for C/EBP (CCAAT/enhancer binding protein), CREB1 (cAMP-response-element-binding protein 1), and c-Jun in this minimal element for transcriptional control. A small interfering RNA (siRNA) knockdown approach revealed that silencing of c-Jun expression significantly reduced GNA12 5′ regulatory region reporter activity. In addition, chromatin immunoprecipitation assays confirmed that c-Jun binds to the GNA12 5′ regulatory region in PC3 cells. Silencing of c-Jun expression reduced mRNA and protein levels of GNA12, but not the closely-related GNA13, in prostate cancer cells. Understanding the mechanisms by which GNA12 expression is controlled may aid in the development of therapies that target key elements responsible for GNA12-mediated tumor progression.

Menin induces endodermal differentiation in aggregated P19 stem cells by modulating the retinoic acid receptors

Menin, a ubiquitously expressed protein, is the product of the multiple endocrine neoplasia type I (Men1) gene, mutations of which cause tumors primarily of the parathyroid, endocrine pancreas, and anterior pituitary. Menin-null mice display early embryonic lethality, and thus imply a critical role for menin in early development. In this study, using the P19 embryonic carcinoma stem cells, we studied menin's role in cell differentiation. Menin expression is induced in P19 cell aggregates by retinoic acid (RA). Menin over-expressing stable clones proliferated in a significantly reduced rate compared to the empty vector harboring cells. RA induced cell death in aggregated menin over-expressing cells. However, in the absence of RA, specific populations of the aggregated menin over-expressing cells displayed the characteristic of an endodermal phenotype by the acquisition of cytokeratin Endo A expression (TROMA-1), a marker for the primitive endoderm, with a concomitant loss of the stem cell marker SSEA-1. Menin's ability to induce endodermal differentiation in specific populations of the aggregated cells in the absence of RA implied that menin could substitute RA by inducing a set of target genes that are RA responsive. Menin over-expressing cells upon aggregation showed a robust expression of RA receptors (RAR), RARα, β, and γ relative to the empty vector-harboring cells. Moreover, endodermal differentiation was inhibited by the pan-RAR antagonist Ro41-5253, suggesting that menin could induce endodermal differentiation of uncommitted cells by functionally modulating the RARs.

G12 signaling through c-Jun NH2-terminal kinase promotes breast cancer cell invasion

Signaling through the heterotrimeric G protein, G12, via Rho induces a striking increase in breast cancer cell invasion. In this study, evidence is provided that the c-Jun NH₂-terminal kinase (JNK) is a key downstream effector of G12 on this pathway. Expression of constitutively-active Gα12 or activation of G12 signaling by thrombin leads to increased JNK and c-Jun phosphorylation. Pharmacologic inhibition of JNK or knockdown of JNK expression by siRNA significantly decreases G12-induced JNK activation as well as the ability of breast cancer cells to invade a reconstituted basement membrane. Furthermore, expression of dominant-negative Rho or treatment of cells with an inhibitor of the Rho kinase, ROCK, reduces G12-induced JNK and c-Jun activation, and ROCK inhibitor treatment also inhibits G12-induced cellular invasion. JNK knockdown or ROCK inhibitor treatment has no effect on activation of Rho by G12. Taken together, our data indicate that JNK activation is required for G12-induced invasion of breast cancer cells and that JNK is downstream of Rho and ROCK on this pathway. This study implicates a G12-stimulated mitogen-activated protein kinase cascade in cancer cell invasion, and supports a role for JNK in cancer progression.

Neoplastic transformation induced by the gep oncogenes involves the scaffold protein JNK-interacting leucine zipper protein

The activated mutants of the α-subunits of G proteins G(12) and G(13) have been designated as the gep oncogenes owing to their ability to stimulate diverse oncogenic signaling pathways that lead to neoplastic transformation of fibroblast cell lines and tumorigenesis in nude mice models. Studies from our laboratory as well as others have shown that the growth-promoting activities of Gα(12) and Gα(13) involve potent activation of c-Jun N-terminal kinases (JNKs). Our previous studies have indicated that the JNK-interacting leucine zipper protein (JLP), a scaffold protein involved in the structural and functional organization of the JNK/p38 mitogen-activated protein kinase module, tethers Gα(12) and Gα(13) to the JNK signaling module. In the present study, in addition to demonstrating the physical association between JLP and Gα(12), we show that this interaction is enhanced by the receptor- or mutation-mediated activation of Gα(12). We also establish that JLP interacts with Gα(12) through the C-terminal domain that has been previously identified to be involved in binding to Gα(13). Furthermore, using this C-terminal domain as a competitively inhibitor of JLP that can disrupt Gα(12)-JLP interaction, we demonstrate that JLP is required for the stimulation of JNK by Gα(12). Our results also indicate that such JLP interaction is required for Gα(12) as well as Gα(13)-mediated neoplastic transformation of JLP. These studies demonstrate for the first time a functional role for JLP in the gep oncogene-regulated neoplastic signaling pathway.

Hypothermia-induced increase of oligodendrocyte precursor cells: Possible involvement of plasmalemmal voltage-dependent anion channel 1

Hypothermia is believed to suppress cell proliferation by inducing apoptosis/necrosis and phase-specific/nonspecific cell cycle arrest, which are, directly or indirectly, related to a reduced energy supply. Intriguingly, hypothermia is known to improve neurological recovery of animals and humans exposed to focal brain hypoxic-ischemic injury. The underlying mechanism of the neuroprotective effect of hypothermia is unclear, although the prevention of neural cell apoptosis is thought to play a role. Herein we demonstrate that in vitro cell culture of oligodendrocyte precursor cells (OPCs) under conditions of mild hypothermia (31.5°C) results in an increase in cell number relative to cells cultured under normothermic conditions (37°C). Cell cycle analysis, immunoblotting of cyclins, TUNEL assay, and immunocytochemistry of OPC differentiation markers suggest that hypothermia shifts the balance between proliferation and apoptosis/differentiation toward proliferation. A combination of transcriptome analysis, pharmacological intervention, and immunoaffinity-based assays suggests a possible involvement of the Gα13-Rho GTPase Cdc42-ERK1/2 signaling cascade and voltage-dependent anion channel 1 (VDAC1), which associate or dissociate with Gα13 protein at 37°C and 31.5°C, respectively. Immunoelectron microscopy revealed the presence of VDAC1 in the plasma membrane of OPCs. Furthermore, the exogenous addition of impermeable VDAC1 inhibitors enhanced proliferation of OPCs at 37°C. These results may contribute to the elucidation of the mechanism of hypothermic neuroprotection as well as the possible novel role of plasmalemmal VDAC1.

The G12 family proteins upregulate matrix metalloproteinase-2 via p53 leading to human breast cell invasion

Although mounting evidence suggests a role for G(12) proteins, G(α12) and G(α13), in tumor progression, a direct role of G(12) proteins has not been determined. This study aims to elucidate the molecular mechanism for a tumorigenic and invasive potential of G(α12) and G(α13) in MCF10A human breast epithelial cells. Here, we report, for the first time, that G(α12) and G(α13) induce upregulation of matrix metalloproteinase (MMP)-2 leading to the invasive and migratory phenotypes in MCF10A cells. We further show that p53 is an important transcription factor for induction of MMP-2 transcriptional activation by G(α12/13). G(α12/13)-induced MMP-2 upregulation, invasion, and migration are dependent on the activation of Ras, Rac1, MKK3/6, p38, and Akt. Using human breast tissue samples, we demonstrate that the expression levels of G(α12) and MMP-2 are strongly correlated with the pathogenically diagnosed cancer (P < 0.0001). Moreover, the expression of G(α12) shows a strong correlation with that of MMP-2 in human breast cancer tissues, implicating the in vivo tumorigenic potential of G(α12). Taken together, this study elucidated the role of G(12) proteins in regulating processes for MMP-2 expression and malignant phenotypic conversion of MCF10A human breast epithelial cells, providing a molecular basis for the promoting role of G(α12) and G(α13) in breast cell invasion.

Coordinate Gα13 and Wnt6-β-catenin signaling in F9 embryonal carcinoma cells is required for primitive endoderm differentiation

The mouse F9 embryonal carcinoma cell line is ideally suited to study the epithelial-to-mesenchymal transition accompanying the differentiation of primitive to parietal extraembryonic endoderm. In F9 cells, the application of exogenous agents including retinoic acid or activation of signal transduction cascades downstream of G-proteins triggers widespread changes in gene expression and leads to the formation of primitive endoderm. The epithelial-to-mesenchymal transition is completed and parietal endoderm develops as of result of increasing PKA activity in primitive endoderm cells. Expression of a constitutively active form of Gα13(Q226L) is sufficient to induce F9 cells into parietal endoderm and a model is emerging that a signaling axis linking G-protein signaling to RhoA and the ERM protein moesin is required for differentiation. In this study, we found that expression of either p115RhoGEF or a constitutively active, GTPase-deficient form of RhoA(L63) promoted primitive, but not parietal, endoderm formation. The overexpression of Gα13(Q226L) or p115RhoGEF, but not Rho(L63), caused β-catenin to translocate to the nucleus. Surprisingly, the stimulation of the Wnt-β-catenin pathway was accompanied by nuclear β-catenin and primitive endoderm formation, even when a dominant negative was used to block the signaling axis at the level of p115RhoGEF or when ROCK activity was inhibited using the pharmacological agent Y-27632. Together, results indicate that the coordinate signaling by two independent pathways, one involving canonical Wnt-β-catenin activation of target genes and the other with Gα13 signaling to ERM proteins to modulate cytoarchitectural changes, is required during the retinoic acid induced differentiation of F9 cells to primitive endoderm.

Role of G12 proteins in oncogenesis and metastasis

The G12 subfamily of heterotrimeric guanine nucleotide-binding proteins consists of two alpha subunits, G alpha12 and G alpha13. These proteins mediate signalling via G protein-coupled receptors and have been implicated in various physiological and pathophysiological processes. A number of direct and indirect effectors of G alpha12 and G alpha13 have been identified that mediate, or have been proposed to mediate, the diverse cellular responses accompanying activation of G12 proteins. This review describes the signalling pathways and cellular events stimulated by G12 proteins, with a particular emphasis on processes that are important in regulating cell migration and invasion, and could potentially be involved in the pathophysiology of cancer metastasis. Experimental findings directly implicating G12 proteins in the spread of metastatic disease are also summarized, indicating the importance of targeted inhibition of G12 signalling as a potential therapeutic option for locally advanced and metastatic disease.

Regulation and physiological functions of G12/13-mediated signaling pathways

Accumulating data indicate that G12 subfamily (Galpha12/13)-mediated signaling pathways play pivotal roles in a variety of physiological processes, while aberrant regulation of this pathway has been identified in various human diseases. It has been demonstrated that Galpha12/13-mediated signals form networks with other signaling proteins at various levels, from cell surface receptors to transcription factors, to regulate cellular responses. Galpha12/13 have slow rates of nucleotide exchange and GTP hydrolysis, and specifically target RhoGEFs containing an amino-terminal RGS homology domain (RH-RhoGEFs), which uniquely function both as a GAP and an effector for Galpha12/13. In this review, we will focus on the mechanisms regulating the Galpha12/13 signaling system, particularly the Galpha12/13-RH-RhoGEF-Rho pathway, which can regulate a wide variety of cellular functions from migration to transformation.

Scaffold proteins of MAP-kinase modules

Mitogen-activated protein kinases (MAPKs) regulate critical signaling pathways involved in cell proliferation, differentiation and apoptosis. Recent studies have shown that a novel class of scaffold proteins mediates the structural and functional organization of the three-tier MAPK module. By linking the MAP3K, MAP2K and MAPK into a multienzyme complex, these MAPK-specific scaffold proteins provide an insulated physical conduit through which signals from the respective MAPK can be transmitted to the appropriate spatiotemporal cellular loci. Scaffold proteins play a determinant role in modulating the signaling strength of their cognate MAPK module by regulating the signal amplitude and duration. The scaffold proteins themselves are finely regulated resulting in dynamic intra- and inter-molecular interactions that can modulate the signaling outputs of MAPK modules. This review focuses on defining the diverse mechanisms by which these scaffold proteins interact with their respective MAPK modules and the role of such interactions in the spatiotemporal organization as well as context-specific signaling of the different MAPK modules.

G protein regulation of MAPK networks

G proteins provide signal-coupling mechanisms to heptahelical cell surface receptors and are critically involved in the regulation of different mitogen-activated protein kinase (MAPK) networks. The four classes of G proteins, defined by the G(s), G(i), G(q) and G(12) families, regulate ERK1/2, JNK, p38MAPK, ERK5 and ERK6 modules by different mechanisms. The alpha- as well as betagamma-subunits are involved in the regulation of these MAPK modules in a context-specific manner. While the alpha- and betagamma-subunits primarily regulate the MAPK pathways via their respective effector-mediated signaling pathways, recent studies have unraveled several novel signaling intermediates including receptor tyrosine kinases and small GTPases through which these G-protein subunits positively as well as negatively regulate specific MAPK modules. Multiple mechanisms together with specific scaffold proteins that can link G-protein-coupled receptors or G proteins to distinct MAPK modules contribute to the context-specific and spatio-temporal regulation of mitogen-activated protein signaling networks by G proteins.

Lysophosphatidic acid regulates trafficking of beta2-adrenergic receptors: the Galpha13/p115RhoGEF/JNK pathway stimulates receptor internalization

Lysophosphatidic acid is an important lipid ligand regulating many aspects of cell function, including proliferation and migration. Operating via heterotrimeric G proteins to downstream effectors, lysophosphatidic acid was shown to regulate the function and trafficking of the G protein-coupled beta(2)-adrenergic receptor. C3 exotoxin, expression of dominant negative RhoA, and inhibition of c-Jun N-terminal kinase blocked the ability of lysophosphatidic acid to sequester the beta(2)-adrenergic receptor, whereas expression of constitutively active Galpha(13), p115RhoGEF, or RhoA mimicked lysophosphatidic acid (LPA) action, stimulating the internalization of the Galpha(s)-coupled beta(2)-adrenergic receptor. This study revealed a novel cross-talk exerted from the LPA/Galpha(13)/p115RhoGEF/RhoA pathway to the beta(2)-adrenergic receptor/Galpha(s)/adenylyl cyclase pathway, attenuating the ability of beta-adrenergic agonists to act following stimulation of cells by LPA as may occur during beta-adrenergic therapy of an inflammatory response.

Concise Review: Regulation of Embryonic Stem Cell Lineage Commitment by Mitogen-Activated Protein Kinases

Embryonic stem (ES) cells can give rise, in vivo, to the ectodermal, endodermal, and mesodermal germ layers and, in vitro, can differentiate into multiple cell lineages, offering broad perspectives in regenerative medicine. Understanding the molecular mechanisms governing ES cell commitment is an essential challenge in this field. The mitogen-activated protein kinase (MAPK) pathways extracellular signal-regulated kinase (ERK), c-Jun amino-terminal kinase (JNK), and p38MAPK are able to regulate ES commitment from early steps of the process to mature differentiated cells. Whereas the ERK pathway inhibits the self-renewal of ES cells, upon commitment this pathway is involved in the development of extraembryonic tissues, in early mesoderm differentiation, and in the formation of mature adipocytes; p38MAPK displays a large spectrum of action from neurons to adipocytes, and JNK is involved in both ectoderm and primitive endoderm differentiations. Furthermore, for a given pathway, several of these effects are isoform-dependent, revealing the complexity of the cellular response to activation of MAPK pathways. Regarding tissue regeneration, the potential outcome of systematic analysis of the function of different MAPKs in different ES cell differentiation programs is discussed. Disclosure of potential conflicts of interest is found at the end of this article.

Gα13 activation rescues moesin-depletion induced apoptosis in F9 teratocarcinoma cells

Mouse F9 cells differentiate into primitive endoderm when treated with retinoic acid (RA) and into parietal endoderm in response to RA and dibutyryl (db-) cAMP. G protein signaling either blocks or mimics RA-induced differentiation, the latter signaling through the Wnt-beta-catenin pathway. In our study, we found that a constitutively active Galpha13 mutant induces F9 cells to differentiate into parietal endoderm in the absence of exogenous agents. Galpha13 expression and subsequent differentiation are accompanied by beta-catenin translocation to the nucleus. Differentiation and changes in cell morphology are supported by rearrangements to the F-actin cytoskeleton. ERM (ezrin-radixin-moesin) proteins, known to link F-actin to transmembrane receptors, are also redistributed during differentiation. Furthermore, morpholino antisense and shRNA approaches show that moesin expression is essential since its knockdown leads to altered F-actin distribution and subsequent apoptosis. Moesin-depleted cells, however, remain attached to the substrate when Galpha13 is constitutively expressed, but they do not differentiate into extraembryonic endoderm. Our study demonstrates a link between Galpha13 signaling that regulates differentiation of F9 cells through primitive to parietal endoderm and a moesin requirement for cell survival.

p38 Mitogen-Activated Protein Kinase Activity Commits Embryonic Stem Cells to Either Neurogenesis or Cardiomyogenesis

Mouse embryonic stem (ES) cells can be differentiated, in vitro into a variety of cell types including cardiac cells and neurons. This process is strictly controlled by the potent morphogen retinoic acid (RA). At a concentration of 10(-7) M, RA induces ES cell differentiation into neurons and, conversely, inhibits cardiomyogenesis. We found that p38 mitogen-activated protein kinase (p38MAPK) activity peaked spontaneously, between day 3 and day 5, during ES cell differentiation and that RA completely inhibited this peak of activity. In contrast to wild-type cells, which required RA treatment, p38alpha(-/-) ES cells differentiated spontaneously into neurons and did not form cardiomyocytes. Moreover, inhibition of the peak of p38MAPK activity by a specific inhibitor, PD169316, committed ES cells into the neuronal lineage and blocked cardiomyogenesis. By genetic and biochemical approaches, we demonstrate that, in two different ES cell lines, the control of p38MAPK activity constitutes an early switch, committing ES cells into either neurogenesis (p38 off) or cardiomyogenesis (p38 on).

Defective expression of Gα12 in the testes of azoospermia patients and in the spermatozoa with low motility

Antibody to the Galpha12-subunit of guanine nucleotide regulatory proteins was used to determine whether the Galpha12 is present in adult human spermatogenic cells and to determine its role in dyszoospermia. Immunoblots from testes and spermatozoa demonstrated the presence of Galpha12 in the samples. Immunohistochemical analyses of testes found that Galpha12 was expressed in the cytoplasm of Leydig cells and was expressed in spermatids from the elongating Sb phase to mature sperm. Indirect immunofluorescence of human spermatozoa revealed the presence of Galpha12 in the neck region and the midpiece of the sperm. Galpha12 in spermatids and spermatozoa partially co-localized with F-actin and alpha-tubulin. Immunohistochemical analyses of tissues from three patients with non-obstructive azoospermia showed abnormal expression of Galpha12 in more than 45% of spermatids. Furthermore, Western blots and indirect immunofluorescence found defective expression of Galpha12 in low-motility spermatozoa with midpieces that were bent on themselves. Therefore, it suggests that Galpha12 plays a role in polarity and tail formation as spermatids mature. Furthermore, Galpha12 may be a candidate protein responsible for azoospermia caused by spermatogenic disturbance or midpiece deformities.

Endodermal differentiation of murine embryonic carcinoma cells by retinoic acid requires JLP, a JNK-scaffolding protein

Retinoic acid (RA) is a morphogen that induces endodermal differentiation of murine P19 embryonic carcinoma cells. RA-induced differentiation of P19 cells has been used as a model system to define the differentiation programs of pluripotent stem cells. Using this system it has been shown that G alpha13--the alpha-subunit of the heterotrimeric G protein G13--and its activation of JNK-module are critically required for the endodermal differentiation of P19 cells. However, the mechanism through which G alpha13 is linked to JNK-module is unknown. Here, we report that RA stimulates the expression of JNK-interacting leucine zipper protein (JLP), a newly identified JNK-scaffolding protein and its critical role in RA-mediated endodermal differentiation. Our results indicate that there is a physical association between JLP and G alpha13 in RA-stimulated P19 cells. More interestingly, silencing JLP abrogates RA-mediated endodermal differentiation of P19 cells analogous to the effects seen with the silencing of G alpha13 or JNK. Therefore, our studies presented here identify for the first time, a novel role for a newly identified scaffolding protein in RA-mediated endodermal differentiation, providing a new signaling conduit to transmit signals from RA to JNK module.

G proteins in development

The focus of developmental biologists has expanded from the analysis of gene expression to include the analysis of cell signalling. Heterotrimeric G proteins (G proteins) mediate signalling from a superfamily of heptahelical receptors (G-protein-coupled receptors) to a smaller number of effector units that include adenylyl cyclases, phospholipase C and various ion channels. The convergence of developmental biology with cell signalling has now revealed overlaps in which G proteins mediate complex pathways in embryonic development.

Selective Uncoupling of Gα12 from Rho-mediated Signaling

The heterotrimeric G protein G(12) has been implicated in such cellular regulatory processes as cytoskeletal rearrangement, cell-cell adhesion, and oncogenic transformation. Although the activated alpha-subunit of G(12) has been shown to interact directly with a number of protein effectors, the roles of many of these protein-protein interactions in G(12)-mediated cell physiology are poorly understood. To begin dissecting the specific cellular pathways engaged upon G(12) activation, we produced a series of substitution mutants in the regions of Galpha(12) predicted to play a role in effector binding. Here we report the identification and characterization of an altered form of Galpha(12) that is functionally uncoupled from signaling through the monomeric G protein Rho, a protein known to propagate several Galpha(12)-mediated signals. This mutant of Galpha(12) fails to bind the Rho-specific guanine nucleotide exchange factors p115RhoGEF and LARG (leukemia-associated RhoGEF), fails to stimulate Rho-dependent transcriptional activation, and fails to trigger activation of RhoA and the Rho-mediated cellular responses of cell rounding and c-jun N-terminal kinase activation. Importantly, this mutant of Galpha(12) retains coupling to the effector protein E-cadherin, as evidenced by its ability both to bind E-cadherin in vitro and to disrupt E-cadherin-mediated cell-cell adhesion. Furthermore, this mutant retains the ability to trigger beta-catenin release from the cytoplasmic domain of cadherin. This identification of a variant of Galpha(12) that is selectively uncoupled from one signaling pathway while retaining signaling capacity through a separate pathway will facilitate investigations into the mechanisms through which G(12) proteins mediate diverse biological responses.

Differential effects of all-trans-retinoic acid (RA) on Erk1/2 phosphorylation and cAMP accumulation in normal and malignant human prostate epithelial cells: Erk1/2 inhibition restores RA-induced decrease of cell growth in malignant prostate cells

OBJECTIVE

All-trans-retinoic acid (RA) regulates cellular growth, differentiation and apoptosis in human prostate by binding to RA receptors. Non-genomic retinoid effects on signal transduction kinases in the cytoplasm are also described in several cells but they are still unknown in prostate cells.

METHODS

Using an epithelial cell line derived from normal human prostate (EPN), and normal (NPEC) and malignant (CPEC) epithelial primary cultures of human prostate, we have examined effects of RA on both extracellular signal-regulated kinase 1/2 (Erk1/2) and cAMP accumulation. Then we have verified the effect of the inhibition of Erk1/2 on RA-induced growth arrest and apoptosis in malignant cells.

RESULTS

In NPEC and in EPN treated with RA for up to 24 h, Western blot analyses of Erk1/2 phosphorylation show that RA causes a rapid activation of Erk1/2 within 5 min, which is maintained for 30 min, followed by a return to basal levels. In CPEC, the activated phosphorylation levels persist up to 24 h. While basal cAMP levels are not affected by 30 min treatment with RA in both EPN and NPEC, levels are increased in CPEC. Forskolin-induced cAMP levels are decreased by RA in all cell types. CPEC were incubated for up to 96 h with RA with and without the inhibitor of Erk1/2, UO126. CPEC incubated with RA and UO126 for 72 h showed a significant arrest of cell growth and after 96 h apoptosis in 11% of cells.

CONCLUSIONS

We show rapid effects of RA on cytoplasmic messenger pathways in human prostate, and that responses can differ between normal and malignant cells. The inhibition of these pathways could improve the efficiency of RA in prostate cancer growth control.

Transcriptional Activation of the Human Gαi2 Gene Promoter through Nuclear Factor-κB and Antioxidant Response Elements

Very little is known regarding molecular mechanism(s) underlying transcriptional regulation of any G-protein gene despite the importance of G-protein expression in modulating cellular processes. Here we show that phorbol myristate acetate (PMA) and tert-butylhydroquinone (tBHQ), which induce oxidative stress in cells, up-regulate transcription of Galpha(i2) in K562 cells. Redox-sensing chemicals abrogated this transcriptional effect. A dominant negative I-kappaB double mutant (S32A/S36A) suppressed PMA-induced transcription by 54-62%, suggesting involvement of nuclear factor-kappaB (NF-kappaB). SN50, a cell-permeable peptide that inhibits nuclear import of stress-responsive transcription factors (such as NF-kappaB), inhibited PMA- and tBHQ-induced transcription. Deletion of an NF-kappaB-binding motif that maps at +10/+19 in the promoter resulted in 55-60% suppression of PMA-induced transcription, and 81% suppression of tBHQ-induced transcription. Mutation of an antioxidant response element (ARE) that maps at -84/-76 in the promoter resulted in 51 and 86% decrease in PMA- and tBHQ-induced transcription, respectively. In electrophoretic mobility shift assays, this element formed complexes with the transcription factors NF-E2p45 and Nrf2 that are prototypic for binding to the ARE, as well as with c-Fos, which can also interact with the ARE. Chromatin immunoprecipitation analysis demonstrated recruitment of these transcription factors to the promoter. Exogenously transfected Nrf2 transactivated the Galpha(i2) gene promoter; the cytoskeleton-associated protein, Keap1, abrogated this effect. Taken together, the present studies reveal that transcription factors that bind NF-kappaB and/or antioxidant response elements play an activating role in the transcription of the human Galpha(i2) gene.

Gα13 Signals via p115RhoGEF Cascades Regulating JNK1 and Primitive Endoderm Formation

The heterotrimeric G-protein G(13) mediates the formation of primitive endoderm from mouse P19 embryonal carcinoma cells in response to retinoic acid, signaling to the level of activation of c-Jun N-terminal kinase. The signal linkage map from MEKK1/MEKK4 to MEK1/MKK4 to JNK is obligate in this G alpha(13)-mediated pathway, whereas that between G alpha(13) and MEKKs is not known. The overall pathway to primitive endoderm formation was shown to be inhibited by treatment with Clostridium botulinum C3 exotoxin, a specific inactivator of RhoA family members. Constitutively active G alpha(13) was found to activate RhoA as well as Cdc42 and Rac1 in these cells. Although constitutively active Cdc42, Rac1, and RhoA all can activate JNK1, only the RhoA mutant was able to promote formation of primitive endoderm, mimicking expression of the constitutively activated G alpha(13). Expression of the constitutively active mutant form of p115RhoGEF (guanine nucleotide exchange factor) was found to activate RhoA and JNK1 activities. Expression of the dominant negative p115RhoGEF was able to inhibit activation of both RhoA and JNK1 in response to either retinoic acid or the expression of a constitutively activated mutant of G alpha(13). Expression of the dominant negative mutants of RhoA as well as those of either Cdc42 or Rac1, but not Ras, attenuated G alpha(13)-stimulated as well as retinoic acid-stimulated activation of all three of these small molecular weight GTPases, suggesting complex interrelationships among the three GTPases in this pathway. The formation of primitive endoderm in response to retinoic acid also could be blocked by expression of dominant negative mutants of RhoA, Cdc42, or Rac1. Thus, the signal propagated from G alpha(13) to JNK requires activation of p115RhoGEF cascades, including p115RhoGEF itself, RhoA, Cdc42, and Rac1. In a concerted effort, RhoA in tandem with Cdc42 and Rac1 activates the MEKK1/4, MEK1/MKK4, and JNK cascade, thereby stimulating formation of primitive endoderm.

Signaling pathways in retinoid chemoprevention and treatment of cancer

The Vitamin A metabolite, retinoic acid, has been shown to have chemopreventive and therapeutic activity for certain cancers such as head and neck, cervical, neuroblastoma and promyelocytic leukemia. Retinoic acid achieves these activities by inducing differentiation and/or growth arrest. A large number of studies have investigated the mechanism(s) by which retinoic acid alters the behavior of premalignant and tumor cells. Although much important data has been obtained, the exact signaling pathways required for retinoic acid to exert its biological effects remains elusive. In this review, we outline the role and function of retinoid nuclear receptors, followed by a discussion of how major signaling pathways are affected in different tumor types by retinoids. We conclude by examining the effect of retinoic acid on G1 cell cycle regulatory proteins in various tumors.

Ku80 is required but not sufficient for Galpha13-mediated endodermal differentiation in P19 embryonic carcinoma cells

We have shown that a constitutively active Galpha13 (Galpha13Q226L) induces differentiation in P19 embryonic carcinoma cells to an endodermal phenotype. In this report, we demonstrate that Ku, a heterodimer of p80 (Ku80) and p70 (Ku70), is upregulated in P19 cells overexpressing Galpha13Q226L. Ku is the regulatory subunit of the DNA-dependent protein kinase and is primarily involved in DNA repair and recombination. Ku80 also is a somatostatin receptor. We show that while overexpression of Ku80 drastically reduced P19 cell proliferation, it was not sufficient to induce endodermal differentiation. However, coexpression of Galpha13Q226L and an antisense Ku80 abrogated the retarded growth rate and endodermal differentiation observed in cells expressing only Galpha13Q226L. Overexpression of Galpha13Q226L or Ku80 downregulated RNA polymerase I-mediated transcriptional activity and overexpression of antisense Ku80 restored the activity to control level. These results suggest that Ku80 is required for Galpha13-mediated endodermal differentiation in P19 cells.

Activation Mechanism of c-Jun Amino-terminal Kinase in the Course of Neural Differentiation of P19 Embryonic Carcinoma Cells

P19 embryonic carcinoma cells, a model system for studying early development and differentiation, can differentiate into neurons and primitive endoderm-like cells depending on the culture conditions. We have previously reported that the activation of c-Jun amino-terminal kinase (JNK) is required for the retinoic acid-induced neural differentiation of P19 cells. However, the signaling pathway(s) responsible for the activation of JNK has not been known. In this study, we demonstrated that activities of MAPK kinase 4 (MKK4) and TAK1, one of the upstream kinases of MKK4, were enhanced in the neurally differentiating cells. Inhibition of the neural differentiation by an overexpression of protein phosphatase 2Cepsilon, an inactivator of TAK1, suggested a critical role of the TAK1 signaling pathway during the differentiation. Confocal microscopic analysis indicated that TAK1, phospho-MKK4, and phospho-JNK were colocalized with tubulin in the neurites and localized also in the nuclei of the differentiating cells. In contrast, two TAK1-binding proteins, TAB1 and TAB2, which are involved in the activation of TAK1, were localized in the neurites and the nuclei of the differentiating cells, respectively. These results suggest that two distinct TAK1-MKK4-JNK signaling pathways are independently activated at the different intracellular locations and may participate in the regulation of the neural differentiation of P19 cells.

Retinoic acid decreases fetal lung mesenchymal cell proliferation in vivo and in vitro

Retinoic acid (RA) is an important coordinator of mammalian organogenesis. RA is implicated in critical lung developmental events. Cell proliferation is precisely regulated during development. We investigated the effect of RA on proliferating mesenchymal cells in both whole organ lung cultures and cell cultures. The potential pathways required for the response were studied in cultures of lung mesenchymal cells from embryonic day (e) 12. We observed an RA-dependent reduction in proliferation of mesenchymal cells in both whole organ and in cell culture. In mesenchymal cell cultures, RA decreased proliferation in lung mesenchymal cells by 72%. This was associated with a decrease of erk-1/2 activity by 68%. Mesenchymal cell proliferation is erk-1/2 dependent. Erk-1/2 can be activated by G-protein coupled receptors (GPCR) or tyrosine kinase receptors (RTK). RA treatment altered both the RTK and the GPCR pathways in primary lung mesenchymal cells. The Epidermal Growth Factor (EGF) dependent erk-1/2 activation was increased by 35% whereas the G(i)-protein cascade was inhibited by 44% in cells treated with RA. Our results suggest that RA decreases proliferation of lung mesenchyme via a G(i)-protein and the erk-1/2 signaling cascade.

Jun N-terminal kinase pathway enhances signaling of monocytic differentiation of human leukemia cells induced by 1,25-dihydroxyvitamin D3

Recent studies revealed that the MEK/ERK module of the mitogen-activated protein kinase (MAPK) signaling cascades is up-regulated in the early stages of 1alpha,25-dihydroxyvitamin D(3) (1,25D(3))-induced monocytic differentiation of human leukemia cells HL60. In the present study, we investigated whether another MAPK module, the JNK pathway, also participates in this form of differentiation. We found that the dependence on the concentration of the inducer, the vitamin-hormone 1,25D(3), in two types of human leukemia cells, HL60 and U937, and the kinetics of monocytic differentiation in HL60 cells, parallel the degree of the activation of the JNK pathway. A blockade of JNK signaling by a stable expression of dominant negative (dn) JNK1 mutant in U937 cells resulted in reduced c-jun phosphorylation, and the differentiation of these cells was markedly decreased. Similarly, inhibition of JNK1 and JNK2 activities by the selective inhibitor SP600125 led to both dose-dependent reduction of c-jun and ATF-2 phosphorylation, and of the differentiation of HL60 cells. In addition, we found that JNK activity is essential for the AP-1 DNA binding induced by 1,25D(3) in HL60 and U937 cells. The results indicate that in cultured human leukemia cells, the JNK pathway participates in the induction of monocytic differentiation by 1,25D(3), probably by activating the AP-1 transcription factor.

Thrombin induces nitric-oxide synthase via Galpha12/13-coupled protein kinase C-dependent I-kappaBalpha phosphorylation and JNK-mediated I-kappaBalpha degradation

An imbalance between thrombin and antithrombin III contributed to vascular hyporeactivity in sepsis, which can be attributed to excess NO production by inducible nitric-oxide synthase (iNOS). In view of the importance of the thrombin-activated coagulation pathway and excess NO as the culminating factors in vascular hyporeactivity, this study investigated the effects of thrombin on the induction of iNOS and NO production in macrophages. Thrombin induced iNOS protein in the Raw264.7 cells, which was inhibited by a thrombin inhibitor, LB30057. Thrombin increased NF-kappaB DNA binding, whose band was supershifted with anti-p65 and anti-p50 antibodies. Thrombin elicited the phosphorylation and degradation of I-kappaBalpha prior to the nuclear translocation of p65. The NF-kappaB-mediated iNOS induction was stimulated by the overexpression of activated mutants of Galpha(12/13) (Galpha(12/13)QL). Protein kinase C depletion inhibited I-kappaBalpha degradation, NF-kappaB activation, and iNOS induction by thrombin or the iNOS induction by Galpha(12/13)QL. JNK, p38 kinase, and ERK were all activated by thrombin. JNK inhibition by the stable transfection with a dominant negative mutant of JNK1 (JNK1(-)) completely suppressed the NF-kappaB-mediated iNOS induction by thrombin. Conversely, the inhibition of p38 kinase enhanced the expression of iNOS. In addition, JNK and p38 kinase oppositely controlled the NF-kappaB-mediated iNOS induction by Galpha(12/13)QL. Hence, iNOS induction by thrombin was regulated by the opposed functions of JNK and p38 kinase downstream of Galpha(12/13). In the JNK1(-) cells, thrombin did not increase either the NF-kappaB binding activity or I-kappaBalpha degradation despite I-kappaBalpha phosphorylation. These results demonstrated that thrombin induces iNOS in macrophages via Galpha(12) and Galpha(13), which leads to NF-kappaB activation involving the protein kinase C-dependent phosphorylation of I-kappaBalpha and the JNK-dependent degradation of phosphorylated I-kappaBalpha.

Activation mechanism of c-Jun amino-terminal kinase in the course of endodermal differentiation of P19 embryonic carcinoma cells

c-Jun amino-terminal kinase (JNK) is known to be activated and play critical roles during neural and endodermal differentiation of P19 embryonic carcinoma cells. In this study we demonstrated that of the two upstream protein kinases of JNK, only MKK4 activity was substantially enhanced in the endodermally differentiating P19 cells. This enhanced activity of MKK4 stemmed from the increased expression of MKK4 and its activation by phosphorylation. Activated MKK4 and JNK were localized in both nucleus and cytoplasm of the differentiating cells, while they were localized only in the nucleus in the undifferentiated cells, suggesting multiple roles of JNK in the course of the endodermal differentiation of P19 cells.

pp60Src mediates insulin-stimulated sequestration of the beta(2)-adrenergic receptor: insulin stimulates pp60Src phosphorylation and activation

Insulin stimulates a rapid phosphorylation and sequestration of the beta(2)-adrenergic receptor. Analysis of the signaling downstream of the insulin receptor with enzyme inhibitors revealed roles for both phosphatidylinositol 3-kinase and pp60Src. Inhibition of Src with PP2, like the inhibition of phosphatidylinositol 3-kinase with LY294002 [2-(4-morpholynyl)-8-phenyl-4H-1-benzopyran-4-one], blocked the activation of Src as well as insulin-stimulated sequestration of the beta(2)-adrenergic receptor. Depletion of Src with antisense morpholinos also suppressed insulin-stimulated receptor sequestration. Src is shown to be phosphorylated/activated in response to insulin in human epidermoid carcinoma A431 cells as well as in mouse 3T3-L1 adipocytes and their derivative 3T3-F422A cells, well-known models of insulin signaling. Inhibition of Src with PP2 blocks the ability of insulin to sequester beta(2)-adrenergic receptors and the translocation of the GLUT4 glucose transporters. Insulin stimulates Src to associate with the beta(2)-adrenergic receptor/AKAP250/protein kinase A/protein kinase C signaling complex. We report a novel positioning of Src, mediating signals from insulin to phosphatidylinositol 3-kinase and to beta(2)-adrenergic receptor trafficking.

Galpha12 and Galpha13 negatively regulate the adhesive functions of cadherin

Cadherins function to promote adhesion between adjacent cells and play critical roles in such cellular processes as development, tissue maintenance, and tumor suppression. We previously demonstrated that heterotrimeric G proteins of the G12 subfamily comprised of Galpha12 and Galpha13 interact with the cytoplasmic domain of cadherins and cause the release of the transcriptional activator beta-catenin (Meigs, T. E., Fields, T. A., McKee, D. D., and Casey, P. J. (2001) Proc. Natl. Acad. Sci. U. S. A. 98, 519-524). Because of the importance of beta-catenin in cadherin-mediated cell-cell adhesion, we examined whether G12 subfamily proteins could also regulate cadherin function. The introduction of mutationally activated G12 proteins into K562 cells expressing E-cadherin blocked cadherin-mediated cell adhesion in steady-state assays. Also, in breast cancer cells, the introduction of activated G12 proteins blocked E-cadherin function in a fast aggregation assay. Aggregation mediated by a mutant cadherin that lacks G12 binding ability was not affected by activated G12 proteins, indicating a requirement for direct G12-cadherin interaction. Furthermore, in wound-filling assays in which ectopic expression of E-cadherin inhibits cell migration, the expression of activated G12 proteins reversed the inhibition via a mechanism that was independent of G12-mediated Rho activation. These results validate the G12-cadherin interaction as a potentially important event in cell biology and suggest novel roles for G12 proteins in the regulation of cadherin-mediated developmental events and in the loss of cadherin function that is characteristic of metastatic tumor progression.

Expression of Galpha 13 (Q226L) induces P19 stem cells to primitive endoderm via MEKK1, 2, or 4

Galpha13 mediates the ability of the morphogen retinoic acid to promote primitive endoderm formation from mouse P19 embryonal carcinoma stem cells, a process that includes the obligate activation of Jun N-terminal kinase. Expression of the constitutively activated (Q226L) GTPase-deficient form of Galpha13 mimics retinoic acid and was used to investigate the signaling upstream of primitive endoderm formation. Jun N-terminal kinase 1 activity, MEK1,2, MKK4, and MEKK1 were constitutively activated in clones stably transfected to express Q226L Galpha13. Dominant negative forms of MEKK1 and MEKK4 were expressed stably in the clones harboring Q226L Galpha13. Expression of dominant negative versions of either MEKK1 or MEKK4 effectively blocks both the activation of Jun N-terminal kinase as well as the formation of primitive endoderm. Depletion of MEKK1, -2, or -4 by antisense oligodeoxynucleotides suppressed signaling from Q226L Galpha13 to JNK1 and primitive endoderm formation. We demonstrate that the signal linkage map from Galpha13 activation to primitive endoderm formation in these stem cells requires activation at three levels of the mitogen-activated protein kinase cascade: MEKK1, -2, or -4 for MAP kinase kinase kinase; MKK4 and/or MEK1 for MAP kinase kinase; and JNK1 for MAP kinase.

Retinoic acid activation of the ERK pathway is required for embryonic stem cell commitment into the adipocyte lineage

Mouse embryonic stem (ES) cells are pluripotent cells that differentiate into multiple cell lineages. The commitment of ES cells into the adipocyte lineage is dependent on an early 3-day treatment with all-trans retinoic acid (RA). To characterize the molecular mechanisms underlying this process, we examined the contribution of the extracellular-signal-regulated kinase (ERK) pathway. Treatment of ES cell-derived embryoid bodies with RA resulted in a prolonged activation of the ERK pathway, but not the c-Jun N-terminal kinase, p38 mitogen-activated protein kinase or phosphoinositide 3-kinase pathways. To investigate the role of ERK activation, co-treatment of RA with PD98059, a specific inhibitor of the ERK signalling pathway, prevented both adipocyte formation and expression of the adipogenic markers, adipocyte lipid-binding protein and peroxisome-proliferator-activated receptor gamma. Furthermore, we show that ERK activation is required only during RA treatment. PD98059 does not interfere with the commitment of ES cells into other lineages, such as neurogenesis, myogenesis and cardiomyogenesis. As opposed to the controversial role of the ERK pathway in terminal differentiation, our results clearly demonstrate that this pathway is specifically required at an early stage of adipogenesis, corresponding to the RA-dependent commitment of ES cells.

Galphaq-dependent activation of mitogen-activated protein kinase kinase 4/c-Jun N-terminal kinase cascade

G-protein-coupled receptors (GPCRs) typically activate c-Jun N-terminal kinase (JNK) through the G protein betagamma subunit (Gbetagamma), in a manner dependent on Rho family small GTPases, in mammalian cells. Here we show that JNK activation by the prototypic Gq-coupled alpha1B-adrenergic receptor is mediated by the alpha subunit of Gq (Galphaq), not by Gbetagamma, using a transient transfection system in human embryonic kidney cells. JNK activation by the alpha1B-adrenergic receptor/Galphaq was selectively mediated by mitogen-activated protein kinase kinase 4 (MKK4), but not MKK7. Also, MKK4 activation by the alpha1B-adrenergic receptor/Galphaq required c-Src and Rho family small GTPases. Furthermore, activation of the alpha1B-adrenergic receptor stimulated JNK activity through Src family tyrosine kinases and Rho family small GTPases in hamster smooth muscle cells that natively express the alpha1B-adrenergic receptor. Together, these results suggest that the alpha1B-adrenergic receptor/Galphaq may up-regulate JNK activity through a MKK4 pathway dependent on c-Src and Rho family small GTPases in mammalian cells.

Wnt signaling and heterotrimeric G-proteins: strange bedfellows or a classic romance?

Wnts are secreted ligands with diverse roles in animal development. Wnts bind to cell surface membrane proteins termed Frizzleds. Molecular cloning of members of the Frizzled family revealed hydropathy plots with seven putative, transmembrane-spanning regions, conserved in Frizzleds characterized in mice, humans, flies, and worms. Understanding how Frizzled translates binding of their cognate Wnts into intracellular signals controlling aspects of development has been an elusive goal. Earlier observations gathered from a variety of model systems provided compelling, but indirect, support that the Frizzled receptors may be members of the superfamily of G-protein-coupled receptors that possess seven transmembrane-spanning domains. Search for a linkage between Frizzled and possible downstream heterotrimeric G-proteins has been advanced by the use of bacterial toxins, antisense DNA, and novel chimeric receptor constructs. New data establish that Frizzleds are indeed bona fide G-protein-coupled receptors. Frizzled-1 couples via G-proteins Go and Gq to the canonical beta-catenin-Lef-Tcf pathway. Frizzled-2 couples via Gq and Gt to downstream effectors including calcium mobilization. Frizzleds and G-proteins might once have been considered strange bedfellows, not likely partners in signaling. The new data, consistent with the properties known for virtually all members of the G-protein-coupled receptors, reveal a more classic romance of signaling elements controlling aspects of early development.

Activation of c-Jun amino-terminal kinase is required for retinoic acid-induced neural differentiation of P19 embryonal carcinoma cells

P19 embryonal carcinoma cells are known to differentiate into neurons and glia when treated with relatively high concentrations (>100 nM) of retinoic acid (RA). Concomitant with this RA-induced neural differentiation, we observed an activation of the c-Jun amino-terminal kinase (JNK). JNK was required for the RA-induced neural differentiation, because dominant-negative JNK blocked the differentiation. Studies using protein phosphatase inhibitors and protein kinase inhibitors suggested that both okadaic acid-sensitive protein phosphatase(s) and protein kinase C participate in the RA-induced activation of JNK.

Sodium butyrate induces transcription from the G alpha(i2) gene promoter through multiple Sp1 sites in the promoter and by activating the MEK-ERK signal transduction pathway

Sodium butyrate, an erythroid differentiation inducer and a histone deacetylase inhibitor, increases G alpha(i2) levels in differentiating K562 cells. Here we show that sodium butyrate induces G alpha(i2) gene transcription via sequences at -50/-36 and -92/-85 in the G alpha(i2) gene promoter. Both sequences contain core sequence motif for Sp1 binding; electrophoretic mobility shift as well as supershift assays confirmed binding to Sp1. Transcription from the G alpha(i2) gene promoter was also activated by two other histone deacetylase inhibitors, trichostatin A and Helminthsporium carbonium toxin (HC toxin), which also induce erythroblastic differentiation in K562 cells. However, hydroxyurea, a potent erythroid differentiation inducer in these cells, did not activate transcription from this gene promoter, indicating that promoter activation is inducer-specific. Mutations within the Sp1 sites at -50/-36 and -92/-85 in the G alpha(i2) gene promoter substantially decreased transcriptional activation by sodium butyrate, trichostatin A, or HC toxin. Transfection with constitutively activated ERKs indicated that this promoter can be activated through the MEK-ERK signal transduction pathway. Inhibition of the MEK-ERK pathway with U0126 or reduction in the expression of endogenous ERK with an antisense oligonucleotide to ERK significantly inhibited sodium butyrate- and HC toxin-induced transcription but had no effect on trichostatin A-induced transcription. Inhibition of the JNK and p38 MAPKs, using selective inhibitors, had no effect on sodium butyrate-induced transcription. In cells in which sodium butyrate induction of promoter activation had been inhibited by various concentrations of U0126, constitutively activated ERK2 reversed this inhibition. These results show that the MEK-ERK signal transduction pathway is important in butyrate signaling, which eventually converges in the cell nucleus.

Expression of JNK cascade scaffold protein JSAP1 in the mouse nervous system

The mitogen-activated protein kinase (MAPK) cascades consist of MAPK, MAPK kinase (MAPKK), and MAPKK kinase (MAPKKK). The specificity of activation of MAPK cascades may be determined, in part, by scaffold proteins that organize multi-enzyme complexes. We have earlier reported a scaffold protein JSAP1 (also known as JIP3) in the JNK MAPK cascade. We also showed that, of the adult mouse tissues tested, JSAP1 mRNA was predominantly expressed in brain. Here we report the localization of JSAP1 protein in mouse embryos and adult brain by immunohistochemical analysis. In embryos (E11-16), JSAP1 immunoreactivity was mainly found in the central and peripheral nervous systems, where it was localized to the cell bodies and/or axons of developing neurons, but not neural precursor cells. In the adult brain, immunoreactive JSAP1 was localized mostly to cell bodies in almost all neurons. We also showed that the expression of JSAP1 transcripts and proteins gradually increased during the neural differentiation of mouse P19 embryonal carcinoma (EC) cells. Furthermore, we showed that overexpressed JSAP1 facilitated the efficient activation of JNK by MEKK1 in P19 cells. These results suggest that JSAP1 may function as a scaffold protein for the JNK signaling module in neuronal cells.

Insulin activation of mitogen-activated protein kinases Erk1,2 is amplified via beta-adrenergic receptor expression and requires the integrity of the Tyr350 of the receptor

Insulin activates a complex set of intracellular responses, including the activation of mitogen-activated protein kinases Erk1,2. The counterregulatory actions of insulin on catecholamine action are well known and include phosphorylation of the beta(2)-adrenergic receptor on Tyr(350), Tyr(354), and Tyr(364) in the C-terminal cytoplasmic domain, as well as enhanced sequestration of the beta(2)-adrenergic receptor. Both beta-adrenergic agonists and insulin provoke sequestration of beta(2)-adrenergic receptors in a synergistic manner. In the current work, cross-talk between insulin action and beta(2)-adrenergic receptors revealed that insulin activation of Erk1,2 was amplified via beta(2)-adrenergic receptors. In Chinese hamster ovary cells, expression of beta(2)-adrenergic receptors enhanced 5-10-fold the activation of Erk1,2 by insulin and prolonged the activation, the greatest enhancement occurring at 5 min post-insulin. The potentiation of insulin signaling on Erk1,2 was proportional to the level of expression of beta(2)-adrenergic receptor. The potentiation of insulin signaling requires the integrity of Tyr(350) of the beta(2)-adrenergic receptor, a residue phosphorylated in response to insulin. beta(2)-adrenergic receptors with a Y350F mutation failed to potentiate insulin activation of Erk1,2. Expression of the C-terminal domain of the beta(2)-adrenergic receptor (Pro(323)-Leu(418)) in cells expressing the intact beta(2)-adrenergic receptor acts as a dominant negative, blocking the potentiation of insulin activation of Erk1,2 via the beta(2)-adrenergic receptor. Blockade of beta(2)-adrenergic receptor sequestration does not alter the ability of the beta(2)-adrenergic receptor to potentiate insulin action on Erk1,2. We propose a new paradigm in which a G-protein-linked receptor, such as the beta(2)-adrenergic receptor, itself acts as a receptor-based scaffold via its binding site for Src homology 2 domains, facilitating signaling of the mitogen-activated protein kinase pathway by insulin.

c-Jun promotes neurite outgrowth and survival in PC12 cells

We investigated the function of c-Jun in PC12 cells by transfecting them with a plasmid containing a c-Jun cDNA transcription cassette. Transfected cells expressed high levels of c-Jun mRNA and protein and demonstrated an increase in both AP-1 DNA binding and gene activation. The c-Jun over-expressing cells showed marked neurite outgrowth but no evidence of spontaneous cell death. In fact, c-Jun over-expressing cells were more resistant to okadaic acid-induced apoptosis. The process outgrowth was not indicative of a full neuronal differentiation response as the transfected PC12 cells did not display action potentials when examined with whole-cell patch-clamping. The phosphorylation of c-Jun on serine 73 appears to be important for this neurite sprouting effect as mutagenesis at this site reduced sprouting whereas a serine 63 mutant tended to increase sprouting. Thus, in PC12 cells c-Jun expression does not induce apoptosis, but rather functions as a neurite outgrowth and neuronal survival signal.

Ajuba, a cytosolic LIM protein, shuttles into the nucleus and affects embryonal cell proliferation and fate decisions

Cellular adhesive events affect cell proliferation and differentiation decisions. How cell surface events mediating adhesion transduce signals to the nucleus is not well understood. After cell-cell or cell-substratum contact, cytosolic proteins are recruited to clustered adhesion receptor complexes. One such family of cytosolic proteins found at sites of cell adhesion is the Zyxin family of LIM proteins. Here we demonstrate that the family member Ajuba was recruited to the cell surface of embryonal cells, upon aggregate formation, at sites of cell-cell contact. Ajuba contained a functional nuclear export signal and shuttled into the nucleus. Importantly, accumulation of the LIM domains of Ajuba in the nucleus of P19 embryonal cells resulted in growth inhibition and spontaneous endodermal differentiation. The differentiating effect of Ajuba mapped to the third LIM domain, whereas regulation of proliferation mapped to the first and second LIM domains. Ajuba-induced endodermal differentiation of these cells correlated with the capacity to activate c-Jun kinase and required c-Jun kinase activation. These results suggest that the cytosolic LIM protein Ajuba may provide a new mechanism to transduce signals from sites of cell adhesion to the nucleus, regulating cell growth and differentiation decisions during early development.

MEKK4 mediates differentiation in response to retinoic acid via activation of c-Jun N-terminal kinase in rat embryonal carcinoma P19 cells

Differentiation of P19 embryonal carcinoma cells in response to the morphogen retinoic acid is regulated by Galpha(12/13) and is associated with activation of c-Jun N-terminal kinase. The role of MEKK1 and MEKK4 upstream of the c-Jun N-terminal kinase was investigated in P19 cells. P19 clones stably expressing constitutively active and dominant negative mutants of MEKK1 and MEKK4 were created and characterized. Expression of the constitutively active form of either MEKK1 or MEKK4 mimicked the action of retinoic acid, inducing these embryonal carcinoma cells to primitive endoderm. Expression of the dominant negative form of MEKK1 had no influence on the ability of retinoic acid to induce either JNK activation or primitive endoderm formation in P19 stem cells. Expression of the dominant negative form of MEKK4, in contrast, effectively blocks both morphogen-induced activation of JNK and cellular differentiation. These data identify MEKK4 as upstream of c-Jun N-terminal kinase in the pathway mediating differentiation of P19 stem cells to primitive endoderm.

Retinoic acid selectively activates the ERK2 but not JNK/SAPK or p38 MAP kinases when inducing myeloid differentiation

Among the three major mitogen-activated protein kinase (MAPK) cascades--the extracellular signal regulated kinase (ERK) pathway, the c-JUN N-terminal/stress-activated protein kinase (JNK/SAPK) pathway, and the reactivating kinase (p38) pathway--retinoic acid selectively utilizes ERK but not JNK/SAPK or p38 when inducing myeloid differentiation of HL-60 human myeloblastic leukemia cells. Retinoic acid is known to activate ERK2. The present data show that the activation is selective for this MAPK pathway. JNK/SAPK or p38 are not activated by retinoic acid. Presumably because it activates relevant signaling pathways including MAPK, the polyoma middle T antigen, as well as certain transformation defective mutants thereof, is known to promote retinoic acid-induced differentiation, although the mechanism of action is not well understood. The present results show that consistent with the selective involvement of ERK2, ectopic expression of either the polyoma middle T antigen or its dl23 mutant, which is defective for PLCgamma and PI-3 kinase activation, or the delta205 mutant, which in addition is also weakened for activation of src-like kinases, caused no enhanced JNK/SAPK or p38 kinase activity that promoted the effects of retinoic acid. However, all three of these polyoma antigens are known to enhance ERK2 activation and promote differentiation induced by retinoic acid. Polyoma-activated MAPK signaling relevant to retinoic acid-induced differentiation is thus restricted to ERK2 and does not involve JNK/SAPK or p38. Taken together, the data indicate that among the three parallel MAPK pathways, retinoic acid-induced HL-60 myeloid differentiation selectively depends on activating ERK but not the other two MAPK pathways, JNK/SAPK or p38, with no apparent cross talk between pathways. Furthermore, the striking ability of polyoma middle T antigens to promote retinoic acid-induced differentiation appears to utilize ERK, but not JNK/SPK or p38 signaling.

Physiological regulation of G protein-linked signaling

Heterotrimeric G proteins in vertebrates constitute a family molecular switches that transduce the activation of a populous group of cell-surface receptors to a group of diverse effector units. The receptors include the photopigments such as rhodopsin and prominent families such as the adrenergic, muscarinic acetylcholine, and chemokine receptors involved in regulating a broad spectrum of responses in humans. Signals from receptors are sensed by heterotrimeric G proteins and transduced to effectors such as adenylyl cyclases, phospholipases, and various ion channels. Physiological regulation of G protein-linked receptors allows for integration of signals that directly or indirectly effect the signaling from receptor-->G protein-->effector(s). Steroid hormones can regulate signaling via transcriptional control of the activities of the genes encoding members of G protein-linked pathways. Posttranscriptional mechanisms are under physiological control, altering the stability of preexisting mRNA and affording an additional level for regulation. Protein phosphorylation, protein prenylation, and proteolysis constitute major posttranslational mechanisms employed in the physiological regulation of G protein-linked signaling. Drawing upon mechanisms at all three levels, physiological regulation permits integration of demands placed on G protein-linked signaling.

Requirement of Activation of JNK and p38 for Environmental Stress-Induced Erythroid Differentiation and Apoptosis and of Inhibition of ERK for Apoptosis

C-Jun amino terminal kinase/stress-activated protein kinases (JNK/SAPK) and p38 subgroups of mitogen-activated protein kinases have been suggested to play a critical role in apoptosis, cell growth, and/or differentiation. We found that a short exposure of SKT6 cells, which respond to erythropoietin (Epo) and induce erythroid differentiation, to osmotic or heat shock induced transient activation of JNK/SAPK and p38 and inactivation of ERK and resulted in erythroid differentiation without Epo, whereas long exposure of the cells to these stresses induced prolonged activation/inactivation of the same kinases and caused apoptosis. Inhibition of JNK/SAPK and p38 resulted in inhibition of stress-induced erythroid differentiation and apoptosis. Inhibition of ERK had no effect on stress-induced erythroid differentiation, but stimulated apoptosis. Activation of p38 and/or JNK/SAPK for a short time caused erythroid differentiation without Epo, although its prolonged activation induced apoptosis. Activation of ERK suppressed stress-induced apoptosis. These results indicate that short cellular stresses, inducing transient activation of JNK/SAPK and p38, lead to cell differentiation rather than apoptosis. Furthermore, activation of JNK/SAPK and p38 is required for both cell differentiation and apoptosis, and the duration of their activation may determine the cell fate, cell differentiation, and apoptosis. In contrast, inactivation of ERK is required for stress-induced apoptosis but not cell differentiation.