Phosphoprotein enriched in astrocytes-15 is expressed in mouse testis and protects spermatocytes from apoptosis

Phosphoprotein enriched in astrocytes (PEA-15) is a 15 kDa acidic serine-phosphorylated protein expressed in different cell types, especially in the CN. We initially detected the expression of PEA-15 in primary cultures of Sertoli cells. To assess the presence and localization of PEA-15 in the mouse testis, we studied the expression pattern of the PEA-15 protein by immunohistochemistry and mRNA byin situhybridization. Both the protein and the mRNA of PEA-15 were localized in the cytoplasm of Sertoli cells, all types of spermatogonia, and spermatocytes up till zygotene phase of the meiotic prophase. Subsequently, with ongoing development of the spermatocytes, the expression decreased and was very low in the cytoplasm of diplotene spermatocytes. To analyze the possible role of PEA-15 in the developing testis, null mutants for PEA-15 were examined. As the PEA-15 C terminus contains residues for ERK binding, we studied possible differences between the localization of the ERK2 protein in wild type (WT) andPEA-15−/−mice. In the WT testis, ERK2 was localized in the cytoplasm of Sertoli cells, B spermatogonia, preleptotene, leptotene, and zygotene spermatocytes, whereas in the KO testis, ERK2 was primarily localized in the nuclei of these cells and only little staining remained in the cytoplasm. Moreover, in PEA-15-deficient mice, significantly increased numbers of apoptotic spermatocytes were found, indicating an anti-apoptotic role of PEA-15 during the meiotic prophase. The increased numbers of apoptotic spermatocytes were not found at a specific step in the meiotic prophase.

Expression profiling of MAP kinase-mediated meiotic progression in Caenorhabditis elegans

The LET-60 (Ras)/LIN-45 (Raf)/MPK-1 (MAP kinase) signaling pathway plays a key role in the development of multiple tissues in Caenorhabditis elegans. For the most part, the identities of the downstream genes that act as the ultimate effectors of MPK-1 signaling have remained elusive. A unique allele of mpk-1, ga111, displays a reversible, temperature-sensitive, tissue-specific defect in progression through meiotic prophase I. We performed gene expression profiling on mpk-1(ga111) animals to identify candidate downstream effectors of MPK-1 signaling in the germ line. This analysis delineated a cohort of genes whose expression requires MPK-1 signaling in germ cells in the pachytene stage of meiosis I. RNA in situ hybridization analysis shows that these genes are expressed in the germ line in an MPK-1-dependent manner and have a spatial expression pattern consistent with the location of activated MPK-1. We found that one MPK-1 signaling-responsive gene encoding a C₂H₂ zinc finger protein plays a role in meiotic chromosome segregation downstream of MPK-1. Additionally, discovery of genes responsive to MPK-1 signaling permitted us to order MPK-1 signaling relative to several events occurring in pachytene, including EFL-1/DPL-1 gene regulation and X chromosome reactivation. This study highlights the utility of applying global gene expression methods to investigate genes downstream of commonly used signaling pathways in vivo.

In many tissues in developing organisms, signaling pathways interpret extracellular cues that change how genes are expressed inside the nucleus and thus direct the appropriate developmental choice. Identification of the genes that are responsive to signaling pathways is critical for understanding how these pathways can promote the correct cell fate. Additionally, understanding the relationships between different regulatory pathways will also help to decipher the network of gene expression that underlies development. The nematode Caenorhabditis elegans has many signaling pathways that are highly similar to those acting in mammals. In particular, the Ras/Raf/MAP kinase signaling pathway acts in many tissues in C. elegans to direct a diverse set of cell fates. Here, we identify a set of genes whose expression alters in response to Ras/Raf/MAP kinase signaling in the germ line during meiosis. We show that this set of genes is primarily expressed in the germ line and that at least one of these genes is important for proper germ cell fate downstream of Ras/Raf/MAP kinase signaling. We also find that the Ras/Raf/MAP kinase signaling pathway functions independently of a second regulatory pathway, the E2F pathway, that acts at a similar time during germ cell development.

C. elegans RBX-2-CUL-5- and RBX-1-CUL-2-based complexes are redundant for oogenesis and activation of the MAP kinase MPK-1

Cul5-based complex is a member of ECS (Elongin B/C-Cul2/Cul5-SOCS-box protein) ubiquitin ligase family. The cellular function of the Cul5-based complex is poorly understood. In this study, we found that oocyte septum formation and egg production did not occur in either cul-5- or rbx-2-depleted cul-2 homozygotes, although control cul-2 homozygotes laid approximately 50 eggs. These phenotypes are reminiscent of those caused by the MAP kinase mpk-1 depletion. In fact, activation of MPK-1 was significantly inhibited in cul-5-depleted cul-2 mutant and cul-2-depleted cul-5 mutant. Yeast two-hybrid analysis and RNAi-knockdown experiments suggest that oocyte maturation from pachytene exit and MPK-1 activation are redundantly controlled by the RBX-2-CUL-5- and RBX-1-CUL-2-based complexes.

Genistein, a soy isoflavone, up-regulates expression of antioxidant genes: involvement of estrogen receptors, ERK1/2, and NFkappaB

We have previously reported that estrogens up-regulate longevity-associated genes. As recent evidence has shown that estrogen replacement therapy is associated with an increased risk of cardiovascular disease, we have studied the effects of genistein, a soy isoflavone with a similar structure to estradiol, on the expression of antioxidant, longevity-related genes. MCF-7 cells (human mammary gland tumor cell line) were incubated for 48 h with 0.5 microM genistein, a concentration found in the plasma of populations consuming diets rich in soy protein. Peroxide levels were determined by fluorimetry, activation of extracellular-signal regulated kinase (ERK1/2), and nuclear factor kappaB (NFkappaB)-signaling pathways by Western blot analysis and ELISA, respectively, and mRNA expression of antioxidant genes by real-time reverse transcriptase-polymerase chain reaction (RT-PCR). Inhibition of basal peroxide levels in MCF-7 cells by genistein was prevented by pretreatment of cells with the estrogen receptor antagonist tamoxifen. Phosphorylation of extracellular regulated kinase (ERK)1/2 led to an activation of NFkappaB, as indicated by increased p50 subunit expression in nuclear extracts, and increased mRNA levels of the antioxidant enzyme manganese-superoxide dismutase (MnSOD). Inhibition of ERK1/2 abrogated genistein-mediated NFkappaB activation and elevated expression of MnSOD. Our molecular studies may provide a basis to determine the effects of genistein and other soy protein-derived products on longevity in both animals and the human population.

C. elegans GLA-3 is a novel component of the MAP kinase MPK-1 signaling pathway required for germ cell survival

During oocyte development in Caenorhabditis elegans, approximately half of all developing germ cells undergo apoptosis. While this process is evolutionarily conserved from worms to humans, the regulators of germ cell death are still largely unknown. In a genetic screen for novel genes involved in germline apoptosis in Caenorhabditis elegans, we identified and cloned gla-3. Loss of gla-3 function results in increased germline apoptosis and reduced brood size due to defective pachytene exit from meiosis I. gla-3 encodes a TIS11-like zinc-finger-containing protein that is expressed in the germline, from the L4 larval stage to adulthood. Biochemical evidence and genetic epistasis analysis revealed that GLA-3 participates in the MAPK signaling cascade and directly interacts with the C. elegans MAPK MPK-1, an essential meiotic regulator. Our results show that GLA-3 is a new component of the MAPK cascade that controls meiotic progression and apoptosis in the C. elegans germline and functions as a negative regulator of the MAPK signaling pathway during vulval development and in muscle cells.

The Src kinase Lyn is required for CCR5 signaling in response to MIP-1beta and R5 HIV-1 gp120 in human macrophages

CCR5 is a receptor for several beta chemokines and the entry coreceptor used by macrophage-tropic (R5) strains of HIV-1. In addition to supporting viral entry, CCR5 ligation by the HIV-1 envelope glycoprotein 120 (gp120) can activate intracellular signals in macrophages and trigger inflammatory mediator release. Using a combination of in vitro kinase assay, Western blotting for phospho-specific proteins, pharmacologic inhibition, CCR5 knockout (CCR5Delta32) cells, and kinase-specific blocking peptide, we show for the first time that signaling through CCR5 in primary human macrophages is linked to the Src kinase Lyn. Stimulation of human monocyte-derived macrophages with either HIV-1 gp120 or MIP-1beta results in the CCR5-mediated activation of Lyn and the concomitant Lyn-dependent activation of the mitogen-activated protein (MAP) kinase ERK-1/2. Furthermore, activation of the CCR5/Lyn/ERK-1/2 pathway is responsible for gp120-triggered production of TNF-alpha by macrophages, which is believed to contribute to HIV-1 pathogenesis. Thus, Lyn kinase may play an important role both in normal CCR5 function in macrophages and in AIDS pathogenesis in syndromes such as AIDS dementia where HIV-1 gp120 contributes to inappropriate macrophage activation, mediator production, and secondary injury.

lin-1 has both positive and negative functions in specifying multiple cell fates induced by Ras/MAP kinase signaling in C. elegans

lin-1 encodes an ETS domain transcription factor that functions downstream of a Ras/MAP kinase pathway mediating induction of the 1 degrees cell fate during vulval development in the C. elegans hermaphrodite. Mutants lacking lin-1 activity display a phenotype similar to that caused by mutations that constitutively activate let-60 Ras consistent with a model in which lin-1 is a repressor of the 1 degree fate whose activity is inhibited by phosphorylation by MPK-1 MAP kinase. Here, we show that, contrary the current model, lin-1 is required positively for the proper expression of several genes regulated by the pathway in cells adopting the 1 degrees cell fate. We show that the positive requirement for lin-1 is downstream of let-60 Ras and mpk-1 MAP kinase, and that it has a focus in the vulval precursor cells themselves. lin-1 alleles encoding proteins lacking a docking site for MPK-1 MAP kinase are defective in the positive function. We also show that lin-1 apparently has both positive and negative functions during the specification of the fates of other cells in the worm requiring Ras/MAP kinase signaling.

LIP-1 phosphatase controls the extent of germline proliferation in Caenorhabditis elegans

Caenorhabditis elegans germline cells are maintained in an undifferentiated and mitotically dividing state by Notch signaling and the FBF (for fem-3 binding factor) RNA-binding protein. Here, we report that the LIP-1 phosphatase, a proposed homolog of mitogen-activated protein (MAP) kinase phosphatases, is required for the normal extent of germline proliferation, and that lip-1 controls germline proliferation by regulating MAP kinase activity. In wild-type germ lines, LIP-1 protein is present in the proximal third of the mitotic region, consistent with its effect on germline proliferation. We provide evidence that lip-1 expression in the germline mitotic region is controlled by a combination of GLP-1/Notch signaling and FBF repression. Unexpectedly, FBF controls the accumulation of lip-1 mRNA, and therefore is likely to control its stability or 3'-end formation. In a sensitized mutant background, LIP-1 can function as a pivotal regulator of the decision between proliferation and differentiation. The control of germline proliferation by LIP-1 has intriguing parallels with the control of stem cells and progenitor cells in vertebrates.

Selective rather than inductive mechanisms favour specific replacement of Purkinje cells by embryonic cerebellar cells transplanted to the cerebellum of adult Purkinje cell degeneration (pcd) mutant mice

Cell replacement after neuronal degeneration in the adult CNS depends on the availability of specific cues to direct specification, differentiation and integration of newly born neurons into mature circuits. Following recent reports indicating that neurogenic signals may be reactivated in the adult injured CNS, here we asked whether such signals are expressed in the cerebellum after Purkinje cell degeneration. Thus, we compared the fate of embryonic cerebellar cells transplanted to the cerebella of adult wild-type and Purkinje cell degeneration (pcd) mutant mice. Donor cells were dissected from beta-actin-enhanced green fluorescent protein (EGFP) transgenic mice and transplanted as a single cell suspension. In both hosts, grafted cells generated all major cerebellar phenotypes, with a precise localization in the recipient cortex or white matter. Nevertheless, the phenotypic distributions showed striking quantitative differences. Most notably, in the pcd cerebellum there was a higher amount of Purkinje cells, while other phenotypes were less frequent. Analysis of cell proliferation by 5-bromo-2'-deoxyuridine (BrDU) incorporation revealed that in both hosts mitotic activity was strongly reduced shortly after transplantation, and virtually all donor Purkinje cells were actually generated before grafting. Together, these results indicate that some compensatory mechanisms operate in the pcd environment. However, the very low mitotic rate of transplanted cells suggests that the adult cerebellum, either wild-type or mutant, does not provide instructive neurogenic cues to direct the specification of uncommitted progenitors. Rather, specific replacement in mutant hosts is achieved through selective mechanisms that favour the survival and integration of donor Purkinje cells at the expense of other phenotypes.

T-cadherin mediates low-density lipoprotein-initiated cell proliferation via the Ca(2+)-tyrosine kinase-Erk1/2 pathway

The GPI-anchored protein T-cadherin was found to be an atypical LDL binding site that is expressed in various types of cells, including endothelial cells, smooth muscle cells, and neurons. Notably, the expression of T-cadherin was reduced in numerous types of cancers, although it was up-regulated in tumor-penetrating blood vessels, atherosclerotic lesions, and during neointima formation. Despite these intriguing findings, our knowledge of the physiological role and the signal transduction pathways associated with this protein is limited. Therefore, T-cadherin was overexpressed in the human umbilical vein-derived endothelial cell line EA.hy926, the human embryonic kidney cell line HEK293, and LDL-initiated signal transduction, and its consequences were elucidated. Our data revealed that T-cadherin serves as a receptor specifically for LDL. Following LDL binding to T-cadherin, mitogenic signal transduction was initiated that involved activation of PLC and IP3 formation, which subsequently yielded intracellular Ca2+ mobilization. Downstream to these early phenomena, activation of tyrosine kinase(s) Erk 1/2 kinase, and the translocation of NF kappa B toward the nucleus were found. Finally, overexpression of T-cadherin in HEK293 cells resulted in accelerated cell proliferation in an LDL-dependent manner, although cell viability was not influenced. Because LDL uptake was not facilitated by T-cadherin, our data suggest that T-cadherin serves as a signaling receptor for LDL that facilitates an LDL-dependent mitogenic signal in the vasculature.

Leishmania lipophosphoglycan activates the transcription factor activating protein 1 in J774A.1 macrophages through the extracellular signal-related kinase (ERK) and p38 mitogen-activated protein kinase

Leishmania donovani is an obligatory intracellular pathogen that resides and multiplies in the phagolysosomes of macrophages. The outcome of this infection depends on the balance between the host ability to activate macrophage killing and the parasite ability to suppress or evade this host immune response. Lipophosphoglycan (LPG) glycoconjugate, the surface molecule of the protozoan parasite is a virulence determinant and a major parasite molecule involved in this process. In this study, we examined the ability of Leishmania and its surface molecule, lipophosphoglycan to activate activating protein 1 (AP-1) through the mitogen-activated protein kinase (MAPK) cascade. We report here that the Leishmania surface molecule, lipophosphoglycan stimulates the simultaneous activation of all three classes of MAP kinases, extracellular signal-related kinases (ERKs), the c-jun amino-terminal kinase (JNK) and the p38 MAP kinase with differential kinetics in J774A.1 macrophage cell line. Furthermore, both L. donovani and its surface molecule lipophosphoglycan resulted in a dose- and time-dependent induction of AP-1 DNA-binding activity. We have also shown a dose-dependent increase of AP-1 binding activity in both low and high virulent strains of parasite. The use of inhibitors selective for ERK (PD98059) and p38 (SB203580) pathway showed that pre-incubation of cells with either SB203580 or PD98059 affected the binding activity of AP-1 suggesting that both p38 and ERK MAP kinase activation appear to be necessary for AP-1 activation by LPG. Lipophosphoglycan induced IL-12 production and generation of nitric oxide in murine macrophages. These results demonstrate that L. donovani LPG activates pro-inflammatory, endotoxin-like response pathway in J774A.1 macrophages and the interaction may play a pivotal role in the elimination of the parasite.

Androgen Receptor Pathway in Rats with Autosomal Dominant Polycystic Kidney Disease

Androgens have been implicated in mediating disease escalation in autosomal dominant polycystic kidney disease (ADPKD). Dihydrotestosterone (DHT), an agonist, and flutamide (FLT), an antagonist, were administered to Han:SPRD rats with ADPKD, and the role of androgen receptor (AR) abundance and activation on the enlargement and function of cystic kidneys was evaluated. Renal AR abundance determined by immunoblots in 8- to 10-wk-old Cy/+ male rats was naturally increased four-fold above that of littermate +/+ controls. In male Cy/+, castration decreased AR abundance below control +/+ by -89.4%, and AR expression within cyst mural epithelial cells was strikingly decreased. Castration of Cy/+ male rats also reduced the usual increases in kidney weight by -49.7%, kidney cyst area by -34.0%, and serum urea nitrogen by -72.8%; these indices were restored to precastration levels by DHT. In Cy/+ male rats, FLT administration reduced the increase in kidney weight by -27.6% and serum urea nitrogen by -53.7% and decreased the increment in AR expression by -84.2% in comparison with untreated +/+ controls. There was no effect of FLT in female rats. Immunoblot expression of phospho-extracellular signal-regulated kinase 1/2 (P-ERK) and B-Raf, key intermediates in the mitogen-activated protein kinase pathway that are abnormally elevated in Cy/+, was unaffected by castration and/or administration of DHT or FLT. AR was not expressed in renal epithelial cell nuclei of androgen-deficient rats but was displayed in most tubule and mural cyst cell nuclei of androgen-replete rats. In androgen-deficient Cy/+, 80.6% of renal epithelial cells that had entered the cell cycle (proliferating cell nuclear antigen positive) also expressed P-ERK. In androgen-replete rats, proliferating cell nuclear antigen-positive cells co-expressed AR (12.7%), P-ERK (36.4%), and P-ERK + AR (45.0%); 5.9% were probably stimulated by other mitogenic mechanisms. It is concluded that androgens potentiate renal cell proliferation and cyst enlargement through ERK1/2-dependent and ERK1/2-independent signaling mechanisms in Han:SPRD. It is suggested that the basal rate of cell proliferation is determined by ERK1/2 signaling to a major extent and that androgens have additive effects.

The P34G mutation reduces the transforming activity of K-Ras and N-Ras in NIH 3T3 cells but not of H-Ras

Ras proteins (H-, N-, and K-Ras) operate as molecular switches in signal transduction cascades controlling cell proliferation, differentiation, or apoptosis. The interaction of Ras with its effectors is mediated by the effector-binding loop, but different data about Ras location to plasma membrane subdomains and new roles for some docking/scaffold proteins point to signaling specificities of the different Ras proteins. To investigate the molecular mechanisms for these specificities, we compared an effector loop mutation (P34G) of three Ras isoforms (H-, N-, and K-Ras4B) for their biological and biochemical properties. Although this mutation diminished the capacity of Ras proteins to activate the Raf/ERK and the phosphatidylinositol 3-kinase/AKT pathways, the H-Ras V12G34 mutant retained the ability to cause morphological transformation of NIH 3T3 fibroblasts, whereas both the N-Ras V12G34 and the K-Ras4B V12G34 mutants were defective in this biological activity. On the other hand, although both the N-Ras V12G34 and the K-Ras4B V12G34 mutants failed to promote activation of the Ral-GDS/Ral A/PLD and the Ras/Rac pathways, the H-Ras V12G34 mutant retained the ability to activate these signaling pathways. Interestingly, the P34G mutation reduced specifically the N-Ras and K-Ras4B in vitro binding affinity to Ral-GDS, but not in the case of H-Ras. Thus, independently of Ras location to membrane subdomains, there are marked differences among Ras proteins in the sensitivity to an identical mutation (P34G) affecting the highly conserved effector-binding loop.

FGF signaling functions in the hypodermis to regulate fluid balance in C. elegans

Signaling by the Caenorhabditis elegans fibroblast growth factor receptor EGL-15 is activated by LET-756, a fibroblast growth factor, and attenuated by CLR-1, a receptor tyrosine phosphatase. Hyperactive EGL-15 signaling results in a dramatic Clr phenotype characterized by the accumulation of clear fluid within the pseudocoelomic space, suggesting that regulated EGL-15 signaling is essential for fluid homeostasis in C. elegans. To determine the cellular focus of EGL-15 signaling, we identified an enhancer element (e15) within the egl-15 promoter, which is both necessary for the promoter activity and sufficient when duplicated to drive either egl-15 or clr-1 rescue activity. This enhancer drives GFP expression in hypodermal cells. Consistent with this finding, immunofluorescence studies of EGL-15 indicate that EGL-15 is expressed in hypodermal cells, and hypodermal promoters can drive full clr-1 and egl-15 rescue activity. Moreover, a mosaic analysis of mpk-1, which acts downstream of egl-15, suggests that its suppression of Clr (Soc) function is required in the hypodermis. These results suggest that EGL-15 and CLR-1 act in the hypodermis to regulate fluid homeostasis in worms.

C. elegansSUR-6/PR55 cooperates with LET-92/protein phosphatase 2A and promotes Raf activity independently of inhibitory Akt phosphorylation sites

Protein phosphatase 2A (PP2A) can both positively and negatively influence the Ras/Raf/MEK/ERK signaling pathway, but its relevant substrates are largely unknown. In C. elegans, the PR55/B regulatory subunit of PP2A, which is encoded by sur-6, positively regulates Ras-mediated vulval induction and acts at a step between Ras and Raf. We show that the catalytic subunit (C) of PP2A, which is encoded by let-92, also positively regulates vulval induction. Therefore SUR-6/PR55 and LET-92/PP2A-C probably act together to dephosphorylate a Ras pathway substrate. PP2A has been proposed to activate the Raf kinase by removing inhibitory phosphates from Ser259 from Raf-1 or from equivalent Akt phosphorylation sites in other Raf family members. However, we find that mutant forms of C. elegansLIN-45 RAF that lack these sites still require sur-6. Therefore,SUR-6 must influence Raf activity via a different mechanism. SUR-6 and KSR(kinase suppressor of Ras) function at a similar step in Raf activation but our genetic analysis suggests that KSR activity is intact in sur-6mutants. We identify the kinase PAR-1 as a negative regulator of vulval induction and show that it acts in opposition to SUR-6 and KSR-1. In addition to their roles in Ras signaling, SUR-6/PR55 and LET-92/PP2A-C cooperate to control mitotic progression during early embryogenesis.

Caspase inhibitors induce a switch from apoptotic to proinflammatory signaling in CD95-stimulated T lymphocytes

CD95 is a major apoptosis receptor that induces caspase activation and programmed cell death in susceptible cells. CD95-induced apoptosis can be blocked by peptidic caspase inhibitors such as benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone or Ile-Glu-Thr-Asp-fluoromethyl ketone. Here we show that stimulation of CD95 in the presence of these inhibitors induces necrosis and expression of various proinflammatory cytokines in primary T lymphocytes, such as TNF-alpha, IFN-gamma and granulocyte/macrophage colony-stimulating factor. In the absence of caspase inhibition CD95 stimulation did not result in cytokine expression, indicating that this proinflammatory signaling pathway is suppressed by active caspases. Further analysis with A3.01 T cells revealed that the proinflammatory signaling activity of CD95 was mediated by MEK/ERK, p38 and NF-kappaB signaling pathways. These findings point to a pivotal role of caspases not only as mediators of apoptosis but also as enzymes that prevent proinflammatory signaling during CD95-induced apoptosis. Moreover, our findings may be useful for the development of novel pharmacological strategies.

The C.elegans MAPK phosphatase LIP-1 is required for the G(2)/M meiotic arrest of developing oocytes

In the Caenorhabditis elegans hermaphrodite germline, spatially restricted mitogen-activated protein kinase (MAPK) signalling controls the meiotic cell cycle. First, the MAPK signal is necessary for the germ cells to progress through pachytene of meiotic prophase I. As the germ cells exit pachytene and enter diplotene/diakinesis, MAPK is inactivated and the developing oocytes arrest in diakinesis (G(2)/M arrest). During oocyte maturation, a signal from the sperm reactivates MAPK to promote M phase entry. Here, we show that the MAPK phosphatase LIP-1 dephosphorylates MAPK as germ cells exit pachytene in order to maintain MAPK in an inactive state during oocyte development. Germ cells lacking LIP-1 fail to arrest the cell cycle at the G(2)/M boundary, and they enter a mitotic cell cycle without fertilization. LIP-1 thus coordinates oocyte cell cycle progression and maturation with ovulation and fertilization.

C. elegans EOR-1/PLZF and EOR-2 positively regulate Ras and Wnt signaling and function redundantly with LIN-25 and the SUR-2 Mediator component

In Caenorhabditis elegans, Ras/ERK and Wnt/beta-catenin signaling pathways cooperate to induce P12 and vulval cell fates in a Hox-dependent manner. Here we describe eor-1 and eor-2, two new positively acting nuclear components of the Ras and Wnt pathways. eor-1 and eor-2 act downstream or in parallel to ERK and function redundantly with the Mediator complex gene sur-2 and the functionally related gene lin-25, such that removal of both eor-1/eor-2 and sur-2/lin-25 mimics the removal of a main Ras pathway component. Furthermore, the eor-1 and eor-2 mutant backgrounds reveal an essential role for the Elk1-related gene lin-1. eor-1 and eor-2 also act downstream or in parallel to pry-1 Axin and therefore act at the convergence of the Ras and Wnt pathways. eor-1 encodes the ortholog of human PLZF, a BTB/zinc-finger transcription factor that is fused to RARalpha in acute promyelocytic leukemia. eor-2 encodes a novel protein. EOR-1/PLZF and EOR-2 appear to function closely together and cooperate with Hox genes to promote the expression of Ras- and Wnt-responsive genes. Further studies of eor-1 and eor-2 may provide insight into the roles of PLZF in normal development and leukemogenesis.

C. elegans ksr-1 and ksr-2 have both unique and redundant functions and are required for MPK-1 ERK phosphorylation

Kinase Suppressor of Ras (KSR) is a conserved protein that positively regulates Ras signaling and may function as a scaffold for Raf, MEK, and ERK. However, the precise role of KSR is not well understood, and some observations have suggested that KSR might act in a parallel pathway. In C. elegans, ksr-1 is only required for a specific Ras-mediated process (sex myoblast migration) and is a nonessential positive regulator of other Ras-mediated developmental events. We report the existence of a second C. elegans ksr gene, ksr-2, which is required for Ras-mediated signaling during germline meiotic progression and functions redundantly with ksr-1 during development of the excretory system, hermaphrodite vulva, and male spicules. Thus, while the ksr-1 and ksr-2 genes are individually required only for specific Ras-dependent processes, together these two genes appear necessary for most aspects of Ras-mediated signaling in C. elegans. The finding that ksr-2; ksr-1 double mutants have strong ras-like phenotypes and severely reduced or absent levels of diphosphorylated MPK-1 ERK strongly supports models where KSR acts to promote the activation or maintenance of the Raf/MEK/ERK kinase cascade.

Mechanisms of lectin (phytohemagglutinin)-induced growth in small intestinal epithelial cells

BACKGROUND/AIMS

The lectin phytohemagglutinin is a mitogen for intestinal epithelial cells in vivo. The mechanisms of action are unknown and were therefore analyzed in vitro.

METHODS

Human (Intestine-407) and rat (IEC-6; IEC-18) intestinal epithelial cell lines were stimulated with phytohemagglutinin. Proliferation was assayed by (3)H-thymidine incorporation, activation of mitogen-activated protein kinase (MAPK) by Western blotting, and induction of c-fos mRNA expression by semiquantitative polymerase chain reaction. Control experiments were performed with phenyl-N-acetyl-alpha-D-galactosaminide or the tyrosine kinase inhibitor tyrphostin A25.

RESULTS

Phytohemagglutinin (0.1 microg/ml) significantly stimulated proliferation in all three cell lines after 48-72 h. MAPK activation was detected after 15-30 min, and an induction of c-fos mRNA expression after 15- 30 min of stimulation. Mitogenic effects were blocked by preincubation with phenyl-N-acetyl-alpha-D-galactosaminide or tyrphostin A25.

CONCLUSION

Phytohemagglutinin stimulated proliferation, MAPK activation and induction of c-fos mRNA expression. The lectin may contribute to intestinal mucosal growth and regeneration thereby preventing gut atrophy.

Tethering of the Platelet-derived Growth Factor β Receptor to G-protein-coupled Receptors

Here we provide evidence to show that the platelet-derived growth factor beta receptor is tethered to endogenous G-protein-coupled receptor(s) in human embryonic kidney 293 cells. The tethered receptor complex provides a platform on which receptor tyrosine kinase and G-protein-coupled receptor signals can be integrated to produce more efficient stimulation of the p42/p44 mitogen-activated protein kinase pathway. This was based on several lines of evidence. First, we have shown that pertussis toxin (which uncouples G-protein-coupled receptors from inhibitory G-proteins) reduced the platelet-derived growth factor stimulation of p42/p44 mitogen-activated protein kinase. Second, transfection of cells with inhibitory G-protein alpha subunit increased the activation of p42/p44 mitogen-activated protein kinase by platelet-derived growth factor. Third, platelet-derived growth factor stimulated the tyrosine phosphorylation of the inhibitory G-protein alpha subunit, which was blocked by the platelet-derived growth factor kinase inhibitor, tyrphostin AG 1296. We have also shown that the platelet-derived growth factor beta receptor forms a tethered complex with Myc-tagged endothelial differentiation gene 1 (a G-protein-coupled receptor whose agonist is sphingosine 1-phosphate) in cells co-transfected with these receptors. This facilitates platelet-derived growth factor-stimulated tyrosine phosphorylation of the inhibitory G-protein alpha subunit and increases p42/p44 mitogen-activated protein kinase activation. In addition, we found that G-protein-coupled receptor kinase 2 and beta-arrestin I can associate with the platelet-derived growth factor beta receptor. These proteins play an important role in regulating endocytosis of G-protein-coupled receptor signal complexes, which is required for activation of p42/p44 mitogen-activated protein kinase. Thus, platelet-derived growth factor beta receptor signaling may be initiated by G-protein-coupled receptor kinase 2/beta-arrestin I that has been recruited to the platelet-derived growth factor beta receptor by its tethering to a G-protein-coupled receptor(s). These results provide a model that may account for the co-mitogenic effect of certain G-protein-coupled receptor agonists with platelet-derived growth factor on DNA synthesis.

HC fragment (C-terminal portion of the heavy chain) of tetanus toxin activates protein kinase C isoforms and phosphoproteins involved in signal transduction

A recent report [Gil, Chaib-Oukadour, Pelliccioni and Aguilera (2000) FEBS Lett. 481, 177-182] describes activation of signal transduction pathways by tetanus toxin (TeTx), a Zn(2+)-dependent endopeptidase synthesized by the Clostridium tetani bacillus, which is responsible for tetanus disease. In the present work, specific activation of protein kinase C (PKC) isoforms and of intracellular signal-transduction pathways, which include nerve-growth-factor (NGF) receptor trkA, phospholipase C(PLC)gamma-1 and extracellular regulated kinases (ERKs) 1 and 2, by the recombinant C-terminal portion of the TeTx heavy chain (H(C)-TeTx) is reported. The activation of PKC isoforms was assessed through their translocation from the soluble (cytosolic) compartment to the membranous compartment, showing that clear translocation of PKC-alpha, -beta, -gamma and -delta isoforms exists, whereas PKC-epsilon showed a slight decrease in its soluble fraction immunoreactivity. The PKC-zeta isoform showed no consistent response. Using immunoprecipitation assays against phosphotyrosine residues, time- and dose-dependent increases in tyrosine phosphorylation were observed in the trkA receptor, PLCgamma-1 and ERK-1/2. The effects shown by the H(C)-TeTx fragment on tyrosine phosphorylation were compared with the effects produced by NGF. The trkA and ERK-1/2 activation were corroborated using phospho-specific antibodies against trkA phosphorylated on Tyr(490), and antibodies against Thr/Tyr phosphorylated ERK-1/2. Moreover, PLCgamma-1 phosphorylation was supported by its H(C)-TeTx-induced translocation to the membranous compartment, an event related to PLCgamma-1 activation. Since H(C)-TeTx is the domain responsible for membrane binding and lacks catalytic activity, the activations described here must be exclusively triggered by the interaction of TeTx with a membrane component.

Two distinct signalling pathways are involved in FGF2-stimulated proliferation of choriocapillary endothelial cells: a comparative study with VEGF

In the retina, angiogenesis is an important component of normal physiological events such as embryonic vascular development. It is also involved in pathological processes including diabetic retinopathies and age-related macular degeneration, and tumour growth such as choroidal melanoma. Fibroblast growth factor (FGF) 2 and vascular endothelial cell growth factor (VEGF) are the two major angiogenic factors in the retina. We investigated the mechanism of proliferation and the regulation of the mitogenic properties of FGF2 and VEGF in cultures of chorocapillary endothelial cells (CEC). FGF2 is a strong mitogen for CEC and induced a 2.5-fold increase in cell proliferation after 4 days in culture in the absence of serum. In contrast, VEGF is a poor mitogen for CEC. FGF2, but not VEGF induces a large activation of MEK1, ERK1/2 and P90(RSK) during CEC proliferation. Pharmacological inhibition of Ras processing, and of MEK1 and ERK1/2 activation reduced only by 50% FGF2-induced cell proliferation, suggesting that there is another signalling pathway for CEC proliferation. Pharmacological inhibition of the PI 3-Kinase also inhibits by half FGF2-induced CEC proliferation. FGF2 stimulates the activation of the PI 3-K, P70(S6K) and Akt. Inhibition of both ERK1/2 and PI 3-K activities suppressed FGF2-induced CEC proliferation, demonstrating that CEC proliferation requires both ERKs and PI 3-K pathways. These data on the molecular mechanism and signalling may have important implications for providing more selective methods for anti-angiogenic and anti-tumoural therapy.

Notch inhibition of RAS signaling through MAP kinase phosphatase LIP-1 during C. elegans vulval development

During Caenorhabditis elegans vulval development, a signal from the anchor cell stimulates the RTK/RAS/MAPK (receptor tyrosine kinase/RAS/mitogen-activated protein kinase) signaling pathway in the closest vulval precursor cell P6.p to induce the primary fate. A lateral signal from P6.p then activates the Notch signaling pathway in the neighboring cells P5.p and P7.p to prevent them from adopting the primary fate and to specify the secondary fate. The MAP kinase phosphatase LIP-1 mediates this lateral inhibition of the primary fate. LIN-12/NOTCH up-regulates lip-1 transcription in P5.p and P7.p where LIP-1 inactivates the MAP kinase to inhibit primary fate specification. LIP-1 thus links the two signaling pathways to generate a pattern.