A signaling pathway to translational control

Involvement of S6 kinase and p38 mitogen activated protein kinase pathways in strain-induced alignment and proliferation of bovine aortic smooth muscle cells

Bovine aortic smooth muscle cell (SMC) phenotype can be altered by physical forces. This has been demonstrated by cyclic strain-induced changes in proliferation and alignment. However, the intracellular coupling pathways remain ill defined. In the present study, we examined whether the p38 and S6 kinase pathway were involved in the mitogenic and morphological changes seen in SMCs exposed to cyclic strain. We seeded bovine aortic SMCs on silastic membranes that were deformed with 150-mmHg vacuum. Cyclic strain induced both alignment and proliferation of SMCs. SB202190, a specific inhibitor of p38, hindered SMC alignment, but not proliferation. Rapamycin, a specific inhibitor of the mTOR-S6 kinase pathway, attenuated strain-induced proliferation, but not alignment. Peak activation of p38 and S6 kinase was 351 +/- 76.9% at 5 min and 363 +/- 56.2% at 60 min compared with static control, respectively (P < 0.05). The results suggest that strain-induced SMC alignment is dependent on activation of p38, but not S6 kinase. Strain induced SMC proliferation is S6 kinase, but not p38 activation, dependent.

Molecular actions of sirolimus: sirolimus and mTor

Recent therapeutic strategies to combat organ allograft rejection have focused on T-cell signaling pathways and the molecules that comprise them. The macrolide antibiotic produced by the bacterium Streptomyces hygroscopicus, known as sirolimus or rapamycin, has shown great therapeutic potential in the transplant setting. Sirolimus alone or in combination with other immunosuppressive agents can block acute rejection, chronic graft destruction, and promote permanent allograft acceptance. Sirolimus targets a unique serine-threonine kinase, mammalian target of rapamycin (mTor), which plays a key role in mitogenic and nutritional cells signals. Within T cells, mTor regulates a number of proteins likely dependent on T cell growth factors such as interleukin 2. This review is focused on the molecular mechanisms by which mTor may regulate T-cell signaling cascades and affect T-cell responsiveness, and how sirolimus likely uncouples this activity.

Clustering of microarray data reveals transcript patterns associated with somatic embryogenesis in soybean

Globular somatic embryos can be induced from immature cotyledons of soybean (Glycine max L. Merr. cv Jack) placed on high levels of the auxin 2,4-dichlorophenoxyacetic acid (2,4-D). Somatic embryos develop from the adaxial side of the cotyledon, whereas the abaxial side evolves into a callus. Using a 9,280-cDNA clone array, we have compared steady-state RNA from the adaxial side from which embryos develop and from the abaxial callus at five time points over the course of the 4 weeks necessary for the development of globular embryos. In a second set of experiments, we have profiled the expression of each clone in the adaxial side during the same period. A total of 495 genes differentially expressed in at least one of these experiments were grouped according to the similarity of their expression profiles using a nonhierarchical clustering algorithm. Our results indicate that the appearance of somatic embryos is preceded by dedifferentiation of the cotyledon during the first 2 weeks on auxin. Changes in mRNA abundance of genes characteristic of oxidative stress and genes indicative of cell division in the adaxial side of the cotyledons suggest that the arrangement of the new cells into organized structures might depend on a genetically controlled balance between cell proliferation and cell death. Our data also suggest that the formation of somatic globular embryos is accompanied by the transcription of storage proteins and the synthesis of gibberellic acid.

A reelin-integrin receptor interaction regulates Arc mRNA translation in synaptoneurosomes

Reelin is synthesized and secreted into extracellular matrix by cortical gamma-aminobutyric acid (GABA)ergic interneurons and binds with high affinity to the extracellular domain of integrin receptors expressed in dendritic shaft and spine postsynaptic densities (DSPSD) of pyramidal neurons. In heterozygous reeler mice, reelin bound to DSPSD, and the expression of Arc (activity-regulated cytoskeletal protein) is lower than in wild-type mice. We studied the effect of reelin on Arc and total protein synthesis in synaptoneurosomes (SNSs) prepared from mouse neocortex. Recombinant full-length mouse reelin displaces the high affinity (K(D) = 60 fM) binding of [(125)I]echistatin (a competitive integrin receptor antagonist) to integrin receptors with a K(i) of 22 pM and with a Hill slope close to 1. Echistatin (50-100 nM) competitively antagonizes and abates reelin binding. The addition of reelin (2-40 pM) to SNSs enhances the incorporation of [(35)S]methionine into Arc and other rapidly translated proteins in a concentration-dependent manner. This incorporation is virtually abolished by 50-100 nM echistatin or by 5-10 nM rapamycin, a blocker of the mammalian target of rapamycin kinase. We conclude that reelin binds with high affinity to integrin receptors expressed in SNSs and thereby activates Arc protein synthesis.

Protein phosphorylation networks in motor neuron death

The disorder amyotrophic lateral sclerosis (ALS) is characterized by the death of specific groups of neurons, especially motor neurons, which innervate skeletal muscle, and neurons connecting the cerebral cortex with motor neurons, such as corticospinal tract neurons. There have been numerous attempts to elucidate why there is selective involvement of motor neurons in ALS. Recent observations have demonstrated altered activities and protein levels of diverse kinases in the brain and spinal cord of transgenic mice that overexpress a mutant superoxide dismutase (mSOD) gene that is found in patients with the familial form of ALS, as well as in patients who have died with ALS. These results suggest that the alteration of protein phosphorylation may be involved in the pathogenesis of ALS. The changes in protein kinase and phosphatase expression and activity can affect the activation of important neuronal neurotransmitter receptors such as NMDA receptors or other signaling proteins and can trigger, or modify, the process producing neuronal loss in ALS. These various kinases, phosphatases and signaling proteins are involved in many signaling pathways; however, they have close interactions with each other. Therefore, an understanding of the role of protein kinases and protein phosphatases and the molecular organization of protein phosphorylation networks are useful to determine the mechanisms of selective motor neuron death.

Transcript profiling of human platelets using microarray and serial analysis of gene expression

Human platelets are anucleate blood cells that retain cytoplasmic mRNA and maintain functionally intact protein translational capabilities. We have adapted complementary techniques of microarray and serial analysis of gene expression (SAGE) for genetic profiling of highly purified human blood platelets. Microarray analysis using the Affymetrix HG-U95Av2 approximately 12 600-probe set maximally identified the expression of 2147 (range, 13%-17%) platelet-expressed transcripts, with approximately 22% collectively involved in metabolism and receptor/signaling, and an overrepresentation of genes with unassigned function (32%). In contrast, a modified SAGE protocol using the Type IIS restriction enzyme MmeI (generating 21-base pair [bp] or 22-bp tags) demonstrated that 89% of tags represented mitochondrial (mt) transcripts (enriched in 16S and 12S ribosomal RNAs), presumably related to persistent mt-transcription in the absence of nuclear-derived transcripts. The frequency of non-mt SAGE tags paralleled average difference values (relative expression) for the most "abundant" transcripts as determined by microarray analysis, establishing the concordance of both techniques for platelet profiling. Quantitative reverse transcription-polymerase chain reaction (PCR) confirmed the highest frequency of mt-derived transcripts, along with the mRNAs for neurogranin (NGN, a protein kinase C substrate) and the complement lysis inhibitor clusterin among the top 5 most abundant transcripts. For confirmatory characterization, immunoblots and flow cytometric analyses were performed, establishing abundant cell-surface expression of clusterin and intracellular expression of NGN. These observations demonstrate a strong correlation between high transcript abundance and protein expression, and they establish the validity of transcript analysis as a tool for identifying novel platelet proteins that may regulate normal and pathologic platelet (and/or megakaryocyte) functions.

The FKBP-associated protein FAP48 is an antiproliferative molecule and a player in T cell activation that increases IL2 synthesis

FAP48 was identified and cloned thanks to its interaction with FK506-binding proteins (FKBPs) such as FKBP52 and FKBP12, which belong to the large family of immunophilins that bind the macrolide immunosuppressant drugs FK506 and rapamycin. We have previously shown that FAP48-FKBP complexes are dissociated by FK506 and rapamycin, suggesting that FAP48 is an endogenous ligand of FKBP. The present work describes the biochemical consequences of FAP48 overexpression, induced by the tetracycline analogue doxycycline, in an established cell line derived from Jurkat T cells. We report that overexpression of FAP48 results in the inhibition of cellular proliferation as does the exposure of Jurkat T cells to FK506. We also show that the expression levels of argininosuccinate synthetase and the Myc antagonist Mxi1 are modified by overexpression of FAP48, suggesting that these proteins could be good candidates to mediate the antiproliferative effect of FAP48. FAP48 affects neither the calcineurin-dependent nuclear factor of activated T cells (NFAT)1 nor JNKp38-dependent pathways that mediate immunosuppression by FK506. However, contrary to FK506, which blocks IL2 synthesis, we observed that FAP48-FKBP complexes increase IL2 production, thus revealing a previously uncharacterized aspect of the immunosuppressive mechanism of FK506.

Low-energy laser irradiation enhances de novo protein synthesis via its effects on translation-regulatory proteins in skeletal muscle myoblasts

Low-energy laser irradiation (LELI) drives quiescent skeletal muscle satellite cells into the cell cycle and enhances their proliferation, thereby promoting skeletal muscle regeneration. Ongoing protein synthesis is a prerequisite for these processes. Here, we studied the signaling pathways involved in the LELI regulation of protein synthesis. High levels of labeled [35S]methionine incorporation were detected in LELI cells as early as 20 min after irradiation, suggesting translation of pre-existing mRNAs. Induced levels of protein synthesis were detected up until 8 h after LELI implying a role for LELI in de novo protein synthesis. Elevated levels of cyclin D1, associated with augmented phosphorylation of the eukaryotic initiation factor 4E (eIF4E) and its inhibitory binding protein PHAS-I, suggested the involvement of LELI in the initiation steps of protein translation. In the presence of the MEK inhibitor, PD98059, eIF4E phosphorylation was abolished and levels of cyclin D1 were dramatically reduced. The LELI-induced PHAS-I phosphorylation was abolished after preincubation with the PI3K inhibitor, Wortmannin. Concomitantly, LELI enhanced Akt phosphorylation, which was attenuated in the presence of Wortmannin. Taken together, these results suggest that LELI induces protein translation via the PI3K/Akt and Ras/Raf/ERK pathways.

Activation of a tyrosine kinase-MAPK cascade enhances the induction of long-term synaptic facilitation and long-term memory in Aplysia

Tyrosine kinases have been implicated in cellular processes thought to underlie learning and memory. Here we show that tyrosine kinases play a direct role in long-term synaptic facilitation (LTF) and long-term memory (LTM) for sensitization in Aplysia. Tyrosine kinase activity is required for serotonin-induced LTF of sensorimotor (SN-MN) synapses, and enhancement of endogenous tyrosine kinase activity facilitates the induction of LTF. These effects are mediated, at least in part, through mitogen-activated protein kinase (MAPK) activation and are blocked by transcriptional and translational inhibitors. Moreover, brain-derived neurotrophic factor (BDNF) also enhances the induction of LTF in a MAPK-dependent fashion. Finally, activation of endogenous tyrosine kinases enhances the induction of long-term memory for sensitization, and this enhancement also requires MAPK activation. Thus, tyrosine kinases, acting through MAPK, play a pivotal role in LTF and LTM formation.

The transformation suppressor Pdcd4 is a novel eukaryotic translation initiation factor 4A binding protein that inhibits translation

Pdcd4 is a novel transformation suppressor that inhibits tumor promoter-induced neoplastic transformation and the activation of AP-1-dependent transcription required for transformation. A yeast two-hybrid analysis revealed that Pdcd4 associates with the eukaryotic translation initiation factors eIF4AI and eIF4AII. Immunofluorescent confocal microscopy showed that Pdcd4 colocalizes with eIF4A in the cytoplasm. eIF4A is an ATP-dependent RNA helicase needed to unwind 5' mRNA secondary structure. Recombinant Pdcd4 specifically inhibited the helicase activity of eIF4A and eIF4F. In vivo translation assays showed that Pdcd4 inhibited cap-dependent but not internal ribosome entry site (IRES)-dependent translation. In contrast, Pdcd4(D418A), a mutant inactivated for binding to eIF4A, failed to inhibit cap-dependent or IRES-dependent translation or AP-1 transactivation. Recombinant Pdcd4 prevented eIF4A from binding to the C-terminal region of eIF4G (amino acids 1040 to 1560) but not to the middle region of eIF4G(amino acids 635 to 1039). In addition, both Pdcd4 and Pdcd4(D418A) bound to the middle region of eIF4G. The mechanism by which Pdcd4 inhibits translation thus appears to involve inhibition of eIF4A helicase, interference with eIF4A association-dissociation from eIF4G, and inhibition of eIF4A binding to the C-terminal domain of eIF4G. Pdcd4 binding to eIF4A is linked to its transformation-suppressing activity, as Pdcd4-eIF4A binding and consequent inhibition of translation are required for Pdcd4 transrepression of AP-1.

Rapamycin-induced G1 arrest in cycling B-CLL cells is associated with reduced expression of cyclin D3, cyclin E, cyclin A, and survivin

In B-cell chronic lymphocytic leukemia (B-CLL), malignant cells seem to be arrested in the G(0)/early G(1) phase of the cell cycle, and defective apoptosis might be involved in disease progression. However, increasing evidence exists that B-CLL is more than a disease consisting of slowly accumulating resting B cells: a proliferating pool of cells has been described in lymph nodes and bone marrow and might feed the accumulating pool in the blood. Rapamycin has been reported to inhibit cell cycle progression in a variety of cell types, including human B cells, and has shown activity against a broad range of human tumor cell lines. Therefore, we investigated the ability of rapamycin to block cell cycle progression in proliferating B-CLL cells. We have recently demonstrated that stimulation with CpG-oligonucleotides and interleukin-2 provides a valuable model for studying cell cycle regulation in malignant B cells. In our present study, we demonstrated that rapamycin induced cell cycle arrest in proliferating B-CLL cells and inhibited phosphorylation of p70s6 kinase (p70(s6k)). In contrast to previous reports on nonmalignant B cells, the expression of the cell cycle inhibitor p27 was not changed in rapamycin-treated leukemic cells. Treatment with rapamycin prevented retinoblastoma protein (RB) phosphorylation in B-CLL cells without affecting the expression of cyclin D2, but cyclin D3 was no longer detectable in rapamycin-treated B-CLL cells. In addition, rapamycin treatment inhibited cyclin-dependent kinase 2 activity by preventing up-regulation of cyclin E and cyclin A. Interestingly, survivin, which is expressed in the proliferation centers of B-CLL patients in vivo, is not up-regulated in rapamycin-treated cells. Therefore, rapamycin interferes with the expression of many critical molecules for cell cycle regulation in cycling B-CLL cells. We conclude from our study that rapamycin might be an attractive substance for therapy for B-CLL patients by inducing a G(1) arrest in proliferating tumor cells.

Protein kinase B/Akt prevents fatty acid-induced apoptosis in pancreatic beta-cells (INS-1)

Free fatty acids (FFA) have been reported to reduce pancreatic beta-cell mitogenesis and to increase apoptosis. Here we show that the FFA, oleic acid, increased apoptosis 16-fold in the pancreatic beta-cell line, INS-1, over a 18-h period as assessed by Hoechst 33342/propidium iodide staining and caspase-3 and -9 activation, with negligible necrosis. A parallel analysis of the phosphorylation activation of protein kinase B (PKB) showed this was reduced in the presence of FFA that correlated with the incidence of apoptosis. At stimulatory 15 mm glucose and/or in the added presence of insulin-like growth factor 1, FFA-induced beta-cell apoptosis was lessened compared with that at a basal 5 mm glucose. However, most strikingly, adenoviral mediated expression of a constitutively active PKB, but not a "kinase-dead" PKB variant, essentially prevented FFA-induced beta-cell apoptosis under all glucose/insulin-like growth factor 1 conditions. Further analysis of pro-apoptotic downstream targets of PKB, implicated a role for PKB-mediated phosphorylation inhibition of glycogen synthase kinase-3alpha/beta and the forkhead transcription factor, FoxO1, in protection of FFA-induced beta-cell apoptosis. In addition, down-regulation of the pro-apoptotic tumor suppressor protein, p53, via PKB-mediated phosphorylation of MDM2 might also play a role in partially protecting beta-cells from FFA-induced apoptosis. Adenoviral mediated expression of wild type p53 potentiated FFA-induced beta-cell apoptosis, whereas expression of a dominant negative p53 partly inhibited beta-cell apoptosis by approximately 50%. Hence, these data demonstrate that PKB activation plays an important role in promoting pancreatic beta-cell survival in part via inhibition of the pro-apoptotic proteins glycogen synthase kinase-3alpha/beta, FoxO1, and p53. This, in turn, provides novel insight into the mechanisms involved in FFA-induced beta-cell apoptosis.

The group I metabotropic glutamate receptor agonist (S)-3,5-dihydroxyphenylglycine induces a novel form of depotentiation in the CA1 region of the hippocampus

The ability of activation of group I metabotropic glutamate receptor (mGluR) to induce depotentiation was investigated at Schaffer collateral-CA1 synapses of rat hippocampal slices. Brief bath application (5 min) of group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG) (10 microm) induced a long-term depression of synaptic transmission or depotentiation (DEP) of previously established long-term potentiation (LTP), which was independent of NMDA or A(1) adenosine receptor activation. This DHPG-DEP was observed when DHPG was delivered 3 min after LTP induction. However, when DHPG was applied at 10 or 30 min after LTP induction, significantly less depotentiation was found. DHPG-DEP (1) is reversible and has the ability to unsaturate LTP, (2) is synapse specific, (3) does not require concurrent synaptic stimulation, (4) is mechanistically distinct from NMDA receptor-dependent depotentiation, (5) requires mGluR5 activation, (6) requires rapamycin-sensitive mRNA translation signaling, (7) does not require phospholipase C or protein phosphatase activation, and (8) is not associated with a change in paired-pulse (PP) facilitation. In addition, the ability of DHPG to reverse LTP was mimicked by a long train of low-frequency (1 Hz/15 min) PP stimulation. Moreover, the expression of DHPG-DEP is associated with a reduction in the increase of the surface expression of AMPA receptors seen with LTP. These results suggest that the activation of mGluR5 and in turn the triggering of a protein synthesis-dependent internalization of synaptic AMPA receptors may contribute to the DHPG-DEP in the CA1 region of the hippocampus.

GABAA receptors and benzodiazepines: a role for dendritic resident subunit mRNAs

This review is designed to describe the evolution of the seminal observation made simultaneously in 1975 by Dr. W. Haefely's laboratory (Hoffman La Roche, Basel, Switzerland) and in the Laboratory of Preclinical Pharmacology (NIH, St. Elizabeths Hospital, Washington DC), that benzodiazepine action was mediated by a modulation of GABA action at GABA(A) receptors. In fact, our suggestion was that the benzodiazepine receptor was "a receptor on a receptor" and that this receptor was GABA(A). Needless to say, this suggestion created opposition, but we did not abandon the original idea, in fact, as shown in this review, there is now universal agreement with our hypothesis on the mode of action of benzodiazepines. Hence, this review deals with the allosteric modulation of GABA(A) receptors by benzodiazepines, the role of GABA(A) receptors and benzodiazepine structure diversities in this modulation, and describes the results of our attempts to establish a benzodiazepine (imidazenil) devoid of tolerance, withdrawal symptoms, and changes in the expression of GABA(A) receptor subunits during tolerance. It also deals with the idea that the synthesis of GABA(A) receptor subunits triggered by tolerance resides in dendrites and spines where mRNAs and the apparatus for this translation is located. New analytic procedures may foster progress in the understanding of tolerance to and withdrawal from benzodiazepines.

The rapamycin-binding domain governs substrate selectivity by the mammalian target of rapamycin

The mammalian target of rapamycin (mTOR) is a Ser/Thr (S/T) protein kinase, which controls mRNA translation initiation by modulating phosphorylation of the translational regulators PHAS-I and p70(S6K). Here we show that in vitro mTOR is able to phosphorylate these two regulators at comparable rates. Both (S/T)P sites, such as Thr36, Thr45, and Thr69 in PHAS-I and the h(S/T)h site (where h is a hydrophobic amino acid) Thr389 in p70(S6K), were phosphorylated. Rapamycin-FKBP12 inhibited mTOR activity. Surprisingly, the extent of inhibition depended on the substrate. Moreover, mutating Ser2035 in the rapamycin-binding domain (FRB) not only decreased rapamycin sensitivity as expected but also dramatically affected the sites phosphorylated by mTOR. The results demonstrate that mutations in Ser2035 are not silent with respect to mTOR activity and implicate the FRB in substrate recognition. The findings also impose new limitations on interpreting results from experiments in which rapamycin and/or rapamycin-resistant forms of mTOR are used to investigate mTOR function in cells.

Post-transcriptional regulation of cyclin D1 expression during G2 phase

During continuous proliferation, cyclin D1 protein is induced to high levels in a Ras-dependent manner as cells progress from S phase to G2 phase. To understand the mechanism of the Ras-dependent cyclin D1 induction, cyclin D1 mRNA levels were determined by quantitative image analysis following fluorescent in situ hybridization. Although a slight increase in mRNA expression levels was detected during the S/G2 transition, this increase could not explain the more robust induction of cyclin D1 protein levels. This suggested the involvement of post-transcriptional regulation as a mechanism of cyclin D1 protein induction. To directly test this hypothesis, the cyclin D1 transcription rate was determined by run-on assays. The transcription rate of cyclin D1 stayed steady during the synchronous transition from S the G2 phase. We further demonstrated that cyclin D1 protein levels could increase during G2 phase in the absence of new mRNA synthesis. alpha-Amanitin, a transcription inhibitor, did not suppress cyclin D1 protein elevation as the cells progressed from S to G2 phase, even though the inhibitor was able to completely block cyclin D1 protein induction during reentry into the cell cycle from quiescence. The half life of cyclin D1 protein was shortest during S phase indicating that a change in protein stability might play a role in post-translational induction of cyclin D1 in G2 phase. These data indicate a fundamental difference in the regulation of cyclin D1 production during continuous cell cycle progression and re-initiation of the cell cycle.

Thymidylate synthase as a translational regulator of cellular gene expression

Studies from our laboratory have shown that the folate-dependent enzyme, thymidylate synthase (TS), functions as an RNA binding protein. There is evidence that TS, in addition to interacting with its own TS mRNA, forms a ribonucleoprotein complex with a number of other cellular mRNAs, including those corresponding to the p53 tumor suppressor gene and the myc family of transcription factors. Using both in vitro and in vivo model systems, we have demonstrated that the functional consequence of binding of TS protein to its own cognate mRNA, as well as binding of TS to the p53 mRNA, is translational repression. Herein, we review current work on the translational autoregulatory control of TS expression and discuss the molecular elements that are required for the TS protein-TS mRNA interaction. TS may play a critical role in regulating the cell cycle and the process of apoptosis through its regulatory effects on expression of p53 and perhaps other cell cycle related proteins. Finally, the ability of TS to function as a translational regulator may have important consequences with regard to the development of cellular resistance to various anticancer drugs.

The effects of rapamycin in murine peripheral nerve isografts and allografts

The FKBP-12-binding ligand FK506 has been successfully used to stimulate nerve regeneration and prevent the rejection of peripheral nerve allografts. The immunosuppressant rapamycin, another FKBP-12-binding ligand, stimulates axonal regeneration in vitro, but its influence on nerve regeneration in peripheral nerve isografts or allografts has not been studied. Sixty female inbred BALB/cJ mice were randomized into six tibial nerve transplant groups, including three isograft and three allograft (C57BL/6J) groups. Grafts were left untreated (groups I and II), treated with FK506 (groups III and IV), or treated with rapamycin (groups V and VI). Nerve regeneration was quantified in terms of histomorphometry and functional recovery, and immunosuppression was confirmed with mixed lymphocyte reactivity assays. Animals treated with FK506 and rapamycin were immunosuppressed and demonstrated significantly less immune cell proliferation relative to untreated recipient animals. Although every animal demonstrated some functional recovery during the study, animals receiving an untreated peripheral nerve allograft were slowest to recover. Isografts treated with FK506 but not rapamycin demonstrated significantly increased nerve regeneration. Nerve allografts in animals treated with FK506, and to a lesser extent rapamycin, however, both demonstrated significantly more nerve regeneration and increased nerve fiber widths relative to untreated controls. The authors suggest that rapamycin can facilitate regeneration through peripheral nerve allografts, but it is not a neuroregenerative agent in this in vivo model. Nerve regeneration in FK506-treated peripheral nerve isografts and allografts was superior to that found in rapamycin-treated animals. Rapamycin may have a role in the treatment of peripheral nerve allografts when used in combination with other medications, or in the setting of renal failure that often precludes the use of calcineurin inhibitors such as FK506.

Signaling inhibitors in the treatment of prostate cancer

Inhibiting androgen receptor (AR) activation through medical or surgical castration and blockade of AR-androgen binding is the cornerstone of treatment for advanced prostate cancer. However, in most cases tumor growth eventually becomes androgen independent. Alternative mechanisms of AR activation, some of which involve growth factor receptor signaling, have been demonstrated in prostate cancer models, and it is likely that a number of autocrine and paracrine growth factor ligand-receptor interactions such as those of epidermal growth factors, fibroblast growth factors, and insulin-like growth factors contribute to the androgen independent phenotype by promoting cell proliferation and survival. Blocking activation and signaling through growth factor receptors and upstream signaling proteins has emerged as a credible approach to cancer treatment. Successful application of this approach in prostate cancer using a growing array of small molecule kinase inhibitors, antibodies, and antisense oligonucleotides will be greatly accelerated by elucidation of the key signaling pathways that maintain the androgen independent phenotype.

Comparative analysis of protein synthesis, transcription and cytoplasmic polyadenylation of mRNA during maturation of bovine oocytes in vitro

In many species, large numbers of macromolecules are accumulated during oocyte growth. The messenger and ribosomal RNAs produced in these cells are far in excess of those necessary to support protein synthesis. Thus, the aim of this study was to elucidate the processes of translational regulation during meiotic maturation. The relationship between transcription, translation and polyadenylation of mRNA during in-vitro maturation (IVM) of bovine oocytes was investigated. The results presented here show that overall protein synthesis is stimulated during meiotic maturation (approximately three times) concomitantly with the onset of germinal vesicle breakdown after 6 to 10 h of IVM. However, in metaphase II, the incorporation of [35S]methionine into proteins showed only basal levels, as in the germinal vesicle (GV) stage. Furthermore, in the course of IVM, de-novo transcription strongly declines as determined by measuring the incorporation of [3H]uridine into RNA. In contrast to this finding, the incorporation of [3H]adenosine increased and showed a peak during the time interval from 6 to 10 h of IVM, parallel with the onset of germinal vesicle breakdown (GVBD) and translation. In the further course of maturation, only a moderate decrease of [3H]adenosine incorporation was observed. These results indicate that (i) translation increased at the time of GVBD; (ii) these processes were accompanied by polyadenylation of mRNA; and (iii) although transcription declines, polyadenylated mRNA is accumulated until metaphase II (as shown by poly(U)-hybridization), in which protein synthesis is low. The correlation of these processes is discussed here. A detailed knowledge of the biochemical and molecular processes which occur during oocyte maturation can be useful for the improvement of IVM conditions.

BCR signal through alpha 4 is involved in S6 kinase activation and required for B cell maturation including isotype switching and V region somatic hypermutation

alpha 4 potentially mediates BCR signals through a rapamycin-sensitive TOR pathway. To investigate a potential role for alpha 4 in B cell activation, the alpha 4 gene was disrupted conditionally in B cells by mating male CD19-Cre mice with female alpha 4-floxed mice. CD19-Cre+/alpha 4flox mice showed loss of alpha 4 protein in B lineage cells and a decreased number of phenotypically normal mature B cells. Compared to normal B cells, alpha 4(-) B cells showed a decreased proliferation in response to the B cell stimulants (anti-IgM antibody plus IL-4, anti-CD40 mAb and lipopolysaccharide), and a reduced S6 kinase activation and rapamycin sensitivity. While CD19-Cre+/alpha 4flox mice showed impaired antibody responses to both T cell-independent and T cell-dependent (TD) antigens, the TD antigen response was markedly impaired as demonstrated by reduced isotype switching, reduced germinal center formation and reduced V region somatic hypermutation. These results show that alpha 4 plays a pivotal role in antigen-specific signal transduction during B cell activation and differentiation in vivo.

Increased activity of c-Src and Csk in fibroblasts transformed by v-src oncogene

When c-Src and v-Src were immunoprecipitated together from hamster fibroblasts transformed by Rous sarcoma virus containing v-src oncogene, the total Src activity was almost threefold higher compared to c-Src activity in the control cells. The activity of v-Src immunoprecipitated separately, however, accounting for only 40% of the total Src activity, indicating that c-Src is activated upon transformation. An increased activity of Csk was also found in RSV-transformed cells. It decreased upon serum stimulation in parallel with an increase in Src kinase activity. In nontransformed cells, serum stimulation induced an enhanced Csk activity, but no changes in c-Src activity were observed. This may suggest that Csk may have more functions in hamster fibroblasts, in addition to its inhibitory effect on c-Src.

A rapamycin-sensitive signaling pathway contributes to long-term synaptic plasticity in the hippocampus

Many forms of long-lasting behavioral and synaptic plasticity require the synthesis of new proteins. For example, long-term potentiation (LTP) that endures for more than an hour requires both transcription and translation. The signal-transduction mechanisms that couple synaptic events to protein translational machinery during long-lasting synaptic plasticity, however, are not well understood. One signaling pathway that is stimulated by growth factors and results in the translation of specific mRNAs includes the rapamycin-sensitive kinase mammalian target of rapamycin (mTOR, also known as FRAP and RAFT-1). Several components of this translational signaling pathway, including mTOR, eukaryotic initiation factor-4E-binding proteins 1 and 2, and eukaryotic initiation factor-4E, are present in the rat hippocampus as shown by Western blot analysis, and these proteins are detected in the cell bodies and dendrites in the hippocampal slices by immunostaining studies. In cultured hippocampal neurons, these proteins are present in dendrites and are often found near the presynaptic protein, synapsin I. At synaptic sites, their distribution completely overlaps with a postsynaptic protein, PSD-95. These observations suggest the postsynaptic localization of these proteins. Disruption of mTOR signaling by rapamycin results in a reduction of late-phase LTP expression induced by high-frequency stimulation; the early phase of LTP is unaffected. Rapamycin also blocks the synaptic potentiation induced by brain-derived neurotrophic factor in hippocampal slices. These results demonstrate an essential role for rapamycin-sensitive signaling in the expression of two forms of synaptic plasticity that require new protein synthesis. The localization of this translational signaling pathway at postsynaptic sites may provide a mechanism that controls local protein synthesis at potentiated synapses.

Tumstatin, an endothelial cell-specific inhibitor of protein synthesis

Tumstatin is a 28-kilodalton fragment of type IV collagen that displays both anti-angiogenic and proapoptotic activity. Here we show that tumstatin functions as an endothelial cell-specific inhibitor of protein synthesis. Through a requisite interaction with alphaVbeta3 integrin, tumstatin inhibits activation of focal adhesion kinase (FAK), phosphatidylinositol 3-kinase (PI3-kinase), protein kinase B (PKB/Akt), and mammalian target of rapamycin (mTOR), and it prevents the dissociation of eukaryotic initiation factor 4E protein (eIF4E) from 4E-binding protein 1. These results establish a role for integrins in mediating cell-specific inhibition of cap-dependent protein synthesis and suggest a potential mechanism for tumstatin's selective effects on endothelial cells.

Modulation of human lymphocyte proliferative response with aging

Previously, we have demonstrated age-associated alterations in transmembrane signaling. One of the most reproducible alterations found in the immune response with aging is the decrease of lymphocyte proliferation on stimulation with various different mitogens. Here, we confirm that proliferative responses to stimulation with phytohaemagglutin (PHA), recombinant human IL-2, or anti-CD3 monoclonal antibody are all greater in the young (20-25 years) than old (60-87 years) population. We attempted to modulate the proliferative response using various agents acting at different levels of transmembrane signaling (pertussis toxin, cholera toxin, isoproterenol, PMA, Ca ionophore A23187), as well as at the level of the lymphocyte plasma membrane (methyl-beta-cyclodextrin, MBCD), or by using antioxidant vitamins (Vitamin E or C). None of these agents was able to restore effectively the proliferative response of lymphocytes from the aged to the level of young subjects. Even the combination of A23187 and PMA acting directly on calcium metabolism and protein kinase C activity was insufficient to restore the decreased mitogenic capacity of T cells from elderly subjects. Cyclodextrin, which decreases the cholesterol content of the membrane, increased the proliferative response of lymphocytes of elderly subjects, but not to the level of the young. Vitamin E had a very strong inhibitory effect on lymphocyte stimulation in both the age groups, except in combination with MBCD in T cells of the elderly, while Vitamin C had no significant modulatory effect. MAPK ERK and p38 activation was found to be decreased with aging in T cells after anti-CD3 mAb stimulation. Vitamin E but not Vitamin C strongly inhibited MAPK ERK or p38 activation. The direct activation of certain molecules or the modulation of the cholesterol content of the membrane seems to be effective immunomodulatory interventions with aging.

Ribosomal protein S6 phosphorylation and function during late gestation liver development in the rat

The phosphorylation of ribosomal protein S6 is thought to be required for biosynthesis of the cell's translational apparatus, a critical component of cell growth and proliferation. We have studied the signal transduction pathways involved in hepatic S6 phosphorylation during late gestation in the rat. This is a period during which hepatocytes show a high rate of proliferation that is, at least in part, independent of mitogenic signaling pathways that are operative in mature hepatocytes. Our initial studies demonstrated that there was low basal activity of two S6 kinases in liver, S6K1 and S6K2, on embryonic day 19 (2 days preterm). In addition, insulin- and growth factor-mediated S6K1 and S6K2 activation was markedly attenuated compared with that in adult liver. Nonetheless, two-dimensional gel electrophoresis demonstrated that fetal liver S6 itself was highly phosphorylated. To characterize the fetal hepatocyte pathway for S6 phosphorylation, we went on to study the sensitivity of hepatocyte proliferation to the S6 kinase inhibitor rapamycin. Unexpectedly, administration of rapamycin to embryonic day 19 fetuses in situ did not affect hepatocyte DNA synthesis. This resistance to the growth inhibitory effect of rapamycin occurred even though S6K1 and S6K2 were inhibited. Furthermore, fetal hepatocyte proliferation was sustained even though rapamycin administration resulted in the dephosphorylation of ribosomal protein S6. In contrast, rapamycin blocked hepatic DNA synthesis in adult rats following partial hepatectomy coincident with S6 dephosphorylation. We conclude that hepatocyte proliferation in the late gestation fetus is supported by a rapamycin-resistant mechanism that can function independently of ribosomal protein S6 phosphorylation.

7SK small nuclear RNA binds to and inhibits the activity of CDK9/cyclin T complexes

The transcription of eukaryotic protein-coding genes involves complex regulation of RNA polymerase (Pol) II activity in response to physiological conditions and developmental cues. One element of this regulation involves phosphorylation of the carboxy-terminal domain (CTD) of the largest polymerase subunit by a transcription elongation factor, P-TEFb, which comprises the kinase CDK9 and cyclin T1 or T2 (ref. 1). Here we report that in human HeLa cells more than half of the P-TEFb is sequestered in larger complexes that also contain 7SK RNA, an abundant, small nuclear RNA (snRNA) of hitherto unknown function. P-TEFb and 7SK associate in a specific and reversible manner. In contrast to the smaller P-TEFb complexes, which have a high kinase activity, the larger 7SK/P-TEFb complexes show very weak kinase activity. Inhibition of cellular transcription by chemical agents or ultraviolet irradiation trigger the complete disruption of the P-TEFb/7SK complex, and enhance CDK9 activity. The transcription-dependent interaction of P-TEFb with 7SK may therefore contribute to an important feedback loop modulating the activity of RNA Pol II.

Translational activation and repression by distinct elements within the 5'-UTR of ENaC alpha-subunit mRNA

The rat amiloride-sensitive epithelial Na(+) channel (rENaC), the rate-limiting step in epithelial Na(+) transport, consists of three subunits, alpha, beta, and gamma. We hypothesized that alpha-rENaC translation is regulated via its 5'-untranslated region (UTR). Transient transfections of alpha-rENaC promoter-reporter constructs in representative epithelial cell lines demonstrated up to fivefold differences in activity among constructs containing different amounts of the alpha-rENaC 5'-UTR sequence. Differences in reporter protein activity did not parallel differences in reporter mRNA, demonstrating that 5'-UTR regulation must be at the level of translation. Specifically, translation was enhanced by a region extending from +53 to +211 bp downstream from the transcription start site and repressed by the region between +367 and +499 bp. Examination of the 5'-UTR sequence revealed an out-of-frame initiation codon within the repressive region, 43 bp upstream from the start of the alpha-rENaC open reading frame. Mutational analysis of this upstream start codon indicated that it plays, at most, a minor role in impeding translation both in vitro and in vivo, suggesting that additional mechanisms of translational regulation are operative.

Estrogen and rapamycin effects on cell cycle progression in T47D breast cancer cells

The contribution of estrogen (and progesterone) in driving cell cycle progression of hormone dependent breast cancer cells is well documented, however, the roles of the various relevant signal transduction pathways remain unclear. The immunosuppressant rapamycin is a potent inhibitor of cell cycle progression and has been used to define signal transduction pathways. In this study we have determined rapamycin's effects on cell cycle progression in estrogen dependent breast cancer cells using a novel method of inducing S-phase. In this method estradiol-17-beta alone induced S-phase without mitogen support. In our studies the T47D cells were quite sensitive to estradiol-17-beta, with half-maximal induction in the picomolar range. indicating that the estrogen can induce S-phase in the absence of mitogens such as insulin. The estrogen response does not seem to be particularly specific because estriol estrone and estradiol-17-beta-BSA were about as effective as estradiol-17-beta. R5020, a progestin also induced S-Phase, while rapamycin blocked steroid driven transition of cells from G1 to S-phase.

Immunosuppressant rapamycin inhibits protein kinase C alpha and p38 mitogen-activated protein kinase leading to the inhibition of chondrogenesis

Immunosuppressants are now known to modulate bone metabolism, including bone formation and resorption. Because cartilage, formed by differentiated chondrocytes, serves as a template for endochondral bone formation, we examined the effects of the immunosuppressant rapamycin on the chondrogenesis of mesenchymal cells and on the cell signaling that is required for chondrogenesis, such as protein kinase C, extracellular signal-regulated kinase-1 (ERK-1), and p38 mitogen-activated protein (MAP) kinase pathways. Rapamycin inhibited the expression of type II collagen and the accumulation of sulfate glycosaminoglycan, indicating inhibition of the chondrogenesis of mesenchymal cells. Rapamycin treatment did not affect precartilage condensation, but it prevented cartilage nodule formation. Exposure of chondrifying mesenchymal cells to rapamycin blocked activation of the protein kinase C alpha and p38 MAP kinase, but had no discernible effect on ERK-1 signaling. Selective inhibition of PKCalpha or p38 MAP kinase activity, which is dramatically increased during chondrogenesis, with specific inhibitors in the absence of rapamycin blocked the chondrogenic differentiation of mesenchymal cells. Taken together, our data indicate that the immunosuppressant rapamycin inhibits the chondrogenesis of mesenchymal cells at the post-precartilage condensation stage by modulating signaling pathways including those of PKCalpha and p38 MAP kinase.

Protein synthesis at synaptic sites on dendrites

Studies over the past 20 years have revealed that gene expression in neurons is carried out by a distributed network of translational machinery. One component of this network is localized in dendrites, where polyribosomes and associated membranous elements are positioned beneath synapses and translate a particular population of dendritic mRNAs. The localization of translation machinery and mRNAs at synapses endows individual synapses with the capability to independently control synaptic strength through the local synthesis of proteins. The present review discusses recent studies linking synaptic plasticity to dendritic protein synthesis and mRNA trafficking and considers how these processes are regulated. We summarize recent information about how synaptic signaling is coupled to local translation and to the delivery of newly transcribed mRNAs to activated synaptic sites and how local translation may play a role in activity-dependent synaptic modification.

Cell adhesion regulates gene expression at translational checkpoints in human myeloid leukocytes

Engagement of adhesion molecules on monocytes and other myeloid leukocytes, which are effector cells of the innate immune system, not only tethers the leukocytes in place but also transmits outside-in signals that induce functional changes and alter gene expression. We found that a subset of mRNAs that are induced or amplified by adhesion of human monocytes to P-selectin via its surface ligand, P-selectin glycoprotein 1, have characteristics that suggest specialized translational control. One of these codes for urokinase plasminogen activator receptor (UPAR), a critical surface protease receptor and regulator of cell adhesion and migration. Although UPAR transcripts are induced by adhesion, rapid synthesis of the protein uses constitutive mRNA without a requirement for new transcription and is regulated by mammalian target of rapamycin, demonstrating new biologic roles for the signal-dependent translation pathway controlled by this intracellular kinase. The synthesis of UPAR in monocytic cells is also regulated by eukaryotic translation initiation factor 4E, a second key translational checkpoint, and phosphorylation of eukaryotic translation initiation factor 4E is induced by adhesion of monocytes to P-selectin. Translationally controlled display of UPAR by monocytes confers recognition of the matrix protein, vitronectin. Adhesion-dependent signaling from the plasma membrane to translational checkpoints represents a previously unrecognized mechanism for regulating surface phenotype that may be particularly important for myeloid leukocytes and other cells that are specialized for rapid inflammatory and vascular responses.

Enhanced sensitivity of PTEN-deficient tumors to inhibition of FRAP/mTOR

Recent evidence places the FRAP/mTOR kinase downstream of the phosphatidyl inositol 3-kinase/Akt-signaling pathway, which is up-regulated in multiple cancers because of loss of the PTEN tumor suppressor gene. We performed biological and biochemical studies to determine whether PTEN-deficient cancer cells are sensitive to pharmacologic inhibition of FRAP/mTOR by using the rapamycin derivative CCI-779. In vitro and in vivo studies of isogenic PTEN(+/+) and PTEN(-/-) mouse cells as well as human cancer cells with defined PTEN status showed that the growth of PTEN null cells was blocked preferentially by pharmacologic FRAP/mTOR inhibition. Enhanced tumor growth caused by constitutive activation of Akt in PTEN(+/+) cells also was reversed by CCI-779 treatment, indicating that FRAP/mTOR functions downstream of Akt in tumorigenesis. Loss of PTEN correlated with increased S6 kinase activity and phosphorylation of ribosomal S6 protein, providing evidence for activation of the FRAP/mTOR pathway in these cells. Differential sensitivity to CCI-779 was not explained by differences in biochemical blockade of the FRAP/mTOR pathway, because S6 phosphorylation was inhibited in sensitive and resistant cell lines. These results provide rationale for testing FRAP/mTOR inhibitors in PTEN null human cancers.

A maize insulin-like growth factor signals to a transduction pathway that regulates protein synthesis in maize

Insulin and insulin-like growth factors (IGFs) are well-characterized regulators in higher eukaryotic cells that control biological processes such as cell growth and survival, and selective translation of mRNAs. This research presents the purification of a 20 kDa protein, isolated from maize tissue, with IGF activity. The protein was purified from 48 h-germinated maize embryonic axes by G-50 Sephadex fractionation followed by affinity chromatography through a bovine insulin antibody-Sepharose column. This protein proved to significantly speed up maize germination and seedling growth. At the molecular level, Zea mays IGF (ZmIGF) enhanced phosphorylation of S6 ribosomal protein (rp) on the 40 S ribosomal subunit, in a similar way as observed when bovine insulin is applied to maize axes during germination. Rapamycin, a specific inhibitor of the insulin-stimulated signal transduction pathway, prevented S6 rp phosphorylation in maize axes. Moreover, ZmIGF stimulated [(35)S]methionine incorporation into rps, above the level of overall cytoplasmic proteins. Either incubation with anti-insulin antibody, heat treatment (60 degrees C) or trypsin digestion abolished this ZmIGF effect. It is proposed that ZmIGF is an endogenous maize growth factor that regulates the synthesis of specific proteins through a pathway similar to that of insulin or IGFs in animal tissues.

Expression profiling of glucocorticoid-treated T-ALL cell lines: rapid repression of multiple genes involved in RNA-, protein- and nucleotide synthesis

To arrive at a better understanding of the effects of the glucocorticoid component of chemotherapy protocols on lymphocytic leukemia cells, we analysed early responses of T-lymphocytic leukemia cell lines Jurkat and CEM-C7, both of which undergo apoptosis in response to dexamethasone, via gene chips. Among genes identified as repressed, a notable cluster seemed to be of importance for the processes of transcription, mRNA splicing and protein synthesis. Consequently, we assessed time-resolved uptake of uridine and methionine to monitor RNA and protein synthesis, along with parameters quantifying apoptosis. Repression of uptake to about 65% of that in untreated cells preceded the first sign of apoptosis by several hours in both cell lines. In addition to this general repression of RNA and protein synthesis, several genes were found to be regulated that may contribute to synergistic action of glucocorticoids with other components of frequently used chemotherapy protocols such as antimetabolites, methotrexate and alkylating agents.

Angiotensin II regulates phosphorylation of translation elongation factor-2 in cardiac myocytes

Increased protein synthesis is the cardinal feature of cardiac hypertrophy. We have studied angiotensin II (ANG II)-dependent regulation of eukaryotic elongation factor-2 (eEF-2), an essential component of protein translation required for polypeptide elongation, in rat neonatal cardiac myocytes. eEF2 is fully active in its dephosphorylated state and is inhibited following phosphorylation by eEF2 kinase. ANG II treatment (10(-10) - 10(-7) M) for 30 min produced an AT(1) receptor-specific and concentration- and time-dependent reduction in the phosphorylation of eEF-2. Protein phosphatase 2A (PP2A) inhibitors okadaic acid and fostriecin, but not the PP2B inhibitor FK506, attenuated ANG II-dependent dephosphorylation of eEF-2. ANG II activated mitogen-activated protein kinase, (MAPK) within 10 min of treatment, and blockade of MAPK activation with PD-98059 (1--20 nM) inhibited eEF-2 dephosphorylation. The effect of ANG II on eEF-2 dephosphorylation was also blocked by LY-29004 (1-20 nM), suggesting a role for phosphoinositide 3-kinase, but the mammalian target rapamycin inhibitor rapamycin (10--100 nM) had no effect. Together these results suggest that the ANG II-dependent increase in protein synthesis includes activation of eEF-2 via dephosphorylation by PP2A by a process that involves both PI3K and MAPK.

Cap-independent translation conferred by the 5'-untranslated region of human thymidine kinase mRNA

Translational control is one of the mechanisms that regulate thymidine kinase (TK) expression in the cell cycle. Evidence for the TK mRNA sequence that is involved in its own translation has been lacking. In this report, we show that TK-deficient mouse fibroblasts transfected with pFLAG-TK express a TK mRNA containing the 5'-untranslated region (5'UTR) and produce two polypeptides, FLAG-TK and TK, resulting from an alternative initiation of translation. Most interestingly, the 5'UTR of TK allowed the translation of FLAG-TK mRNA to become cap-independent in an in vitro translation system. Furthermore, this 5'UTR sequence decreased significantly the efficiency of translation from the AUG codon of FLAG when the concentration of FLAG-TK RNA was low. Here, we also show that in normal human IMR-90 fibroblasts the induction of TK polypeptide by serum stimulation is insensitive to rapamycin treatment, which is known to inhibit the translations of transcripts of some growth-controlled genes by affecting the cap-binding efficiency. Taken together, we propose that the 5'UTR in TK mRNA might actually confer a secondary structure to regulate ribosome binding during translation in a cap-independent manner.

Differential activation of protein kinase B and p70(S6)K by glucose and insulin-like growth factor 1 in pancreatic beta-cells (INS-1)

It has been shown that IGF-1-induced pancreatic beta-cell proliferation is glucose-dependent; however, the mechanisms responsible for this glucose dependence are not known. Adenoviral mediated expression of constitutively active phosphatidylinositol 3-kinase (PI3K) in the pancreatic beta-cells, INS-1, suggested that PI3K was not necessary for glucose-induced beta-cell proliferation but was required for IGF-1-induced mitogenesis. Examination of the signaling components downstream of PI3K, 3-phosphoinositide-dependent kinase 1, protein kinase B (PKB), glycogen synthase kinase-3, and p70-kDa-S6-kinase (p70(S6K)), suggested that a major part of glucose-dependent beta-cell proliferation requires activation of mammalian target of rapamycin/p70(S6K), independent of phosphoinositide-dependent kinase 1/PKB activation. Adenoviral expression of the kinase-dead form of PKB in INS-1 cells decreased IGF-1-induced beta-cell proliferation. However, a surprisingly similar decrease was also observed in adenoviral wild type and constitutively active PKB-infected cells. Upon analysis of extracellular signal-regulated protein kinase 1 and 2 (ERK1/ERK2), an increase in ERK1/ERK2 phosphorylation activation by glucose and IGF-1 was observed in kinase-dead PKB-infected cells, but this phosphorylation activation was inhibited in the constitutively active PKB-infected cells. Hence, there is a requirement for the activation of both ERK1/ERK2 and mammalian target of rapamycin/p70(S6K) signal transduction pathways for a full commitment to glucose-induced pancreatic beta-cell mitogenesis. However, for IGF-1-induced activation, these pathways must be carefully balanced, because chronic activation of one (PI3K/PKB) can lead to dampening of the other (ERK1/2), reducing the mitogenic response.

mTOR inhibitors: an overview

Inhibitors of the mammalian target of rapamycin are a new class of immunosuppressants. In contrast to other macrolides, such as tacrolimus and cyclosporine A, they do not inhibit calcineurin and thus signal I of T-cell activation. By inhibiting signal III, the mechanism of action and side effects of sirolimus (rapamycin) and its derivative RAD are distinct from other immunosuppressants. Reports of synergism with cyclosporine A and tacrolimus in preclinical and clinical studies, avoidance of nephrotoxicity, and possible treatment or prevention of chronic allograft rejection are leading to high expectations for this new class of immunosuppressants. Furthermore, studies evaluating tolerance induction are being conducted. This review summarizes preclinical and clinical results published to date and exploits the future value of sirolimus and RAD for clinical transplantation.

Insulin-like growth factor-I augments prolactin and inhibits growth hormone release through distinct as well as overlapping cellular signaling pathways

We recently discovered a new role for insulin-like growth factor-I (IGF-I) as a specific and direct stimulator of prolactin (PRL) release in addition to its recognized function as an inhibitor of growth hormone (GH) release and synthesis. Little is known of the mechanisms that transduce the actions of IGF-I on PRL and GH release in vertebrates. The present study was undertaken to determine the cellular pathways that mediate the disparate actions of IGF-I on PRL and GH release in hybrid striped bass (Morone saxatilis X M. chrysops). When regulating cellular function, IGF-I may activate two primary pathways, phosphatidylinositol 3-kinase (PI 3-K) and mitogen-activated protein kinase (MAPK). The specific MAPK inhibitor, PD98059, blocked IGF-I-evoked PRL release as well as GH release inhibition over an 18-20-h incubation. LY294002, a specific PI 3-K inhibitor, overcame IGF-I's inhibition of GH release but was ineffective in blocking PRL release stimulated by IGF-I. These studies suggest IGF-I disparately alters PRL and GH by activating distinct as well as overlapping signaling pathways central for mediating actions of growth factors on secretory activity as well as cell proliferation. These results further support a role for IGF-I as a physiological regulator of PRL and GH.

Signaling pathways leading to transcription and translation cooperatively regulate the transient increase in expression of c-Fos protein

The mechanisms by which growth factors trigger signal transduction pathways leading to the regulation of c-Fos expression are of great interest. In this study we investigated the effect of hepatocyte growth factor (HGF/SF) and epidermal growth factor (EGF) on the expression of c-fos and its product, c-Fos, in human epithelial cell line MKN74. The expression level of c-Fos protein in HGF/SF-stimulated cells was 5--10-fold higher than that in EGF-stimulated cells, whereas the level of c-fos mRNA induced by HGF/SF was similar to that by EGF. The hyperphosphorylation of eukaryotic initiation factor 4E-binding protein 1 (4E-BP1), indicative of an increased number of translation initiation complexes, was detected only in HGF/SF-induced MKN74 cells. Activation of phosphatidylinositol-3'-OH kinase and FKBP12-rapamycin associated mammalian target of rapamycin (FRAP/mTOR) was observed after the treatment with HGF/SF. Pretreatment with an inhibitor of either one, i.e. LY294002 for phosphatidylinositol-3'-OH kinase or rapamycin for FRAP/mTOR, completely inhibited 4E-BP1 phosphorylation and decreased the c-Fos synthesis induced by HGF/SF down to the level found in EGF-induced cells. These results suggest that the phosphorylation of 4E-BP1 is stimulated by HGF/SF in a manner requiring both phosphatidy-linositol-3'-OH kinase-dependent and FRAP/mTOR-dependent pathways, thereby stimulating c-fos mRNA translation. Regulation of the translation process of c-fos mRNA in addition to the immediate activation of c-fos transcription is necessary for the transient increase in the level of c-Fos protein to stimulate cell proliferation.

An integrated approach in the discovery and characterization of a novel nuclear protein over-expressed in liver and pancreatic tumors

An integrated approach in protein discovery through the use of multidisciplinary tools was reported. A novel protein, Hcc-1, was identified by analysis of the hepatocellular carcinoma (HCC)-M cell proteome. The assembled EST sequence of the 210 amino acid novel protein was subsequently confirmed by rapid amplification of cDNA ends (RACE). A total of 687 bp at the 5' untranslated region of Hcc-1 was identified. Promoter activity and several upstream open reading frames (uORFs) were demonstrated at this region. Bioinformatics prediction showed that the first 42 amino acids of the protein is a SAP domain with sequence matches to hnRNP from various vertebrate species. The Hcc-1 protein was localized to the cell nucleus while the gene was localized to chromosome 7q22.1. Hcc-1 cDNA level was increased in pancreatic adenocarcinoma. The level was also increased in well-differentiated hepatocellular carcinoma but decreases as the carcinoma progressed to a poorly differentiated stage.

Flow-induced DNA synthesis requires signaling to a translational control pathway

BACKGROUND

The mTOR translational control pathway that signals to the P70/P85 S6 kinase (pp70(S6k)) is essential for mitogenesis. We have previously shown that pp70(S6k) is activated by fluid flow. We hypothesized that oscillatory fluid flow in the absence of exogenous mitogens would induce endothelial cells to synthesize DNA via activation of the mTOR pathway. For comparison, we also studied the ERK1/2 transcriptional signaling pathway.

METHODS

Confluent human umbilical vein endothelial cells (HUVECs) were exposed to oscillatory flow (12 dyn/cm(2) peak shear stress; 3.3 Hz) or kept static in serum-deprived culture medium. Rapamycin or PD98059 was used to inhibit pp70(S6k) or ERK1/2 activation, respectively.

RESULTS

Oscillatory flow activated both the pp70(S6k) and ERK1/2 signaling pathways. Rapamycin blocked activation of pp70(S6k) but not ERK1/2, while PD98059 blocked ERK1/2 but not pp70(S6k). DNA synthesis, as measured by [3H]thymidine uptake, increased by approximately twofold (P < 0.01) in HUVEC cultures exposed to oscillatory flow compared with those kept static. Rapamycin completely abolished the flow-induced increase in DNA synthesis while PD98059 did not. Oscillatory flow upregulated expression of cyclin-dependent kinases 1 and 4 mRNA in a temporal pattern consistent with cell cycle entry; rapamycin also inhibited these changes.

CONCLUSIONS

Oscillatory flow activates both the ERK 1/2 and pp70(S6k) signaling pathways in HUVECs and induces DNA synthesis in the absence of other exogenous mitogens. Complete blockade of [3H]thymidine uptake by the mTOR pathway inhibitor rapamycin indicates that separate and distinct signaling to a translational control pathway is necessary to mediate flow-induced DNA synthesis by endothelial cells. Oscillatory flow-induced endothelial proliferation may contribute to atherogenesis.

Dynamic visualization of local protein synthesis in hippocampal neurons

Using pharmacological approaches, several recent studies suggest that local protein synthesis is required for synaptic plasticity. Convincing demonstrations of bona fide dendritic protein synthesis in mammalian neurons are rare, however. We developed a protein synthesis reporter in which the coding sequence of green fluorescent protein is flanked by the 5' and 3' untranslated regions from CAMKII-alpha, conferring both dendritic mRNA localization and translational regulation. In cultured hippocampal neurons, we show that BDNF, a growth factor involved in synaptic plasticity, stimulates protein synthesis of the reporter in intact, mechanically, or "optically" isolated dendrites. The stimulation of protein synthesis is blocked by anisomycin and not observed in untreated neurons. In addition, dendrites appear to possess translational hot spots, regions near synapses where protein synthesis consistently occurs over time.

Serotonin activates S6 kinase in a rapamycin-sensitive manner in Aplysia synaptosomes

The identification of tags that can specifically mark activated synapses is important for understanding how long-term synaptic changes can be restricted to specific synapses. The maintenance of synapse-specific facilitation in Aplysia sensory to motor neuron cultures can be blocked by inhibitors of translation and by the drug rapamycin, which specifically blocks a signaling pathway that regulates phosphorylation of translational regulators. One important target of rapamycin is the phosphorylation and subsequent activation of S6 kinase. To test whether S6 kinase is the target for the ability of rapamycin to block synapse-specific facilitation in Aplysia, we cloned Aplysia S6 kinase, its substrate S6, and the S6 kinase kinase phosphoinositide-dependent kinase 1 (PDK-1). Serotonin, which induces synapse-specific facilitation, increased phosphorylation of Aplysia S6 kinase at threonine 399 in a rapamycin-sensitive manner in Aplysia synaptosomes. The phosphorylation of threonine 399 by 5-HT was independent of phosphoinositide-3 kinase, dependent on PKA and PKC, and occluded by the phosphatase inhibitor calyculin-A. 5-HT also increased S6 kinase activity and led to increased phosphorylation of S6 in synaptosomes. 5-HT increased levels of S6 in synaptosomes because of a rapamycin-sensitive increase in translation-stabilization of S6. Aplysia PDK-1 bound to and phosphorylated Aplysia S6 kinase but only modulated phosphorylation of threonine 399 indirectly. These results suggest a mechanism by which the levels of translation factors can be increased specifically at activated synapses generating a long-lasting synaptic tag.