Improving the mechanical and biological functions of cell sheet constructs: The interplay of human-derived periodontal ligament stem cells, endothelial cells and plasma rich in growth factors

OBJECTIVE

The aim of this study was to produce and characterize triple-layered cell sheet constructs with varying cell compositions combined or not with the fibrin membrane scaffold obtained by the technology of Plasma Rich in Growth Factors (mPRGF).

MATERIALS AND METHODS

Human primary cultures of periodontal ligament stem cells (hPDLSCs) were isolated, and their stemness nature was evaluated. Three types of triple-layered composite constructs were generated, composed solely of hPDLSCs or combined with human umbilical vein endothelial cells (HUVECs), either as a sandwiched endothelial layer or as coculture sheets of both cell phenotypes. These three triple-layered constructs were also manufactured using mPRGF as cell sheets' support. Necrosis, glucose consumption, secretion of extracellular matrix proteins and synthesis of proangiogenic factors were determined. Histological evaluations and proteomic analyses were also performed.

RESULTS

The inclusion of HUVECs did not clearly improve the properties of the multilayered constructs and yet hindered their optimal conformation. The presence of mPRGF prevented the shrinkage of cell sheets, stimulated the metabolic activity and increased the matrix synthesis. At the proteome level, mPRGF conferred a dramatic advantage to the hPDLSC constructs in their ability to provide a suitable environment for tissue regeneration by inducing the expression of proteins necessary for bone morphogenesis and cellular proliferation.

CONCLUSIONS

hPDLSCs' triple-layer construct onto mPRGF emerges as the optimal structure for its use in regenerative therapeutics.

CLINICAL RELEVANCE

These results suggest the suitability of mPRGF as a promising tool to support cell sheet formation by improving their handling and biological functions.

Foreword Special Issue Genomic Instability in Tumor Evolution and Therapy Response

From an evolutionary perspective, mutations in the DNA molecule act as a source of genetic variation and thus, are beneficial to the adaptation and survival of the species [...].

Targeting E2F Sensitizes Prostate Cancer Cells to Drug-Induced Replication Stress by Promoting Unscheduled CDK1 Activity

Simple Summary

E2F1 and E2F2 are highly expressed in many cancer types, but their contribution to malignancy is not well understood. Here we aimed to define the impact of E2F1/E2F2 deregulation in prostate cancer. We show that inhibition of E2F sensitizes prostate cancer cells to drug-induced replication stress and cell death. We found that E2F target genes involved in nucleotide biosynthesis contribute to maintaining genome stability in prostate cancer cells, but their enzymatic activity is insufficient to prevent replication stress after E2F1/E2F2 depletion. Instead, E2F1/E2F2 hinder premature CDK1 activation during S phase, which is key to ensure genome stability and viability of prostate cancer cells. From a therapeutic perspective, inhibiting E2F activity provokes catastrophic levels of replication stress and blunts xenograft growth in combination with drugs targeting nucleotide biosynthesis or DNA repair. Our results highlight the suitability of targeting E2F for the treatment of prostate cancer.

Abstract

E2F1/E2F2 expression correlates with malignancy in prostate cancer (PCa), but its functional significance remains unresolved. To define the mechanisms governed by E2F in PCa, we analyzed the contribution of E2F target genes to the control of genome integrity, and the impact of modulating E2F activity on PCa progression. We show that silencing or inhibiting E2F1/E2F2 induces DNA damage during S phase and potentiates 5-FU-induced replication stress and cellular toxicity. Inhibition of E2F downregulates the expression of E2F targets involved in nucleotide biosynthesis (TK1, DCK, TYMS), whose expression is upregulated by 5-FU. However, their enzymatic products failed to rescue DNA damage of E2F1/E2F2 knockdown cells, suggesting additional mechanisms for E2F function. Interestingly, targeting E2F1/E2F2 in PCa cells reduced WEE1 expression and resulted in premature CDK1 activation during S phase. Inhibition of CDK1/CDK2 prevented DNA damage induced by E2F loss, suggesting that E2F1/E2F2 safeguard genome integrity by restraining CDK1/CDK2 activity. Importantly, combined inhibition of E2F and ATR boosted replication stress and dramatically reduced tumorigenic capacity of PCa cells in xenografts. Collectively, inhibition of E2F in combination with drugs targeting nucleotide biosynthesis or DNA repair is a promising strategy to provoke catastrophic levels of replication stress that could be applied to PCa treatment.

E2f2 Attenuates Apoptosis of Activated T Lymphocytes and Protects from Immune-Mediated Injury through Repression of Fas and FasL

Targeted disruption of E2f2 in mice causes T-cell hyperactivation and a disproportionate cell cycle entry upon stimulation. However, E2f2^−/− mice do not develop a lymphoproliferative condition. We report that E2f2 plays a Fas-dependent anti-apoptotic function in vitro and in vivo. TCR-stimulated murine E2f2^−/− T cells overexpress the proapoptotic genes Fas and FasL and exhibit enhanced apoptosis, which is prevented by treatment with neutralizing anti-FasL antibodies. p53 pathway is activated in TCR-stimulated E2f2^−/− lymphocytes, but targeted disruption of p53 in E2f2^−/− mice does not abrogate Fas/FasL expression or apoptosis, implying a p53-independent apoptotic mechanism. We show that E2f2 is recruited to Fas and FasL gene promoters to repress their expression. in vivo, E2f2^−/− mice are prone to develop immune-mediated liver injury owing to an aberrant lymphoid Fas/FasL activation. Taken together, our results suggest that E2f2-dependent inhibition of Fas/FasL pathway may play a direct role in limiting the development of immune-mediated pathologies.

E2F1 and E2F2-Mediated Repression of CPT2 Establishes a Lipid-Rich Tumor-Promoting Environment

Lipid metabolism rearrangements in nonalcoholic fatty liver disease (NAFLD) contribute to disease progression. NAFLD has emerged as a major risk for hepatocellular carcinoma (HCC), where metabolic reprogramming is a hallmark. Identification of metabolic drivers might reveal therapeutic targets to improve HCC treatment. Here, we investigated the contribution of transcription factors E2F1 and E2F2 to NAFLD-related HCC and their involvement in metabolic rewiring during disease progression. In mice receiving a high-fat diet (HFD) and diethylnitrosamine (DEN) administration, E2f1 and E2f2 expressions were increased in NAFLD-related HCC. In human NAFLD, E2F1 and E2F2 levels were also increased and positively correlated. E2f1 ^-/- and E2f2 ^-/- mice were resistant to DEN-HFD-induced hepatocarcinogenesis and associated lipid accumulation. Administration of DEN-HFD in E2f1 ^-/- and E2f2 ^-/- mice enhanced fatty acid oxidation (FAO) and increased expression of Cpt2, an enzyme essential for FAO, whose downregulation is linked to NAFLD-related hepatocarcinogenesis. These results were recapitulated following E2f2 knockdown in liver, and overexpression of E2f2 elicited opposing effects. E2F2 binding to the Cpt2 promoter was enhanced in DEN-HFD-administered mouse livers compared with controls, implying a direct role for E2F2 in transcriptional repression. In human HCC, E2F1 and E2F2 expressions inversely correlated with CPT2 expression. Collectively, these results indicate that activation of the E2F1-E2F2-CPT2 axis provides a lipid-rich environment required for hepatocarcinogenesis. SIGNIFICANCE: These findings identify E2F1 and E2F2 transcription factors as metabolic drivers of hepatocellular carcinoma, where deletion of just one is sufficient to prevent disease. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/81/11/2874/F1.large.jpg.

Sustained CHK2 activity, but not ATM activity, is critical to maintain a G1 arrest after DNA damage in untransformed cells

BACKGROUND

The G1 checkpoint is a critical regulator of genomic stability in untransformed cells, preventing cell cycle progression after DNA damage. DNA double-strand breaks (DSBs) recruit and activate ATM, a kinase which in turn activates the CHK2 kinase to establish G1 arrest. While the onset of G1 arrest is well understood, the specific role that ATM and CHK2 play in regulating G1 checkpoint maintenance remains poorly characterized.

RESULTS

Here we examine the impact of ATM and CHK2 activities on G1 checkpoint maintenance in untransformed cells after DNA damage caused by DSBs. We show that ATM becomes dispensable for G1 checkpoint maintenance as early as 1 h after DSB induction. In contrast, CHK2 kinase activity is necessary to maintain the G1 arrest, independently of ATM, ATR, and DNA-PKcs, implying that the G1 arrest is maintained in a lesion-independent manner. Sustained CHK2 activity is achieved through auto-activation and its acute inhibition enables cells to abrogate the G1-checkpoint and enter into S-phase. Accordingly, we show that CHK2 activity is lost in cells that recover from the G1 arrest, pointing to the involvement of a phosphatase with fast turnover.

CONCLUSION

Our data indicate that G1 checkpoint maintenance relies on CHK2 and that its negative regulation is crucial for G1 checkpoint recovery after DSB induction.

An E2F7-dependent transcriptional program modulates DNA damage repair and genomic stability

The cellular response to DNA damage is essential for maintaining the integrity of the genome. Recent evidence has identified E2F7 as a key player in DNA damage-dependent transcriptional regulation of cell-cycle genes. However, the contribution of E2F7 to cellular responses upon genotoxic damage is still poorly defined. Here we show that E2F7 represses the expression of genes involved in the maintenance of genomic stability, both throughout the cell cycle and upon induction of DNA lesions that interfere with replication fork progression. Knockdown of E2F7 leads to a reduction in 53BP1 and FANCD2 foci and to fewer chromosomal aberrations following treatment with agents that cause interstrand crosslink (ICL) lesions but not upon ionizing radiation. Accordingly, E2F7-depleted cells exhibit enhanced cell-cycle re-entry and clonogenic survival after exposure to ICL-inducing agents. We further report that expression and functional activity of E2F7 are p53-independent in this context. Using a cell-based assay, we show that E2F7 restricts homologous recombination through the transcriptional repression of RAD51. Finally, we present evidence that downregulation of E2F7 confers an increased resistance to chemotherapy in recombination-deficient cells. Taken together, our results reveal an E2F7-dependent transcriptional program that contributes to the regulation of DNA repair and genomic integrity.

Golgi Oncoprotein GOLPH3 Gene Expression Is Regulated by Functional E2F and CREB/ATF Promoter Elements

The Golgi organelle duplicates its protein and lipid content to segregate evenly between two daughter cells after mitosis. However, how Golgi biogenesis is regulated during interphase remains largely unknown. Here we show that messenger RNA (mRNA) expression of GOLPH3 and GOLGA2, two genes encoding Golgi proteins, is induced specifically in G1 phase, suggesting a link between cell cycle regulation and Golgi growth. We have examined the role of E2F transcription factors, critical regulators of G1 to S progression of the cell cycle, in the expression of Golgi proteins during interphase. We show that promoter activity for GOLPH3, a Golgi protein that is also oncogenic, is induced by E2F1-3 and repressed by E2F7. Mutation of the E2F motifs present in the GOLPH3 promoter region abrogates E2F1-mediated induction of a GOLPH3 luciferase reporter construct. Furthermore, we identify a critical CREB/ATF element in the GOLPH3 promoter that is required for its steady state and ATF2-induced expression. Interestingly, depletion of GOLPH3 with small interfering RNA (siRNA) delays the G1 to S transition in synchronized U2OS cells. Taken together, our results reveal a link between cell cycle regulation and Golgi function, and suggest that E2F-mediated regulation of Golgi genes is required for the timely progression of the cell cycle.

Detection of E2F-Induced Transcriptional Activity Using a Dual Luciferase Reporter Assay

The E2F transcription factors are key targets for the retinoblastoma (RB) tumor suppressor function. The active or inactive status of RB determines the degree by which E2F-dependent gene expression will occur in a given condition. Changes in transcriptional activity in response to extracellular or intracellular stimuli are frequently measured using genetic reporter assays. In particular, dual luciferase reporter assays are most recommended for this purpose because of their improved experimental accuracy. Here we illustrate the usefulness of the dual luciferase reporter assay to detect E2F-mediated transcriptional activity upon overexpression of E2F1 in cultured cells as readout for RB status and function.

Nuclear Phosphoproteomic Screen Uncovers ACLY as Mediator of IL-2-induced Proliferation of CD4+ T lymphocytes

Anti-cancer immunotherapies commonly rely on the use of interleukin-2 (IL-2) to promote the expansion of T lymphocytes. IL-2- dependent proliferation is the culmination of a complex network of phosphorylation-driven signaling events that impact on gene transcription through mechanisms that are not clearly understood. To study the role of IL-2 in the regulation of nuclear protein function we have performed an unbiased mass spectrometry-based study of the nuclear phosphoproteome of resting and IL-2-treated CD4(+) T lymphocytes. We detected 8521distinct phosphosites including many that are not yet reported in curated phosphorylation databases. Although most phosphorylation sites remained unaffected upon IL-2 treatment, 391 sites corresponding to 288 gene products showed robust IL-2-dependent regulation. Importantly, we show that ATP-citrate lyase (ACLY) is a key phosphoprotein effector of IL-2-mediated T-cell responses. ACLY becomes phosphorylated on serine 455 in T lymphocytes upon IL-2-driven activation of AKT, and depletion or inactivation of ACLY compromises IL-2-promoted T-cell growth. Mechanistically, we demonstrate that ACLY is required for enhancing histone acetylation levels and inducing the expression of cell cycle regulating genes in response to IL-2. Thus, the metabolic enzyme ACLY emerges as a bridge between cytokine signaling and proliferation of T lymphocytes, and may be an attractive candidate target for the development of more efficient anti-cancer immunotherapies.

E2F7 regulates transcription and maturation of multiple microRNAs to restrain cell proliferation

E2F transcription factors (E2F1-8) are known to coordinately regulate the expression of a plethora of target genes, including those coding for microRNAs (miRNAs), to control cell cycle progression. Recent work has described the atypical E2F factor E2F7 as a transcriptional repressor of cell cycle-related protein-coding genes. However, the contribution of E2F7 to miRNA gene expression during the cell cycle has not been defined. We have performed a genome-wide RNA sequencing analysis to identify E2F7-regulated miRNAs and show that E2F7 plays as a major role in the negative regulation of a set of miRNAs that promote cellular proliferation. We provide mechanistic evidence for an interplay between E2F7 and the canonical E2F factors E2F1-3 in the regulation of multiple miRNAs. We show that miR-25, -26a, -27b, -92a and -7 expression is controlled at the transcriptional level by the antagonistic activity of E2F7 and E2F1-3. By contrast, let-7 miRNA expression is controlled indirectly through a novel E2F/c-MYC/LIN28B axis, whereby E2F7 and E2F1-3 modulate c-MYC and LIN28B levels to impact let-7 miRNA processing and maturation. Taken together, our data uncover a new regulatory network involving transcriptional and post-transcriptional mechanisms controlled by E2F7 to restrain cell cycle progression through repression of proliferation-promoting miRNAs.

LDLR and PCSK9 Are Associated with the Presence of Antiphospholipid Antibodies and the Development of Thrombosis in aPLA Carriers

Introduction

The identification of the genetic risk factors that could discriminate non- thrombotic from thrombotic antiphospholipid antibodies (aPLA) carriers will improve prognosis of these patients. Several human studies have shown the presence of aPLAs associated with atherosclerotic plaque, which is a known risk factor for thrombosis. Hence, in order to determine the implication of atherosclerosis in the risk of developing thrombosis in aPLA positive patients, we performed a genetic association study with 3 candidate genes, APOH, LDLR and PCSK9.

Material & Methods

For genetic association study we analyzed 190 aPLA carriers -100 with non-thrombotic events and 90 with thrombotic events- and 557 healthy controls. Analyses were performed by χ2 test and were corrected by false discovery rate. To evaluate the functional implication of the newly established susceptibility loci, we performed expression analyses in 86 aPLA carrier individuals (43 with thrombotic manifestations and 43 without it) and in 45 healthy controls.

Results

Our results revealed significant associations after correction in SNPs located in LDLR gene with aPLA carriers and thrombotic aPLA carriers, when compared with healthy controls. The most significant association in LDLR gene was found between SNP rs129083082 and aPLA carriers in recessive model (adjusted P-value = 2.55 x 10⁻³; OR = 2.18; 95%CI = 1.49–3.21). Furthermore, our work detected significant allelic association after correction between thrombotic aPLA carriers and healthy controls in SNP rs562556 located in PCSK9 gene (adjusted P-value = 1.03 x 10⁻²; OR = 1.60; 95%CI = 1.24–2.06). Expression level study showed significantly decreased expression level of LDLR gene in aPLA carriers (P-value <0.0001; 95%CI 0.16–2.10; SE 0.38–1.27) in comparison to the control group.

Discussion

Our work has identified LDLR gene as a new susceptibility gene associated with the development of thrombosis in aPLA carriers, describing for the first time the deregulation of LDLR expression in individuals with aPLAs. Besides, thrombotic aPLA carriers also showed significant association with PCSK9 gene, a regulator of LDLR plasma levels. These results highlight the importance of atherosclerotic processes in the development of thrombosis in patients with aPLA.

The stress of coping with E2F loss

E2F transcription factors are key regulators of cellular proliferation, and altered E2F activity is a common feature of tumor cells. Thus, E2F targeting is being explored as a therapeutic strategy in cancer. Importantly, recent mouse knockout studies show that concomitant loss of E2f1/E2f2 activity is associated with increased genomic instability and oncogenic potential in normal differentiating cells, a finding that might have implications for cancer therapy.

E2F1 and E2F2 prevent replicative stress and subsequent p53-dependent organ involution

Tissue homeostasis requires tight regulation of cellular proliferation, differentiation and apoptosis. E2F1 and E2F2 transcription factors share a critical role in tissue homeostasis, since their combined inactivation results in overall organ involution, specially affecting the pancreatic gland, which subsequently triggers diabetes. We have examined the mechanism by which these E2Fs regulate tissue homeostasis. We show that pancreas atrophy in E2F1/E2F2 double-knockout (DKO) mice is associated with mitochondrial apoptosis and activation of the p53 pathway in young animals, before the development of diabetes. A deregulated expression of E2F target genes was detected in pancreatic cells of young DKO animals, along with unscheduled DNA replication and activation of a DNA damage response. Importantly, suppression of DNA replication in vivo with aphidicolin led to a significant inhibition of the p53 pathway in DKO pancreas, implying a causal link between DNA replication stress and p53 activation in this model. We further show that activation of the p53 pathway has a key role in the aberrant phenotype of DKO mice, since targeted inactivation of p53 gene abrogated cellular apoptosis and prevented organ involution and insulin-dependent diabetes in mice lacking E2F1/E2F2. Unexpectedly, p53 inactivation unmasked oncogenic features of E2F1/E2F2-depleted cells, as evidenced by an accelerated tumor development in triple-knockout mice compared with p53(-/-) mice. Collectively, our data reveal a role for E2F1 and E2F2 as suppressors of replicative stress in differentiating cells, and uncover the existence of a robust E2F-p53 regulatory axis to enable tissue homeostasis and prevent tumorigenesis. These findings have implications in the design of approaches targeting E2F for cancer therapy.

E2F2 and CREB cooperatively regulate transcriptional activity of cell cycle genes

E2F2 is essential for the maintenance of T lymphocyte quiescence. To identify the full set of E2F2 target genes, and to gain further understanding of the role of E2F2 in transcriptional regulation, we have performed ChIP-chip analyses across the genome of lymph node–derived T lymphocytes. Here we show that during quiescence, E2F2 binds the promoters of a large number of genes involved in DNA metabolism and cell cycle regulation, concomitant with their transcriptional silencing. A comparison of ChIP-chip data with expression profiling data on resting E2f2^−/− T lymphocytes identified a subset of 51 E2F2-specific target genes, most of which are upregulated on E2F2 loss. Luciferase reporter assays showed a retinoblastoma-independent role for E2F2 in the negative regulation of these target genes. Importantly, we show that the DNA binding activity of the transcription factor CREB contributes to E2F2-mediated repression of Mcm5 and Chk1 promoters. siRNA-mediated CREB knockdown, expression of a dominant negative KCREB mutant or disruption of CREB binding by mutating a CRE motif on Mcm5 promoter, relieved E2F2-mediated transcriptional repression. Taken together, our data uncover a new regulatory mechanism for E2F-mediated transcriptional control, whereby E2F2 and CREB cooperate in the transcriptional repression of a subset of E2F2 target genes.

Thrombotic antiphospholipid syndrome shows strong haplotypic association with SH2B3-ATXN2 locus

Background

Thrombotic antiphospholipid syndrome is defined as a complex form of thrombophilia that is developed by a fraction of antiphospholipid antibody (aPLA) carriers. Little is known about the genetic risk factors involved in thrombosis development among aPLA carriers.

Methods

To identify new loci conferring susceptibility to thrombotic antiphospholipid syndrome, a two-stage genotyping strategy was performed. In stage one, 19,000 CNV loci were genotyped in 14 thrombotic aPLA+ patients and 14 healthy controls by array-CGH. In stage two, significant CNV loci were fine-mapped in a larger cohort (85 thrombotic aPLA+, 100 non-thrombotic aPLA+ and 569 healthy controls).

Results

Array-CGH and fine-mapping analysis led to the identification of 12q24.12 locus as a new susceptibility locus for thrombotic APS. Within this region, a TAC risk haplotype comprising one SNP in SH2B3 gene (rs3184504) and two SNPs in ATXN2 gene (rs10774625 and rs653178) exhibited the strongest association with thrombotic antiphospholipid syndrome (p-value = 5,9 × 10⁻⁴ OR 95% CI 1.84 (1.32–2.55)).

Conclusion

The presence of a TAC risk haplotype in ATXN2-SH2B3 locus may contribute to increased thrombotic risk in aPLA carriers.

The nuclear protein ALY binds to and modulates the activity of transcription factor E2F2

E2F transcription factors control the expression of genes involved in a variety of essential cellular processes and consequently their activity needs to be tightly regulated. Protein-protein interactions are thought to be key modulators of E2F activity. To gain insight into the mechanisms that regulate the activity of E2F2, we searched for novel proteins that associate with this transcription factor. We show that the nuclear protein ALY (THO complex 4), originally described as a transcriptional co-activator, associates with DNA-bound E2F2 and represses its transcriptional activity. The capacity of ALY to modulate gene expression was analyzed with expression microarrays by characterizing the transcriptome of E2F2 expressing HEK293T cells in which ALY was either overexpressed or silenced. We show that ALY influences the expression of more than 400 genes, including 98 genes bearing consensus E2F motifs. Thus, ALY emerges as a novel E2F2-interacting protein and a relevant modulator of E2F-responsive gene expression.

Rac1 protein regulates glycogen phosphorylase activation and controls interleukin (IL)-2-dependent T cell proliferation

Small GTPases of the Rho family have been implicated in important cellular processes such as cell migration and adhesion, protein secretion, and/or gene transcription. In the lymphoid system, these GTPases participate in the signaling cascades that are activated after engagement of antigen receptors. However, little is known about the role that Rho GTPases play in IL-2-mediated responses. Here, we show that IL-2 induces Rac1 activation in Kit 225 T cells. We identified by mass spectrometry the muscle isoform of glycogen phosphorylase (PYGM) as a novel Rac1 effector molecule in IL-2-stimulated cells. The interaction between the active form of Rac1 (Rac1-GTP) and PYGM was established directly through a domain comprising amino acids 191-270 of PYGM that exhibits significant homology with the Rac binding domain of PAK1. The integrity of this region was crucial for PYGM activation. Importantly, IL-2-dependent cellular proliferation was inhibited upon blocking both the activation of Rac1 and the activity of PYGM. These results reveal a new role for Rac1 in cell signaling, showing that this GTPase triggers T cell proliferation upon IL-2 stimulation by associating with PYGM and modulating its enzymatic activity.

A role for transcription factor E2F2 in hepatocyte proliferation and timely liver regeneration

E2F transcription factors are key regulators of the cell cycle although the relative contribution of each E2F member in regulating cellular proliferation is still poorly defined. Present evidence suggests that E2F2 may act both as a suppressor and promoter of proliferation, depending on the cellular context. We used a loss-of-function mutant mouse model to investigate the function of E2F2 in liver regeneration after partial hepatectomy, a paradigm of cell-cycle progression. Liver mass recovery and histology were examined over 9 days in 70% hepatectomized E2F2(-/-) and wild-type animals. Transcriptome analysis was performed in quiescent and 48-h regenerating liver samples. TIGR MultiExperiment Viewer was used for the statistical analysis of microarray data, significance was determined by Fischer, and P values were adjusted applying Benjamini-Hochberg multiple-testing correction. We show that E2F2 is required for adult hepatocyte proliferation and for timely liver regeneration, as disruption of the E2F2 gene in hepatocytes leads to a reduced rate of S-phase entry and to delayed liver regeneration. Transcriptome analysis followed by ontological classification of differentially expressed genes and gene-interaction network analysis indicated that the majority of genes involved in normal liver regeneration were related to biosynthetic and catabolic processes of all major biomolecules as well as cellular location and intracellular transport, confirming the complex nature of the regeneration process. Remarkably, transcripts of genes included in functional categories that are crucial for cell cycle, apoptosis and wound-healing response, and fibrosis were absent in the transcriptome of posthepatectomized E2F2(-/-) mice. Our results indicate that the transcriptional activity of E2F2 contributes to promote adult hepatocyte proliferation and liver regeneration.

Interleukin-2 signaling pathway analysis by quantitative phosphoproteomics

Interleukin-2 (IL-2) is major cytokine involved in T cell proliferation, differentiation and apoptosis. Association between IL-2 and its receptor (IL-2R), triggers activation of complex signaling cascade governed by tyrosine phosphorylation that culminates in transcription of genes involved in modulation of the immune response. The complete characterization of the IL-2 pathway is essential to understand how aberrant IL-2 signaling results in several diseases such as cancer or autoimmunity and also how IL-2 treatments affect cancer patients. To gain insights into the downstream machinery activated by IL-2, we aimed to define the global tyrosine-phosphoproteome of IL-2 pathway in human T cell line Kit225 using high resolution mass spectrometry combined with phosphotyrosine immunoprecipitation and SILAC. The molecular snapshot at 5min of IL-2 stimulation resulted in identification of 172 proteins among which 79 were found with increased abundance in the tyrosine-phosphorylated complexes, including several previously not reported IL-2 downstream effectors. Combinatorial site-specific phosphoproteomic analysis resulted in identification of 99 phosphorylated sites mapping to the identified proteins with increased abundance in the tyrosine-phosphorylated complexes, of which 34 were not previously described. In addition, chemical inhibition of the identified IL-2-mediated JAK, PI3K and MAPK signaling pathways, resulted in distinct alteration on the IL-2 dependent proliferation.

Differential proteomics analysis reveals a role for E2F2 in the regulation of the Ahr pathway in T lymphocytes

E2F transcription factors (E2F1-8) are best known for their role in cell proliferation, although it is clear that they regulate many other biological processes through the transcriptional modulation of distinct target genes. However, the specific set of genes regulated by each E2F remains to be characterized. To gain insight into the molecular pathways regulated by E2F2, we have analyzed the proteome of antigen receptor-activated T cells lacking E2F2. We report that loss of E2F2 results in a deregulated Aryl-hydrocarbon-receptor pathway. Proliferating E2F2(-/-) T lymphocytes expressed significantly higher levels of Aip, Ahr, and Arnt relative to wild-type (WT)(1) controls. The mechanism for increased levels of Aip appears straightforward, involving direct regulation of the Aip gene promoter by E2F2. Although the Ahr and Arnt promoters also bind E2F2, their regulation appears to be more complex. Nevertheless, exposure to the environmental xenobiotic 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a well-known exogenous ligand of the Ahr pathway, led to overexpression of the Ahr target gene Cyp1a1, and to increased sensitivity to TCDD-triggered apoptosis in E2F2(-/-) T cells compared with WT controls. These results suggest that E2F2 modulates cellular sensitivity to xenobiotic signals through the negative regulation of the Ahr pathway.

Hedgehog signaling is critical for normal liver regeneration after partial hepatectomy in mice

Distinct mechanisms are believed to regulate growth of the liver during fetal development and after injury in adults, because the former relies on progenitors and the latter generally involves replication of mature hepatocytes. However, chronic liver injury in adults increases production of Hedgehog (Hh) ligands, developmental morphogens that control progenitor cell fate and orchestrate various aspects of tissue construction during embryogenesis. This raises the possibility that similar Hh-dependent mechanisms also might regulate adult liver regeneration. The current analysis of murine liver regeneration after 70% partial hepatectomy (PH), an established model of adult liver regeneration, demonstrated that PH induced production of Hh ligands and activated Hh signaling in liver cells. Treatment with a specific Hh signaling inhibitor interfered with several key components of normal liver regeneration, significantly inhibiting progenitor responses, matrix remodeling, proliferation of hepatocytes and ductular cells, and restoration of liver mass. These global inhibitory effects on liver regeneration dramatically reduced survival after PH.

CONCLUSION

Mechanisms that mediate liver organogenesis, such as Hh pathway activation, are retained and promote reconstruction of adult livers after injury.

Microarray analysis of autoimmune diseases by machine learning procedures

Microarray-based global gene expression profiling, with the use of sophisticated statistical algorithms is providing new insights into the pathogenesis of autoimmune diseases. We have applied a novel statistical technique for gene selection based on machine learning approaches to analyze microarray expression data gathered from patients with systemic lupus erythematosus (SLE) and primary antiphospholipid syndrome (PAPS), two autoimmune diseases of unknown genetic origin that share many common features. The methodology included a combination of three data discretization policies, a consensus gene selection method, and a multivariate correlation measurement. A set of 150 genes was found to discriminate SLE and PAPS patients from healthy individuals. Statistical validations demonstrate the relevance of this gene set from an univariate and multivariate perspective. Moreover, functional characterization of these genes identified an interferon-regulated gene signature, consistent with previous reports. It also revealed the existence of other regulatory pathways, including those regulated by PTEN, TNF, and BCL-2, which are altered in SLE and PAPS. Remarkably, a significant number of these genes carry E2F binding motifs in their promoters, projecting a role for E2F in the regulation of autoimmunity.

E2F2 represses cell cycle regulators to maintain quiescence

E2F transcription factors control diverse biological processes through regulation of target gene expression. However, the mechanism by which this regulation is established, and the relative contribution of each E2F member are still poorly defined. We have investigated the role of E2F2 in regulating cellular proliferation. We show that E2F2 is required for the normal G(0)/G(1) phase because targeted disruption of the E2F2 gene causes T cells to enter S phase early and to undergo accelerated cell division. A large set of E2F target genes involved in DNA replication and cell cycle progression (such as Mcm's, cyclins and Cdc2a) that are silent in G(0) and typically transcribed late in G(1) phase are already actively expressed in quiescent T cells and MEFs lacking E2F2. The classic E2F activators, E2F1 and E2F3, are largely dispensable for this process because compound loss of E2F1(-/-) and E2F2(-/-) produces a comparably shortened G(0)/G(1) phase, with early S phase entry. Likewise, shRNA knockdown of E2F3 does not alter significantly the E2F2(-/-) phenotype. Chromatin immunoprecipitation analysis indicates that in wild-type cells the promoters of the aberrantly early-transcribed genes are occupied by E2F2 in G(0), suggesting a direct role for E2F2 in transcriptional repression. We conclude that E2F2 functions to transcriptionally repress cell cycle genes to establish the G(0) state.

Developmental silencing and independency from E2F of apoptotic gene expression in postmitotic tissues

The involvement of caspases in postmitotic cell death is controversial. Here we report that adult brain and heart are devoid of many key pro-apoptotic proteins due to a progressive postnatal silencing event involving a reduction of their transcript levels. E2F has been shown to control cell cycle progression and to be transcriptional activator of apoptotic genes. However, our data demonstrate that apoptotic gene expression in heart, brain and liver, as well as cardiac and neuronal apoptotic gene silencing during development, are E2F-independent events. Therefore, the genes regulating caspase-dependent cell death are expressed in embryonic organs in an E2F-independent manner and a developmental-related silencing event represses these genes in postmitotic adult tissues.

Innate immune response gene expression profiles characterize primary antiphospholipid syndrome

Primary antiphospholipid syndrome (PAPS) is a systemic autoimmune disorder characterized by thromboembolic episodes and pregnant morbidity with an increasing clinical importance. To gain insight into the pathogenesis of PAPS, we have investigated the gene expression profiles that characterize peripheral blood mononuclear cells derived from PAPS patients. We show that the transcriptional activity of genes involved in innate immune responses, such as toll-like receptor 8 and CD14, as well as downstream genes of this pathway, such as STAT1, OAS2, TNFSF13 and PLSCR1 are significantly increased in PAPS patients. In addition, the expression of monocyte-specific cytokines is also elevated in PAPS mononuclear cells stimulated in vitro with lipopolysaccharide. Taken together, these results reveal a 'response to pathogen' signature in PAPS, which could reflect an altered monocyte activity. Finally, microarray analyses also revealed a reduced expression of genes coding for proteins involved in transcriptional control. Interestingly, a significant proportion of them exhibit E2F-binding sites in their promoter, suggesting that a deregulated RB/E2F activity could play a role in the pathogenesis of antiphospholipid syndrome.

Transcriptional activation of the proapoptotic bik gene by E2F proteins in cancer cells

BH3-only proteins are required for execution of apoptotic cell death. We have found that one of these proteins, Bik, is strongly induced in cancer cells treated with chemotherapeutic agents. Furthermore, we showed that chemotherapy-induced expression of bik is independent of p53. Consistent with its pro-apoptotic activity, blockade of bik expression reduces the adriamycin-mediated apoptotic cell death. We also found that the bik gene is transcriptionally activated by E2F proteins. Consistently, adriamycin induces the E2F-bik pathway. In addition, E2Fs transactivate bik by a p53-independent mechanism. Thus, our data indicate that transcriptional regulation of bik contributes to the efficient apoptotic response to chemotherapeutic agents.

Differential proteome profiles in E2F2-deficient T lymphocytes

E2F transcription factors are important regulators of proliferation, differentiation and apoptosis. We have previously shown that E2F2-/- mice develop late-onset autoimmune features, similar to systemic lupus erythematosus. E2F2-deficient T lymphocytes exhibit enhanced T cell receptor (TCR)-stimulated proliferation, which is presumably responsible for causing autoimmunity in E2F2-deficient mice. The comparison of E2F2-/- and wild-type T lymphocyte expression profiles by 2-DE followed by MS identification has revealed a set of deregulated proteins involved in TCR-mediated signaling, cell survival and stress responses. The deregulation of these proteins may account for the hyperproliferative phenotype that characterizes E2F2-/- T cells. Our work shows that proteomic analysis of gene-knockout strains can be a useful methodology to study the functional role of specific genes.

Diabetes and exocrine pancreatic insufficiency in E2F1/E2F2 double-mutant mice

E2F transcription factors are thought to be key regulators of cell growth control. Here we use mutant mouse strains to investigate the function of E2F1 and E2F2 in vivo. E2F1/E2F2 compound-mutant mice develop nonautoimmune insulin-deficient diabetes and exocrine pancreatic dysfunction characterized by endocrine and exocrine cell dysplasia, a reduction in the number and size of acini and islets, and their replacement by ductal structures and adipose tissue. Mutant pancreatic cells exhibit increased rates of DNA replication but also of apoptosis, resulting in severe pancreatic atrophy. The expression of genes involved in DNA replication and cell cycle control was upregulated in the E2F1/E2F2 compound-mutant pancreas, suggesting that their expression is repressed by E2F1/E2F2 activities and that the inappropriate cell cycle found in the mutant pancreas is likely the result of the deregulated expression of these genes. Interestingly, the expression of ductal cell and adipocyte differentiation marker genes was also upregulated, whereas expression of pancreatic cell marker genes were downregulated. These results suggest that E2F1/E2F2 activity negatively controls growth of mature pancreatic cells and is necessary for the maintenance of differentiated pancreatic phenotypes in the adult.

Mutation of E2F2 in mice causes enhanced T lymphocyte proliferation, leading to the development of autoimmunity

E2Fs are important regulators of proliferation, differentiation, and apoptosis. Here we characterize the phenotype of mice deficient in E2F2. We show that E2F2 is required for immunologic self-tolerance. E2F2(-/-) mice develop late-onset autoimmune features, characterized by widespread inflammatory infiltrates, glomerular immunocomplex deposition, and anti-nuclear antibodies. E2F2-deficient T lymphocytes exhibit enhanced TCR-stimulated proliferation and a lower activation threshold, leading to the accumulation of a population of autoreactive effector/memory T lymphocytes, which appear to be responsible for causing autoimmunity in E2F2-deficient mice. Finally, we provide support for a model to explain E2F2's unexpected role as a suppressor of T lymphocyte proliferation. Rather than functioning as a transcriptional activator, E2F2 appears to function as a transcriptional repressor of genes required for normal S phase entry, particularly E2F1.

A role for E2F1 in the induction of apoptosis during thymic negative selection

Thymic negative selection is the process in which maturing thymocytes that express T-cell receptors recognizing self are eliminated by apoptotic cell death. The molecular mechanism by which this occurs is poorly understood. Notably, genes involved in cell death, even thymocyte death, such as Fas, Fas-ligand, p53, caspase-1, caspase-3, and caspase-9, and Bcl-2 have been found to not be required for normal thymic negative selection. We have demonstrated previously that E2F1-deficient mice have a defect in thymocyte apoptosis. Here we show that E2F1 is required for normal thymic negative selection. Furthermore, we observed an E2F1-dependent increase of p53 protein levels during the process of thymic clonal deletion, which suggests that E2F1 regulates activation-induced apoptosis of self-reactive thymocytes by a p53-dependent mechanism. In contrast, other apoptotic pathways operating on developing thymocytes, such as glucocorticoid-induced cell death, are not mediated by E2F1. The T lymphocytes that escape thymic negative selection migrate to the peripheral immune system but do not appear to be autoreactive, indicating that there may exist E2F1-independent mechanisms of peripheral tolerance, which protect mice from developing an autoimmune response. We expect that E2F1-deficient mice will provide a useful tool for understanding the molecular mechanism of and the immunological importance of thymic negative selection.

E2F-1 functions in mice to promote apoptosis and suppress proliferation

Members of the E2F transcription factor family (E2F-1-E2F-5) are believed to be critical positive regulators of cell cycle progression in eukaryotes although the in vivo functions of the individual E2Fs have not been elucidated. Mice were generated that lack E2F-1 and, surprisingly, these mice develop and reproduce normally. However, E2F-1-/- mice exhibit a defect in T lymphocyte development leading to an excess of mature T cells due to a maturation stage-specific defect in thymocyte apoptosis. As E2F-1-/- mice age they exhibit a second phenotype marked by aberrant cell proliferation. These findings suggest that while certain members of the E2F family may positively regulate cell cycle progression, E2F-1 functions to regulate apoptosis and to suppress cell proliferation.

The nonamer UUAUUUAUU is the key AU-rich sequence motif that mediates mRNA degradation

Labile mRNAs that encode cytokine and immediate-early gene products often contain AU-rich sequences within their 3' untranslated region (UTR). These AU-rich sequences appear to be key determinants of the short half-lives of these mRNAs, although the sequence features of these elements and the mechanism by which they target mRNAs for rapid decay have not been fully defined. We have examined the features of AU-rich elements (AREs) that are crucial for their function as determinants of mRNA instability in mammalian cells by testing the ability of various mutant c-fos AREs and synthetic AREs to direct rapid mRNA deadenylation and decay when inserted within the 3' UTR of the normally stable beta-globin mRNA. Evidence is presented that the pentamer AUUUA, which previously was suggested to be the minimal determinant of instability present in mammalian AREs, cannot direct rapid mRNA deadenylation and decay. Instead, the nonomer UUAUUUAUU is the elemental AU-rich sequence motif that destabilizes mRNA. Removal of one uridine residue from either end of the nonamer (UUAUUUAU or UAUUUAUU) results in a decrease of potency of the element, while removal of a uridine residue from both ends of the nonamer (UAUUUAU) eliminates detectable destabilizing activity. The inclusion of an additional uridine residue at both ends of the nonamer (UUUAUUUAUUU) does not further increase the efficacy of the element. Taken together, these findings suggest that the nonamer UUAUUUAUU is the minimal AU-rich motif that effectively destabilizes mRNA. Additional ARE potency is achieved by combining multiple copies of this nonamer in a single mRNA 3' UTR. Furthermore, analysis of poly(A) shortening rates for ARE-containing mRNAs reveals that the UUAUUUAUU sequence also accelerates mRNA deadenylation and suggests that the UUAUUUAUU motif targets mRNA for rapid deadenylation as an early step in the mRNA decay process.

Interleukin-1 induces c-fos and c-jun gene expression in T helper type II cells through different signal transmission pathways

Interleukin (IL)-1 induces proliferation and expression of several protooncogenes in the T helper 2 cell line D10A. We have analyzed the signal transmission pathways activated by IL-1 in these cells, leading to the expression of c-jun and c-fos. IL-1 induced c-jun gene transcription and mRNA expression by means of a pathway dependent on protein tyrosine kinase activity since tyrphostin, a specific inhibitor of tyrosine kinase, inhibited this induction. This mechanism of transmission signaling was independent of protein kinase C (PKC) and was linked to the 80-kDa IL-1 receptor (IL-1R). In addition, phorbol esters did not induce c-jun mRNA expression, whereas c-fos mRNA expression mediated by IL-1 dependent on PKC; this pathway was linked to a different, still unidentified IL-1R that was functional in the D10A cell line. Accumulation of intracellular cAMP generated by IL-1 through the 80-kDa IL-1R negatively regulated c-fos expression which was induced by IL-1 through PKC activation. We conclude that IL-1 modulates the expression of c-fos in D10A cells by occupying of two independent IL-1R that are linked to different signal transduction pathways.

IL-4 and IL-2 selectively rescue Th cell subsets from glucocorticoid-induced apoptosis

It is well established that T cell maturation and activation are negatively regulated by a mechanism termed apoptosis. We now present evidence that glucocorticoids, known to possess immunosuppressive properties, cause apoptosis in mature Th cells, similarly to what has been reported for thymocytes. Th cells treated with the synthetic glucocorticoid dexamethasone show genome fragmentation into oligonucleosomal fragments, and proliferation of growth factor stimulated Th cells is inhibited by glucocorticoids. We show that IL-4 specifically rescues Th2 cells from dexamethasone-mediated apoptosis, whereas IL-2 and IL-1 are ineffective in these cells. However, IL-2 is the relevant rescue-factor of glucocorticoid-treated Th1 cells. The rescue induced by IL-4 and IL-2 is thought to be mediated by protein kinases (possibly protein kinase C), as evidenced by the fact that the protein kinase inhibitor H7 blocks the action of IL-4 and IL-2 in glucocorticoid-treated cells. Our in vitro data show that mature T cells can be protected by their own growth factors from the deleterious effects of the synthetic glucocorticoid dexamethasone, and suggest that specific interactions occur between lymphokines and naturally produced glucocorticoids in vivo, which may play a role in the regulation of the immune response.

IL-4 and IL-2 selectively rescue Th cell subsets from glucocorticoid-induced apoptosis

It is well established that T cell maturation and activation are negatively regulated by a mechanism termed apoptosis. We now present evidence that glucocorticoids, known to possess immunosuppressive properties, cause apoptosis in mature Th cells, similarly to what has been reported for thymocytes. Th cells treated with the synthetic glucocorticoid dexamethasone show genome fragmentation into oligonucleosomal fragments, and proliferation of growth factor stimulated Th cells is inhibited by glucocorticoids. We show that IL-4 specifically rescues Th2 cells from dexamethasone-mediated apoptosis, whereas IL-2 and IL-1 are ineffective in these cells. However, IL-2 is the relevant rescue-factor of glucocorticoid-treated Th1 cells. The rescue induced by IL-4 and IL-2 is thought to be mediated by protein kinases (possibly protein kinase C), as evidenced by the fact that the protein kinase inhibitor H7 blocks the action of IL-4 and IL-2 in glucocorticoid-treated cells. Our in vitro data show that mature T cells can be protected by their own growth factors from the deleterious effects of the synthetic glucocorticoid dexamethasone, and suggest that specific interactions occur between lymphokines and naturally produced glucocorticoids in vivo, which may play a role in the regulation of the immune response.

Superinduction of IL-2 gene transcription in the presence of cycloheximide

Lymphokine production is regulated both at the transcriptional and the posttranscriptional level. To date, it has been shown that the protein synthesis inhibitor cycloheximide (CHX) up-regulates IL-2 expression in T cells by stabilizing its mRNA. In this report we have examined the effect of CHX on IL-2 at the transcriptional level. We have found that CHX has a positive regulatory function in IL-2 transcription, which is dependent on prior activation of this gene. This is not due to posttranslational conversion of inactive NFkB into its active form by CHX, because a clustered mutation in the kB-like sequence in the IL-2 enhancer that abrogates NFkB binding does not affect the up-regulation of IL-2 transcription. These results favor the hypothesis that, in addition to positive factors, negative elements regulate IL-2 transcription. Furthermore, we have tested the effect of CHX on IL-4 and granulocyte-macrophage-CSF transcription of both lymphokines. These results suggest that transcriptional up-regulation by CHX may be specific for IL-2 with respect to lymphokine expression.

Production of IL-1 alpha by activated Th type 2 cells. Its role as an autocrine growth factor

Autocrine growth of Th type 2 cells has been reported to be mediated by the lymphokine IL-4. In this report we present evidence that in addition to IL-4 Th2 cells also produce IL-1 alpha in its active form in the absence of APC. We have found that this cytokine is an autocrine growth factor, because proliferation of Th2 cells in response to several stimuli is inhibited by anti-IL-1 alpha or anti-IL-1R mAb, or by an IL-1 alpha antisense oligodeoxynucleotide. However, Th1 cells do not produce this cytokine. We have investigated the role of endogenous IL-1 alpha on the induction of c-myc and c-myb, two protooncogenes involved in T cell activation. Here we show that endogenous IL-1 alpha is involved in the activation of both protooncogenes. Our results suggest that a possible function of IL-1 alpha, and perhaps other growth factors, might be to sustain or amplify the initial second messengers derived through the TCR. The possible implications of this finding with respect to interactions between T cell subsets and B cells or macrophages are discussed.

Production of IL-1 alpha by activated Th type 2 cells. Its role as an autocrine growth factor

Autocrine growth of Th type 2 cells has been reported to be mediated by the lymphokine IL-4. In this report we present evidence that in addition to IL-4 Th2 cells also produce IL-1 alpha in its active form in the absence of APC. We have found that this cytokine is an autocrine growth factor, because proliferation of Th2 cells in response to several stimuli is inhibited by anti-IL-1 alpha or anti-IL-1R mAb, or by an IL-1 alpha antisense oligodeoxynucleotide. However, Th1 cells do not produce this cytokine. We have investigated the role of endogenous IL-1 alpha on the induction of c-myc and c-myb, two protooncogenes involved in T cell activation. Here we show that endogenous IL-1 alpha is involved in the activation of both protooncogenes. Our results suggest that a possible function of IL-1 alpha, and perhaps other growth factors, might be to sustain or amplify the initial second messengers derived through the TCR. The possible implications of this finding with respect to interactions between T cell subsets and B cells or macrophages are discussed.

Superinduction of IL-2 gene transcription in the presence of cycloheximide

Lymphokine production is regulated both at the transcriptional and the posttranscriptional level. To date, it has been shown that the protein synthesis inhibitor cycloheximide (CHX) up-regulates IL-2 expression in T cells by stabilizing its mRNA. In this report we have examined the effect of CHX on IL-2 at the transcriptional level. We have found that CHX has a positive regulatory function in IL-2 transcription, which is dependent on prior activation of this gene. This is not due to posttranslational conversion of inactive NFkB into its active form by CHX, because a clustered mutation in the kB-like sequence in the IL-2 enhancer that abrogates NFkB binding does not affect the up-regulation of IL-2 transcription. These results favor the hypothesis that, in addition to positive factors, negative elements regulate IL-2 transcription. Furthermore, we have tested the effect of CHX on IL-4 and granulocyte-macrophage-CSF transcription of both lymphokines. These results suggest that transcriptional up-regulation by CHX may be specific for IL-2 with respect to lymphokine expression.

Tyrosine protein phosphorylation is required for protein kinase C-mediated proliferation in T cells

We have studied the role of tyrosine kinase in PMA-stimulated T cells. Protein kinase C (PKC)-mediated D10A cell proliferation is inhibited by the specific inhibitor of tyrosine kinase, tyrphostin. This inhibitor selectively blocks the mRNA expression of the proto-oncogene c-myc in response to the phorbol ester, PMA. On the other hand, the same doses of this inhibitor do not affect the mRNA expression of the proto-oncogene c-fos in PMA-stimulated D10A cells. Phorbol esters induce in this T cell line the tyrosine phosphorylation of a unique protein of 42 kDa and the enzyme PKC is required for this activity.

IL-1 signal transduction pathways. I. Two functional IL-1 receptors are expressed in T cells

We have recently characterized a subline of the Th2 cell line D10.G4.1, D10A, which responds to exogenous IL-1 in the absence of cofactors. In this cell line IL-1 activates two distinct transmission signal pathways, one leading to increased levels of intracellular cAMP, and the other to the phosphorylation of an 80-kDa substrate of protein kinase C (PKC). To determine whether both pathways are activated upon occupancy of a single IL-1R, we used a mAb, M15, which blocks binding of IL-1 to the 80-kDa IL-1R, known to be expressed in Th2 cells. Whereas M15 was able to inhibit the accumulation of cAMP induced by IL-1, it had no effect on the IL-1-induced phosphorylation of the 80-kDa substrate of PKC or the c-fos mRNA expression. In addition, we show that IL-1 induces the expression of the two proto-oncogenes c-myb and c-myc through the 80-kDa IL-1R, and that this induction is PKC independent. The proliferation of D10A cells in response to IL-1 is blocked by M15, but is unaffected by H-7, a potent inhibitor of PKC. These results indicate that the IL-1-induced proliferation of D10A cells is independent of PKC. In addition, IL-1 induces c-fos mRNA expression in thymocytes but not in EL-4 cells.

IL-1 signal transduction pathways. I. Two functional IL-1 receptors are expressed in T cells

We have recently characterized a subline of the Th2 cell line D10.G4.1, D10A, which responds to exogenous IL-1 in the absence of cofactors. In this cell line IL-1 activates two distinct transmission signal pathways, one leading to increased levels of intracellular cAMP, and the other to the phosphorylation of an 80-kDa substrate of protein kinase C (PKC). To determine whether both pathways are activated upon occupancy of a single IL-1R, we used a mAb, M15, which blocks binding of IL-1 to the 80-kDa IL-1R, known to be expressed in Th2 cells. Whereas M15 was able to inhibit the accumulation of cAMP induced by IL-1, it had no effect on the IL-1-induced phosphorylation of the 80-kDa substrate of PKC or the c-fos mRNA expression. In addition, we show that IL-1 induces the expression of the two proto-oncogenes c-myb and c-myc through the 80-kDa IL-1R, and that this induction is PKC independent. The proliferation of D10A cells in response to IL-1 is blocked by M15, but is unaffected by H-7, a potent inhibitor of PKC. These results indicate that the IL-1-induced proliferation of D10A cells is independent of PKC. In addition, IL-1 induces c-fos mRNA expression in thymocytes but not in EL-4 cells.

Cholera toxin discriminates between T helper 1 and 2 cells in T cell receptor-mediated activation: role of cAMP in T cell proliferation

CD4+ T helper (Th) clones can be divided into interleukin 2 (IL-2)-secreting Th1 and IL-4-secreting Th2 cells. We show in the present report that these two Th subsets have different activation requirements for lymphokine production and proliferation: namely, cholera toxin (CT) as well as forskolin inhibit T cell receptor (TCR)-mediated IL-2 production and proliferation in Th1 cells, while the same reagents fail to block IL-4 production and proliferation in Th2 cells. In addition, CT and forskolin differentially influence the proto-oncogene mRNA expression in Th1 vs. Th2 cells after stimulation with Con A. Since both reagents lead to elevated levels of intracellular cAMP, it is likely that Th1 and Th2 cells differ in their sensitivity to an increase in cAMP. Our results indicate that the two Th subsets use different transmission signal pathways upon TCR-mediated activation.

Regulation of IL-4 lymphokine gene expression and cellular proliferation in murine T helper type II cells

Activation of T cells results in the production of lymphokines and cellular proliferation. Protein kinase C (PKC) plays a key role in this process. It has been shown that this enzyme is essential to elicit a response to Con A or specific antigen in CD4+ T helper type 1 (Th 1) cells that secrete IL-2. We have now explored the signal transduction pathway that leads to transcription of the IL-4 gene and proliferation in murine CD4+ T helper type 2 (Th 2) cells. Surprisingly, we have found in two independently derived Th 2 clones that neither cellular proliferation nor IL-4 lymphokine production is affected by blocking or depletion of PKC. This differential mechanism of signal transmission leading to cellular activation implies a new distinction between murine Th 1 and Th 2 cells.

Regulation of interleukin 6 production in T helper cells

The cytokine interleukin 6 (IL-6) is a cellular regulatory molecule that is produced by both lymphoid and non-lymphoid cells in response to several stimuli. In this report we present evidence that within the murine T cell compartment T helper type 2 cells (Th2) produce this lymphokine, whereas unprimed CD4+ T cells and a T helper type 1 clone (Th1) do not. Furthermore, IL-6 is not an autocrine growth factor for in vitro cultured Th cells, in contrast to what occurs in freshly isolated CD4+ and CD8+ T cells. We have examined the signal transduction pathways that lead to IL-6 production in activated Th2 cells. We have found that protein kinase C activators, such as PMA, Con A, or IL-1, increase the IL-6 expression in these cells. On the other hand, activation of the cAMP-dependent pathway does not seem to have an effect on the IL-6 production, since forskolin, 8BrcAMP, or TNF-alpha, which in these cells increases the level of intracellular cAMP, do not lead to an accumulation of IL-6 message. These results indicate that the IL-6 gene is more tightly regulated in T cells than in other systems described previously.