Inactivation of a Cdk2 inhibitor during interleukin 2-induced proliferation of human T lymphocytes

The Not-So-Good, the Bad and the Ugly: HPV E5, E6 and E7 Oncoproteins in the Orchestration of Carcinogenesis

Infection with HPV starts with the access of the viral particles to basal cells in the epidermis, potentially via microtraumas to the skin. The basal cells are able to keep away these pathogens in normal circumstances through a robust immune response from the host, as HPV infections are, in general, cleared within 2 to 3 weeks. However, the rare instances of persistent infection and/or in cases where the host immune system is compromised are major risk factors for the development of lesions potentially leading to malignancy. Evolutionarily, obligatory pathogens such as HPVs would not be expected to risk exposing the host to lethal cancer, as this would entail challenging their own life cycle, but infection with these viruses is highly correlated with cancer and malignancy-as in cancer of the cervix, which is almost always associated with these viruses. Despite this key associative cause and the availability of very effective vaccines against these viruses, therapeutic interventions against HPV-induced cancers are still a challenge, indicating the need for focused translational research. In this review, we will consider the key roles that the viral proteins play in driving the host cells to carcinogenesis, mainly focusing on events orchestrated by early proteins E5, E6 and E7-the not-so-good, the bad and the ugly-and discuss and summarize the major events that lead to these viruses mechanistically corrupting cellular homeostasis, giving rise to cancer and malignancy.

Superoxide Dismutase 3 Controls the Activation and Differentiation of CD4+T Cells

Superoxide dismutase 3 (SOD3), a well-known antioxidant has been shown to possess immunomodulatory properties through inhibition of T cell differentiation. However, the underlying inhibitory mechanism of SOD3 on T cell differentiation is not well understood. In this study, we investigated the effect of SOD3 on anti-CD3/CD28- or phorbol myristate acetate (PMA) and ionomycin (ION)-mediated activation of mouse naive CD4⁺ T cells. Our data showed that SOD3 suppressed the expression of activation-induced surface receptor proteins such as CD25, and CD69, and cytokines production. Similarly, SOD3 was found to reduce CD4⁺T cells proliferation and suppress the activation of downstream pathways such as ERK, p38, and NF-κB. Moreover, naïve CD4⁺T cells isolated from global SOD3 knock-out mice showed higher expression of CD25, CD69, and CD71, IL-2 production, proliferation, and downstream signals compared to wild-type CD4⁺T cells. Whereas, the use of DETCA, a known inhibitor of SOD3 activity, found to nullify the inhibitory effect of SOD3 on CD4⁺T cell activation of both SOD3 KO and wild-type mice. Furthermore, the expression of surface receptor proteins, IL-2 production, and downstream signals were also reduced in Th2 and Th17 differentiated cells upon SOD3 treatment. Overall, our data showed that SOD3 can attenuate CD4⁺T cell activation through modulation of the downstream signalings and restrict CD4⁺T cell differentiation. Therefore, SOD3 can be a promising therapeutic for T cell-mediated disorders.

The cyclin-dependent kinase 2 (CDK2) mediates hematopoiesis through G1-to-S transition in Chinese mitten crab Eriocheir sinensis

Cyclin-dependent kinases (CDKs), a family of cell cycle-related serine/threonine kinases, participate in various biological processes, and play crucial roles in the innate immunity. In the present study, a CDK2 (designed as EsCDK2) with a serine/threonine protein kinase catalytic domain was identified from Chinese mitten crab (Eriocheir sinensis). The full-length cDNA sequence of EsCDK2 was of 2405 bp with an open reading frame (ORF) of 909 bp. EsCDK2 shared 66%-81% sequence similarities with previously identified CDK2s. It was clustered with the CDK2 from Penaeus monodon in the invertebrate branch of the phylogenetic tree. The mRNA transcripts of EsCDK2 were highly expressed in hematopoietic tissue (HPT) and gonad, while lower in hemocytes, heart, gills, and muscle. EsCDK2 protein distributed in both cytoplasm and nucleus of HPT cells. The expression of EsCDK2 mRNA in HPT was significantly up-regulated and peaked at 3 h post stimulations with Aeromonas hydrophila (2.31-fold, p < 0.05) and Lipopolysaccharide (LPS) (2.02-fold, p < 0.05). After exsanguination, the total hemocyte counts (THC) decreased significantly to 0.42 × 10⁷/ml (0.39-fold, p < 0.05) at 0.5 h, then returned to a normal level at 6 h, while the mRNA expression of EsCDK2 in HPT cells was up-regulated at the early phase from 0.5 h to 6 h. After injection of EsCDK2-dsRNA, the mRNA expression level of EsCDK2 in HPT and THC both decreased to 0.53-fold (p < 0.01) and 0.78-fold (p < 0.05) at 24 h, respectively, and the percentage of new-born hemocytes in HPT also decreased significantly from 37.7% to 16.3% (0.43-fold, p < 0.01). After knocking down of EsCDK2, THC decreased dramatically at 6 h (0.65-fold, p < 0.01) post exsanguination, while returned normal at 6 h in PBS group. After interference of EsCDK2 mRNA expression, the percentage of G0-G1 phase cells significantly increased to 85.01% (1.26-fold, p < 0.01), while S phase and G2-M phase cells significantly decreased to 7.92% (0.46-fold, p < 0.01) and 7.07% (0.43-fold, p < 0.01) respectively, indicating that the cell cycle of HPT cells arrested at G1 phase. These results collectively demonstrated that EsCDK2 participated in the regeneration of hemocytes or hematopoiesis by regulating the transition from G1 to S phase in the cell cycle, and involves in the innate immune responses of E. sinensis.

Mutations at multiple CDK phosphorylation consensus sites on Cdt2 increase the affinity of CRL4Cdt2 for PCNA and its ubiquitination activity in S phase

CRL4^Cdt2 ubiquitin ligase plays an important role maintaining genome integrity during the cell cycle. A recent report suggested that Cdk1 negatively regulates CRL4^Cdt2 activity through phosphorylation of its receptor, Cdt2, but the involvement of phosphorylation remains unclear. To address this, we mutated all CDK consensus phosphorylation sites located in the C-terminal half region of Cdt2 (Cdt2-18A) and examined the effect on substrate degradation. We show that both cyclinA/Cdk2 and cyclinB/Cdk1 phosphorylated Cdt2 in vitro and that phosphorylation was reduced by the 18A mutation both in vitro and in vivo. The 18A mutation increased the affinity of Cdt2 to PCNA, and a high amount of Cdt2-18A was colocalized with PCNA foci during S phase in comparison with Cdt2-WT. Poly-ubiquitination activity to Cdt1 was concomitantly enhanced in cells expressing Cdt2-18A. Other CRL4^Cdt2 substrates, Set8 and thymine DNA glycosylase, begin to accumulate around late S phase to G2 phase, but the accumulation was prevented in Cdt2-18A cells. Furthermore, mitotic degradation of Cdt1 after UV irradiation was induced in these cells. Our results suggest that CDK-mediated phosphorylation of Cdt2 inactivates its ubiquitin ligase activity by reducing its affinity to PCNA, an important strategy for regulating the levels of key proteins in the cell cycle.

M(IL-4) Tissue Macrophages Support Efficient Interferon-Gamma Production in Antigen-Specific CD8+ T Cells with Reduced Proliferative Capacity

CD8⁺ cytotoxic T cell (CTL) responses are necessary for the lysis of virally infected cells and control of infection. CTLs are activated when their TCRs bind a major histocompatibility complex (MHC)-I/peptide complex on the surface of antigen presenting cells such as macrophages (MΦ). It is now apparent that MΦ display remarkable plasticity in response to environmental signals to polarize into classically activated M(LPS + IFN-γ) or alternatively activated M(IL-4). However, little is known about how MΦ activation status influences their antigen presentation function to CD8⁺ T cell in models of virus infection. Consequently, we tested how polarization of spleen-derived (Sp)-MΦ impacts direct presentation of viral antigens to influence effector and proliferative CD8⁺ T-cell responses. We show that M(IL-4) Sp-MΦ retain MHC-I surface expression and the ability to stimulate IFN-γ production by CTL following peptide stimulation and lymphocytic choriomeningitis virus infection to levels similar to M0 and M(LPS + IFN-γ) MΦ. However, memory CD8⁺ T cells cultured in the presence of M(IL-4) MΦ underwent significantly reduced proliferation and produced similar IFN-γ levels as coculturing with M0 or M(LPS + IFN-γ) cells. Thus, these results show a novel ability of polarized MΦ to regulate CD8⁺ T-cell proliferation and effector functions during virus infection.

Green tea EGCG, T-cell function, and T-cell-mediated autoimmune encephalomyelitis

Autoimmune diseases are common, disabling immune disorders affecting millions of people. Recent studies indicate that dysregulated balance of different CD4⁺ T-cell subpopulations plays a key role in immune pathogenesis of several major autoimmune diseases. Green tea and its active ingredient, epigallocatechin-3-gallate (EGCG), have been shown to modulate immune cell functions and improve some autoimmune diseases in animal models. In a series of studies we determined EGCG's effect on T-cell functions and its application in autoimmune diseases. We first observed that EGCG inhibited CD4⁺ T-cell expansion induced by polyclonal (mitogens or anti-CD3/CD28) or antigen-specific stimulation. We then showed that EGCG suppressed expansion and cell cycle progression of naïve CD4⁺ T by modulating cell cycle-related proteins. EGCG also inhibited naive CD4⁺ T-cell differentiation into Th1 and Th17 effector subsets by impacting their respective signaling transducers and transcription factors. These results suggest that EGCG may improve T-cell-mediated autoimmune diseases. Using the experimental autoimmune encephalomyelitis (EAE) mice, an animal model for human multiple sclerosis, we found that dietary supplementation with EGCG attenuated the disease's symptoms and pathology. These EGCG-induced changes are associated with findings in the immune and inflammation profiles in lymphoid tissues and the central nervous system: a reduction in proliferation of autoreactive T cells, production of proinflammatory cytokines, and Th1 and Th17 subpopulations, and an increase in regulatory T-cell populations. These results suggest that green tea or its active components may have a preventive and therapeutic potential in dealing with T-cell-mediated autoimmune diseases. However, the translational value of these findings needs to be validated in future human studies.

Requirement of the expression of 3-phosphoglycerate dehydrogenase for traversing S phase in murine T lymphocytes following immobilized anti-CD3 activation

Murine resting (G(0)) T lymphocytes contained no detectable mRNA of 3-phosphoglycerate dehydrogenase (PHGDH) catalyzing the first step in the phosphorylated pathway of l-serine biosynthesis. Immobilized anti-CD3 activation of G(0) T cells expressed the PHGDH mRNA in G(1) with a maximum level in S phase. G(0) T cells activated with either immobilized anti-CD3 plus CsA or PBu(2), which failed to drive the activated T cells to enter S phase, did not express the PHGDH mRNA unless exogenous rIL-2 was added. Blocking of IL-2R signaling by adding anti-IL-2 and anti-IL-2Rα resulted in no expression of the PHGDH mRNA during immobilized anti-CD3 activation of G(0) T cells. Deprivation of l-serine from culture medium or addition of antisense PHGDH oligonucleotide significantly reduced [(3)H]TdR incorporation of activated T cells. These results indicate that the PHGDH gene expression, dictated by IL-2R signaling, is a crucial event for DNA synthesis during S phase of activated T cells.

Epigallocatechin-3-gallate directly suppresses T cell proliferation through impaired IL-2 utilization and cell cycle progression

Previously, we demonstrated that in vitro epigallocatechin-3-gallate (EGCG) supplementation inhibited T cell response in mouse spleen cells. In this study, we confirmed this effect of EGCG in mice fed 0.3% EGCG for 6 wk. A coculture with all the combinations of preincubating antigen-presenting cells and T cells with or without EGCG showed that EGCG suppressed antigen-induced T cell proliferation, mainly through a direct effect on T cells. To determine the mechanisms for this effect of EGCG, we stimulated purified mouse T cells with anti-CD3/CD28 in the presence of EGCG (2.5-15 micromol/L) and found that EGCG dose-dependently inhibited cell division and cell cycle progression and this effect of EGCG was more pronounced in CD4(+) than in CD8(+) T cells. Interleukin (IL)-2 concentrations in EGCG-treated cell cultures showed no difference up to 24 h but were higher in the cultures at 48 h compared with the untreated control cells. However, intracellular staining showed no difference between EGCG-treated and untreated control cells in IL-2 synthesis, but EGCG-treated cells expressed less IL-2 receptor (IL-2R) compared with untreated control cells. EGCG did not affect mRNA expression of IL-2 and IL-2R. These results indicate that EGCG-induced IL-2 accumulation in 48 h cultures is due to its reduced utilization. In summary, EGCG directly inhibits T cell proliferative response to both polyclonal and antigen-specific stimulation. CD4(+) cells are more responsive to EGCG than CD8(+) cells. Future studies should determine the effect of EGCG on CD4(+) cell subsets to assess its application in T cell-mediated autoimmune diseases.

Oncogenic activities of human papillomaviruses

Infectious etiologies for certain human cancers have long been suggested by epidemiological studies and studies with experimental animals. Important support for this concept came from the discovery by Harald zur Hausen's group that human cervical carcinoma almost universally contains certain "high-risk" human papillomavirus (HPV) types. Over the years, much has been learned about the carcinogenic activities of high-risk HPVs. These studies have revealed that two viral proteins, E6 and E7, that are consistently expressed in HPV-associated carcinomas, are necessary for induction and maintenance of the transformed phenotype. Hence, HPV-associated tumors are unique amongst human solid tumors in that they are universally caused by exposure to the same, molecularly defined oncogenic agents, and the molecular signal transduction pathways subverted by these viral transforming agents are frequently disrupted in other, non-virus-associated human cancers.

The human papillomavirus E7 oncoprotein

The human papillomavirus (HPV) E7 oncoprotein shares functional similarities with such proteins as adenovirus E1A and SV40 large tumor antigen. As one of only two viral proteins always expressed in HPV-associated cancers, E7 plays a central role in both the viral life cycle and carcinogenic transformation. In the HPV viral life cycle, E7 disrupts the intimate association between cellular differentiation and proliferation in normal epithelium, allowing for viral replication in cells that would no longer be in the dividing population. This function is directly reflected in the transforming activities of E7, including tumor initiation and induction of genomic instability.

Age-associated changes in immune and inflammatory responses: impact of vitamin E intervention

Aging is associated with dysregulated immune and inflammatory responses. Declining T cell function is the most significant and best-characterized feature of immunosenescence. Intrinsic changes within T cells and extrinsic factors contribute to the age-associated decline in T cell function. T cell defect seen in aging involves multiple stages from early receptor activation events to clonal expansion. Among extrinsic factors, increased production of T cell-suppressive factor PGE(2) by macrophages (Mphi) is most recognized. Vitamin E reverses an age-associated defect in T cells, particularly naïve T cells. This effect of vitamin E is also reflected in a reduced rate of upper respiratory tract infection in the elderly and enhanced clearance of influenza infection in a rodent model. The T cell-enhancing effect of vitamin E is accomplished via its direct effect on T cells and indirectly by inhibiting PGE(2) production in Mphi. Up-regulated inflammation with aging has attracted increasing attention as a result of its implications in the pathogenesis of diseases. Increased PGE(2) production in old Mphi is a result of increased cyclooxygenase 2 (COX-2) expression, leading to higher COX enzyme activity, which in turn, is associated with the ceramide-induced up-regulation of NF-kappaB. Similar to Mphi, adipocytes from old mice have a higher expression of COX-2 as well as inflammatory cytokines IL-1beta, IL-6, and TNF-alpha, which might also be related to elevated levels of ceramide and NF-kappaB activation. This review will discuss the above age-related immune and inflammatory changes and the effect of vitamin E as nutritional intervention with a focus on the work conducted in our laboratory.

Inhibition of normal lymphocyte proliferation by Indirubin-3'-monoxime: a multifactorial process

Indirubin-3'-monoxime (IO) is a derivative of Indirubin, compound of a Chinese medicinal recipe used to treat various diseases including leukemia. In this study, we investigated to what extent IO inhibits the growth of normal human lymphocytes. We defined various experimental conditions of peripheral blood lymphocyte treatment: IO introduced (i) on unstimulated lymphocytes, (ii) or on stimulated lymphocytes at the time of phytohemagglutinin stimulation (L1 protocol), (iii) 48 h after the beginning of stimulation (L2 protocol), and (iv) after nocodazole synchronization of stimulated lymphocytes. IO induces a concentration dependent cytotoxic effect yielding a characteristic sub-G1 peak in normal stimulated lymphocytes. Cell death was partly due to necrosis and apoptosis. Normal unstimulated lymphocytes remained insensitive to the cytotoxic effect of 10 microM IO treatment. A cell cycle inhibition was observed after IO treatment, stronger for the L1 than for the L2 protocol, without induction of polyploidy after Nocodazole synchronization. These cellular consequences were associated with a decrease in CDK activity, and with CDK and cyclin gene expression modifications. The inhibition of lymphocyte proliferation by IO indicates that indirubin derivatives may be potent immunosuppressive agents.

Cyclin-dependent kinase inhibitors in yeast, animals, and plants: a functional comparison

The cell cycle is remarkably conserved in yeast, animals, and plants and is controlled by cyclin-dependent kinases (CDKs). CDK activity can be inhibited by binding of CDK inhibitory proteins, designated CKIs. Numerous studies show that CKIs are essential in orchestrating eukaryotic cell proliferation and differentiation. In yeast, animals, and plants, CKIs act as regulators of the G1 checkpoint in response to environmental and developmental cues and assist during mitotic cell cycles by inhibiting CDK activity required to arrest mitosis. Furthermore, CKIs play an important role in regulating cell cycle exit that precedes differentiation and in promoting differentiation in cooperation with transcription factors. Moreover, CKIs are essential to control CDK activity in endocycling cells. So, in yeast, animals, and plants, CKIs share many functional similarities, but their functions are adapted toward the specific needs of the eukaryote.

All-trans retinoic acid stimulates IL-2-mediated proliferation of human T lymphocytes: early induction of cyclin D3

Vitamin A is established as an important immune regulator, but the mechanisms whereby vitamin A regulates T cell biology are poorly defined. In this study, we show that an active metabolite of vitamin A, all-trans retinoic acid (RA), potently stimulates T cell proliferation by modulating IL-2-mediated signaling downstream of IL-2R and independent of the induction of IL-2. Thus, at concentrations as low as 0.1 nM, RA enhanced the division of normal human T lymphocytes that were simultaneously stimulated with anti-CD3 mAbs and saturating concentrations of IL-2. At the optimal concentration of RA (50 nM), a 3-fold increase in T cell proliferation was observed. The induced proliferation was preceded by increased phosphorylation of the retinoblastoma protein and enhanced G1- to S-phase progression. Interestingly, the promitogenic effect of RA was found to be particularly directed toward increased expression of cyclin D3 at both the mRNA and protein level. Furthermore, the stimulatory effect of RA on cyclin D3 expression as well as on cell proliferation was completely abolished in the presence of the JAK inhibitor AG-490 or blocking IL-2R alpha mAbs, and RA also enhanced cyclin D3 expression and T cell proliferation in the presence of IL-2 alone. Finally, we showed that the proliferative effect of RA was mimicked by agonists of the retinoic acid receptor (RAR) and completely inhibited by a RAR-selective antagonist. In conclusion, our results indicate that RA, via RAR, stimulates IL-2-induced signaling in a JAK-dependent manner to enhance cyclin D3 expression and thereby promote T cell proliferation.

Mutational analysis of the cell cycle inhibitor Kip1/p27 in childhood leukemia

BACKGROUND

Cyclin-dependent kinases (CDKs) and cyclins, their regulatory subunits, govern cell-cycle progression in eukaryotic cells. Kip1/p27 is the main cyclin-dependent kinase inhibitor, which arrests cell division inhibiting G1-S transition. Kip1/p27 seems to play a critical role in the pathogenesis of several human malignancies and its lower expression has been shown to correlate with a poor prognosis in adult solid tumors.

METHODS

Bone marrow blasts from 49 children with leukemia, 37 acute lymphoblastic leukemia (ALL), and 12 acute myeloid leukemia (AML) were studied. Exon 3 of Kip1/p27 was amplified using the polymerase chain reaction technique (PCR). Single strand conformational polymorphism and heterodouplex analysis were performed to detect DNA sequence with altered conformations and were subsequently sequenced to document mutations.

RESULTS

Mutations in Kip1/p27 gene were detected in 2 out of 3 T-ALL, 6 out of 12 AML patients, and only 1 out of 34 B lineage ALL cases. Although the patient groups are small, a highly significant relation of the mutation status with the type of leukemia (P = 0.0037) and the risk group according to treatment protocols (P = 0.00021) was estimated. A statistically significant difference in the white blood count was observed (P = 0.019) between the mutated and non-mutated patient groups although no statistically significant association of the mutation status with the hemoglobin and platelets values, karyotype, age, sex, disease progression, and outcome was determined.

CONCLUSIONS

Based upon these results, the Kip1/p27 mutations should be considered for further prospective testing as an additional parameter for risk stratification and treatment of childhood leukemia.

Mouse models of cell cycle regulators: new paradigms

In yeast, a single cyclin-dependent kinase (Cdk) is able to regulate diverse cell cycle transitions (S and M phases) by associating with multiple stage-specific cyclins. The evolution of multicellular organisms brought additional layers of cell cycle regulation in the form of numerous Cdks, cyclins and Cdk inhibitors to reflect the higher levels of organismal complexity. Our current knowledge about the mammalian cell cycle emerged from early experiments using human and rodent cell lines, from which we built the current textbook model of cell cycle regulation. In this model, the functions of different cyclin/Cdk complexes were thought to be specific for each cell cycle phase. In the last decade, studies using genetically engineered mice in which cell cycle regulators were targeted revealed many surprises. We discovered the in vivo functions of cell cycle proteins within the context of a living animal and whether they are essential for animal development. In this review, we discuss first the textbook model of cell cycle regulation, followed by a global overview of data obtained from different mouse models. We describe the similarities and differences between the phenotypes of different mouse models including embryonic lethality, sterility, hematopoietic, pancreatic, and placental defects. We also describe the role of key cell cycle regulators in the development of tumors in mice, and the implications of these data for human cancer. Furthermore, animal models in which two or more genes are ablated revealed which cell cycle regulators interact genetically and functionally complement each other. We discuss for example the interaction of cyclin D1 and p27 and the compensation of Cdk2 by Cdc2. We also focus on new functions discovered for certain cell cycle regulators such as the regulation of S phase by Cdc2 and the role of p27 in regulating cell migration. Finally, we conclude the chapter by discussing the limitations of animal models and to what extent can the recent findings be reconciled with the past work to come up with a new model for cell cycle regulation with high levels of redundancy among the molecular players.

Inhibition of in vitro and in vivo T cell responses by recombinant human Tim-1 extracellular domain proteins

Members of the T cell, Ig domain and mucin domain (Tim) family of proteins have recently been implicated in the control of T cell-mediated immune responses. Tim-1 (HUGO designation HAVCR1) polymorphisms have been linked to the regulation of atopy in mice and humans, suggestive of a role in immune regulation. Tim-1 is expressed upon activation of T cells. In concert with the increased expression of Tim-1, a binding partner for the extracellular domain of Tim-1 (eTim-1) was induced on activated T cells, and mRNA expression data was consistent with the binding partner being Tim-4. We found that co-immobilized recombinant eTim-1 was able to inhibit T cell activation mediated by CD3 + CD28 mAb. eTim-1 mediated its inhibitory effects on proliferation by arresting cell cycle at G(0)/G(1) phase through regulation of cell cycle proteins. In vivo, administration of eTim-1 proteins led to a decrease in both ear (contact hypersensitivity to oxazolone) and joint (methylated BSA antigen-induced arthritis) swelling. The inhibitory activity of eTim-1 in the T(h)1-dependent models was evidence that eTim-1 is able to modulate T cell responses. Manipulation of the Tim-1 interaction with its binding partner on T cells may therefore provide a novel target for therapeutic intervention in T cell-mediated diseases.

Cyclin T1 expression is regulated by multiple signaling pathways and mechanisms during activation of human peripheral blood lymphocytes

Stimulation of primary human T lymphocytes results in up-regulation of cyclin T1 expression, which correlates with phosphorylation of the C-terminal domain of RNA polymerase II (RNAP II). Up-regulation of cyclin T1 and concomitant stabilization of cyclin-dependent kinase 9 (CDK9) may facilitate productive replication of HIV in activated T cells. We report that treatment of PBLs with two mitogens, PHA and PMA, results in accumulation of cyclin T1 via distinct mechanisms. PHA induces accumulation of cyclin T1 mRNA and protein, which results from cyclin T1 mRNA stabilization, without significant change in cyclin T1 promoter activity. Cyclin T1 mRNA stabilization requires the activation of both calcineurin and JNK because inhibition of either precludes cyclin T1 accumulation. In contrast, PMA induces cyclin T1 protein up-regulation by stabilizing cyclin T1 protein, apparently independently of the proteasome and without accumulation of cyclin T1 mRNA. This process is dependent on Ca2+-independent protein kinase C activity but does not require ERK1/2 activation. We also found that PHA and anti-CD3 Abs induce the expression of both the cyclin/CDK complexes involved in RNAP II C-terminal domain phosphorylation and the G1-S cyclins controlling cell cycle progression. In contrast, PMA alone is a poor inducer of the expression of G1-S cyclins but often as potent as PHA in inducing RNAP II cyclin/CDK complexes. These findings suggest coordination in the expression and activation of RNAP II kinases by pathways that independently stimulate gene expression but are insufficient to induce S phase entry in primary T cells.

Structure-activity relationship between carboxylic acids and T cell cycle blockade

This study was designed to examine the potential structure-activity relationship between carboxylic acids, histone acetylation and T cell cycle blockade. Toward this goal a series of structural homologues of the short-chain carboxylic acid n-butyrate were studied for their ability to block the IL-2-stimulated proliferation of cloned CD4+ T cells. The carboxylic acids were also tested for their ability to inhibit histone deacetylation. In addition, Western blotting was used to examine the relative capacity of the carboxlic acids to upregulate the cyclin kinase-dependent inhibitor p21cip1 in T cells. As shown earlier n-butyrate effectively inhibited histone deacetylation. The increased acetylation induced by n-butyrate was associated with the upregulation of the cyclin-dependent kinase inhibitor p21cip1 and the cell cycle blockade of CD4+ T cells. Of the other carboxylic acids studied, the short chain acids, C3-C5, without branching were the best inhibitors of histone deacetylase. This inhibition correlated with increased expression of the cell cycle blocker p21cip1, and the associated suppression of CD4+ T cell proliferation. The branched-chain carboxylic acids tested were ineffective in all the assays. These results underline the relationship between the ability of a carboxylic acid to inhibit histone deacetylation, and their ability to block T cell proliferation, and suggests that branching inhibits these effects.

Modulation of p53 expression and its role in the conversion to a fully immortalized chicken embryo fibroblast line

We have established a spontaneously immortalized chicken embryo fibroblast (CEF) cell line (SC-1) that has been in continuous culture for more than three years. This is only the second report of a spontaneously immortalized reverse transcriptase (RT)-negative chicken cell line. The SC-1 cells emerged from crisis (at about passage 29-31) with a slower growth rate than primary cells. Passage 50 SC-1 cells expressed similar levels of p53 mRNA, but slightly lower levels of p53 protein than passage 6 CEF cells. By passage 120, p53 mRNA levels were significantly decreased in the SC-1 cells, while protein levels were slightly increased compared to passage 6 CEF cells. However, functional analysis of p53 revealed reduced activity in later passage SC-1 cells. Other p53-related genes including p21WAF1, p27Kip1, MDM-2, and the p16INK4a alternate reading frame (ARF) sequence showed similar patterns of differential mRNA expression. Levels of p15INK4b mRNA and protein were dramatically decreased in SC-1 cells, suggesting that the Rb pathway also has been compromised. Telomerase expression was undetectable in SC-1 cells. Fluorescence-activated cell sorting analysis showed that SC-1 and primary cells contained a similar proportion of G0/G1 phase cells, unlike the only other spontaneously immortalized chicken cell line (DF-1). The present study suggests that alterations in the p53 and Rb pathways cause fluctuations in expression levels of important cell-cycle regulatory genes during crucial transition periods as the SC-1 spontaneously immortalized chicken fibroblast cells progress toward becoming a fully committed cell line.

A mouse model for cyclin E-dependent genetic instability and tumorigenesis

Ubiquitination of murine cyclin E is triggered by phosphorylation on threonine 393. Cyclin E(T393A) knockin mice exhibited increased cyclin E stability, but no phenotypic abnormalities. Importantly, loss of the p53 pathway exacerbated the effect of the T393A mutation. Thus, in p21(-/-) cells the T393A mutation had an exaggerated effect on cyclin E abundance and its associated kinase activity, which caused abnormal cell cycle progression, and genetic instability involving chromosome breaks and translocations. Moreover, cyclin E(T393A) acted synergistically with p53 deficiency to accelerate tumorigenesis in cyclin E(T393A) p53(-/-) mice; Ras more readily transformed cyclin E(T393A) p53(-/-) cells than p53(-/-) cells in vitro; and cyclin E(T393A) mice had a greatly increased susceptibility to Ras-induced lung cancer.

T cell receptor induced intracellular redistribution of type I protein kinase A

The productive activation of CD4(+) T lymphocytes, leading to proliferation and cytokine secretion, requires precise temporal regulation of intracellular cyclic AMP concentrations. The major effector molecule activated by cyclic AMP in mammalian cells is the cyclic AMP-dependent protein kinase A (PKA). The type I PKA isozyme mediates the inhibitory effects of cyclic AMP on T-cell activation. Using laser scanning confocal microscopy, we demonstrated that the regulation of PKA type I activity involves spatial redistribution of PKA type I molecules following T-cell receptor (TCR) stimulation. In resting T cells, PKA type I was located in membrane proximal regions and distributed equally across the cell. Shortly after antigen engagement, T cells and antigen-presenting cells formed an area of intense contact, known as the immunological synapse. TCR concentrated at the synapse, whereas PKA type I molecules redistributed to the opposite cell pole within 10 min after T-cell stimulation. Type I PKA redistribution was solely dependent on TCR signalling, because we observed the same temporal and spatial distribution after antibody-mediated cross-linking of the TCR-associated CD3 complex. Segregation of TCR and PKA type I molecules was maintained for at least 20 min. Thirty minutes after stimulation, PKA type I partially colocalized with the TCR. After 60 min, PKA type I distribution again approached the resting state. Considering that initial TCR signals lead to increases in intracellular cyclic AMP, PKA type I molecules may be targeted towards localized cyclic AMP accumulations or transported away from these areas, depending on the requirements of the cellular response.

The proliferation inhibitory proteins p27Kip1 and retinoblastoma are involved in the control of equine lymphocyte proliferation

Observations in early equine pregnancy clearly reveal maternal immune recognition of and response to the presence of the conceptus. Nevertheless, both maternal cellular and humoral responses appear ineffective in destroying the developing placenta and fetus in early pregnancy. Our previous studies had shown that the pre-conditioned medium generated from the culture of equine invasive trophoblast inhibited mitogen-induced lymphocyte proliferation and the expression of cytokine messenger RNA in vitro. Those findings also suggested that lymphocytes might have been halted in the G0/G1 phase of the cell cycle. To characterize the cell cycle and the intracellular mechanisms involved in the inhibition of lymphocyte proliferation, equine peripheral blood lymphocytes were cultured in the presence or absence of pokeweed mitogen (PWM) in fresh medium, or in medium pre-conditioned through cell culture of invasive trophoblast cells or fetal fibroblasts. Two-color flow cytometric analysis for bromodeoxyuridine (BrdU) incorporation by stimulated lymphocytes, and concomitant DNA staining with 7-amino-actinomycin D (7-AAD), indicated that a greater proportion of lymphocytes were found in the G0/G1 phase of the cell cycle when cultured in the invasive trophoblast cell pre-conditioned medium compared to controls. Analysis using carboxyfluorescein diacetate succinimidyl ester (CFSE) fluorescence intensity demonstrated that lymphocytes cultured in the presence of invasive trophoblast cell pre-conditioned medium had fewer cells going through division, but that those fewer cells sustained similar numbers of cell divisions as in control cultures. Hypophosphorylated retinoblastoma (Rb) protein expression was increased and p27Kip1 expression was maintained at higher levels in lymphocytes cultured in invasive trophoblast pre-conditioned medium compared to fresh medium. In agreement with these data, flow cytometric measurement of the Ki-67 protein expression in lymphocytes cultured in invasive trophoblast pre-conditioned medium was lower in comparison to controls. These findings suggest that the equine lymphocyte proliferation is at least partially regulated by the expression of proliferation inhibitory proteins such as p27Kip1 and hypophosphorylated Rb. These proteins seem to be important regulators of cell cycle transition between G1 and S phase in equine lymphocytes.

Identification of multiple cell cycle regulatory functions of p57Kip2 in human T lymphocytes

The specific functions of p57(Kip2) in lymphocytes have not yet been fully elucidated. In this study, it is shown that p57(Kip2), which is a member of the Cip/Kip family of cyclin-dependent kinase inhibitors, is present in the nuclei of normal resting (G(0)) T cells from peripheral blood and in the nuclei of the T cell-derived Jurkat cell line. Activation through the TCR results in rapid transport of cytoplasmic cyclin-dependent kinase 6 (cdk6) to nuclei, where it associates with cyclin D and p57(Kip2) in active enzyme complexes. Using purified recombinant proteins, it was shown in vitro that addition of p57(Kip2) protein to a mixture of cyclin D2 and cdk6 enhanced the association of the latter two proteins and resulted in phosphorylation of p57(Kip2). To probe further the function of p57(Kip2), Jurkat cells stably transfected with a plasmid encoding p57(Kip2) under control of an inducible (tetracycline) promoter were made. Induction of p57(Kip2) resulted in increased association of cdk6 with cyclin D3, without receptor-mediated T cell stimulation. The overall amounts of cdk6 and cyclin D3, and also of cdk4 and cyclin E, remained unchanged. Most notably, increased p57(Kip2) levels resulted in marked inhibition of both cyclin E- and cyclin A-associated cdk2 kinase activities and a decrease in cyclin A amounts. Therefore, although facilitating activation of cdk6, the ultimate outcome of p57(Kip2) induction was a decrease in DNA synthesis and cell proliferation. The results indicate that p57(Kip2) is involved in the regulation of several aspects of the T cell cycle.

HCV core/gC1qR interaction arrests T cell cycle progression through stabilization of the cell cycle inhibitor p27Kip1

Hepatitis C virus (HCV) is efficient in the establishment of persistent infection. We have previously shown that HCV core protein inhibits T cell proliferation through its interaction with the complement receptor, gC1qR. Here we show that HCV core-induced inhibition of T cell proliferation involves a G(0)/G(1) cell cycle arrest, which is reversible upon addition of anti-gC1qR antibody. Correspondingly, the expression of cyclin-dependent kinases (Cdk) 2/4 and cyclin E/D, as well as subsequent phosphorylation of retinoblastoma (pRb), is reduced in core-treated T cells in response to mitogenic stimulation. Remarkably, degradation of p27(Kip1), a negative regulator of both Cdk4/cyclin D and Cdk2/cyclin E complexes, is significantly diminished in T cells treated with HCV core upon mitogenic stimulation. These data indicate that the stability of p27(Kip1) by HCV core is associated with blocking activated T cells for the G(1) to S phase transition and inhibiting T cell proliferation.

Deregulation of p27 by oncogenic signaling and its prognostic significance in breast cancer

p27 is a key regulator of progression from G1 to S phase. Although the gene encoding p27 is rarely mutated in human cancers, p27 is functionally inactivated in a majority of human cancers through accelerated p27 proteolysis, through sequestration by cyclin D-cyclin-dependent kinase complexes and by cytoplasmic mislocalization. Here we review mechanisms whereby oncogenic activation of receptor tyrosine kinase and Ras pathways lead to accelerated p27 proteolysis and p27 mislocalization in cancer cells. The prognostic significance of p27 in human breast cancer is also reviewed.

CDK2 is involved in the S-phase lengthening induced by glucocorticoids in normal human lymphocytes

Involvement of CDK2 in glucocorticoid-mediated S-phase lengthening was analyzed in this work. Dexamethasone (DXM) treatment of PHA-stimulated lymphocytes induced a decrease in CDK2 mRNA expression without any change in mRNA stability. This glucocorticoid-induced decrease in CDK2 mRNA expression could be suppressed by cycloheximide treatment. These results support the hypothesis of an indirect effect of DXM at the transcriptional level. Furthermore, CDK2 protein expression also decreased while the rate of protein synthesis and stability remained unchanged, suggesting a second post-translational level of regulation with the preferential degradation of a CDK2 protein stock fraction. The analysis of co-precipitated proteins showed that glucocorticoids induced modifications of protein complex composition. We found i) a preservation of the linkage capability of CDK2 for cyclin E and A, ii) a relative increase in p27kip1 linkage, and iii) a decrease in p21waf1 complexed with CDK2. As a consequence of CDK2 decrease and modifications of protein complex composition, pRb and histone H1 kinase activity of CDK2 was profoundly decreased. All these results pinpoint the role of CDK2 in glucocorticoid-induced S-phase lengthening and the potential activator role of p21waf1 for CDK2 in human lymphocytes.

P27kip1 regulates the cell cycle arrest and survival of activated T lymphocytes in response to interleukin-2 withdrawal

The majority of activated T lymphocytes undergo cell death at the end of a primary immune response, while a minority survive as memory cells. The mechanisms that control the decision between these two fates are unknown. In the present study we examined the response of activated T cells to interleukin-2 (IL-2) withdrawal. Within hours, the percentage of T lymphocytes in cell cycle showed a steady decrease, while the percentage arrested in G1 increased proportionally. Deprivation of IL-2 resulted in upregulation of the cell cycle inhibitor p27kip1. Comparison with resting T-cell populations revealed that the highest expression of p27kip1 occurs in activated T cells undergoing cell cycle arrest following IL-2 withdrawal. T cells deficient in p27kip1 expression showed an impaired ability to undergo cell cycle arrest in response to IL-2 deprivation. Moreover, T cells deficient in p27kip1 showed significantly more apoptosis after IL-2 withdrawal. Collectively, this study demonstrates that p27kip1 regulates both the cell cycle arrest and the apoptosis of antigen-specific T lymphocytes.

Down-modulation of responses to type I IFN upon T cell activation

The immunomodulatory role of type I IFNs (IFN-alpha/beta) in shaping T cell responses has been demonstrated, but the direct effects of IFN on T cells are still poorly characterized. Particularly, because IFN exert an antiproliferative activity, it remains elusive how the clonal expansion of effector T cells can paradoxically occur in the event of an infection when large amounts of IFN are produced. To address this issue, we have studied the effects of type I IFN in an in vitro differentiation model of human primary CD4(+) T cells. We found that IFN-alpha treatment of resting naive T cells delayed their entry into the cell cycle after TCR triggering. Conversely, the ongoing expansion of effector T cells was not inhibited by the presence of IFN. Moreover, activated T cells showed a significantly reduced induction of IFN-sensitive genes, as compared with naive precursors, and this decline occurred independently of subset-specific polarization. The residual type I IFN response measured in activated T cells was found sufficient to inhibit replication of the vesicular stomatitis virus. Our data suggest that the activation of T lymphocytes includes regulatory processes that restrain the transcriptional response to IFN and allow the proliferation of effector cells in the presence of this cytokine.

Loss of the cell cycle inhibitors p21(Cip1) and p27(Kip1) enhances tumorigenesis in knockout mouse models

Events that contribute to tumor formation include mutations in the ras gene and loss or inactivation of cell cycle inhibitors such as p21(Cip1) and p27(Kip1). In our previous publication, we showed that mice expressing the MMTV/v-Ha-ras transgene developed tumors earlier and at higher multiplicities in the absence than in the presence of p21(Cip1). To further evaluate the combinatorial role of genetic alterations and loss of cell cycle inhibitors in tumorigenesis, we performed two companion studies. In the first study, wild type and p21(Cip1)-null mice were exposed to the chemical carcinogen, urethane. Similar to its effects in v-Ha-ras mice, loss of p21(Cip1) accelerated tumor onset and increased tumor multiplicity in urethane-treated mice. Lung tumors were the predominant tumor type in urethane-treated mice regardless of p21(Cip1) status. In the second study, tumor formation was monitored in v-Ha-ras mice expressing or lacking p27(Kip1). Unlike p21(Cip1), the absence of p27(Kip1) had no effect on the timing or multiplicity of tumor formation, which was largely restricted to mammary and salivary glands. However, once tumors appeared, they grew faster in p27(Kip1)-null mice than in p27(Kip1)-wild type mice. Increases in growth rate were particularly striking for salivary tumors in ras/p27(-/-) mice. Loss of p21(Cip1), on the other hand, had no effect on tumor growth rate in v-Ha-ras mice. Collectively, our data suggest that p21(Cip1) suppresses tumor formation elicited by multiple agents and that p21(Cip1) and p27(Kip1) suppress tumor formation in different ways.

Antigen-specific dose-dependent system for the study of an inheritable and reversible phenotype in mouse CD4+ T cells

The transgenic T-cell receptor in mouse TEa CD4+ lymphocytes recognizes an endogenous peptide, Ealpha52-68, presented in the context of the major histocompatibility complex class II molecule I-Ab. In response to an optimal peptide concentration TEa cells enter the cell cycle and proliferate. However, a single exposure to high doses of the specific peptide diminished cell expansion upon subsequent restimulation. This hyporesponsive, or anergic, phenotype can still be detected after multiple restimulations indicating that the hyporesponsiveness persists despite cell division and it was inherited by daughter cells. Furthermore, we demonstrated that this hypoproliferative response is associated with high p27Kip1 and cyclin E protein levels, and reduced intracellular interleukin-2 (IL-2) expression. Addition of exogenous IL-2 was required to reset p27Kip1 levels in the progeny derived from hyporesponsive TEa cells. Thus, we have established antigen dose-dependent induction of a reversible, inheritable (i.e. epigenetic) phenotype and we have identified at least three components of the network of interactions: p27Kip1 cyclin E, and IL-2 expression.

IL-12 Provides Proliferation and Survival Signals to Murine CD4+T Cells Through Phosphatidylinositol 3-Kinase/Akt Signaling Pathway

IL-12 is a pleiotropic cytokine that plays an important role in innate and adaptive immunity. IL-12 induces T cell proliferation and IFN-gamma secretion from activated T cells. It was also reported that IL-12 prevents apoptosis of CD4(+) T cells. However, the signaling mechanism that regulates these IL-12-induced responses is poorly understood yet. In this study, we demonstrated that IL-12 activates phosphatidylinositol 3-kinase (PI3K)/Akt pathway in murine CD4(+) T cells, and that this signaling pathway is required for IL-12-induced T cell proliferation and antiapoptotic function, but not for IFN-gamma induction. Through PI3K/Akt pathway, IL-12 up-regulates the expression of cell cycle-related molecule such as cyclin D3, and antiapoptotic molecules such as Bcl-2 and cellular inhibitors of apoptosis proteins-2, followed by down-regulation of active caspase-3. These results suggest that PI3K/Akt pathway is critical for mediating IL-12-induced CD4(+) T cell responses such as T cell proliferation and survival.

Mycophenolic acid inhibits IL-2-dependent T cell proliferation, but not IL-2-dependent survival and sensitization to apoptosis

Mycophenolic acid (MPA), the active metabolite of the immunosuppressive drug mycophenolate mofetil, is a selective inhibitor of inosine 5'-monophosphate dehydrogenase type II, a de novo purine nucleotide synthesis enzyme expressed in T and B lymphocytes and up-regulated upon cell activation. In this study, we report that the blockade of guanosine nucleotide synthesis by MPA inhibits mitogen-induced proliferation of PBL, an effect fully reversed by addition of guanosine and shared with mizoribine, another inhibitor of inosine 5'-monophosphate dehydrogenase. Because MPA does not inhibit early TCR-mediated activation events, such as CD25 expression and IL-2 synthesis, we investigated how it interferes with cytokine-dependent proliferation and survival. In activated lymphoblasts that are dependent on IL-2 or IL-15 for their proliferation, MPA does not impair signaling events such as of the extracellular signal-regulated kinase 2 and Stat5 phosphorylation, but inhibits down-regulation of the cyclin-dependent kinase inhibitor p27(Kip1). Therefore, in activated lymphoblasts, MPA specifically interferes with cytokine-dependent signals that control cell cycle and blocks activated T cells in the mid-G(1) phase of the cell cycle. Although it blocks IL-2-mediated proliferation, MPA does not inhibit cell survival and Bcl-x(L) up-regulation by IL-2 or other cytokines whose receptors share the common gamma-chain (CD132). Finally, MPA does not interfere with IL-2-dependent acquisition of susceptibility to CD95-mediated apoptosis and degradation of cellular FLIP. Therefore, MPA has unique functional properties not shared by other immunosuppressive drugs interfering with IL-2R signaling events such as rapamycin and CD25 mAbs.

A serum factor after intestinal resection stimulates epidermal growth factor receptor signaling and proliferation in intestinal epithelial cells

BACKGROUND

In vivo, intestinal adaptation after massive small bowel resection (SBR) requires a functional epidermal growth factor (EGF) receptor (EGFR). In vitro studies have shown that serum from mice after SBR induces rat intestinal epithelial cells to proliferate. This study tested the hypothesis that the proliferative response to SBR serum is mediated by EGFR signaling.

METHODS

Serum was collected from male Sprague-Dawley rats 7 days after 75% SBR or sham operation. Rat intestinal epithelial cells were incubated in the presence of sham or SBR serum. Total EGFR expression and phosphorylation of several EGFR downstream pathways were determined by Western blotting. In other experiments, a specific EGFR inhibitor (ZD1839) was added and cell growth determined over 5 days.

RESULTS

SBR serum significantly increased total EGFR expression (3-fold) over sham operation and consistently activated the phosphatidylinositol 3-kinase pathway. Furthermore, SBR serum markedly augmented rat intestinal epithelial cell growth, an effect that was abolished by EGFR inhibition.

CONCLUSIONS

SBR serum contains a factor or factors that stimulates proliferation of intestinal epithelial cells by an EGFR and phosphatidylinositol 3-kinase signaling mechanism. These data recapitulate in vivo studies supporting the hypothesis that EGFR is a central mediator of postresection intestinal adaptation. This in vitro model may provide a novel means to gain insight into the pathophysiology of intestinal adaptation.

CDKs and CKIs: molecular targets for tissue remodelling

Effective tissue remodelling is essential to the survival of adult organs. Many of the signalling pathways that control these cellular decisions are regulated by nuclear interactions of cell-cycle proteins. Molecules that target cyclin-dependent kinases (CDKs) or CDK inhibitors (CKIs) represent a new class of therapeutic agents that influence tissue remodelling in several organ systems. An understanding of their cell-specific functions is leading to the development of exciting and bold approaches to the treatment cancer, cardiovascular disease and other diseases.

The homeostasis but not the differentiation of T cells is regulated by p27(Kip1)

The cyclin-dependent kinase inhibitor p27(Kip1) is a critical regulator of T cell proliferation. To further examine the relationship of T cell proliferation and differentiation, we examined the ability of T cells deficient in p27(Kip1) to differentiate into Th subsets. We observed increased Th2 differentiation in p27(Kip1)-deficient cultures. In addition to increases in CD4(+) and CD8(+) T cells, there is a similar increase in gamma delta T cells in p27(Kip1)-deficient mice compared with wild-type mice. The increase in Th2 differentiation is correlated to an increase of IL-4 secretion by CD4(+)DX5(+)TCR alpha beta(+)CD62L(low) T cells but not to increased expansion of differentiating Th2 cells. While STAT4- and STAT6-deficient T cells have diminished proliferative responses to IL-12 and IL-4, respectively, proliferative responses are increased in T cells doubly deficient in p27(Kip1) and STAT4 or STAT6. In contrast, the increased proliferation and differentiative capacity of p27(Kip1)-deficient T cells has no effect on the ability of STAT4/p27(Kip1)- or STAT6/p27(Kip1)-deficient CD4(+) cells to differentiate into Th1 or Th2 cells, respectively. Thus, while p27(Kip1) regulates the expansion and homeostasis of several T cell subsets, it does not affect the differentiation of Th subsets.

Paradigms of growth control: relation to Cdk activation

The cyclin-dependent kinases (CDKs) play a key role in cell cycle control, and in this review, we focus on the events that regulate their activities. Emphasis is placed on the CDKs that function during the G(1) phase of the cell cycle and on the CDK inhibitor p27(Kip1). We discuss how CDK activation relates to two basic concepts of cell cycle regulation: (i) the need for multiple mitogens for the proliferation of nontransformed cells and (ii) the inhibitory effect of high culture density on proliferative capacity. We also describe how Cdk2 modulates the expression of the alpha subunit of the interleukin-2 receptor in T cells, and address the question of whether p27(Kip1) functions as an activator or inhibitor of the CDKs associated with the D cyclins.

Convergence of mitogenic and DNA damage signaling in the G1 phase of the cell cycle

Research into the molecular basis of cancer has a central tenet. Cancer arises from genetic alterations that disconnect growth and differentiation signaling pathways from the machinery that regulates cellular proliferation. In multi-cellular eukaryotes, proliferation is regulated by external signals, such as the availability of growth factors and nutrients and by internal signals, such as those sensing cellular integrity. Cellular stress created either by lack of mitogens or damage to cellular components, such as DNA, stimulates responses that enforce temporal or permanent withdrawal from the cell cycle. Although these stress responses stem from different sources and activate distinct pathways, they converge on the same components of the cell cycle machinery in the G1 phase of the cell cycle. This review will highlight and compare aspects of the G1 arrest in response to stress generated either by lack of mitogens or damage to DNA.

High levels of the onco-protein Gfi-1 accelerate T-cell proliferation and inhibit activation induced T-cell death in Jurkat T-cells

Gfi-1 is a nuclear zinc finger protein with the activity of a transcriptional repressor and the ability to predispose for the development of T-cell lymphoma when expressed constitutively at high levels. Whereas thymic T-cell precursors express endogenous Gfi-1, mature peripheral T-cells lack Gfi-1 but upregulate its expression transiently after antigenic stimulation and activation of Erk1/2 demonstrating a role of Gfi-1 in T-cell activation. Here we show that constitutive expression of Gfi-1 accelerates S phase entry of primary, resting T-cells upon antigenic stimulation. In addition, high level Gfi-1 expression inhibits phorbol ester induced G1 arrest and activation induced cell death in Jurkat T-cells. We demonstrate that these effects of Gfi-1 concur with lower absolute levels and hyperphosphorylation of the pocket protein pRb. Moreover, phorbol ester induced expression of the negative cell cycle regulator p21(WAF1) is blocked in the presence of Gfi-1. These findings suggest that Gfi-1 contributes to T-cell lymphomagenesis by overriding a late G1 cell cycle checkpoint which controls activation induced death and S phase entry of T-cells.

Regulation of RNA polymerase II activity by CTD phosphorylation and cell cycle control

The carboxyl-terminal domain (CTD) of the largest subunit of mammalian RNA polymerase II (RNAP II) consists of 52 repeats of a consensus heptapeptide and is subject to phosphorylation and dephosphorylation events during each round of transcription. RNAP II activity is regulated during the cell cycle and cell cycle-dependend changes in RNAP II activity correlate well with CTD phosphorylation. In addition, global changes in the CTD phosphorylation status are observed in response to mitogenic or cytostatic signals such as growth factors, mitogens and DNA-damaging agents. Several CTD kinases are members of the cyclin-dependent kinase (CDK) superfamily and associate with transcription initiation complexes. Other CTD kinases implicated in cell cycle regulation include the mitogen-activated protein kinases ERK-1/2 and the c-Abl tyrosine kinase. These observations suggest that reversible RNAP II CTD phosphorylation may play a key role in linking cell cycle regulatory events to coordinated changes in transcription.

Rapamycin blocks IL-2-driven T cell cycle progression while preserving T cell survival

Effective cellular immune responses require increases in antigen-specific T lymphocytes; IL-2 drives antigen-stimulated T cell proliferation and is largely responsible for the increases observed. We used microarrays containing approximately 9000 mouse cDNAs to study IL-2-induced gene expression. IL-2 induces the expression of genes that regulate cell cycle progression, control cell survival, and increase synthetic and metabolic processes during proliferation. IL-2 also suppresses expression of genes that block cell cycle progression and promote cell death. Rapamycin inhibits IL-2-driven proliferation by downregulating the expression of genes required for key processes required for cell cycle progression. Rapamycin also preserves cell survival by keeping intact the IL-2-induced cell survival programs. These complex multifaceted programs of gene expression permit a dynamic regulation of cellular proliferation and cellular survival.

Interferon-alpha inhibits Stat5 DNA-binding in IL-2 stimulated primary T-lymphocytes

It has previously been shown that IFN-alpha is a potent inhibitor of IL-2 induced proliferation in primary T-lymphocytes, by selectively abrogating the downstream effects of IL-2 on the core cell cycle machinery regulating the G1/S transition. Theoretically this could be mediated through cross-talk between the signalling cascades activated by these cytokines, as several signalling components are known to be shared. IL-2 activates multiple signalling pathways that are important for T-cell proliferation and differentiation. In the present study, the effects of IFN-alpha on IL-2 signal transduction was investigated. The IFN-alpha induced inhibition of IL-2 induced proliferation in activated T-lymphocytes, was associated with a suppressed Jak3 protein expression as well as an inhibited prolonged Stat5 DNA binding, and a partially reduced expression of the Stat5 inducible gene IL-2R alpha. Our results provide a possible molecular link between the prominent antiproliferative effects of IFN-alpha on IL-2 induced T-cell proliferation and the signal transduction pathways emerging from the IL-2 receptor.

Effects of dominant-negative c-Jun on platelet-derived growth factor-induced vascular smooth muscle cell proliferation

Although platelet-derived growth factor (PDGF)-BB is thought to participate in vascular disorders, the mechanism of PDGF-induced vascular smooth muscle cell (SMC) proliferation is not fully understood. This study was undertaken to examine the role of c-Jun in PDGF-BB-induced proliferation of rat aortic SMCs. PDGF-BB (10 ng/mL) significantly increased activator protein (AP)-1 DNA binding activity in SMCs, followed by the increase in [(3)H]thymidine incorporation and cell number. SMCs were infected with recombinant adenovirus containing TAM67, a dominant-negative c-Jun lacking the transactivation domain of wild c-Jun (Ad-DN-c-Jun), to inhibit endogenous AP-1. Ad-DN-c-Jun, which specifically blocked AP-1 transcriptional activity, significantly inhibited PDGF-BB-induced increases in [(3)H]thymidine incorporation or cell number. As shown by flow cytometric analysis, Ad-DN-c-Jun inhibited PDGF-BB-induced entrance of SMCs into S phase, leading to a G(1) arrest. Ad-DN-c-Jun attenuated PDGF-BB-induced downregulation of p27(Kip1), as shown by Western blot analysis, and the prevented PDGF-BB-induced decrease in cyclin E/cyclin-dependent kinase 2 complex-associated p27(Kip1), as shown by immunoprecipitation study. Furthermore, protein kinase assay showed that Ad-DN-c-Jun blocked PDGF-BB-induced activation of cyclin-dependent kinase 2. Our results provide the first evidence that dominant-negative c-Jun inhibits PDGF-BB-induced vascular SMC proliferation by preventing the downregulation of p27(Kip1), thereby supporting the important role of c-Jun in vascular SMC proliferation.

Regulation of p27KIP1 in Epstein-Barr virus-immortalized lymphoblastoid cell lines involves non-apoptotic caspase cleavage

The cyclin-dependent kinase inhibitor p27KIP1 plays a key role in controlling cell proliferation. Here we show that p27KIP1 is commonly down-regulated in B-cells immortalized by Epstein-Barr virus (EBV) (lymphoblastoid cell lines, LCLs). The significance of this event for the immortal phenotype of LCLs is implied by a requirement for active cdk2-containing complexes for continued proliferation, and by the ability of the residual p27KIP1 to associate with cdk2. The mechanism of p27KIP1 attenuation is post-translational, but inhibitor studies reveal that the mechanism does not rely heavily on the proteasome. Instead we find that LCLs contain an activity that cleaves a caspase recognition site present in p27KIP1 (DPSD139). The activity is not associated with apoptosis and closely resembles a proliferation-associated caspase activity we previously described in the EBV-negative B-lymphoma-derived cell line BJAB. Importantly, proliferating LCLs contain a p27KIP1 product that is consistent with cleavage at this site. Inhibition of caspase(s) in vivo modulates p27KIP1 expression and strongly inhibits proliferation of IB4 cells. This inhibitor profile is identical to that displayed by the DPSD-directed caspase present in BJAB cells, suggesting that the caspase may fulfil a general role in controlling p27KIP1 expression in immortal lymphoid cell lines. Thus, apoptosis-independent cleavage appears to contribute to the maintenance of the low basal levels of p27KIP1 in B-cells immortalized by EBV.