Cell Cycling through Cdc25A: Transducer of Cytokine Proliferative Signals

Iron overload induces G1 phase arrest and autophagy in murine preosteoblast cells

This study aimed to investigate the cell cycle arrest and autophagy induced by iron overload in MC3T3-E1 cells. MC3T3-E1 cells were cultured in different concentrations of ferric ammonium citrate (FAC), and Perls' Prussian blue reaction was used to detect the iron levels of the cells. CCK-8 assays were used to detect the growth of MC3T3-E1. The level of reactive oxygen species (ROS) within cells was investigated with DCFH-DA. PI staining was used to analyze the cell cycle distribution of MC3T3-E1 cells. Finally, the expression levels of cell cycle related proteins, autophagy related proteins, AKT, p38 MAPK, Stat3, and their downstream proteins were detected with Western blot assays. The results showed that the iron levels of MC3T3-E1 cells increased with increasing concentrations of FAC. High levels of ferric ion inhibited proliferation of MC3T3-E1 cells and increased their ROS levels. Additionally, iron overload induced G1arrest in MC3T3-E1 cells and down-regulated the expression of Cyclin D₁ , Cyclin D₃ , CDK2, CDK4 and CDK6, but up-regulated p27 Kip1. In addition, the expression levels of Beclin-1 and LC3 II increased, but that of p62 decreased. Further experiments showed that the phosphorylation of AKT and its downstream proteins p-GSK-3β(Ser9) and p-mTOR (Ser2448) were decreased. The levels of p-p38 and p53 were up-regulated while those of cdc25A and p-ERK 1/2 were down-regulated. Phosphorylation of Stat3 and its downstream proteins was all decreased. These results show that iron overload generates ROS, blocks the PI3K/AKT and Jak/Stat3 signal pathways, and activates p38 MAPK, subsequently inducing G1 arrest and autophagy in MC3T3-E1 cells.

Imatinib mesylate inhibits STAT5 phosphorylation in response to IL-7 and promotes T cell lymphopenia in chronic myelogenous leukemia patients

Imatinib mesylate (IM) therapy has been shown to induce lower T cell counts in chronic myelogenous leukemia (CML) patients and an interference of IM with T cell receptor (TCR) signaling has been invoked to explain this observation. However, IL-7 and TCR signaling are both essential for lymphocyte survival. This study was undertaken to determine whether IM interferes with IL-7 or TCR signaling to explain lower T cell counts in patients. At diagnosis, CML patients have typically lower CD4⁺ counts in their blood, yet CD8⁺ counts are normal or even increased in some. Following the initiation of IM treatment, CD4⁺ counts were further diminished and CD8⁺ T lymphocytes were dramatically decreased. In vitro studies confirmed IM interference with TCR signaling through the inhibition of ERK phosphorylation and we showed a similar effect on IL-7 signaling and STAT5 phosphorylation (STAT5-p). Importantly however, using an in vivo mouse model, we demonstrated that IM impaired T cell survival through the inhibition of IL-7 and STAT5-p but not TCR signaling which remained unaffected during IM therapy. Thus, off-target inhibitory effects of IM on IL-7 and STAT5-p explain how T cell lymphopenia occurs in patients treated with IM.

CyclinD-CDK4/6 complexes phosphorylate CDC25A and regulate its stability

The phosphatase CDC25A is a key regulator of cell cycle progression by dephosphorylating and activating cyclin-CDK complexes. CDC25A is an unstable protein expressed from G1 until mitosis. CDC25A overexpression, which can be caused by stabilization of the protein, accelerates the G1/S and G2/M transitions, leading to genomic instability and promoting tumorigenesis. Thus, controlling CDC25A protein levels by regulating its stability is a critical mechanism for timing cell cycle progression and to maintain genomic integrity. Herein, we show that CDC25A is phosphorylated on Ser40 throughout the cell cycle and that this phosphorylation is established during the progression from G1 to S phase. We demonstrate that CyclinD-CDK4/CDK6 complexes mediate the phosphorylation of CDC25A on Ser40 during G1 and that these complexes directly phosphorylate this residue in vitro. Importantly, we also find that CyclinD1-CDK4 decreases CDC25A stability in a ßTrCP-dependent manner and that Ser40 and Ser88 phosphorylations contribute to this regulation. Thus our results identify cyclinD-CDK4/6 complexes as novel regulators of CDC25A stability during G1 phase, generating a negative feedback loop allowing control of the G1/S transition.

PD-1 inhibits T cell proliferation by upregulating p27 and p15 and suppressing Cdc25A

The programmed cell death-1 (PD)-1 receptor (CD279) is a potent T cell inhibitor with a critical role in peripheral tolerance, but it can also compromise anti-viral and antitumor T cell responses. The effects of PD-1 on the cell cycle leading to inhibition of T cell expansion are poorly understood. Recently, we examined the effects of PD-1 on the molecular control of the cell cycle machinery and on TCR-activated signaling pathways that regulate these downstream outcomes. Our studies showed that PD-1 blocks cell cycle progression in the G 1 phase. PD-1 did not alter the expression of G 1 phase cyclins or cyclin-dependent kinases (Cdks) but, instead, suppressed the transcription of SKP2, the substrate recognition component of the SCF (Skp2) ubiquitin ligase that leads p27 (kip1) to degradation and resulted in accumulation of p27 (kip1) . Subsequently, T cells receiving PD-1 signals displayed impaired Cdk2 activation and failed to phosphorylate two critical Cdk2 substrates, the retinoblastoma gene product (Rb) and the TGFβ-specific transcription factor Smad3, leading to suppression of E2F target genes but enhanced Smad3 transactivation. These events resulted in upregulation of the Cdk4/6 inhibitor p15 (INK4B) and repression of the Cdk-activating phosphatase Cdc25A. The suppressive effect of PD-1 on Skp2 expression was mediated by inhibition of both PI3K/Akt and Ras/MEK/Erk pathways and was only partially reversed by IL-2, which restored activation of MEK/Erk but not Akt. Thus, PD-1 targets Ras and PI3K/Akt signaling to inhibit transcription of Skp2 and to activate Smad3 as an integral component of a pathway that regulates blockade of cell cycle progression in T lymphocytes. Here, we discuss the detailed sequence of these signaling events and their implications in mediating cell-intrinsic and -extrinsic mechanisms that inhibit proliferation of T effector cells in response to PD-1-mediated signaling.

Cdc25A-driven proliferation regulates CD62L levels and lymphocyte movement in response to interleukin-7

OBJECTIVE

Interleukin-7 (IL-7) is a multifunctional cytokine and a promising immunotherapeutic agent. However, because transient T-cell depletion is an immediate outcome of IL-7 administration at supraphysiological doses, we investigated the mechanism by which the IL-7 proliferative signal transduced through Cdc25A, a key activator of cyclin-dependent kinases, could modulate lymphocyte movement.

MATERIALS AND METHODS

Employing novel methods of manipulating Cdc25A gene expression, combined with in vitro and in vivo evaluation of IL-7 application, we assessed the expression of activation and homing markers and identified the mechanism by which IL-7 could induce T-cell expansion and alter lymphocyte motility.

RESULTS

Constitutively active Cdc25A drove T-cell proliferation independently of IL-7 and resulted in an activated phenotype (CD69(hi), CD44(hi)). Conversely, inhibition of Cdc25A resulted in decreased proliferation, reduced expression of activation markers, and upregulation of the lymph node homing molecule, CD62L, which promoted cell adhesion when engaged by ligand. We found that IL-7 prevented the nuclear translocation of the transcription factor, Foxo1, in a manner dependent on the activity of Cdc25A, resulting in decreased levels of CD62L. In vivo administration of IL-7 decreased lymph node cellularity, while treatment with IL-7, premixed with a neutralizing IL-7 antibody (M25), increased total lymph node cells--with more nuclear Foxo1 detected in cells from mice receiving IL-7 + M25.

CONCLUSIONS

These results are consistent with the model that IL-7 drives Cdc25A-mediated T-cell proliferation, which prevents the nuclear translocation of Foxo1, leading to reduced expression of CD62L and the migration of T cells into circulation.

Comparison of AAV/IL-7 autocrine (T cell) versus paracrine (DC) gene delivery for enhancing CTL stimulation and function

Adoptive transfer of antigen-specific cytotoxic T lymphocyte (CTL) into patients holds promise in treating cancer. Such anti-cancer CTL are stimulated by professional antigen-presenting dendritic cells (DC). We hypothesize the gene delivery of various Th1-response cytokines, such as interleukin 7 (IL-7), should further enhance CTL stimulation and activity. However, the issue as to which cell type, DC (paracrine) or the T cell (autocrine), should express a particular Th1 cytokine gene for optimal CTL stimulation has never been addressed. We used adeno-associated virus-2 (AAV) to compare delivery of IL-7 and IL-2 genes into DC or T cells and to exogenous commercial cytokines for generating robust carcinoembryonic antigen (CEA)-specific CTL. AAV/IL-7 transduction of T cells (autocrine delivery) generated CTL with the highest killing capability. Consistent with this, AAV/IL-7 delivery generated T cell populations with the highest proliferation, highest interferon gamma expression, highest CD8(+):CD4(+) ratio, highest CD8(+), CD69(+) levels, and lowest CD4(+), CD25(+) (Treg) levels. These data are consistent with higher killing by the AAV/IL-7-altered CTL. These data strongly suggest that IL-7 autocrine gene delivery is optimal for CTL generation. These data also suggest Th1 cytokine autocrine versus paracrine delivery is an important issue for immuno-gene therapy and uncovers new questions into cytokine mechanism of action.

The interaction of LCK and the CD4 co-receptor alters the dose response of T-cells to interleukin-7

CD8 and CD4 T-cells grow optimally under different concentrations of the cytokine, interleukin-7 (IL-7). While CD8 T-cells expand at high doses of IL-7, CD4 T-cells favor low doses. To examine the reason for the preference of CD4 T-cells for lower doses of the cytokine, we used IL-7 dependent T-cells to study signal transduction upon a range of IL-7 concentrations. We found that the high dose responsiveness of CD8 T-cells to IL-7 could be altered if these cells also expressed CD4. Using the phosphorylation of STAT5 as an indicator of growth, we found that the co-receptor associated kinase, LCK, contributed to phospho-STAT5 levels. Phospho-STAT5 was elevated at high dose IL-7 for CD8 T-cells and at low dose IL-7 for CD4 T-cells, which was reversed upon LCK inhibition. Examining the direct association of LCK with CD4 using a T- cell line that over-expresses CD4, we determined that CD4 could directly sequester LCK. Non-CD4 T-cells were not restricted in this manner and levels of phospho-STAT5 increased proportionally to the IL-7 dose. Our studies, therefore, show that the response of a T-cell to IL-7 can be modulated by the availability of LCK.

Ex vivo expansion of memory CD8 T cells from lymph nodes or spleen through in vitro culture with interleukin-7

Interleukin-7 (IL-7) increases lymphocyte numbers, a critical feature of immune reconstitution, through mechanisms that are still poorly understood. Part of the problem is that IL-7 is produced in limited amounts by non-lymphoid cells, making in vivo studies of the cytokine's activity a challenge. To overcome this, we developed an in vitro system by which lymphocytes from secondary immune organs could be cultured to produce IL-7 responsive cells. Using this method, we showed that CD8(hi)CD44(hi) T cells accumulate in culture with IL-7 from a population of lymph node or splenic cells. These results were validated when a similar lymphocyte subset was found in mice expressing a constitutively active form of STAT5b, a key transducer of IL-7 signals. Interestingly, IL-7-expanded cells also up regulated the activation marker, CD69. The IL-7-derived CD44(hi)CD69(hi) cells were not generated from naïve cells, but expanded from an existing population, since culture in IL-7 of naïve lymphocytes from OT-1/Rag1(-/-) mice did not produce CD44(hi)CD69(hi) cells. Using the in vitro culture system to study lymphocytes from mice deficient in the apoptotic protein, BIM, we were able to attribute the expansion of CD8(hi)CD44(hi)CD69(hi) T cells to the proliferative and not survival activity of IL-7. The in vitro culture system provides an important new methodology to examine the activities of this essential as well as immunotherapeutic cytokine.

Short 42 degrees C heat shock induces phosphorylation and degradation of Cdc25A which depends on p38MAPK, Chk2 and 14.3.3

The effects of heat shock (HS; 42 degrees C) on the cell cycle and underlying molecular mechanisms are astonishingly unexplored. Here, we show that HS caused rapid Cdc25A degradation and a reduction of cell cycle progression. Cdc25A degradation depended on Ser75-Cdc25A phosphorylation caused by p38MAPK and Chk2, which phosphorylated Ser177-Cdc25A that is specific for 14.3.3 binding. Upon HS, Cdc25A rapidly co-localized with 14.3.3 in the perinuclear space that was accompanied with a decrease of nuclear Cdc25A protein levels. Consistently, a 14.3.3 binding-deficient Cdc25A double mutant (Ser177/Ala-Tyr507/Ala) was not degraded in response to HS and there was no evidence for an increased co-localization of Cdc25A with 14.3.3 in the cytosol. Therefore, upon HS, p38, Chk2 and 14.3.3 were antagonists of Cdc25A stability. On the other hand, Cdc25A was protected by Hsp90 in HEK293 cells because the specific inhibition of Hsp90 with Geldanamycin caused Cdc25A degradation in HEK293 implicating that Cdc25A is an Hsp90 client. Specific inhibition of Hsp90 together with HS caused and accelerated degradation of Cdc25A and was highly cytotoxic. The results presented here show for the first time that Cdc25A is degraded by moderate heat shock and protected by Hsp90. We describe the mechanisms explaining HS-induced cell cycle retardation and provide a rationale for a targeted hyperthermia cancer therapy.

Mitogen-activated protein (MAP) kinase/MAP kinase phosphatase regulation: roles in cell growth, death, and cancer

Mitogen-activated protein kinase dual-specificity phosphatase-1 (also called MKP-1, DUSP1, ERP, CL100, HVH1, PTPN10, and 3CH134) is a member of the threonine-tyrosine dual-specificity phosphatases, one of more than 100 protein tyrosine phosphatases. It was first identified approximately 20 years ago, and since that time extensive investigations into both mkp-1 mRNA and protein regulation and function in different cells, tissues, and organs have been conducted. However, no general review on the topic of MKP-1 exists. As the subject matter pertaining to MKP-1 encompasses many branches of the biomedical field, we focus on the role of this protein in cancer development and progression, highlighting the potential role of the mitogen-activated protein kinase (MAPK) family. Section II of this article elucidates the MAPK family cross-talk. Section III reviews the structure of the mkp-1 encoding gene, and the known mechanisms regulating the expression and activity of the protein. Section IV is an overview of the MAPK-specific dual-specificity phosphatases and their role in cancer. In sections V and VI, mkp-1 mRNA and protein are examined in relation to cancer biology, therapeutics, and clinical studies, including a discussion of the potential role of the MAPK family. We conclude by proposing an integrated scheme for MKP-1 and MAPK in cancer.

p38 pathway kinases as anti-inflammatory drug targets

Mitogen-activated protein kinases (MAPK) are intracellular signaling molecules involved in cytokine synthesis. Several classes of mammalian MAPK have been identified, including extracellular signal-regulated kinase, c-jun N-terminal kinase, and p38 MAP kinase. p38alpha is a key MAPK involved in tumor necrosis factor alpha and other cytokine production, as well as enzyme induction (cyclooxygenase-2, inducible nitric oxide synthase, and matrix metalloproteinases) and adhesion molecule expression. An understanding of the broad biologic and pathophysiological roles of p38 MAPK family members has grown significantly over the past decade, as has the complexity of the signaling network leading to their activation. Downstream substrates of MAPK include other kinases (e.g., mitogen-activated protein-kinase-activated protein kinase 2) and factors that regulate transcription, nuclear export, and mRNA stability and translation. The high-resolution crystal structure of p38alpha has led to the design of selective inhibitors that have good pharmacological activity. Despite the strong rationale for MAPK inhibitors in human disease, direct proof of concept in the clinic has yet to be demonstrated, with most compounds demonstrating dose-limiting adverse effects. The role of MAPK in inflammation makes them attractive targets for new therapies, and efforts are continuing to identify newer, more selective inhibitors for inflammatory diseases.

Interlinking interleukin-7

The signaling processes that maintain the homeostatic proliferation of peripheral T-cells and result in their self-renewal largely remain to be elucidated. Much focus has been placed on the anti-apoptotic function of the cytokine, interleukin-7 (IL-7), during T-cell development. But a more critical role has been ascribed to IL-7 as a mediator of peripheral T-cell maintenance. The biological effects responsive to IL-7 signaling are transduced through only a few well-known pathways. In this review we will focus on the signals transduced by IL-7 and similar cytokines, examining how proliferative signals originate from cytokine receptors, are amplified and eventually alter gene expression. In this regard we will highlight the crosstalk between pathways that promote survival, drive cell cycle progression and most importantly provide the needed energy to sustain these critical cellular activities. Though this review showcases much of what has been learned about IL-7 proliferative signaling, it also reveals the significant gaps in our knowledge about cytokine signaling in the very relevant context of peripheral T-cell homeostasis.

p38 MAP-kinases pathway regulation, function and role in human diseases

Mammalian p38 mitogen-activated protein kinases (MAPKs) are activated by a wide range of cellular stresses as well as in response to inflammatory cytokines. There are four members of the p38MAPK family (p38alpha, p38beta, p38gamma and p38delta) which are about 60% identical in their amino acid sequence but differ in their expression patterns, substrate specificities and sensitivities to chemical inhibitors such as SB203580. A large body of evidences indicates that p38MAPK activity is critical for normal immune and inflammatory response. The p38MAPK pathway is a key regulator of pro-inflammatory cytokines biosynthesis at the transcriptional and translational levels, which makes different components of this pathway potential targets for the treatment of autoimmune and inflammatory diseases. However, recent studies have shed light on the broad effect of p38MAPK activation in the control of many other aspects of the physiology of the cell, such as control of cell cycle or cytoskeleton remodelling. Here we focus on these emergent roles of p38MAPKs and their implication in different pathologies.

An ageing question: do embryonic stem cells protect their genomes?

Many physiological and cellular processes contribute to the ageing of individuals. One hypothesis argues that the genomes of somatic cells accumulate mutations, which, in turn, alter the metabolism of the cells and contribute to the ageing process. The frequency of somatic mutation approaches 10(-4) and the majority of mutagenic events at heterozygous loci is due to loss of heterozygosity as a consequence of mitotic recombination. A corollary to the argument that somatic cells accumulate mutations is that cells of the germ line and ES cells have a greater requirement for maintaining the integrity of their genomes. In the former case, a high somatic mutation frequency predicts an increase in somatic disease, which limits our lifespan. The corollary is that cells of the germline and ES cells must minimize the mutational burden to limit the frequency of congenital disease and to ensure the proper transmission of undamaged DNA to the gene pool. This report describes two mechanisms utilized by murine ES cells to minimize DNA damage within the proliferative pool. In the first case, murine ES cells display a frequency of mutation and mitotic recombination that is about 100-fold lower than that observed in somatic cells. Second, ES cells lack a G1 checkpoint following DNA damage. When subjected to ionizing radiation, the fraction of apoptotic cells increases to about 40%. Ectopic expression of Chk2 is sufficient to establish a G1 arrest and the concomitant protection from cell death.