Cell cycle-specific induction of Cdk2 expression in B lymphocytes following antigen receptor cross-linking

Cellular and Molecular Mechanisms of R/S-Roscovitine and CDKs Related Inhibition under Both Focal and Global Cerebral Ischemia: A Focus on Neurovascular Unit and Immune Cells

Ischemic stroke is the second leading cause of death worldwide. Following ischemic stroke, Neurovascular Unit (NVU) inflammation and peripheral leucocytes infiltration are major contributors to the extension of brain lesions. For a long time restricted to neurons, the 10 past years have shown the emergence of an increasing number of studies focusing on the role of Cyclin-Dependent Kinases (CDKs) on the other cells of NVU, as well as on the leucocytes. The most widely used CDKs inhibitor, (R)-roscovitine, and its (S) isomer both decreased brain lesions in models of global and focal cerebral ischemia. We previously showed that (S)-roscovitine acted, at least, by modulating NVU response to ischemia. Interestingly, roscovitine was shown to decrease leucocytes-mediated inflammation in several inflammatory models. Specific inhibition of roscovitine majors target CDK 1, 2, 5, 7, and 9 showed that these CDKs played key roles in inflammatory processes of NVU cells and leucocytes after brain lesions, including ischemic stroke. The data summarized here support the investigation of roscovitine as a potential therapeutic agent for the treatment of ischemic stroke, and provide an overview of CDK 1, 2, 5, 7, and 9 functions in brain cells and leucocytes during cerebral ischemia.

Ribonucleotide reductase inhibitors suppress SAMHD1 ara-CTPase activity enhancing cytarabine efficacy

The deoxycytidine analogue cytarabine (ara-C) remains the backbone treatment of acute myeloid leukaemia (AML) as well as other haematological and lymphoid malignancies, but must be combined with other chemotherapeutics to achieve cure. Yet, the underlying mechanism dictating synergistic efficacy of combination chemotherapy remains largely unknown. The dNTPase SAMHD1, which regulates dNTP homoeostasis antagonistically to ribonucleotide reductase (RNR), limits ara-C efficacy by hydrolysing the active triphosphate metabolite ara-CTP. Here, we report that clinically used inhibitors of RNR, such as gemcitabine and hydroxyurea, overcome the SAMHD1-mediated barrier to ara-C efficacy in primary blasts and mouse models of AML, displaying SAMHD1-dependent synergy with ara-C. We present evidence that this is mediated by dNTP pool imbalances leading to allosteric reduction of SAMHD1 ara-CTPase activity. Thus, SAMHD1 constitutes a novel biomarker for combination therapies of ara-C and RNR inhibitors with immediate consequences for clinical practice to improve treatment of AML.

Cell cycle control mechanisms in B-1 and B-2 lymphoid subsets

An effective humoral response requires that a given B lymphocyte population express a repertoire of receptors capable of recognizing a distinct array of antigens, while at the same time disregarding self-antigens. Mature B cells interacting with antigen via their B cell antigen receptors (BCRs) enter G(1) phase of the cell cycle and, depending on the strength of the signal, can commit to S phase entry. Input from co-receptors, which may function to either enhance or inhibit BCR signals, also influence the decision to proliferate. We review herein recent advances in the biochemistry of G(1)-cyclin holoenzymes that function to integrate BCR-coupled signaling pathways to the phosphorylation (and inactivation) of the retinoblastoma gene product (pRb) in splenic B lymphocytes (B-2 cells). We also highlight differences in the control of G(1)-to-S phase progression between B-2 cells and peritoneal CD5+ B cells (B-1 cells).

Cloning and characterization of a p53-related protein kinase expressed in interleukin-2-activated cytotoxic T-cells, epithelial tumor cell lines, and the testes

A human protein kinase, p53-related protein kinase (PRPK), was cloned from an interleukin-2-activated cytotoxic T-cell subtraction library. PRPK appears to be a homologue of a growth-related yeast serine/threonine protein kinase, YGR262c. However, a complementation assay using YGR262c-disrupted yeast indicated that PRPK is not functionally identical to the yeast enzyme. PRPK expression was observed in interleukin-2-activated cytotoxic T-cells, some human epithelial tumor cell lines, and the testes. The intrinsic transcriptional activity of p53 was up-regulated by a transient transfection of PRPK to COS-7 cells. PRPK was shown to bind to p53 and to phosphorylate p53 at Ser-15. These results indicate that PRPK may play an important role in the cell cycle and cell apoptosis through phosphorylation of p53.

Indole-3-carbinol inhibits CDK6 expression in human MCF-7 breast cancer cells by disrupting Sp1 transcription factor interactions with a composite element in the CDK6 gene promoter

Indole-3-carbinol (I3C), a compound naturally occurring in Brassica vegetables, can induce a G(1) cell cycle arrest of human MCF-7 breast cancer cells that is accompanied by the selective inhibition of cyclin-dependent kinase 6 (CDK6) expression. Reverse transcriptase-polymerase chain reaction analysis of CDK6 mRNA decay rates revealed that I3C had no effect on CDK6 transcript stability. We report the first identification and functional characterization of the CDK6 promoter in order to determine whether I3C inhibits CDK6 transcription. In MCF-7 cells stably transfected with CDK6 promoter-linked luciferase reporter plasmids, I3C inhibited CDK6 promoter activity in an I3C-specific response that was not a consequence of the growth-arrested state of the cells. Deletion analysis revealed a 167-base pair I3C-responsive region of the CDK6 promoter between -805 and -638. Site-specific mutations within this region revealed that both Sp1 and Ets-like sites, which are spaced 5 base pairs apart, were necessary for I3C responsiveness in the context of the CDK6 promoter. Electrophoretic mobility shift analysis of protein-DNA complexes formed with nuclear proteins isolated from I3C-treated and -untreated cells, in combination with supershift assays using Sp1 antibodies, demonstrated that the Sp1-binding site in the CDK6 promoter forms a specific I3C-responsive DNA-protein complex that contains the Sp1 transcription factor. Taken together, our results suggest that I3C down-regulates CDK6 transcription by targeting Sp1 at a composite DNA site in the CDK6 promoter.

Identification of a membrane Ig-induced p38 mitogen-activated protein kinase module that regulates cAMP response element binding protein phosphorylation and transcriptional activation in CH31 B cell lymphomas

The cAMP response element (CRE) binding protein (CREB) is emerging as a key regulatory factor of gene transcription in B lymphocytes; however, the postreceptor pathways that regulate CREB activity and CRE-dependent gene transcription remain largely undefined. We investigated B cell Ag receptor (BCR)-mediated phosphorylation and activation of CREB in the surface IgM+ CH31 B cell lymphoma, which undergoes Ag-dependent cell death. The activity of p38 mitogen-activated protein kinase (MAPK) was increased in response to BCR ligation. Phosphorylation of CREB on serine 133, a modification that positively regulates its trans-activation, was concomitantly increased. Inhibition of p38 MAPK by pretreating CH31 B cells with the highly specific bicyclic imidazole inhibitor, SB203580, reduced BCR-induced CREB phosphorylation. BCR cross-linking also led to increased MAPK-activated protein kinase-2 activity, an enzyme that lies immediately downstream from p38 MAPK; MAPK-activated protein kinase-2 immune complexes phosphorylated a peptide substrate containing the CREB serine 133 phosphoacceptor motif. Given the role of CREB in regulating junB gene expression in mature B lymphocytes, we examined whether p38 MAPK activity was necessary for CRE-dependent junB transcription in CH31 B cells. BCR ligation led to increased junB mRNA levels, which were significantly reduced in CH31 B cells pretreated with SB203580. Activation of a CRE-dependent junB promoter/chloramphenicol acetyltransferase (CAT) reporter gene by the BCR was also blocked by SB203580. Similarly, inhibition of p38 MAPK in surface IgM+ WEHI-231 B cell lymphomas resulted in reduced BCR-induced junB mRNA expression and junB promoter activation. The results implicate a p38 MAPK pathway in BCR-mediated CREB phosphorylation and junB transcriptional activation in B cell lymphomas.

Modulation of E2F activity in primary mouse B cells following stimulation via surface IgM and CD40 receptors

Since signals via CD40 and the B cell receptor are known to synergize to induce B cell activation, we have analyzed the pocket protein/E2F complexes in mouse B lymphocytes following stimulation by anti-IgM, anti-CD40, alone or together. We find that E2F4 and DP1 form the predominant E2F heterodimers in the G0 and G1 phases of the cell cycle, complexed with hypophosphorylated p130. During late G1 and S phase this complex is replaced by at least three different E2F complexes, one of which is an E2F complex containing p107 or pRB as well as two "free" E2F complexes consisting of E2F4/DP1 and E2F1-3/DP1. These effects were mirrored by the levels and phosphorylation status of the three pocket proteins. We also observed an increase in electrophoretic mobility of DP1 and E2F4 as B cells progressed from G0 into early G1, resulting from their dephosphorylation. This is known to correlate with a decrease in DNA binding capacity of these proteins and could also be important for derepression of genes negatively regulated through E2F sites in their promoters. These results therefore indicate that the pRB/E2F pathway integrates proliferative signals emanating from the sIgM and CD40 receptors.

B Cell Antigen Receptor-Mediated Activation of Cyclin-Dependent Retinoblastoma Protein Kinases and Inhibition by Co-Cross-Linking with Fcγ Receptors

Cross-linking the B cell Ag receptor (BCR) to surface Fc receptors for IgG (FcγR) inhibits G1-to-S progression; the mechanism by which this occurs is not completely known. We investigated the regulation of three key cell cycle regulatory components by BCR-FcγR co-cross-linking: G1-cyclins, cyclin-dependent kinases (Cdks), and the retinoblastoma gene product (Rb). Rb functions to suppress G1-to-S progression in mammalian cells. Rb undergoes cell-cycle-dependent phosphorylation, leading to its inactivation and thereby promoting S phase entry. We demonstrate in this paper for the first time that BCR-induced Rb phosphorylation is abrogated by co-cross-linking with FcγR. The activation of Cdk4/6- and Cdk2-dependent Rb protein kinases is concomitantly blocked. FcγR-mediated inhibition of Cdk2 activity results in part from an apparent failure to express Cdk2 protein. By contrast, inhibition of Cdk4/6 activities is not due to suppression of Cdk4/6 or cyclins D2/D3 expression or inhibition of Cdk-activating kinase activity. Cdk4- and Cdk6-immune complexes recovered from B cells following BCR-FcγR co-cross-linking are devoid of coprecipitated D-type cyclins, indicating that inhibition of their Rb protein kinase activities is due in part to the absence of bound D-type cyclin. Thus, BCR-derived activation signals that up-regulate D-type cyclin and Cdk4/6 protein expression remain intact; however, FcγR-mediated signals block cyclin D-Cdk4/6 assembly or stabilization. These results suggest that assembly or stabilization of D-type cyclin holoenzyme complexes 1) is an important step in the activation of Cdk4/6 by BCR signals, and 2) suffice in providing a mechanism to account for inhibition of BCR-stimulated Rb protein phosphorylation by FcγR.

CD40-mediated induction of p21 accumulation in resting and cycling B cells

The accumulation of G1 cell cycle-related proteins by resting or cycling B cells stimulated with B cell antigen receptor (BCR)- and T helper (Th) cell-derived signals is documented. Resting B cells constitutively express cyclin dependent kinase (cdk)4, cdk2 and the cyclin dependent kinase inhibitor (CKI), p27. The initiation of optimal proliferation with F(ab')2 anti-mu plus paraformaldehyde-fixed CD40 ligand-baculovirus-infected Sf9 cells (CD40L/Sf9 cells) increases accumulation of both cdk4 and cdk2 while decreasing p27 levels. B cells express cyclin D2 early during cycle progression, while cyclin D3 and E are not expressed until 18 h poststimulation and cyclin A by 24 h poststimulation. Cycling B cells express heightened levels of all these cyclins and cdks. Although neither BCR- nor CD40-mediated signals appreciably alter cycling B cell accumulation of cyclins D2, cdk4 and cdk2, the absence of BCR-derived signals results in a decreased accumulation of cyclins D3 and E. Finally, CD40-mediated signals induce resting B cells to accumulate the CKI, p21, while cycling B cells require both BCR- and CD40-mediated signals to maintain increased expression of p21. Thus, a Th cell-derived signal may impact upon both resting and cycling B cell cycle progression, at least in part, by regulating the accumulation of p21. The functional consequences of p21 accumulation as cells enter and move through the cell cycle are discussed.

p27KIP1, an inhibitor of cyclin-dependent kinases

The identification of a family of proteins that stoichiometrically regulate the activation of the G1 cyclin-dependent kinases has added to our understanding of the process of commitment to the mitotic cycle. The properties of p27 as a CDK binding protein, the ability of environmental signals to regulate the expression of p27, and the observation that p27 may link the accumulation of G1 CDK complexes with activation of the CDK2 kinase, suggest it may have a critical role in establishing a threshold for G1 cyclin/CDK accumulation prior to activation of CDK2 kinase and entry into the mitotic cycle.

The complexity of signaling pathways activated by the BCR

Cross-linking of the B cell antigen receptor (BCR) leads to the activation of three types of intracellular protein tyrosine kinases. These tyrosine kinases then phosphorylate signaling components to activate a variety of signaling reactions, including phosphatidylinositol 4,5-bisphosphate hydrolysis, Ras activation, and phosphatidylinositol 3-kinase activation. Each of these signaling reactions, and also the signaling molecules Vav and HS1, appears to be important for at least some of the many types of B cell responses to antigen. The complexity of BCR signaling reactions may be required to allow the B cell to respond in a number of distinct ways to antigen (proliferation, survival, apoptosis, maturational arrest, etc.) depending on the maturation state of the B cell, the location in the body, the physical nature of the antigen, and the possible presence of the antigen in complex with antibody or complement components.

The regulated expression of cell cycle-related proteins as B-lymphocytes enter and progress through the G1 cell cycle stage following delivery of complete versus partial activation stimuli

Resting B-cells lack both cyclins D and E while constitutively expressing low levels of cdk4 and cdk2. B-cells receiving a complete growth stimulus express cyclin D2 by 10hr and cyclin E by 10-24hr poststimulation while increasing their protein levels of cdk4 and cdk2. B-cells receiving partial growth stimuli move into G1 without passing the G1 restriction point and transiently increase cyclin D2 mRNA levels without accumulating cyclin D2 protein. In the absence of cyclin D2 accumulation, cdk4 is not activated, and cyclin E is not expressed. These results suggest that signals responsible for moving B-cells through the G1 restriction point impact at the level of cyclin D2 protein accumulation. The possible implications of these results are discussed.

Induction of cell cycle regulatory proteins in anti-immunoglobulin-stimulated mature B lymphocytes

Progression through the cell cycle is a tightly controlled process that integrates signals generated at the plasma membrane with the proteins that form the cell cycle machinery. The current study chronicles the induction of cyclins, cyclin-dependent kinases (cdk), and cdk inhibitors in low density primary mouse B lymphocytes after anti-immunoglobulin plus interleukin 4 (IgM + IL-4) stimulation. In this system, > 85% of cells remain in the G0/G1 phase of cell cycle for an initial 24-h period, followed by entry of up to 50% of the cells into S phase, commencing around 30 h and peaking at 48 h. Extensive time course analyses of these anti-IgM + IL-4-stimulated B cells revealed that the G1-associated D-type cyclins D2 and D3 were induced by 3 h after stimulation, and that cyclins E, A, and B were subsequently induced sequentially, beginning at mid-G1, G1/S transition, and S phase, respectively. The G1-associated cyclin D1 was not expressed at any stage of the anti-Ig + IL-4-induced B cell cycle. cdk2, cdk4, and cdk6 were induced during G1, whereas cell division cycle-2 (cdc2) was induced concomitantly with S phase. Irrespective of their expression, the kinases cdk2 and cdc2 were only active from S phase onwards, suggesting that productive cyclin/kinase complex formation did not occur until that time. Cell cycle inhibitors p21 and p19 were induced by anti-Ig + IL-4, peaking in expression at mid-G1 and S phase, respectively. Stimulation of low density B cells with anti-Ig + IL-4 caused rapid down regulation of the p27 inhibitor, however this protein was reexpressed at 54-96 h after stimulation. In contrast, B cells stimulated with anti-CD40, a stimulus which induces long-term B cell proliferation, permanently down regulated p27. These findings are consistent with the concept that p27 reexpression contributes to the G1 arrest that follows antigen receptor crosslinking. Low density B cells cultured in the viability-enhancing cytokine IL-4 alone also showed induction of D2 and D3 cyclin expression. However, the D2 expression was transient, and the D3 expression was substantially lower than that observed in B cells induced to proliferate by anti-Ig + IL-4. This partial induction of D2 and D3 expression may explain IL-4's ability to promote B cell entry into G1 but not S phase of cell cycle, and furthermore, its ability to truncate G1 progression when B cells are subsequently stimulated with anti-Ig.

Characterization of the human cyclin-dependent kinase 2 gene. Promoter analysis and gene structure

Cyclin-dependent kinase 2 is a serine/threonine protein kinase essential for progression of the mammalian cell cycle from G1 to S phase. CDK2 mRNA has been shown to be induced by serum in several cultured cell types. Therefore, we set out to identify elements that regulate the transcription of the human CDK2 gene and to characterize its structure. This paper describes the cloning of approximately 2.4-kilobase pair genomic DNA fragment from the upstream region of the human CDK2 gene. This fragment contains five transcription initiation sites within a 72-nucleotide stretch. A 200-base pair sub-fragment that confers 70% of maximal basal promoter activity was shown to contain two synergistically acting Sp1 sites. However, a much larger DNA fragment containing approximately 1.7 kilobase pairs of upstream sequence is required for induction of promoter activity following serum stimulation. The intron exon boundaries of seven exons in this gene were also identified, and this information will be useful for analyzing genomic abnormalities associated with CDK2.

Role of cyclin A and p27 in anti-IgM induced G1 growth arrest of murine B-cell lymphomas

Cross-linking surface immunoglobulin (Ig)M on the WEHI-231 B-cell lymphoma results in decreased cell size, G1/S growth arrest, and finally DNA cleavage into oligonucleosomal fragments that are the classical features of apoptotic cells. Treatment of WEHI-231 cells with anti-IgM in early G1 phase prevents phosphorylation of the retinoblastoma gene product (pRb) and inhibits entry into S phase. Using unsynchronized cells, we previously demonstrated that cyclin A-associated and Cdk2-dependent GST-pRb kinase activity were inhibited in WEHI-231 cells treated with anti-IgM. We now show that progression of elutriated early G1 phase WEHI-231 cells from early into late G1 phase is accompanied by an increase in the abundance of cyclin A protein and cyclin A-associated kinase activity. Treatment of early G1 cells with anti-IgM prevented this increase in cyclin A-associated kinase activity at late G1, despite minimal changes in the overall level of cyclin A and Cdk2 proteins. Late G1 cells, which already possess high cyclin A-associated kinase activity, were insensitive to anti-IgM treatment and were able to complete the cell cycle. We also found that anti-IgM-treated cells contained increased amounts of the Cdk inhibitor protein p27Kip1. Essentially all of the cyclin A in treated cells was associated with p27, a result which we propose explains the lack of cyclin A/Cdk2 kinase activity. Accumulation of p27 in cyclin A kinase complexes, however, did not decrease the amount of Cdk2 bound to cyclin A. Thus, cross-linking IgM on growth-inhibitable B-cell lymphomas affects cyclin A kinase activity by increasing the levels of p27 in this complex, thus preventing productive pRb phosphorylation and leading to cell cycle arrest and subsequent apoptosis. These results are discussed in terms of the cell cycle restriction points that regulate lymphocyte function, as well as the lineage-specific differences in cell cycle control.

Regulation of B cell function by the immunosuppressive agent leflunomide

Leflunomide is an immunosuppressive drug capable of inhibiting cellular and humoral mediated responses in vivo. The mechanism responsible for suppression of B cell antibody responses in vivo has not been identified. In this study we demonstrate that leflunomide functions to inhibit murine B cell antibody production by directly acting on the B cell. Experiments performed in vivo showed that both T cell-dependent as well as T cell-independent antigen responses were suppressed by leflunomide. Initial in vitro experiments demonstrated that leflunomide inhibited B cell antibody production by decreasing B cell proliferation. The suppression of B cell proliferation induced by a variety of stimuli that use different signal cascade components suggested that leflunomide was acting on a common component required for B cell proliferation. Kinetic studies with LPS activated B cells revealed that leflunomide retained its inhibitory activity when added as late as 24 hr after stimulation in an 88-hr assay. By analyzing the cell cycle of LPS-stimulated B cells we observed that leflunomide targets two different stages in cell cycle transition: (1) from G1 to S phase and (2) from S phase to G2/M phase. Analysis of one of the cyclin-dependent kinases, Cdk2 protein, by Western blot revealed that Cdk2 levels were decreased, in the presence of leflunomide, 48 hr after stimulation. These data further confirmed that leflunomide inhibited B cell progression through the S phase. We also present evidence that the addition of exogenous uridine reversed the antiproliferative activity of leflunomide. This indicated that leflunomide acted as a pyrimidine synthesis inhibitor, thereby inhibiting B cell proliferation and cell cycle progression.

Fusobacterium nucleatum inhibits human T-cell activation by arresting cells in the mid-G1 phase of the cell cycle

Fusobacterium nucleatum has been implicated in the pathogenesis of several diseases, including urinary tract infections, bacteremia, pericarditis, otitis media, and disorders of the oral cavity such as pulpal infections, alveolar bone abscesses, and periodontal disease. We have previously demonstrated that sonic extracts of F. nucleatum FDC 364 were capable of inhibiting human T-cell responses to mitogens and antigens. In this study, we have further characterized this immunosuppressive protein (FIP) and initiated experiments to determine its mode of action. The purified FIP has an apparent molecular mass of 90 to 100 kDa; sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicates that the FIP is actually composed of two subunits with molecular masses of 48 and 44 kDa. Purified FIP retained its biological activity and was capable of inhibiting mitogen-induced proliferation of human T cells. Inhibition was dose dependent, and the FIP exhibited a specific activity approximately 250-fold greater than that of the crude extract. Cell cycle analysis indicates that FIP-treated cells were prevented from exiting the G0/G1 phase of the cell cycle. However, FIP did not alter the expression of activation markers (CD69, CD25, and CD71) or interleukin-2 secretion. The latter observations suggest that the T cells did indeed become activated and had entered the G1 phase of the cell cycle. Analysis of the expression of cyclins indicates that the phase of the cell cycle that is FIP sensitive resides somewhere beyond the restriction point of cyclin D2 (early to mid-G1) but prior to that of cyclins D3 and E (mid- to late G1). Finally, analysis of the expression of the proliferating cell nuclear antigen indicates that this is the earliest detectable defect in T cells exposed to FIP. We propose that if a block in the G1 phase of the cell cycle occurs in vivo in lymphocytes, it may result in a state of local and/or systemic immunosuppression. These suppressive effects could alter the nature and consequences of host-parasite interactions, thereby enhancing the pathogenicity of F. nucleatum itself or that of some other opportunistic organisms.