p27Kip1 and p130 cooperate to regulate hematopoietic cell proliferation in vivo

A replicative self-renewal model for long-lived plasma cells: questioning irreversible cell cycle exit

Plasma cells are heterogenous in terms of their origins, secretory products, and lifespan. A current paradigm is that cell cycle exit in plasma cell differentiation is irreversible, following a pattern familiar in short-lived effector populations in other hemopoietic lineages. This paradigm no doubt holds true for many plasma cells whose lifespan can be measured in days following the completion of differentiation. Whether this holds true for long-lived bone marrow plasma cells that are potentially maintained for the lifespan of the organism is less apparent. Added to this the mechanisms that establish and maintain cell cycle quiescence in plasma cells are incompletely defined. Gene expression profiling indicates that in the transition of human plasmablasts to long-lived plasma cells a range of cell cycle regulators are induced in a pattern that suggests a quiescence program with potential for cell cycle re-entry. Here a model of relative quiescence with the potential for replicative self-renewal amongst long-lived plasma cells is explored. The implications of such a mechanism would be diverse, and the argument is made here that current evidence is not sufficiently strong that the possibility should be disregarded.

Cyclin D1 and cdk4 mediate development of neurologically destructive oligodendroglioma

Although the molecular changes that characterize gliomas have been studied, the pathogenesis of tumor development remains unclear. p21 contributes to gliomagenesis by stabilizing cyclin D1-cdk4 kinase complexes, suggesting that cyclin D1 and cdk4 may also be required for glial tumor development. In this study, we used a mouse model to attempt to confirm this hypothesis, finding that cyclin D1 and cdk4 played active roles in not only the tumor but also the tumor microenvironment. Loss of cdk4 blocked tumor development, but loss of cyclin D1 did not prevent gliomas from developing. Instead, loss of cyclin D1 impeded progression to higher stages of malignancy. Enforcing expression of cyclin D1 was insufficient to correct the progression defect observed in cyclin D1-deficient animals. In contrast, restoration of cdk4 in the cdk4-deficient animals restored cell proliferation and tumor formation, although at lower tumor grades. Notably, the failure of tumors in the cyclin D1- and cdk4-deficient animals to progress to higher grades was correlated with a failure to fully activate microglia in the tumor microenvironment. Moreover, when platelet-derived growth factor-transformed glial cells were engrafted orthotopically into the mice, the tumors that formed progressed to high grades in wild-type mice but not cyclin D1-deficient animals. Together, our findings establish that the cyclin D1-cdk4 axis is not only critical in glial tumor cells but also in stromal-derived cells in the surrounding tumor microenvironment that are vital to sustain tumor outgrowth.

Chemical genetics approach to restoring p27Kip1 reveals novel compounds with antiproliferative activity in prostate cancer cells

BACKGROUND

The cyclin-dependent kinase (CDK) inhibitor p27(Kip)¹ is downregulated in a majority of human cancers due to ectopic proteolysis by the ubiquitin-proteasome pathway. The expression of p27 is subject to multiple mechanisms of control involving several transcription factors, kinase pathways and at least three different ubiquitin ligases (SCF(SKP)², KPC, Pirh2), which regulate p27 transcription, translation, protein stability and subcellular localization. Using a chemical genetics approach, we have asked whether this control network can be modulated by small molecules such that p27 protein expression is restored in cancer cells.

RESULTS

We developed a cell-based assay for measuring the levels of endogenous nuclear p27 in a high throughput screening format employing LNCaP prostate cancer cells engineered to overexpress SKP2. The assay platform was optimized to Z' factors of 0.48 - 0.6 and piloted by screening a total of 7368 chemical compounds. During the course of this work, we discovered two small molecules of previously unknown biological activity, SMIP001 and SMIP004, which increase the nuclear level of p27 at low micromolar concentrations. SMIPs (small molecule inhibitors of p27 depletion) also upregulate p21(Cip)¹, inhibit cellular CDK2 activity, induce G1 delay, inhibit colony formation in soft agar and exhibit preferential cytotoxicity in LNCaP cells relative to normal human fibroblasts. Unlike SMIP001, SMIP004 was found to downregulate SKP2 and to stabilize p27, although neither SMIP is a proteasome inhibitor. Whereas the screening endpoint - nuclear p27 - was robustly modulated by the compounds, SMIP-mediated cell cycle arrest and apoptosis were not strictly dependent on p27 and p21 - a finding that is explained by parallel inhibitory effects of SMIPs on positive cell cycle regulators, including cyclins E and A, and CDK4.

CONCLUSIONS

Our data provide proof-of-principle that the screening platform we developed, using endogenous nuclear p27 as an endpoint, presents an effective means of identifying bioactive molecules with cancer selective antiproliferative activity. This approach, when applied to larger and more diverse sets of compounds with refined drug-like properties, bears the potential of revealing both unknown cellular pathways globally impinging on p27 and novel leads for chemotherapeutics targeting a prominent molecular defect of human cancers.

PIM1 kinase is destabilized by ribosomal stress causing inhibition of cell cycle progression

PIM1 is a constitutively active serine/threonine kinase regulated by cytokines, growth factors and hormones. It has been implicated in the control of cell cycle progression and apoptosis and its overexpression has been associated with various kinds of lymphoid and hematopoietic malignancies. The activity of PIM1 is dependent on the phosphorylation of several targets involved in transcription, cell cycle and apoptosis. We have recently observed that PIM1 interacts with ribosomal protein (RP)S19 and cosediments with ribosomes. Defects in ribosome synthesis (ribosomal stress) have been shown to activate a p53-dependent growth arrest response. To investigate if PIM1 could have a role in the response to ribosomal stress, we induced ribosome synthesis alterations in TF-1 and K562 erythroid cell lines. We found that RP deficiency, induced by RNA interference or treatment with inhibitor of nucleolar functions, causes a drastic destabilization of PIM1. The lower level of PIM1 induces an increase in the cell cycle inhibitor p27(Kip1) and blocks cell proliferation even in the absence of p53. Notably, restoring PIM1 level by transfection causes a recovery of cell growth. Our data indicate that PIM1 may act as a sensor for ribosomal stress independently of or in concert with the known p53-dependent mechanisms.

Single round of antigen receptor signaling programs naive B cells to receive T cell help

To simulate transient B cell activation that is the likely initiator of T-dependent responses, we examined the molecular and functional consequences of a single round of immunoglobulin M (IgM) signaling. This form of activation triggered early cytosolic signaling and the transcription factor NF-kappaB activation indistinguishably from conventional continuous IgM crosslinking but did not induce G1 progression. However, single round IgM signaling changed the expression of chemokine and chemokine receptor genes implicated in initiating T-dependent responses, as well as accentuated responsiveness to CD40 signaling. Several features of single-round IgM signaling in vitro were recapitulated in B cells after short-term exposure to antigen in vivo. We propose that transient BCR signals prime B cells to receive T cell help by increasing the probability of B-T encounter and creating a cellular environment that is hyper-responsive to CD40 signaling.

Development of a malignancy-associated proteomic signature for diffuse large B-cell lymphoma

The extreme pathological diversity of non-Hodgkin's lymphomas has made their accurate histological assessment difficult. New diagnostics and treatment modalities are urgently needed for these lymphomas, particularly in drug development for cancer-specific targets. Previously, we showed that a subset of B cell lymphoma, diffuse large B cell lymphoma, may be characterized by two major, orthogonal axes of gene expression: one set of transcripts that is differentially expressed between resting and proliferating, nonmalignant cells (ie, a "proliferative signature") and another set that is expressed only in proliferating malignant cells (ie, a "cancer signature"). A differential proteomic analysis of B cell proliferative states, similar to previous transcriptional profiling analyses, holds great promise either to reveal novel factors that participate in lymphomagenesis or to define biomarkers of onset or progression. Here, we use a murine model of diffuse large B cell lymphoma to conduct unbiased two-dimensional gel electrophoresis and mass spectrometry-based comparative proteomic analyses of malignant proliferating B cells and tissue-matched, normal resting, or normal proliferating cells. We show that the expression patterns of particular proteins or isoforms across these states fall into eight specific trends that provide a framework to identify malignancy-associated biomarkers and potential drug targets, a signature proteome. Our results support the central hypothesis that clusters of proteins of known function represent a panel of expression markers uniquely associated with malignancy and not normal proliferation.

Regulation of B-cell proliferation and differentiation by pre-B-cell receptor signalling

The pre-B-cell receptor (pre-BCR) is expressed following the productive recombination of the immunoglobulin heavy chain gene. Signals through the pre-BCR are required for initiating diverse processes in pre-B cells, including proliferation and recombination of the light chain gene, which eventually lead to the differentiation of pre-B cells to immature B cells. However, the molecular mechanisms by which the pre-BCR promotes these processes remain largely unresolved. Recent findings suggest that forkhead box O (FOXO) transcription factors connect pre-BCR signalling to the activation of the recombination machinery. In this Review, we discuss how FOXO transcription factors are regulated by the pre-BCR to allow the progression of the cell cycle and the recombination of the light chain gene.

Many forks in the path: cycling with FoxO

FoxO transcription factors are an evolutionary conserved subfamily of the forkhead transcription factors, characterized by the forkhead DNA-binding domain. FoxO factors regulate a number of cellular processes involved in cell-fate decisions in a cell-type- and environment-specific manner, including metabolism, differentiation, apoptosis and proliferation. A key mechanism by which FoxO determines cell fate is through regulation of the cell cycle machinery, and as such the cellular consequence of FoxO deregulation is often manifested through perturbation of the cell cycle. Consequently, the deregulation of FoxO factors is implicated in the development of numerous proliferative diseases, in particular cancer.

Functional regulation of D-type cyclins by insulin-like growth factor-I and serum in multiple myeloma cells

D-type cyclin genes are universally dysregulated in multiple myeloma (MM), but the functional consequences are unclear as D-type cyclin gene expression does not correlate with proliferation or disease progression. We examined the protein expression and regulation of D-type cyclins and other cell cycle regulators in human myeloma cell lines and primary CD138(+) plasma cells (PCs). Cyclin D1, cyclin D2, cyclin dependent kinase (CDK) 4, CDK6, p27(Kip1) p18(INK4C) and retinoblastoma protein (pRb) were absent in normal PCs, heterogeneously expressed in primary MM cells and positively correlated with disease activity/progression. Cyclins D1 and D2 complexed with both CDK4 and CDK6, suggesting that both phosphorylate pRb in MM. Furthermore, cyclin D2 expressed via either t(14;16) or t(4;14) IgH translocations was functionally upregulated by fetal calf serum or insulin-like growth factor-I, leading to pRb phosphorylation and cell cycle entry/progression, and in some cases inversely correlated with p27(Kip1). However, pRb phosphorylation and cell cycle progression mediated by cyclin D1 expressed via t(11;14) was less dependent on exogenous stimuli. These data suggest that the presence or absence of specific IgH translocations underlying aberrant D-type cyclin expression may influence their response to mitogens in the bone marrow microenvironment. We showed for the first time that D-type cyclins are functionally regulated in MM, differentially responsive to exogenous growth factors and upregulated with disease progression.

Promiscuous and lineage-specific roles of cell cycle regulators in haematopoiesis

Haematopoietic cell number is maintained by a delicate balance between cell proliferation, differentiation and death. Gene knockout studies in mice have revealed the complex roles of cyclins, CDKs, and CDK inhibitors in regulating cell proliferation and differentiation in the haematopoietic system. These studies point to families of cell cycle regulators which display both redundant and unique roles within a lineage and developmental-stage specific manner. Moreover, the promiscuity of these cell cycle regulators is critical for haematopoietic cell proliferation and differentiation. In this review, we discuss the current evidence from mouse models that the complexity and multifarious nature of the haematopoietic system is critical for its form and function.