Cyclin D1-dependent regulation of B-myb activity in early stages of neuroblastoma differentiation

Emerging role of MYB transcription factors in cancer drug resistance

Decades ago, the viral myeloblastosis oncogene v-myb was identified as a gene responsible for the development of avian leukemia. However, the relevance of MYB proteins for human cancer diseases, in particular for solid tumors, remained basically unrecognized for a very long time. The human family of MYB transcription factors comprises MYB (c-MYB), MYBL2 (b-MYB), and MYBL1 (a-MYB), which are overexpressed in several cancers and are associated with cancer progression and resistance to anticancer drugs. In addition to overexpression, the presence of activated MYB-fusion proteins as tumor drivers was described in certain cancers. The identification of anticancer drug resistance mediated by MYB proteins and their underlying mechanisms are of great importance in understanding failures of current therapies and establishing new and more efficient therapy regimens. In addition, new drug candidates targeting MYB transcription factor activity and signaling have emerged as a promising class of potential anticancer therapeutics that could tackle MYB-dependent drug-resistant cancers in a more selective way. This review describes the correlation of MYB transcription factors with the formation and persistence of cancer resistance to various approved and investigational anticancer drugs.

PCLAF promotes neuroblastoma G1/S cell cycle progression via the E2F1/PTTG1 axis

PCLAF (PCNA clamp-associated factor), also known as PAF15/ KIAA0101, is overexpressed in most human cancers and is a predominant regulator of tumor progression. However, its biological function in neuroblastoma remains unclear. PCLAF is extremely overexpressed in neuroblastoma and is associated with poor prognosis. Through the analysis of various data sets, we found that the high expression of PCLAF is positively correlated with increased stage and high risk of neuroblastoma. Most importantly, knocking down PCLAF could restrict the proliferation of neuroblastoma cells in vitro and in vitro. By analyzing RNA-seq data, we found that the enrichment of cell cycle-related pathway genes was most significant among the differentially expressed downregulated genes after reducing the expression of PCLAF. In addition, PCLAF accelerated the G1/S transition of the neuroblastoma cell cycle by activating the E2F1/PTTG1 signaling pathway. In this study, we reveal the mechanism by which PCLAF facilitates cell cycle progression and recommend that the PCLAF/E2F1/PTTG1 axis is a therapeutic target in neuroblastoma.

B-Myb participated in ionizing radiation-induced apoptosis and cell cycle arrest in human glioma cells

As a common treatment of human glioma, ionizing radiation (IR) was reported to result in cell cycle arrest. However, the mechanisms underlying IR-induced abnormal cell cycle remain largely unclear. Here we found that IR caused an elevated expression of B-Myb and cell cycle-related proteins, as well as G2/M phase arrest in U251 cells instead of U87 cells. However, the knockdown of B-Myb by small interfering RNAs ameliorated the increasing of cell cycle-related proteins and G2/M phase arrest induced by IR. Further analysis demonstrated that decreased-B-Myb enhanced the sensitivity of U251 cells to IR. Moreover, the establishment of H1299 cell line proved that B-Myb expression was associated with the status of p53. Immunoprecipitation (IP) and chromatin immunoprecipitation (CHIP) assay results indicated that mutant p53 and SP1 regulated the expression of B-Myb via different mechanisms. This study not only elucidated the role of B-Myb in IR-induced cell cycle alternation, but also provided insight into mechanism of B-Myb expression.

Less-well known functions of cyclin/CDK complexes

Cyclin-dependent kinases (CDKs) are activated by cyclins, which play important roles in dictating the actions of CDK/cyclin complexes. Cyclin binding influences the substrate specificity of these complexes in addition to their susceptibility to inhibition or degradation. CDK/cyclin complexes are best known to promote cell cycle progression in the mitotic cell cycle but are also crucial for important cellular processes not strictly associated with cellular division. This chapter primarily explores the understudied topic of CDK/cyclin complex functionality during the DNA damage response. We detail how CDK/cyclin complexes perform dual roles both as targets of DNA damage checkpoint signaling as well as effectors of DNA repair. Additionally, we discuss the potential CDK-independent roles of cyclins in these processes and the impact of such roles in human diseases such as cancer. Our goal is to place the spotlight on these important functions of cyclins either acting as independent entities or within CDK/cyclin complexes which have attracted less attention in the past. We consider that this will be important for a more complete understanding of the intricate functions of cell cycle proteins in the DNA damage response.

Cyclin D1 in human neuroblastic tumors recapitulates its developmental expression: An immunohistochemical study

The protein cyclin D1 (CD1), which belongs to a family of proteins functioning as regulators of CDKs (cyclin-dependent kinases) throughout the cell cycle, has been immunohistochemically detected in a wide variety of human malignant tumors. The aim of the present study was to investigate immunohistochemically the expression and distribution of CD1 in the developing human peripheral sympathetic nervous system (PSNS) and in childhood peripheral neuroblastic tumors (neuroblastomas, ganglioneuroblastomas, and ganglioneuromas). The above mentioned fetal and neoplastic tissues represent an in vivo model in which undifferentiated neuroblastic cells undergo ganglion cell differentiation. During development, a strong nuclear expression of CD1 was restricted to neuroblasts, disappearing progressively from the maturing ganglion cells with increasing gestational age. In neoplastic tissues, CD1 immunoreactivity was restricted to neuroblastic cell component of all neuroblastomas and ganglioneuroblastomas, whereas it was absent or only focally detectable in maturing/mature ganglion cell component of differentiating neuroblastomas, ganglioneuroblastomas, and ganglioneuromas. We conclude that CD1 is a reliable marker, which can be used routinely to stain neuroblastic cells in both developing and neoplastic tissues. Furthermore, our results indicate that CD1 expression in childhood peripheral neuroblastic tumors recapitulates the changes during normal development of PSNS, as previously reported for Bcl-2 oncoprotein, c-ErbB2, insulin-like growth factor 2, β-2-microglobulin, and cathepsin D. This is consistent with the current view that childhood peripheral neuroblastic tumors exhibit gene expression profiles mirroring those occurring during PSNS development.

Functional Interactions between BM88/Cend1, Ran-binding protein M and Dyrk1B kinase affect cyclin D1 levels and cell cycle progression/exit in mouse neuroblastoma cells

BM88/Cend1 is a neuronal-lineage specific modulator with a pivotal role in coordination of cell cycle exit and differentiation of neuronal precursors. In the current study we identified the signal transduction scaffolding protein Ran-binding protein M (RanBPM) as a BM88/Cend1 binding partner and showed that BM88/Cend1, RanBPM and the dual specificity tyrosine-phosphorylation regulated kinase 1B (Dyrk1B) are expressed in mouse brain as well as in cultured embryonic cortical neurons while RanBPM can form complexes with either of the two other proteins. To elucidate a potential mechanism involving BM88/Cend1, RanBPM and Dyrk1B in cell cycle progression/exit, we transiently co-expressed these proteins in mouse neuroblastoma Neuro 2a cells. We found that the BM88/Cend1-dependent or Dyrk1B-dependent down-regulation of cyclin D1 is reversed following their functional interaction with RanBPM. More specifically, functional interaction of RanBPM with either BM88/Cend1 or Dyrk1B stabilizes cyclin D1 in the nucleus and promotes 5-bromo-2'-deoxyuridine (BrdU) incorporation as a measure of enhanced cell proliferation. However, the RanBPM-dependent Dyrk1B cytosolic retention and degradation is reverted in the presence of Cend1 resulting in cyclin D1 destabilization. Co-expression of RanBPM with either BM88/Cend1 or Dyrk1B also had a negative effect on Neuro 2a cell differentiation. Our results suggest that functional interactions between BM88/Cend1, RanBPM and Dyrk1B affect the balance between cellular proliferation and differentiation in Neuro 2a cells and indicate that a potentially similar mechanism may influence cell cycle progression/exit and differentiation of neuronal precursors.

Withania somnifera water extract as a potential candidate for differentiation based therapy of human neuroblastomas

Neuroblastoma is an aggressive childhood disease of the sympathetic nervous system. Treatments are often ineffective and have serious side effects. Conventional therapy of neuroblastoma includes the differentiation agents. Unlike chemo-radiotherapy, differentiation therapy shows minimal side effects on normal cells, because normal non-malignant cells are already differentiated. Keeping in view the limited toxicity of Withania somnifera (Ashwagandha), the current study was aimed to investigate the efficacy of Ashwagandha water extract (ASH-WEX) for anti-proliferative potential in neuroblastoma and its underlying signalling mechanisms. ASH-WEX significantly reduced cell proliferation and induced cell differentiation as indicated by morphological changes and NF200 expression in human IMR-32 neuroblastoma cells. The induction of differentiation was accompanied by HSP70 and mortalin induction as well as pancytoplasmic translocation of the mortalin in ASH-WEX treated cells. Furthermore, the ASH-WEX treatment lead to induction of neural cell adhesion molecule (NCAM) expression and reduction in its polysialylation, thus elucidating its anti-migratory potential, which was also supported by downregulation of MMP 2 and 9 activity. ASH-WEX treatment led to cell cycle arrest at G0/G1 phase and increase in early apoptotic population. Modulation of cell cycle marker Cyclin D1, anti-apoptotic marker bcl-xl and Akt-P provide evidence that ASH-WEX may prove to be a promising phytotherapeutic intervention in neuroblatoma related malignancies.

miR-34a induces the downregulation of both E2F1 and B-Myb oncogenes in leukemic cells

PURPOSE

To elucidate new molecular mechanisms able to downregulate the mRNA levels of key oncogenes, such as B-Myb and E2F1, in a therapeutic perspective.

EXPERIMENTAL DESIGN

B-Myb and E2F1 mRNA levels were evaluated in primary B chronic lymphocytic leukemia (B-CLL, n = 10) and acute myeloid leukemia (AML, n = 5) patient cells, in a variety of p53(wild-type) and p53(mutated/deleted) leukemic cell lines, as well as in primary endothelial cells and fibroblasts. Knockdown experiments with siRNA for p53 and E2F1 and overexpression experiments with miR34a were conducted to elucidate the role of these pathways in promoting B-Myb downregulation.

RESULTS

In vitro exposure to Nutlin-3, a nongenotoxic activator of p53, variably downregulated the expression of B-Myb in primary leukemic cells and in p53(wild-type) myeloid (OCI, MOLM) and lymphoblastoid (SKW6.4, EHEB) but not in p53(mutated) (NB4, BJAB, MAVER) or p53(deleted) (HL-60) leukemic cell lines. The transcriptional repression of B-Myb was also observed in primary normal endothelial cells and fibroblasts. B-Myb downregulation played a critical role in the cell-cycle block in G(1) phase induced by Nutlin-3, as shown by transfection experiments with specific siRNA. Moreover, we have provided experimental evidence suggesting that miR-34a is a central mediator in the repression of B-Myb both directly and through E2F1.

CONCLUSIONS

Owing to the role of B-Myb and E2F1 transcription factors in controlling cell-cycle progression of leukemic cells, the downregulation of these oncogenes by miR-34a suggests the usefulness of therapeutic approaches aimed to modulate the levels of miR-34a.

Impaired cerebellar development and deficits in motor coordination in mice lacking the neuronal protein BM88/Cend1

During nervous system development, neural progenitors arise in proliferative zones, then exit the cell cycle and differentiate as they migrate away from these zones. The neuronal protein BM88/Cend1 has been implicated in coordination of cell cycle exit and differentiation of neuronal precursors. To further elucidate its function we generated Cend1 knock-out mice and analyzed their phenotype during postnatal cerebellar development. Cend1(-/-) mice showed no overt abnormalities in the gross anatomy of the cerebellum or other brain regions. However, detailed analysis revealed alterations in cerebellar layering arising from increased proliferation of granule cell precursors, delayed radial granule cell migration and impaired Purkinje cell differentiation. Accordingly, expression of Patched1, cyclin D1, reelin and brain-derived neurotrophic factor, which correlate with morphological development of the cerebellum, was altered in Cend1(-/-) mice. The observed anatomical and molecular alterations were accompanied by deficits in motor behaviour. Our results suggest that Cend1 is required for normal cerebellar development.

Post-translational modification of an R2R3-MYB transcription factor by a MAP Kinase during xylem development

Despite the pivotal role played by R2R3-MYB family members in the regulation of plant gene expression, little is known about post-translational regulation of these proteins. In animals, the MYB family member, c-MYB, is post-translationally modified by a mitogen-activated protein kinase (MAPK), p42(mapk). In order to test the hypothesis that R2R3-MYB proteins may be regulated by MAPK activity, interplay between a R2R3-MYB family member expressed in differentiating pine xylem (Pinus taeda MYB4, PtMYB4) and MAPK proteins expressed in the same tissue was examined. One of the MAPK proteins expressed in pine xylem, PtMAPK6, phosphorylated PtMYB4. Recombinant PtMAPK6 phosphorylated PtMYB4 on serine-236, located in the C-terminal activation domain of this transcription factor in a context that is found in other plant MYB proteins. Modification of the PtMAPK6 target serine in PtMYB4 did not appear to alter DNA binding in vitro but did alter the ability of PtMYB4 to promote transcriptional activation in yeast. PtMAPK6 activity was detected in developing xylem cells that had ceased cell division and formed secondary walls. Together, the data support a role for PtMAPK6 during early xylem development and suggest a function for this kinase in regulating gene expression through phosphorylation of PtMYB4.

Cyclin D1 and CDK4 activity contribute to the undifferentiated phenotype in neuroblastoma

Genomic aberrations of Cyclin D1 (CCND1), CDK4, and CDK6 in neuroblastoma indicate that dysregulation of the G(1) entry checkpoint is an important cell cycle aberration in this pediatric tumor. Here, we report that analysis of Affymetrix expression data of primary neuroblastic tumors shows an extensive overexpression of Cyclin D1, which correlates with histologic subgroups. Immunohistochemical analysis showed overexpression of Cyclin D1 in neuroblasts and low Cyclin D1 expression in all cell types in ganglioneuroma. This suggests an involvement of G(1)-regulating genes in neuronal differentiation processes which we further evaluated using RNA interference against Cyclin D1 and its kinase partners CDK4 and CDK6 in several neuroblastoma cell lines. The Cyclin D1 and CDK4 knockdown resulted in pRb pathway inhibition as shown by an almost complete disappearance of CDK4/CDK6-specific pRb phosphorylation, reduction of E2F transcriptional activity, and a decrease of Cyclin A protein levels. Phenotype analysis showed a significant reduction in cell proliferation, a G(1)-specific cell cycle arrest, and, moreover, an extensive neuronal differentiation. Affymetrix microarray profiling of small interfering RNA-treated cells revealed a shift in expression profile toward a neuronal phenotype. Several new potential downstream players are identified. We conclude that neuroblastoma functionally depend on overexpression of G(1)-regulating genes to maintain their undifferentiated phenotype.

Explorative data analysis of MCL reveals gene expression networks implicated in survival and prognosis supported by explorative CGH analysis

Background

Mantle cell lymphoma (MCL) is an incurable B cell lymphoma and accounts for 6% of all non-Hodgkin's lymphomas. On the genetic level, MCL is characterized by the hallmark translocation t(11;14) that is present in most cases with few exceptions. Both gene expression and comparative genomic hybridization (CGH) data vary considerably between patients with implications for their prognosis.

Methods

We compare patients over and below the median of survival. Exploratory principal component analysis of gene expression data showed that the second principal component correlates well with patient survival. Explorative analysis of CGH data shows the same correlation.

Results

On chromosome 7 and 9 specific genes and bands are delineated which improve prognosis prediction independent of the previously described proliferation signature. We identify a compact survival predictor of seven genes for MCL patients. After extensive re-annotation using GEPAT, we established protein networks correlating with prognosis. Well known genes (CDC2, CCND1) and further proliferation markers (WEE1, CDC25, aurora kinases, BUB1, PCNA, E2F1) form a tight interaction network, but also non-proliferative genes (SOCS1, TUBA1B CEBPB) are shown to be associated with prognosis. Furthermore we show that aggressive MCL implicates a gene network shift to higher expressed genes in late cell cycle states and refine the set of non-proliferative genes implicated with bad prognosis in MCL.

Conclusion

The results from explorative data analysis of gene expression and CGH data are complementary to each other. Including further tests such as Wilcoxon rank test we point both to proliferative and non-proliferative gene networks implicated in inferior prognosis of MCL and identify suitable markers both in gene expression and CGH data.

BM88/CEND1 coordinates cell cycle exit and differentiation of neuronal precursors

During development, coordinate regulation of cell cycle exit and differentiation of neuronal precursors is essential for generation of appropriate number of neurons and proper wiring of neuronal circuits. BM88 is a neuronal protein associated in vivo with terminal neuron-generating divisions, marking the exit of proliferative cells from the cell cycle. Here, we provide functional evidence that BM88 is sufficient to initiate the differentiation of spinal cord neural precursors toward acquisition of generic neuronal and subtype-specific traits. Gain-of-function approaches show that BM88 negatively regulates proliferation of neuronal precursors, driving them to prematurely exit the cell cycle, down-regulate Notch1, and commit to a neuronal differentiation pathway. The combined effect on proliferation and differentiation results in precocious induction of neurogenesis and generation of postmitotic neurons within the ventricular zone. The dual action of BM88 is not recapitulated by the cell cycle inhibitor p27Kip1, suggesting that cell cycle exit does not induce differentiation by default. Mechanistically, induction of endogenous BM88 by forced expression of the proneural gene Mash1 indicates that BM88 is part of the differentiation program activated by proneural genes. Furthermore, BM88 gene silencing conferred by small interfering RNA in spinal cord neural progenitor cells enhances cell cycle progression and impairs neuronal differentiation. Our results implicate BM88 in the synchronization of cell cycle exit and differentiation of neuronal precursors in the developing nervous system.

Comparative proteomic expression profile in all-trans retinoic acid differentiated neuroblastoma cell line

Neuroblastoma (NB) is an infant tumor which frequently differentiates into neurons. We used two-dimensional differential in-gel electrophoresis (2D-DIGE) to analyze the cytosolic and nuclear protein expression patterns of LAN-5 cells following neuronal differentiating agent all-trans-retinoic acid treatment. We identified several candidate proteins, from which G beta2 and Prefoldin 3 may have a role on NB development. These results strength the use of proteomics to discover new putative protein targets in cancer.

BM88 is a dual function molecule inducing cell cycle exit and neuronal differentiation of neuroblastoma cells via cyclin D1 down-regulation and retinoblastoma protein hypophosphorylation

Control of cell cycle progression/exit and differentiation of neuronal precursors is of paramount importance during brain development. BM88 is a neuronal protein associated with terminal neuron-generating divisions in vivo and is implicated in mechanisms underlying neuronal differentiation. Here we have used mouse neuroblastoma Neuro 2a cells as an in vitro model of neuronal differentiation to dissect the functional properties of BM88 by implementing gain- and loss-of-function approaches. We demonstrate that stably transfected cells overexpressing BM88 acquire a neuronal phenotype in the absence of external stimuli, as judged by enhanced expression of neuronal markers and neurite outgrowth-inducing signaling molecules. In addition, cell cycle measurements involving cell growth assays, BrdUrd incorporation, and fluorescence-activated cell sorting analysis revealed that the BM88-transfected cells have a prolonged G(1) phase, most probably corresponding to cell cycle exit at the G(0) restriction point, as compared with controls. BM88 overexpression also results in increased levels of the cell cycle regulatory protein p53, and accumulation of the hypophosphorylated form of the retinoblastoma protein leading to cell cycle arrest, with concomitant decreased levels and, in many cells, cytoplasmic localization of cyclin D1. Conversely, BM88 gene silencing using RNA interference experiments resulted in acceleration of cell proliferation accompanied by impairment of retinoic acid-induced neuronal differentiation of Neuro 2a cells. Taken together, our results suggest that BM88 plays an essential role in regulating cell cycle exit and differentiation of Neuro 2a cells toward a neuronal phenotype and further support its involvement in the proliferation/differentiation transition of neural stem/progenitor cells during embryonic development.

Phosphorylation of progesterone receptor serine 400 mediates ligand-independent transcriptional activity in response to activation of cyclin-dependent protein kinase 2

Human progesterone receptors (PR) are phosphorylated by cyclin-dependent protein kinase 2 (CDK2) at multiple sites, including Ser400. Herein, we have addressed the significance of phosphorylation of this residue. PR phospho-Ser400-specific antibodies revealed regulated phosphorylation of Ser400 in response to progestins and mitogens, and this correlated with increased CDK2 levels and activity. Expression of cyclin E elevated CDK2 activity and downregulated PR independently of ligand. Similarly, overexpression of activated mutant CDK2 increased PR transcriptional activity in the absence and presence of progestin. Mutation of PR Ser400 to alanine (S400A) blocked CDK2-induced PR activity in the absence, but not in the presence, of progestin. PR was unresponsive to activated CDK2 in breast cancer cells with elevated p27, and RNA interference knock-down of p27 partially restored CDK2-induced ligand-independent PR activation. Similarly, in p27(-/-) mouse embryonic fibroblasts, elevated CDK2 activity increased wild-type (wt) but not S400A PR transcriptional activity in the absence of progestin. CDK2 induced nuclear localization of unliganded wt but not S400A PR; liganded S400A PR exhibited delayed nuclear accumulation. These studies demonstrate that CDK2 regulates PR in the absence of progestins via phosphorylation of Ser400, thus revealing a novel mechanism for upregulated PR transcriptional activity in human breast cancer cells expressing altered cell cycle regulatory molecules.

Guanine nucleotide depletion induces differentiation and aberrant neurite outgrowth in human dopaminergic neuroblastoma lines: a model for basal ganglia dysfunction in Lesch-Nyhan disease

Lesch-Nyhan disease (LND), caused by complete deficiency of hypoxanthine guanine phosphoribosyltransferase (HPRT), is characterized by a neurological deficit, the etiology of which is unknown. Evidence has accumulated indicating that it might be related to dysfunction of the basal ganglia with a prominent loss of striatal dopamine fibers. Guanine nucleotide depletion has been shown to occur in cells from Lesch-Nyhan patients. In this study we demonstrate that chronic guanine nucleotide depletion induced by inhibition of inosine monophosphate dehydrogenase with low levels (50 nM) of mycophenolic acid (MPA) lead human neuroblastoma cell lines to differentiate toward the neuronal phenotype. The MPA-induced morphological changes were more evident in the dopaminergic line LAN5, than in the cholinergic line IMR32. MPA-induced differentiation, unlike that induced by retinoic acid, caused a less extensive neurite outgrowth and branching (similar to that observed in cultured HPRT-deficient dopaminergic neurons) and involved up-regulation of p53, p21 and bax, and bcl-2 down-regulation without p27 protein accumulation. These results suggest that guanine nucleotide depletion following HPRT deficiency, might lead to earlier and abnormal brain development mainly affecting the basal ganglia, displaying the highest HPRT activity, and could be responsible for the specific neurobehavioral features of LND.