A unique role for the Rb protein in controlling E2F accumulation during cell growth and differentiation

Transcription factor E2F4 facilitates SUMOylation to promote HCC progression through interaction with LIN9

SUMOylation plays a crucial role in numerous cellular biological and pathophysiological processes associated with human disease; however, the mechanisms regulating the genes involved in SUMOylation remain unclear. In the present study, E2F transcription factor 4 (E2F4) was identified as an E2F member related to hepatocellular carcinoma (HCC) progression by public database analysis. It was found that E2F4 promoted the proliferation and invasiveness of HCC cells via SUMOylation using Soft agar and Transwell migration assays. Mechanistically, it was demonstrated that E2F4 upregulated the transcript and protein expression levels of baculoviral IAP repeat containing 5, cell division cycle associated 8 and DNA topoisomerase II α using western blotting. Furthermore, the interaction between E2F4 with lin‑9 DREAM multi‑vulva class B core complex component (LIN9) was explored by co‑immunoprecipitation, immunofluorescence co‑localization and bimolecular fluorescence complementation assays. Moreover, it was demonstrated that E2F4 promoted the progression of HCC cells via LIN9. Rescue experiments revealed that LIN9 facilitated the SUMOylation and proliferation of HCC cells, which was prevented by knocking down E2F4 expression. In conclusion, the findings of the present study indicated that E2F4 plays a major role in the proliferation of HCC cells and may be a potential therapeutic target in the future.

Expanding Roles of the E2F-RB-p53 Pathway in Tumor Suppression

The transcription factor E2F links the RB pathway to the p53 pathway upon loss of function of pRB, thereby playing a pivotal role in the suppression of tumorigenesis. E2F fulfills a major role in cell proliferation by controlling a variety of growth-associated genes. The activity of E2F is controlled by the tumor suppressor pRB, which binds to E2F and actively suppresses target gene expression, thereby restraining cell proliferation. Signaling pathways originating from growth stimulative and growth suppressive signals converge on pRB (the RB pathway) to regulate E2F activity. In most cancers, the function of pRB is compromised by oncogenic mutations, and E2F activity is enhanced, thereby facilitating cell proliferation to promote tumorigenesis. Upon such events, E2F activates the Arf tumor suppressor gene, leading to activation of the tumor suppressor p53 to protect cells from tumorigenesis. ARF inactivates MDM2, which facilitates degradation of p53 through proteasome by ubiquitination (the p53 pathway). P53 suppresses tumorigenesis by inducing cellular senescence or apoptosis. Hence, in almost all cancers, the p53 pathway is also disabled. Here we will introduce the canonical functions of the RB-E2F-p53 pathway first and then the non-classical functions of each component, which may be relevant to cancer biology.

Regulatory Mechanisms That Guide the Fetal to Postnatal Transition of Cardiomyocytes

Heart disease remains a global leading cause of death and disability, necessitating a comprehensive understanding of the heart's development, repair, and dysfunction. This review surveys recent discoveries that explore the developmental transition of proliferative fetal cardiomyocytes into hypertrophic postnatal cardiomyocytes, a process yet to be well-defined. This transition is key to the heart's growth and has promising therapeutic potential, particularly for congenital or acquired heart damage, such as myocardial infarctions. Although significant progress has been made, much work is needed to unravel the complex interplay of signaling pathways that regulate cardiomyocyte proliferation and hypertrophy. This review provides a detailed perspective for future research directions aimed at the potential therapeutic harnessing of the perinatal heart transitions.

D-Type Cyclins in Development and Disease

D-type cyclins encode G1/S cell cycle checkpoint proteins, which play a crucial role in defining cell cycle exit and progression. Precise control of cell cycle exit is vital during embryonic development, with defects in the pathways regulating intracellular D-type cyclins resulting in abnormal initiation of stem cell differentiation in a variety of different organ systems. Furthermore, stabilisation of D-type cyclins is observed in a wide range of disorders characterized by cellular over-proliferation, including cancers and overgrowth disorders. In this review, we will summarize and compare the roles played by each D-type cyclin during development and provide examples of how their intracellular dysregulation can be an underlying cause of disease.

Regulation of RAD51 at the Transcriptional and Functional Levels: What Prospects for Cancer Therapy?

The RAD51 recombinase is a critical effector of Homologous Recombination (HR), which is an essential DNA repair mechanism for double-strand breaks. The RAD51 protein is recruited onto the DNA break by BRCA2 and forms homopolymeric filaments that invade the homologous chromatid and use it as a template for repair. RAD51 filaments are detectable by immunofluorescence as distinct foci in the cell nucleus, and their presence is a read out of HR proficiency. RAD51 is an essential gene, protecting cells from genetic instability. Its expression is low and tightly regulated in normal cells and, contrastingly, elevated in a large fraction of cancers, where its level of expression and activity have been linked with sensitivity to genotoxic treatment. In particular, BRCA-deficient tumors show reduced or obliterated RAD51 foci formation and increased sensitivity to platinum salt or PARP inhibitors. However, resistance to treatment sets in rapidly and is frequently based on a complete or partial restoration of RAD51 foci formation. Consequently, RAD51 could be a highly valuable therapeutic target. Here, we review the multiple levels of regulation that impact the transcription of the RAD51 gene, as well as the post-translational modifications that determine its expression level, recruitment on DNA damage sites and the efficient formation of homofilaments. Some of these regulation levels may be targeted and their impact on cancer cell survival discussed.

Dissecting the novel partners of nuclear c-Raf and its role in all-trans retinoic acid (ATRA)-induced myeloblastic leukemia cells differentiation

All-trans retinoic acid (ATRA) is an anti-cancer differentiation therapy agent effective for acute promyelocytic leukemia (APL) but not acute myeloid leukemia (AML) in general. Using the HL-60 human non-APL AML model where ATRA causes nuclear enrichment of c-Raf that drives differentiation and G1/G0 cell cycle arrest, we now observe that c-Raf in the nucleus showed novel interactions with several prominent regulators of the cell cycle and cell differentiation. One is cyclin-dependent kinase 2 (Cdk2). ATRA treatment caused c-Raf to dissociate from Cdk2. This was associated with enhanced binding of Cdk2 with retinoic acid receptor α (RARα). Consistent with this novel Raf/CDK2/RARα axis contributing to differentiation, CD38 expression per cell, which is transcriptionally regulated by a retinoic acid response element (RARE), is enhanced. The RB tumor suppressor, a fundamental regulator of G1 cell cycle progression or arrest, was also targeted by c-Raf in the nucleus. RB and specifically the S608 phosphorylated form (pS608RB) complexed with c-Raf. ATRA treatment induced S608RB-hypophosphorylation associated with G1/G0 cell cycle arrest and release of c-Raf from RB. We also found that nuclear c-Raf interacted with SMARCD1, a pioneering component of the SWI/SNF chromatin remodeling complex. ATRA treatment diminished the amount of this protein bound to c-Raf. The data suggest that ATRA treatment to HL-60 human cells re-directed c-Raf from its historically pro-proliferation functions in the cytoplasm to pro-differentiation functions in the nucleus.

Restorative Mechanism of Neural Progenitor Cells Overexpressing Arginine Decarboxylase Genes Following Ischemic Injury

Cell replacement therapy using neural progenitor cells (NPCs) following ischemic stroke is a promising potential therapeutic strategy, but lacks efficacy for human central nervous system (CNS) therapeutics. In a previous in vitro study, we reported that the overexpression of human arginine decarboxylase (ADC) genes by a retroviral plasmid vector promoted the neuronal differentiation of mouse NPCs. In the present study, we focused on the cellular mechanism underlying cell proliferation and differentiation following ischemic injury, and the therapeutic feasibility of NPCs overexpressing ADC genes (ADC-NPCs) following ischemic stroke. To mimic cerebral ischemia in vitro , we subjected the NPCs to oxygen-glucose deprivation (OGD). The overexpressing ADC-NPCs were differentiated by neural lineage, which was related to excessive intracellular calcium-mediated cell cycle arrest and phosphorylation in the ERK1/2, CREB, and STAT1 signaling cascade following ischemic injury. Moreover, the ADC-NPCs were able to resist mitochondrial membrane potential collapse in the increasingly excessive intracellular calcium environment. Subsequently, transplanted ADC-NPCs suppressed infarct volume, and promoted neural differentiation, synapse formation, and motor behavior performance in an in vivo tMCAO rat model. The results suggest that ADC-NPCs are potentially useful for cell replacement therapy following ischemic stroke.

Aurora kinase A revives dormant laryngeal squamous cell carcinoma cells via FAK/PI3K/Akt pathway activation

Revival of dormant tumor cells may be an important tumor metastasis mechanism. We hypothesized that aurora kinase A (AURKA), a cell cycle control kinase, promotes the transition of laryngeal squamous cell carcinoma (LSCC) cells from G0 phase to active division. We therefore investigated whether AURKA could revive dormant tumor cells to promote metastasis. Western blotting revealed that AURKA expression was persistently low in dormant laryngeal cancer Hep2 (D-Hep2) cells and high in non-dormant (T-Hep2) cells. Decreasing AURKA expression in T-Hep2 cells induced dormancy and reduced FAK/PI3K/Akt pathway activity. Increasing AURKA expression in D-Hep2 cells increased FAK/PI3K/Akt pathway activity and enhanced cellular proliferation, migration, invasion and metastasis. In addition, FAK/PI3K/Akt pathway inhibition caused dormancy-like behavior and reduced cellular mobility, migration and invasion. We conclude that AURKA may revive dormant tumor cells via FAK/PI3K/Akt pathway activation, thereby promoting migration and invasion in laryngeal cancer. AURKA/FAK/PI3K/Akt inhibitors may thus represent potential targets for clinical LSCC treatment.

RRD-251 enhances all-trans retinoic acid (RA)-induced differentiation of HL-60 myeloblastic leukemia cells

All-trans-retinoic acid (RA) is known to induce terminal granulocytic differentiation and cell cycle arrest of HL-60 cells. Responding to an RA-induced cytosolic signaling machine, c-Raf translocates to the nucleus, providing propulsion for RA-induced differentiation. This novel mechanism is not understood, but presumably reflects c-Raf binding with nuclear gene regulatory proteins. RRD-251 is a small molecule that prevents the interaction of c-Raf and RB, the retinoblastoma tumor suppressor protein. The involvement of c-Raf and RB in RA-induced differentiation motivates interest in the effects of combined RA and RRD-251 treatment on leukemic cell differentiation.

We demonstrate that RRD-251 enhances RA-induced differentiation. Mechanistically, we find that nuclear translocated c-Raf associates with pS608 RB. RA causes loss of pS608 RB, where cells with hypophosphorylated S608 RB are G_0/G₁ restricted. Corroborating the pS608 RB hypophosphorylation, RB sequestration of E2F increased with concomitant loss of cdc6 expression, which is known to be driven by E2F. Hypophosphorylation of S608 RB releases c-Raf from RB sequestration to bind other nuclear targets. Release of c-Raf from RB sequestration results in enhanced association with GSK-3 which is phosphorylated at its S21/9 inhibitory sites. c-Raf binding to GSK-3 is associated with dissociation of GSK-3 and RARα, thereby relieving RARα of GSK-3 inhibition. RRD-251 amplifies each of these RA-induced events. Consistent with the posited enhancement of RARα transcriptional activity by RRD-251, RRD-251 increases the RARE-driven CD38 expression per cell. The RA/c-Raf/GSK-3/RARα axis emerges as a novel differentiation regulatory mechanism susceptible to RRD-251, suggesting enhancing RA-effects with RRD-251 in therapy.

The c-Raf modulator RRD-251 enhances nuclear c-Raf/GSK-3/VDR axis signaling and augments 1,25-dihydroxyvitamin D3-induced differentiation of HL-60 myeloblastic leukemia cells

Differentiation therapy is used in cancer treatment. Epidemiologic studies showed that higher vitamin D levels are associated with reduced cancer risks. However, the therapeutic doses needed for differentiation are accompanied by hypercalcemia and intolerable pathological sequelae. In the present work we evaluated if RRD-251, a small-molecule, can enhance vitamin D3-induced differentiation of leukemic cells, in the hope of decreasing the needed vitamin D3-dose.

We demonstrate that RRD-251 enhances vitamin D3-induced differentiation of leukemic cells, the enrichment of the c-Raf kinase in the nucleus, the binding of nuclear c-Raf to GSK-3, increased phosphorylation of GSK-3 ser 21/9 inhibitory sites, and the binding of GSK-3 to VDR, where GSK-3 inhibition is known to enhance transcriptional activation by VDR. Enhancement of D3-induced p-GSK-3 ser 21/9 by RRD-251 was associated with enhanced Akt-GSK-3 binding, Akt being a known GSK-3 inhibitor, and diminished Erk1/2 binding. Diminishing Erk interaction with GSK-3 was associated with enhanced interaction with Vav1, a known driver of myeloid differentiation. This is redolent of an ATRA/c-Raf/GSK-3/RARα axis we previously reported, although the phosphorylation effects to enhance transcriptional activation on RARα vs VDR diverge. Taken together this indicates potential therapeutic significance for a c-Raf/GSK-3/VDR or RARα axis for leukemic myelo-monocytic differentiation.

Altered stoichiometry and nuclear delocalization of NonO and PSF promote cellular senescence

While cellular senescence is a critical mechanism to prevent malignant transformation of potentially mutated cells, persistence of senescent cells can also promote cancer and aging phenotypes. NonO/p54nrb and PSF are multifunctional hnRNPs typically found as a complex exclusively within the nuclei of all mammalian cells. We demonstrate here that either increase or reduction of expression of either factor results in cellular senescence. Coincident with this, we observe expulsion of NonO and PSF-containing nuclear paraspeckles and posttranslational modification at G2/M. That senescence is mediated most robustly by overexpression of a cytoplasmic C-truncated form of NonO further indicated that translocation of NonO and PSF from the nucleus is critical to senescence induction. Modulation of NonO and PSF expression just prior to or coincident with senescence induction disrupts the normally heterodimeric NonO-PSF nuclear complex resulting in a dramatic shift in stoichiometry to heterotetramers and monomer with highest accumulation within the cytoplasm. This is accompanied by prototypic cell cycle checkpoint activation and chromatin condensation. These observations identify yet another role for these multifunctional factors and provide a hitherto unprecedented mechanism for cellular senescence and nuclear-cytoplasmic trafficking.

Molecular mechanisms of human papillomavirus-related carcinogenesis in head and neck cancer

This review examines the general cellular and molecular underpinnings of human papillomavirus (HPV)-related carcinogenesis in the context of head and neck squamous cell carcinoma (HNSCC) and focuses on HPV-positive oropharyngeal squamous cell carcinoma in areas for which specific data is available. It covers the major pathways dysregulated in HPV-positive HNSCC and the genome-wide changes associated with this disease.

Targeting nuclear thymidylate biosynthesis

Thymidylate (dTMP) biosynthesis plays an essential and exclusive function in DNA synthesis and proper cell division, and therefore has been an attractive therapeutic target. Folate analogs, known as antifolates, and nucleotide analogs that inhibit the enzymatic action of the de novo thymidylate biosynthesis pathway and are commonly used in cancer treatment. In this review, we examine the mechanisms by which the antifolate 5-fluorouracil, as well as other dTMP synthesis inhibitors, function in cancer treatment in light of emerging evidence that dTMP synthesis occurs in the nucleus. Nuclear localization of the de novo dTMP synthesis pathway requires modification of the pathway enzymes by the small ubiquitin-like modifier (SUMO) protein. SUMOylation is required for nuclear localization of the de novo dTMP biosynthesis pathway, and disruption in the SUMO pathway inhibits cell proliferation in several cancer models. We summarize evidence that the nuclear localization of the dTMP biosynthesis pathway is a critical factor in the efficacy of antifolate-based therapies that target dTMP synthesis.

Ablation of the transcription factors E2F1-2 limits neuroinflammation and associated neurological deficits after contusive spinal cord injury

Traumatic spinal cord injury (SCI) induces cell cycle activation (CCA) that contributes to secondary injury and related functional impairments such as motor deficits and hyperpathia. E2F1 and E2F2 are members of the activator sub-family of E2F transcription factors that play an important role in proliferating cells and in cell cycle-related neuronal death, but no comprehensive study have been performed in SCI to determine the relative importance of these factors. Here we examined the temporal distribution and cell-type specificity of E2F1 and E2F2 expression following mouse SCI, as well as the effects of genetic deletion of E2F1-2 on neuronal cell death, neuroinflammation and associated neurological dysfunction. SCI significantly increased E2F1 and E2F2 expression in active caspase-3(+) neurons/oligodendrocytes as well as in activated microglia/astrocytes. Injury-induced up-regulation of cell cycle-related genes and protein was significantly reduced by intrathecal injection of high specificity E2F decoy oligodeoxynucleotides against the E2F-binding site or in E2F1-2 null mice. Combined E2F1+2 siRNA treatment show greater neuroprotection in vivo than E2F1 or E2F2 single siRNA treatment. Knockout of both E2F1 and E2F2 genes (E2Fdko) significantly reduced neuronal death, neuroinflammation, and tissue damage, as well as limiting motor dysfunction and hyperpathia after SCI. Both CCA reduction and functional improvement in E2Fdko mice were greater than those in E2F2ko model. These studies demonstrate that SCI-induced activation of E2F1-2 mediates CCA, contributing to gliopathy and neuronal/tissue loss associated with motor impairments and post-traumatic hyperesthesia. Thus, E2F1-2 provide a therapeutic target for decreasing secondary tissue damage and promoting recovery of function after SCI.

Elevated cyclin A associated kinase activity promotes sensitivity of metastatic human cancer cells to DNA antimetabolite drug

Drug resistance is a major obstacle in successful systemic therapy of metastatic cancer. We analyzed the involvement of cell cycle regulatory proteins in eliciting response to N (phosphonoacetyl)-L-aspartate (PALA), an inhibitor of de novo pyrimidine synthesis, in two metastatic variants of human cancer cell line MDA-MB-435 isolated from lung (L-2) and brain (Br-1) in nude mouse, respectively. L-2 and Br-l cells markedly differed in their sensitivity to PALA. While both cell types displayed an initial S phase delay/arrest, Br-l cells proliferated but most L-2 cells underwent apoptosis. There was distinct elevation in cyclin A, and phosphorylated Rb proteins concomitant with decreased expression of bcl-2 protein in the PALA treated L-2 cells undergoing apoptosis. Markedly elevated cyclin A associated and cdk2 kinase activities together with increased E2F1-DNA binding were detected in these L-2 cells. Induced ectopic cyclin A expression sensitized Br-l cells to PALA by activating an apoptotic pathway. Our findings demonstrate that elevated expression of cyclin A and associated kinase can activate an apoptotic pathway in cells exposed to DNA antimetabolites. Abrogation of this pathway can lead to resistance against these drugs in metastatic variants of human carcinoma cells.

Rb and p107 are required for alpha cell survival, beta cell cycle control and glucagon-like peptide-1 action

AIMS/HYPOTHESIS

Diabetes mellitus is characterised by beta cell loss and alpha cell expansion. Analogues of glucagon-like peptide-1 (GLP-1) are used therapeutically to antagonise these processes; thus, we hypothesised that the related cell cycle regulators retinoblastoma protein (Rb) and p107 were involved in GLP-1 action.

METHODS

We used small interfering RNA and adenoviruses to manipulate Rb and p107 expression in insulinoma and alpha-TC cell lines. In vivo we examined pancreas-specific Rb knockout, whole-body p107 knockout and Rb/p107 double-knockout mice.

RESULTS

Rb, but not p107, was downregulated in response to the GLP-1 analogue, exendin-4, in both alpha and beta cells. Intriguingly, this resulted in opposite outcomes of cell cycle arrest in alpha cells but proliferation in beta cells. Overexpression of Rb in alpha and beta cells abolished or attenuated the effects of exendin-4 supporting the important role of Rb in GLP-1 modulation of cell cycling. Similarly, in vivo, Rb, but not p107, deficiency was required for the beta cell proliferative response to exendin-4. Consistent with this finding, Rb, but not p107, was suppressed in islets from humans with diabetes, suggesting the importance of Rb regulation for the compensatory proliferation that occurs under insulin resistant conditions. Finally, while p107 alone did not have an essential role in islet homeostasis, when combined with Rb deletion, its absence potentiated apoptosis of both alpha and beta cells resulting in glucose intolerance and diminished islet mass with ageing.

CONCLUSIONS/INTERPRETATION

We found a central role of Rb in the dual effects of GLP-1 in alpha and beta cells. Our findings highlight unique contributions of individual Rb family members to islet cell proliferation and survival.

The heart: mostly postmitotic or mostly premitotic? Myocyte cell cycle, senescence, and quiescence

The concept of myocyte division and myocyte-mediated regeneration has re-emerged in the past 5 years through development of sophisticated transgenic mice and carbon-dating of cells. Although recently, a couple of studies have been conducted as an attempt to intervene in myocyte division, the efficiency in adult animals remains discouragingly low. Re-enforcing myocyte division is a vision that has been desired for decades, leading to years of experience in myocyte resistance to proproliferative stimuli. Previous attempts have indeed provided a platform for basic knowledge on molecular players and signalling in myocytes. However, natural biological processes such as hypertrophy and binucleation provide layers of complexity in interpretation of previous and current findings. A major hurdle in mediating myocyte division is a lack of insight in the myocyte cell cycle. To date, no knowledge is gained on myoycte cell cycle progression and/or duration. This review will include an overview of previous and current literature on myocyte cell cycle and division. Furthermore, the limitations of current approaches and basic questions that might be essential in understanding myocardial resistance to division will be discussed.

The DNA damage response induced by infection with human cytomegalovirus and other viruses

Viruses use different strategies to overcome the host defense system. Recent studies have shown that viruses can induce DNA damage response (DDR). Many of these viruses use DDR signaling to benefit their replication, while other viruses block or inactivate DDR signaling. This review focuses on the effects of DDR and DNA repair on human cytomegalovirus (HCMV) replication. Here, we review the DDR induced by HCMV infection and its similarities and differences to DDR induced by other viruses. As DDR signaling pathways are critical for the replication of many viruses, blocking these pathways may represent novel therapeutic opportunities for the treatment of certain infectious diseases. Lastly, future perspectives in the field are discussed.

Skin Tumors Rb(eing) Uncovered

The Rb1 gene was the first bona fide tumor suppressor identified and cloned more than 25 years ago. Since then, a plethora of studies have revealed the functions of pRb and the existence of a sophisticated and strictly regulated pathway that modulates such functional roles. An emerging paradox affecting Rb1 in cancer connects the relatively low number of mutations affecting Rb1 gene in specific human tumors, compared with the widely functional inactivation of pRb in most, if not in all, human cancers. The existence of a retinoblastoma family of proteins pRb, p107, and p130 and their potential unique and overlapping functions as master regulators of cell cycle progression and transcriptional modulation by similar processes, may provide potential clues to explain such conundrum. Here, we will review the development of different genetically engineered mouse models, in particular those affecting stratified epithelia, and how they have offered new avenues to understand the roles of the Rb family members and their targets in the context of tumor development and progression.

E2F6 associates with BRG1 in transcriptional regulation

The E2F6 protein functions as an Rb-independent repressor of gene transcription. We have previously provided evidence suggesting a role for E2F6 in repression of E2F-responsive genes at S phase. Here, we have identified BRG1, the ATPase subunit of the SWI/SNF chromatin-remodeling complex, as an E2F6 interacting protein. Immunoprecipitation experiments demonstrate that BRG1 binds specifically to E2F6 and E2F4 but not the activator E2Fs. E2F6 was also able to interact with BAF155, a BRG1-associated factor, in the SWI/SNF complex. Chromatin immunoprecipitation assays demonstrate the binding of BRG1 coincident with E2F6 on G1/S gene promoters during S phase. Collectively, our studies suggest that E2F6 may recruit BRG1 in transcriptional regulation of genes important for G1/S phase transition of the cell cycle.

Covariation of copy number located at 16q22.1: new evidence in mammary ductal carcinoma

Copy number variation (CNV) is crucial for gene regulation in humans. A number of studies have revealed that CNV contributes to the initiation and progression of cancer. In this study, we analysed four breast cancer cell lines and six fresh frozen tissues from patients to evaluate the CNV present in the genome using microarray-based comparative genomic hybridization (aCGH). Six genes located at 16q22.1 were analysed by real-time PCR. The real-time PCR analysis revealed that the loss of CDH1/E2F4 may be associated with worse clinical and pathological findings. Interestingly, covariation of CDH1, CDH3, CTCF and E2F4 was found to be associated with triple negative breast cancer and HER-2 receptor status. In conclusion, our study supports the idea that CNV at 16q22.1 in breast cancer is a frequent event; furthermore, it reveals the covariation of CDH1, CDH3, CTCF and E2F4. The role of the covariation is more complex than a simple additive effect of these four separate genes, which may provide a novel target for breast cancer.

Rbf1 degron dysfunction enhances cellular DNA replication

The E2F family of transcription factors contributes to oncogenesis through activation of multiple genes involved in cellular proliferation, a process that is opposed by the Retinoblastoma tumor suppressor protein (RB). RB also increases E2F1 stability by inhibiting its proteasome-mediated degradation, but the consequences of this post-translational regulation of E2F1 remain unknown. To better understand the mechanism of E2F stabilization and its physiological relevance, we examined the streamlined Rbf1-dE2F1 network in Drosophila. During embryonic development, Rbf1 is insulated from ubiquitin-mediated turnover by the COP9 signalosome, a multi-protein complex that modulates E3 ubiquitin ligase activity. Here, we report that the COP9 signalosome also protects the Cullin4-E3 ligase that is responsible for dE2F1 proteasome-mediated destruction. This dual role of the COP9 signalosome may serve to buffer E2F levels, enhancing its turnover via Cul4 protection and its stabilization through protection of Rbf1. We further show that Rbf1-mediated stabilization of dE2F1 and repression of dE2F1 cell cycle-target genes are distinct properties. Removal of an evolutionarily conserved Rbf1 C terminal degron disabled Rbf1 repression without affecting dE2F1 stabilization. This mutant form of Rbf1 also enhanced G(1)-to-S phase progression when expressed in Rbf1-containing S2 embryonic cells, suggesting that such mutations may generate gain-of-function properties relevant to cellular transformation. Consistent with this idea, several studies have identified mutations in the homologous C terminal domains of RB and p130 in human cancer.

Hwanggeumchal sorghum induces cell cycle arrest, and suppresses tumor growth and metastasis through Jak2/STAT pathways in breast cancer xenografts

BACKGROUND

Cancer is one of the highly virulent diseases known to humankind with a high mortality rate. Breast cancer is the most common cancer in women worldwide. Sorghum is a principal cereal food in many parts of the world, and is critical in folk medicine of Asia and Africa. In the present study, we analyzed the effects of HSE in metastatic breast cancer.

METHODOLOGY/PRINCIPAL FINDINGS

Preliminary studies conducted on MDA-MB 231 and MCF-7 xenograft models showed tumor growth suppression by HSE. Western blotting studies conducted both in vivo and in vitro to check the effect of HSE in Jak/STAT pathways. Anti-metastatic effects of HSE were confirmed using both MDA-MB 231 and MCF-7 metastatic animal models. These studies showed that HSE can modulate Jak/STAT pathways, and it hindered the STAT5b/IGF-1R and STAT3/VEGF pathways not only by down-regulating the expression of these signal molecules and but also by preventing their phosphorylation. The expression of angiogenic factors like VEGF, VEGF-R2 and cell cycle regulators like cyclin D, cyclin E, and pRb were found down-regulated by HSE. In addition, it also targets Brk, p53, and HIF-1α for anti-cancer effects. HSE induced G1 phase arrest and migration inhibition in MDA-MB 231 cells. The metastasis of breast cancer to the lungs also found blocked by HSE in the metastatic animal model.

CONCLUSIONS/SIGNIFICANCE

Usage of HS as a dietary supplement is an inexpensive natural cancer therapy, without any side effects. We strongly recommend the use of HS as an edible therapeutic agent as it possesses tumor suppression, migration inhibition, and anti-metastatic effects on breast cancer.

Target genes of microsatellite sequences in head and neck squamous cell carcinoma: mononucleotide repeats are not detected

Microsatellite instability (MSI) is detected in a wide variety of tumors. It is thought that mismatch repair gene mutation or inactivation is the major cause of MSI. Microsatellite sequences are predominantly distributed in intergenic or intronic DNA. However, MSI is found in the exonic sequences of some genes, causing their inactivation. In this report, we searched GenBank for candidate genes containing potential MSI sequences in exonic regions. Twenty seven target genes were selected for MSI analysis. Instability was found in 70% of these genes (14/20) with head and neck squamous cell carcinoma (HNSCC). Interestingly, no instability was detected in mononucleotide repeats in genes or in intergenic sequences. We conclude that instability of mononucleotide repeats is a rare event in HNSCC. High MSI phenotype in young HNSCC patients is limited to noncoding regions only. MSI percentage in HNSCC tumor is closely related to the repeat type, repeat location and patient's age.

Cooperation between ARID3A and p53 in the transcriptional activation of p21WAF1 in response to DNA damage

ARID3A/DRIL1/Bright is a family member of the AT rich interaction domain (ARID) DNA-binding proteins that are involved in diverse biological processes. We have reported that p53 activates ARID3A transcription, and ARID3A overexpression induces G1 arrest. However, the role of ARID3A in the p53 pathway remains unclear. Here, we show that ARID3A cooperates with p53 to transcriptionally activate p21(WAF1), a p53-target gene important for cell-cycle arrest. ARID3A bound to its binding sites in the p21(WAF1) promoter in vivo and in vitro, and induced p21(WAF1) transcription in U2OS cells expressing wild-type p53 but not Saos-2 cells lacking p53. The co-expression of ARID3A with p53 cooperates to activate p21(WAF1) transcription and the stably transfected p21(WAF1) promoter. Mutation of the ARID3A binding sites reduced the p21(WAF1) promoter activity, and siRNA-based ARID3A knockdown suppressed the transcription of p21(WAF1), but not the proapoptotic NOXA and PUMA in response to DNA damage. Furthermore, p53 knockdown decreased ARID3A transcription, and, conversely, ARID3A overexpression and knockdown resulted in an increase or decrease in p53 stability, respectively. These results indicate both cooperative and interdependent roles for ARID3A and p53 in the transcriptional activation of p21(WAF1) in response to DNA damage.

Mad2 is a critical mediator of the chromosome instability observed upon Rb and p53 pathway inhibition

Multiple mechanisms have been proposed to explain how Rb and p53 tumor suppressor loss lead to chromosome instability (CIN). It was recently shown that Rb pathway inhibition causes overexpression of the mitotic checkpoint gene Mad2, but whether Mad2 overexpression is required to generate CIN in this context is unknown. Here, we show that CIN in cultured cells lacking Rb family proteins requires Mad2 upregulation and that this upregulation is also necessary for CIN and tumor progression in vivo. Mad2 is also repressed by p53 and its upregulation is required for CIN in a p53 mutant tumor model. These results demonstrate that Mad2 overexpression is a critical mediator of the CIN observed upon inactivation of two major tumor suppressor pathways.

S-phase-coupled apoptosis in tumor suppression

DNA replication is essential for accurate transmission of genomic information from parental to daughter cells. DNA replication is licensed once per cell division cycle. This process is highly regulated by both positive and negative regulators. Over-replication, under-replication, as well as DNA damage in a cell all induce the activation of checkpoint control pathways such as ATM/ATR, CHK kinases, and the tumor suppressor protein p53, which provide "damage controls" via either DNA repairs or apoptosis. This review focuses on accumulating evidence, with the emphasis on recently discovered Killin, that S-phase checkpoint control is crucial for a mammalian cell to make a life and death decision in order to safeguard genome integrity.

Potent anti-tumor effect generated by a novel human papillomavirus (HPV) antagonist peptide reactivating the pRb/E2F pathway

Human papillomavirus type 16 (HPV16) E7 is a viral oncoprotein believed to play a major role in cervical cancer. In this study, an antagonist peptide against HPV16E7 protein was first identified from screening the c7c phage display peptide library. The binding specificity and affinity of the selected peptide to HPV16E7 were tested by competitive enzyme-linked immunosorbent assay (ELISA). The antagonist peptide showed obvious anti-tumor efficacy both in cell lines and animal tumor models. Significant cell proliferation inhibition with high specificity was noted when HPV16-positive cells were treated with the peptide. This anti-tumor efficacy was resulted from overriding the activities of HPV16E7 and reactivating the pRb/E2F pathway, as shown by a series of experiments. Flow cytometry analysis revealed that the selected peptide induced G1 arrest in a dose-dependent manner. Competitive ELISA, pull down, and Co-IP experiments indicated that the selected peptide disrupted the interaction between HPV16E7 and pRb proteins both in vitro and in vivo. Luciferase reporter assay verified that transcription activities of E2F were suppressed by the peptide through restoration of pRb. RT-PCR and Western blot revealed that it reduced cyclins A, D1, and E1 expression, and led to HPV16E7 protein degradation, but pRb protein stabilization. The current study suggests that this specific peptide may serve as a potential therapeutic agent for HPV16-positive cervical cancer.

Wide-ranging functions of E2F4 in transcriptional activation and repression revealed by genome-wide analysis

The E2F family of transcription factors has important roles in cell cycle progression. E2F4 is an E2F family member that has been proposed to be primarily a repressor of transcription, but the scope of its binding activity and functions in transcriptional regulation is not fully known. We used ChIP sequencing (ChIP-seq) to identify around 16 000 E2F4 binding sites which potentially regulate 7346 downstream target genes with wide-ranging functions in DNA repair, cell cycle regulation, apoptosis, and other processes. While half of all E2F4 binding sites (56%) occurred near transcription start sites (TSSs), ∼20% of sites occurred more than 20 kb away from any annotated TSS. These distal sites showed histone modifications suggesting that E2F4 may function as a long-range regulator, which we confirmed by functional experimental assays on a subset. Overexpression of E2F4 and its transcriptional cofactors of the retinoblastoma (Rb) family and its binding partner DP-1 revealed that E2F4 acts as an activator as well as a repressor. E2F4 binding sites also occurred near regulatory elements for miRNAs such as let-7a and mir-17, suggestive of regulation of miRNAs by E2F4. Taken together, our genome-wide analysis provided evidence of versatile roles of E2F4 and insights into its functions.

Expression of cyclin E in postmitotic neurons during development and in the adult mouse brain

Cyclin E, a member of the G1 cyclins, is essential for the G1/S transition of the cell cycle in cultured cells, but its roles in vivo are not fully defined. The present study characterized the spatiotemporal expression profile of cyclin E in two representative brain regions in the mouse, the cerebral and cerebellar cortices. Western blotting showed that the levels of cyclin E increased towards adulthood. In situ hybridization and immunohistochemistry showed the distributions of cyclin E mRNA and protein were comparable in the cerebral cortex and the cerebellum. Immunohistochemistry for the proliferating cell marker, proliferating cell nuclear antigen (PCNA) revealed that cyclin E was expressed by both proliferating and non-proliferating cells in the cerebral cortex at embryonic day 12.5 (E12.5) and in the cerebellum at postnatal day 1 (P1). Subcellular localization in neurons was examined using immunofluorescence and western blotting. Cyclin E expression was nuclear in proliferating neuronal precursor cells but cytoplasmic in postmitotic neurons during embryonic development. Nuclear cyclin E expression in neurons remained faint in newborns, increased during postnatal development and was markedly decreased in adults. In various adult brain regions, cyclin E staining was more intense in the cytoplasm than in the nucleus in most neurons. These data suggest a role for cyclin E in the development and function of the mammalian central nervous system and that its subcellular localization in neurons is important. Our report presents the first detailed analysis of cyclin E expression in postmitotic neurons during development and in the adult mouse brain.

Cell-context specific role of the E2F/Rb pathway in development and disease

Development of the central nervous system (CNS) requires the generation of neuronal and glial cell subtypes in appropriate numbers, and this demands the careful coordination of cell-cycle exit, survival, and differentiation. The E2F/Rb pathway is critical for cell-cycle regulation and also modulates survival and differentiation of distinct cell types in the developing and adult CNS. In this review, we first present the specific temporal patterns of expression of the E2F and Rb family members during CNS development and then discuss the genetic ablation of single or multiple members of these two families. Overall, the available data suggest a time-dependent and cell-context specific role of E2F and Rb family members in the developing and adult CNS.

pRB and E2F4 play distinct cell-intrinsic roles in fetal erythropoiesis

The retinoblastoma tumor suppressor protein pRB functions, at least in part, by directly binding to and modulating the activity of the E2F transcription factors. Previous studies have shown that both E2F4 and pRB play important roles in fetal erythropoiesis. Given that these two proteins interact directly we investigated the overlap of E2F4 and pRB function in this process by analyzing E2f4(-/-), conditional Rb knockout (Rb(1lox/1lox)), and compound E2f4(-/-);Rb(1lox/1lox) embryos. At E15.5 E2f4(-/-) and Rb(1lox/1lox) fetal erythroid cells display distinct abnormalities in their differentiation profiles. When cultured in vitro, both E2f4(-/-) and Rb(1lox/1lox) erythroid cells show defects in cell cycle progression. Surprisingly, analysis of cell cycle profiling suggests that E2F4 and pRB control cell cycle exit through different mechanisms. Moreover, only pRB, but not E2F4, promotes cell survival in erythroid cells. We observed an additive rather than a synergistic impact upon the erythroid defects in the compound E2f4(-/-);Rb(1lox/1lox) embryos. We further found that fetal liver macrophage development is largely normal regardless of genotype. Taken together, our results show that E2F4 and pRB play independent cell-intrinsic roles in fetal erythropoiesis.

E2F4 expression is required for cell cycle progression of normal intestinal crypt cells and colorectal cancer cells

The generation of knock-out mice for E2F4 gene expression has suggested a role for this transcription factor in establishing and/or maintaining the intestinal crypt compartment. Having previously demonstrated that E2F4 is cytoplasmic in quiescent-differentiated cells but nuclear in growth factor-stimulated proliferative cells, the present study was aimed at determining the role of E2F4 in the control of human intestinal epithelial proliferation. Results herein demonstrate that lentiviral infection of an shRNA which specifically knocked-down E2F4 expression slowed down G1/S phase transition and the proliferation rate of normal human intestinal epithelial cells (HIEC) and of colon cancer cells. Protein expression of Cdk2, cyclins D1 and A, Cdc25A and c-myc was markedly down-regulated in shE2F4-expressing cells; by contrast, expression of the cell cycle inhibitors p21(Cip/Waf) and p27(Kip1) was increased. In addition, the expression of many genes involved in DNA synthesis was down-regulated in shE2F4-expressing cells, whereas no modulation in E2F1 expression was observed. A decrease in E2F4 in colon cancer cell lines also resulted in a reduction in soft-agar growth capacity. Immunofluorescence experiments in human fetal intestine revealed that cells expressing high nuclear levels of E2F4 also expressed cyclin A protein. Lastly, E2F4 and its target cyclin A were up-regulated and mostly nuclear in human colorectal tumor cells in comparison to the corresponding benign epithelium. These results indicate that nuclear E2F4 may be determinant in the promotion of proliferation of human intestinal epithelial crypt cells and colorectal cancer cells.

A combinatorial mechanism for determining the specificity of E2F activation and repression

Various studies have detailed the role of E2F proteins in both transcription activation and repression. Further work has shown that distinct promoter elements, but comprising the same E2F recognition motif, confer positive or negative E2F control and that this reflects binding of either activator or repressor E2F proteins respectively. We now show that the specificity of binding of an activator or repressor E2F protein is determined by adjacent sequences that bind a cooperating transcription factor. We propose that the functional E2F element is a module comprising not only the E2F binding site but also the adjacent site for the cooperating transcription factor.

E2f3a and E2f3b contribute to the control of cell proliferation and mouse development

The E2f3 locus encodes two Rb-binding gene products, E2F3a and E2F3b, which are differentially regulated during the cell cycle and are thought to be critical for cell cycle progression. We targeted the individual inactivation of E2f3a or E2f3b in mice and examined their contributions to cell proliferation and development. Chromatin immunoprecipitation and gene expression experiments using mouse embryo fibroblasts deficient in each isoform showed that E2F3a and E2F3b contribute to G(1)/S-specific gene expression and cell proliferation. Expression of E2f3a or E2f3b was sufficient to support E2F target gene expression and cell proliferation in the absence of other E2F activators, E2f1 and E2f2, suggesting that these isoforms have redundant functions. Consistent with this notion, E2f3a(-/-) and E2f3b(-/-) embryos developed normally, whereas embryos lacking both isoforms (E2f3(-/-)) died in utero. We also find that E2f3a and E2f3b have redundant and nonredundant roles in the context of Rb mutation. Analysis of double-knockout embryos suggests that the ectopic proliferation and apoptosis in Rb(-/-) embryos is mainly mediated by E2f3a in the placenta and nervous system and by both E2f3a and E2f3b in lens fiber cells. Together, we conclude that the contributions of E2F3a and E2F3b in cell proliferation and development are context dependent.

A small-molecule E2F inhibitor blocks growth in a melanoma culture model

HLM006474 was identified using a computer-based virtual screen and the known crystal structure of the DNA-bound E2F4/DP2 heterodimer. Treatment of multiple cell lines with HLM006474 resulted in the loss of intracellular E2F4 DNA-binding activity as measured by electrophoretic mobility shift assay within hours. Overnight exposure to HLM006474 resulted in down-regulation of total E2F4 protein as well as known E2F targets. The effects of HLM006474 treatment on different cell lines varied but included a reduction in cell proliferation and an increase in apoptosis. HLM006474 induced apoptosis in a manner distinct from cisplatin and doxorubicin. E2F4-null mouse embryonic fibroblasts were less sensitive than wild-type counterparts to the apoptosis-inducing activity of the compound, revealing its biological specificity. A375 cells were extremely sensitive to the apoptosis-inducing activity of the compound in two-dimensional culture, and HLM006474 was a potent inhibitor of melanocytes proliferation and subsequent invasion in a three-dimensional tissue culture model system. Together, these results suggest that interference with E2F activity using small molecules may have clinical application in cancer therapy.

Role of human T-cell leukemia virus type I Tax in expression of the human telomerase reverse transcriptase (hTERT) gene in human T-cells

The viral product Tax encoded by human T-cell leukemia virus type I (HTLV-I) is thought to play a central role in leukemogenesis. Clonal expansion of HTLV-I-infected cells requires the extension of cell division with telomere maintenance, which is regulated by the ribonucleoprotein enzyme telomerase. However, the roles of Tax in the expression of telomerase activity in T-cells remains controversial. Our previous study indicated that expression of the human telomerase reverse transcriptase subunit (hTERT) gene, which determines telomerase activity, is tightly regulated in human T-cells. In the present study, we investigated Tax-mediated regulation of hTERT gene expression by Tax in human T-cells. HTLV-I Tax induced expression of the hTERT gene in human peripheral blood leukocytes. Reporter assays revealed that Tax activated the hTERT promoter in quiescent Kit 225 cells, while the promoter activity was repressed by Tax in proliferating Jurkat cells. Both up-regulation and down-regulation by Tax were mediated through the 43-bp sequences in the promoter, which carried at least two elements that independently functioned as repressors. The two elements bound distinct factors. G1 to S phase transition induced by introduction of either cyclin D2 with cdk4 or p130-specific shRNA also activated the hTERT promoter, implying that activation of the hTERT promoter in quiescent Kit 225 cells is associated with cell cycle progression. Our findings suggest that the cell cycle state critically influences Tax-mediated regulation of hTERT expression.

Interrogating domain-domain interactions with parsimony based approaches

Background

The identification and characterization of interacting domain pairs is an important step towards understanding protein interactions. In the last few years, several methods to predict domain interactions have been proposed. Understanding the power and the limitations of these methods is key to the development of improved approaches and better understanding of the nature of these interactions.

Results

Building on the previously published Parsimonious Explanation method (PE) to predict domain-domain interactions, we introduced a new Generalized Parsimonious Explanation (GPE) method, which (i) adjusts the granularity of the domain definition to the granularity of the input data set and (ii) permits domain interactions to have different costs. This allowed for preferential selection of the so-called "co-occurring domains" as possible mediators of interactions between proteins. The performance of both variants of the parsimony method are competitive to the performance of the top algorithms for this problem even though parsimony methods use less information than some of the other methods. We also examined possible enrichment of co-occurring domains and homo-domains among domain interactions mediating the interaction of proteins in the network. The corresponding study was performed by surveying domain interactions predicted by the GPE method as well as by using a combinatorial counting approach independent of any prediction method. Our findings indicate that, while there is a considerable propensity towards these special domain pairs among predicted domain interactions, this overrepresentation is significantly lower than in the iPfam dataset.

Conclusion

The Generalized Parsimonious Explanation approach provides a new means to predict and study domain-domain interactions. We showed that, under the assumption that all protein interactions in the network are mediated by domain interactions, there exists a significant deviation of the properties of domain interactions mediating interactions in the network from that of iPfam data.

TGFbeta-mediated formation of pRb-E2F complexes in human myeloid leukemia cells

TGFbeta is well known for its inhibitory effect on cell cycle G1 checkpoint kinases. However, its role in the control of pRb-E2F complexes is not well established. TGFbeta inhibits phosphorylation of pRb at several serine and threonine residues and regulates the association of E2F transcription factors with pRb family proteins. Recent studies found that predominantly E2F-4, p130, and histone deacetylase (HDAC) are found to bind to corresponding E2F-responsive promoters in G0/G1 phase. As cells progress through mid-G1, p130-E2F4 complex are replaced by p107-E2F4 followed by activators E2F1, 2, and 3. pRb was not detectable in the promoters containing the E2F-responsive site in cycling cells but was associated with E2F4-p130 complexes or E2F4-p107 complexes during G0/G1 phase. In human myeloid leukemia cell line, MV4-11, TGFbeta upregulated pRb-E2F-4 and p130-E2F-4, and downregulated p107-E2F-4 complexes. However, pRB-E2F1 and pRb-E2F3 complexes were found in proliferating cells but not in TGFbeta arrested G1 cells. In addition, electrophoretic gel mobility shift assay (EMSA) could not detect pRb-E2F DNA-binding activities either in S or G1 phase but exhibited the existence of p107-E2F4 in proliferating cells and p130-E2F4 complexes in TGFbeta-arrested G1 cells, respectively. Our data suggest that p107 and p130, but not pRb, and the repressor E2F, but not activator E2Fs, play a critical role in regulating E2F-responsive gene expression in TGFbeta-mediated cell cycle control in human myeloid leukemia cells.

Interaction of E2F-Rb family members with corepressors binding to the adjacent E2F site

Cell cycle-dependent transcriptional repression of the E2F1 and B-myb promoters is mediated through E2F-binding sites and adjacent corepressor site (cell cycle gene homology region (CHR)/downstream repression site (DRS)). Here, we show that a factor binding to the B-myb CHR is co-purified with E2F DNA-binding activity, and coimmunoprecipitated with components of E2F/Rb-family repressor complexes, E2F4 and retinoblastoma (Rb) family proteins. In spite of structural and functional similarities, however, the E2F1 and B-myb CHRs exhibited distinct factor-binding specificities. Furthermore, substitution of E2F1 CHR with the B-myb CHR in the E2F1 promoter revealed that the B-myb CHR was unable to repress the E2F1 promoter completely in the G0 phase. These results suggest that transcriptional repression of the E2F1 and B-myb promoters is mediated by physical interaction of E2F/Rb-family repressor complexes with promoter-specific corepressors.

The significance of p53 and retinoblastoma pathways in canine hemangiosarcoma

To investigate whether inactivation of the p53 and retinoblastoma (Rb) protein pathways contributes to the development of canine hemangiosarcoma (HSA), we examined immunohistochemically the expression of p53, Rb, phosphorylated Rb (phospho-Rb), p16, and cyclin D1 in 39 spontaneous canine HSAs and 10 hemangiomas. In addition, mutations in the p53 gene were analyzed by polymerase chain reaction (PCR)-single-stranded conformation polymorphism and PCR direct sequencing; furthermore, we quantified cyclin D1 mRNA by semiquantitative real-time reverse transcription-PCR. Positive immunoreactivity for p53 was observed in 17.9% of HSAs. However, mutations were not detected in these cases. The labeling indices for Rb, phospho-Rb, and cyclin D1 were markedly higher in all HSAs than in hemangiomas. Of the 7 cases with cyclin D1-positive immunoreactivity, 4 overexpressed cyclin D1 mRNA (to a level more than 10-fold higher than that of GAPDH mRNA). The p16 protein was clearly detected in all hemangiomas; however, 82% of the neoplastic cells in HSA showed a loss of or low immunoreactivity. These results suggest that alteration of the p16-cyclin D1-Rb pathway, rather than the p53 pathway, may be associated with the pathogenesis of canine HSA.

Ab initio phenomenological simulation of the growth of large tumor cell populations

In a previous paper we have introduced a phenomenological model of cell metabolism and of the cell cycle to simulate the behavior of large tumor cell populations (Chignola and Milotti 2005 Phys. Biol. 2 8). Here we describe a refined and extended version of the model that includes some of the complex interactions between cells and their surrounding environment. The present version takes into consideration several additional energy-consuming biochemical pathways such as protein and DNA synthesis, the tuning of extracellular pH and of the cell membrane potential. The control of the cell cycle, which was previously modeled by means of ad hoc thresholds, has been directly addressed here by considering checkpoints from proteins that act as targets for phosphorylation on multiple sites. As simulated cells grow, they can now modify the chemical composition of the surrounding environment which in turn acts as a feedback mechanism to tune cell metabolism and hence cell proliferation: in this way we obtain growth curves that match quite well those observed in vitro with human leukemia cell lines. The model is strongly constrained and returns results that can be directly compared with actual experiments, because it uses parameter values in narrow ranges estimated from experimental data, and in perspective we hope to utilize it to develop in silico studies of the growth of very large tumor cell populations (10(6) cells or more) and to support experimental research. In particular, the program is used here to make predictions on the behavior of cells grown in a glucose-poor medium: these predictions are confirmed by experimental observation.

Unique requirement for Rb/E2F3 in neuronal migration: evidence for cell cycle-independent functions

The cell cycle regulatory retinoblastoma (Rb) protein is a key regulator of neural precursor proliferation; however, its role has been expanded to include a novel cell-autonomous role in mediating neuronal migration. We sought to determine the Rb-interacting factors that mediate both the cell cycle and migration defects. E2F1 and E2F3 are likely Rb-interacting candidates that we have shown to be deregulated in the absence of Rb. Using mice with compound null mutations of Rb and E2F1 or E2F3, we asked to what extent either E2F1 or E2F3 interacts with Rb in neurogenesis. Here, we report that E2F1 and E2F3 are both functionally relevant targets in neural precursor proliferation, cell cycle exit, and laminar patterning. Each also partially mediates the Rb requirement for neuronal survival. Neuronal migration, however, is specifically mediated through E2F3, beyond its role in cell cycle regulation. This study not only outlines overlapping and distinct functions for E2Fs in neurogenesis but also is the first to establish a physiologically relevant role for the Rb/E2F pathway beyond cell cycle regulation in vivo.

Limited redundancy in phosphorylation of retinoblastoma tumor suppressor protein by cyclin-dependent kinases in acute lymphoblastic leukemia

Cyclin-dependent kinases (CDKs) successively phosphorylate the retinoblastoma protein (RB) at the restriction point in G1 phase. Hyperphosphorylation results in functional inactivation of RB, activation of the E2F transcriptional program, and entry of cells into S phase. RB unphosphorylated at serine 608 has growth suppressive activity. Phosphorylation of serines 608/612 inhibits binding of E2F-1 to RB. In Nalm-6 acute lymphoblastic leukemia extracts, serine 608 is phosphorylated by CDK4/6 complexes but not by CDK2. We reasoned that phosphorylation of serines 608/612 by redundant CDKs could accelerate phospho group formation and determined which G1 CDK contributes to serine 612 phosphorylation. Here, we report that CDK4 complexes from Nalm-6 extracts phosphorylated in vitro the CDK2-preferred serine 612, which was inhibited by p16INK4a, and fascaplysin. In contrast, serine 780 and serine 795 were efficiently phosphorylated by CDK4 but not by CDK2. The data suggest that the redundancy in phosphorylation of RB by CDK2 and CDK4 in Nalm-6 extracts is limited. Serine 612 phosphorylation by CDK4 also occurred in extracts of childhood acute lymphoblastic leukemia cells but not in extracts of mobilized CD34+ hemopoietic progenitor cells. This phenomenon could contribute to the commitment of childhood acute lymphocytic leukemia cells to proliferate and explain their refractoriness to differentiation-inducing agents.

Bok, Bcl-2-related Ovarian Killer, Is Cell Cycle-regulated and Sensitizes to Stress-induced Apoptosis

Bok/Mtd (Bcl-2-related ovarian killer/Matador) is considered a pro-apoptotic member of the Bcl-2 family. Although identified in 1997, little is known about its biological role. We have previously demonstrated that Bok mRNA is up-regulated following E2F1 overexpression. In the current work, we demonstrate that Bok RNA is low in quiescent cells and rises upon serum stimulation. To determine the mechanism underlying this regulation, we cloned and characterized the mouse Bok promoter. We find that the mouse promoter contains a conserved E2F binding site (-43 to -49) and that a Bok promoter-driven luciferase reporter is activated by serum stimulation dependent on this site. Chromatin immunoprecipitation assays demonstrate that endogenous E2F1 and E2F3 associate with the Bok promoter in vivo. Surprisingly, we find that H1299 cells can stably express high levels of exogenous Bok protein. However, these cells are highly sensitive to chemotherapeutic drug treatment. Taken together these results demonstrate that Bok represents a cell cycle-regulated pro-apoptotic member of the Bcl-2 family, which may predispose growing cells to chemotherapeutic treatment.

Trans-10,cis-12, not cis-9,trans-11, conjugated linoleic acid inhibits G1-S progression in HT-29 human colon cancer cells

Commercial preparations of conjugated linoleic acid (CLA) contain both positional and geometric isomers of octadecadienoic acid, with cis-9,trans-11 CLA (c9t11) and trans-10,cis-12 CLA (t10c12) as the principal isomers. We showed previously that CLA reduced the incidence of colon tumors in rats treated with 1,2-dimethylhydrazine. In addition, our previous in vitro studies showed that t10c12 inhibited the growth of HT-29 and Caco-2 human colon cancer cells, whereas c9t11 had no effect on cell growth. In the present study, to examine the effects of the CLA isomers on cell cycle and cell cycle regulatory proteins, we treated HT-29 cells with various concentrations (0-4 micromol/L) of the individual CLA isomers. A DNA flow cytometric analysis revealed that t10c12 induced a G1 arrest, whereas c9t11 had no effect on the cell cycle. Western blot analysis of total cell lysates revealed no alteration in the protein expression of cyclin A, cyclin D, cyclin E, cyclin-dependent kinase (CDK) 2, or CDK4 due to t10c12 treatment. However, t10c12 substantially increased the protein expression and mRNA accumulation of the CDK inhibitor p21(CIP1/WAF1). The t10c12 isomer increased the association of p21(CIP1/WAF1) with CDK2 and proliferating cell nuclear antigen, but decreased the levels of phosphorylated retinoblastoma protein (Rb), with an increase in the levels of hypophosphorylated Rb protein. An in vitro kinase assay using histone H1 as a substrate showed that the activities of CDK2 were significantly decreased by t10c12. These results indicate that t10c12 exerts its growth inhibitory effects in colon cancer cells through the induction of G1 cell cycle arrest. The induction of p21(CIP1/WAF1) may be one of the mechanisms by which t10c12 inhibits cell cycle progression in HT-29 cells.

Pocket proteins p107 and p130 exhibit increased expression in macrophages during SIV encephalitis

Progression of HIV encephalitis (HIVE) is associated with neuronal damage and loss because of infiltration of infected and/or activated macrophages into the CNS. We have previously observed increased inactivation of the retinoblastoma susceptibility gene product (pRb) by phosphorylation in neurons and glia of HIVE and the simian model of HIVE (SIVE). To determine if other pRb family members are altered in response to increased macrophage-secreted factors, we investigated expression of pRb family members p107 and p130 in SIVE. Both p130 and p107 exhibited increased staining in macrophages, but not neurons, astrocytes or T-cells in SIVE. Increased p130 and p107 immunostaining was not limited to virally infected or PCNA-expressing macrophages. Most p107-positive staining was observed in perivascular macrophages, suggesting p107 may indicate macrophages at a specific stage of differentiation soon after migration. In contrast, cytoplasmic p130 was found in the majority of macrophages present in SIVE cases and may indicate activation as it was not seen in microglia in control CNS. These findings suggest that p107 and p130 are differentially expressed in CNS macrophage populations which may have multiple derivations and/or roles in lentiviral encephalitis.