Activation of cyclin-dependent kinases by Myc mediates induction of cyclin A, but not apoptosis

Wnt Signaling Pathway Collapse upon β-Catenin Destruction by a Novel Antimicrobial Peptide SKACP003: Unveiling the Molecular Mechanism and Genetic Activities Using Breast Cancer Cell Lines

Despite progress in breast cancer treatment, the survival rate for patients with metastatic breast cancer remains low due to chemotherapeutic agent resistance and the lack of specificity of the current generation of cancer drugs. Our previous findings indicated that the antimicrobial peptide SKACP003 exhibited anticancer properties, particularly against the MCF-7, MDA-MB-231, and MDA-MB-453 breast cancer cell lines. However, the mechanism of SKACP003-induced cancer cell death is unknown. Here, we investigated the molecular mechanism by which SKACP003 inhibits the cell cycle, cell proliferation, and angiogenesis in breast cancer cell lines. The results revealed that all the breast cancer cell lines treated at their IC₅₀ values significantly inhibited the replicative phase of the cell cycle. The SKACP003-induced growth inhibition induced apoptosis, as evidenced by a decrease in BCL-2 and an increase in BAX and caspase gene (Cas-3, Cas-8, and Cas-9) expression. Reduced expression of the β-Catenin signaling pathway was associated with the SKACP003-induced apoptosis. SKACP003-treated breast cancer cells showed decreased expression of Wnt/β-Catenin targeting genes such as C-Myc, P⁶⁸, and COX-2 and significant downregulation of CDK-4 and CDK-6 genes. Furthermore, cytoplasmic β-catenin protein levels in SKACP003-treated cell lines were significantly lower than in control cell lines. The results of the current study suggest that the newly identified antimicrobial peptide SKACP003 has great potential as a candidate for specifically targeting the β-catenin and thus significantly reducing the progression and prognosis of breast cancer cell lines.

Exploring major signaling cascades in melanomagenesis: a rationale route for targetted skin cancer therapy

Although most melanoma cases may be treated by surgical intervention upon early diagnosis, a significant portion of patients can still be refractory, presenting low survival rates within 5 years after the discovery of the illness. As a hallmark, melanomas are highly prone to evolve into metastatic sites. Moreover, melanoma tumors are highly resistant to most available drug therapies and their incidence have increased over the years, therefore leading to public health concerns about the development of novel therapies. Therefore, researches are getting deeper in unveiling the mechanisms by which melanoma initiation can be triggered and sustained. In this context, important progress has been achieved regarding the roles and the impact of cellular signaling pathways in melanoma. This knowledge has provided tools for the development of therapies based on the intervention of signal(s) promoted by these cascades. In this review, we summarize the importance of major signaling pathways (mitogen-activated protein kinase (MAPK), phosphoinositide 3-kinase (PI3K)-Akt, Wnt, nuclear factor κ-light-chain-enhancer of activated B cell (NF-κB), Janus kinase (JAK)-signal transducer and activator of transcription (STAT), transforming growth factor β (TGF-β) and Notch) in skin homeostasis and melanoma progression. Available and developing melanoma therapies interfering with these signaling cascades are further discussed.

Overview of Cell Synchronization

The widespread interest in cell synchronization is maintained by the studies of control mechanism involved in cell cycle regulation. During the synchronization distinct subpopulations of cells are obtained representing different stages of the cell cycle. These subpopulations are then used to study regulatory mechanisms of the cycle at the level of macromolecular biosynthesis (DNA synthesis, gene expression, protein synthesis), protein phosphorylation, development of new drugs, etc. Although several synchronization methods have been described, it is of general interest that scientists get a compilation and an updated view of these synchronization techniques. This introductory chapter summarizes: (1) the basic concepts and principal criteria of cell cycle synchronizations, (2) the most frequently used synchronization methods, such as physical fractionation (flow cytometry, dielectrophoresis, cytofluorometric purification), chemical blockade, (3) synchronization of embryonic cells, (4) synchronization at low temperature, (5) comparison of cell synchrony techniques, (6) synchronization of unicellular organisms, and (7) the effect of synchronization on transfection.

The T-Box factor TBX3 is important in S-phase and is regulated by c-Myc and cyclin A-CDK2

The transcription factor, TBX3, is critical for the formation of, among other structures, the heart, limbs and mammary glands and haploinsufficiency of the human TBX3 gene result in ulnar-mammary syndrome which is characterized by hypoplasia of these structures. On the other hand, the overexpression of TBX3 is a feature of a wide range of cancers and it has been implicated in several aspects of the oncogenic process. This includes its ability to function as an immortalizing gene and to promote proliferation through actively repressing negative cell cycle regulators. Together this suggests that TBX3 levels may need to be tightly regulated during the cell cycle. Here we demonstrate that this is indeed the case and that TBX3 mRNA and protein levels peak at S-phase and that the TBX3 protein is predominantly localized to the nucleus of S-phase cells. The increased levels of TBX3 in S-phase are shown to occur transcriptionally through activation by c-Myc at E-box motifs located at -1210 and -701 bps and post-translationally by cyclin A-CDK2 phosphorylation. Importantly, when TBX3 is depleted by shRNA the cells accumulate in S-phase. These results suggest that TBX3 is required for cells to transit through S-phase and that this function may be linked to its role as a pro-proliferative factor.

Nujiangexathone A, a novel compound from Garcinia nujiangensis, suppresses cervical cancer growth by targeting hnRNPK

Cervical cancer is among the most frequently diagnosed cancers in females worldwide. Nujiangexathone A (NJXA), a novel compound from Garcinia nujiangensis, has been shown to have anti-cancer potential. In this study, the anti-tumor effects and the underlying mechanisms of NJXA action were investigated. Our results suggested that NJXA induced G0/G1 cell cycle arrest in HeLa and SiHa cells by down-regulating cyclins B1, E1, and A and cyclin-dependent kinases 2, 4 and 6, while selectively restoring p27. Using two-dimensional gel electrophoresis, we showed that NJXA reduced the expression of heterogeneous nuclear ribonucleoprotein K (hnRNPK) by accelerating ubiquitin-proteasome-dependent hnRNPK degradation, which then induced cell cycle arrest through the c-Myc-cyclin/Cdk-Rb-E2F1 pathway. The loss-of-function study showed NJXA induced cell cycle arrest was mediated by down regulation of hnRNPK protein. In vivo results further confirmed the tumor inhibitory effect of NJXA via the down-regulation of hnRNPK, and NJXA induced no apparent toxicity. Our study suggests that NJXA may be a novel anti-cancer drug candidate, especially for treating cancers with abnormally high hnRNPK expression.

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.

Functional interactions among members of the MAX and MLX transcriptional network during oncogenesis

The transcription factor MYC and its related family members MYCN and MYCL have been implicated in the etiology of a wide spectrum of human cancers. Compared to other oncoproteins, such as RAS or SRC, MYC is unique because its protein coding region is rarely mutated. Instead, MYC's oncogenic properties are unleashed by regulatory mutations leading to unconstrained high levels of expression. Under both normal and pathological conditions MYC regulates multiple aspects of cellular physiology including proliferation, differentiation, apoptosis, growth and metabolism by controlling the expression of thousands of genes. How a single transcription factor exerts such broad effects remains a fascinating puzzle. Notably, MYC is part of a network of bHLHLZ proteins centered on the MYC heterodimeric partner MAX and its counterpart, the MAX-like protein MLX. This network includes MXD1-4, MNT, MGA, MONDOA and MONDOB proteins. With some exceptions, MXD proteins have been functionally linked to cell cycle arrest and differentiation, while MONDO proteins control cellular metabolism. Although the temporal expression patterns of many of these proteins can differ markedly they are frequently expressed simultaneously in the same cellular context, and potentially bind to the same, or similar DNA consensus sequence. Here we review the activities and interactions among these proteins and propose that the broad spectrum of phenotypes elicited by MYC deregulation is intimately connected to the functions and regulation of the other network members. Furthermore, we provide a meta-analysis of TCGA data suggesting that the coordinate regulation of the network is important in MYC driven tumorigenesis. This article is part of a Special Issue entitled: Myc proteins in cell biology and pathology.

WNT signaling: an emerging mediator of cancer cell metabolism?

WNT signaling was discovered in tumor models and has been recognized as a regulator of cancer development and progression for over 3 decades. Recent work has highlighted a critical role for WNT signaling in the metabolic homeostasis of mammals, where its misregulation has been heavily implicated in diabetes. While the majority of WNT metabolism research has focused on nontransformed tissues, the role of WNT in cancer metabolism remains underinvestigated. Cancer is also a metabolic disease where oncogenic signaling pathways regulate energy production and macromolecular synthesis to fuel rapidly proliferating tumors. This review highlights the emerging evidence for WNT signaling in the reprogramming of cancer cell metabolism and examines the role of these signaling pathways as mediators of tumor bioenergetics.

MYC-y mice: from tumour initiation to therapeutic targeting of endogenous MYC

MYC is one of the best-studied oncogenes in terms of mouse models of malignancy. MYC overexpression has been targeted to several tissues using transgenic constructs, and more recently as mouse models have evolved, conditional systems have been developed to allow the regulation of MYC expression or activity in vivo. The ability to target MYC expression to specific tissues and cell lineages, as well as the ability to regulate that expression, has made genetically engineered mouse models (GEMM) a valuable resource for studying the importance of MYC in the process of tumourigenesis. Here we review how these models have been used to address the role of MYC in tumour initiation and maintenance, how subtle changes in levels of MYC can influence tumourigenesis, and finally the ongoing efforts to target endogenous MYC genetically and with novel therapies.

The MYC-associated protein CDCA7 is phosphorylated by AKT to regulate MYC-dependent apoptosis and transformation

Cell division control protein A7 (CDCA7) is a recently identified target of MYC-dependent transcriptional regulation. We have discovered that CDCA7 associates with MYC and that this association is modulated in a phosphorylation-dependent manner. The prosurvival kinase AKT phosphorylates CDCA7 at threonine 163, promoting binding to 14-3-3, dissociation from MYC, and sequestration to the cytoplasm. Upon serum withdrawal, induction of CDCA7 expression in the presence of MYC sensitized cells to apoptosis, whereas CDCA7 knockdown reduced MYC-dependent apoptosis. The transformation of fibroblasts by MYC was reduced by coexpression of CDCA7, while the non-MYC-interacting protein Δ(156-187)-CDCA7 largely inhibited MYC-induced transformation. These studies provide insight into a new mechanism by which AKT signaling to CDCA7 could alter MYC-dependent growth and transformation, contributing to tumorigenesis.

Adeno-associated virus type 2 infection activates caspase dependent and independent apoptosis in multiple breast cancer lines but not in normal mammary epithelial cells

Background

In normal cells proliferation and apoptosis are tightly regulated, whereas in tumor cells the balance is shifted in favor of increased proliferation and reduced apoptosis. Anticancer agents mediate tumor cell death via targeting multiple pathways of programmed cell death. We have reported that the non-pathogenic, tumor suppressive Adeno-Associated Virus Type 2 (AAV2) induces apoptosis in Human Papillomavirus (HPV) positive cervical cancer cells, but not in normal keratinocytes. In the current study, we examined the potential of AAV2 to inhibit proliferation of MCF-7 and MDA-MB-468 (both weakly invasive), as well as MDA-MB-231 (highly invasive) human breast cancer derived cell lines. As controls, we used normal human mammary epithelial cells (nHMECs) isolated from tissue biopsies of patients undergoing breast reduction surgery.

Results

AAV2 infected MCF-7 line underwent caspase-independent, and MDA-MB-468 and MDA-MB-231 cell lines underwent caspase-dependent apoptosis. Death of MDA-MB-468 cells was marked by caspase-9 activation, whereas death of MDA-MB-231 cells was marked by activation of both caspase-8 and caspase-9, and resembled a mixture of apoptotic and necrotic cell death. Cellular demise was correlated with the ability of AAV2 to productively infect and differentially express AAV2 non-structural proteins: Rep78, Rep68 and Rep40, dependent on the cell line. Cell death in the MCF-7 and MDA-MB-231 lines coincided with increased S phase entry, whereas the MDA-MB-468 cells increasingly entered into G2. AAV2 infection led to decreased cell viability which correlated with increased expression of proliferation markers c-Myc and Ki-67. In contrast, nHMECs that were infected with AAV2 failed to establish productive infection or undergo apoptosis.

Conclusion

AAV2 regulated enrichment of cell cycle check-point functions in G1/S, S and G2 phases could create a favorable environment for Rep protein expression. Inherent Rep associated endonuclease activity and AAV2 genomic hair-pin ends have the potential to induce a cellular DNA damage response, which could act in tandem with c-Myc regulated/sensitized apoptosis induction. In contrast, failure of AAV2 to productively infect nHMECs could be clinically advantageous. Identifying the molecular mechanisms of AAV2 targeted cell cycle regulation of death inducing signals could be harnessed for developing novel therapeutics for weakly invasive as well as aggressive breast cancer types.

GSK-3 inactivation or depletion promotes β-cell replication via down regulation of the CDK inhibitor, p27 (Kip1)

Diabetes (T1DM and T2DM) is characterized by a deficit in β-cell mass. A broader understanding of human β-cell replication mechanism is thus important to increase β-cell proliferation for future therapeutic interventions. Here, we show that p27 (Kip1), a CDK inhibitor, is expressed abundantly in isolated adult human islets and interacts with various positive cell cycle regulatory proteins including D-type cyclins (D1, D2 and D3) and their kinase partners, CDK4 and CDK6. Also, we see interaction of cyclin E and its kinase partner, CDK2, with p27 suggesting a critical role of p27 as a negative cell cycle regulator in human islets. Our data demonstrate interaction of p27 with GSK-3 in β-cells and show, employing rodent β-cells (INS-1), isolated human islets and purified β-cells derived from human islets, that siRNA-mediated depletion of GSK-3 or p27 or 1-AKP / BIO - mediated GSK-3 inhibition results in increased β-cell proliferation. We also see reduction of p27 levels following GSK-3 inactivation or depletion. Our data show that serum induction of quiescent INS-1 cells leads to sequential phosphorylation of p27 on its S10 and T187 residues with faster kinetics for S10 corresponding with the decreased levels of p27. Altogether our findings indicate that p27 levels in β-cells are stabilized by GSK-3 and thus p27 down regulation following GSK-3 depletion / inactivation plays a critical role in promoting β-cell replication.

Overview of cell synchronization

Widespread interest in cell synchronization is maintained by the studies of control mechanisms involved in cell cycle regulation. During the synchronization distinct subpopulations of cells are obtained representing different stages of the cell cycle. These subpopulations are then used to study regulatory mechanisms of the cycle at the level of macromolecular biosynthesis (DNA synthesis, gene expression, protein synthesis), protein phosphorylation, development of new drugs, etc. Although several synchronization methods have been described, it is of general interest that scientists get a compilation and an updated view of these synchronization techniques. This introductory chapter summarizes: (1) the basic concepts and principal criteria of cell cycle synchronizations, (2) the most frequently used synchronization methods, such as physical fractionation (flow cytometry, dielectrophoresis, cytofluorometric purification), chemical blockade, (3) synchronization of embryonic cells, (4) synchronization at low temperature, (5) comparison of cell synchrony techniques, (6) synchronization of unicellular organisms, and (7) the effect of synchronization on transfection.

The DNA damage response to non-replicating adeno-associated virus: Centriole overduplication and mitotic catastrophe independent of the spindle checkpoint

Adeno-associated virus (AAV) type 2 or UV-inactivated AAV (UV-AAV2) infection provokes a DNA damage response that leads to cell cycle arrest at the G2/M border. p53-deficient cells cannot sustain the G2 arrest, enter prolonged impaired mitosis, and die. Here, we studied how non-replicating AAV2 kills p53-deficient osteosarcoma cells. We found that the virus uncouples centriole duplication from the cell cycle, inducing centrosome overamplification that is dependent on Chk1, ATR and CDK kinases, and on G2 arrest. Interference with spindle checkpoint components Mad2 and BubR1 revealed unexpectedly that mitotic catastrophe occurs independently of spindle checkpoint function. We conclude that, in the p53-deficient cells, UV-AAV2 triggers mitotic catastrophe associated with a dramatic Chk1-dependent overduplication of centrioles and the consequent formation of multiple spindle poles in mitosis. As AAV2 acts through cellular damage response pathways, the results provide information on the role of Chk1 in mitotic catastrophe after DNA damage signaling in general.

A chemical compound commonly used to inhibit PKR, {8-(imidazol-4-ylmethylene)-6H-azolidino[5,4-g] benzothiazol-7-one}, protects neurons by inhibiting cyclin-dependent kinase

Activation of the double-stranded RNA-dependent protein kinase (PKR) has been implicated in the pathogenesis of several neurodegenerative diseases. We find that a compound widely used as a pharmacological inhibitor of this enzyme, referred to as PKR inhibitor (PKRi), {8-(imidazol-4-ylmethylene)-6H-azolidino[5,4-g]benzothiazol-7-one}, protects against the death of cultured cerebellar granule and cortical neurons. PKRi also prevents striatal neurodegeneration and improves behavioral outcomes in a chemically induced mouse model of Huntington's disease. Surprisingly, PKRi fails to block the phosphorylation of eIF2alpha, a downstream target of PKR, and does not reduce the autophosphorylation of PKR enzyme immunoprecipitated from neurons. Furthermore, neurons lacking PKR are fully protected from apoptosis by PKRi, demonstrating that neuroprotection by this compound is not mediated by PKR inhibition. Using in vitro kinase assays we investigated whether PKRi affects any other protein kinase. These analyses demonstrated that PKRi has no major inhibitory effect on pro-apoptotic kinases such as the c-Jun N-terminal kinases, the p38 MAP kinases and the death-associated protein kinases, or on other kinases including c-Raf, MEK1, MKK6 and MKK7. PKRi does, however, inhibit the activity of certain cyclin-dependent kinases (CDKs), including CDK1, CDK2 and CDK5 both in vitro and in low potassium-treated neurons. Consistent with its inhibitory action on mitotic CDKs, the treatment of HT-22 and HEK293T cell lines with PKRi sharply reduces the rate of cell cycle progression. Taken together with the established role of CDK activation in the promotion of neurodegeneration, our results suggest that PKRi exerts its neuroprotective action by inhibiting CDK.

The estrogen and c-Myc target gene HSPC111 is over-expressed in breast cancer and associated with poor patient outcome

INTRODUCTION

Estrogens play a pivotal role in the initiation and progression of breast cancer. The genes that mediate these processes are not fully defined, but potentially include the known mammary oncogene MYC. Characterization of estrogen-target genes may help to elucidate further the mechanisms of estrogen-induced mitogenesis and endocrine resistance.

METHODS

We used a transcript profiling approach to identify targets of estrogen and c-Myc in breast cancer cells. One previously uncharacterized gene, namely HBV pre-S2 trans-regulated protein 3 (HSPC111), was acutely upregulated after estrogen treatment or inducible expression of c-Myc, and was selected for further functional analysis using over-expression and knock-down strategies. HSPC111 expression was also analyzed in relation to MYC expression and outcome in primary breast carcinomas and published gene expression datasets.

RESULTS

Pretreatment of cells with c-Myc small interfering RNA abrogated estrogen induction of HSPC111, identifying HSPC111 as a potential c-Myc target gene. This was confirmed by the demonstration of two functional E-box motifs upstream of the transcription start site. HSPC111 mRNA and protein were over-expressed in breast cancer cell lines and primary breast carcinomas, and this was positively correlated with MYC mRNA levels. HSPC111 is present in a large, RNA-dependent nucleolar complex, suggesting a possible role in ribosomal biosynthesis. Neither over-expression or small interfering RNA knock-down of HSPC111 affected cell proliferation rates or sensitivity to estrogen/antiestrogen treatment. However, high expression of HSPC111 mRNA was associated with adverse patient outcome in published gene expression datasets.

CONCLUSION

These data identify HSPC111 as an estrogen and c-Myc target gene that is over-expressed in breast cancer and is associated with an adverse patient outcome.

Ras triggers ataxia-telangiectasia-mutated and Rad-3-related activation and apoptosis through sustained mitogenic signaling

Genetic evidence indicates that Ras plays a critical role in the initiation and progression of human thyroid tumors. Paradoxically, acute expression of activated Ras in normal rat thyroid cells induced deregulated cell cycle progression and apoptosis. We investigated whether cell cycle progression was required for Ras-stimulated apoptosis. Ras increased CDK-2 activity following its introduction into quiescent cells. Apoptotic cells exhibited a sustained increase in CDK-2 activity, accompanied by the loss of CDK-2-associated p27. Blockade of Ras-induced CDK-2 activity and S phase entry via overexpression of p27 inhibited apoptosis. Inactivation of the retinoblastoma protein in quiescent cells through expression of HPV-E7 stimulated cell cycle progression and apoptosis, indicating that deregulated cell cycle progression is sufficient to induce apoptosis. Ras failed to induce G1 phase growth arrest in normal rat thyroid cells. Rather, Ras-expressing thyroid cells progressed into S and G2 phases and evoked a checkpoint response characterized by the activation of ATR. Ras-stimulated ATR activity, as evidenced by Chk1 and p53 phosphorylation, was blocked by p27, suggesting that cell cycle progression triggers checkpoint activation, likely as a consequence of replication stress. These data reveal that Ras is capable of inducing a DNA damage response with characteristics similar to those reported in precancerous lesions. Our findings also suggest that the frequent mutational activation of Ras in thyroid tumors reflects the ability of Ras-expressing cells to bypass checkpoints and evade apoptosis rather than to simply increase proliferative potential.

Control of cell proliferation and growth by Myc proteins

Myc proteins act as signal transducers that alter cell proliferation in dependence on signals from the extracellular environment. In normal cells, the expression of MYC genes is therefore under tight control by growth factor dependent signals. The enormous interest in the function of these proteins is motivated by the observation that the close control of MYC expression is disrupted in a large percentage of human tumors, leading to deregulated expression of Myc proteins. A large body of evidence shows that this deregulation is a major driving force of human tumorigenesis; in cells with deregulated Myc, proliferation often takes place in the complete absence of external stimuli. We will discuss current models to understand Myc function and also potential avenues to selectively interfere with the proliferation of Myc-transformed cells.

Cyclin a but not cyclin D1 is essential for c-myc-modulated cell-cycle progression

The proto-oncogene c-myc is a key player in cell-cycle regulation and is deregulated in a broad range of human cancers and cell proliferation disorders. Here we reported that overexpression of c-myc in human embryonic lung fibroblasts (HEL) that have low endogenous c-myc enriched S phase cells with increased expression of cyclin D3, E, A, Cdk2, and Cdk4, and decreased expression of p21 and p27. To the opposite, using RNAi to downregulate c-myc expression in A549 cells that have high endogenous c-myc enriched G1 phase cells with decreased expression of cyclin D3, E, A, Cdk2, Cdk4, and increased expression of p21 and p27. We found that cyclin A expression was the most susceptive to changes in c-myc levels and essential in c-myc-modulated cell cycle pathway via co-transfection, however, cyclin D1 showed no change between treated and control groups in either HEL or A549 cells. Our results indicated that upregulation of c-myc expression promotes cell cycling in HEL cells, whereas downregulation of c-myc expression causes G1 phase arrest in A549 cells, and the c-myc-mediated cell-cycle regulation pathway was dependent on cyclin A and involved cyclin D3, E, Cdk2, Cdk4, p21, and p27, but not cyclin D1.

Cdk2 activity is dispensable for triggering replicon initiation after transient hypoxia in T24 cells

We examined whether the fast release of replicon initiation after sudden O2 recovery of hypoxically incubated mammalian cells depends on kinase activity of Cdk2. We used a system based on starved/refed T24 cells elaborated previously for such investigations [van Betteraey-Nikoleit M, Eisele KH, Stabenow D and Probst H (2003) Eur J Biochem270, 3880-3890]. Cells subjected to hypoxia concurrently with refeeding accumulate the G1 DNA content within 5-6 h. In this state they are ready to perform, within 1-2 min after O2 recovery, a burst of replicon initiations that marks the start of a synchronous S-phase. We found that Cdk2 binds to the chromatin fraction within 4-6 h after refeeding with fresh medium, irrespective of whether the cells were incubated normoxically or hypoxically. However, inhibition of Cdk2 by olomoucine, roscovitine or the Cdk2/cyclin inhibitory peptide II had no influence on the synchronous burst of replicon initiations. Cdc6 and pRb, possible targets of Cdk2 phosphorylation, behaved differentially. Inhibition did not affect phosphorylation of Cdc6 after reoxygenation, whilst chromatin bound pRb remained hypophosphorylated beyond the initiation burst. Thus, neither Cdk2 activity, though present at the end of the hypoxic period, nor pRb phosphorylation are necessary for releasing the burst of replicon initiations upon oxygen recovery. Consequentially, Cdk2 dependent phosphorylation(s) cannot be a critical trigger of replicon initiation in response to reoxygenation after several hours of hypoxia, at least in the T24 cells studied.

Cell cycle-dependent nuclear retention of p53 by E2F1 requires phosphorylation of p53 at Ser315

We show here that the cell cycle-dependent DNA-binding and transcriptional activity of p53 correlates with E2F expression in human primary fibroblasts. E2F1 binds and stimulates DNA-binding, transactivation and apoptotic functions of p53 but not p63 and p73. E2F1 binds residues 347-370 of p53 and enhances nuclear retention of Ser315 phosphorylated p53. This regulation of p53 by E2F1 is cell cycle dependent, as the cellular distribution of Ser315 phosphorylated p53 is associated with the periodic expression of E2F and cyclin A throughout the cell cycle. This is the first demonstration that the activities of p53 are regulated during the cell cycle by E2F/p53 interactions and that phosphorylation of p53 at Ser315 is required for this regulation.

Critical role of CDK2 for melanoma growth linked to its melanocyte-specific transcriptional regulation by MITF

The genomic organization of the CDK2 gene, which overlaps the melanocyte-specific gene SILV/PMEL17, poses an interesting regulatory challenge. We show that, despite its ubiquitous expression, CDK2 exhibits tissue-specific regulation by the essential melanocyte lineage transcription factor MITF. In addition, functional studies revealed this regulation to be critical for maintaining CDK2 kinase activity and growth of melanoma cells. Expression levels of MITF and CDK2 are tightly correlated in primary melanoma specimens and predict susceptibility to the CDK2 inhibitor roscovitine. CDK2 depletion suppressed growth and cell cycle progression in melanoma, but not other cancers, corroborating previous results. Collectively, these data indicate that CDK2 activity in melanoma is largely maintained at the transcriptional level by MITF, and unlike other malignancies, it may be a suitable drug target in melanoma.

Absence of p21WAF1 cooperates with c-myc in bypassing Ras-induced senescence and enhances oncogenic cooperation

The absence of p21waf1 combined with an ectopic expression of c-myc prevents ras-induced senescence in mouse embryo fibroblasts. Extension of lifespan after c-myc transduction into p21-null cells was followed at later passages by apoptosis of a large fraction of c-myc-overexpressing p21-null cells. This apoptotic effect could be overridden by inactivation of the p53 tumor suppressor or oncogenic ras expression. Ras-induced inhibition of apoptosis is mediated by PI3K activation. These results suggest a functional relationship between ras and myc that may explain their oncogenic cooperation. The number of foci formed by myc+ras increased cooperatively in the absence of p21waf1. Thus, the reciprocal cooperation between myc and ras in a p21-null background during cellular immortalization lead to increased oncogenic cooperation between ras and myc.

Functions of myc:max in the control of cell proliferation and tumorigenesis

Deregulation and elevated expression of members of the Myc family of bHLHZip transcription factors are observed in a high percentage of tumors. This close association with human cancers has led to a tremendous effort to define their biological and biochemical activities. Although Myc family proteins have the capacity to elicit a wide range of cell behaviors, their principal function appears to be to drive cells into the cell cycle and to keep them there. However, forced expression of Myc profoundly sensitizes normal cells to apoptosis. Therefore, tumor formation caused by deregulated Myc expression requires cooperating events that disrupt pathways that mediate apoptosis. Myc-dependent tumor formation may also be impeded by a set of related bHLHZip proteins with the demonstrated potential to act as Myc antagonists in cell culture experiments. In this review, we examine the complex activities of Myc family proteins and how their actions might be regulated in the context of a network of bHLHZip proteins.

Regulation of ania-6 splice variants by distinct signaling pathways in striatal neurons

The striatum is a brain region involved in motor control and in diverse forms of implicit memory. It is also involved in the pathogenesis of many significant human disorders, including drug addiction, that are thought to involve adaptive changes in gene expression. We have previously shown that the cyclin L, ania-6, is expressed as at least two splice forms, which are differentially regulated in striatal neurons by different neurotransmitters. Here, we report that ania-6 transcription is mostly regulated via cAMP response element binding protein (CREB), but that signaling pathways that converge on CREB at the transcriptional level produce different effects on splicing and neuronal gene expression. Glutamate induced a long ania-6 mRNA that encodes a truncated form of the cyclin. This effect depended on the activation of NMDA receptors but was independent of both calcium/calmodulin-dependent protein kinases (CaMK) and extracellular regulated kinase (ERK). Forskolin or brain-derived neurotropic factor (BDNF) induced a short ania-6 mRNA, that encodes the full-length cyclin, and this induction depended on ERK. However, KCl-mediated induction of ania-6 short mRNA, which required activation of L-type calcium channels, was independent of ERK but depended on CaMK. These data suggest that different neuronal signals can differentially regulate splicing and that different intracellular pathways can be recruited to yield a given splice variant.

Cadherins and synaptic plasticity: activity-dependent cyclin-dependent kinase 5 regulation of synaptic beta-catenin-cadherin interactions

Cyclin-dependent kinase 5 (Cdk5)/p35 kinase activity is known to decrease the affinity of beta-catenin for cadherin in developing cortical neurons. Our recent work demonstrated that depolarization causes an increased affinity between beta-catenin and cadherin. Here, we examine whether Cdk5/p35 regulates beta-catenin-cadherin affinity in response to neural activity. In hippocampal neurons depolarization caused a significant decrease in Cdk5 kinase activity, without changing the protein levels of either Cdk5 or p35, suggesting that the proteasome pathway is not involved. Decreasing Cdk5 kinase activity with the inhibitor roscovitine increased the amount of beta-catenin that was co-immunoprecipitated with cadherin. Inhibiting Cdk5 activity also resulted in a redistribution of EGFP-beta-catenin from the dendritic shaft to the spines, where cadherins are highly concentrated. The redistribution of beta-catenin induced by roscovitine is similar to that induced by depolarization. Interestingly, the redistribution induced by the Cdk5 inhibitor was completely blocked by either a tyrosine phosphatase inhibitor, orthovanadate or by point mutations of beta-catenin Tyr-654 to Glu or Phe. Immunoprecipitation studies further revealed that roscovitine increases the affinity of the wild-type, but not mutated, EGFP-beta-catenin for cadherin. These results suggest that Cdk5 activity regulates the affinity of beta-catenin for cadherin by changing the phosphorylation level of beta-catenin Tyr-654.

Senescence, apoptosis and therapy--cutting the lifelines of cancer

Apoptosis and senescence are cellular failsafe programmes that counteract excessive mitogenic signalling from activated oncogenes. Cancellation of apoptosis or senescence is therefore a prerequisite for tumour formation, and the ability of the cancer cell to disrupt these processes can be considered its 'lifeline'. Ironically, the efficacy of anticancer agents also depends on the activation of apoptosis or an acutely inducible form of cellular senescence. Understanding how the 'lifelines' of the cancer cell interfere with treatment sensitivity is of crucial importance for developing safer and more effective treatment strategies.

The Medicago species A2-type cyclin is auxin regulated and involved in meristem formation but dispensable for endoreduplication-associated developmental programs

Phytohormones as well as temporal and spatial regulation of the cell cycle play a key role in plant development. Here, we investigated the function and regulation of an alfalfa (Medicago sativa) A2-type cyclin in three distinct root developmental programs: in primary and secondary root development, nodule development, and nematode-elicited gall formation. Using transgenic plants carrying the Medsa;cycA2;2 promoter-beta-glucuronidase gene fusion, in combination with other techniques, cycA2;2 expression was localized in meristems and proliferating cells in the lateral root and nodule primordia. Rapid induction of cycA2;2 by Nod factors demonstrated that this gene is implicated in cell cycle activation of differentiated cells developing to nodule primordia. Surprisingly, cycA2;2 was repressed in the endoreduplicating, division-arrested cells both during nodule development and formation of giant cells in nematode-induced galls, indicating that CycA2;2 was dispensable for S-phase in endoreduplication cycles. Overexpression of cycA2;2 in transgenic plants corresponded to wild type protein levels and had no apparent phenotype. In contrast, antisense expression of cycA2;2 halted regeneration of somatic embryos, suggesting a role for CycA2;2 in the formation or activity of apical meristems. Expression of cycA2;2 was up-regulated by auxins, as expected from the presence of auxin response elements in the promoter. Moreover, auxin also affected the spatial expression pattern of this cyclin by shifting the cycA2;2 expression from the phloem to the xylem poles.

Retinoic acid decreases targeting of p27 for degradation via an N-myc-dependent decrease in p27 phosphorylation and an N-myc-independent decrease in Skp2

Poor prognosis neuroblastoma (NB) tumors are marked by amplification and overexpression of N-myc. Retinoic acid (RA) decreases N-myc levels and induces cell cycle arrest in vitro and increases event-free survival in advanced stage NB patients. In this study, we investigated the mechanism(s) by which RA regulates cell cycle and how N-myc affects NB cell cycle progression. Constitutive N-myc overexpression stimulates increases in cyclin E-dependent kinase activity and decreases in p27 resulting in increased DNA synthesis. N-myc regulates p27 levels through an increase in targeting of p27 to the proteasome via cyclin E kinase-dependent phosphorylation of p27 and its ubiquitination. N-myc also stimulates an increase in proteasome activity. In RA-treated cells in which N-myc levels decline as p27 levels increase, degradation of p27 is also decreased. However, RA does not affect the activity of proteasome. The decrease in the degradation of p27 in RA-treated cells is due in part to a decrease in the N-myc stimulated phosphorylation of p27. However, RA also decreases Skp2 levels thus impairing the ability of p27 to be ubiquitinated. Thus, RA induces both N-myc-dependent and -independent mechanisms to minimize the degradation of p27 and arrest NB cell growth.

Loss of a FYN-regulated differentiation and growth arrest pathway in advanced stage neuroblastoma

Tumor stage, age of patient, and amplification of MYCN predict disease outcome in neuroblastoma. To gain insight into the underlying molecular pathways, we have obtained expression profiles from 94 primary neuroblastoma specimens. Advanced tumor stages show a characteristic expression profile that includes downregulation of multiple genes involved in signal transduction through Fyn and the actin cytoskeleton. High expression of Fyn and high Fyn kinase activity are restricted to low-stage tumors. In culture, expression of active Fyn kinase induces differentiation and growth arrest of neuroblastoma cells. Expression of Fyn predicts long-term survival independently of MYCN amplification. Amplification of MYCN correlates with deregulation of a distinct set of genes, many of which are target genes of Myc. Our data demonstrate a causal role for Fyn kinase in the genesis of neuroblastoma.

E7 abolishes raf-induced arrest via mislocalization of p21(Cip1)

The cellular response to oncogenic Ras depends upon the presence or absence of cooperating mutations. In the absence of immortalizing oncogenes or genetic lesions, activation of the Ras/Raf pathway results in a p21(Cip1)-dependent cellular arrest. The human papillomavirus oncoprotein E7 transforms primary cells in cooperation with Ras and abolishes p21(Cip1)-mediated growth arrest in the presence of various antimitogenic signals. Here we have utilized a conditional Raf molecule to investigate the effects of E7 on p21(Cip1) function in the context of Raf-induced cellular arrest. E7 bypassed Raf-induced arrest and alleviated inhibition of cyclin E-CDK2 without suppressing Raf-specific synthesis of p21(Cip1) or derepressing p21(Cip1)-associated CDK2 complexes. Activation of Raf led to nuclear accumulation of p21(Cip1), and we provide evidence that this effect is mediated by inhibition of Akt, a regulator of p21(Cip1) localization. Loss of Akt activity appears to be an important event in the cellular arrest associated with Raf-induction, since maintenance of Akt activity was necessary and sufficient to bypass Raf-induced arrest. In agreement, expression of E7 sustained Akt activity and reduced nuclear accumulation of p21(Cip1), resulting in decreased association between p21(Cip1) and cyclin E-CDK2. Taken together, these data suggest that E7 inhibits p21(Cip1) function in the context of Raf signaling by altering Raf-Akt antagonism and preventing the proper subcellular localization of p21(Cip1). We propose that E7 elicits a proliferative response to Raf signaling by targeting p21(Cip1) function via a novel mechanism.

Use of radiolabelled choline as a pharmacodynamic marker for the signal transduction inhibitor geldanamycin

There is an urgent need to develop non-invasive pharmacodynamic endpoints for the evaluation of new molecular therapeutics that inhibit signal transduction. We hypothesised that, when labelled appropriately, changes in choline kinetics could be used to assess geldanamycin pharmacodynamics, which involves inhibition of the HSP90 molecular chaperone-->Raf1-->Mitogenic Extracellular Kinase-->Extracellular Signal-Regulated Kinase 1 and 2 signal transduction pathway. Towards identifying a potential pharmacodynamic marker response, we have studied radiolabelled choline metabolism in HT29 human colon carcinoma cells following treatment with geldanamycin. We studied the effects of geldanamycin, on net cellular accumulation of (methyl-(14)C)choline and (methyl-(14)C)phosphocholine production. In parallel experiments, the effects of geldanamycin on extracellular signal-regulated kinase 1 and 2 phosphorylation and cell viability were also assessed. Additional validation studies were carried out with the mitogenic extracellular kinase inhibitor U0126 as a positive control; a cyclin-dependent kinase-2 inhibitor roscovitine and the phosphatidylinositol 3-kinase inhibitor LY294002 as negative controls. Hemicholinium-3, an inhibitor of choline transport and choline kinase activity was included as an additional control. In exponentially growing HT29 cells, geldanamycin inhibited extracellular signal-regulated kinase 1 and 2 phosphorylation in a concentration- and time-dependent manner. These changes were associated with a reduction in (methyl-(14)C)choline uptake, (methyl-(14)C) phosphocholine production and cell viability. Brief exposure to U0126, suppressed phosphocholine production to the same extent as Hemicholinium-3. In contrast to geldanamycin and U0126, which act upstream of extracellular signal-regulated kinase 1 and 2, roscovitine and LY294002 failed to suppress phosphocholine production. Our results suggest that when labelled with carbon-11 isotope, (methyl-(11)C)choline may be a useful pharmacodynamic marker for the non-invasive evaluation of geldanamycin analogues.

E2F activity is essential for survival of Myc-overexpressing human cancer cells

Effective cell cycle completion requires both Myc and E2F activities. However, whether these two activities interact to regulate cell survival remains to be tested. Here we have analysed survival of inducible c-Myc-overexpressing cell lines derived from U2OS human osteosarcoma cells, which carry wild-type pRb and p53 and are deficient for p16 and ARF expression. Induced U2OS-Myc cells neither underwent apoptosis spontaneously nor upon reconstitution of the ARF-p53 axis and/or serum-starvation. However, they died massively when concomitantly exposed to inhibitors of E2F activity, including a constitutively active pRb (RbDeltacdk) mutant, p16, a stable p27 (p27T187A) mutant, a dominant-negative (dn) CDK2, or dnDP-1. Similar apoptotic effect was observed upon down-modulation of endogenous E2Fs through overexpression of E2F binding site oligonucleotides in U2OS-Myc cells, upon expression of RbDeltacdk or dnDP-1 in the Myc-amplified HL-60 (ARF-; p53-) human leukemia cells, and upon co-transfection of Myc and RbDeltacdk in SAOS-2 (ARF+; p53-) human osteosarcoma cells but not in human primary fibroblasts. Consistent with these results, a dnp53 mutant did not abrogate the Myc-induced apoptotic phenotype, which instead strictly depended on caspase-3-like proteases and on Myc transcriptional activity. Our data indicate that in contrast to normal cells, Myc-overexpressing human cancer cells need E2F activity for their survival, regardless of their ARF and p53 status, a notion that may have important implications for antineoplastic treatment strategies.

Crosstalk between Myc and activating transcription factor 2 (ATF2): Myc prolongs the half-life and induces phosphorylation of ATF2

Myc is a key regulator of cell growth, differentiation and apoptosis, and affects cell fate decisions by activating as well as by inhibiting the expression of cellular genes. Myc is a member of the basic region-helix-loop-helix-leucine zipper (b-HLH-Zip) class of transcription factors, which heterodimerizes with the Max protein and recognizes a consensus Myc binding motif. Stimulation of gene expression by Myc is thought to be mediated by direct binding of Myc-Max heterodimers to specific target genes. So far, only a few genes have been identified as direct binding targets of Myc, raising the possibility that Myc affects gene expression also by indirect mechanisms. In this work we present evidence that v-Myc encoded by the avian retrovirus MC29 stimulates activating transcription factor 2 (ATF2)-dependent transcription. Analysis of the effect of Myc on ATF2 shows that v-Myc prolongs the half-life of ATF2 and induces the phosphorylation of N-terminal sites of ATF2 (Thr-69 and Thr-71) which have previously been identified as the target sites of stress-activated protein kinases and implicated in the regulation of ATF2 activity. Taken together, our results suggest that v-Myc can affect gene expression indirectly by modulating the activity of ATF2.

Cyclin A-CDK Phosphorylation Regulates MDM2 Protein Interactions

The product of the MDM2 gene interacts with and regulates a number of proteins, in particular the tumor suppressor p53. The MDM2 protein is likely to be extensively modified in vivo, and such modification may regulate its functions in cells. We identified a potential cyclin-dependent kinase (CDK) site in murine MDM2, and found the protein to be efficiently phosphorylated in vitro by cyclin A-containing complexes (cyclin A-CDK2 and cyclin A-CDK1), but MDM2 was either weakly or not phosphorylated by other cyclin-containing complexes. Moreover, a peptide containing a putative MDM2 cyclin recognition motif specifically inhibited phosphorylation by cyclin A-CDK2. The site of cyclin A-CDK2 phosphorylation was identified as Thr-216 by two-dimensional phosphopeptide mapping and mutational analysis. Phosphorylation of MDM2 at Thr-216 both weakens its interaction with p53 and modestly augments its binding to p19(ARF). Interestingly, an MDM2-specific monoclonal antibody, SMP14, cannot recognize MDM2 phosphorylated at Thr-216. Changes in SMP14 reactivity of MDM2 in staged cell extracts indicate that phosphorylation of MDM2 at Thr-216 in vivo is most prevalent at the onset of S phase when cyclin A first becomes detectable.

Adhesion-regulated G1 cell cycle arrest in epithelial cells requires the downregulation of c-Myc

Adhesion to the extracellular matrix is required for the expression and activation of the cyclin-cyclin-dependent kinase (CDK) complexes, and for G1 phase progression of non-transformed cells. However, in non-adherent cells no molecular mechanism has yet been proposed for the cell adhesion-dependent up-regulation of the p27 cyclin-dependent kinase inhibitor (CKI), and the associated inhibition of cyclin E-CDK2. We now show that in epithelial cells the expression of c-Myc is tightly regulated by cell-substrate adhesion. When deprived of adhesion, two independently derived mammary epithelial cell lines, 184A1N4 and MCF-10A, rapidly decrease their level of c-Myc mRNA and protein. This decrease in levels of c-Myc correlates with G1 phase arrest, as indicated by hypophosphorylation of pRb and inhibition of the activity of the cyclin E-CDK2 complex. In 184A1N4 cells, cell-substrate adhesion is required for the suppression of p27, and induction of cyclin E, E2F-1, but not cyclins D1 and D3. Enforced expression of c-Myc in non-adherent 184A1N4 and MCF-10A cells reverses the adhesion-dependent inhibition of cell cycle progression. Restoration of c-Myc in non-adherent cells induces the expression of E2F-1, and hyperphosphorylation of pRb in response to EGF treatment. In addition, expression of c-Myc results in the anchorage-independent activation of the CDK2 complex, the associated upregulation of cyclin E, and the destabilization and degradation of p27 by the ubiquitin-proteasome pathway. Our study thus suggests that c-Myc is the link between cell adhesion and the regulation of p27 and cyclin E-CDK2. Furthermore, we describe a role for c-Myc in adhesion-mediated regulation of E2F-1.

Myc requires distinct E2F activities to induce S phase and apoptosis

Previous work has shown that the Myc transcription factor induces transcription of the E2F1, E2F2, and E2F3 genes. Using primary mouse embryo fibroblasts deleted for individual E2F genes, we now show that Myc-induced S phase and apoptosis requires distinct E2F activities. The ability of Myc to induce S phase is impaired in the absence of either E2F2 or E2F3 but not E2F1 or E2F4. In contrast, the ability of Myc to induce apoptosis is markedly reduced in cells deleted for E2F1 but not E2F2 or E2F3. From this data, we propose that the induction of specific E2F activities is an essential component in the Myc pathways that control cell proliferation and cell fate decisions.

Genetic dissection of melanoma pathways in the mouse

The frequent loss of the INK4a/ARF locus, encoding for both p16(INK4a)and p19(ARF)in human melanoma, raises the question as to which INK4a/ARF gene product functions to suppress melanoma-genesis in vivo. Studies in the mouse have shown that activated RAS mutation can cooperate with INK4a(Delta 2/3)deficiency (null for both p16(INK4a)and p19(ARF)) to promote development of melanoma, and these melanomas retain wild-type p53. Given the functional link between p19(ARF)and p53, we have now shown that activated RAS can also cooperate with p53 deficiency to produce melanoma in the mouse. Moreover, genome-wide analysis of RAS-induced p53 mutant melanomas reveals alterations of key components governing RB-regulated G1/S transition, such as c-Myc. These experimental findings suggest that both RB and p53 pathways function to suppress melanocyte transformation in vivo in the mouse.

Expression of P27(KIP1) is prognostic and independent of MYCN amplification in human neuroblastoma

Amplification of the MYCN gene is significantly associated with an unfavorable prognosis and rapid progression in human neuroblastoma tumors. One potential mechanism by which MYCN may cause these effects is by deregulating cell proliferation. Tissue culture experiments support a model in which MYC genes stimulate cell cycle progression by antagonizing the function of the cell cycle inhibitor p27(kip1). In culture, activation of MYC induces both sequestration of p27(kip1) by cyclin D complexes and its subsequent proteolytic degradation. We have tested whether this model applies to human neuroblastoma in a retrospective study of 100 primary tumor biopsy samples from neuroblastoma patients with a documented follow-up. Consistent with this hypothesis, MYCN-amplified tumors express high levels of both cyclin A and proliferating cell nuclear antigen, 2 marker proteins of cell proliferation. Further, expression levels of p27(kip1) are of prognostic significance in human neuroblastoma patients. Similar to tissue culture systems, p27(kip1) is sequestered by cyclin D complexes in a subset of human neuroblastoma samples. Surprisingly, however, expression levels of p27(kip1) are prognostic independent of MYCN amplification, and tumors that have an amplified MYCN gene do not express elevated levels of D-type cyclins or contain significantly lower levels of p27(kip1). Our data do not support a model in which regulation of p27(kip1) function is an important mechanism by which amplified MYCN deregulates cell proliferation in neuroblastoma.

Inhibition of the cyclin D1/E2F pathway by PCA-4230, a potent repressor of cellular proliferation

1. Tight control of cellular growth is essential to ensure normal tissue patterning and prevent pathological responses. Excessive vascular smooth muscle cell (VSMC) proliferation is associated with the pathophysiology of atherosclerosis and restenosis post-angioplasty. Thus, drug targeting of pathological VSMC growth may be a suitable therapeutic intervention in vascular proliferative diseases. 2. In the present study, we investigated the mechanisms underlying VSMC growth arrest induced by the pharmacological agent PCA-4230. Addition of PCA-4230 to cultured VSMCs blocked the induction of cyclin D1 and cyclin A expression normally seen in serum-restimulated cells. Moreover, PCA-4230 inhibited cyclin-dependent kinase 2 (CDK2) activity and abrogated hyperphosphorylation of the retinoblastoma (Rb) gene product. Similarly, PCA-4230-dependent growth arrest of transformed cell lines correlated with reduced level of cyclin D1 protein and inhibition of CDK2 activity. Consistent with these findings, PCA-4230 repressed serum-inducible cyclin A promoter activity, and overexpression of either cyclin D1 or E2F1 efficiently circumvented this inhibitory effect. Importantly, adenovirus-mediated overexpression of E2F1 restored S-phase entry in PCA-4230-treated VSMCs, demonstrating that PCA-4230 represses cyclin A gene expression and VSMC growth via inhibition of the cyclin D1/E2F pathway. 3. Because of its ability to inhibit the growth of human VSMCs and transformed cell lines, future studies are warranted to assess whether PCA-4230 may be a suitable therapeutic intervention for the treatment of hyperproliferative disorders, including cardiovascular disease and cancer.

Expression of Mad1 in T cells leads to reduced thymic cellularity and impaired mitogen-induced proliferation

To investigate Mad1 function in vivo, transgenic mice were generated that express a Mad1 transgene in T lineage cells under the control of the proximal lck promoter. Thymus size in lck-Mad1 transgenic mice is drastically reduced although representation of the various thymocyte sub populations appears normal. To investigate more closely any effects of Mad1 expression on thymocytes, we examined thymic selection using MHC class I-restricted H-Y-TCR transgenic mice. Mad1 expression in vivo reduces the efficiency of positive selection. Furthermore, thymocytes and splenic T cells from lck-Mad1 transgenic mice display a profound proliferative defect in response to activation with either PMA/Ionomycin or immobilized anti-CD3/CD28 antibody. This proliferative defect is not reversed by addition of exogenous IL-2 and is p53-independent. The growth inhibition caused by Mad1 is overcome by expression of active c-Myc.

Analysis of C-MYC function in normal cells via conditional gene-targeted mutation

Germline inactivation of c-myc in mice causes embryonic lethality. Therefore, we developed a LoxP/Cre-based conditional mutation approach to test the role of c-myc in mouse embryonic fibroblasts (MEFs) and mature B lymphocytes. Cre expression resulted in reduced proliferation of wild-type MEFs, but c-Myc-deficient MEFs showed a further reduction. In contrast to fibroblasts, Cre expression had no apparent affect on wild-type B cell proliferation. Deletion of both c-Myc genes in B cells led to severely impaired proliferation in response to anti-CD40 plus IL-4. However, treated cells did upregulate several early activation markers but not CD95 or CD95 ligand. We discuss these findings with respect to potential c-Myc functions in proliferation and apoptosis and also discuss potential limitations in the Cre-mediated gene inactivation approach.

Myc-enhanced expression of Cul1 promotes ubiquitin-dependent proteolysis and cell cycle progression

The c-Myc oncoprotein plays an important role in the growth and proliferation of normal and neoplastic cells. To execute these actions, c-Myc is thought to regulate functionally diverse sets of genes that directly govern cellular mass and progression through critical cell cycle transitions. Here, we provide several lines of evidence that c-Myc promotes ubiquitin-dependent proteolysis by directly activating expression of the Cul1 gene, encoding a critical component of the ubiquitin ligase SCF(SKP2). The cell cycle inhibitor p27(kip1) is a known target of the SCF(SKP2) complex, and Myc-induced Cul1 expression matched well with the kinetics of declining p27(kip1) protein. Enforced Cul1 expression or antisense neutralization of p27(kip1) was capable of overcoming the slow-growth phenotype of c-Myc null primary mouse embryonic fibroblasts (MEFs). In reconstitution assays, the addition of in vitro translated Cul1 protein alone was able to restore p27(kip1) ubiquitination and degradation in lysates derived from c-myc(-/-) MEFs or density-arrested human fibroblasts. These functional and biochemical data provide a direct link between c-Myc transcriptional regulation and ubiquitin-mediated proteolysis and together support the view that c-Myc promotes G(1) exit in part via Cul1-dependent ubiquitination and degradation of the CDK inhibitor, p27(kip1).

Identification and characterization of SA/Scc3p subunits in the Xenopus and human cohesin complexes

A multisubunit protein complex, termed cohesin, plays an essential role in sister chromatid cohesion in yeast and in Xenopus laevis cell-free extracts. We report here that two distinct cohesin complexes exist in Xenopus egg extracts. A 14S complex (x-cohesin(SA1)) contains XSMC1, XSMC3, XRAD21, and a newly identified subunit, XSA1. In a second 12.5S complex (x-cohesin(SA2)), XSMC1, XSMC3, and XRAD21 associate with a different subunit, XSA2. Both XSA1 and XSA2 belong to the SA family of mammalian proteins and exhibit similarity to Scc3p, a recently identified component of yeast cohesin. In Xenopus egg extracts, x-cohesin(SA1) is predominant, whereas x-cohesin(SA2) constitutes only a very minor population. Human cells have a similar pair of cohesin complexes, but the SA2-type is the dominant form in somatic tissue culture cells. Immunolocalization experiments suggest that chromatin association of cohesin(SA1) and cohesin(SA2) may be differentially regulated. Dissociation of x-cohesin(SA1) from chromatin correlates with phosphorylation of XSA1 in the cell-free extracts. Purified cdc2-cyclin B can phosphorylate XSA1 in vitro and reduce the ability of x-cohesin(SA1) to bind to DNA or chromatin. These results shed light on the mechanism by which sister chromatid cohesion is partially dissolved in early mitosis, far before the onset of anaphase, in vertebrate cells.

Protooncogenes as mediators of apoptosis

Apoptosis has been well established as a vital biological phenomenon that is important in the maintenance of cellular homeostasis. Three major protooncogene families and their encoded proteins function as mediators of apoptosis in various cell types and are the subject of this chapter. Protooncogenic proteins such as c-Myc/Max, c-Fos/c-Jun, and Bcl-2/Bax utilize a synergetic effect to enhance their roles in the pro- or antiapoptotic action. These family members activate and repress the expression of their target genes, control cell cycle progression, and execute programmed cell death. Repression or overproduction of these protooncogenic proteins induces apoptosis, which may vary as a result of either cell type specificity or the nature of the apoptotic stimuli. The proapoptotic and antiapoptotic proteins exert their effects in the membrane of cellular organelles. Here they generate cell-type-specific signals that activate the caspase family of proteases and their regulators for the execution of apoptosis.

The oncoprotein kinase chaperone CDC37 functions as an oncogene in mice and collaborates with both c-myc and cyclin D1 in transformation of multiple tissues

CDC37 encodes a 50-kDa protein that targets intrinsically unstable oncoprotein kinases including Cdk4, Raf-1, and v-src to the molecular chaperone Hsp90, an interaction that is thought to be important for the establishment of signaling pathways. CDC37 is required for proliferation in budding yeast and is coexpressed with cyclin D1 in proliferative zones during mouse development, a finding consistent with a positive role in cell proliferation. CDC37 expression may not only be required to support proliferation in cells that are developmentally programmed to proliferate but may also be required in cells that are inappropriately induced to initiate proliferation by oncogenes. Here we report that mouse mammary tumor virus (MMTV)-CDC37 transgenic mice develop mammary gland tumors at a rate comparable to that observed previously in MMTV-cyclin D1 mice. Moreover, CDC37 was found to collaborate with MMTV-c-myc in the transformation of multiple tissues, including mammary and salivary glands in females and testis in males, and also collaborates with cyclin D1 to transform the female mammary gland. These data indicate that CDC37 can function as an oncogene in mice and suggests that the establishment of protein kinase pathways mediated by Cdc37-Hsp90 can be a rate-limiting event in epithelial cell transformation.