v-Jun overrides the mitogen dependence of S-phase entry by deregulating retinoblastoma protein phosphorylation and E2F-pocket protein interactions as a consequence of enhanced cyclin E-cdk2 catalytic activity

Alcohol-mediated calcium signals dysregulate pro-survival Snai2/PUMA/Bcl2 networks to promote p53-mediated apoptosis in avian neural crest progenitors

BACKGROUND

Prenatal alcohol exposure causes distinctive craniofacial anomalies that arise, in part, from the apoptotic elimination of neural crest (NC) progenitors that form the face. This vulnerability of NC to alcohol is puzzling as they normally express the transcriptional repressor Snail1/2 (in chick Snai2), which suppresses apoptosis and promotes their migration. Here, we investigate alcohol's impact upon Snai2 function.

METHODS

Chick cranial NC cells were treated with acute alcohol (52 mM, 2 hr). We evaluated NC migration, gene expression, proliferation, and apoptosis thereafter.

RESULTS

Transient alcohol exposure induced Snai2 (191% ± 23%; p = .003) and stimulated NC migration (p = .0092). An alcohol-induced calcium transient mediated this Snai2 induction, and BAPTA-AM blocked whereas ionomycin mimicked these pro-migratory effects. Alcohol suppressed CyclinD1 protein content (59.1 ± 12%, p = .007) and NC proliferation (19.7 ± 5.8%, p < .001), but these Snai2-enriched cells still apoptosed in response to alcohol. This was explained because alcohol induced p53 (198 ± 29%, p = .023), and the p53 antagonist pifithrin-α prevented their apoptosis. Moreover, alcohol counteracted Snai2's pro-survival signals, and Bcl2 was repressed (68.5 ± 6.0% of controls, p = .016) and PUMA was not induced, while ATM (1.32-fold, p = .01) and PTEN (1.30-fold, p = .028) were elevated.

CONCLUSIONS

Alcohol's calcium transient uncouples the Snai2/p53 regulatory loop that normally prevents apoptosis during EMT. This represents a novel pathway in alcohol's neurotoxicity, and complements demonstrations that alcohol suppresses PUMA in mouse NC. We propose that the NCs migratory behavior, and their requirement for Snai2/p53 co-expression, makes them vulnerable to stressors that dysregulate Snai2/p53 interactions, such as alcohol.

Downregulation of Cdc2/CDK1 kinase activity induces the synthesis of noninfectious human papillomavirus type 31b virions in organotypic tissues exposed to benzo[a]pyrene

Epidemiological studies suggest that human papillomavirus (HPV)-infected women who smoke face an increased risk for developing cervical cancer. We have previously reported that exposure of HPV-positive organotypic cultures to benzo[a]pyrene (BaP), a major carcinogen in cigarette smoke, resulted in enhanced viral titers. Since BaP is known to deregulate multiple pathways of cellular proliferation, enhanced virion synthesis could result from carcinogen/host cell interaction. Here, we report that BaP-mediated upregulation of virus synthesis is correlated to an altered balance between cell cycle-specific cyclin-dependent kinase (CDK) activity profile compared with controls. Specifically, BaP treatment increased accumulation of hyperphosphorylated retinoblastoma protein (pRb) which coincided with increased cdc2/CDK1 kinase activity, but which further conflicted with the simultaneous upregulation of CDK inhibitors p16(INK4) and p27(KIP1), which normally mediate pRb hypophosphorylation. In contrast, p21(WAF1) and p53 levels remained unchanged. Under these conditions, CDK6 and CDK2 kinase activities were decreased, whereas CDK4 kinase activity remained unchanged. The addition of purvalanol A, a specific inhibitor of CDK1 kinase, to BaP-treated cultures, resulted in the production of noninfectious HPV type 31b (HPV31b) particles. In contrast, infectivity of control virus was unaffected by purvalanol A treatment. BaP targeting of CDK1 occurred independently of HPV status, since BaP treatment also increased CDK1 activity in tissues derived from primary keratinocytes. Our data indicate that HPV31b virions synthesized in the presence of BaP were dependent on BaP-mediated alteration in CDK1 kinase activity for maintaining their infectivity.

Runx2 disruption promotes immortalization and confers resistance to oncogene-induced senescence in primary murine fibroblasts

The Runx genes play paradoxical roles in cancer where they can function either as dominant oncogenes or tumor suppressors according to context. We now show that the ability to induce premature senescence in primary murine embryonic fibroblasts (MEF) is a common feature of all three Runx genes. However, ectopic Runx-induced senescence contrasts with Ras oncogene-induced senescence, as it occurs directly and lacks the hallmarks of proliferative stress. Moreover, a fundamental role for Runx function in the senescence program is indicated by the effects of Runx2 disruption, which renders MEFs prone to spontaneous immortalization and confers an early growth advantage that is resistant to stress-induced growth arrest. Runx2(-/-) cells are refractory to H-Ras(V12)-induced premature senescence, despite the activation of a cascade of growth inhibitors and senescence markers, and are permissive for oncogenic transformation. The aberrant behavior of Runx2(-/-) cells is associated with signaling defects and elevated expression of S-G(2)-M cyclins and their associated cyclin dependent kinase activities that may override the effects of growth inhibitory signals. Coupling of stress responses to the cell cycle represents a novel facet of Runx tumor suppressor function and provides a rationale for the lineage-specific effects of loss of Runx function in cancer.

Global regulation of genome duplication in eukaryotes: an overview from the epifluorescence microscope

In eukaryotes, DNA replication is initiated along each chromosome at multiple sites called replication origins. Locally, each replication origin is "licensed" or specified at the end of the M and the beginning of the G1 phases of the cell cycle. During the S phase when DNA synthesis takes place, origins are activated in stages corresponding to early and late-replicating domains. The staged and progressive activation of replication origins reflects the need to maintain a strict balance between the number of active replication forks and the rate at which DNA synthesis proceeds. This suggests that origin densities (frequency of initiation) and replication fork movement (rates of elongation) must be coregulated to guarantee the efficient and complete duplication of each subchromosomal domain. Emerging evidence supports this proposal and suggests that the ATM/ATR intra-S phase checkpoint plays an important role in the coregulation of initiation frequencies and rates of elongation. In this paper, we review recent results concerning the mechanisms governing the global regulation of DNA replication and discuss the roles these mechanisms play in maintaining genome stability during both a normal and perturbed S phase.

xCT expression reduces the early cell cycle requirement for calcium signaling

Calcium has long been recognized as an important regulator of cell cycle transitions although the mechanisms are largely unknown. A functional genomic screen has identified genes involved in the regulation of early cell cycle progression by calcium. These genes when overexpressed confer the ability to bypass the G1/S arrest induced by Ca(2+)-channel antagonists in mouse fibroblasts. Overexpression of the cystine-glutamate exchanger, xCT, had the greatest ability to evade calcium antagonist-induced cell cycle arrest. xCT carries out the rate limiting step of glutathione synthesis in many cell types and is responsible for the uptake of cystine in most human cancer cell lines. Functional analysis indicates that the cystine uptake activity of xCT overcomes the G1/S arrest induced by Ca(2+)-channel antagonists by bypassing the requirement for calcium signaling. Since cells overexpressing xCT were found to have increased levels and activity of the AP-1 transcription factor in G1, redox stimulation of AP-1 activity accounts for the observed growth of these cells in the presence of calcium channel antagonists. These results suggest that reduced calcium signaling impairs AP-1 activation and that xCT expression may directly affect cell proliferation.

Adeno-associated virus type 2 increases proteosome-dependent degradation of p21WAF1 in a human papillomavirus type 31b-positive cervical carcinoma line

Adeno-associated virus type 2 (AAV2) seropositivity is negatively correlated with the development of human papillomavirus (HPV)-associated cervical cancer. We have begun analysis of the molecular mechanisms underlying AAV2-mediated onco-suppression through cell cycle regulation in HPV-infected keratinocytes isolated from a low-grade cervical lesion. AAV2 superinfection of HPV type 31b (HPV31b)-positive cells at early times postinfection resulted in degradation of the cyclin-dependent kinase (CDK) inhibitor p21(WAF1) protein in a proteosome-dependent manner. Downstream consequences of lowering p21(WAF1) levels included a proportional loss of cyclin E/CDK2 complexes bound to p21(WAF1). The loss of stable p21(WAF1)/cyclin E/CDK2 complexes coincided with an increase in CDK2-associated kinase activity and cyclin E levels. Both events have the potential to enhance the G(1)/S transition point mediated by active cyclin E/CDK2 complexes. Concurrently, cyclin A and E2F levels were decreased, conditions reminiscent of delayed entrance into the S phase of the cell cycle. On the other hand, infection of primary human foreskin keratinocytes with AAV2 resulted in upregulation of p21(WAF1) protein levels, reminiscent of a block in G(1) phase progression. We propose that by down regulating p21(WAF1), AAV2 initiates cell cycle activities leading to enhanced G(1)/S phase-like conditions which may be favorable for AAV2-specific functions and may lead to downstream interference with HPV-associated cervical cancer progression.

Inactivation of MAP kinase signalling in Myc transformed cells and rescue by LiCl inhibition of GSK3

c-Myc oncogene is an important regulator of cell cycle and apoptosis, and its dysregulated expression is associated with many malignancies. Myc is instrumental in directly or indirectly regulating the progression through the G1 phase and G1/S transition, and transformation by Myc results in perturbed cell cycle. Also contributory to the control of G1 is the Ras effector pathway Raf/MEK/ERK MAP kinase. Together with GSK3, ERK plays an important role in the critical hierarchical phosphorylation of S62/T58 controlling Myc protein levels. Therefore, our main aim was to examine the levels of MAPK in Myc transformed cells in light of the roles of ERK in cell cycle and control of Myc protein levels. We found that active forms of ERK were barely detectable in v-Myc (MC29) transformed cells. Furthermore, we could only detect reduced levels of activated ERK in c-Myc transformed cells compared to the non-transformed primary chick embryo fibroblast cells. The addition of LiCl inhibited GSK3 and successfully restored the levels of ERK in v-Myc and c-Myc transformed cells to those found in non-transformed cells. In addition, LiCl stabilised Myc protein in the non-transformed and c-Myc transformed cells but not in v-Myc transformed cells. These results can provide an important insight into the role of MAPK in the mechanism of Myc induced transformation and carcinogenesis.

Chk1-dependent S-M checkpoint delay in vertebrate cells is linked to maintenance of viable replication structures

We investigated mitotic delay during replication arrest (the S-M checkpoint) in DT40 B-lymphoma cells deficient in the Chk1 or Chk2 kinase. We show here that cells lacking Chk1, but not those lacking Chk2, enter mitosis with incompletely replicated DNA when DNA synthesis is blocked, but only after an initial delay. This initial delay persists when S-M checkpoint failure is induced in Chk2-/- cells with the Chk1 inhibitor UCN-01, indicating that it does not depend on Chk1 or Chk2 activity. Surprisingly, dephosphorylation of tyrosine 15 did not accompany Cdc2 activation during premature entry to mitosis in Chk1-/- cells, although mitotic phosphorylation of cyclin B2 did occur. Previous studies have shown that Chk1 is required to stabilize stalled replication forks during replication arrest, and strikingly, premature mitosis occurs only in Chk1-deficient cells which have lost the capacity to synthesize DNA as a result of progressive replication fork inactivation. These results suggest that Chk1 maintains the S-M checkpoint indirectly by preserving the viability of replication structures and that it is the continued presence of such structures, rather than the activation of Chk1 per se, which delays mitosis until DNA replication is complete.

Cyclin A is a c-Jun target gene and is necessary for c-Jun-induced anchorage-independent growth in RAT1a cells

Overexpression of c-Jun enables Rat1a cells to grow in an anchorage-independent manner. We used an inducible c-Jun system under the regulation of doxycycline in Rat1a cells to identify potential c-Jun target genes necessary for c-Jun-induced anchorage-independent growth. Induction of c-Jun results in sustained expression of cyclin A in the nonadherent state with only minimal expression in the absence of c-Jun. The promoter activity of cyclin A2 was 4-fold higher in Rat1a cells in which c-Jun expression was induced compared with the control cells. Chromatin immunoprecipitation demonstrated that c-Jun bound directly to the cyclin A2 promoter. Mutation analysis of the cyclin A2 promoter mapped the c-Jun regulatory site to an ATF site at position -80. c-Jun was able to bind to this site both in vitro and in vivo, and mutation of this site completely abolished promoter activity. Cyclin A1 was also elevated in c-Jun-overexpressing Rat1a cells; however, c-Jun did not regulate this gene directly, since it did not bind directly to the cyclin A1 promoter. Suppression of cyclin A expression via the introduction of a cyclin A antisense sequences significantly reduced the ability of c-Jun-overexpressing Rat1a cells to grow in an anchorage-independent fashion. Taken together, these results suggest that cyclin A is a target of c-Jun and is necessary but not sufficient for c-Jun-induced anchorage-independent growth. In addition, we demonstrated that the cytoplasmic oncogenes Ras and Src transcriptionally activated the cyclin A2 promoter via the ATF site at position -80. Using a dominant negative c-Jun mutant, TAM67, we showed that this transcriptional activation of cyclin A2 requires c-Jun. Thus, our results suggest that c-Jun is a mediator of the aberrant cyclin A2 expression associated with Ras/Src-induced transformation.

Microarray analysis identifies Autotaxin, a tumour cell motility and angiogenic factor with lysophospholipase D activity, as a specific target of cell transformation by v-Jun

We have used chicken cDNA microarrays to investigate gene-expression changes induced during transformation of chick embryo fibroblasts (CEF) by the viral Jun oncoprotein encoded by ASV17. This analysis reveals that v-Jun induces increases and decreases of varying magnitude in the expression of genes involved in diverse cellular functions, most of which have not been detected in previous screens for putative v-Jun targets. In all, 27 individual genes were identified, whose expression is increased threefold or more in v-Jun-transformed cells, including genes involved in energy generation, protein synthesis, and gene transcription. Interestingly, this group includes the hypoxia-inducible factor-1 alpha (Hif-1alpha) transcription factor and the glycolytic enzyme enolase, suggesting that adaptation to hypoxia could play a role in tumorigenesis by v-Jun. We also identified 32 genes whose expression is decreased threefold or more, including chaperones, components of the cytoskeleton, and, unexpectedly, DNA replication factors. The gene whose expression is upregulated most dramatically (approximately 100-fold) encodes Autotaxin (ATX), a secreted tumor motility-promoting factor with lysophospholipase D activity. Strikingly, v-Jun-transformed CEF secrete catalytically active ATX and chemotactic activity, which can be detected in conditioned medium. ATX is not detectably expressed in normal CEF or CEF transformed by the v-Src or v-Myc oncoproteins, indicating that induction of this putative autocrine/paracrine factor is a specific consequence of cell transformation by v-Jun. ATX has been implicated in both angiogenesis and invasion, and could therefore play an important role in tumorigenesis by v-Jun in vivo.

Accelerated cell cycle progression in osteoblasts overexpressing the c-fos proto-oncogene: induction of cyclin A and enhanced CDK2 activity

Transgenic mice overexpressing the c-Fos oncoprotein develop osteosarcomas that are associated with deregulated expression of cell cycle genes. Here we have generated osteoblast cell lines expressing c-fos under the control of a tetracycline-regulatable promoter to investigate the role of c-Fos in osteoblast cell cycle control in vitro. Three stable subclones, AT9.2, AT9.3, and AT9.7, derived from MC3T3-E1 mouse osteoblasts, expressed high levels of exogenous c-fos mRNA and protein in the absence of tetracycline. Functional contribution of ectopic c-Fos to AP-1 complexes was confirmed by electromobility shift assays and transactivation of AP-1 reporter constructs. Induction of exogenous c-Fos in quiescent AT9.2 cells caused accelerated S-phase entry following serum stimulation, resulting in enhanced growth rate. Ectopic c-Fos resulted in increased expression of cyclins A and E protein levels, and premature activation of cyclin A-, cyclin E-, and cyclin-dependent kinase (CDK) 2-associated kinase activities, although cyclin D levels and CDK4 activity were not affected significantly in these cell lines. The enhanced CDK2 kinase activity was associated with a rapid, concomitant dissociation of p27 from CDK2-containing complexes. Deregulated cyclin A expression and CDK2 activity was also observed in primary mouse osteoblasts overexpressing c-Fos, but not in fibroblasts, and c-Fos transgenic tumor-derived osteosarcoma cells constitutively expressed high levels of cyclin A protein. These data suggest that overexpression of c-Fos in osteoblasts results in accelerated S phase entry as a result of deregulated cyclin A/E-CDK2 activity. This represents a novel role for c-Fos in osteoblast growth control and may provide c-Fos-overexpressing osteoblasts with a growth advantage during tumorigenesis.

Lipoxygenase inhibitors enhance tumor suppressive effects of Jun proteins on v-myb-transformed monoblasts BM2

Inhibitors of arachidonic acid (AA) conversion were described as suppressors of proliferation and inducers of differentiation of various leukemic cells. Certain AA metabolites have been shown to cooperate with Jun proteins that are important factors controlling cell proliferation, differentiation and apoptosis. Using lipoxygenase (LOX) inhibitors of various specifity we studied possible participation of lipoxygenase pathway in regulation of proliferation and apoptosis of v-myb-transformed chicken monoblasts BM2 and its functional interaction with Jun proteins. We found that nordihydroguaiaretic acid (NDGA) and esculetin (Esc) negatively regulate proliferation of BM2 cells causing accumulation in either G0/G1-phase (nordihydroguaiaretic acid) or S-phase (esculetin) of the cell cycle. BM2 cells can be also induced to undergo growth arrest and partial differentiation by ectopic expression of Jun proteins. We demonstrated that lipoxygenase inhibitors further enforce tumor suppressive capabilities of Jun proteins by inducing either more efficient cell cycle block and/or apoptosis in BM2 cells. This suggests that there is a cross-talk between the lipoxygenase- and Jun-directed pathways in regulation of differentiation and proliferation of monoblastic cells. Thus pharmacologic agents that specifically block lipoxygenase-catalyzed activity and enforce the effects of differentiation-inducers may be important components in anti-tumor therapies.

Estrogen receptor activation function 2 (AF-2) is essential for hormone-dependent transactivation and cell transformation induced by a v-Jun DNA binding domain-estrogen receptor chimera

A chimeric protein consisting of the estrogen receptor alpha ligand binding domain (ER-alpha LBD) fused to the DNA binding domain (DBD) of the v-Jun oncoprotein, deltavJ-hER, was previously shown to elicit estradiol-dependent transcriptional activation and cell transformation. Remarkably, in the unliganded state deltavJ-hER is not inert, but rather inhibits cell proliferation. To understand the molecular basis for these opposite effects on cell growth, we investigated the effect of estradiol on deltavJ-hER function. We find that deltavJ-hER is localised to the cell nucleus and capable of binding TPA-response element (TRE) DNA recognition sites in the presence and absence of estradiol, indicating that these properties are unlikely to be the targets of hormonal regulation. In contrast, a mutant derivative of deltavJ-hER in which amino acid substitutions selectively disrupt activation function 2 (AF-2) function is unable to elicit estradiol-dependent transcription or cell transformation, even though DNA binding is not impaired. Taken together, these observations establish that estrogen receptor AF-2 activity is essential for cell transformation by deltavJ-hER.

Signal transduction mediated by the Ras/Raf/MEK/ERK pathway from cytokine receptors to transcription factors: potential targeting for therapeutic intervention

The Ras/Raf/Mitogen-activated protein kinase/ERK kinase (MEK)/extracellular-signal-regulated kinase (ERK) cascade couples signals from cell surface receptors to transcription factors, which regulate gene expression. Depending upon the stimulus and cell type, this pathway can transmit signals, which result in the prevention or induction of apoptosis or cell cycle progression. Thus, it is an appropriate pathway to target for therapeutic intervention. This pathway becomes more complex daily, as there are multiple members of the kinase and transcription factor families, which can be activated or inactivated by protein phosphorylation. The diversity of signals transduced by this pathway is increased, as different family members heterodimerize to transmit different signals. Furthermore, additional signal transduction pathways interact with the Raf/MEK/ERK pathway to regulate positively or negatively its activity, or to alter the phosphorylation status of downstream targets. Abnormal activation of this pathway occurs in leukemia because of mutations at Ras as well as genes in other pathways (eg PI3K, PTEN, Akt), which serve to regulate its activity. Dysregulation of this pathway can result in autocrine transformation of hematopoietic cells since cytokine genes such as interleukin-3 and granulocyte/macrophage colony-stimulating factor contain the transacting binding sites for the transcription factors regulated by this pathway. Inhibitors of Ras, Raf, MEK and some downstream targets have been developed and many are currently in clinical trials. This review will summarize our current understanding of the Ras/Raf/MEK/ERK signal transduction pathway and the downstream transcription factors. The prospects of targeting this pathway for therapeutic intervention in leukemia and other cancers will be evaluated.

Identification of cJun-responsive genes in Rat-1a cells using multiple techniques: increased expression of stathmin is necessary for cJun-mediated anchorage-independent growth

cJun is a major component of the transcription factor AP-1 and mediates a diverse set of biologic properties including proliferation, differentiation, and apoptosis. To identify cJun-responsive genes, we inducibly expressed cJun in Rat-1a cells and observed two distinct phenotypes: changes in cellular morphology with adherent growth and anchorage-independent growth. The biologic effects of cJun were entirely reversible demonstrating that they require the continued presence of cJun. To determine the genes, which mediate the biologic effects of cJun, we employed multiple methods including differential gene analysis, suppression subtractive hybridization, and cDNA microarrays. We identified 38 cJun-responsive genes including three uncharacterized genes under adherent and/or nonadherent conditions. Half of the known 36 genes were cytoskeleton- and adhesion-related genes, suggesting a major role of cJun in the regulation of the genes related to cell morphology. As proof of the principle that this approach could identify genes whose upregulation was necessary for nonadherent growth, we investigated one gene, stathmin whose upregulation by cJun was observed only under these conditions. Although overexpression of stathmin did not result in nonadherent growth, inhibition of stathmin protein expression by antisense oligonucleotides in cJun-induced Rat-1a cells prevented nonadherent growth. These results suggest that stathmin plays an essential role in anchorage-independent growth by cJun and may be a potential target for specific inhibitors for AP-1-dependent processes involved in carcinogenesis.

v-Jun stimulates both cdk2 kinase activity and G1/S progression via transcriptional repression of p21 CIP1

Previous studies have shown that the viral Jun (v-Jun) oncoprotein induces marked alterations in cell cycle control, which are associated with, and may be caused by, increased cdk2 kinase activity. Since p21 CIP1 is an important regulator of cdk2, we investigated whether aberrant expression of this cyclin-dependent kinase inhibitor might contribute to cell cycle deregulation by v-Jun. We find that the basal levels of p21 CIP1 mRNA and protein expression are greatly reduced in chick embryo fibroblasts (CEF) transformed by v-Jun, and that v-Jun blocks the increases in p21 CIP1 expression that normally accompany growth inhibition induced by serum deprivation or confluency in untransformed CEF. Importantly, ectopic expression of p21 CIP1 in v-Jun-transformed CEF inhibits both cdk2 kinase activity and cell cycle progression, indicating that these alterations in p21 CIP1 expression are likely to be functionally significant for growth deregulation. We also investigated the mechanism through which v-Jun disturbs p21 CIP1 expression and the possible involvement of a known p21 CIP1 regulator, p53, as an intermediate in this process. This analysis revealed that repression is mediated primarily at the level of p21 CIP1 gene transcription, however the mechanism is complex; both p53-dependent and -independent mechanisms contribute as judged by analysis of p21 CIP1 promoter mutants and other assays of p53 transcriptional activity.

Chk1-deficient tumour cells are viable but exhibit multiple checkpoint and survival defects

The conserved protein kinase Chk1 is believed to play an important role in checkpoint responses to aberrant DNA structures; however, genetic analysis of Chk1 functions in metazoans is complicated by lethality of Chk1-deficient embryonic cells. We have used gene targeting to eliminate Chk1 function in somatic DT40 B-lymphoma cells. We find that Chk1-deficient DT40 cells are viable, but fail to arrest in G(2)/M in response to and are hypersensitive to killing by ionizing radiation. Chk1-deficient cells also fail to maintain viable replication forks or suppress futile origin firing when DNA polymerase is inhibited, leading to incomplete genome duplication and diminished cell survival after release from replication arrest. In contrast to embryonic cells, however, Chk1 is not required to delay mitosis when DNA synthesis is inhibited. Thus, Chk1 is dispensable for normal cell division in somatic DT40 cells but is essential for DNA damage-induced G(2)/M arrest and a subset of replication checkpoint responses. Furthermore, Chk1-dependent processes promote tumour cell survival after perturbations of DNA structure or metabolism.

A new c-Jun N-terminal kinase (JNK)-interacting protein, Sab (SH3BP5), associates with mitochondria

We have identified a novel c-Jun N-terminal kinase (JNK)-interacting protein, Sab, by yeast two-hybrid screening. Sab binds to and serves as a substrate for JNK in vitro, and was previously found to interact with the Src homology 3 (SH3) domain of Bruton's tyrosine kinase (Btk). Inspection of the sequence of Sab reveals the presence of two putative mitogen-activated protein kinase interaction motifs (KIMs) similar to that found in the JNK docking domain of the c-Jun transcription factor, and four potential serine-proline JNK phosphorylation sites in the C-terminal half of the molecule. Using deletion and site-directed mutagenesis, we demonstrate that the most N-terminal KIM in Sab is essential for JNK binding, and that, as with c-Jun, physical interaction with JNK is necessary for Sab phosphorylation. Interestingly, confocal immunocytochemistry and cell fractionation studies indicate that Sab is associated with mitochondria, where it co-localizes with a fraction of active JNK. These and previously reported properties of Sab suggest a possible role in targeting JNK to this subcellular compartment and/or mediating cross-talk between the Btk and JNK signal transduction pathways.

Cell transformation by v-Jun deactivates ERK MAP kinase signalling

Previous studies have shown that v-Jun accelerates G1 progression and enables cells to sustain S phase entry in the absence of serum growth factors. Since growth factor-dependent ERK MAP kinase signalling plays an important role in regulating the G1/S transition, we investigated whether aberrant ERK regulation might contribute to cell cycle deregulation by v-Jun. Contrary to expectation, we find that cells transformed by v-Jun exhibit a profound reduction in the basal level of active, dual-phosphorylated ERK. In addition, ERK becomes refractory to stimulation by a subset of agonists including serum, LPA, and EGF, but remains partially responsive to the phorbol ester, TPA. Biochemical analysis indicates that these defects are attributable to a combination of inefficient signal propagation between Ras and Raf within the ERK pathway and increased tonic deactivation by MAP kinase phosphatases. Taken together, these results demonstrate that cell transformation by v-Jun induces alterations in cell physiology which antagonize ERK signalling at multiple levels. The potential significance of this phenotype for oncogenesis by v-Jun is discussed.

AP-1 as a regulator of cell life and death

The transcription factor AP-1 (activator protein-1) is involved in cellular proliferation, transformation and death. Using mice and cells lacking AP-1 components, the target-genes and molecular mechanisms mediating these processes were recently identified. Interestingly, the growth-promoting activity of c-Jun is mediated by repression of tumour suppressors, as well as upregulation of positive cell cycle regulators. Mostly, c-Jun is a positive regulator of cell proliferation, whereas JunB has the converse effect. The intricate relationships between the different Jun proteins, their activities and the mechanisms that mediate them will be discussed.

Perspectives for cancer therapies with cdk2 inhibitors

Modern anticancer strategies are designed against specific molecular targets with the goal of sparing normal, non-neoplastic tissues. Choosing specific molecular targets, however, is problematic. Cdk2 (Cyclin dependent kinase 2, cell division kinase 2, p33) is an important candidate target for therapeutic intervention. Phosphorylation of retinoblastoma protein (pRb) by Cdk2 is the penultimate step in the transition from G1 to S phase. Inhibition of this step could potentially result in inhibition of proliferation, cytostasis and possibly apoptosis in human tumors. Cdk2 also plays a critical role in the transition through S phase and the S to G2 transition as well. Inhibitors of the cyclin dependent kinases, such as flavopiridol and UCN-01, are currently in clinical trials. While demonstrating clinical activity, neither acts specifically against Cdk2. Other more specific Cdk2 inhibitors are currently in preclinical development. Further studies to explore the therapeutic worth of such agents are warranted.

Dominant negative c-jun inhibits activation of the cyclin D1 and cyclin E kinase complexes

The AP-1 transcription factor is activated by oncogenic signal transduction cascades and its function is critical for both mitogenesis and carcinogenesis. To define the role of AP-1 in the context of a human fibrosarcoma cell line, HT1080, we expressed a dominant negative c-jun mutant fused to the green fluorescent protein in an ecdysone-inducible system. We demonstrated that high levels of this mutant, GFP-TAM67, inhibit AP-1 activity and arrest cells predominantly in the G1 phase of the cell cycle. This arrest is reversible and occurs only above a threshold concentration; low to moderate levels of GFP-TAM67 are insufficient for growth arrest. Contrary to expectations based on the literature, GFP-TAM67 does not inhibit expression of cyclin D1, cyclin E, or their respective cyclin-dependent kinases. However, pRB is hypophosphorylated in GFP-TAM67-arrested cells and the activity of both the cyclin D1:cdk and the cyclin E:cdk complexes are impaired. Both of these complexes show an increased association with p21(CIP1/WAF1), concomitantly with induction of the p21 mRNA by GFP-TAM67. These results suggest a novel function of AP-1 in the activation of the G1 cyclin:cdk complexes in human tumor cells by regulating the expression of the p21(CIP1/WAF1) gene.

Distinct roles of Jun : Fos and Jun : ATF dimers in oncogenesis

Jun : Fos and Jun : ATF complexes represent two classes of AP-1 dimers that (1) preferentially bind to either heptameric or octameric AP-1 binding sites, and (2) are differently regulated by cellular signaling pathways and oncogene products. To discriminate between the functions of Jun : Fos, Jun : ATF and Jun : Jun, mutants were developed that restrict the ability of Jun to dimerize either to itself, or to Fos(-like) or ATF(-like) partners. Introduction of these mutants in chicken embryo fibroblasts shows that Jun : Fra2 and Jun : ATF2 dimers play distinct, complementary roles in in vitro oncogenesis by inducing either anchorage independence or growth factor independence, respectively. v-Jun : ATF2 rather than v-Jun : Fra2 triggers the development of primary fibrosarcomas in the chicken wing. Genes encoding extracellular matrix components seem to constitute an important subset of v-Jun : ATF2-target genes. Repression of the matrix component SPARC by Jun is essential for the induction of fibrosarcomas. Avian primary cells transformed by either Jun : Fra2 or Jun : ATF2 thus provide powerful tools for the investigation of the downstream pathways involved in oncogenesis. Further genetic studies with Jun dimerization mutants will be required to be precise and extend the specific roles of the Jun : Fos and Jun : ATF dimers during cancer progression in avian and mammalian systems.

v-Jun sensitizes cells to apoptosis by a mechanism involving mitochondrial cytochrome C release

v-Jun shares the ability of the Myc, E1A, and E2F oncogenes to both sustain cell cycle progression and promote apoptosis in the absence of mitogenic stimulation. To gain an insight into the mechanism of apoptosis sensitization, we examined the possible involvement of key regulatory proteins previously implicated in oncogene-induced cell death during v-Jun-induced apoptosis triggered by serum withdrawal. We observed that ectopic expression of the anti-apoptotic Bcl-2 protein, or of two downstream effectors of growth factor signalling, v-PI 3-Kinase and v-Src, partially or completely suppressed apoptosis. Apoptosis was also observed in the presence of serum growth factors when endogenous PI3K activity was blocked using the synthetic inhibitor LY294002, further suggesting an important role for PI3-K in cell survival. Cytochrome C was released into the cytosol of apoptotic v-Jun expressing cells, and this release was inhibited by Bcl-2, suggesting an important role for mitochondrial dysfunction in v-Jun induced apoptosis. In contrast, inhibition of Fas signalling using dominant negative FADD did not inhibit apoptosis, nor was there any evidence for accumulation or activation of p53 in v-Jun transformed cells. Consistent with this latter observation, inhibition of p53 function by HPV16 E6 protein had no effect on v-Jun induced cell death. Taken together, these results suggest that mitochondrial dysfunction is an important component of the mechanism through which v-Jun sensitizes cells to apoptosis, but that the apoptotic signals elicited by v-Jun upstream of the mitochondria do not depend on increased levels of p53 activity or Fas signalling.