Phosphorylation at serine 331 is required for Aurora B activation

Aurora B kinase activity is required for successful cell division. In this paper, we show that Aurora B is phosphorylated at serine 331 (Ser331) during mitosis and that phosphorylated Aurora B localizes to kinetochores in prometaphase cells. Chk1 kinase is essential for Ser331 phosphorylation during unperturbed prometaphase or during spindle disruption by taxol but not nocodazole. Phosphorylation at Ser331 is required for optimal phosphorylation of INCENP at TSS residues, for Survivin association with the chromosomal passenger complex, and for complete Aurora B activation, but it is dispensable for Aurora B localization to centromeres, for autophosphorylation at threonine 232, and for association with INCENP. Overexpression of Aurora B(S331A), in which Ser331 is mutated to alanine, results in spontaneous chromosome missegregation, cell multinucleation, unstable binding of BubR1 to kinetochores, and impaired mitotic delay in the presence of taxol. We propose that Chk1 phosphorylates Aurora B at Ser331 to fully induce Aurora B kinase activity. These results indicate that phosphorylation at Ser331 is an essential mechanism for Aurora B activation.

Chk2 is required for optimal mitotic delay in response to irradiation-induced DNA damage incurred in G2 phase

Whether Chk2 contributes to DNA damage-induced arrest in G2 has been controversial. To investigate this issue further, we generated Chk2-deficient DT40 B-lymphoma cells by gene targeting and compared their cell cycle response to ionizing radiation (IR) with wild-type (WT) and isogenic Chk1-deficient counterparts. After moderate doses of IR (4 Gy), we find that Chk2-/- cells which are in G1 or S phase at the time of irradiation arrest efficiently in G2. In contrast, Chk2-/- cells which are in G2 when DNA damage is incurred exhibit an impaired mitotic delay compared to WT, with the result that cells enter mitosis with damaged DNA as judged by the presence of numerous gamma-H2AX foci on condensed chromosomes. Impaired G2 delay as the result of Chk2 deficiency can be detected at very low doses of radiation (0.1 Gy), and may allow division with spontaneous DNA damage, since a higher proportion of mitotic Chk2-/- cells bear spontaneous gamma-H2AX foci and damaged chromosomes during unperturbed growth compared to WT. The contribution of Chk2 to G2/M delay is epistatic to that of Chk1, since Chk1-/- cells exhibit no measurable mitotic delay at any radiation dose tested. We suggest that this function of Chk2 could contribute to tumour suppression, since cell division with low levels of spontaneous damage is likely to promote genetic instability and thus carcinogenesis.

Analysing the DNA damage and replication checkpoints in DT40 cells

Eukaryotic cells respond to DNA damage or blocks to DNA replication by triggering a variety of "checkpoint" responses which delay cell cycle progression, modulate DNA replication, and facilitate DNA repair. Checkpoints play a vital role in maintaining genome integrity, particularly under conditions of genotoxic stress, and mutations in checkpoint genes can predispose to cancer and aging. Checkpoints are best understood at the molecular level in model organisms such as fission yeast, where the presence of aberrant DNA structures is sensed and relayed via signal transduction pathways to activate the checkpoint effector kinases, Chk1 and Cds1/ Chk2, which implement appropriate responses. Many of the yeast checkpoint sensor, transducer, and effector proteins are conserved in vertebrate cells, raising the question of whether they function in a similar or analogous way. DT40 cells provide a particularly tractable experimental system for genetic and biochemical dissection of checkpoints in vertebrates. Thus far, gene knockouts in DT40 have revealed that the Chk1 and Chk2 checkpoint effector kinases control a very different range of checkpoint responses in vertebrates compared to yeast. In future, these and other DT40 mutants will provide powerful tools for understanding the molecular basis of these unexpected differences and detailed studies of checkpoint mechanisms.

Centrifugal elutriation as a means of cell cycle phase separation and synchronisation

The ability to purify cells in specific phases of the cell cycle in sufficient quantities for biochemical analysis or to obtain a "synchronized" population of living cells enriched in a particular phase is of great value for studying a wide range of cell cycle processes. Transformed tumour cell lines, such as DT40, can be arrested and released from specific points in the cell cycle using drugs which inhibit DNA or protein synthesis or interfere with mitotic spindle function, however such drugs are frequently toxic and may themselves perturb the cell cycle process or phenomenon under study. Centrifugal elutriation provides a means of separating an unperturbed culture of living cells into highly enriched G1-, S-, and G2/ M-phase fractions. The resulting cell fractions can be analysed directly for protein and mRNA expression or returned to culture and their subsequent progression through the cell cycle monitored by flow cytometry, microscopy, or other techniques.

Mitotic index determination by flow cytometry

Chromosome condensation during mitosis is associated with phosphorylation of histone H3 at serine 10 (pS10H3). Detection of pS10H3 phosphorylation using phospho-specific antibodies in combination with DNA content flow cytometry provides a rapid and accurate means of determining the percentage of mitotic cells in a population. Using the spindle poison nocodazole to trap mitotic cells it is possible to determine the rate at which cells accumulate in mitosis with time. By comparing the rates of mitotic accumulation before and after DNA damage it is possible to gauge the efficiency with which the G2 checkpoint blocks entry to mitosis. Because DNA content is measured simultaneously with pS10H3 fluorescence this method can also be used to identify cells which enter mitosis with incompletely replicated DNA as a result of S-M checkpoint failure during DNA synthesis inhibition.

Chk1 regulates the density of active replication origins during the vertebrate S phase

The checkpoint kinase 1 (Chk1) preserves genome integrity when replication is performed on damaged templates. Recently, Chk1 has also been implicated in regulating different aspects of unperturbed S phase. Using mammalian and avian cells with compromised Chk1 activity, we show that an increase in active replicons compensates for inefficient DNA polymerisation. In the absence of damage, loss of Chk1 activity correlates with the frequent stalling and, possibly, collapse of active forks and activation of adjacent, previously suppressed, origins. In human cells, super-activation of replication origins is restricted to pre-existing replication factories. In avian cells, in contrast, Chk1 deletion also correlates with the super-activation of replication factories and loss of temporal continuity in the replication programme. The same phenotype is induced in wild-type avian cells when Chk1 or ATM/ATR is inhibited. These observations show that Chk1 regulates replication origin activation and contributes to S-phase progression in somatic vertebrate cells.

Exercising restraints: role of Chk1 in regulating the onset and progression of unperturbed mitosis in vertebrate cells

In vertebrate cells Chk1 is essential for multiple checkpoint responses to acute DNA damage or replication blocks, however potential functions for Chk1 during unperturbed cell cycles have remained less well characterised. In the past few years a role for Chk1 in timing the onset of mitosis in the absence of exogenous perturbations via regulation of Cdc25 family phosphatases has been documented. Furthermore, a recent report shows that Chk1 is also required for the spindle checkpoint which protects against spontaneous chromosome mis-segregation during mitotic cell division. Specifically, Chk1 is required for proper regulation of the mitotic Aurora-B kinase which ensures that anaphase proceeds only once all kinetochores have achieved bipolar attachment to microtubules and are under tension.

DNA mismatch repair and Chk1-dependent centrosome amplification in response to DNA alkylation damage

Centrosome amplification is frequently observed in tumor cells exposed to genotoxic stress, however the underlying mechanisms and biological consequences are poorly understood. Here, we show that the anti-metabolite and alkylating agent 6-thioguanine (6-TG) induces centrosome amplification resulting in the formation of multi-polar spindles when damaged cells subsequently enter mitosis. These aberrant, multi-polar mitoses are frequently resolved by asymmetric cell divisions causing unequal segregation of genetic material and cell death in one or both daughter products. We show that this phenomenon is associated with transient cell cycle delay in S- and G(2)-phase and is dependent on DNA mismatch repair (DNA MMR) proficiency and Chk1 protein kinase activity. Although Chk1-deficient cells do not exhibit cell cycle delay, centrosome amplification, or multi-polar spindle formation, continued cell cycle progression in the presence of 6-TG eventually results in increased levels of mitotic catastrophe, most probably due to mitosis with incompletely replicated DNA. Taken together, these results reveal novel mechanisms of cell killing by 6-TG and underscore the importance of interactions between cell cycle checkpoints and DNA MMR in determining the fate of cells bearing DNA damage.

Chk1 requirement for high global rates of replication fork progression during normal vertebrate S phase

Chk1 protein kinase maintains replication fork stability in metazoan cells in response to DNA damage and DNA replication inhibitors. Here, we have employed DNA fiber labeling to quantify, for the first time, the extent to which Chk1 maintains global replication fork rates during normal vertebrate S phase. We report that replication fork rates in Chk1(-/-) chicken DT40 cells are on average half of those observed with wild-type cells. Similar results were observed if Chk1 was inhibited or depleted in wild-type DT40 cells or HeLa cells by incubation with Chk1 inhibitor or small interfering RNA. In addition, reduced rates of fork extension were observed with permeabilized Chk1(-/-) cells in vitro. The requirement for Chk1 for high fork rates during normal S phase was not to suppress promiscuous homologous recombination at replication forks, because inhibition of Chk1 similarly slowed fork progression in XRCC3(-/-) DT40 cells. Rather, we observed an increased number of replication fibers in Chk1(-/-) cells in which the nascent strand is single-stranded, supporting the idea that slow global fork rates in unperturbed Chk1(-/-) cells are associated with the accumulation of aberrant replication fork structures.

Chk1-dependent slowing of S-phase progression protects DT40 B-lymphoma cells against killing by the nucleoside analogue 5-fluorouracil

Chk1 plays a crucial role in the DNA damage and replication checkpoints in vertebrates and may therefore be an important determinant of tumour cell responses to genotoxic anticancer drugs. To evaluate this concept we compared the effects of the nucleoside analogue 5-fluorouracil (5FU) on cell cycle progression and clonogenic survival in DT40 B-lymphoma cells with an isogenic mutant derivative in which Chk1 function was ablated by gene targeting. We show that 5FU activates Chk1 in wild-type DT40 cells and that 5FU-treated cells accumulate in the S phase of the cell cycle due to slowing of the overall rate of DNA replication. In marked contrast, Chk1-deficient DT40 cells fail to slow DNA replication upon initial exposure to 5FU, despite equivalent inhibition of the target enzyme thymidylate synthase, and instead accumulate progressively in the G1 phase of the following cell cycle. This G1 accumulation cannot be reversed rapidly by exogenous thymidine or removal of 5FU, and is associated with increased incorporation of 5FU into genomic DNA and severely diminished clonogenic survival. Taken together, these results demonstrate that a Chk1-dependent replication checkpoint which slows S phase progression can protect tumour cells against the cytotoxic effects of 5FU.

Viral mutations enhance the Max binding properties of the vMyc b-HLH-LZ domain

Interaction with Max via the helix-loop-helix/leucine zipper (HLH-LZ) domain is essential for Myc to function as a transcription factor. Myc is commonly upregulated in tumours, however, its activity can also be potentiated by virally derived mutations. vMyc, derived from the virus, MC29 gag-Myc, differs from its cellular counterpart by five amino acids. The N-terminal mutation stabilizes the protein, however, the significance of the other mutations is not known. We now show that vMyc can sustain longer deletions in the LZ domain than cMyc before complete loss in transforming activity, implicating the viral mutations in contributing to Myc:Max complex formation. We confirmed this both in vitro and in vivo, with loss of Max binding correlating with a loss in the biological activity of Myc. A specific viral mutation, isoleucine383>leucine (I383>L) in helix 2 of the HLH domain, extends the LZ domain from four to five heptad repeats. Significantly, introduction of I383>L into a Myc mutant that is defective for Max binding substantially restored its ability to complex with Max in vitro and in vivo. We therefore propose that this virally derived mutation is functional by significantly contributing to establishing a more hydrophobic interface between the LZs of Myc and Max.

Sab (SH3BP5), a novel mitochondria-localized JNK-interacting protein

The JNK (c-Jun N-terminal kinase) pathway is activated by diverse stresses and can have an effect on a number of different cellular processes. Protein-protein interactions are critical for efficient signalling from JNK to multiple targets; through a screen for interacting proteins, we identified a novel JNK-interacting protein, Sab (SH3BP5). Sab has previously been found to interact with the Src homology 3 domain of Bruton's tyrosine kinase; however, the interaction with JNK occurs through a mitogen-activated protein KIM (kinase interaction motif) in a region distinct from the Bruton's tyrosine kinase-binding domain. As with c-Jun, the presence of this KIM is essential for Sab to act as a JNK substrate. Interestingly, Sab is associated with the mitochondria and co-localizes with a portion of active JNK after stress treatment. The present study and previously reported work may suggest a possible role for Sab in targeting JNK to this subcellular compartment and/or mediating crosstalk between different signal-transduction pathways.

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.

c-Jun-deficient cells undergo premature senescence as a result of spontaneous DNA damage accumulation

Mouse embryo fibroblasts deficient for the c-Jun proto-oncogene (c-Jun-/- MEF) undergo p53-dependent premature senescence in conventional culture. This phenotype becomes evident only after several cell divisions, suggesting that senescence may result from exposure to unknown environmental factors. Here, we show that c-Jun-/- MEF can proliferate successfully in low oxygen (3% O2), indicating that premature senescence under conventional culture conditions is a consequence of hyperoxic stress. c-Jun-/- MEF exhibit higher basal levels of DNA damage compared to normal fibroblasts in high but not low oxygen, implying that senescence results from chronic accumulation of spontaneous DNA damage. This accumulation may be attributable, at least in part, to inefficient repair, since DNA damage induced by gamma ionizing radiation and H2O2 persists for longer in c-Jun-/- MEF than in wild-type MEF. Unexpectedly, p53 expression, phosphorylation, and transcriptional activity are largely unaffected by oxygen exposure, indicating that the accumulation of spontaneous DNA damage does not result in chronic activation of p53 as judged by conventional criteria. Finally, we find that c-Jun associates with nuclear foci containing gammaH2AX and ATM following irradiation, suggesting a potential role for c-Jun in DNA repair processes per se.

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.

Invasion of normal human fibroblasts induced by v-Fos is independent of proliferation, immortalization, and the tumor suppressors p16INK4a and p53

Invasion is generally perceived to be a late event during the progression of human cancer, but to date there are no consistent reports of alterations specifically associated with malignant conversion. We provide evidence that the v-Fos oncogene induces changes in gene expression that render noninvasive normal human diploid fibroblasts highly invasive, without inducing changes in growth factor requirements or anchorage dependence for proliferation. Furthermore, v-Fos-stimulated invasion is independent of the pRb/p16(INK4a) and p53 tumor suppressor pathways and telomerase. We have performed microarray analysis using Affymetrix GeneChips, and the gene expression profile of v-Fos transformed cells supports its role in the regulation of invasion, independent from proliferation. We also demonstrate that invasion, but not proliferation, is dependent on the activity of the up-regulated epidermal growth factor receptor. Taken together, these results indicate that AP-1-directed invasion could precede deregulated proliferation during tumorigenesis and that sustained activation of AP-1 could be the epigenetic event required for conversion of a benign tumor into a malignant one, thereby explaining why many malignant human tumors present without an obvious premalignant hyperproliferative dysplastic lesion.

The Secret Life of Histones

Recent evidence reveals an unexpected role for the linker histone H1.2 in DNA damage-induced apoptosis. DNA double strand breaks induce translocation of nuclear H1.2 to the cytoplasm, where it promotes release of cytochrome c from mitochondria by activating the Bcl-2 family protein, Bak.

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.

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.

Interactions of the DNA mismatch repair proteins MLH1 and MSH2 with c-MYC and MAX

MSH2 and MLH1 have a central role in correcting mismatches in DNA occurring during DNA replication and have been implicated in the engagement of apoptosis induced by a number of cytotoxic anticancer agents. The function of MLH1 is not clearly defined, although it is required for mismatch repair (MMR) and engagement of apoptosis after certain types of DNA damage. In order to identify other partners of MLH1 that may be involved in signalling MMR or apoptosis, we used human MLH1 in yeast two-hybrid screens of normal human breast and ovarian cDNA libraries. As well as known partners of MLH1 such as PMS1, MLH3 and MBD4, we identified the carboxy terminus of the human c-MYC proto-oncogene as an interacting sequence. We demonstrate, both in vitro by yeast two-hybrid and GST-fusion pull-down experiments, as well as in vivo by coimmunoprecipitation from human tumour cell extracts, that MLH1 interacts with the c-MYC protein. We further demonstrate that the heterodimeric partner of c-MYC, MAX, interacts with a different MMR protein, MSH2, both in vitro and in vivo. Using an inducible c-MYC-ER fusion gene, we show that elevated c-MYC expression leads to an increased HGPRT mutation rate of Rat1 cells and an increase in the number of frameshift mutants at the HGPRT locus. The effect on HGPRT mutation rate is small (2-3-fold), but is consistent with deregulated c-MYC expression partially inhibiting MMR activity.

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.

Molecular mechanism and biological functions of c-Jun N-terminal kinase signalling via the c-Jun transcription factor

The regulation of c-Jun transcriptional activity by Jun N-terminal kinase (JNK) has become a paradigm for understanding how mitogen-activated protein (MAP) kinase signalling pathways elicit specific changes in gene transcription through selective phosphorylation of nuclear transcription factors. Selective phosphorylation of c-Jun by JNK is determined by a specific docking motif in c-Jun, the delta region, which enables JNK to associate physically with c-Jun. Analogous MAP kinase docking motifs have subsequently been found in several other transcription factors, indicating that this is a general mechanism for ensuring specificity of signal transduction. Genetic and biochemical studies in mice, flies and cultured cells have provided evidence that signals relayed by JNK through c-Jun regulate a range of cellular processes including cell proliferation, tumourigenesis, apoptosis and embryonic development. Despite these advances, in most cases, the genes or programs of gene expression downstream of JNK and c-Jun, which control these processes, have not been defined. Here, we review the current understanding of the molecular basis and biological consequences of JNK signalling via c-Jun and highlight some of the mechanistic issues, which remain to be resolved.