Ignacio Palmero

p19ARF links the tumour suppressor p53 to Ras

Normal healthy cells possess safeguard mechanisms that sense oncogenic signals and trigger anti-tumorigenic responses that limit the proliferative potential of cells harbouring active oncogenes. In particular, expression of the Ras oncogene in normal primary cells causes a cell-cycle arrest that involves the activation of the tumoursuppressor protein p53 (ref. 2). It has recently been reported that the tumour-suppressor p19ARF can activate p53 (refs 3–5). Here we show that p19ARF in the mouse (the human homologue is called p14ARF) is essential for the activation of p53 in response to oncogenic Ras. These results, together with the finding that p19ARF does not mediate the activation of p53 by DNA damage, dissociate the activation of p53 into two pathways: one pathway is induced by DNA damage and is independent of p19ARF, whereas the other pathway is induced by oncogenic Ras and is dependent on p19ARF.

Accumulation of p16INK4a in mouse fibroblasts as a function of replicative senescence and not of retinoblastoma gene status.

Viral transformation of mouse and human fibroblasts has very different effects on the composition of cyclin-dependent kinase (Cdk) complexes. In human cells transformed by the large T-antigen of simian virus 40 (SV40 T-Ag) and human tumour cell lines that lack a functional retinoblastoma gene product (pRb) no cyclin D1-Cdk4 complexes can be detected because all the available Cdk4 is associated with the Cdk-inhibitor p16INK4a. In contrast, SV40-transformed mouse cells and fibroblasts from Rb1-nullizygous mouse embryos contain normal levels of cyclin D1-Cdk4 complexes. To investigate this species difference, we have compared the biochemical properties and expression of mouse p16INK4a with that of its human counterpart. There is a marked increase in p16 RNA and protein levels as primary embryo fibroblasts approach their finite lifespan in culture, but mouse p16 expression does not appear to be influenced by the status of pRb. Transformed or spontaneously immortalized mouse cells therefore do not achieve the very high levels of p16 characteristic of pRb-negative human cell lines. We suggest that these differences may be related to the different frequencies with which mouse and human cells can be immortalized in culture.

Induction of senescence by oncogenic ras

Publisher Summary This chapter discusses the induction of senescence by oncogenic Ras. Oncogenic Ras is able to transform efficiently most immortal rodent cell lines. In contrast, transformation of primary cells by Ras requires the cooperation of an immortalizing event. Transformation of primary cells through the combined action of immortalizing alterations and oncogenic Ras is the basis for several models of multistep carcinogenesis both in culture cells and in animal models. Normal, nonimmortal cells are therefore refractile to transformation by Ras, presumably because of the existence of built-in protection mechanisms against unlimited proliferation. A detailed study of the effects of Ras in primary cells has not been achieved until recently, mainly because of the lack of efficient gene transfer techniques. Improvements in retrovirus-based gene transfer methods have made it possible to analyze the effect of the sustained expression of oncogenic Ras in primary cells. Interestingly, it was observed that after an initial mitogenic response, prolonged action of Ras provokes a permanent cell cycle arrest.

Growth inhibition by the tumor suppressor p33ING1 in immortalized and primary cells: involvement of two silencing domains and effect of Ras.

ABSTRACT ING1 was identified as an inhibitor of growth and has been described as a tumor suppressor. Furthermore, the expression of ING1 is induced in senescent cells and antisense ING1 extends the proliferative life span of primary human fibroblasts. Cooperation of p33ING1 with p53 has been suggested to be an important function of ING1 in cell cycle control. Intriguingly, it has been shown that p33ING1 is associated with histone acetylation as well as with histone deacetylation function. Here we show that p33ING1 is a potent transcriptional silencer in various cell types. However, the silencing function is independent of the presence of p53. By use of deletion mutants two potent autonomous and transferable silencing domains were identified, but no evidence of an activation domain was found. The amino (N)-terminal silencing domain is sensitive to the histone deacetylase inhibitor trichostatin A (TSA) whereas the carboxy-terminal silencing function is resistant to TSA, suggesting that p33ING1 confers gene silencing through both HDAC-dependent and -independent mechanisms. Interestingly, the presence of oncogenic Ras, which is able to induce premature senescence, increases the p33ING1-mediated silencing function. Moreover, ING1-mediated silencing was reduced by coexpressing dominant-negative Ras or by treatment with the mitogen-activated protein kinase inhibitor PD98059 but not by treatment with SB203580, an inhibitor of the p38 pathway. In addition, we show that both silencing domains of ING1 are involved in cell cycle control, as measured by inhibition of colony formation of immortalized cells and by thymidine incorporation of primary human diploid fibroblasts (HDF). Interestingly, p33ING1 expression induces features of cellular senescence in HDFs.

A functional link between the tumour suppressors ARF and p33ING1.

The ARF tumour suppressor protein plays a critical role in the activation of p53 in response to oncogenic stress. ARF can activate p53 through nucleolar sequestration of Mdm2. However, several lines of evidence indicate that this is not the only way of action of ARF, and alternative mechanisms must exist. p33ING1 is a putative tumour suppresor, which induces cell-cycle arrest and apoptosis in a p53-dependent manner. Here, we describe that ARF and p33ING1 can interact in vivo. We also show that the subcellular localization of ING1 can be modulated by ARF protein levels, causing a displacement from nuclear to nucleolar localization. Finally, the ability of p33ING1 to cause cell-cycle arrest and induction of p21CIP1, or Mdm2, is impaired in ARF-deficient primary mouse fibroblasts. Based on these observations, we propose that the interaction with p33ING1 represents a novel mechanism for the tumour suppression function of ARF.

Activation of ARF by oncogenic stress in mouse fibroblasts is independent of E2F1 and E2F2

The ARF tumour suppressor protein (p14ARF in human and p19ARF in mouse) is a major mediator of the activation of p53 in response to oncogenic stress. Little is known about the signalling pathways connecting oncogenic stimuli to the activation of ARF. Regulation of ARF occurs primarily at the transcriptional level and several modulators of ARF transcription have been identified. Notably, ectopic expression of E2F1 upregulates ARF transcriptionally, and both E2F1 and ARF have been implicated in apoptosis and cell-cycle arrest. We have used primary mouse fibroblasts deficient for E2F1, E2F2, or both to determine the possible role of these E2F proteins as upstream regulators of ARF in response to oncogenic stimuli and other stresses. In particular, we have studied the effects of oncogenic Ras and the viral oncoprotein E1A on ARF levels, neoplastic transformation, and sensitization to apoptosis. We have also examined the behaviour of the E2F-deficient MEFs with respect to immortalization and sensitivity to DNA damage. None of the ARF-mediated responses that we have analysed is significantly affected in E2F1−/−, E2F2−/− or E2F1/2−/− MEFs, and ARF is upregulated normally in all cases. Taken together, our results indicate that the activation of ARF in response to oncogenic stress can occur by E2F1- and E2F2-independent mechanisms. This challenges previous suggestions implicating E2F factors as key mediators in the activation of ARF by oncogenic stress.

The Dimeric Structure and the Bivalent Recognition of H3K4me3 by the Tumor Suppressor ING4 Suggests a Mechanism for Enhanced Targeting of the HBO1 Complex to Chromatin

The INhibitor of Growth (ING) family of tumor suppressors regulates the transcriptional state of chromatin by recruiting remodeling complexes to sites with histone H3 trimethylated at position K4 (H3K4me3). This modification is recognized by the plant homeodomain (PHD) present at the C-terminus in the five members of the ING family. ING4 facilitates histone H3 acetylation by the HBO1 complex. Here, we show that ING4 forms homodimers through its N-terminal domain, which folds independently into an elongated coiled-coil structure. The central region of ING4, which contains the nuclear localization sequence, is disordered and flexible and does not directly interact with p53, or does it with very low affinity, in contrast to previous findings. The NMR analysis of the full-length protein reveals that the two PHD fingers of the dimer are chemically equivalent and independent of the rest of the molecule. The detailed NMR analysis of the full-length dimeric protein binding to histone H3K4me3 shows essentially the same binding site and affinity as the isolated PHD finger. Therefore, the ING4 dimer has two identical and independent binding sites for H3K4me3 tails, which, in the context of the chromatin, could belong to the same or to different nucleosomes. These results show that ING4 is a bivalent reader of the chromatin H3K4me3 modification and suggest a mechanism for enhanced targeting of the HBO1 complex to specific chromatin sites. This mechanism could be common to other ING-containing remodeling complexes.

The tumor suppressor ING1 contributes to epigenetic control of cellular senescence

Cellular senescence is an effective tumor‐suppressive mechanism that causes a stable proliferative arrest in cells with potentially oncogenic alterations. Here, we have investigated the role of the p33ING1 tumor suppressor in the regulation of cellular senescence in human primary fibroblasts. We show that p33ING1 triggers a senescent phenotype in a p53‐dependent fashion. Also, endogenous p33ING1 protein accumulates in chromatin in oncogene‐senescent fibroblasts and its silencing by RNA interference impairs senescence triggered by oncogenes. Notably, the ability to induce senescence is lost in a mutant version of p33ING1 present in human tumors. Using specific point mutants, we further show that recognition of the chromatin mark H3K4me3 is essential for induction of senescence by p33ING1. Finally, we demonstrate that ING1‐induced senescence is associated to a specific genetic signature with a strong representation of chemokine and cytokine signaling factors, which significantly overlaps with that of oncogene‐induced senescence. In summary, our results identify ING1 as a critical epigenetic regulator of cellular senescence in human fibroblasts and highlight its role in control of gene expression in the context of this tumor‐protective response.

Regulation of the MicroRNA Processor DGCR8 by the Tumor Suppressor ING1

The ING family of tumor suppressor proteins controls several cellular functions relevant to antitumor protection, such as cell cycle control, apoptosis, senescence, or migration. ING proteins are functionally linked to the p53 pathway, and they participate in transcriptional control via the recognition of histone marks and recruitment of protein complexes with chromatin-modifying activity to specific promoters. Here, we have investigated the global effect of ING1 in gene regulation through genome-wide analysis of expression profiles in primary embryonic fibroblasts deficient for the Ing1 locus. We find that Ing1 has a predominant role as transcriptional repressor in this setting, affecting the expression of genes involved in a variety of cellular functions. Within the subset of genes showing differential expression, we have identified DGCR8, a protein involved in the early steps of microRNA biogenesis. We show that ING1 binds to the DGCR8 promoter and controls its transcription through chromatin regulation. We also find that ING1 and DGCR8 can cooperate in restraining proliferation. In summary, this study reveals a novel connection between ING1 and a regulator of microRNA biogenesis and identifies new links between tumor suppressor proteins and the microRNA machinery.

Direct promoter induction of p19Arf by Pit-1 explains the dependence receptor RET/Pit-1/p53-induced apoptosis in the pituitary somatotroph cells

Somatotrophs produce growth hormone (GH) and are the most abundant secretory cells of the pituitary. Somatotrophs express the transcription factor Pit-1 and the dependence receptor RET, its co-receptor GFRa1 and ligand GDNF. Pit-1 is a transcription factor essential for somatotroph proliferation and differentiation and for GH expression. GDNF represses excess Pit-1 expression preventing excess GH. In the absence of GDNF, RET behaves as a dependence receptor, becomes intracellularly processed and induces strong Pit-1 expression leading to p53 accumulation and apoptosis. How accumulation of Pit-1 leads to p53 expression is unknown. We have unveiled the relationship of Pit-1 with the p19Arf gene. There is a parallel correlation of RET processing, Pit-1 increase and ARF protein and mRNA expression. Interfering the pathway with RET, Pit-1 or p19Arf siRNA blocked apoptosis. We have found a Pit-1 DNA-binding element within the ARF promoter. Pit-1 directly regulates the CDKN2A locus and binds to the p19Arft promoter inducing p19Arf gene expression. The Pit-1-binding element is conserved in rodents and humans. RET/Pit-1 induces p19Arf/p53 and apoptosis not only in a somatotroph cell line but also in primary cultures of pituitary somatotrophs, where ARF siRNA interference also blocks p53 and apoptosis. Analyses of the somatotrophs in whole pituitaries supported the above findings. Thus Pit-1, a differentiation factor, activates the oncogene-induced apoptosis (OIA) pathway as oncogenes exerting a tight control in somatotrophs to prevent the disease due to excess of GH (insulin-resistance, metabolic disease, acromegaly).