High Intensity ras Signaling Induces Premature Senescence by Activating p38 Pathway in Primary Human Fibroblasts

Abstract

Although oncogenic ras plays a pivotal role in neoplastic transformation, it triggers an anti-oncogenic defense mechanism known as premature senescence in normal cells. In this study, we investigated the induction of cellular responses by different expression levels of oncogenic ras in primary human fibroblasts. We found that a moderate, severalfold increase in ras expression promoted cell growth. Further elevation of ras expression initially enhanced proliferation but eventually induced p16INK4A expression and senescence. The induction of these opposing cellular responses by ras signals of different intensity was achieved through differential activation of the MAPK pathways that mediated these responses. Whereas moderate ras activities only stimulated the mitogenic MEK-ERK pathway, high intensity ras signals induced MEK and ERK to higher levels, leading to stimulation of the MKK3/6-p38 pathway, which had been shown previously to act downstream of Ras-MEK to trigger the senescence response. Thus, these studies have revealed a mechanism for the differential effects of ras on cell proliferation. Furthermore, moderate ras activity mediated transformation in cooperation with E6E7 and hTERT, suggesting that a moderate intensity ras signal can provide sufficient oncogenic activities for tumorigenesis. This result also implies that the ability of ras to promote proliferation and oncogenic transformation can be uncoupled with that to induce senescence in cell culture and that the development of tumors with relatively low ras activities may not need to acquire genetic alterations that bypass premature senescence.Neoplastic transformation is caused by multiple and successive genetic alterations (1), among which the oncogenic activation of ras genes seems to be one of the critical steps. The ras proto-oncogenes encode a family of small GTP-binding proteins that transduce mitogenic signals from the cell surface. Amplification of the ras genes and constitutively active ras mutant alleles has been frequently found in a wide variety of tumors (2), suggesting that the growth signals elicited by ras are crucial for tumorigenesis. Whereas wild-type ras transduces mitogenic signals in response to extracellular stimuli, activated ras provides cells with constitutive growth signals. The key role of activated ras in promoting oncogenic transformation has been established by numerous studies. In both in vitro cell culture systems and animal models, activated ras genes can cooperate with other oncogenic genetic alterations to lead to transformation (3-8). In addition, ras genes have been found to be activated in a variety of carcinogen-induced tumors in animals, suggesting that ras genes are targets of carcinogens and that they are likely to participate in the initiation of neoplastic transformation (9). The transforming activity of activated ras depends on its interaction with multiple downstream effectors, which mediate different aspects of oncogenic transformation (10, 11). One of the best characterized ras downstream effectors is ....

Busulfan-induced senescence is dependent on ROS production upstream of the MAPK pathway

“…The lack of involvement of p53 in BU-induced senescence prompted us to hypothesize that activation of the Erk-p38 MAPK pathway may mediate the induction, because activation of the Erk-p38 MAPK pathway has also been implicated in the induction of cellular senescence [13,15,34,35]. Indeed, it was found that treatment of WI38 cells with BU activated Erk and p38 in a time-dependent manner and inhibition of Erk or p38 with a specific inhibitor significantly suppressed BU-induced WI38 cell senescence.…”

“…To elucidate the mechanisms of action whereby BU induces senescence, we first evaluated activation of the two key pathways that have been implicated in senescence induction: (1) the p53-p21 pathway triggered by DNA damage and (2) the p16-Rb pathway activated by the MAPK cascades [1,3,15,27]. The activation of these pathways was determined by analysis of phosphorylation of p53, Erk, p38, and JNK and expression of p21 and p16 using Western blot.…”

“…Activation of the Erk-p38 MAPK pathway has been implicated in oncogenic stress-induced senescence as seen in cells ectopically transfected with Ras or Raf oncogene [13,14]. Activation of p38 also contributes to the induction of senescence in response to DNA damage and telomere shortening [15].…”

Transcriptional factor HBP1 targets P16INK4A, upregulating its expression and consequently is involved in Ras-induced premature senescence

“…p16 INK4A promoter activity is enhanced by Ras and MKK6E It has previously been shown that p38 MAPK activation is necessary for Ras-induced senescence and that p38 MAPK activation is sufficient to induce senescence (Haq et al, 2002;Iwasa et al, 2003;Deng et al, 2004;Nelyudova et al, 2007;Kim et al, 2008). Our previous data revealed that HBP1 is a downstream effector of Ras, and p38 MAPK .…”

Phenotype-assisted transcriptome analysis identifies FOXM1 downstream from Ras–MKK3–p38 to regulate in vitro cellular invasion

“…It confers cellular migration and invasion, but at the same time apoptosis (Haq et al, 2002;Deng et al, 2004;Behren et al, 2005;Qi et al, 2006;Sun et al, 2007). In this study we attempted to identify downstream regulators of this pathway that specifically convey signals culminating in an in vitro invasive phenotype.…”