p38α and p38γ Mediate Oncogenic ras-induced Senescence through Differential Mechanisms

Abstract

Oncogene-induced senescence is a tumor-suppressive defense mechanism triggered upon activation of certain oncogenes in normal cells. Recently, the senescence response to oncogene activation has been shown to act as a bona fide barrier to cancer development in vivo. Multiple previous studies have implicated the importance of the p38 MAPK pathway in oncogene-induced senescence. However, the contribution of each of the four p38 isoforms (encoded by different genes) to senescence induction is unclear. In the current study, we demonstrated that p38␣ and p38␥, but not p38␤, play an essential role in oncogenic ras-induced senescence. Both p38␣ and p38␥ are expressed in primary human fibroblasts and are activated upon transduction of oncogenic ras. Small hairpin RNA-mediated silencing of p38␣ or p38␥ expression abrogated ras-induced senescence, whereas constitutive activation of p38␣ and p38␥ caused premature senescence. Furthermore, upon activation by oncogenic ras, p38␥ stimulated the transcriptional activity of p53 by phosphorylating p53 at Ser 33 , suggesting that the ability of p38␥ to mediate senescence is at least partly achieved through p53. However, p38␣ contributed to ras-inducted senescence via a p53-indepdendent mechanism in cells by mediating ras-induced expression of p16 INK4A , another key senescence effector. These findings have identified p38␣ and p38␥ as essential components of the signaling pathway that regulates the tumor-suppressing senescence response, providing insights into the molecular mechanisms underlying the differential involvement of the p38 isoforms in senescence induction.The ras proto-oncogenes encode small GTP-binding proteins that transduce growth signals from cell surface (1-3). Aberrant activation of ras is a crucial step in tumor formation. Constitutive activation of ras genes, either through point mutations or overexpression, is associated with a wide variety of human tumors at high frequency and contributes to the initiation and maintenance of multiple tumorigenic phenotypes in these cancers (4 -11). However, in early-passage primary human and rodent cells, activated ras causes a permanent proliferative arrest known as premature senescence, because of its phenotypic similarities to replicative senescence observed in late-passage cells (12). Other oncogenes, such as E2F1 and raf, or inactivation of certain tumor suppressor genes, also induce senescence in normal human cells (13-15). The existence of the premature senescence response to oncogene activation implies that like apoptosis, oncogene-induced senescence serves as an anti-tumorigenic defense mechanism. Indeed, it has been well documented that cellular transformation by ras requires cooperation from immortalizing oncogenes that overcome the senescence response, such as those inactivating p53 (8,16,17). Recent studies have also demonstrated that senescent cells can be detected in early-stage premalignant lesions of lung, pancreas, skin, and prostate in both human cancer patients and mouse tumor models and that disruption of s...

Human regulatory T cells induce T-lymphocyte senescence

“…Recent studies have demonstrated that MAPK signaling pathways play a major role in regulating cell-cycle reentry and oncogenic ras-induced senescence. [39][40][41] The results of the present study show clearly that human Treg cells modulated the specific MAPK p38 and ERK1/2 signaling pathways selectively in responder T cells. In addition, our in vitro and in vivo studies suggest that specific MAPK signaling pathway inhibition in responder T cells can prevent the induction of T-cell senescence mediated by Treg cells.…”

“…39,40 It has been reported that ERK1/2 and p38 activation can induce p21-dependent G 1 cell-cycle arrest. 41 Therefore, in the present study, we investigated whether human Treg-induced naive T-cell cycle G 0 /G 1 arrest and conversion of naive CD4 ϩ T cells into senescent T cells involved MAPK signaling modulation.…”

Inhibition of p38 Mitogen-Activated Protein Kinase Promotes Ex Vivo Hematopoietic Stem Cell Expansion

“…This hypothesis is supported by the finding that the progeny of LKS + cells cultured with SB exhibited significantly less SA-bgal staining and expression of p16 mRNA than those without SB. SA-b-gal is a widely used biomarker of senescent cells [34] and increased expression of p16 has been implicated in the induction of cellular senescence down-stream of p38 activation [26,33]. In addition, a significant fewer apoptotic cells were detected in LKS + cells cultured with SB than the cells without SB, indicating that p38 inhibition can also inhibit HSC apoptosis.…”

“…The mechanisms by which p38 inhibition promotes HSC self-renewal and ex vivo expansion may be attributable primarily to the inhibition of cellular senescence, because it has been shown that p38 functions as a key molecule mediating diverse stimuli-induced cellular senescence via upregulation of p16 in a variety of cells, including HSCs [14,15,32,33]. This hypothesis is supported by the finding that the progeny of LKS + cells cultured with SB exhibited significantly less SA-bgal staining and expression of p16 mRNA than those without SB.…”

p38β Mitogen-Activated Protein Kinase Modulates Its Own Basal Activity by Autophosphorylation of the Activating Residue Thr180 and the Inhibitory Residues Thr241 and Ser261

“…These variants had acquired increased autophosphorylation activity (Fig. 6B) (13) sufficient to elevate their catalytic activities (15,18,30,31). We thus coincubated the Histagged WT or active variants of each p38 isoform with increasing amounts of their GST-tagged KD counterpart (Fig.…”