Depletion of ERK2 but not ERK1 abrogates oncogenic Ras-induced senescence

Abstract

In response to oncogenic activation, cells initially undergo proliferation followed by an irreversible growth arrest called oncogene-induced senescence (OIS), an endogenous defense mechanism against tumorigenesis. Oncogenic activation of ERK1/2 is essential for both the initial phase of cellular proliferation as well as subsequent premature senescence, but little is known about the specific contribution of ERK1 versus 2 to OIS. Here we show that depletion of ERK2 but not ERK1 by shRNA knockdown in MEFs leads to continuous proliferation bypassing senescence even in the presence of oncogenic HRASV12. Upon depletion of ERK2, induction of both p19Arf and p16Ink4a was significantly compromised after oncogenic HRASV12 expression, attenuating activation of the key tumor suppressors p53 and pRb. Here we demonstrate that ERK2 but not ERK1 indirectly regulates p19Arf and p16Ink4a both at the transcriptional and translational level. Oncogenic Ras expression after ERK2 knockdown downregulates Fra-1 and c-Jun, components of the activator protein-1 (AP-1) heterodimer essential for transactivation of p19Arf. Similarly we show a significant decrease in the activation of p38 MAPK and ETS family members which are involved in the induction of p16Ink4a. The role of ERK2 in translational regulation is observed by the lack of tuberin (TSC2) and p70 ribosomal S6 kinase 1 (p70S6K1) phosphorylation, components of the mTOR pathway, which enhances p19Arf mRNA translation during oncogenic Ras-induced senescence. These observations suggest that ERK2 but not ERK1 contributes to upregulation of p19Arf and p16Ink4a in a transcription- and translation-dependent manner during oncogenic Ras-induced senescence. Taken together, our data indicate that ERK2 is the key ERK isoform mediating the senescence signaling pathway downstream of oncogenic Ras.