MEK1/2 Overactivation Can Promote Growth Arrest by Mediating ERK1/2-Dependent Phosphorylation of p70S6K

Abstract

The extracellular signal-regulated kinase (ERK)1/2 mitogen-activated protein (MAP) kinase pathway has been involved in the positive and negative regulation of cell proliferation. Upon mitogen stimulation, ERK1/ERK2 activation is necessary for G1- to S-phase progression whereas when hyperactived, this pathway could elicit cell cycle arrest. The mechanisms involved are not fully elucidated but a kinase-independent function of ERK1/2 has been evidenced in the MAPK-induced growth arrest. Here, we show that p70S6K, a central regulator of protein biosynthesis, is essential for the cell cycle arrest induced by overactivation of ERK1/2. Indeed, whereas MEK1 silencing inhibits cell cycle progression, we demonstrate that active mutant form of MEK1 or MEK2 triggers a G1 phase arrest by stimulating an activation of p70S6K by ERK1/2 kinases. Silencing of ERK1/2 activity by shRNA efficiently suppresses p70S6K phosphorylation on Thr421/Ser424 and S6 phosphorylation on Ser240/244 as well as p21 expression, but these effects can be partially reversed by the expression of kinase-dead mutant form of ERK1 or ERK2. In addition, we demonstrate that the kinase p70S6K modulates neither the p21 gene transcription nor the stability of the protein but enhances the translation of the p21 mRNA. In conclusion, our data emphasizes the importance of the translational regulation of p21 by the MEK1/2-ERK1/2-p70S6K pathway to negatively control the cell cycle progression.

ERK1/2 can feedback-regulate cellular MEK1/2 levels

“…MEK1 and MEK2 are >86% identical at the amino acid level and evaluation of their constitutively active mutants has revealed functional redundancy in a large extent of cell physiology [15, 34–36], although MEK1 and MEK2 can exhibit distinct physical interactions with certain proteins, e.g., A-Raf [37] and the scaffold MP1 [38]. In the context of p21 CIP1 regulation, we and others previously showed that overexpression of constitutively active MEK1 or MEK2 mutants was equally effective for inducing p21 CIP1 expression in LNCaP and other cell lines [15, 36]. Therefore, it is hardly conceivable that the differential effects of MEK1 and MEK2 knockdown on p21 CIP1 mRNA levels represent distinct intrinsic properties of MEK1 and MEK2.…”

Growth arrest signaling of the Raf/MEK/ERK pathway in cancer

“…It has long been known that expression of constitutively active Ras, Raf, or MEK mutants could induce irreversible growth arrest in different malignant tumor cells which were not transformed by Ras or Raf. These cell lines were derived from diverse tumor types, including small cell lung carcinoma (Mabry et al, 1989; Ravi et al, 1998, 1999a), medullary thyroid carcinoma (MTC) (Nakagawa et al, 1987; Carson et al, 1995; Carson-Walter et al, 1998; Park et al, 2003; Vaccaro et al, 2006), glioma (Fanton et al, 2001), pheochromocytoma (Wood et al, 1993; Park et al, 2005b), gastrointestinal carcinoid (Sippel et al, 2003), prostate carcinoma (Ravi et al, 1999b; Hong et al, 2011), hepatocarcinoma (Guégan et al, 2013b) and breast carcinoma (Taylor et al, 2011). In some of these tumor cell lines, Raf/MEK/ERK activation could lead to expression of senescence-associated β-galactosidase (Ravi et al, 1999b; Arthan et al, 2010; Taylor et al, 2011), a key marker used for senescence determination (Gupta and Wajapeyee, 2013).…”

“…Gene deletion in mice also revealed a critical difference in their requirement at an early developmental stage (Giroux et al, 1999; Bélanger et al, 2003; Nadeau et al, 2009). Further, their unique functions in epidermal neoplasia or hepatocarcinoma growth have been reported (Scholl et al, 2009; Guégan et al, 2013b), although activation of MEK1 or MEK2 was equally sufficient to transform intestinal epithelial cells and to induce the formation of metastatic tumors (Voisin et al, 2008). It is noteworthy that determination of functional specificity of MEK1 and MEK2 can be affected by their intrinsic properties as well as gene dosage effects.…”

“…However, Raf-induced cell cycle arrest was abrogated by the MEK1/2 specific inhibitors or RNA interference, indicating that MEK1/2 activation is necessary for Raf-induced growth arrest (Carson-Walter et al, 1998; Ravi et al, 1998; Zhu et al, 1998; Park et al, 2003; Hong et al, 2009). In addition, ectopic expression of constitutively active MEK1 or MEK2 mutant was equally sufficient to induce growth arrest in different cell types (Hong et al, 2009; Guégan et al, 2013b), suggesting that MEK1 and MEK2 have redundant roles in the context of growth arrest signaling.…”

“…Kinase-independent effects of ERK1/2 were also determined in the context of Raf/MEK-induced growth arrest signaling (Hong et al, 2009; Guégan et al, 2013b). Briefly, decreases in ERK1/2 activity can arrest proliferation of many cell types, as determined by expression of kinase-deficient ERK mutants (Pagés et al, 1993; Kortenjann et al, 1994) or gene knockdown (Vantaggiato et al, 2006; Bessard et al, 2008; Lefloch et al, 2008).…”