MEK1 and MEK2, Different Regulators of the G1/S Transition

Abstract

The ERK cascade is activated by hormones, cytokines, and growth factors that result in either proliferation or growth arrest depending on the duration and intensity of the ERK activation. Here we provide evidence that the MEK1/ERK module preferentially provides proliferative signals, whereas the MEK2/ERK module induces growth arrest at the G 1 /S boundary. Depletion of either MEK subtype by RNA interference generated a unique phenotype. The MEK1 knock down led to p21 cip1 induction and to the appearance of cells with a senescencelike phenotype. Permanent ablation of MEK1 resulted in reduced colony formation potential, indicating the importance of MEK1 for long term proliferation and survival. MEK2 deficiency, in contrast, was accompanied by a massive induction of cyclin D expression and, thus, CDK4/6 activation followed by nucleophosmin hyperphosphorylation and centrosome over-amplification. Our results suggest that the two MEK subtypes have distinct ways to contribute to a regulated ERK activity and cell cycle progression.The extracellular-regulated kinase (ERK) 1 cascade is one of three major mitogen-activated protein kinase cascades. It is predominantly involved in the control of cell proliferation, migration, cell division, and differentiation (1-3). This pathway, consisting of Raf, MEK1/2, and ERK1/2, regulates cell proliferation via its impact on cell cycle control (4). A number of observations demonstrate the importance of this cascade in the regulation of cell cycle progression. Activated variants of Raf and MEK are capable of promoting cell cycle reentry and even transformation (5-7), whereas inhibition of the ERK pathway by different means results in G 1 arrest in a variety of cell types (5,8). Activation of the ERK cascade has also been shown under certain conditions to induce cell cycle arrest (4) and even differentiation (9). Activated ERK1/2 translocates to the nucleus (10) and phosphorylates transcription factors like Ets and AP-1 family members, thereby positively regulating their activity (11). The magnitude and duration of ERK activity are, therefore, the crucial parameters determining the quality of gene expression (4).Activation of this mitogen-activated protein kinase pathway promotes the expression of cyclin D1 (12, 13) and the CDK2 inhibitory protein p21 cip1 (14). These key events result in the activation of the cyclin-dependent protein kinases CDK4/6, which promote cell cycle entry by phosphorylating the retinoblastoma tumor suppressor (Rb), leading to the release of the transcription factor E2F (15). This in turn promotes the transcription of cyclins A and E, resulting in the activation of CDK2. Activated CDK2 phosphorylates Rb at additional sites and, thus, enables DNA synthesis and centrosome duplication via further liberation of E2F (16). Furthermore, sustained expression of p21 cip1 has been shown to be responsible for the ERK-mediated proliferation inhibition (6).MEK1 and MEK2 show high homology only in their kinase domains. The N termini and the characteristic proline-rich inserts...

Mitogen-activated Protein Kinase Kinase 1-dependent Golgi Unlinking Occurs in G2Phase and Promotes the G2/M Cell Cycle Transition

“…Cells treated with U0126 progressed out of M phase within 1-2 h, whereas MEK1-depleted cells did not. Knockdown of MEK2 did not delay mitotic entry, which is consistent with evidence for nonoverlapping MEK1 and MEK2 roles (Ussar and Voss, 2004).…”

Endothelial Nitric-oxide Synthase Activation Generates an Inducible Nitric-oxide Synthase-like Output of Nitric Oxide in Inflamed Endothelium

“…RNA Interference-Small interfering RNA (siRNA) from Sigma was synthesized using previously published and validated sequences for eNOS (30), MEK1 (31), MEK2 (31), and JNK1/2 (32). Sequences (sense) are as follows: eNOS, 5Ј-GGU-CUGCACAGGAAAUGUU(dT)(dT)-3Ј; MEK1, 5Ј-AAGUC-CUGAAGAAAGCUGGAA(dT)(dT)-3Ј; MEK2, 5Ј-AAGGU-CGGCGAACUCAAAGAC(dT)(dT)-3Ј; and JNK1/2, 5Ј-TGA-AAGAATGTCCTACCTT(dT)(dT)-3Ј.…”

Le rôle des kinasesMek1etMek2dans la formation de la barrière hématoplacentaire chez la souris

“…La voie ERK/MAPK est impliquée dans la détermination cellulaire chez plusieurs espèces. Chez les mammifères, cette cascade est impliquée dans la prolifération, la différenciation, la migration, la survie et la mort cellulaires [3,4]. La voie d'activation ERK/MAPK débute par la liaison de facteurs de croissance aux récepteurs membranaires à activité tyrosine kinase, tels les récepteurs du FGF (fibroblast growth factor), de l'EGF (epidermal growth factor) et du PDGF (platelet derived growth factor).…”