The Control Operated by the Cell Cycle Machinery on MEF2 Stability Contributes to the Downregulation of CDKN1A and Entry into S Phase

Abstract

MEF2s are pleiotropic transcription factors (TFs) which supervise multiple cellular activities. During the cell cycle, MEF2s are activated at the G₀/G₁ transition to orchestrate the expression of the immediate early genes in response to growth factor stimulation. Here we show that, in human and murine fibroblasts, MEF2 activities are downregulated during late G₁. MEF2C and MEF2D interact with the E3 ligase F-box protein SKP2, which mediates their subsequent degradation through the ubiquitin-proteasome system. The cyclin-dependent kinase 4 (CDK4)/cyclin D1 complex phosphorylates MEF2D on serine residues 98 and 110, and phosphorylation of these residues is an important determinant for SKP2 binding. Unscheduled MEF2 transcription during the cell cycle reduces cell proliferation, whereas its containment sustains DNA replication. The CDK inhibitor p21/CDKN1A gene is a MEF2 target gene required to exert this antiproliferative influence. MEF2C and MEF2D bind a region within the first intron of CDKN1A, presenting epigenetic markers of open chromatin. Importantly, H3K27 acetylation within this regulative region depends on the presence of MEF2D. We propose that following the initial engagement in the G₀/G₁ transition, MEF2C and MEF2D must be polyubiquitylated and degraded during G₁ progression to diminish the transcription of the CDKN1A gene, thus favoring entry into S phase.

Supplemental material for this article may be found at http://dx.doi.org/10.1128/MCB.01461-14.

ACKNOWLEDGMENTS

This work was supported by AIRC (IG-10437) and MIUR Progetto PRIN (Progetto 2010W4J4RM_002) grants to C.B.

We thank Michele Pagano (New York University) for the SKP2DN plasmid, Mauro Giacca (ICGEB, Trieste, Italy) for the Rb plasmid, Eric Olson (Dallas, TX) for the plasmid encoding MEF2-ENG, and Kevin Ryan (Glasgow, United Kingdom) and Peiqing Sun (La Jolla, CA) for E1A plasmids.