PP2A-Mediated Regulation of Ras Signaling in G2 Is Essential for Stable Quiescence and Normal G1 Length

Abstract

SUMMARY Quiescence (G0) allows cycling cells to reversibly cease proliferation. A decision to enter quiescence is suspected of occurring early in G1, before the restriction point, R. Surprisingly, we have identified G2 as an interval during which inhibition of the protein phosphatase, PP2A, results in failure to exhibit stable quiescence. This effect is accompanied by shortening of the ensuing G1. The PP2A subcomplex required for stable G0 contains the B56γ B subunit. Following PP2A inhibition in G2, aberrant overexpression of cyclin E occurs during mitosis and is responsible for overriding quiescence. Strikingly, suppression of Ras signaling re-establishes normal cyclin E levels during M and restores G0. These data point to PP2A-B56γ-driven Ras signaling-modulation in G2 as essential for suppressing aberrant cyclin E expression during mitosis and, thereby, achieving normal G0 control. Thus, G2 is an interval during which the length and growth factor dependence of the next G1 interval are established.

The same, only different – DNA damage checkpoints and their reversal throughout the cell cycle

“…Although cells normally retain some cyclin-E-Cdk2 activity as they complete mitosis and swiftly progress through G1 without the requirement for mitogenic stimulation, the induction of p21 in late G2 or mitosis gives rise to quiescent daughters with hypophosphorylated retinoblastoma protein (pRb) (Spencer et al, 2013). Similarly, PP2A-B56c-dependent dephosphorylation of pRb in G2 results in quiescent daughter cells that lack residual Cdk2 activity (Naetar et al, 2014). Inhibition of PP2A-B56c in G2 results in increased levels of cyclin E1 in mitotic cells and accelerates Cdk2-dependent S-phase entry in daughter cells.…”

LB100, a small molecule inhibitor of PP2A with potent chemo- and radio-sensitizing potential

“…24 Furthermore, PP2A silencing has been shown to upregulate downstream Cdk1 transcriptional targets and promote mitosis. 27,28 As such, the Greatwall-PP2A network has been proposed as a key signaling axis that promotes normal Cdk1-driven entry through mitosis. 29 PP2A also acts on other mitotic mediators, including the key mitosis-specific kinase, Polo-like kinase 1 (PLK1), which localizes to centrosomes during mitosis and when inactivated by PP2A is an important hallmark of G2/M arrest and activation of the DNA damage response.…”

“…50,51 Essential to cell senescence is proper functioning of PP2A during the G2 phase, during which Ras signaling is modulated to achieve stable quiescence. 28,52 Inhibition of PP2A leads to hyperactivation of Ras signaling and stabilization of c-Myc during G2, which drives cells into mitosis through accumulation of cyclin E: Cdk1 complexes. 28,53 The efficacy of PP2A inhibition in overcoming cellular senescence has been corroborated in myelodysplastic syndromes and colon cancer, among others.…”

“…28,52 Inhibition of PP2A leads to hyperactivation of Ras signaling and stabilization of c-Myc during G2, which drives cells into mitosis through accumulation of cyclin E: Cdk1 complexes. 28,53 The efficacy of PP2A inhibition in overcoming cellular senescence has been corroborated in myelodysplastic syndromes and colon cancer, among others. 12,54,55 Pharmacological inhibition of PP2A has been primarily studied using naturally produced, albeit toxic compounds.…”

Protein phosphatase 2A promotes the transition to G0 during terminal differentiation in Drosophila

“…Another group recently found a second mechanism for PP2A to promote quiescence, whereby PP2A/B56 inhibits Ras signaling during G2 phase, which limits subsequent Myc expression and reduces CycE expression in the following G1 (Naetar et al, 2014). This promotes quiescence by limiting CycE, which would otherwise disrupt the association of RB family members with E2F/DP complexes by phosphorylation.…”