Escape from Cellular Quiescence

Sotillo, Elena; Graña, Xavier

doi:10.1007/978-1-4419-1770-6_1

Cited by 5 publications

(5 citation statements)

References 92 publications

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“…The 3 members of the pocket protein family—pRB, p107, and p130—are inactivated by phosphorylation in a cell cycle dependent manner. 3,5,68,69 Pocket proteins are hypophosphorylated in quiescent cells or in the G1 phase of the cell cycle preceding the restriction point. Mitogenic stimulation of D-type cyclins′ expression and downregulation and/or sequestration of p27 result in the sequential activation of D-type cyclin/CDK4/6 and cyclin E/CDK2 complexes.…”

Section: Alterations On B55α May Be Linked To Inactivation Of the Pocmentioning

confidence: 99%

“…These G1/S cyclin/CDK complexes cooperate to hyperphosphorylate and inactivate pocket proteins in mid- to late G1, resulting in disruption of E2F/pocket protein transcriptional repressor complexes and the initiation of a wave of E2F dependent gene transcription required for progression through S phase and mitosis. 69,70 Pocket proteins remain hyperphosphorylated through S, G2, and most M phase, as other cyclin/ CDK complexes are active through these phases. Coinciding with CDK inactivation in mitosis, pocket proteins are abruptly dephosphorylated and reset to their active state, as cells exit mitosis and enter the next G1 phase.…”

Section: Alterations On B55α May Be Linked To Inactivation Of the Pocmentioning

confidence: 99%

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PP2A Counterbalances Phosphorylation of pRB and Mitotic Proteins by Multiple CDKs: Potential Implications for PP2A Disruption in Cancer

Kurimchak¹,

Graña²

2012

Genes & Cancer

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Protein Phosphatase 2A (PP2A) consists of a collection of heterotrimeric serine/threonine phosphatase holoenzymes that play multiple roles in cell signaling via dephosphorylation of numerous substrates of a large family of serine/threonine kinases. PP2A substrate specificity is mediated by B regulatory subunits of four different families, which selectively recognize diverse substrates by mechanisms that are not well understood. Among the many signaling pathways with critical PP2A functions are several deregulated in cancer cells, and PP2A is a know tumor suppressor. However, the precise composition of the heterotrimeric PP2A complexes with tumor supressor activity is not well understood. This review is centered on the emerging role of the B regulatory subunit B55a and related subfamilly members in the modulation of the phosphorylation state of pocket proteins and mitotic CDK substrates, as well as the implications of PP2A function disruption in cancer in the context of these activities.

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Section: Alterations On B55α May Be Linked To Inactivation Of the Pocmentioning

confidence: 99%

Section: Alterations On B55α May Be Linked To Inactivation Of the Pocmentioning

confidence: 99%

PP2A Counterbalances Phosphorylation of pRB and Mitotic Proteins by Multiple CDKs: Potential Implications for PP2A Disruption in Cancer

Kurimchak¹,

Graña²

2012

Genes & Cancer

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“…Pocket proteins remain hyperphosphorylated through the S and G 2 phases and part of mitosis as they become targets of cyclin-CDK complexes that operate at these cell cycle stages (reviewed in Refs. [2][3][4].…”

mentioning

confidence: 99%

B55α PP2A Holoenzymes Modulate the Phosphorylation Status of the Retinoblastoma-related Protein p107 and Its Activation

Jayadeva

Kurimchak

Garriga

et al. 2010

Journal of Biological Chemistry

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. These phosphorylations are reversed by the action of two families of Ser/Thr phosphatases: PP1, which has been implicated in abrupt dephosphorylation of retinoblastoma protein (pRB) in mitosis, and PP2A, which plays a role in an equilibrium that counteracts cyclin-dependent kinase (CDK) action throughout the cell cycle. However, the identity of the trimeric PP2A holoenzyme(s) functioning in this process is unknown. Here we report the identification of a PP2A trimeric holoenzyme containing B55␣, which plays a major role in restricting the phosphorylation state of p107 and inducing its activation in human cells. Our data also suggest targeted selectivity in the interaction of pocket proteins with distinct PP2A holoenzymes, which is likely necessary for simultaneous pocket protein activation.The retinoblastoma family of growth suppressor proteins, designated pocket proteins, includes the product of the retinoblastoma susceptibility gene and the functionally and structurally related proteins p107 and p130 (reviewed in Ref. 1). Pocket proteins suppress the G 0 /G 1 cell cycle transition and passage through the restriction point by repressing E2F-dependent transcription via a direct interaction of the hypophosphorylated pocket protein with members of the E2F family of transcription factors. Mitogen-dependent activation of D-type cyclin-CDK 2 complexes cooperates with cyclin E-CDK2 complexes to hyperphosphorylate pocket proteins starting in midto late G 1 disrupting the pocket protein-E2F interaction and relieving repression of many genes whose products are required for cell cycle progression. Pocket proteins remain hyperphosphorylated through the S and G 2 phases and part of mitosis as they become targets of cyclin-CDK complexes that operate at these cell cycle stages (reviewed in Refs. 2-4).As cyclin-CDK complexes are inactivated during mitotic exit, the three pocket proteins become simultaneously hypophosphorylated. PP1 has been implicated in dephosphorylation of pRB from mitosis to a point in G 1 , where pRB becomes hyperphosphorylated by G 1 cyclin-CDKs (reviewed in Refs. 3 and 5). However, a second phosphatase plays a role in reversing the action of CDKs throughout the cell cycle and in quiescent cells (6). When the activity of cyclin-CDK complexes is pharmacologically inhibited, pocket proteins become rapidly dephosphorylated, and the severity of the hypophosphorylation depends on the subsets of CDKs that become inactivated. For instance, cycloheximide, an inhibitor of protein synthesis, causes rapid down-regulation of short lived cyclins such as D-type cyclins, resulting in potent dephosphorylation of p107, but only partial dephosphorylation of p130 and pRB, as CDK2 remains active for several hours. In contrast, treatment with the potent CDK inhibitor flavopiridol leads to rapid and complete dephosphorylation of pocket proteins as all CDKs become inactive (6).PP2A, a second Ser/Thr phosphatase, has been implicated in dephosphorylation of pocket proteins throughout the cell cycle by using selective pharmacolog...

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“…FVP inhibited expression of Cyclin D1, and completely blocked upregulation of p107, an E2F-dependent gene product. This indicates that cells are likely blocked prior to the restriction point in G1 or earlier, as passage through the restriction point is associated with activation of E2F dependent gene expression (Figure 3B ) [ 23 ]. The effects of FVP in transcription were monitored by determining the phosphorylation state of RNAPII.…”

Section: Resultsmentioning

confidence: 99%

“…Since EGR1, FOS, JUNB and GADD45B are all responsive to serum stimulation [ 21 , 28 , 31 ], we selected this system. Our data clearly shows that FVP, at concentrations that do not inhibit pocket protein phosphorylation, completely blocks DNA synthesis and the upregulation of cell cycle markers, such as the expression of the E2F-dependent product p107, suggesting that cells are arrested before the restriction point in G1 or even earlier at the G0/G1 transition [ 23 ]. This arrest is preceded by potent inhibition of total Ser-2 and Ser-5 phosphorylation on the CTD of RNAPII, which remains low throughout the time course.…”

Section: Discussionmentioning

confidence: 99%

Complex effects of flavopiridol on the expression of primary response genes

et al. 2012

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BackgroundThe Positive Transcription Elongation Factor b (P-TEFb) is a complex of Cyclin Dependent Kinase 9 (CDK9) with either cyclins T1, T2 or K. The complex phosphorylates the C-Terminal Domain of RNA polymerase II (RNAPII) and negative elongation factors, stimulating productive elongation by RNAPII, which is paused after initiation. P-TEFb is recruited downstream of the promoters of many genes, including primary response genes, upon certain stimuli. Flavopiridol (FVP) is a potent pharmacological inhibitor of CDK9 and has been used extensively in cells as a means to inhibit CDK9 activity. Inhibition of P-TEFb complexes has potential therapeutic applications.ResultsIt has been shown that Lipopolysaccharide (LPS) stimulates the recruitment of P-TEFb to Primary Response Genes (PRGs) and proposed that P-TEFb activity is required for their expression, as the CDK9 inhibitor DRB prevents localization of RNAPII in the body of these genes. We have previously determined the effects of FVP in global gene expression in a variety of cells and surprisingly observed that FVP results in potent upregulation of a number of PRGs in treatments lasting 4-24 h. Because inhibition of CDK9 activity is being evaluated in pre-clinical and clinical studies for the treatment of several pathologies, it is important to fully understand the short and long term effects of its inhibition. To this end, we determined the immediate and long-term effect of FVP in the expression of several PRGs. In exponentially growing normal human fibroblasts, the expression of several PRGs including FOS, JUNB, EGR1 and GADD45B, was rapidly and potently downregulated before they were upregulated following FVP treatment. In serum starved cells re-stimulated with serum, FVP also inhibited the expression of these genes, but subsequently, JUNB, GADD45B and EGR1 were upregulated in the presence of FVP. Chromatin Immunoprecipitation of RNAPII revealed that EGR1 and GADD45B are transcribed at the FVP-treatment time points where their corresponding mRNAs accumulate. These results suggest a possible stress response triggered by CDK9 inhibition than ensues transcription of certain PRGs.ConclusionsWe have shown that certain PRGs are transcribed in the presence of FVP in a manner that might be independent of CDK9, suggesting a possible alternative mechanism for their transcription when P-TEFb kinase activity is pharmacologically inhibited. These results also show that the sensitivity to FVP is quite variable, even among PRGs.

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Escape from Cellular Quiescence

Cited by 5 publications

References 92 publications

PP2A Counterbalances Phosphorylation of pRB and Mitotic Proteins by Multiple CDKs: Potential Implications for PP2A Disruption in Cancer

PP2A Counterbalances Phosphorylation of pRB and Mitotic Proteins by Multiple CDKs: Potential Implications for PP2A Disruption in Cancer

B55α PP2A Holoenzymes Modulate the Phosphorylation Status of the Retinoblastoma-related Protein p107 and Its Activation

Complex effects of flavopiridol on the expression of primary response genes

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