Activation of p107 by Fibroblast Growth Factor, Which Is Essential for Chondrocyte Cell Cycle Exit, Is Mediated by the Protein Phosphatase 2A/B55 alpha Holoenzyme

作者：Kurimchak Alison; Haines Dale S; Garriga Judit; Wu Shufang; De Luca Francesco; Sweredoski Michael J; Deshaies Raymond J; Hess Sonja; Grana Xavier^*

来源：Molecular and Cellular Biology, 2013, 33(16): 3330-3342.

DOI：10.1128/MCB.00082-13

The phosphorylation state of pocket proteins during the cell cycle is determined at least in part by an equilibrium between inducible cyclin-dependent kinases (CDKs) and serine/threonine protein phosphatase 2A (PP2A). Two trimeric holoenzymes consisting of the core PP2A catalytic/scaffold dimer and either the B55 alpha or PR70 regulatory subunit have been implicated in the activation of p107/p130 and pRB, respectively. While the phosphorylation state of p107 is very sensitive to forced changes of B55 alpha levels in human cell lines, regulation of p107 in response to physiological modulation of PP2A/B55 alpha has not been elucidated. Here we show that fibroblast growth factor 1 (FGF1), which induces maturation and cell cycle exit in chondrocytes, triggers rapid accumulation of p107-PP2A/B55 alpha complexes coinciding with p107 dephosphorylation. Reciprocal solution-based mass spectrometric analysis identified the PP2A/B55 alpha complex as a major component in p107 complexes, which also contain E2F/DPs, DREAM subunits, and/or cyclin/CDK complexes. Of note, p107 is one of the preferred partners of B55 alpha, which also associates with pRB in RCS cells. FGF1-induced dephosphorylation of p107 results in its rapid accumulation in the nucleus and formation of larger complexes containing p107 and enhances its interaction with E2F4 and other p107 partners. Consistent with a key role of B55 alpha in the rapid activation of p107 in chondrocytes, limited ectopic expression of B55 alpha results in marked dephosphorylation of p107 while B55 alpha knockdown results in hyperphosphorylation. More importantly, knockdown of B55 alpha dramatically delays FGF1-induced dephosphorylation of p107 and slows down cell cycle exit. Moreover, dephosphorylation of p107 in response to FGF1 treatment results in early recruitment of p107 to the MYC promoter, an FGF1/E2F-regulated gene. Our results suggest a model in which FGF1 mediates rapid dephosphorylation and activation of p107 independently of the CDK activities that maintain p130 and pRB hyperphosphorylation for several hours after p107 dephosphorylation in maturing chondrocytes.

• 出版日期2013-8