Distinct Protein Phosphatase 2A Heterotrimers Modulate Growth Factor Signaling to Extracellular Signal-regulated Kinases and Akt

Abstract

At least 99% of protein phosphorylation in eukaryotic cells occurs on Ser and Thr residues. The greater than 300 protein Ser/Thr kinases in the human genome are opposed by less than 30 protein Ser/Thr phosphatases, of which protein phosphatase 1 and protein phosphatase 2A (PP2A) 2 contribute the bulk of activity in most cell types. The notion of Ser/Thr phosphatases as promiscuous and constitutively active enzymes that simply provide the substrates for regulated kinase signaling has been challenged by the discovery of batteries of catalytic subunit-interacting proteins, which impart substrate specificity, subcellular localization, and responsiveness to phosphorylation (1, 2). In the case of PP2A, the predominant holoenzyme is formed by association of a core dimer of catalytic and scaffold subunits with one of a least 12 regulatory subunits in vertebrates. Since vertebrate A and C subunits are each encoded by two genes and since many regulatory subunits are diversified by alternative splicing, several dozen different PP2A heterotrimers are likely to exist in any given cell type. The three unrelated regulatory subunit gene families (B, BЈ, BЉ) are almost certain to utilize different mechanisms to control enzymatic activity and cellular localization of PP2A. The B-family (PR55) of regulatory subunits consists of predicted ␤-propellers with divergent N-terminal tails that act as subcellular targeting signals (3-5). In addition to interacting with phosphatase substrates (6 -12), BЈ-family (also referred to as B56, PR61) subunits are heavily phosphorylated, which may confer regulation by second messengers (13-16). Members of the BЉ-family of PP2A subunits (PR72/ 130, PR59, PR48) feature two calcium binding EF hands with presumed structural rather than regulatory functions (17). Striatin and SG2NA have been referred to as Bٞ regulatory subunits (18). It may be more appropriate to refer to them as PP2A dimer-associated proteins since their stability does not depend on association with the PP2A core enzyme (17, 19) and since they lack the A subunit binding consensus motif common to B, BЈ, and BЉ subunits (20).A growing body of evidence indicates that PP2A has complex inhibitory and stimulatory effects on hormone and growth factor signaling, in particular the extracellular-signal regulated (ERK)/mitogen-activated protein kinase (MAPK) cascade (21). PP2A substrates include G-protein-coupled receptors and receptor tyrosine kinases (22-24), receptorassociated proteins (25-28), and all three kinases of the ERK/MAPK cascade core module , MAPK/ERK kinase (MEK) (33)(34)(35) and ERK (36,37)).Here, we have begun to dissect the contribution of different PP2A holoenzymes to growth factor signaling in PC12 cells. The net effect of total PP2A silencing or inhibition was ERK and Akt hyperphosphorylation, most likely as a consequence of direct dephosphorylation of these kinases by PP2A. Cautioning against exclusive reliance on silencing approaches, protracted inhibition of PP2A by RNA interference (RNAi) resulted in a compensatory unco...

S6K1 Phosphorylates and Regulates Fragile X Mental Retardation Protein (FMRP) with the Neuronal Protein Synthesis-dependent Mammalian Target of Rapamycin (mTOR) Signaling Cascade

“…These data are consistent with previous studies of mitogenic signaling where PP2A was found to inhibit S6K1 and mTOR was found to regulate both PP2A and S6K1 (33,34). Our data extend this interplay to neurons, further indicating that mGluR acts synergistically on a pre-existing inhibition of S6K1 by PP2A.…”

“…To further position S6K1 phosphorylation of FMRP within the known mGluR-triggered pathway (13,23,24), we sought a link between S6K1 and PP2A, the FMRP phosphatase (22), since kinase-phosphatase modules regulate synaptic activity (41,42) and PP2A inhibits S6K1 in mitogenic signaling (33,34). We found that FMRP-IPs, at 1 or 5 min following DHPG stimulation, revealed an inverse correlation between the association of PP2A and S6K1 with FMRP in agreement with the pattern of FMRP phosphorylation (Fig.…”

Negative regulation of Pim-1 protein kinase levels by the B56β subunit of PP2A

“…Figure 8b demonstrates (Qian et al, 2005b). It is interesting that Pim-1 has been shown to transform cells in concert with both c-Myc (Verbeek et al, 1991;Shirogane et al, 1999;Ellwood-Yen et al, 2003) or Akt (Amaravadi and Thompson, 2005), and that the levels of both of these proteins are regulated by PP2A (Sears, 2004;Ugi et al, 2004;Van Kanegan et al, 2005). Using a model in which hematopoietic cells are grown in an IL-3-dependent fashion, we find that knocking down a PP2A B subunit increases viability only in Pim containing, but not in Pim knockout cells.…”

Calpain small-1 modulates Akt/FoxO3A signaling and apoptosis through PP2A

“…Every family accounts for a number of isoforms, in particular the B56 family involves the alpha, beta, gamma, delta and epsilon members, which share high homology. Interestingly, B56 has been implicated in the modulation of Akt phosphorylation (Van Kanegan et al, 2005;Yin et al, 2006;Rocher et al, 2007).…”