Protein phosphatase‐2A restricts migration of Lewis lung carcinoma cells by modulating the phosphorylation of focal adhesion proteins

Abstract

The process of metastasis requires migration of tumor cells from the primary tumor into the vasculature and the target tissue. 1 Studies with a variety of tumor types have shown that metastatic tumor cells are more highly motile than are nonmetastatic cells. [2][3][4][5] Cellular migration is influenced by the degree of adhesiveness to a substratum, signaling through integrins, and active turnover of focal adhesions. 6,7 There is, however, a fine balance between adherence/detachment and motility. A strong shift toward focal adhesion formation results in strong adherence with little migration, whereas excessive loss of adherence results in the inability to navigate across the substratum. Interactions with a substratum leads to the formation and stabilization of focal adhesions, where the actin skeleton converges with integrins and an interconnection of protein complexes. 6,8 Among the focal adhesion proteins that mediates communication through integrins during interaction with the substratum is the cytosolic tyrosine kinase FAK. 6,9 When FAK activity is reduced, cellular adhesiveness is diminished 10. During motility, the levels of membrane-associated FAK decline. 11 In adherent cells, FAK is complexed with Src within stable focal adhesions. 6 The tyrosine kinase Src (pp60c-Src) is a prototype of a family of tyrosine kinases that also includes Fyn, Lyn, Yes. Src Y527 residue (Y530 in humans) regulates Src activity. 7,12 When phosphorylated, Y527 inactivates Src by undergoing a conformational change and binding with the Src SH2 domain. FAK complexing with Src is important in Src activation and stabilization of focal adhesions, Src activation can also increase turnover of focal adhesions and increase motility. [13][14][15] An in vivo role of FAK or Src in cancer invasion has been suggested by their increased levels in malignancies. 16 Key in the formation of stable adhesions is association of FAK and Src with paxillin. Paxillin is a 68 kDa adaptor protein that binds a multitude of structural and signaling molecules within focal adhesions. The ability of paxillin to facilitate interaction between molecules of the focal adhesions is regulated by its level of serine and tyrosine phosphorylation. 17 Focal adhesion formation and cellular adhesion are accompanied by tyrosine phosphorylation of paxillin, 18,19 whereas the breakdown of focal adhesions and loss of adherence are accompanied by tyrosine dephosphorylation of paxillin. 20,21 Blocking tyrosine phosphorylation of paxillin results in failure to form organized stress fibers and focal adhesions, and defective cellular adherence. 22 In contrast, serine phosphorylation of paxillin has been associated with redistribution of paxillin from the tyrosine-phosphorylated focal adhesions and increased cell scattering. 23 Our studies with a murine Lewis lung carcinoma model have shown regulation of cell migration by the serine/threonine protein phosphatase PP-2A. 3,24 More highly motile and metastatic tumor cells have diminished PP-2A activity. Inhibition of PP-2A activity wit...

Protein Phosphatase 2A Negatively Regulates Integrin αIIbβ3 Signaling

“…5E). Other investigators have reported that pharmacological inhibition of PP2Ac leads to increased migration of endothelial and carcinoma cells (32,33). Thus, the enhanced ERK1/2 and p38 signaling in PP2Ac ␣ -depleted cells would be predicted to exhibit greater adhesive and migratory properties.…”

Regulation of cell adhesion by PP2A and SV40 small tumor antigen: An important link to cell transformation

“…In these cells, there is increased Ser phosphorylation of paxillin and reduced Tyr phosphorylation of paxillin, FAK and Src. PP2A holoenzymes containing the B'/B56γ1 isoform co-localize with, interact with, and dephosphorylate paxillin [14], possibly at Ser-457 and Ser-481 residues [39], the phosphorylation of which is critical for cell migration [39]. In contrast, the regulation of Tyr phosphorylation by PP2A is indirect, and may involve Tyr phosphatases like Shp-2 [40].…”

“…1). PP2A inhibition induces the disorganization of focal adhesion sites and increases cell migratory activity in endothelial cells [16], non-metastatic Lewis Lung carcinoma (LLC-C8) tumor variants [38][39][40] and keratinocytes [15]. In these cells, there is increased Ser phosphorylation of paxillin and reduced Tyr phosphorylation of paxillin, FAK and Src.…”

Paxillin: Adapting to Change

“…Regardless, mutagenesis of the LIM domains that mediate PTP-PEST binding decreases cell adhesion and motility on fibronectin, perhaps indicating a role for paxillin binding to PTP-PEST in focal adhesion dynamics (27, 41). Finally, paxillin also interacts with the serine/threonine phosphatase PP2A, and although it is not known if paxillin is a physiological protein phosphatase 2A substrate, perturbation of this association and consequent increase in paxillin LIM3 serine phosphorylation enhances cell metastasis (110,323).…”