Protein phosphatases limit tumor motility

Abstract

Elevators of cAMP, such as prostaglandin E2 (PGE2), activate protein kinase A (PKA) and induce PKA-stimulated motility and metastasis by metastatic Lewis lung carcinoma cells (LLC-LN7). Non-metastatic LLC (LLC-C8) are unresponsive to cAMP elevation even though they are not deficient in the PKA enzymes. To determine whether this PKA unresponsiveness might be due to increased dephosphorylation by serine/threonine protein phosphatases (PP-1/2A) within non-metastatic LLC-C8, the effects of the PP-1/2A inhibitor okadaic acid on the migration and invasion by non-metastatic LLC-C8 cells was measured. Okadaic acid stimulated motility of non-metastatic LLC-C8 cells to a level that was comparable to that of metastatic LLC-LN7 cells. PGE2 further increased the motility of the non-metastatic LLC-C8 cells when okadaic acid was present, although not in the absence of okadaic acid. The stimulation of motility by okadaic acid was diminished when PKA activity was inhibited. Dose-response studies with concentrations of okadaic acid that selectively inhibited PP-2A or both PP-2A and PP-1 showed a progressive increase in migration of non-metastatic LLC-C8 cells, suggesting that both PP-1 and PP-2A limit their motility. By contrast, metastatic LLC-LN7 cells were more motile than were non-metastatic LLC-C8 cells, but this motility was only marginally affected by okadaic acid. Comparisons of the levels of PP-1/2A enzyme activities in the LLC variants showed more activity in non-metastatic LLC-C8 than in metastatic LLC-LN7 cells. The identity of the PP whose activity was increased in the non-metastatic LLC-C8 was assessed by using okadaic acid, which selectively inhibits PP-2A activity at low concentrations and PP-1 and PP-2A at high concentrations, and calyculin A, which inhibits PP-2A at a similar concentration to that affected by okadaic acid but is more potent at inhibiting PP-1. The inhibition of PP activities by okadaic acid and by calyculin A showed a pattern which suggested the presence both of PP-1 and of PP-2A in non-metastatic LLC-C8 cells, but the presence of PP-1 and a reduction in PP-2A in metastatic LLC-LN7 cells. The sum of these data suggests that PKA-stimulated motility is restricted both by PP-1 and by PP-2A in non-metastatic LLC, and that a deficiency in this restriction results in increased migration and invasion.

Dynamic Regulation of Extracellular Signal-Regulated Kinase (ERK) by Protein Phosphatase 2A Regulatory Subunit B56γ1 in Nuclei Induces Cell Migration

“…PP2A influences tumor progression [40]–[42]. It is also well-known to be involved in one of the most important anchorage-dependent cellular events, i.e., cell motility [43], [44], which is in turn involved in cancer invasion and metastasis.…”

Profiling the Downstream Genes of Tumor Suppressor PTEN in Lung Cancer Cells by Complementary DNA Microarray

“…PI3 kinase activity is required for the formation of lamellae, dynamic sites of motility in carcinoma cells (19). Protein phosphatase 2A is one of the enzymes involved in the maintenance of cytoskeletal polymerization and also influences tumor invasion and metastasis (32,33). Reduced phosphatase 2A enzyme activity has been demonstrated in metastatic cells as compared with nonmetastatic cells in Lewis lung carcinoma (34).…”

Protein phosphatase‐2A regulates endothelial cell motility and both the phosphorylation and the stability of focal adhesion complexes

“…2 Our studies have shown that endothelial cells exposed to tumor-derived products and more highly metastatic tumor cells have diminished serine/threonine dephosphorylating activity of PP-2A. [15][16][17] Pharmacologic inhibition of PP-2A activity stimulates the motility of endothelial cells and nonmetastatic tumor cells. 18 -20 However, the pathways downstream of the diminished PP-2A activity that lead to increased motility by either tumor cells or endothelial cells have not been defined.…”

“…In vitro studies have suggested that these pathways include p130Cas/Crk and PI3K. 27,[31][32][33][34] While serine/threonine and tyrosine phosphorylation signaling pathways have been shown to regulate the motility of normal and tumor cells, 15,16,18,27,[35][36][37][38][39][40] "crossover" from serine/threonine to tyrosine pathways has also been described. Serine/threonine phosphorylation by protein kinase A in colon cancer cells and adrenal cells reduces tyrosine phosphorylation of paxillin and causes cell rounding.…”