Protein phosphatases-1 and -2A regulate tumor cell migration, invasion and cytoskeletal organization

Sds22/PP1 links epithelial integrity and tumor suppression via regulation of myosin II and JNK signaling

Loss of epithelial integrity often correlates with the progression of malignant tumors. Sds22, a regulatory subunit of protein phosphatase 1 (PP1), has recently been linked to regulation of epithelial polarity in Drosophila. However, its role in tumorigenesis remains obscure. In this study, using Drosophila imaginal tissue as an in vivo model system, we show that sds22 is a new potential tumor suppressor gene in Drosophila. Without sds22, cells lose epithelial architecture, and become invasive and tumorigenic when combined with Ras overexpression; conversely, sds22 overexpression can largely suppress tumorigenic growth of Ras(V12)scrib(-/-) mutant cells. Mechanistically, we show that sds22 prevents cell invasion and metastasis by inhibiting myosin II and Jun N-terminal kinase (JNK) activity downstream of PP1. Loss of this inhibition causes cells to lose epithelial organization and promotes cell invasion. Finally, human Sds22 is focally deleted and downregulated in multiple carcinomas, and this downregulation correlates with tumor progression, suggesting that sds22 inactivation may contribute to tumorigenesis and metastatic potential in human cancers via a similar mechanism.

PP2A activation by beta2-adrenergic receptor agonists: novel regulatory mechanism of keratinocyte migration

Understanding the mechanisms that regulate cell migration is important for devising novel therapies to control metastasis or enhance wound healing. Previously, we demonstrated that beta2-adrenergic receptor (beta2-AR) activation in keratinocytes inhibited their migration by decreasing the phosphorylation of a critical promigratory signaling component, the extracellular signal-related kinase (ERK). Here we demonstrate that beta2-AR-induced inhibition of migration is mediated by the activation of the serine/threonine phosphatase PP2A. Pretreating human keratinocytes with the PP2A inhibitor, okadaic acid, prevented the beta2-AR-induced inhibition of migration, either as isolated cells or as a confluent sheet of cells repairing an in vitro "wound" and also prevented the beta2-AR-induced reduction in ERK phosphorylation. Similar results were obtained with human corneal epithelial cells. In keratinocytes, immunoprecipitation studies revealed that beta2-AR activation resulted in the rapid association of beta2-AR with PP2A as well as a 37% increase in association of PP2A with ERK2. Finally, beta2-AR activation resulted in a rapid and transient 2-fold increase in PP2A activity. Thus, we provide the first evidence that beta2-AR activation in keratinocytes modulates migration via a novel pathway utilizing PP2A to alter the promigratory signaling cascade. Exploiting this pathway may result in novel therapeutic approaches for control of epithelial cell migration.

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

Compared to metastatic Lewis lung carcinoma (LLC) cells, nonmetastatic LLC cells have increased levels of activity of the protein phosphatase PP-2A, which functions to limit their migration through transwell chambers. Inhibition of PP-2A in nonmetastatic LLC stimulates their transmigration to levels similar to those of metastatic LLC cells. Studies to define the signaling pathways intermediate between diminished PP-2A activity and stimulated migration showed that inhibiting PP-2A activity resulted in paxillin serine hyperphosphorylation and tyrosine dephosphorylation. Paxillin was important for the stimulated migration because the increased transmigration in response to PP-2A inhibition was dampened by expression of mutant paxillin at the LIM3 S457 and S481 residues. Inhibition of PP-2A also led to the dissolution of FAK/Src/paxillin focal adhesion complexes, which was also dependent on paxillin S457 and S481 residues. In addition, inhibition of PP-2A resulted in dephosphorylation of Src inhibitory Y527 residue, suggesting increased Src activity. The stimulated transmigration of cells with diminished PP-2A was in part dependent on this Src activity. These studies show the importance of PP-2A in limiting tumor cell migration through its modulation of proteins of the focal adhesions.

Cyclic AMP-dependent protein kinase A plays a role in the directed migration of human keratinocytes in a DC electric field

Skin wound healing requires epithelial cell migration for re-epithelialization, wound closure, and re-establishment of normal function. We believe that one of the earliest signals to initiate wound healing is the lateral electric field generated by the wound current. Normal human epidermal keratinocytes migrate towards the negative pole, representing the center of the wound, in direct currents of a physiological strength, 100 mV/mm. Virtually nothing is known about the signal transduction mechanisms used by these cells to sense the endogenous electric field. To elucidate possible protein kinase (PK) involvement in the process, PK inhibitors were utilized. Two important findings have been described. Firstly, addition of 50 nM KT5720, an inhibitor of PKA, resulted in a 53% percent reduction in the directional response of keratinocytes in the electric field, while not significantly affecting general cell motility. The reduction was dose-dependent, there was a gradual decrease in the directional response from 5 to 50 nM. Secondly, addition of 1 microM ML-7, a myosin light chain kinase inhibitor, resulted in an approximate 31% decrease in the distance the cells migrated without affecting directional migration. The PKC inhibitors GF109203X at 4 microM and H-7 at 20 microM and W-7, a CaM kinase inhibitor, did not significantly alter either directed migration or cell migration, although they all resulted in a slight reduction in directional migration. D-erythro-sphingosine at 15 microM, a PKC inhibitor, had virtually no effect on either migration distance or directed migration. These findings demonstrate that divergent kinase signaling pathways regulate general cell motility and sustained directional migration and highlight the complexity of the signal transduction mechanisms involved. The inhibitor studies described in this paper implicate a role for PKA in the regulation of the directional migratory response to applied electric fields, galvanotaxis.

Differences in association of the serine/threonine protein phosphatase PP-2A with microtubules of metastatic and nonmetastatic tumor cells

Motility and adhesiveness are regulated by a multitude of factors, including cytoskeletal polymerization and phosphorylation of cytoskeletal and associated proteins. The metastatic Lewis lung carcinoma variant, LLC-LN7, was highly motile in vitro and had lower levels of the serine/threonine protein phosphatase PP-2A than did the nonmetastatic variant, LLC-C8. Reducing PP-2A activity of the nonmetastatic cells pharmacologically or with catalytic (Calpha) subunit antisense increased their in vitro motility. Nonmetastatic LLC-C8 cells had a greater proportion of polymerized tubulin which co-purified with PP-2A as compared to the metastatic LLC-LN7 cells. The PP-2A that was associated with the microtubules of these cells showed similar ratios of the Aalpha structural subunit to the Calpha/beta catalytic subunits. In contrast, the proportion of the regulatory subunit B56alpha was lower in the nonmetastatic LLC-C8 cells as compared to the metastatic LLC-LN7 cells. These studies show the role of PP-2A in restricting the motility of nonmetastatic tumor cells and suggest that the loss of this regulatory control in metastatic LLC-LN7 cells may be due to both a reduction in microtubule-associated PP-2A and a difference in the composition of the subunits of PP-2A that is associated with the microtubules.

Protein phosphatases 1 and 2A maintain endothelial cells in a resting state, limiting the motility that is needed for the morphogenic process of angiogenesis

Angiogenesis that is induced by cancers, including those of the head and neck, requires endothelial cells to shift from a nonmotile resting state to an increased level of motility. Using a human microvascular endothelial cell line, this study shows the importance of the serine/threonine protein phosphatases 1 (PP1) and 2A (PP2A) in restricting endothelial cell motility. Treatment of endothelial cells with increasing concentrations of the PP1 and PP2A inhibitor okadaic acid resulted in cell rounding and increased motility, which was accompanied by cytoskeletal disorganization involving a loss of filamentous beta-tubulin and F-actin. These effects occurred at okadaic acid levels that selectively inhibit PP2A and became more prominent with higher levels that inhibit both PP2A and PP1. This study shows the importance of PP1 and PP2A in maintaining cytoskeletal organization, thereby limiting endothelial cell motility, and suggests that pharmacologic approaches to enhance PP1 and PP2A activities may be useful in preventing key events of the angiogenic process.