Src-dependent phosphorylation of ROCK participates in regulation of focal adhesion dynamics

Protein tyrosine phosphatase SHP2 suppresses podosome rosette formation in Src-transformed fibroblasts

Podosomes are actin-enriched membrane protrusions that play important roles in extracellular matrix degradation and invasive cell motility. Podosomes undergo self-assembly into large rosette-like structures in Src-transformed fibroblasts, osteoclasts and certain highly invasive cancer cells. Several protein tyrosine kinases have been shown to be important for the formation of podosome rosettes, but little is known regarding the role of protein tyrosine phosphatases in this process. We found that knockdown of the Src homolog domain-containing phosphatase 2 (SHP2) significantly increased podosome rosette formation in Src-transformed fibroblasts. By contrast, SHP2 overexpression suppressed podosome rosette formation in these cells. The phosphatase activity of SHP2 was essential for the suppression of podosome rosette formation. SHP2 selectively suppressed the tyrosine phosphorylation of Tks5, a scaffolding protein required for podosome formation. The inhibitory effect of SHP2 on podosome rosette formation was associated with the increased activation of Rho-associated kinase (ROCK) and the enhanced polymerization of vimentin filaments. A higher content of polymerized vimentin filaments was correlated with a lower content of podosome rosettes. Taken together, our findings indicate that SHP2 serves as a negative regulator of podosome rosette formation through the dephosphorylation of Tks5 and the activation of ROCK-mediated polymerization of vimentin in Src-transformed fibroblasts.

Rho-associated kinases in tumorigenesis: re-considering ROCK inhibition for cancer therapy

The Rho-associated (ROCK) serine/threonine kinases have emerged as central regulators of the actomyosin cytoskeleton, their main purpose being to promote contractile force generation. Aided by the discovery of effective inhibitors such as Y27632, their roles in cancer have been extensively explored with particular attention focused on motility, invasion and metastasis. Recent studies have revealed a surprisingly diverse range of functions of ROCK. These insights could change the way ROCK inhibitors might be used in cancer therapy to include the targeting of stromal rather than tumour cells, the concomitant blocking of ROCK and proteasome activity in K-Ras-driven lung cancers and the combination of ROCK with tyrosine kinase inhibitors for treating haematological malignancies such as chronic myeloid leukaemia. Despite initial optimism for therapeutic efficacy of ROCK inhibition for cancer treatment, no compounds have progressed into standard therapy so far. However, by carefully defining the key cancer types and expanding the appreciation of ROCK's role in cancer beyond being a cell-autonomous promoter of tumour cell invasion and metastasis, the early promise of ROCK inhibitors for cancer therapy might still be realized.

ROCKII Ser1366 phosphorylation reflects the activation status

ROCK (Rho-associated protein kinase), a downstream effector of RhoA, plays an important role in many cellular processes. Accumulating evidence has shown the involvement of ROCK activation in the pathogenesis of many diseases. However, a reagent capable of detecting ROCK activation directly is lacking. In the present study, we show autophosphorylation of ROCKII in an in vitro kinase reaction. The phosphorylation sites were identified by MS, and the major phosphorylation site was found to be at the highly conserved residue Ser1366. A phospho-specific antibody was generated that can specifically recognize ROCKII Ser1366 phosphorylation. We found that the extent of Ser1366 phosphorylation of endogenous ROCKII is correlated with that of myosin light chain phosphorylation in cells in response to RhoA stimulation, showing that Ser1366 phosphorylation reflects its kinase activity. In addition, ROCKII Ser1366 phosphorylation could be detected in human breast tumours by immunohistochemical staining. The present study provides a new approach for revealing the ROCKII activation status by probing ROCKII Ser1366 phosphorylation directly in cells or tissues.

The metastasis gene NEDD9 product acts through integrin β3 and Src to promote mesenchymal motility and inhibit amoeboid motility

Neural precursor expressed, developmentally down-regulated 9 (NEDD9), a member of the Cas family of signal transduction molecules, is amplified at the genetic level in melanoma, and elevated expression levels have been shown to correlate with melanoma progression and metastasis. NEDD9 interacts with the guanine nucleotide exchange factor DOCK3 to promote Rac activation and the elongated, mesenchymal-type of tumour cell invasion, but the molecular mechanisms through which NEDD9 promotes melanoma metastasis are not fully understood. We show that signalling through increased NEDD9 levels requires integrin β3 signalling, which leads to elevated phosphorylation of integrin β3. This results in increased Src and FAK but decreased ROCK signalling to drive elongated, mesenchymal-type invasion in environments that contain vitronectin. NEDD9 overexpression does not affect ROCK signalling through activation of RhoA but decreases ROCKII signalling through Src-dependent phosphorylation of a negative regulatory site Tyr722. In NEDD9-overexpressing melanoma cells, inhibition of Src with dasatinib results in a switch from Rac-driven elongated, mesenchymal-type invasion to ROCK-dependent rounded, amoeboid invasion. These findings brings into question whether dasatinib would work as a therapeutic agent to block melanoma invasion and metastasis. On the basis of the in vitro data presented here, a combination treatment of dasatinib and a ROCK inhibitor might be a better alternative in order to inhibit both elongated, mesenchymal-type and rounded, amoeboid motility.

Lysophosphatidic acid induces a migratory phenotype through a crosstalk between RhoA-Rock and Src-FAK signalling in colon cancer cells

Lysophosphatidic acid (LPA) acts as a potent stimulator of tumorigenesis. Cell-cell adhesion disassembly, actin cytoskeletal alterations, and increased migratory potential are initial steps of colorectal cancer progression. However, the role that LPA plays in these events in this cancer type is still unknown. We explored this question by using Caco-2 cells, as colon cancer model, and treatment with LPA or pretreatment with different cell signalling inhibitors. Changes in the location of adherent junction proteins were examined by immunofluorescence and immunoblotting. The actin cytoskeleton organisation and focal adhesion were analysed by confocal microscopy. Rho-GTPase activation was analysed by the pull-down assay, FAK and Src activation by immunoblotting, and cell migration by the wound healing technique. We show that LPA induced adherent junction disassembly, perijunctional actin cytoskeletal reorganisation, and increased cell migration. These events were dependent on Src, Rho and Rock because their chemical inhibitors PP2, toxin A and Y27632, respectively, abrogated the effects of LPA. Moreover, we showed that Src acts upstream of RhoA in this signalling cascade and that LPA induces focal adhesion formation and FAK redistribution and activation in confluent monolayers. Focal adhesion formation was also observed in the front of migrating cells in response to LPA, and Rock inhibitor abolished this effect. In conclusion, our findings show that LPA modulates adherent junction disassembly, actin cytoskeletal disorganisation, and focal adhesion formation, conferring a migratory phenotype in colon tumour cells. We suggest a functional regulatory cascade that integrates RhoA-Rock and Src-FAK signalling to control these events during colorectal cancer progression.

Integrins and cadherins join forces to form adhesive networks

Cell-cell and cell-extracellular-matrix (cell-ECM) adhesions have much in common, including shared cytoskeletal linkages, signaling molecules and adaptor proteins that serve to regulate multiple cellular functions. The term 'adhesive crosstalk' is widely used to indicate the presumed functional communication between distinct adhesive specializations in the cell. However, this distinction is largely a simplification on the basis of the non-overlapping subcellular distribution of molecules that are involved in adhesion and adhesion-dependent signaling at points of cell-cell and cell-substrate contact. The purpose of this Commentary is to highlight data that demonstrate the coordination and interdependence of cadherin and integrin adhesions. We describe the convergence of adhesive inputs on cell signaling pathways and cytoskeletal assemblies involved in regulating cell polarity, migration, proliferation and survival, differentiation and morphogenesis. Cell-cell and cell-ECM adhesions represent highly integrated networks of protein interactions that are crucial for tissue homeostasis and the responses of individual cells to their adhesive environments. We argue that the machinery of adhesion in multicellular tissues comprises an interdependent network of cell-cell and cell-ECM interactions and signaling responses, and not merely crosstalk between spatially and functionally distinct adhesive specializations within cells.

Kinase activity profiling of gram-negative pneumonia

Pneumonia is a severe disease with high morbidity and mortality. A major causative pathogen is the Gram-negative bacterium Klebsiella (K.) pneumoniae. Kinases play an integral role in the transduction of intracellular signaling cascades and regulate a diverse array of biological processes essential to immune cells. The current study explored signal transduction events during murine Gram-negative pneumonia using a systems biology approach. Kinase activity arrays enable the analysis of 1,024 consensus sequences of protein kinase substrates. Using a kinase activity array on whole lung lysates, cellular kinase activities were determined in a mouse model of K. pneumoniae pneumonia. Notable kinase activities also were validated with phospho-specific Western blots. On the basis of the profiling data, mitogen-activated protein kinase (MAPK) signaling via p42 mitogen-activated protein kinase (p42) and p38 mitogen-activated protein kinase (p38) and transforming growth factor β (TGFβ) activity were reduced during infection, whereas v-src sarcoma (Schmidt-Ruppin A-2) viral oncogene homolog (avian) (SRC) activity generally was enhanced. AKT signaling was represented in both metabolic and inflammatory (mitogen-activated protein kinase kinase 2 [MKK], apoptosis signal-regulating kinase/mitogen-activated protein kinase kinase kinase 5 [ASK] and v-raf murine sarcoma viral oncogene homolog B1 [b-RAF]) context. This study reaffirms the importance of classic inflammation pathways, such as MAPK and TGFβ signaling and reveals less known involvement of glycogen synthase kinase 3β (GSK-3β), AKT and SRC signaling cassettes in pneumonia.