Quantification of ruffle area and dynamics in live or fixed lung adenocarcinoma cells

Ruffles are actin-rich membrane protrusions implicated in actin reorganization and initiation of cell motility. Here, we describe methods for measuring and analyzing ruffle dynamics in live cells and average ruffle area per cell in fixed samples. The specific steps described are for the analysis of A549 lung adenocarcinoma cells, but the protocol can be applied to other cell types. The protocol has applications for dissecting the signaling events linked to ruffling. For complete details on the use and execution of this protocol, please refer to Cooke et al. (2021).

SGEF forms a complex with Scribble and Dlg1 and regulates epithelial junctions and contractility

The canonical Scribble polarity complex is implicated in regulation of epithelial junctions and apical polarity. Here, we show that SGEF, a RhoG-specific GEF, forms a ternary complex with Scribble and Dlg1, two members of the Scribble complex. SGEF targets to apical junctions in a Scribble-dependent fashion and functions in the regulation of actomyosin-based contractility and barrier function at tight junctions as well as E-cadherin-mediated formation of adherens junctions. Surprisingly, SGEF does not control the establishment of polarity. However, in 3D cysts, SGEF regulates the formation of a single open lumen. Interestingly, SGEF's nucleotide exchange activity regulates the formation and maintenance of adherens junctions, and in cysts the number of lumens formed, whereas SGEF's scaffolding activity is critical for regulation of actomyosin contractility and lumen opening. We propose that SGEF plays a key role in coordinating junctional assembly and actomyosin contractility by bringing together Scribble and Dlg1 and targeting RhoG activation to cell-cell junctions.

Regulation of circular dorsal ruffles, macropinocytosis, and cell migration by RhoG and its exchange factor, Trio

The small GTPase RhoG and its exchange factor, Trio, regulate the formation and size of circular dorsal ruffles and associated functions, including macropinocytosis and cell migration.

Circular dorsal ruffles (CDRs) are actin-rich structures that form on the dorsal surface of many mammalian cells in response to growth factor stimulation. CDRs represent a unique type of structure that forms transiently and only once upon stimulation. The formation of CDRs involves a drastic rearrangement of the cytoskeleton, which is regulated by the Rho family of GTPases. So far, only Rac1 has been consistently associated with CDR formation, whereas the role of other GTPases in this process is either lacking or inconclusive. Here we show that RhoG and its exchange factor, Trio, play a role in the regulation of CDR dynamics, particularly by modulating their size. RhoG is activated by Trio downstream of PDGF in a PI3K- and Src-dependent manner. Silencing RhoG expression decreases the number of cells that form CDRs, as well as the area of the CDRs. The regulation of CDR area by RhoG is independent of Rac1 function. In addition, our results show the RhoG plays a role in the cellular functions associated with CDR formation, including macropinocytosis, receptor internalization, and cell migration. Taken together, our results reveal a novel role for RhoG in the regulation of CDRs and the cellular processes associated with their formation.

A RhoG-mediated signaling pathway that modulates invadopodia dynamics in breast cancer cells

One of the hallmarks of cancer is the ability of tumor cells to invade surrounding tissues and metastasize. During metastasis, cancer cells degrade the extracellular matrix, which acts as a physical barrier, by developing specialized actin-rich membrane protrusion structures called invadopodia. The formation of invadopodia is regulated by Rho GTPases, a family of proteins that regulates the actin cytoskeleton. Here, we describe a novel role for RhoG in the regulation of invadopodia disassembly in human breast cancer cells. Our results show that RhoG and Rac1 have independent and opposite roles in the regulation of invadopodia dynamics. We also show that SGEF (also known as ARHGEF26) is the exchange factor responsible for the activation of RhoG during invadopodia disassembly. When the expression of either RhoG or SGEF is silenced, invadopodia are more stable and have a longer lifetime than in control cells. Our findings also demonstrate that RhoG and SGEF modulate the phosphorylation of paxillin, which plays a key role during invadopodia disassembly. In summary, we have identified a novel signaling pathway involving SGEF, RhoG and paxillin phosphorylation, which functions in the regulation of invadopodia disassembly in breast cancer cells.

Palladin expression is a conserved characteristic of the desmoplastic tumor microenvironment and contributes to altered gene expression

The stroma surrounding solid tumors contributes in complex ways to tumor progression. Cancer-associated fibroblasts (CAFs) are the predominant cell type in the tumor stroma. Previous studies have shown that the actin-binding protein palladin is highly expressed in the stroma of pancreas tumors, but the interpretation of these results is complicated by the fact that palladin exists as multiple isoforms. In the current study, the expression and localization of palladin isoform 4 was examined in normal specimens and adenocarcinomas of human pancreas, lung, colon, and stomach samples. Immunohistochemistry with isoform-selective antibodies revealed that expression of palladin isoform 4 was higher in adenocarcinomas versus normal tissues, and highest in CAFs. Immunohistochemistry staining revealed that palladin was present in both the cytoplasm and the nucleus of CAFs, and this was confirmed using immunofluorescence staining and subcellular fractionation of a pancreatic CAF cell line. To investigate the functional significance of nuclear palladin, RNA Seq analysis of palladin knockdown CAFs versus control CAFs was performed, and the results showed that palladin regulates the expression of genes involved in the biosynthesis and assembly of collagen, and organization of the extracellular matrix. These results suggested that palladin isoform 4 may play a conserved role in establishing the phenotype of CAFs in multiple tumor types.

I'm coming to GEF you: Regulation of RhoGEFs during cell migration

Cell migration is a highly regulated multistep process that requires the coordinated regulation of cell adhesion, protrusion, and contraction. These processes require numerous protein–protein interactions and the activation of specific signaling pathways. The Rho family of GTPases plays a key role in virtually every aspect of the cell migration cycle. The activation of Rho GTPases is mediated by a large and diverse family of proteins; the guanine nucleotide exchange factors (RhoGEFs). GEFs work immediately upstream of Rho proteins to provide a direct link between Rho activation and cell–surface receptors for various cytokines, growth factors, adhesion molecules, and G protein-coupled receptors. The regulated targeting and activation of RhoGEFs is essential to coordinate the migratory process. In this review, we summarize the recent advances in our understanding of the role of RhoGEFs in the regulation of cell migration.

Structure and function of palladin's actin binding domain

Here, we report the NMR structure of the actin-binding domain contained in the cell adhesion protein palladin. Previously, we demonstrated that one of the immunoglobulin domains of palladin (Ig3) is both necessary and sufficient for direct filamentous actin binding in vitro. In this study, we identify two basic patches on opposite faces of Ig3 that are critical for actin binding and cross-linking. Sedimentation equilibrium assays indicate that the Ig3 domain of palladin does not self-associate. These combined data are consistent with an actin cross-linking mechanism that involves concurrent attachment of two actin filaments by a single palladin molecule by an electrostatic mechanism. Palladin mutations that disrupt actin binding show altered cellular distributions and morphology of actin in cells, revealing a functional requirement for the interaction between palladin and actin in vivo.

Palladin promotes invasion of pancreatic cancer cells by enhancing invadopodia formation in cancer-associated fibroblasts

The stromal compartment surrounding epithelial-derived pancreatic tumors is thought to have a key role in the aggressive phenotype of this malignancy. Emerging evidence suggests that cancer-associated fibroblasts (CAFs), the most abundant cells in the stroma of pancreatic tumors, contribute to the tumor's invasion, metastasis and resistance to therapy, but the precise molecular mechanisms that regulate CAFs behavior are poorly understood. In this study, we utilized immortalized human pancreatic CAFs to investigate molecular pathways that control the matrix-remodeling and invasion-promoting activity of CAFs. We showed previously that palladin, an actin-associated protein, is expressed at high levels in CAFs of pancreatic tumors and other solid tumors, and also in an immortalized line of human CAFs. In this study, we found that short-term exposure of CAFs to phorbol esters reduced the number of stress fibers and triggered the appearance of individual invadopodia and invadopodial rosettes in CAFs. Molecular analysis of invadopodia revealed that their composition resembled that of similar structures (that is, invadopodia and podosomes) described in other cell types. Pharmacological inhibition and small interfering RNA knockdown experiments demonstrated that protein kinase C, the small GTPase Cdc42 and palladin were necessary for the efficient assembly of invadopodia by CAFs. In addition, GTPase activity assays showed that palladin contributes to the activation of Cdc42. In mouse xenograft experiments using a mixture of CAFs and tumor cells, palladin expression in CAFs promoted the rapid growth and metastasis of human pancreatic tumor cells. Overall, these results indicate that high levels of palladin expression in CAFs enhance their ability to remodel the extracellular matrix by regulating the activity of Cdc42, which in turn promotes the assembly of matrix-degrading invadopodia in CAFs and tumor cell invasion. Together, these results identify a novel molecular signaling pathway that may provide new molecular targets for the inhibition of pancreatic cancer metastasis.

The actin associated protein palladin is important for the early smooth muscle cell differentiation

Palladin, an actin associated protein, plays a significant role in regulating cell adhesion and cell motility. Palladin is important for development, as knockdown in mice is embryonic lethal, yet its role in the development of the vasculature is unknown. We have shown that palladin is essential for the expression of smooth muscle cells (SMC) marker genes and force development in response to agonist stimulation in palladin deficient SMCs. The goal of the study was to determine the molecular mechanisms underlying palladin's ability to regulate the expression of SMC marker genes. Results showed that palladin expression was rapidly induced in an A404 cell line upon retinoic acid (RA) induced differentiation. Suppression of palladin expression with siRNAs inhibited the expression of RA induced SMC differentiation genes, SM α-actin (SMA) and SM22, whereas over-expression of palladin induced SMC gene expression. Chromatin immunoprecipitation assays provided evidence that palladin bound to SMC genes, whereas co-immunoprecipitation assays also showed binding of palladin to myocardin related transcription factors (MRTFs). Endogenous palladin was imaged in the nucleus, increased with leptomycin treatment and the carboxyl-termini of palladin co-localized with MRTFs in the nucleus. Results support a model wherein palladin contributes to SMC differentiation through regulation of CArG-SRF-MRTF dependent transcription of SMC marker genes and as previously published, also through actin dynamics. Finally, in E11.5 palladin null mouse embryos, the expression of SMA and SM22 mRNA and protein is decreased in the vessel wall. Taken together, our findings suggest that palladin plays a key role in the differentiation of SMCs in the developing vasculature.

Isoform-specific upregulation of palladin in human and murine pancreas tumors

Pancreatic ductal adenocarcinoma (PDA) is a lethal disease with a characteristic pattern of early metastasis, which is driving a search for biomarkers that can be used to detect the cancer at an early stage. Recently, the actin-associated protein palladin was identified as a candidate biomarker when it was shown that palladin is mutated in a rare inherited form of PDA, and overexpressed in many sporadic pancreas tumors and premalignant precursors. In this study, we analyzed the expression of palladin isoforms in murine and human PDA and explored palladin's potential use in diagnosing PDA. We performed immunohistochemistry and immunoblot analyses on patient samples and tumor-derived cells using an isoform-selective monoclonal antibody and a pan-palladin polyclonal antibody. Immunoblot and real-time quantitative reverse transcription-PCR were used to quantify palladin mRNA levels in human samples. We show that there are two major palladin isoforms expressed in pancreas: 65 and 85–90 kDa. The 65 kDa isoform is expressed in both normal and neoplastic ductal epithelial cells. The 85–90 kDa palladin isoform is highly overexpressed in tumor-associated fibroblasts (TAFs) in both primary and metastatic tumors compared to normal pancreas, in samples obtained from either human patients or genetically engineered mice. In tumor-derived cultured cells, expression of palladin isoforms follows cell-type specific patterns, with the 85–90 kDa isoform in TAFs, and the 65 kDa isoform predominating in normal and neoplastic epithelial cells. These results suggest that upregulation of 85–90 kDa palladin isoform may play a role in the establishment of the TAF phenotype, and thus in the formation of a desmoplastic tumor microenvironment. Thus, palladin may have a potential use in the early diagnosis of PDA and may have much broader significance in understanding metastatic behavior.

Cytoplasmic Ig-domain proteins: cytoskeletal regulators with a role in human disease

Immunoglobulin domains are found in a wide variety of functionally diverse transmembrane proteins, and also in a smaller number of cytoplasmic proteins. Members of this latter group are usually associated with the actin cytoskeleton, and most of them bind directly to either actin or myosin, or both. Recently, studies of inherited human disorders have identified disease-causing mutations in five cytoplasmic Ig-domain proteins: myosin-binding protein C, titin, myotilin, palladin, and myopalladin. Together with results obtained from cultured cells and mouse models, these clinical studies have yielded novel insights into the unexpected roles of Ig domain proteins in mechanotransduction and signaling to the nucleus. An emerging theme in this field is that cytoskeleton-associated Ig domain proteins are more than structural elements of the cell, and may have evolved to fill different needs in different cellular compartments. Cell Motil. Cytoskeleton 2009. (c) 2009 Wiley-Liss, Inc.

Palladin expression contributes to invasive motiliy in metastatic breast cancer cells

Abstract #6018

Background: Cancer metastasis involves multiple actin-dependent steps, including intravasation , extravasation, and adhesion. Therefore, it is important to understand the molecular mechanisms that control cytoskeletal dynamics in metastatic cells. Palladin is an actin-binding protein that functions as a key molecular scaffold involved in actin organization. Recent studies showed that (1) a mutation in palladin is linked to a rare form of familial pancreatic cancer, (2) palladin is overexpressed in many sporadic pancreatic tumors, and (3) palladin is upregulated in pancreatic tumor-associated fibroblasts. However, palladin's functions in other invasive cancers have not been well elucidated.
 Methods: Three primary human breast cancer specimens, three metastatic breast cancer specimens and three samples of benign breast tissue were obtained from the UNC Tissue Procurement Facility. Extracts were analyzed using western blots. Eight human breast cancer cell lines, which differ in their metastatic potential, were used: T47D, BT474, ZR75.1 and MCF-7, BT549, Hs578T, MDA-MB-231 and SUM159. Western blots were performed to determine the level of palladin in all cells lines. siRNA and virus infection approaches were used to reduce and overexpress palladin levels, respectively. Cell migration was analyzed in transwell inserts. Invasion experiments were conducted in Matrigel invasion chambers. Podosome formation was induced with the phorbol ester PDBu.
 Results: Immunoblot analysis revealed that palladin levels are higher overall in primary tumors and metastases, when compared to benign breast tissue. Immunohistochemistry showed that palladin staining is associated with ductal epithelial cells and tumor cells. In breast cancer cells, palladin expression closely correlates with metastatic potential: palladin levels were ∼16 fold higher in invasive cells than in non-invasive cells. Furthermore, knockdown of palladin resulted in a significant reduction in the ability of SUM159 cells to migrate through a transwell filter, and to invade through a layer of Matrigel. Phorbol ester treatment stimulated the formation of palladin-containing podosomes in SUM159 and MDA-MB-231, but not MCF7 cells. Palladin knockdown also impaired the ability of SUM159 cells to assemble podosomes. Palladin overexpression promotes podosome formation but does not affect cell migration.
 Discussion: Our results show that palladin plays an important role in podosome formation, and thus high levels of palladin expression may contribute to the invasive motility of metastatic breast cancer cells by facilitating the assembly of these structures. Our results also show that overexpression of palladin alone is not sufficient to trigger enhanced invasive motility in cultured human breast cancer cells. These results support a model in which a cohort of genes are coordinately upregulated in metastatic cells, contributing to specific aspects of invasive cell motility.

Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 6018.

Palladin contributes to invasive motility in human breast cancer cells

Cancer metastasis involves multiple steps including detachment of the metastatic cells from neighboring cells, the acquisition of motility and invasion to other tissue. All of these steps require the reorganization of the actin cytoskeleton. In this study, we found that the protein palladin, a molecular scaffold with an important function in actin organization, is expressed at higher overall levels in tumors compared to benign breast tissue, and also significantly higher in four invasive breast cancer cell lines when compared to four non-invasive cell lines. In addition, we found that palladin plays a key role in the formation of podosomes. Podosomes are actin-rich structures that function in adhesion and matrix degradation and have been found in many invasive cell types. Our results show that phorbol ester treatment stimulated the formation of palladin-containing podosomes in invasive, but not in non-invasive cell lines. More importantly, palladin knockdown resulted in decreased podosome formation and a significant reduction in transwell migration and invasive motility. Palladin overexpression induced podosome formation in the non-invasive MCF7 cells, which are otherwise unable to form podosomes, suggesting that palladin plays a critical role in the assembly of podosomes. Overall, these results indicate that palladin overexpression contributes to the invasive behavior of metastatic cells.

The role of palladin in actin organization and cell motility

Palladin is a widely expressed protein found in stress fibers, focal adhesions, growth cones, Z-discs, and other actin-based subcellular structures. It belongs to a small gene family that includes the Z-disc proteins myopalladin and myotilin, all of which share similar Ig-like domains. Recent advances have shown that palladin shares with myotilin the ability to bind directly to F-actin, and to crosslink actin filaments into bundles, in vitro. Studies in a variety of cultured cells suggest that the actin-organizing activity of palladin plays a central role in promoting cell motility. Correlative evidence also supports this hypothesis, as palladin levels are typically up-regulated in cells that are actively migrating: in developing vertebrate embryos, in cells along a wound edge, and in metastatic cancer cells. Recently, a mutation in the human palladin gene was implicated in an unusually penetrant form of inherited pancreatic cancer, which has stimulated new ideas about the role of palladin in invasive cancer.

Nck-2 interacts with focal adhesion kinase and modulates cell motility

Nck-2 is a ubiquitously expressed adaptor protein comprising primarily three N-terminal SH3 domains and one C-terminal SH2 domain. We report here that Nck-2 interacts with focal adhesion kinase (FAK), a cytoplasmic protein tyrosine kinase critically involved in the cellular control of motility. Using a mutational strategy, we have found that the formation of the Nck-2-FAK complex is mediated by interactions involving multiple SH2 and SH3 domains of Nck-2. The Nck-2 SH2 domain-mediated interaction with FAK is dependent on phosphorylation of Tyr397, a site that is involved in the regulation of cell motility. A fraction of Nck-2 co-localizes with FAK at cell periphery in spreading cells. Furthermore, overexpression of Nck-2 modestly decreased cell motility, whereas overexpression of a mutant form of Nck-2 containing the SH2 domain but lacking the SH3 domains significantly promoted cell motility. These results identify a novel interaction between Nck-2 and FAK and suggest a role of Nck-2 in the modulation of cell motility.

Influence of protein nutrition on calpain activity of mouse kidney: modulation by calpastatin

The effect of protein depletion and refeeding with a normal diet on calpain activity was examined in mouse kidney soluble homogenate. In terms of units per gram of protein, it increased 2.9 times with depletion and decreased upon refeeding. After a DEAE-Sephacel chromatography, the homogenate yielded three enzymatic activities. Their sum, assessed as total calpain activity, was higher than the activity measured before fractionation and did not appreciably change during protein depletion and refeeding. Because the proportion of total activity displayed by the complete homogenate increased with depletion and decreased with refeeding, a low calpastatin content in depleted kidney was envisaged. This was confirmed by direct estimations: depleted kidney had 6 times less calpastatin compared to both normal and 16 h refed tissue. We concluded that a decrease in calpastatin content contributes to an increased calpain activity related to degradable protein in protein depleted kidney. In view of this, it seems not unlikely that the in vivo rate of protein breakdown depicted by kidney during protein depletion and refeeding is in part effected through modulation of the calpain proteolytic system.

Effect of dietary level of protein on the activity of mouse liver calpain

The effect of protein depletion and refeeding with a normal diet on mouse liver soluble homogenate calpain activity was studied. It was unchanged when expressed in terms of whole liver (units/liver). However, when expressed in terms of degradable protein (units/mg protein) it increased with depletion and decreased with refeeding. DEAE Sephacel chromatography of soluble homogenate yielded three calpain activities which were eluted at 0.04, 0.16 and 0.23 M NaCl, respectively. On the basis of whole liver, they decreased with depletion and increased with refeeding. Immunochemical analysis revealed similar changes in the mass of the calpain eluted with 0.16 M NaCl. The sum of these three activities (total liver calpain activity) was higher than the activity displayed by the soluble homogenate, indicating that they were separated from calpastatin. Furthermore, the percentage of total calpain activity displayed by soluble homogenate increased with depletion and decreased with refeeding, suggesting that depleted liver had the lowest calpastatin content. This was confirmed by direct measurements which indicated that depleted homogenate had in average 5.5 and 3.1 times less calpastatin compared to normal and 16 hours refed liver, respectively. It is concluded that a remarkable decrease in calpastatin content maintained unchanged whole liver soluble homogenate calpain activity during protein depletion and refeeding and contributes to an increased calpain activity related to degradable protein in depleted livers. This increase is in accordance with the high in vivo rate of protein breakdown depicted by these livers.