A novel mechanoeffector role of fibroblast S100A4 in myofibroblast transdifferentiation and fibrosis

Fibroblast to myofibroblast transdifferentiation mediates numerous fibrotic disorders, such as idiopathic pulmonary fibrosis (IPF). We have previously demonstrated that non-muscle myosin II (NMII) is activated in response to fibrotic lung extracellular matrix, thereby mediating myofibroblast transdifferentiation. NMII-A is known to interact with the calcium-binding protein S100A4, but the mechanism by which S100A4 regulates fibrotic disorders is unclear. In this study, we show that fibroblast S100A4 is a calcium-dependent, mechanoeffector protein that is uniquely sensitive to pathophysiologic-range lung stiffness (8-25 kPa) and thereby mediates myofibroblast transdifferentiation. Re-expression of endogenous fibroblast S100A4 rescues the myofibroblastic phenotype in S100A4 KO fibroblasts. Analysis of NMII-A/actin dynamics reveals that S100A4 mediates the unraveling and redistribution of peripheral actomyosin to a central location, resulting in a contractile myofibroblast. Furthermore, S100A4 loss protects against murine in vivo pulmonary fibrosis, and S100A4 expression is dysregulated in IPF. Our data reveal a novel mechanosensor/effector role for endogenous fibroblast S100A4 in inducing cytoskeletal redistribution in fibrotic disorders such as IPF.

Macropinocytosis: mechanisms and regulation

Macropinocytosis is defined as an actin-dependent but coat- and dynamin-independent endocytic uptake process, which generates large intracellular vesicles (macropinosomes) containing a non-selective sampling of extracellular fluid. Macropinocytosis provides an important mechanism of immune surveillance by dendritic cells and macrophages, but also serves as an essential nutrient uptake pathway for unicellular organisms and tumor cells. This review examines the cell biological mechanisms that drive macropinocytosis, as well as the complex signaling pathways - GTPases, lipid and protein kinases and phosphatases, and actin regulatory proteins - that regulate macropinosome formation, internalization, and disposition.

PIP3 abundance overcomes PI3K signaling selectivity in invadopodia

PI3Kβ is required for invadopodia-mediated matrix degradation by breast cancer cells. Invadopodia maturation requires GPCR activation of PI3Kβ and its coupling to SHIP2 to produce PI(3,4)P2 . We now test whether selectivity for PI3Kβ is preserved under conditions of mutational increases in PI3K activity. In breast cancer cells where PI3Kβ is inhibited, short-chain diC8-PIP3  rescues gelatin degradation in a SHIP2-dependent manner; rescue by diC8-PI(3,4)P2  is SHIP2-independent. Surprisingly, the expression of either activated PI3Kβ or PI3Kα mutants rescued the effects of PI3Kβ inhibition. In both cases, gelatin degradation was SHIP2-dependent. These data confirm the requirement for PIP3 conversion to PI(3,4)P2 for invadopodia function and suggest that selectivity for distinct PI3K isotypes may be obviated by mutational activation of the PI3K pathway.

Absence of S100A4 in the mouse lens induces an aberrant retina-specific differentiation program and cataract

S100A4, a member of the S100 family of multifunctional calcium-binding proteins, participates in several physiological and pathological processes. In this study, we demonstrate that S100A4 expression is robustly induced in differentiating fiber cells of the ocular lens and that S100A4 (-/-) knockout mice develop late-onset cortical cataracts. Transcriptome profiling of lenses from S100A4 (-/-) mice revealed a robust increase in the expression of multiple photoreceptor- and Müller glia-specific genes, as well as the olfactory sensory neuron-specific gene, S100A5. This aberrant transcriptional profile is characterized by corresponding increases in the levels of proteins encoded by the aberrantly upregulated genes. Ingenuity pathway network and curated pathway analyses of differentially expressed genes in S100A4 (-/-) lenses identified Crx and Nrl transcription factors as the most significant upstream regulators, and revealed that many of the upregulated genes possess promoters containing a high-density of CpG islands bearing trimethylation marks at histone H3K27 and/or H3K4, respectively. In support of this finding, we further documented that S100A4 (-/-) knockout lenses have altered levels of trimethylated H3K27 and H3K4. Taken together, our findings suggest that S100A4 suppresses the expression of retinal genes during lens differentiation plausibly via a mechanism involving changes in histone methylation.

Abstract A08: PI3Kbeta regulates beta-1 integrin signaling in invadopodia through formation of PI(3,4)P2

The PI3Kbeta isoform of PI 3-kinase, which is activated downstream from both RTKs and GPCRs, is an important regulator of invadopodia formation. We have previously reported that in MDA-MB-231 breast cancer cells, replacement of endogenous PI3Kbeta with kinase dead (KD) or GPCR-uncoupled (KK-DD) mutants leads to defects in invadopodia-mediated gelatin degradation. We have now studied the mechanism by which loss of PI3Kbeta signaling affects invadopodia. While the number of invadopodia precursors is unaffected by mutation of PI3Kbeta, formation of mature invadopodia is reduced in cells expressing KK-DD and KD PI3Kbeta mutants. Previous studies have demonstrated a role for beta-1 integrins in invadopodia maturation, and PI3Kbeta has been implicated in integrin signaling in platelets. We therefore tested whether mutation of PI3Kbeta would affect integrin signaling in breast cancer cells. In both haptotaxis and cell spreading assays on collagen I, cells expressing KD or KK-DD PI3Kbeta showed a significant impairment. Haptotaxis and cell spreading was also inhibited by TGX221 (PI3Kbeta inhibitor) and pertussis toxin (PTX), but not by specific inhibitors of other Class I PI3Ks. To distinguish whether PI3Kbeta was acting on integrins by inside-out versus outside-in mechanisms, we treated cells with the activating beta-1 integrin antibody TS2/16, which bypasses inside-out regulation of integrin binding. TS2/16 increased cell spreading on gelatin, but this was blocked by TGX221, suggesting that PI3Kbeta acts downstream of beta-1 integrins. Importantly, haptotaxis and cell spreading were not affected by an Akt inhibitor, but were blocked by an inhibitor of SHIP2, which hydrolyzes PIP3 to produce PI[3,4]P2. These data suggest that integrin signaling requires the coupling of Gbeta/gamma-mediated activation of PI3Kbeta to the production of PI[3,4]P2. Finally, to determine whether PI3Kbeta was involved in beta-1 integrin signaling in invadopodia, we plated starved cells on high-density fibrillar collagen (HDFC), which drives invadopodia formation through beta-1 integrin activation. Collagen degradation on HDFC was blocked by inhibition of PI3Kbeta or SHIP2, but not PI3Kalpha. Moreover, recruitment of the PI(3,4)P2 binding protein lamellipodin to invadopodia was reduced by inhibition of PI3Kbeta. In summary, beta-1 integrin signaling in the context of invadopodia maturation requires signaling through PI3Kbeta. We propose that heterotrimeric G-protein activation of PI3Kbeta is necessary for formation of PI(3,4)P2 in invadopodia, which is required for invadopodial maturation.

Citation Format: Zahra Erami, Anne R. Bresnick, Jonathan M. Backer. PI3Kbeta regulates beta-1 integrin signaling in invadopodia through formation of PI(3,4)P2 [abstract]. In: Proceedings of the AACR Special Conference on Targeting PI3K/mTOR Signaling; 2018 Nov 30-Dec 8; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Res 2020;18(10_Suppl):Abstract nr A08.

PI3Kβ is selectively required for growth factor-stimulated macropinocytosis

Macropinocytosis is an actin-dependent but clathrin-independent endocytic process by which cells nonselectively take up large aliquots of extracellular material. Macropinocytosis is used for immune surveillance by dendritic cells, as a route of infection by viruses and protozoa, and as a nutrient uptake pathway in tumor cells. In this study, we explore the role of class I phosphoinositide 3-kinases (PI3Ks) during ligand-stimulated macropinocytosis. We find that macropinocytosis in response to receptor tyrosine kinase activation is strikingly dependent on a single class I PI3K isoform, namely PI3Kβ (containing the p110β catalytic subunit encoded by PIK3CB). Loss of PI3Kβ expression or activity blocks macropinocytosis at early steps, before the formation of circular dorsal ruffles, but also plays a role in later steps, downstream from Rac1 activation. PI3Kβ is also required for the elevated levels of constitutive macropinocytosis found in tumor cells that are defective for the PTEN tumor suppressor. Our data shed new light on PI3K signaling during macropinocytosis, and suggest new therapeutic uses for pharmacological inhibitors of PI3Kβ.

PI3Kβ links integrin activation and PI(3,4)P2 production during invadopodial maturation

The invasion of tumor cells from the primary tumor is mediated by invadopodia, actin-rich protrusive organelles that secrete matrix metalloproteases and degrade the extracellular matrix. This coupling between protrusive activity and matrix degradation facilitates tumor invasion. We previously reported that the PI3Kβ isoform of PI 3-kinase, which is regulated by both receptor tyrosine kinases and G protein-coupled receptors, is required for invasion and gelatin degradation in breast cancer cells. We have now defined the mechanism by which PI3Kβ regulates invadopodia. We find that PI3Kβ is specifically activated downstream from integrins, and is required for integrin-stimulated spreading and haptotaxis as well as integrin-stimulated invadopodia formation. Surprisingly, these integrin-stimulated and PI3Kβ-dependent responses require the production of PI(3,4)P2 by the phosphoinositide 5'-phosphatase SHIP2. Thus, integrin activation of PI3Kβ is coupled to the SHIP2-dependent production of PI(3,4)P2, which regulates the recruitment of PH domain-containing scaffolds such as lamellipodin to invadopodia. These findings provide novel mechanistic insight into the role of PI3Kβ in the regulation of invadopodia in breast cancer cells.

Abstract 3431: PI3Kbeta regulates macropinocytosis in PTEN-null tumor cells

Class I phosphoinositide 3-kinases (PI3Ks) have been previously implicated in the regulation of vesicular trafficking, including receptor-mediated endocytosis and macropinocytosis. Using NIH3T3 cells in which the genes for the PI3Kbeta or PI3Kalpha catalytic subunits were deleted by CRISPR/Cas9, we were unable to detect any defects in receptor-mediated endocytosis of [125I]-PDGF or in pinocytosis of Lucifer yellow. However, we observed a marked inhibition of macropinocytosis in PI3Kbeta knockout cells; deletion of the other major PI3K isoform in these cells, PI3Kalpha, had no effect. These studies were confirmed using isoform-selective inhibitors: only inhibition of PI3Kbeta blocked macropinocytosis. Similarly, in MDA-MB-231 breast cancer cells, HGF-stimulated macropinocytosis was markedly reduced by inhibition of PI3Kbeta, but not the other PI3K isoforms. Notably, expression of a mutant PI3Kbeta that cannot be activated by Gbeta-gamma, or treatment of WT cells with pertussis toxin, also inhibited macropinocytosis in MDA-MB-231 cells. A requirement for Gbeta-gamma binding to PI3Kbeta was also seen in primary macrophages from mice expressing WT or Gbeta-gamma-uncoupled PI3Kbeta, as both CSF-1 and C5a-stimulated macropinocytosis was inhibited by mutant PI3Kbeta. To define the mechanism by which PI3Kbeta regulates macropinocytosis, we measured circular dorsal ruffle (CDR) formation. Genetic ablation or inhibition of PI3Kbeta completely suppressed CDR formation in PDGF-stimulated NIH3T3 cells, whereas inhibition or ablation of other PI3K isoforms had no effect. While previous studies have suggested that Class I PI3Ks act late in macropinosome formation, just prior to macropinosome sealing, our data suggest that PI3Kbeta functions early in macropinocytosis, as it mediates CDR formation in a Gbeta-gamma-dependent manner. Given previous work showing a positive feedback loop involving PI3Kbeta and Rac1, we tested whether macropinosome formation was rescued by expression of CA-Rac1. CA-Rac1 expression potently stimulated macropinocytosis in MDA-MB-231 cells. Strikingly, this was completely blocked by expression of the Gbeta-gamma-uncoupled PI3Kbeta, and partially blocked by expression of a mutant PI3Kbeta that cannot bind Rac1. Finally, given the well established role of PI3Kbeta in the growth of tumor cells lacking the PTEN tumor suppressor, as well as recent studies showing that high rates of basal macropinocytosis can serve as a nutrient a uptake mechanism in PTEN null cells, we measured macropinocytosis in two PTEN-deficient tumor lines, BT549 (breast) and PC3 (prostate). Both lines showed elevated basal levels of macropinocytosis, which were specifically blocked by inhibition of PI3Kbeta. Our data support a highly specific role for PI3Kbeta in the regulation of macropinocytosis, which may contribute the role of PI3Kbeta in the growth of PTEN-null tumor cells.

Citation Format: Gilbert Salloum, Charles Jakubik, Anne R. Bresnick, Jonathan M. Backer. PI3Kbeta regulates macropinocytosis in PTEN-null tumor cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3431.

CCL2 mobilizes ALIX to facilitate Gag-p6 mediated HIV-1 virion release

Cellular ESCRT machinery plays pivotal role in HIV-1 budding and release. Extracellular stimuli that modulate HIV-1 egress are currently unknown. We found that CCL2 induced by HIV-1 clade B (HIV-1B) infection of macrophages enhanced virus production, while CCL2 immuno-depletion reversed this effect. Additionally, HIV-1 clade C (HIV-1C) was refractory to CCL2 levels. We show that CCL2-mediated increase in virus production requires Gag late motif LYPX present in HIV-1B, but absent in HIV-1C, and ALIX protein that recruits ESCRT III complex. CCL2 immuno-depletion sequestered ALIX to F-actin structures, while CCL2 addition mobilized it to cytoplasm facilitating Gag-ALIX binding. The LYPX motif improves virus replication and its absence renders the virus less fit. Interestingly, novel variants of HIV-1C with PYRE/PYKE tetrapeptide insertions in Gag-p6 conferred ALIX binding, CCL2-responsiveness and enhanced virus replication. These results, for the first time, indicate that CCL2 mediates ALIX mobilization from F-actin and enhances HIV-1 release and fitness.

PI3Kβ-A Versatile Transducer for GPCR, RTK, and Small GTPase Signaling

The phosphoinositide 3-kinase (PI3K) family includes eight distinct catalytic subunits and seven regulatory subunits. Only two PI3Ks are directly regulated downstream from G protein-coupled receptors (GPCRs): the class I enzymes PI3Kβ and PI3Kγ. Both enzymes produce phosphatidylinositol 3,4,5-trisposphate in vivo and are regulated by both heterotrimeric G proteins and small GTPases from the Ras or Rho families. However, PI3Kβ is also regulated by direct interactions with receptor tyrosine kinases (RTKs) and their tyrosine phosphorylated substrates, and similar to the class II and III PI3Ks, it binds activated Rab5. The unusually complex regulation of PI3Kβ by small and trimeric G proteins and RTKs leads to a rich landscape of signaling responses at the cellular and organismic levels. This review focuses first on the regulation of PI3Kβ activity in vitro and in cells, and then summarizes the biology of PI3Kβ signaling in distinct tissues and in human disease.

S100 proteins as therapeutic targets

The human genome codes for 21 S100 protein family members, which exhibit cell- and tissue-specific expression patterns. Despite sharing a high degree of sequence and structural similarity, the S100 proteins bind a diverse range of protein targets and contribute to a broad array of intracellular and extracellular functions. Consequently, the S100 proteins regulate multiple cellular processes such as proliferation, migration and/or invasion, and differentiation, and play important roles in a variety of cancers, autoimmune diseases, and chronic inflammatory disorders. This review focuses on the development of S100 neutralizing antibodies and small molecule inhibitors and their potential therapeutic use in controlling disease progression and severity.

Myosin-IIA heavy chain phosphorylation on S1943 regulates tumor metastasis

Nonmuscle myosin-IIA (NMHC-IIA) heavy chain phosphorylation has gained recognition as an important feature of myosin-II regulation. In previous work, we showed that phosphorylation on S1943 promotes myosin-IIA filament disassembly in vitro and enhances EGF-stimulated lamellipod extension of breast tumor cells. However, the contribution of NMHC-IIA S1943 phosphorylation to the modulation of invasive cellular behavior and metastasis has not been examined. Stable expression of phosphomimetic (S1943E) or non-phosphorylatable (S1943A) NMHC-IIA in breast cancer cells revealed that S1943 phosphorylation enhances invadopodia function, and is critical for matrix degradation in vitro and experimental metastasis in vivo. These studies demonstrate a novel link between NMHC-IIA S1943 phosphorylation, the regulation of extracellular matrix degradation and tumor cell invasion and metastasis.

Abstract 3165: PI 3-kinase-beta regulates invadopodia maturation and beta-1 integrin signaling

The invasion of tumors cells during metastasis is mediated by invadopodia, actin rich protrusive organelles that secrete matrix metalloproteases and mediate matrix degradation. The PI3K-beta isoform of PI 3-kinase, which is regulated by both receptor tyrosine kinases and GPCRs, is an important regulator of invadopodia formation. We have previously reported that in MDA-MB-231 breast cancer cells, replacement of endogenous PI3K-beta with kinase dead (p110-betaK799R; KD) or GPCR-uncoupled (p110-beta526KK-DD; KK-DD) mutants leads to defects in gelatin degradation. We have now studied the mechanism by which loss of PI3K-beta signaling affects invadopodia formation and matrix degradation. While the number of invadopodia precursors (cortactin-Tks5 positive, degradation negative) is unaffected by mutation of PI3K-beta, formation of mature invadopodia (cortactin-Tks5 positive, degradation positive) is reduced in cells expressing KK-DD and KD PI3K-beta. Previous studies have demonstrated a role for beta-1 integrins in invadopodia maturation, and PI3K-beta has been implicated in integrin signaling in platelets. We therefore tested whether mutation of PI3K-beta would affect integrin signaling in breast cancer cells. In a haptotaxis assay, MDA-MB-231 cells expressing KD or KK-DD PI3K-beta showed a significant decrease in migration. Consistent with these data, cell spreading on collagen I was significantly impaired in cells expressing KD or KK-DD PI3K-beta, and was inhibited by TGX221 (PI3K-beta inhibitor) and LY294002 (pan-PI3K inhibitor), but not by specific inhibitors of other Class I PI3Ks (alpha, delta and gamma). Spreading was also impaired in cells treated with pertussis toxin, which blocks GPCR activation of PI3K-beta. To distinguish whether PI3K-beta was acting on integrins by inside-out versus outside-in mechanisms, we treated cells with the activating beta-1 integrin antibody TS2/16, which bypasses inside-out regulation of integrin binding. TS2/16 increased cell spreading on collagen I, but TS2/16-stimulated spreading was blocked by treatment of cells with TGX221, suggesting that PI3K-beta acts downstream of beta-1 integrins. Finally, to determine whether PI3K-beta was involved in beta-1 integrin signaling in invadopodia, we plated cells on high density fibrillar collagen (HDFC), which enhances invadopodia formation through beta-1 integrin activation. Invadopodia formation on HDFC was blocked by treatment of cells with TGX221. In summary, beta-1 integrin signaling in the context of invadopodia requires GPCR signaling to PI3K-beta. We propose that GPCR activation of PI3K-beta is a critical downstream component of beta-1 integrin signaling in invadopodia.

Citation Format: Zahra Erami, Anne R. Bresnick, Jonathan M. Backer. PI 3-kinase-beta regulates invadopodia maturation and beta-1 integrin signaling [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3165.

Abstract 3168A: S100A4 mediates paracrine interactions with breast tumor cells

S100A4, a member of the S100 family of Ca2+-binding proteins, has a direct and causative role in tumor metastasis. In murine models of breast cancer, S100A4 overexpression in tumor cells promotes an aggressive metastatic phenotype, while inhibition of S100A4 expression significantly reduces metastatic burden. We have now evaluated the contribution of stromal S100A4 to mammary tumor progression. The growth of E0771 orthotopic mammary tumors was reduced in S100A4-/- mice as compared to wild type mice. In addition, in a 3D collagen assay that recapitulates macrophage-tumor cell paracrine interactions that drive metastasis in vivo, S100A4-/- bone marrow-derived macrophages failed to stimulate breast tumor cell invasion. Our cell biological studies on bone marrow-derived macrophages from wild type and S100A4-/- mice show that loss of S100A4 produces two mechanistically distinct phenotypes in macrophages: a myosin-IIA-dependent defect in matrix degradation, due to an inhibition of podosome rosettes (actin-rich degradative structures that allow macrophages to penetrate through endothelial layers and their associated basement membranes), and a defect in invasion, due to a myosin-IIA-independent increase in microtubule acetylation and stability. These studies suggest that S100A4, which has well described pro-metastatic functions in tumor cells, also plays a critical role in immune cells in the tumor microenvironment.

Citation Format: Natalya G. Dulyaninova, Adriana Levine, Jonathan M. Backer, Anne R. Bresnick. S100A4 mediates paracrine interactions with breast tumor cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3168A.

S100A4 regulates macrophage invasion by distinct myosin-dependent and myosin-independent mechanisms

S100A4, a member of the S100 family of Ca²⁺-binding proteins, is a key regulator of cell migration and invasion. Our previous studies showed that bone marrow–derived macrophages from S100A4^−/− mice exhibit defects in directional motility and chemotaxis in vitro and reduced recruitment to sites of inflammation in vivo. We now show that the loss of S100A4 produces two mechanistically distinct phenotypes with regard to macrophage invasion: a defect in matrix degradation, due to a disruption of podosome rosettes caused by myosin-IIA overassembly, and a myosin-independent increase in microtubule acetylation, which increases podosome rosette stability and is sufficient to inhibit macrophage invasion. Our studies point to S100A4 as a critical regulator of matrix degradation, whose actions converge on the dynamics and degradative functions of podosome rosettes.

Deficiency in Calcium-Binding Protein S100A4 Impairs the Adjuvant Action of Cholera Toxin

The calcium-binding protein S100A4 has been described to promote pathological inflammation in experimental autoimmune and inflammatory disorders and in allergy and to contribute to antigen presentation and antibody response after parenteral immunization with an alum-adjuvanted antigen. In this study, we extend these findings by demonstrating that mice lacking S100A4 have a defective humoral and cellular immune response to mucosal (sublingual) immunization with a model protein antigen [ovalbumin (OVA)] given together with the strong mucosal adjuvant cholera toxin (CT), and that this impairment is due to defective adjuvant-stimulated antigen presentation by antigen-presenting cells. In comparison to wild-type (WT) mice, mice genetically lacking S100A4 had reduced humoral and cellular immune responses after immunization with OVA plus CT, including a complete lack of detectable germinal center reaction. Further, when stimulated in vitro with OVA plus CT, S100A4^−/− dendritic cells (DCs) showed impaired responses in several CT-stimulated immune regulatory molecules including the co-stimulatory molecule CD86, inflammasome-associated caspase-1 and IL-1β. Coculture of OVA-specific OT-II T cells with S100A4^−/− DCs that had been pulse incubated with OVA plus CT resulted in impaired OT-II T cell proliferation and reduced production of Th1, Th2, and Th17 cytokines compared to similar cocultures with WT DCs. In accordance with these findings, transfection of WT DCs with S100A4-targeting small interfering RNA (siRNA) but not mock-siRNA resulted in significant reductions in the expression of caspase-1 and IL-1β as well as CD86 in response to CT. Importantly, also engraftment of WT DCs into S100A4^−/− mice effectively restored the immune response to immunization in the recipients. In conclusion, our results demonstrate that deficiency in S100A4 has a strong impact on the development of both humoral and cellular immunity after mucosal immunization using CT as adjuvant.

Abstract 5060: Myosin-IIA heavy chain phosphorylation on S1943 regulates tumor cell invasion

Nonmuscle myosin IIA (NMHC-IIA) heavy chain phosphorylation has been shown to regulate motility and chemotaxis in invasive breast cancer cells; phosphorylation on S1943 promotes myosin-IIA filament disassembly in vitro and enhances two-dimensional cell migration. To test the role of NMHC-IIA S1943 phosphorylation in regulating the invasive properties of tumor cells, we produced MDA-MB-231 breast cancer cells in which the endogenous NMHC-IIA was stably knocked down via shRNA and replaced with exogenously expressed murine GFP-tagged wild type, phosphomimetic (S1943E) or non-phosphorylatable (S1943A) NMHC-IIA. Since previous studies have shown that the matrix-degrading activity of invadopodia is regulated via a myosin II-FAK-Cas pathway (Alexander et al, Curr Biol 18:1295, 2009), we examined the ability of MDA-MB-231 cells expressing wild type, S1943E or S1943A NMHC-IIA to form invadopodia and degrade matrix. Only 39.4% of S1943A NMHC-IIA expressing cells exhibited matrix degradation as compared to wild type NMHC-IIA (90.5%) and S1943E NMHC-IIA (90.3%). In addition, the average percent degradation area for cells expressing S1943A NMHC-IIA was 15-fold lower than for cells expressing wild type NMHC-IIA, whereas S1943E NMHC-IIA cells exhibited a 2-fold increase in degradation area. There was no significant difference in the numbers of immature invadopodia between the three cell lines. However, S1943A NMHC-IIA cells had significantly fewer mature invadopodia, while S1943E NMHC-IIA cells had increased numbers of mature invadopodia compared to wild type NMHC-IIA cells. To investigate the consequences of altered invadopodia maturation and activity, we examined the secretion of MMP-9 from cells expressing wild type, S1943E or S1943A NMHC-IIA. Gelatin zymography and MMP-9 activity assays of conditioned medium showed that S1943E NMHC-IIA expression enhanced MMP-9 secretion by 2-fold, whereas S1943A NMHC-IIA expression reduced MMP-9 secretion by 4-fold. In experimental metastasis assays in SCID mice, S1943A NMHC-IIA cells rarely formed lung metastases as compared to wild type NMHC-IIA cells, which formed poorly differentiated metastatic foci with some pleural localization. The S1943E NMHC-IIA cells also produced poorly differentiated carcinomas, which primarily localized along the pleural lining. Overall, S1943A NMHC-IIA cells produced approximately 2-fold fewer and S1943E NMHC-IIA cells produced 2-fold greater metastases than wild type NMHC-IIA cells. Together, these observations indicate that NMHC-IIA S1943 phosphorylation contributes to tumor cell invasion and metastasis via the regulation of extracellular matrix degradation.

Citation Format: Laura E. Norwood Toro, Jonathan M. Backer, Anne R. Bresnick. Myosin-IIA heavy chain phosphorylation on S1943 regulates tumor cell invasion. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 5060.

Abstract 4838: Peptide prodrugs targeting the SH2 domains of p85 block PI3K signaling

Class IA phosphoinositol-3-kinases (PI3Ks) play pivotal roles in cancer and inflammation and are targets for anti-cancer therapies. p85 was first described as a regulatory subunit of Class I PI3Ks which complexes with the p110 catalytic domain to form the heterodimeric enzyme. PI3Ks are recruited to cytokine and growth factor receptors via the SH2 domains of p85. While p85 represses the kinase activity of p110 subunits in the cytosol, binding to pTyr residues on receptors or adapters activates p110, resulting in the phosphorylation of the 3’-hydroxyl group of phosphatidylinositol-4’,5’-diphosphate (PIP2) to form phosphatidylinositol-3’,4’,5’-triphosphate (PIP3). Membrane-associated PIP3 subsequently recruits kinases such as PDK1 and Akt to the cell membrane. PDK1 phosphorylates and activates Akt, which propagates growth and survival signals. In biochemical assays, doubly phosphorylated peptides enhance catalytic activity of p85/p110 dimers, which led to the dogma that phosphopeptides should activate PI3K in cells.

Due to the challenges of cellular entry of negatively charged phosphopeptides, blocking protein-protein interactions mediated by p85 has been largely unexplored in intact cells. We targeted the SH2 domains of p85 with phosphatase-stable, cell-permeable prodrug analogs of Y(p)VPML, a high-affinity ligand derived from Tyr751 of PDGF. To allow cell entry and to protect against phosphatase degradation, we replaced pTyr with bis-pivaloyloxymethyl esters of 4-phosphonodifluoromethylphenylalanine (F2Pmp(POM2)), which have not been reported. We synthesized Z-NMe-F2Pmp(POM2)-OPcp with the á-amino group methylated to enhance cell penetration and the carboxyl group esterified with pentachlorophenol. Z-protection was necessary for phosphonate protecting group manipulation and for stability of the Pcp ester. A series of inhibitors was prepared by coupling Z-NMe-F2Pmp(POM2)-OPcp to analogs of VPML. Hydrogenation in the presence of Ac2O gave an N-terminal acetyl prodrug. Screening for inhibition of EGF-stimulation of pAkt in serum starved MDA-MB-468 cells led to the identification of the lead inhibitor, PM-190I. In serum starved MDB-MB-231 cells, PM-190I inhibited EGF-stimulated phosphorylation of Akt with an IC50 of 5 ìM. At 30 ìM Akt phosphorylation was negligible. These results suggest that the phosphopeptide mimic blocks recruitment to p85/p110 dimers to EGFR or its substrates, thereby preventing PI3K pathway signaling. Further, in intact cells, a mono-phosphopeptide does not stimulate PI3K activity, which contradicts current dogma.

In addition to its role as a regulatory subunit, non-catalytic roles of p85 include complexing Rab4 and Rab5, stabilizing PTEN, invadopodium formation, cytokinesis, and nuclear trafficking of proteins such as XBP-1 and OPNi. Phosphopeptide mimics targeted to the SH2 domain of p85 have potential applications both in blocking PI3K signaling as well as in modulating non-catalytic functions.

Citation Format: Pijus K. Mandal, Yanhua Yao, Anne R. Bresnick, Jonathan M. Backer, John S. McMurray. Peptide prodrugs targeting the SH2 domains of p85 block PI3K signaling. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4838.

Abstract A13: Macrophage FLT1 mediated inflammatory response determines breast cancer distal metastasis

Macrophages are abundantly found in the tumor microenvironment and enhance malignancy. At distal metastatic sites, our previous studies identified a distinct population of metastasis associated macrophages (MAMs) that promotes tumor cell extravasation, seeding and persistent growth. These macrophages were derived from circulating inflammatory monocytes recruited by CCL2/CCR2 chemokine signaling and directly promote tumor cell extravasation and metastatic seeding in vivo through VEGF production. Our recent studies identified that MAMs express high levels of cell surface FMS-like tyrosine kinase 1 (FLT1, also known as VEGFR1) after their recruitment. Blockade of FLT1 signaling using specific inhibitory antibodies significantly inhibited the metastatic seeding and persistent growth. Using several genetic models of Flt1 deficiency, we show that macrophage specific FLT1 signaling is critical for breast tumor distal metastatic potential. FLT1 is not expressed by other hematopoietic cells and its inhibition did not affect the recruitment of MAMs, which indicated that specific FLT1 signaling in MAMs are important for their metastasis promoting functions. Indeed, we identified that FLT1 regulates a set of inflammatory response genes including Colony Stimulating Factor 1 (CSF1) a central regulator of macrophage biology. Using a genetic gain-of-function approach we show that CSF1 mediated autocrine signaling in MAMs is downstream of FLT1 and can restore the tumor-promoting activity in MAMs even when FLT1 has been inhibited. Together, our data established a link between inflammation and cancer metastasis and suggested the therapeutic potential of targeting these pathways in treating metastatic disease.

Citation Format: Bin-Zhi Qian, Hui Zhang, Jiufeng Li, Eun-Jin Yeo, Neil O. Carragher, Anne R. Bresnick, Richard A. Lang, Jeffrey W. Pollard. Macrophage FLT1 mediated inflammatory response determines breast cancer distal metastasis. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Metastasis; 2015 Nov 30-Dec 3; Austin, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(7 Suppl):Abstract nr A13.

GPCR Signaling Mediates Tumor Metastasis via PI3Kβ

Inappropriate activation of PI3K signaling has been implicated strongly in human cancer. Although studies on the role of PI3K signaling in breast tumorigenesis and progression have focused most intensively on PI3Kα, a role for PI3Kβ has begun to emerge. The PI3Kβ isoform is unique among class IA PI3K enzymes in that it is activated by both receptor tyrosine kinases and G-protein-coupled receptors (GPCR). In previous work, we identified a mutation that specifically abolishes PI3Kβ binding to Gβγ (p110(526KK-DD)). Expression of this mutant in p110β-silenced breast cancer cells inhibits multiple steps of the metastatic cascade in vitro and in vivo and causes a cell autonomous defect in invadopodial matrix degradation. Our results identify a novel link between GPCRs and PI3Kβ in mediating metastasis, suggesting that disruption of this link might offer a novel therapeutic target to prevent the development of metastatic disease. Cancer Res; 76(10); 2944-53. ©2016 AACR.

Identification of the Rab5 binding site in p110β: assays for PI3Kβ binding to Rab5

Isoform-specific signaling by Class IA PI 3-kinases depends in part on the interactions between distinct catalytic subunits and upstream regulatory proteins. From among the class IA catalytic subunits (p110α, p110β, and p110δ), p110β has unique properties. Unlike the other family members, p110β directly binds to Gβγ subunits, downstream from activated G-protein coupled receptors, and to activated Rab5. Furthermore, the Ras-binding domain (RBD) of p110β binds to Rac and Cdc42 but not to Ras. Defining mutations that specifically disrupt these regulatory interactions is critical for defining their role in p110β signaling. This chapter describes the approach that was used to identify the Rab5 binding site in p110β, and discusses methods for the analysis of p110β-Rab5 interactions.

Assembly and Molecular Architecture of the Phosphoinositide 3-Kinase p85α Homodimer

Phosphoinositide 3-kinases (PI3Ks) are a family of lipid kinases that are activated by growth factor and G-protein-coupled receptors and propagate intracellular signals for growth, survival, proliferation, and metabolism. p85α, a modular protein consisting of five domains, binds and inhibits the enzymatic activity of class IA PI3K catalytic subunits. Here, we describe the structural states of the p85α dimer, based on data from in vivo and in vitro solution characterization. Our in vitro assembly and structural analyses have been enabled by the creation of cysteine-free p85α that is functionally equivalent to native p85α. Analytical ultracentrifugation studies showed that p85α undergoes rapidly reversible monomer-dimer assembly that is highly exothermic in nature. In addition to the documented SH3-PR1 dimerization interaction, we identified a second intermolecular interaction mediated by cSH2 domains at the C-terminal end of the polypeptide. We have demonstrated in vivo concentration-dependent dimerization of p85α using fluorescence fluctuation spectroscopy. Finally, we have defined solution conditions under which the protein is predominantly monomeric or dimeric, providing the basis for small angle x-ray scattering and chemical cross-linking structural analysis of the discrete dimer. These experimental data have been used for the integrative structure determination of the p85α dimer. Our study provides new insight into the structure and assembly of the p85α homodimer and suggests that this protein is a highly dynamic molecule whose conformational flexibility allows it to transiently associate with multiple binding proteins.

FLT1 signaling in metastasis-associated macrophages activates an inflammatory signature that promotes breast cancer metastasis

Although the link between inflammation and cancer initiation is well established, its role in metastatic diseases, the primary cause of cancer deaths, has been poorly explored. Our previous studies identified a population of metastasis-associated macrophages (MAMs) recruited to the lung that promote tumor cell seeding and growth. Here we show that FMS-like tyrosine kinase 1 (Flt1, also known as VEGFR1) labels a subset of macrophages in human breast cancers that are significantly enriched in metastatic sites. In mouse models of breast cancer pulmonary metastasis, MAMs uniquely express FLT1. Using several genetic models, we show that macrophage FLT1 signaling is critical for metastasis. FLT1 inhibition does not affect MAM recruitment to metastatic lesions but regulates a set of inflammatory response genes, including colony-stimulating factor 1 (CSF1), a central regulator of macrophage biology. Using a gain-of-function approach, we show that CSF1-mediated autocrine signaling in MAMs is downstream of FLT1 and can restore the tumor-promoting activity of FLT1-inhibited MAMs. Thus, CSF1 is epistatic to FLT1, establishing a link between FLT1 and inflammatory responses within breast tumor metastases. Importantly, FLT1 inhibition reduces tumor metastatic efficiency even after initial seeding, suggesting that these pathways represent therapeutic targets in metastatic disease.

S100 proteins in cancer

In humans, the S100 protein family is composed of 21 members that exhibit a high degree of structural similarity, but are not functionally interchangeable. This family of proteins modulates cellular responses by functioning both as intracellular Ca(2+) sensors and as extracellular factors. Dysregulated expression of multiple members of the S100 family is a common feature of human cancers, with each type of cancer showing a unique S100 protein profile or signature. Emerging in vivo evidence indicates that the biology of most S100 proteins is complex and multifactorial, and that these proteins actively contribute to tumorigenic processes such as cell proliferation, metastasis, angiogenesis and immune evasion. Drug discovery efforts have identified leads for inhibiting several S100 family members, and two of the identified inhibitors have progressed to clinical trials in patients with cancer. This Review highlights new findings regarding the role of S100 family members in cancer diagnosis and treatment, the contribution of S100 signalling to tumour biology, and the discovery and development of S100 inhibitors for treating cancer.

S100 proteins in cancer

In humans, the S100 protein family is composed of 21 members that exhibit a high degree of structural similarity, but are not functionally interchangeable. This family of proteins modulates cellular responses by functioning both as intracellular Ca(2+) sensors and as extracellular factors. Dysregulated expression of multiple members of the S100 family is a common feature of human cancers, with each type of cancer showing a unique S100 protein profile or signature. Emerging in vivo evidence indicates that the biology of most S100 proteins is complex and multifactorial, and that these proteins actively contribute to tumorigenic processes such as cell proliferation, metastasis, angiogenesis and immune evasion. Drug discovery efforts have identified leads for inhibiting several S100 family members, and two of the identified inhibitors have progressed to clinical trials in patients with cancer. This Review highlights new findings regarding the role of S100 family members in cancer diagnosis and treatment, the contribution of S100 signalling to tumour biology, and the discovery and development of S100 inhibitors for treating cancer.

The heavy chain has its day: regulation of myosin-II assembly

Nonmuscle myosin-II is an actin-based motor that converts chemical energy into force and movement, and thus functions as a key regulator of the eukaryotic cytoskeleton. Although it is established that phosphorylation on the regulatory light chain increases the actin-activated MgATPase activity of the motor and promotes myosin-II filament assembly, studies have begun to characterize alternative mechanisms that regulate filament assembly and disassembly. These investigations have revealed that all three nonmuscle myosin-II isoforms are subject to additional regulatory controls, which impact diverse cellular processes. In this review, we discuss current knowledge on mechanisms that regulate the oligomerization state of nonmuscle myosin-II filaments by targeting the myosin heavy chain.

Structure of the S100A4/myosin-IIA complex

Background

S100A4, a member of the S100 family of Ca²⁺-binding proteins, modulates the motility of both non-transformed and cancer cells by regulating the localization and stability of cellular protrusions. Biochemical studies have demonstrated that S100A4 binds to the C-terminal end of the myosin-IIA heavy chain coiled-coil and disassembles myosin-IIA filaments; however, the mechanism by which S100A4 mediates myosin-IIA depolymerization is not well understood.

Results

We determined the X-ray crystal structure of the S100A4Δ8C/MIIA^1908-1923 peptide complex, which showed an asymmetric binding mode for the myosin-IIA peptide across the S100A4 dimer interface. This asymmetric binding mode was confirmed in NMR studies using a spin-labeled myosin-IIA peptide. In addition, our NMR data indicate that S100A4Δ8C binds the MIIA^1908-1923 peptide in an orientation very similar to that observed for wild-type S100A4. Studies of complex formation using a longer, dimeric myosin-IIA construct demonstrated that S100A4 binding dissociates the two myosin-IIA polypeptide chains to form a complex composed of one S100A4 dimer and a single myosin-IIA polypeptide chain. This interaction is mediated, in part, by the instability of the region of the myosin-IIA coiled-coil encompassing the S100A4 binding site.

Conclusion

The structure of the S100A4/MIIA^1908-1923 peptide complex has revealed the overall architecture of this assembly and the detailed atomic interactions that mediate S100A4 binding to the myosin-IIA heavy chain. These structural studies support the idea that residues 1908–1923 of the myosin-IIA heavy chain represent a core sequence for the S100A4/myosin-IIA complex. In addition, biophysical studies suggest that structural fluctuations within the myosin-IIA coiled-coil may facilitate S100A4 docking onto a single myosin-IIA polypeptide chain.

G protein-coupled receptor-mediated activation of p110β by Gβγ is required for cellular transformation and invasiveness

Synergistic activation by heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs) and receptor tyrosine kinases distinguishes p110β from other class IA phosphoinositide 3-kinases (PI3Ks). Activation of p110β is specifically implicated in various physiological and pathophysiological processes, such as the growth of tumors deficient in phosphatase and tensin homolog deleted from chromosome 10 (PTEN). To determine the specific contribution of GPCR signaling to p110β-dependent functions, we identified the site in p110β that binds to the Gβγ subunit of G proteins. Mutation of this site eliminated Gβγ-dependent activation of PI3Kβ (a dimer of p110β and the p85 regulatory subunit) in vitro and in cells, without affecting basal activity or phosphotyrosine peptide-mediated activation. Disrupting the p110β-Gβγ interaction by mutation or with a cell-permeable peptide inhibitor blocked the transforming capacity of PI3Kβ in fibroblasts and reduced the proliferation, chemotaxis, and invasiveness of PTEN-null tumor cells in culture. Our data suggest that specifically targeting GPCR signaling to PI3Kβ could provide a therapeutic approach for tumors that depend on p110β for growth and metastasis.

Abstract 2145: Coupling S100A4 to Rhotekin alters Rho signaling output in breast cancer cells

S100A4 is a calcium binding protein and tumor metastasis associated factor that has been suggested to promote motility and invasiveness of different types of cancer. This migratory promoting effect, in part, is due to the interaction of S100A4 with actin and actin binding proteins, such as myosin IIA. We found that S100A4 interacts with Rho binding domain of Rhotekin (TRBD), one of the Rho effectors, thus suggesting a connection between the Rho and S100A4 pathways. To test whether this interaction is specific, we performed GST pull-down assays using Rho binding domain from different Rho effectors such as Rhotekin, Rhophilin, PKN, ROCK II, and Citron. These results showed that S100A4 specifically interacts with Rho binding domain of Rhotekin but not of other Rho effectors. We further determined that this interaction is direct and calcium-dependant by performing GST-TRBD pull-down assays with purified S100A4 in the presence and absence of calcium. The in vivo interaction of these two proteins was further confirmed by immunoprecipitation of exogenously expressed mutated forms of Rhotekin with endogenous S100A4 from MDA-MB-231 cells. Confocal microscopy showed that Rhotekin and S100A4 co-localize at the edge of membrane ruffles in response to EGF. We demonstrated that S100A4 did not bind TRBD using the same residues as Rho, as determined by using a triple-A mutant of Rhotekin. Interestingly, we found that active Rho, S100A4 and Rhotekin are in a complex by using constitutively active GST-RhoQ63L pull-down assay. To examine the function of this interaction, RNAi was used to suppress Rhotekin and/or S100A4 in MDA-MB-231. F-actin staining showed that suppression of both RTKN and S100A4 leads to the loss of Rho-dependent membrane ruffling in response to EGF stimulation, an increase in contractile F-actin “stress” fibers in the cell body and a reduction in invasive growth in three-dimensional culture. Taken together, we showed that S100A4 specifically and directly interacts with Rhotekin in a calcium-dependent manner. This coupling permits S100A4 to complex with RhoA and switch RhoA function from stress fiber formation to membrane ruffling. Based on the function of this interaction, we propose that interaction of S100A4 and Rhotekin alters the functional output of Rho signaling to confer an invasive phenotype in breast cancer cells.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2145. doi:1538-7445.AM2012-2145

Direct inhibition of myosin II effectively blocks glioma invasion in the presence of multiple motogens

Anaplastic gliomas, the most common and malignant of primary brain tumors, frequently contain activating mutations and amplifications in promigratory signal transduction pathways. However, targeting these pathways with individual signal transduction inhibitors does not appreciably reduce tumor invasion, because these pathways are redundant; blockade of any one pathway can be overcome by stimulation of another. This implies that a more effective approach would be to target a component at which these pathways converge. In this study, we have investigated whether the molecular motor myosin II represents such a target by examining glioma invasion in a series of increasingly complex models that are sensitive to platelet-derived growth factor, epidermal growth factor, or both. Our results lead to two conclusions. First, malignant glioma cells are stimulated to invade brain through the activation of multiple signaling cascades not accounted for in simple in vitro assays. Second, even though there is a high degree of redundancy in promigratory signaling cascades in gliomas, blocking tumor invasion by directly targeting myosin II remains effective. Our results thus support our hypothesis that myosin II represents a point of convergence for signal transduction pathways that drive glioma invasion and that its inhibition cannot be overcome by other motility mechanisms.

The suppression of myosin light chain (MLC) phosphorylation during the response to lipopolysaccharide (LPS): beneficial or detrimental to endothelial barrier?

Sepsis-induced vascular leakage is a major underlying cause of the respiratory dysfunction seen in severe sepsis. Here, we studied the role of MLC phosphorylation in LPS-induced endothelial hyperpermeability and assessed how the changes in phospho-MLC distribution affect LPS-induced barrier dysfunction. We demonstrated that the changes in human lung microvascular endothelial permeability are preceded by the increase in intracellular calcium level, and increase in MYPT and MLC phosphorylation. Using the siRNA approach, we showed that both LPS-induced barrier dysfunction and MLC phosphorylation are attenuated by the depletion of the smooth muscle isoform of MLC kinase (MLCK) and Rho kinase 2 (ROCK2). Surprisingly, pharmacological inhibition of both ROCK1 and 2 with Y-27632 exacerbated LPS-induced drop in transendothelial resistance, although significantly decreasing MLC phosphorylation level. We next studied the involvement of protein kinase A (PKA)-dependent pathways in LPS-induced barrier dysfunction. We showed that LPS decreased the level of PKA-dependent phosphorylation in endothelial cells; and the pretreatment with forskolin or PKA activator bnz-cAMP counteracted this effect. Forskolin and bnz-cAMP also attenuated LPS-induced increase in MLC phosphorylation level. As we have shown earlier (Bogatcheva et al., 2009), forskolin and bnz-cAMP provide protection from LPS-induced barrier dysfunction. We compared the effects of bnz-cAMP and Y-27632 on phospho-MLC distribution and observed that while bnz-cAMP increased the association of the phospho-MLC signal with the cortical structures, Y-27632 decreased this association. These data indicate that an overall decrease in MLC phosphorylation could be either beneficial or detrimental to endothelial barrier, depending on the intracellular locale of major phospho-MLC changes.

Cysteine 81 is critical for the interaction of S100A4 and myosin-IIA

Overexpression of S100A4, a member of the S100 family of Ca(2+)-binding proteins, is associated with a number of human pathologies, including fibrosis, inflammatory disorders, and metastatic disease. The identification of small molecules that disrupt S100A4/target interactions provides a mechanism for inhibiting S100A4-mediated cellular activities and their associated pathologies. Using an anisotropy assay that monitors the Ca(2+)-dependent binding of myosin-IIA to S100A4, NSC 95397 was identified as an inhibitor that disrupts the S100A4/myosin-IIA interaction and inhibits S100A4-mediated depolymerization of myosin-IIA filaments. Mass spectrometry demonstrated that NSC 95397 forms covalent adducts with Cys81 and Cys86, which are located in the canonical target binding cleft. Mutagenesis studies showed that covalent modification of just Cys81 is sufficient to inhibit S100A4 function with respect to myosin-IIA binding and depolymerization. Remarkably, substitution of Cys81 with serine or alanine significantly impaired the ability of S100A4 to promote myosin-IIA filament disassembly. As reversible covalent cysteine modifications have been observed for several S100 proteins, we propose that modification of Cys81 may provide an additional regulatory mechanism for mediating the binding of S100A4 to myosin-IIA.

Two functional S100A4 monomers are necessary for regulating nonmuscle myosin-IIA and HCT116 cell invasion

S100A4, a member of the Ca(2+)-activated S100 protein family, regulates the motility and invasiveness of cancer cells. Moreover, high S100A4 expression levels correlate with poor patient survival in several cancers. Although biochemical, biophysical, and structural data indicate that S100A4 is a noncovalent dimer, it is unknown if two functional S100A4 monomers are required for the productive recognition of protein targets and the promotion of cell invasion. To address this question, we created covalently linked S100A4 dimers using a glycine rich flexible linker. The single-chain S100A4 (sc-S100A4) proteins exhibited wild-type affinities for calcium and nonmuscle myosin-IIA, retained the ability to regulate nonmuscle myosin-IIA assembly, and promoted tumor cell invasion when expressed in S100A4-deficient colon carcinoma cells. Mutation of the two calcium-binding EF-hands in one monomer, while leaving the other monomer intact, caused a 30-60-fold reduction in binding affinity for nonmuscle myosin-IIA concomitant with a weakened ability to regulate the monomer-polymer equilibrium of nonmuscle myosin-IIA. Moreover, sc-S100A4 proteins with one monomer deficient in calcium responsiveness did not support S100A4-mediated colon carcinoma cell invasion. Cross-linking and titration data indicate that the S100A4 dimer binds a single myosin-IIA target peptide. These data are consistent with a model in which a single peptide forms interactions in the vicinity of the canonical target binding cleft of each monomer in such a manner that both target binding sites are required for the efficient interaction with myosin-IIA.

Mena invasive (MenaINV) promotes multicellular streaming motility and transendothelial migration in a mouse model of breast cancer

We have shown previously that distinct Mena isoforms are expressed in invasive and migratory tumor cells in vivo and that the invasion isoform (Mena(INV)) potentiates carcinoma cell metastasis in murine models of breast cancer. However, the specific step of metastatic progression affected by this isoform and the effects on metastasis of the Mena11a isoform, expressed in primary tumor cells, are largely unknown. Here, we provide evidence that elevated Mena(INV) increases coordinated streaming motility, and enhances transendothelial migration and intravasation of tumor cells. We demonstrate that promotion of these early stages of metastasis by Mena(INV) is dependent on a macrophage-tumor cell paracrine loop. Our studies also show that increased Mena11a expression correlates with decreased expression of colony-stimulating factor 1 and a dramatically decreased ability to participate in paracrine-mediated invasion and intravasation. Our results illustrate the importance of paracrine-mediated cell streaming and intravasation on tumor cell dissemination, and demonstrate that the relative abundance of Mena(INV) and Mena11a helps to regulate these key stages of metastatic progression in breast cancer cells.

S100A4 regulates macrophage chemotaxis

Using a targeted genetic deletion, we show that the S100A4 metastasis factor is required for macrophage recruitment to sites of inflammation in vivo. S100A4^−/− primary macrophages display defects in chemotaxis due to myosin-IIA overassembly and altered CSF-1 receptor signaling. These studies establish S100A4 as a regulator of macrophage motility.

S100A4, a member of the S100 family of Ca²⁺-binding proteins, is directly involved in tumor metastasis. In addition to its expression in tumor cells, S100A4 is expressed in normal cells and tissues, including fibroblasts and cells of the immune system. To examine the contribution of S100A4 to normal physiology, we established S100A4-deficient mice by gene targeting. Homozygous S100A4^−/− mice are fertile, grow normally and exhibit no overt abnormalities; however, the loss of S100A4 results in impaired recruitment of macrophages to sites of inflammation in vivo. Consistent with these observations, primary bone marrow macrophages (BMMs) derived from S100A4^−/− mice display defects in chemotactic motility in vitro. S100A4^−/− BMMs form unstable protrusions, overassemble myosin-IIA, and exhibit altered colony-stimulating factor-1 receptor signaling. These studies establish S100A4 as a regulator of physiological macrophage motility and demonstrate that S100A4 mediates macrophage recruitment and chemotaxis in vivo.

Differential enhancement of breast cancer cell motility and metastasis by helical and kinase domain mutations of class IA phosphoinositide 3-kinase

Class IA (p85/p110) phosphoinositide 3-kinases play a major role in regulating cell growth, survival, and motility. Activating mutations in the p110alpha isoform of the class IA catalytic subunit (PIK3CA) are commonly found in human cancers. These mutations lead to increased proliferation and transformation in cultured cells, but their effects on cell motility and tumor metastasis have not been evaluated. We used lentiviral-mediated gene transfer and knockdown to produce stable MDA-MB-231 cells in which the endogenous human p110alpha is replaced with either wild-type bovine p110alpha or the two most common activating p110alpha mutants, the helical domain mutant E545K and the kinase domain mutant H1047R. The phosphoinositide 3-kinase/Akt pathway was hyperactivated in cells expressing physiologic levels of helical or kinase domain mutants. Cells expressing either mutant showed increased motility in vitro, but only cells expressing the helical domain mutant showed increased directionality in a chemotaxis assay. In severe combined immunodeficient mice, xenograft tumors expressing either mutant showed increased rates of tumor growth compared with tumors expressing wild-type p110alpha. However, tumors expressing the p110alpha helical domain mutant showed a marked increase in both tumor cell intravasation into the blood and tumor cell extravasation into the lung after tail vein injection compared with tumors expressing wild-type p110alpha or the kinase domain mutant. Our observations suggest that, when compared with kinase domain mutations in a genetically identical background, expression of helical domain mutants of p110alpha produce a more severe metastatic phenotype.

Integrin alpha6beta4 controls the expression of genes associated with cell motility, invasion, and metastasis, including S100A4/metastasin

The integrin alpha6beta4 is associated with carcinoma progression by contributing to apoptosis resistance, invasion, and metastasis, due in part to the activation of select transcription factors. To identify genes regulated by the alpha6beta4 integrin, we compared gene expression profiles of MDA-MB-435 cells that stably express integrin alpha6beta4 (MDA/beta4) and vector-only-transfected cells (MDA/mock) using Affymetrix GeneChip analysis. Our results show that integrin alpha6beta4 altered the expression of 538 genes (p < 0.01). Of these genes, 36 are associated with pathways implicated in cell motility and metastasis, including S100A4/metastasin. S100A4 expression correlated well with integrin alpha6beta4 expression in established cell lines. Suppression of S100A4 by small interference RNA resulted in a reduced capacity of alpha6beta4-expressing cells to invade a reconstituted basement membrane in response to lysophosphatidic acid. Using small interference RNA, promoter analysis, and chromatin immunoprecipitation, we demonstrate that S100A4 is regulated by NFAT5, thus identifying the first target of NFAT5 in cancer. In addition, several genes that are known to be regulated by DNA methylation were up-regulated dramatically by integrin alpha6beta4 expression, including S100A4, FST, PDLIM4, CAPG, and Nkx2.2. Notably, inhibition of DNA methyltransferases stimulated expression of these genes in cells lacking the alpha6beta4 integrin, whereas demethylase inhibitors suppressed expression in alpha6beta4 integrin-expressing cells. Alterations in DNA methylation were confirmed by bisulfate sequencing, thus suggesting that integrin alpha6beta4 signaling can lead to the demethylation of select promoters. In summary, our data suggest that integrin alpha6beta4 confers a motile and invasive phenotype to breast carcinoma cells by regulating proinvasive and prometastatic gene expression.

Rap1 activation in collagen phagocytosis is dependent on nonmuscle myosin II-A

Rap1 enhances integrin-mediated adhesion but the link between Rap1 activation and integrin function in collagen phagocytosis is not defined. Mass spectrometry of Rap1 immunoprecipitates showed that the association of Rap1 with nonmuscle myosin heavy-chain II-A (NMHC II-A) was enhanced by cell attachment to collagen beads. Rap1 colocalized with NM II-A at collagen bead-binding sites. There was a transient increase in myosin light-chain phosphorylation after collagen-bead binding that was dependent on myosin light-chain kinase but not Rho kinase. Inhibition of myosin light-chain phosphorylation, but not myosin II-A motor activity inhibited collagen-bead binding and Rap activation. In vitro binding assays demonstrated binding of Rap1A to filamentous myosin rods, and in situ staining of permeabilized cells showed that NM II-A filaments colocalized with F-actin at collagen bead sites. Knockdown of NM II-A did not affect talin, actin, or beta1-integrin targeting to collagen beads but targeting of Rap1 and vinculin to collagen was inhibited. Conversely, knockdown of Rap1 did not affect localization of NM II-A to beads. We conclude that MLC phosphorylation in response to initial collagen-bead binding promotes NM II-A filament assembly; binding of Rap1 to myosin filaments enables Rap1-dependent integrin activation and enhanced collagen phagocytosis.

Structure of Ca2+-bound S100A4 and its interaction with peptides derived from nonmuscle myosin-IIA

S100A4, also known as mts1, is a member of the S100 family of Ca2+-binding proteins that is directly involved in tumor invasion and metastasis via interactions with specific protein targets, including nonmuscle myosin-IIA (MIIA). Human S100A4 binds two Ca2+ ions with the typical EF-hand exhibiting an affinity that is nearly 1 order of magnitude tighter than that of the pseudo-EF-hand. To examine how Ca2+ modifies the overall organization and structure of the protein, we determined the 1.7 A crystal structure of the human Ca2+-S100A4. Ca2+ binding induces a large reorientation of helix 3 in the typical EF-hand. This reorganization exposes a hydrophobic cleft that is comprised of residues from the hinge region,helix 3, and helix 4, which afford specific target recognition and binding. The Ca2+-dependent conformational change is required for S100A4 to bind peptide sequences derived from the C-terminal portion of the MIIA rod with submicromolar affinity. In addition, the level of binding of Ca2+ to both EF-hands increases by 1 order of magnitude in the presence of MIIA. NMR spectroscopy studies demonstrate that following titration with a MIIA peptide, the largest chemical shift perturbations and exchange broadening effects occur for residues in the hydrophobic pocket of Ca2+-S100A4. Most of these residues are not exposed in apo-S100A4 and explain the Ca2+ dependence of formation of theS100A4-MIIA complex. These studies provide the foundation for understanding S100A4 target recognition and may support the development of reagents that interfere with S100A4 function.

Visual snapshots of intracellular kinase activity at the onset of mitosis

Visual snapshots of intracellular kinase activity can be acquired with exquisite temporal control by using a light-activatable (caged) sensor, thereby providing a means to interrogate enzymatic activity at any point during the cell-division cycle. Robust protein kinase activity transpires just prior to, but not immediately after, nuclear envelope breakdown (NEB). Furthermore, kinase activity is required for the progression from prophase into metaphase. Finally, the application of selective protein kinase C (PKC) inhibitors, in combination with the caged sensor, correlates the action of the PKC beta isoform with subsequent NEB.

A biosensor of S100A4 metastasis factor activation: inhibitor screening and cellular activation dynamics

S100A4, a member of the S100 family of Ca2+-binding proteins, displays elevated expression in malignant human tumors compared with benign tumors, and increased expression correlates strongly with poor patient survival. S100A4 has a direct role in metastatic progression, likely due to the modulation of actomyosin cytoskeletal dynamics, which results in increased cellular motility. We developed a fluorescent biosensor (Mero-S100A4) that reports on the Ca2+-bound, activated form of S100A4. Direct attachment of a novel solvatochromatic reporter dye to S100A4 results in a sensor that, upon activation, undergoes a 3-fold enhancement in fluorescence, thus providing a sensitive assay for use in vitro and in vivo. In cells, localized activation of S100A4 at the cell periphery is observed during random migration and following stimulation with lysophosphatidic acid, a known activator of cell motility and proliferation. Additionally, a screen against a library of FDA-approved drugs with the biosensor identified an array of phenothiazines as inhibitors of myosin-II associated S100A4 function. These data demonstrate the utility of the new biosensor both for drug discovery and for probing the cellular dynamics controlled by the S100A4 metastasis factor.

Myosin-IIA heavy-chain phosphorylation regulates the motility of MDA-MB-231 carcinoma cells

In mammalian nonmuscle cells, the mechanisms controlling the localized formation of myosin-II filaments are not well defined. To investigate the mechanisms mediating filament assembly and disassembly during generalized motility and chemotaxis, we examined the EGF-dependent phosphorylation of the myosin-IIA heavy chain in human breast cancer cells. EGF stimulation of MDA-MB-231 cells resulted in transient increases in both the assembly and phosphorylation of the myosin-IIA heavy chains. In EGF-stimulated cells, the myosin-IIA heavy chain is phosphorylated on the casein kinase 2 site (S1943). Cells expressing green fluorescent protein-myosin-IIA heavy-chain S1943E and S1943D mutants displayed increased migration into a wound and enhanced EGF-stimulated lamellipod extension compared with cells expressing wild-type myosin-IIA. In contrast, cells expressing the S1943A mutant exhibited reduced migration and lamellipod extension. These observations support a direct role for myosin-IIA heavy-chain phosphorylation in mediating motility and chemotaxis.

Increase in production of matrix metalloproteinase 13 by human articular chondrocytes due to stimulation with S100A4: Role of the receptor for advanced glycation end products

OBJECTIVE

S100 proteins have been implicated in various inflammatory conditions, including arthritis. The aims of this study were to determine whether chondrocytes produce S100A4 and whether S100A4 can stimulate the production of matrix metalloproteinase 13 (MMP-13) by articular chondrocytes via receptor for advanced glycation end products (RAGE)-mediated signaling.

METHODS

The expression of chondrocyte S100A4 was analyzed by immunohistochemistry using normal and osteoarthritic (OA) cartilage and by immunoblotting of chondrocyte cell lysates. RAGE signaling was examined by stimulating chondrocytes with S100A4 and monitoring for the activation of MAP kinases and NF-kappaB. Production of MMP-13 was determined in the conditioned medium. A pulldown assay using biotin-labeled S100A4 was used to demonstrate binding to RAGE.

RESULTS

S100A4 expression was detected in human articular chondrocytes by immunoblotting and appeared to increase in the cell lysates from OA tissue. Marked positive immunostaining for S100A4 was also noted in sections of human cartilage with changes due to OA. Stimulation of chondrocytes with S100A4 increased the phosphorylation of Pyk-2, MAP kinases, and activated NF-kappaB, followed by increased production of MMP-13 in the conditioned medium. This signaling was inhibited in cells pretreated with soluble RAGE, advanced glycation end product-bovine serum albumin, or the antioxidant Mn(III)tetrakis (4-benzoic acid) porphyrin, or by overexpression of a dominant-negative RAGE construct. A pulldown assay showed that S100A4 binds to RAGE in chondrocytes.

CONCLUSION

This is the first study to demonstrate that S100A4 binds to RAGE and stimulates a RAGE-mediated signaling cascade, leading to increased production of MMP-13. Since both S100A4 and RAGE are up-regulated in OA cartilage, this signaling pathway could contribute to cartilage degradation in OA.

A global, myosin light chain kinase-dependent increase in myosin II contractility accompanies the metaphase-anaphase transition in sea urchin eggs

Myosin II is the force-generating motor for cytokinesis, and although it is accepted that myosin contractility is greatest at the cell equator, the temporal and spatial cues that direct equatorial contractility are not known. Dividing sea urchin eggs were placed under compression to study myosin II-based contractile dynamics, and cells manipulated in this manner underwent an abrupt, global increase in cortical contractility concomitant with the metaphase-anaphase transition, followed by a brief relaxation and the onset of furrowing. Prefurrow cortical contractility both preceded and was independent of astral microtubule elongation, suggesting that the initial activation of myosin II preceded cleavage plane specification. The initial rise in contractility required myosin light chain kinase but not Rho-kinase, but both signaling pathways were required for successful cytokinesis. Last, mobilization of intracellular calcium during metaphase induced a contractile response, suggesting that calcium transients may be partially responsible for the timing of this initial contractile event. Together, these findings suggest that myosin II-based contractility is initiated at the metaphase-anaphase transition by Ca2+-dependent myosin light chain kinase (MLCK) activity and is maintained through cytokinesis by both MLCK- and Rho-dependent signaling. Moreover, the signals that initiate myosin II contractility respond to specific cell cycle transitions independently of the microtubule-dependent cleavage stimulus.

The S100A4 metastasis factor regulates cellular motility via a direct interaction with myosin-IIA

S100A4, a member of the Ca(2+)-dependent S100 family of proteins, is a metastasis factor that is thought to regulate the motility and invasiveness of cancer cells. Previously, we showed that S100A4 specifically binds to nonmuscle myosin-IIA and promotes the unassembled state. S100A4, thus, provides a connection between the actomyosin cytoskeleton and the regulation of cellular motility; however, the step or steps in the motility cycle that are affected by S100A4 expression have not been investigated. To examine how the biochemical properties of S100A4 affect cell motility, we determined the effect of S100A4 expression on protrusive behavior during chemoattractant-stimulated motility. Our studies show that S100A4 modulates cellular motility by affecting cell polarization, with S100A4 expressing cells displaying few side protrusions and extensive forward protrusions during chemotaxis compared with control cells. To establish a direct link between S100A4 and the regulation of myosin-IIA function, we prepared an antibody to the S100A4 binding site on the myosin-IIA heavy chain that has comparable effects on myosin-IIA assembly as S100A4. Microinjection experiments show that the antibody elicits the same effects on cell polarization as S100A4. Our studies show for the first time that S100A4 promotes directional motility via a direct interaction with myosin-IIA. These findings establish S100A4 as a critical regulator of myosin-II function and metastasis-associated motility.

The activity status of cofilin is directly related to invasion, intravasation, and metastasis of mammary tumors

Understanding the mechanisms controlling cancer cell invasion and metastasis constitutes a fundamental step in setting new strategies for diagnosis, prognosis, and therapy of metastatic cancers. LIM kinase1 (LIMK1) is a member of a novel class of serine–threonine protein kinases. Cofilin, a LIMK1 substrate, is essential for the regulation of actin polymerization and depolymerization during cell migration. Previous studies have made opposite conclusions as to the role of LIMK1 in tumor cell motility and metastasis, claiming either an increase or decrease in cell motility and metastasis as a result of LIMK1 over expression (Zebda, N., O. Bernard, M. Bailly, S. Welti, D.S. Lawrence, and J.S. Condeelis. 2000. J. Cell Biol. 151:1119–1128; Davila, M., A.R. Frost, W.E. Grizzle, and R. Chakrabarti. 2003. J. Biol. Chem. 278:36868–36875; Yoshioka, K., V. Foletta, O. Bernard, and K. Itoh. 2003. Proc. Natl. Acad. Sci. USA. 100:7247–7252; Nishita, M., C. Tomizawa, M. Yamamoto, Y. Horita, K. Ohashi, and K. Mizuno. 2005. J. Cell Biol. 171:349–359). We resolve this paradox by showing that the effects of LIMK1 expression on migration, intravasation, and metastasis of cancer cells can be most simply explained by its regulation of the output of the cofilin pathway. LIMK1-mediated decreases or increases in the activity of the cofilin pathway are shown to cause proportional decreases or increases in motility, intravasation, and metastasis of tumor cells.