Abstract P2-05-01: Chemotherapy-induced metastasis: Mechanisms and translational opportunities

1 Department of Anatomy and Structural Biology, Albert Einstein College of Medicine, NY, USA

2 Integrated Imaging Program, Albert Einstein College of Medicine, NY, USA

3 Gruss-Lipper Biophotonics Center, Albert Einstein College of Medicine, NY, USA

4 Department of Pathology, Albert Einstein College of Medicine, NY, USA

Background: Chemotherapy has become essential in the care of patients with advanced breast cancer. However, as we have previously discovered, chemotherapy may induce pro-metastatic changes in the breast cancer microenvironment by promoting the assembly and function of cancer cell intravasation sites called tumor microenvironment of metastasis (TMEM), and by increasing the proportion of the highly-invasive MenaINV-HI tumor cells that utilize TMEM sites for hematogenous dissemination. Consequently, chemotherapy-treated animals demonstrate increased numbers of circulating-tumor cells and micrometastases. Since the formation of metastases depends on both the presence of functional doorways for dissemination (TMEM) and cancer cells capable of using these doorways (MenaINV-HI tumor cells), we investigated the cellular and molecular contexts required for chemotherapy-mediated induction of TMEM function and MenaINV expression. Since TMEM function depends on Tie2 expressing macrophages and MenaINV expression is inducible by cancer cell -macrophage contact, we focused on inhibiting Tie2 receptors and chemotherapy-induced macrophage influx to inhibit chemotherapy-induced metastasis.

Materials and Methods: We used spontaneous models of mouse breast carcinoma (MMTV-PyMT) and patient-derived xenografts treated with chemotherapy with or without co-treatment with either Tie2 inhibitor (rebastinib), inhibitors of Cxcr4+ macrophage chemotaxis (Cxcl12 inhibitors) or macrophage depletion agents (clodronate). Pro-metastatic endpoints were measured by intravital imaging, multichannel immunofluorescence and standard metastasis dissemination assays.

Results: We demonstrated, using multiple methods of macrophage suppression, that chemotherapy-mediated induction of MenaINV-HI tumor cells depends on the presence of macrophages. In particular, depletion of either the entire macrophage lineage using clodronate liposomes or the use of specific inhibitors of the Cxcl12/Cxcr4 chemotactic pathway, resulted in a significant suppression of the MenaINV-hi cancer cell subpopulation in all mammary tumors examined. Moreover, inhibition of Tie2 by rebastinib blocked TMEM function and decreased the number of circulating tumor cells and metastatic foci, despite the chemotherapy-mediated induction of MenaINV-HI tumor cells.

Conclusions: Our data indicate that both the MenaINV-HI disseminating cancer cell population and the TMEM doorways are necessary but not individually sufficient for metastasis. As such, suppression of either the MenaINV-HI population or TMEM function can suppress chemotherapy-induced metastasis, thus providing a target to improve clinical care and eliminate non-beneficial effects of chemotherapy.

Citation Format: Rivera Sanchez LG, Karagiannis GS, Wang Y, Sharma VP, Burt J, Entenberg D, Oktay M, Condeelis JS. Chemotherapy-induced metastasis: Mechanisms and translational opportunities [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P2-05-01.

Abstract P2-08-18: Tumor microenvironment of metastasis (TMEM) score in residual breast carcinoma post-neoadjuvant chemotherapy as an independent prognosticator of distant recurrence

Background: Tumor microenvironment of metastasis (TMEM) is a microanatomical structure composed by 3 cells in direct contact, including a tumor cell expressing the actin-regulatory protein Mammalian-enabled (Mena), a perivascular Tie2hi/Vegfhi-expressing macrophage, and an endothelial cell. TMEM are intravasation sites that function as doorways for hematogenous tumor cell dissemination and metastases (Harney et al. Cancer Discovery 2015). TMEM may be identified and enumerated by triple immunohistochemistry in mouse and human mammary carcinomas. High TMEM score is associated with increased risk of distant metastasis in early stage breast cancer, and provides complementary prognostic information to IHC4 (Rohan et al. JNCI 2014) and Oncotype DX Recurrence Score in ER+, HER2-negative breast cancer (Sparano et al. NPJ Breast Cancer, 2017). Neoadjuvant chemotherapy (NAC) increases TMEM score in breast carcinoma in animal models and humans, indicating a previously unrecognized mechanism of resistance to cytotoxic therapy (Karagiannis et al. Science Trans Med 2017). Intravasation at TMEM sites may be inhibited using agents that block release of VEGF from TMEM-associated TIE2-hi, VEGF-hi macrophages (Harney et al. Mol Cancer Ther, 2017). Here we investigated whether TMEM score in post-NAC treated breast carcinoma is prognostic of distant recurrence in localized breast cancer after NAC, and thus provides a foundation for testing agents that block TMEM function in combination with NAC.

Methods: We determined TMEM score in 80 evaluable patients' post-NAC specimens with residual invasive ductal carcinomas of at least 0.5 cm. Approximately 60% of patients had ER+/HER2-negative, 28% had triple negative and 12% had HER2+ disease. Most of the patients received doxorubicin/cyclophosphamide + taxane and an anti-HER2 therapy if applicable. Tissue sections from residual tumors were stained for TMEM using triple immunohistochemistry for Mena-expressing cancer cells, CD31-expressing endothelial cells and CD68-expressing macrophages. The stained slides were scanned, and the images were analyzed by three pathologists, blinded to outcome, who independently determined the tissue areas appropriate for TMEM scoring. TMEM was scored within these areas using an automated algorithm.

Results: TMEM score was significantly higher in patients with distant recurrence (average TMEM=106), compared to patients without distant recurrence (average TMEM=71) (p<0.01, two-sided t-test). Moreover, in a Cox proportional hazards model that included TMEM score (upper tertile vs. lower 2 tertiles), age (>50 yrs. vs. <50), race (black vs non-black), tumor stage (T 1-3), estrogen receptor (ER) status (+ vs -), high TMEM score was associated with a increased risk of distant recurrence (HR=2.2, 95% CI=1.0 to 4.9, p=0.05)

Conclusion: TMEM score may provide independent prognostic information for distant recurrence in patients with residual invasive carcinoma after NAC. These results support the use of agents that block TMEM function in combination with NAC, as planned in the I-SPY2 trial.

Citation Format: Oktay MH, D'Alfonso T, Ginter P, Lanjewar S, Entenberg D, Pastoriza JM, Wang Y, Lin Y, Karagiannnis GS, Lin J, Ye X, Anampa J, Xue X, Rohan TE, Sparano JA, Condeelis JS. Tumor microenvironment of metastasis (TMEM) score in residual breast carcinoma post-neoadjuvant chemotherapy as an independent prognosticator of distant recurrence [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P2-08-18.

Abstract P2-01-06: Intravital imaging of the lung reveals the efficiency of the metastatic cascade

Background: Breast cancer is the leading cause of cancer related-death in women[1] with over 90% of these deaths due to metastasis. In these cases, the lung is the most common anatomical site of metastasis found at autopsy[2]. For the last 100 years, the process of metastasis has been studied through the use of an experimental metastasis (EM) assay [3] consisting of tail vein injection of tumor cells into tumor-free mice, followed by histopathological analysis of the lung weeks later to gain insight into tumor cell arrival, survival, and the growth of metastases. These studies have concluded that metastasis is an inefficient process[4]. Using a new technology developed at Albert Einstein College of Medicine, called the Window for High Resolution Imaging of the Lung (WHRIL)[5], we have directly compared EM to the more clinically relevant process of tumor cells spontaneously metastasizing (SM) from a primary tumor in situ to the lung and have found significant differences in metastatic efficiency between EM and SM.

Methods: Real-time images of tumor cell dissemination were captured using the WHRIL (Figure 1) in both EM and SM models. Metastatic potential was analyzed, and compared between the models, the percentage of tumor cells surviving in the lung over time, their endothelial crossing-time, their frequency of interaction with macrophages, the fraction of cells which are dormant, and the percent of cells that developed into metastases.

Results: Tumor cells which spontaneously metastasize from primary tumors show a ten-fold higher rate of survival in the lung and three times greater efficiency in forming metastases compared to those directly injected into the lung vasculature. Most of SM tumor cells are dormant indicating that the residual disease phenotype is programmed by the primary tumor either directly in the primary site or indirectly at the secondary site.

Conclusion: These results indicate that experimental metastasis does not accurately reflect the true clinical process and that spontaneous dissemination from a primary tumor has significant influence on the survival and growth of disseminated cells. This suggests that the tumor microenvironment of the primary tumor educates disseminating tumor cells for survival, dormancy and growth at the primary site, and/or prepares the pre-metastatic niche, in the secondary site. Understanding where and how disseminated tumor cells are educated is critical to preventing their survival and growth.at secondary sites. This discovery will open the door to new strategies for the treatment of metastatic tumors to prevent metastatic progression and death.

Citation Format: Coste AH, Boriello L, Wang Y, Oktay M, Condeelis JS, Entenberg D. Intravital imaging of the lung reveals the efficiency of the metastatic cascade [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P2-01-06.

Abstract P6-10-02: Black race is associated with worse distant relapse-free survival in breast cancer patients treated with neoadjuvant compared to adjuvant systemic chemotherapy

Background: Compared to white women, black women with operable breast cancer treated with primary surgical therapy and adjuvant or neoadjuvant systemic chemotherapy have higher recurrence rates and breast cancer mortality. Large randomized prospective studies did not find significant differences in distant-recurrence free survival (DRFS) and overall survival (OS) between breast cancer patients treated in the adjuvant and neoadjuvant setting for predominantly white populations. However, data indicating that neoadjuvant treatment is equivalent to adjuvant treatment for black breast cancer patients are missing. Here, we first examined racial differences in DRFS among breast cancer patients treated in the neoadjuvant setting at Einstein-Montefiore Center for Cancer Care (EMCCC) in the Bronx, and then investigated if DRFS in black patients treated in the neoadjuvant setting is comparable to DRFS in the adjuvant setting.

Methods: We evaluated DRFS in 241 racially diverse patients with localized or regionally advanced breast cancer treated with neoadjuvant chemotherapy between January 2000 and December 2016. In addition, we evaluated DRFS in 474 white and 701 black patients with localized or regionally advanced breast cancer treated with systemic adjuvant (432 whites, 596 blacks) or neoadjuvant (42 whites, 105 blacks) chemotherapy. Using multivariate Cox proportional hazard models, we generated hazard ratios (HR) and 95% confidence intervals (95%CI) for risk of distant recurrence, with adjustment for age (<50 vs >/50 years), stage (I/II vs III), estrogen receptor (ER) status (+ vs -), HER2/neu overexpression (+ vs. -/equivocal/unknown), triple negative (TN) status (yes vs no), and type of systemic chemotherapy (adjuvant vs. neoadjuvant).

Results: Black patients treated with neoadjuvant systemic chemotherapy had significantly worse DRFS than white patients (HR=2.29; 95%CI=1.02-5.15, p=0.04). DRFS in non-black Hispanics and patients from racial backgrounds other than Hispanic or black compared to whites was not statistically different. Neoadjuvant chemotherapy was associated with worse DRFS compared to adjuvant chemotherapy in black (HR=3.72; 95%CI=4.03-5.81; p=<0.0001), but not in white women.

Conclusion: Black patients with localized breast cancer treated with systemic neoadjuvant chemotherapy not only have inferior DRFS compared to white patients, but also worse DRFS when compared to black patients treated with adjuvant chemotherapy, after adjustment for clinical and pathological covariates. This observation needs to be confirmed in further prospective studies and biologic factors contributing to this finding need to be evaluated.

Citation Format: Pastoriza JM, Karagiannis GS, Xue X, Lin J, Condeelis JS, Sparanno JA, Rohan TE, Oktay MH. Black race is associated with worse distant relapse-free survival in breast cancer patients treated with neoadjuvant compared to adjuvant systemic chemotherapy [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P6-10-02.

Abstract P2-05-06: Analytical and clinical validation of a fully automated tissue-based quantitative assay (MetaSite Breast™) to detect the likelihood of distant metastasis in hormone receptor (HR)-positive, HER2-negative early stage breast cancer (ESBC)

Background: MetaSite Breast™ is a validated assay to predict risk of distant breast cancer metastasis in patients with HR+/HER2- ESBC. The assay measures the number of MetaSites defined as tumor microanatomic structures composed of MENA protein expressing tumor cells in contact with CD31+ endothelial cells and CD68+ macrophages. Previous studies have demonstrated that an increased number of these microanatomic structures is associated with distant metastasis (DM) in HR+/HER2- ESBC independent of clinicopathologic features. Analytical validation of MetaSite Breast™ demonstrated precision of 97-99% (repeat image analysis of the same slide) and performance of 91-96% (staining and image analysis of serial tumor sections). We sought to further understand the importance of the MetaSite in predicting distant breast cancer metastasis utilizing a fully automated prognostic assay in an independent large patient cohort.

Methods: We conducted a nested case-control study within a cohort of 3,760 patients diagnosed between 1980 and 2000 with invasive breast cancer from the Kaiser Permanente Northwest health care system. Cases (n=259) were women who developed a subsequent distant metastasis; controls, selected using incidence density sampling, were matched closely to cases (1:1) on age at and calendar year of primary diagnosis. Of the 481 patient tumor samples evaluated in this study, 57% were HR+/HER2-, 19% were triple negative (TN), and 15% were HER2+ disease. Multivariate models were adjusted for clinical factors including: lymph node status, tumor size, tumor grade, and HRT; as well as matching variables: age and year of diagnosis. Odds ratios (ORs) and 95% confidence intervals (CIs) were estimated using logistic regression.

Results: In the HR+/HER2- group, MetaSite Score (MS) ranged from 0-357 and the mean was 44.6. MS was a significant predictor of DM (P=0.039) in patients with HR+/HER2- disease. Cut-points based on tertiles of MS in all 259 controls defined intermediate (13-41) and high (>41) risk groups that were significantly associated with risk of DM versus the low risk group (OR=2.24; 95%CI=1.23-4.13, P=0.009) and (OR=2.94; 95%CI=1.62-5.41, P=0.0005), respectively. Univariate estimates of absolute risk of DM with cutoffs based on 90% sensitivity and specificity were 9.4% for the low risk group (MS<7), 14.1% for the intermediate (MS=7-91), and 23.4% for the high (MS>91). When adjusted for clinical factors, estimates of absolute risk of DM were 6.6%, 14.1%, and 33.0% for the low, intermediate, and high risk groups, respectively. A binary cut-point for the high risk group was determined (MS>14) and was significant with a 2-fold higher risk of DM versus the low risk group and adjusted for clinical covariates (P=0.036). MS was not positively associated with DM in TN or HER2+ disease.

Conclusions: MetaSite Breast™ significantly predicted the risk of distant breast cancer metastasis in ESBC patients with HR+/HER2-disease, independent of classical clinicopathologic features.

Citation Format: Donovan MJ, Jones JG, Entenberg DR, Condeelis JS, D'alfonso TM, Gustavson M, Molinaro A, Oktay MH, Xue X, Sparano JA, Peterson MA, Podznyakova O, Rohan TE, Shuber AP, Gertler FB, Ly A, Divelbiss ME, Hamilton DA. Analytical and clinical validation of a fully automated tissue-based quantitative assay (MetaSite Breast™) to detect the likelihood of distant metastasis in hormone receptor (HR)-positive, HER2-negative early stage breast cancer (ESBC) [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P2-05-06.

Abstract PD5-02: Paclitaxel induced mena- and TMEM-mediated pro-metastatic changes in the breast cancer microenvironment

Background: Breast cancer cell intravasation and dissemination occurs specifically at microanatomical structures that we call tumor-microenvironment of metastasis (TMEM), representing direct physical contact between a tumor cell expressing the actin-regulatory protein Mammalian-enabled (Mena), a perivascular Tie2hi/Vegfhi-expressing macrophage, and an endothelial cell (Harney et al. Cancer Discovery 2015). TMEM sites have been identified in mouse and human mammary carcinomas, and both TMEM density (Rohan et al. JNCI 2014) and invasive Mena isoform expression (Agarwal et al. Breast Cancer Res, 2012; Forse et al. BMC Cancer, 2015]) correlates with metastasis in early stage breast cancer. Since cytotoxic agents such as PTX induce influx of bone marrow-derived progenitors that differentiate into Tie2hi/VEGFhi macrophages in the primary tumor, we hypothesized that PTX may potentiate tumor cell invasion and metastasis by inducing the formation of TMEM sites and/or function.

Methods and Results in humans: We analyzed the effect of chemotherapy on TMEM and invasive Mena isoforms in 10 patients with localized breast cancer who had residual disease after neoadjuvant chemotherapy (NAC: weekly paclitaxel followed by dose-dense doxorubicin-cyclophosphamide [AC]), of whom 7 had more than 2-fold increase in TMEM density in residual disease compared with pretreatment. In a separate cohort of 5 patients, NAC produced an acute increase of up to 150-fold in invasive Mena isoforms after 1-2 doses of NAC.

Methods and Results in mice: After our preliminary data in humans, we evaluated effects of PTX in 4 different models, including 2 mouse models (PyMT-spontaneous & transplantation) and 2 patient-derived xenograft (PDX) triple negative models (HT17, HT33). Although PTX delayed primary tumor growth, tumors in PTX-treated mice had significantly more TMEM sites, circulating tumor cells (CTCs) and metastatic foci when compared to vehicle-treated animals. Using intravital imaging of MMTV-PyMT-Dendra2/Cfms-CFP mice, PTX induced influx of macrophages into primary tumors and intravasation of cancer cells at TMEM sites. Furthermore, PTX treatment significantly increased expression of Mena at the gene and protein levels, including invasive Mena isoforms. Deletion of the Mena gene completely abolished dissemination and metastasis in all cases, including those treated with PTX.

Conclusions: We show in mammary carcinoma mouse models and PDX models that although PTX delays tumor growth, it induces invasive Mena isoform expression and significantly increases the density of TMEM sites that are responsible for cancer cell intravasation, dissemination and metastasis. Thus, our data indicate that PTX paradoxically induces dissemination of breast cancer cells by promoting invasive Mena isoforms and TMEM-mediated cancer cell intravasation, suggesting that blockade of TMEM assembly and/or function could enhance the effectiveness of PTX and possibly other cytotoxic agents commonly used to treat early and advanced stage breast cancer.

Citation Format: Karagiannis GS, Pastoriza JM, Wang Y, Harney AS, Entenberg D, Pignatelli J, Jones JG, Anampa J, Sparano JA, Rohan TE, Condeelis JS, Oktay MH. Paclitaxel induced mena- and TMEM-mediated pro-metastatic changes in the breast cancer microenvironment [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr PD5-02.

Profilin1 regulates invadopodium maturation in human breast cancer cells

Invadopodia are actin-driven membrane protrusions that show oscillatory assembly and disassembly causing matrix degradation to support invasion and dissemination of cancer cells in vitro and in vivo. Profilin1, an actin and phosphoinositide binding protein, is downregulated in several adenocarcinomas and it is been shown that its depletion enhances invasiveness and motility of breast cancer cells by increasing PI(3,4)P2 levels at the leading edge. In this study, we show for the first time that depletion of profilin1 leads to an increase in the number of mature invadopodia and these assemble and disassemble more rapidly than in control cells. Previous work by Sharma et al. (2013a), has shown that the binding of the protein Tks5 with PI(3,4)P2 confers stability to the invadopodium precursor causing it to mature into a degradation-competent structure. We found that loss of profilin1 expression increases the levels of PI(3,4)P2 at the invadopodium and as a result, enhances recruitment of the interacting adaptor Tks5. The increased PI(3,4)P2-Tks5 interaction accelerates the rate of invadopodium anchorage, maturation, and turnover. Our results indicate that profilin1 acts as a molecular regulator of the levels of PI(3,4)P2 and Tks5 recruitment in invadopodia to control the invasion efficiency of invadopodia.

Autocrine HBEGF expression promotes breast cancer intravasation, metastasis and macrophage-independent invasion in vivo

Increased expression of HBEGF in ER negative breast tumors is correlated with enhanced metastasis to distant organ sites and more rapid disease recurrence upon removal of the primary tumor. Our previous work has demonstrated a paracrine loop between breast cancer cells and macrophages in which the tumor cells are capable of stimulating macrophages through the secretion of CSF-1 while the tumor associated macrophages (TAMs) in turn aid in tumor cell invasion by secreting EGF. To determine how the autocrine expression of EGFR ligands by carcinoma cells would affect this paracrine loop mechanism, and in particular whether tumor cell invasion depends on spatial ligand gradients generated by TAMs, we generated cell lines with increased HBEGF expression. We find that autocrine HBEGF expression enhanced in vivo intravasation and metastasis, and resulted in a novel phenomenon in which macrophages were no longer required for in vivo invasion of breast cancer cells. In vitro studies revealed that expression of HBEGF enhanced invadopodium formation, thus providing a mechanism for cell autonomous invasion. The increased invadopodium formation was directly dependent on EGFR signaling, as demonstrated by a rapid decrease in invadopodia upon inhibition of autocrine HBEGF/EGFR signaling as well as inhibition of signaling downstream of EGFR activation. HBEGF expression also resulted in enhanced invadopodium function via upregulation of MMP2 and MMP9 expression. We conclude that high levels of HBEGF expression can short-circuit the tumor cell/macrophage paracrine invasion loop, resulting in enhanced tumor invasion that is independent of macrophage signaling.

Abstract P6-02-04: TMEM (Tumor MicroEnvironment of Metastasis) in human breast cancer is a blood vessel associated intravasation microenvironment unrelated to lymphatics

In breast cancer, both lymph node and distant metastasis represent dissemination of tumor cells from a primary site, but the mechanism of spread and the subsequent risk of mortality may not be the same. Historically, lymphatic spread has been documented both descriptively, as presence or absence of lymphovascular invasion (LVI), and as a formal part of TNM staging. Until recently, however, there has been no way to directly assess the risk of hematogenous dissemination by the primary tumor.

Observations from multiphoton-based intravital imaging of rodent models of breast cancer and the analysis of Mena function in tumor cells in vivo have characterized an intravasation microenvironment (ME) involved in the systemic dissemination of tumor cells from primary breast tumors. We have identified the corresponding structure in FFPE tissue and called it TMEM (Tumor MicroEnvironment of Metastasis). This microanatomic landmark is defined as the direct apposition of a Mena-overexpressing intravasation competent carcinoma cell, a perivascular macrophage, and an endothelial cell. In a case control study of 30 case-control pairs, where each matched pair differed only in their metastatic status – non-metastatic vs. metastatic – we found that the density of TMEM was significantly associated with development of systemic metastasis (p = 0.00006).

The relationship of hematogenous- and lymphatic-mediated tumor cell spread is not understood. Using the previously described cohort in which we showed that TMEM was associated with metastasis, the purpose of this study was to 1) assess intratumoral lymphatic density, 2) determine if TMEM- lymphatic structures associated with lymphatics exist, and 3) determine if TMEM- lymphatic structures correlate with systemic metastatic risk. Cases were stained with a triple immunostain identical to that used in our earlier study except that D2-40 (a lymphatic marker) was used, rather than CD31 (a blood vessel marker). The marker for macrophages (CD68) and invasive tumor cells (Mena) remained the same. Two pathologists, blinded to outcome, evaluated the presence or absence of intratumoral lymphatics and quantitated the number of TMEM-lymphatic structures per 10 high power (400x) fields in areas of highest intratumoral lymphatic density. A TMEM-lymphatic structure was defined as the direct apposition of a lymphatic (D2-40) endothelial cell with a macrophage and invasive tumor cell.

Intratumoral lymphatics were absent in a majority of tumors in each of the 2 groups (18 of 30 non-metastatic, 16 of 30 metastatic; p = 0.6). TMEM-lymphatic structures were rare and were equally present in the 2 groups (3 metastatic and 3 non-metastatic cases). Using the Wilcoxon (paired) signed-rank test, we found no significant difference in the density of these structures between the two groups (p = 0.4). Furthermore, TMEM-lymphatic structures did not correlate with the presence of lymph node metastases (p = 0.8). We conclude that lymphatic vessels do not participate in the TMEM assembly that has been associated with hematogenous metastasis. TMEM density assessment reflects a hematogenous intravasation ME and offers a novel approach to the assessment of metastatic risk.

Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P6-02-04.

Arg/Abl2 promotes invasion and attenuates proliferation of breast cancer in vivo

Tumor progression is a complex, multistep process involving accumulation of genetic aberrations and alterations in gene-expression patterns leading to uncontrolled cell division, invasion into surrounding tissue and finally dissemination and metastasis. We have previously shown that the Arg/Abl2 non-receptor tyrosine kinase acts downstream of the EGF receptor and Src tyrosine kinases to promote invadopodium function in breast cancer cells, thereby promoting their invasiveness. However, whether and how Arg contributes to tumor development and dissemination in vivo has never been investigated. Using a mouse xenograft model, we show that knocking down Arg in breast cancer cells leads to increased tumor cell proliferation and significantly enlarged tumor size. Despite having larger tumors, the Arg knockdown tumor-bearing mice exhibit significant reductions in tumor cell invasion, intravasation into blood vessels, and spontaneous metastasis to lungs. Interestingly, we found that proliferation-associated genes in the Ras-MAPK pathway are upregulated in Arg-knockdown breast cancer cells, as is Ras-MAPK signaling, while invasion-associated genes are significantly downregulated. These data suggest that Arg promotes tumor cell invasion and dissemination, while simultaneously inhibiting tumor growth. We propose that Arg acts as a switch in metastatic cancer cells that governs the decision to “grow or go” (divide or invade).

Evaluation of Mena isoforms as a surrogate for epithelial mesenchymal transformation and erlotinib resistance in breast carcinoma

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Background: Epithelial to mesenchymal transition (EMT) is an important step in invasiveness and has been shown to correlate with metastatic potential in several cancer cell lines, including breast carcinoma. However, given the heterogeneity of tumors in vivo, EMT has not been a reliable marker of metastatic potential in cancer patients. Mena, a member of the enabled (ena)/vasodilator-stimulated phophoprotein (VASP) family, which controls cell motility, is upregulated in the invasive subpopulation of breast cancer cells. Mena is alternatively spliced to include one of four exons: +, ++, invasive (INV), or 11a. In the presence of MenaINV, tumor cells are able to invade even at epidermal growth factor (EGF) concentrations that would otherwise be undetectable by the tumor cells. Currently, there are several clinical and genetic characteristics which can predict sensitivity to erlotinib, an EGF receptor inhibitor, but further studies are necessary. Methods: The animal models used were the polyoma middle T antigen (PyMT) transgenic mouse and severe combined immunodeficiency (SCID) xenografted tumors derived from injection with a human breast carcinoma line, MDA-MB-231, and a rat adenocarcinoma line, MTLn3, with forced expression of MenaINV and Mena11a. Invasive cells and average primary tumor cells were collected using the in vivo invasion assay and Fluorescence activated cell sorting (FACS), respectively, and imaged via immunofluorescence. Results: We show that in both the PyMT and MDA-MB-231 animal models, invasive cells are significantly more mesenchymal than average primary tumor cells, indicating they have undergone EMT. The MTLn3-MenaINV cells are more mesenchymal-like, whereas the Mena11a are more epithelial-like. We also expect that cells with a high Mena INV/11a ratio invade despite the presence of erlotinib.

No significant financial relationships to disclose.

Lamellipodia in invasion

In vivo imaging of GFP-labeled metastatic tumor cells reveals cell orientation towards blood vessels. Orientation of tumor cells during chemotactic responses to ligands such as EGF begins with lamellipod extension. Evaluation of some of the downstream events in lamellipod extension indicates: (1) plasma membrane distribution of the EGF receptor is uniform but internalized receptor accumulates on the side of the cell closest to the source of EGF; (2) the alpha p110 isoform of PI-3 kinase is required; and (3) protrusion of the lamellipod relies upon the combined actions of the Arp2/3 complex and cofilin for generation of filamentous actin.

Protein tyrosine phosphatase phi regulates paxillin tyrosine phosphorylation and mediates colony-stimulating factor 1-induced morphological changes in macrophages

Removal of colony-stimulating factor 1 (CSF-1) causes macrophages to round up and to increase their expression of protein tyrosine phosphatase phi (PTP phi). This is accompanied by the disruption of focal complexes and the formation of ruffles. Here we have overexpressed wild-type (WT) PTP phi and a phosphatase-inactive (C325S) mutant in a macrophage cell line in the presence and absence of CSF-1. In the presence of CSF-1, WT PTP phi induces cell rounding and ruffle formation, while C325S PTP phi has no effect. In contrast, in CSF-1-starved cells, C325S PTP phi behaves in a dominant negative fashion, preventing rounding and ruffling. Furthermore, C325S PTP phi increases adhesion in cycling cells, while WT PTP phi enhances motility. In WT PTP phi-overexpressing cells, the focal contact protein paxillin is selectively depleted from focal complexes and specifically dephosphorylated on tyrosine. In contrast, paxillin is hyperphosphorylated in C325S PTP phi-expressing cells. Moreover, a complex containing PTP phi, paxillin, and a paxillin-associated tyrosine kinase, Pyk2, can be immunoprecipitated from macrophage lysates, and the catalytic domain of PTP phi selectively binds paxillin and Pyk2 in vitro. Although PTP phi and Pyk2 do not colocalize with paxillin in focal complexes, all three proteins are colocalized in dorsal ruffles. The results suggest that paxillin is dephosphorylated by PTP phi in dorsal ruffles, using Pyk2 as a bridging molecule, resulting in a reduced pool of tyrosine-phosphorylated paxillin available for incorporation into focal complexes, thereby mediating CSF-1 regulation of macrophage morphology, adhesion, and motility.

Phosphorylation of ADF/cofilin abolishes EGF-induced actin nucleation at the leading edge and subsequent lamellipod extension

In metastatic rat mammary adenocarcinoma cells, cell motility can be induced by epidermal growth factor. One of the early events in this process is the massive generation of actin barbed ends, which elongate to form filaments immediately adjacent to the plasma membrane at the tip of the leading edge. As a result, the membrane moves outward and forms a protrusion. To test the involvement of ADF/cofilin in the stimulus-induced barbed end generation at the leading edge, we inhibited ADF/cofilin's activity in vivo by increasing its phosphorylation level using the kinase domain of LIM-kinase 1 (GFP-K). We report here that expression of GFP-K in rat cells results in the near total phosphorylation of ADF/cofilin, without changing either the G/F-actin ratio or signaling from the EGF receptor in vivo. Phosphorylation of ADF/cofilin is sufficient to completely inhibit the appearance of barbed ends and lamellipod protrusion, even in the continued presence of abundant G-actin. Coexpression of GFP-K, together with an active, nonphosphorylatable mutant of cofilin (S3A cofilin), rescues barbed end formation and lamellipod protrusion, indicating that the effects of kinase expression are caused by the phosphorylation of ADF/cofilin. These results indicate a direct role for ADF/cofilin in the generation of the barbed ends that are required for lamellipod extension in response to EGF stimulation.

The collection of the motile population of cells from a living tumor

In this study, we report that needles containing chemoattractants can be used to collect the subpopulation of motile and chemotactic tumor cells from a primary tumor in a live rat as a pure population suitable for further analysis. The most efficient cell collection requires the presence of chemotactic cytokines, such as epidermal growth factor and serum components, and occurs with 15-fold higher efficiency in metastatic tumors compared with nonmetastatic tumors. Although tumor cells of the nonmetastatic tumors show a motility response to serum, they were not collected with high efficiency into needles in vivo in response to serum, indicating that additional factors besides motility are required to explain differences in cell collection efficiencies between metastatic and nonmetastatic tumors. The results reported here indicate that needles filled with growth factors and matrigel, when inserted into the primary tumor, can faithfully mimic the environment that supports invasion and intravasation in vivo. Furthermore, the results indicate that the same cell behaviors that contribute to chemotaxis in vitro also contribute to invasion in vivo.

Imaging of cancer invasion and metastasis using green fluorescent protein

The use of green fluorescent protein to fluorescently tag tumour cells has allowed investigators to open the "black box" of metastasis in order to visualise the behaviour of tumour cells in living tissues. Analysis of cells leaving the primary tumour indicates that highly metastatic cells are able to polarise more effectively towards blood vessels while poorly metastatic cells fragment more often when interacting with blood. In addition, there appear to be greater numbers of host immune system cells interacting with metastatic tumours. After arresting in target organs such as the lungs or liver, most tumour cells become dormant or apoptose. A small fraction of the arrested cells form metastases. In some target organs, migration of tumour cells may enhance the ability to form metastases.

A critical step in metastasis: in vivo analysis of intravasation at the primary tumor

Detailed evaluation of all steps in tumor cell metastasis is critical for evaluating the cell mechanisms controlling metastasis. Using green fluorescent protein transfectants of metastatic (MTLn3) and nonmetastatic (MTC) cell lines derived from the rat mammary adenocarcinoma 13762 NF, we have measured tumor cell density in the blood, individual tumor cells in the lungs, and lung metastases. Correlation of blood burden with lung metastases indicates that entry into the circulation is a critical step for metastasis. To examine cell behavior during intravasation, we have used green fluorescent protein technology to view these cells in time lapse images within a single optical section using a confocal microscope. In vivo imaging of the primary tumors of MTLn3 and MTC cells indicates that both metastatic and nonmetastatic cells are motile and show protrusive activity. However, metastatic cells show greater orientation toward blood vessels and larger numbers of host cells within the primary tumor, whereas nonmetastatic cells fragment when interacting with vessels. These results demonstrate that a major difference in intravasation between metastatic and nonmetastatic cells is detected in the primary tumor and illustrate the value of a direct visualization of cell properties in vivo for dissection of the metastatic process.

Actin filaments are severed by both native and recombinant dictyostelium cofilin but to different extents

Cofilin has been reported to depolymerize F-actin alternately by either severing filaments to increase the number of depolymerizing ends or by increasing the off-rate of monomers from F-actin without increasing the number of filament ends. We have compared directly the ability of native and recombinant cofilins from Dictyostelium to sever F-actin. Our results demonstrate that native cofilin has a higher level of severing activity than recombinant cofilin. Significantly, the measurement of cofilin's severing activity by two independent methods, direct visualization with an improved light microscope assay and by scoring of the number of pointed ends by DNase I binding, clearly shows that both native and recombinant cofilins sever F-actin but to different extents. The severing activity in preparations of recombinant cofilin is variable depending on the method of preparation and, in some cases, is difficult to detect by microscopy assays. This latter point is particularly significant because it may lead to the conclusion that cofilin severs weakly or not at all depending on its method of isolation.

Specific requirement for the p85-p110alpha phosphatidylinositol 3-kinase during epidermal growth factor-stimulated actin nucleation in breast cancer cells

We have studied the role of phosphatidylinositol 3-kinases (PI 3-kinases) in the regulation of the actin cytoskeleton in MTLn3 rat adenocarcinoma cells. Stimulation of MTLn3 cells with epidermal growth factor (EGF) induced a rapid increase in actin polymerization, with production of lamellipodia within 3 min. EGF-stimulated lamellipodia were blocked by 100 nM wortmannin, suggesting the involvement of a class Ia PI 3-kinase. MTLn3 cells contain equal amounts of p110alpha and p110beta, and do not contain p110delta. Injection of specific inhibitory antibodies to p110alpha induced cell rounding and blocked EGF-stimulated lamellipod extension, whereas control or anti-p110beta antibodies had no effect. In contrast, both antibodies inhibited EGF-stimulated DNA synthesis. An in situ assay for actin nucleation showed that EGF-stimulated formation of new barbed ends was blocked by injection of anti-p110alpha antibodies. In summary, the p110alpha isoform of PI 3-kinase is specifically required for EGF-stimulated actin nucleation during lamellipod extension in breast cancer cells.

Relationship between Arp2/3 complex and the barbed ends of actin filaments at the leading edge of carcinoma cells after epidermal growth factor stimulation

Using both light and high resolution electron microscopy, we analyzed the spatial and temporal relationships between the Arp2/3 complex and the nucleation activity that is required for lamellipod extension in mammary carcinoma cells after epidermal growth factor stimulation. A rapid two- to fourfold increase in filament barbed end number occurs transiently after stimulation and remains confined almost exclusively to the extreme outer edge of the extending lamellipod (within 100-200 nm of the plasma membrane). This is accompanied by an increase in filament density at the leading edge and a general decrease in filament length, with a specific loss of long filaments. Concomitantly, the Arp2/3 complex is recruited with a 1.5-fold increase throughout the entire cortical filament network extending 1-1.5 microm in depth from the membrane at the leading edge. The recruitment of the Arp2/3 complex at the membrane of the extending lamellipod indicates that Arp2/3 may be involved in initial generation of growing filaments. However, only a small subset of the complex present in the cortical network colocalizes near free barbed ends. This suggests that the 100-200-nm submembraneous compartment at the leading edge of the extending lamellipod constitutes a special biochemical microenvironment that favors the generation and maintenance of free barbed ends, possibly through the locally active Arp2/3 complex, severing or decreasing the on-rate of capping protein. Our results are inconsistent with the hypothesis suggesting uncapping is the dominant mechanism responsible for the generation of nucleation activity. However, they support the hypothesis of an Arp2/3-mediated capture of actin oligomers that formed close to the membrane by other mechanisms such as severing. They also support pointed-end capping by the Arp2/3 complex, accounting for its wide distribution at the leading edge.

Chemoattractant-induced lamellipod extension

The mammary adenocarcinoma cell line MTLn3 is chemotactic towards epidermal growth factor (EGF), and this induced motility is thought to promote breast cancer invasion and metastasis. Stimulation of MTLn3 cells with EGF results in the extension of a flat, thin structure filled with filamentous actin and termed a lamellipod. Lamellipod extension is dependent on actin polymerization and is localized to the border of adherent cells. The structure of EGF-stimulated lamellipods in MTLn3 cells is well suited to analysis of chemoattractant-stimulated protrusion. Actin polymerization occurs within 200 nm of the extending edge of the lamellipod. Although extension of the lamellipod is not dependent upon interaction with the substratum, stabilization of the extended lamellipod is dependent on an adhesive substratum. Dorsal ruffling is suppressed during lamellipod extension. Tyrosine phosphorylation is reduced in preexisting focal contacts compared to new contacts induced by EGF stimulation. The coordination of turnover of focal contacts with lamellipod extension is proposed to result in polarized cell motility in response to gradients of chemoattractants.

Suppression of ruffling by the EGF receptor in chemotactic cells

To clarify the relationship between ruffling and lamellipod extension in growth factor-stimulated chemotactic responses, we utilized cell lines derived from the rat 13762 NF mammary adenocarcinoma. Nonmetastatic MTC cells expressing the human EGF receptor (termed MTC HER cells) demonstrated chemotactic responses to TGF-alpha, an EGF receptor ligand typically present in mammary tissue. In microchemotaxis chambers, peak chemotactic responses occurred in response to 5 nM TGF-alpha. MTC HER cells showed dramatic ruffling edges in the absence of external stimuli, and addition of 5 nM TGF-alpha led to a transient reduction in ruffling concomitant with lamellipod extension. Lamellipod extension correlated with an overall increase in actin polymerization. These responses were blocked by the PI 3 kinase inhibitor wortmannin but not by the MAP kinase inhibitors PD98059 and SB203580. We conclude that the initial chemotactic response to TGF-alpha involves lamellipod extension and that ruffling reflects a dynamic turnover of lamellipodia that is arrested during lamellipod extension. By regulating the dissolution of ruffles and extension of lamellipods, a chemotactic response can be achieved, which may contribute to the metastatic process.

Regulation of protrusion shape and adhesion to the substratum during chemotactic responses of mammalian carcinoma cells

We report here the first direct observation of chemotaxis to EGF by rat mammary carcinoma cells. When exposed to a gradient of EGF diffusing from a micropipette, MTLn3 cells displayed typical ameboid chemotaxis, extending a lamellipod-like protrusion and moving toward the pipette. Using a homogeneous upshift in EGF to model stimulated lamellipod extension (J. E. Segall et al., 1996, Clin. Exp. Metastasis 14, 61-72), we analyzed the relationship between adhesion and chemoattractant-stimulated protrusion. Exposure to EGF led to a rapid remodeling of the adhesive contacts on adherent cells, in synchrony with extension of a flat lamellipod over the substratum. EGF-stimulated lamellipods still extended in the presence of adhesion-blocking peptides or over nonadhesive surfaces. They were, however, slightly shorter and retracted rapidly under those conditions. The major protrusive structure observed on well-spread, adherent cells, after EGF stimulation was a flat broad lamellipod, whether or not in contact with the substratum, while cells in suspension showed transient protrusive activity over the entire cell surface. We conclude that the initial adhesive status of the cell conditions the shape of the outcoming protrusion. Altogether our results suggest that, although adhesive contacts are not necessary for lamellipod extension, they play a role in stabilizing the protrusion as well as in the control of its final shape and amplitude.

Cell motility of tumor cells visualized in living intact primary tumors using green fluorescent protein

Metastasis is the leading cause of death in cancer patients. Cell motility is believed to be a necessary step in the metastatic process (L. Liotta and W. G. Stetler-Stevenson, In: Cancer: Principles and Practice of Oncology, pp. 134-149, 1993). Currently, most methods available to study the behavior of metastatic tumor cells are indirect, e.g., cell motility is examined in vitro and the results are correlated with metastatic capability (A. W. Partin, et al., Cancer Treat. Res., 59: 121-130, 1992). We have developed a model that directly examines the motility of metastatic primary tumor cells in situ. A metastatic rat breast cancer cell line was established that constitutively expresses green fluorescent protein. Upon s.c. injection of these cells into the mammary fat pad of female Fischer 344 rats, primary and metastatic tumors form that fluoresce when they are excited with FITC-filtered light. Animations of metastatic tumor cells moving in live rats were generated by intravital imaging of the primary tumor in situ on a laser scanning confocal microscope. With this model, the behavioral phenotype of metastatic and nonmetastatic tumor cells can be described and determined. This information will allow the effects of genetic manipulations or therapeutic treatments on this phenotype to be determined (D. R. Soll, Int. Rev. Cytol., 163: 43-104, 1995). This is the first time that living primary tumor cells in a live animal have been visualized as part of a clinically relevant model.

EGF stimulates an increase in actin nucleation and filament number at the leading edge of the lamellipod in mammary adenocarcinoma cells

Stimulation of metastatic MTLn3 cells with EGF causes the rapid extension of lamellipods, which contain a zone of F-actin at the leading edge. In order to establish the mechanism for accumulation of F-actin at the leading edge and its relationship to lamellipod extension in response to EGF, we have studied the kinetics and location of EGF-induced actin nucleation activity in MTLn3 cells and characterized the actin dynamics at the leading edge by measuring the changes at the pointed and barbed ends of actin filaments upon EGF stimulation of MTLn3 cells. The major result of this study is that stimulation of MTLn3 cells with EGF causes a transient increase in actin nucleation activity resulting from the appearance of free barbed ends very close to the leading edge of extending lamellipods. In addition, cytochalasin D causes a significant decrease in the total F-actin content in EGF-stimulated cells, indicating that both actin polymerization and depolymerization are stimulated by EGF. Pointed end incorporation of rhodamine-labeled actin by the EGF stimulated cells is 2.12+/−0.47 times higher than that of control cells. Since EGF stimulation causes an increase in both barbed and pointed end incorporation of rhodamine-labeled actin in the same location, the EGF-stimulated nucleation sites are more likely due either to severing of pre-existing filaments or de novo nucleation of filaments at the leading edge thereby creating new barbed and pointed ends. The timing and location of EGF-induced actin nucleation activity in MTLn3 cells can account for the observed accumulation of F-actin at the leading edge and demonstrate that this F-actin rich zone is the primary actin polymerization zone after stimulation.

Capping protein terminates but does not initiate chemoattractant-induced actin assembly in Dictyostelium

The first step in the directed movement of cells toward a chemotactic source involves the extension of pseudopods initiated by the focal nucleation and polymerization of actin at the leading edge of the cell. We have previously isolated a chemoattractant-regulated barbed-end capping activity from Dictyostelium that is uniquely associated with capping protein, also known as cap32/34. Although uncapping of barbed ends by capping protein has been proposed as a mechanism for the generation of free barbed ends after stimulation, in vitro and in situ analysis of the association of capping protein with the actin cytoskeleton after stimulation reveals that capping protein enters, but does not exit, the cytoskeleton during the initiation of actin polymerization. Increased association of capping protein with regions of the cell containing free barbed ends as visualized by exogenous rhodamine-labeled G-actin is also observed after stimulation. An approximate threefold increase in the number of filaments with free barbed ends is accompanied by increases in absolute filament number, whereas the average filament length remains constant. Therefore, a mechanism in which preexisting filaments are uncapped by capping protein, in response to stimulation leading to the generation of free barbed ends and filament elongation, is not supported. A model for actin assembly after stimulation, whereby free barbed ends are generated by either filament severing or de novo nucleation is proposed. In this model, exposure of free barbed ends results in actin assembly, followed by entry of free capping protein into the actin cytoskeleton, which acts to terminate, not initiate, the actin polymerization transient.

A major agonist-regulated capping activity in Dictyostelium is due to the capping protein, cap32/34

Stimulation of starved Dictyostelium amoebae with the chemoattractant cAMP produces a rapid increase in actin nucleation activity at 5 seconds which is cotemporal with an increase in actin assembly and a decrease in Ca(2+)-insensitive capping activity [1]. Further characterization of this capping activity, called aginactin, led to the isolation of an Hsc70 [2]. Here, we demonstrate that purified aginactin contains both Hsc70 and the heterodimeric barbed-end capping protein, cap32/34. Immunoprecipitation of cap32/34 from purified aginactin removes all capping activity while immunoprecipitation of Hsc70 does not, indicating that the capping activity of aginactin is an intrinsic property of cap32/34. Gel filtration and immunoprecipitation assays fail to demonstrate the existence of a stable, high affinity complex between Hsc70 and cap32/34 in either lysate supernatants or aginactin pools but indicate the presence of a transient, ATP-sensitive interaction in cell lysates. Reconstitution experiments with purified Hsc70 and cap32/34 demonstrate that Hsc70 neither stimulates nor inhibits the capping activity of native cap32/34. Furthermore, we measured a Kd of approx. 0.8 nM for the binding of cap32/34 to barbed ends of actin filaments in the absence or presence of Hsc70, in agreement with Kd values measured for purified capping protein from other sources. We conclude, therefore, that cap32/34 is responsible for the capping activity called aginactin and that Hsc70 is not a regulatory cofactor for cap32/34 in Dictyostelium but may function as a chaperone during assembly of the cap32/34 heterodimer.

Aginactin, an agonist-regulated F-actin capping activity is associated with an Hsc70 in Dictyostelium

We have previously isolated an agonist-regulated actin filament capping activity, called aginactin, that is associated with a 70-kDa protein (Sauterer, R. A., Eddy, R. J., Hall, A. L., and Coneelis, J. S. (1991) J. Biol. Chem. 266, 24533-24539). A 2.0-kilobase clone isolated from a Dictyostelium lambda gt11 cDNA library screened with affinity-purified aginactin antibodies displays an overall sequence identity of 73% to the 70-kDa heat shock cognate protein, Hsc70, from various species. Aginactin capping activity and the 70-kDa protein bind to ATP-agarose columns and are quantitatively depleted from the load, indicating that an Hsc70 is associated with aginactin activity. Moderate stringency Southern blots indicate the presence of no fewer than six Hsc70-related sequences. Immunofluorescent staining of vegetative Dictyostelium AX3 cells with aginactin antibodies reveals a colocalization of aginactin-associated Hsc70 in F-actin-rich regions of the cell cortex and cell protrusions. Nuclei and organelles lacked positive staining indicating that the aginactin-associated Hsc70 is cytosolic. The levels of cytoskeletal-associated Hsc70 correlate with the loss of barbed end capping activity following cAMP stimulation, suggesting that the uncapping of barbed filament ends through an Hsc70-associated process may account for the increase in nucleation activity observed at 5 s following agonist stimulation.

Purification and characterization of aginactin, a newly identified agonist-regulated actin-capping protein from Dictyostelium amoebae

Amoeboid chemotaxis involves a regulated increase in actin nucleation activity that is correlated with an increase in actin polymerization occurring seconds after chemotactic stimulation (Carson, M., Weber, A., and Zigmond, S. H. (1986) J. Cell Biol. 103, 2707-2714; Hall, A. L., Warren, V., Dharmawardhane, S., and Condeelis, J. (1989) J. Cell Biol. 109, 2207-2213). We report the isolation and characterization of an agonist-regulated capping protein, aginactin, from Dictyostelium that may regulate these changes in actin nucleation activity. Aginactin is isolated from low speed supernatants of starved amoebae by sequential anion exchange, hydrophobic interaction, fast protein liquid chromatography anion exchange, and hydroxyapatite chromatography. Aginactin migrates with an apparent molecular weight of 70,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels and gel filtration columns, suggesting that it is a globular monomer. Aginactin is a barbed-end capping protein by several criteria. It inhibits the rate and final extent of actin polymerization and increases the apparent critical concentration at substoichiometric ratios to actin. It also inhibits depolymerization of F-actin and inhibits polymerization at the barbed end of Limulus acrosomal bundles. Aginactin is unaffected by micromolar Ca2+, and it neither severs F-actin nor nucleates actin polymerization in either the presence or absence of Ca2+. Aginactin binds to and cosediments with F-actin and has an apparent Kd for capping F-actin of 2.7 nM.

Actin-associated proteins in Dictyostelium discoideum

The cellular slime mold Dictyostelium discoideum is becoming the premier system for the explication of the biochemical and cellular events that occur during motile processes. Proteins associated with the actin cytoskeleton, in particular, appear to play key roles in cellular responses to many external stimuli. This review summarizes our present understanding of the actin-associated proteins in Dictyostelium, including their in vitro activities and their structural and/or functional analogues in mammalian cells.

Isolation of a 240-kilodalton actin-binding protein from Dictyostelium discoideum

A high molecular weight actin-binding protein with subunit mass of 240 kilodaltons has been purified from vegetative amoebae of Dictyostelium discoideum. Briefly, a cell extract was prepared by homogenizing vegetative amoebae in 5 mM EGTA, 5 mM 1,4-piperazineethanesulfonic acid, 1 mM dithiothreitol, 0.02% NaN3, pH 7.0, followed by ultracentrifugation at 114,000 X g for 1 h. The 240-kDa protein in this extract was separated from actin by chromatography on ATP-saturated DEAE-cellulose and further purified by chromatography on hydroxylapatite and Sephacryl S-300. The 240-kDa protein increases the low shear viscosity of F-actin. Covalent cross-linking with dimethyl suberimidate demonstrates that the 240-kDa protein can form dimers in high salt (500 mM NaCl). Hydrodynamic studies in high salt demonstrate the presence of an asymmetric dimer (Stokes' radius = 8.6 nm, sedimentation coefficient = 12 S, native molecular weight = 434,000, and frictional ratio = 1.7). Rotary shadowing demonstrates that the monomer is a flexible rod of approximately 70 nm in length that can associate end to end to form a dimer of approximately 140 nm in length. The 240-kDa protein cross-reacts with antibodies to chicken gizzard filamin. The properties of the 240-kDa protein suggest that it is a member of the filamin class of actin-associated proteins.

Ligand-induced changes in the location of actin, myosin, 95K (alpha-actinin), and 120K protein in amebae of Dictyostelium discoideum

In this study we investigated concanavalin A (Con A) induced changes in the locations of actin, myosin, 120K, and 95K (alpha-actinin) to determine the extent to which actin and myosin are reorganized during capping and the roles that 120K and 95K might play in this reorganization. We observed the location of each protein by indirect immunofluorescence using affinity purified antibodies. Four morphological states were distinguished in vegetative Dictyostelium amebae: ameboid cells before Con A binding, patched cells, capped cells, and ameboid cells with caps. The location of each protein was distinct in ameboid cells both before and after capping Actin and 120K were found in the cell cortex usually associated with surface projections, and myosin and 95K were diffusely distributed. Myosin was excluded from surface projections in ameboid cells. During patching, all four proteins were localized below Con A patches. During capping, actin, myosin, and 95K protein moved with the Con A patches into the cap whereas 120K protein was excluded from the cap. During the late stages of cap formation actin and myosin were progressively lost from the cap, and 120K became concentrated in new actin-filled projections that formed away from the cap. However, 95K remained tightly associated with the cap. Poisoning cells with sodium azide inhibited capping but not patching of ligand. In azide-poisoned cells, myosin and 95K did not co-patch with Con A, whereas copatching of 120K and actin with Con A occurred as usual. Our results support the hypothesis that capping is an actomyosin-mediated motile event that involves a sliding interaction between actin filaments, which are anchored through the membrane to ligand patches, and myosin in the cortex. They are also consistent with a role for 120K in the formation of surface projections by promoting growth and/or cross-linking of actin filaments within projections, and with a role for 95K in regulating actomyosin-mediated contractility, earlier proposals based on the in vitro properties of these two proteins (Condeelis, J., M. Vahey, J. M. Carboni, J. DeMey, S. Ogihara, 1984, J. Cell Biol., 99:119s-126s).

Role of coated vesicles, microfilaments, and calmodulin in receptor-mediated endocytosis by cultured B lymphoblastoid cells

Cell surface receptor IgM molecules of cultured human lymlphoblastoid cells (WiL2) patch and redistribute into a cap over the Golgi region of the cell after treatment with multivalent anti-IgM antibodies. During and after the redistribution, ligand-receptor clusters are endocytosed into coated pits and coated vesicles. Morphometric analysis of the distribution of ferritin-labeled ligand at EM resolution reveals the following sequence of events in the endocytosis of cell surface IgM: (a) binding of the multivalent ligand in a diffuse cell surface distribution, (b) clustering of the ligand-receptor complexes, (c) recruitment of clathrin coats to the cytoplasmic surface of the cell membrane opposite ligand-receptor clusters, (d) assembly and (e) internalization of coated vesicles, and (f) delivery of label into a large vesicular compartment, presumably partly lysosomal. Most of the labeled ligand enters this pathway. The recruitment of clathrin coats to the membrane opposite ligand-receptor clusters is sensitive to the calmodulin-directed drug Stelazine (trifluoperazine dihydrochloride). In addition, Stelazine inhibits an alternate pathway of endocytosis that does not involve coated vesicle formation. The actin-directed drug dihydrocytochalasin B has no effect on the recruitment of clathrin to the ligand-receptor clusters and the formation of coated pits and little effect on the alternate pathway, but this drug does interfere with subsequent coated vesicle formation and it inhibits capping. Cortical microfilaments that decorate with heavy meromyosin with constant polarity are observed in association with the coated regions of the plasma membrane and with coated vesicles. SDS-polyacrylamide gel electrophoresis analysis of a coated vesicle preparation isolated from WiL2 cells demonstrates that the major polypeptides in the fraction are a 175-kdalton component that comigrates with calf brain clathrin, a 42-kdalton component that comigrates with rabbit muscle actin and a 18.5-kdalton minor component that comigrates with calmodulin as well as 110-, 70-, 55-, 36-, 30-, and 17-kdalton components. These results clarify the pathways of endocytosis in this cell and suggest functional roles for calmodulin, especially in the formation of clathrin-coated pits, and for actin microfilaments in coated vesicle formation and in capping.

The contractile basis of amoeboid movement. V. The control of gelation, solation, and contraction in extracts from Dictyostelium discoideum

Motile extracts have been prepared from Dictyostelium discoideum by homogenization and differential centrifugation at 4 degrees C in a stabilization solution (60). These extracts gelled on warming to 25 degrees Celsius and contracted in response to micromolar Ca++ or a pH in excess of 7.0. Optimal gelation occurred in a solution containing 2.5 mM ethylene glycol-bis (beta-aminoethyl ether)N,N,N',N'-tetraacetate (EGTA), 2.5 mM piperazine-N-N'-bis [2-ethane sulfonic acid] (PIPES), 1 mM MgC1(2), 1 mM ATP, and 20 mM KCI at ph 7.0 (relaxation solution), while micromolar levels of Ca++ inhibited gelation. Conditions that solated the gel elicited contraction of extracts containing myosin. This was true regardless of whether chemical (micromolar Ca++, pH >7.0, cytochalasin B, elevated concentrations of KCI, MgC1(2), and sucrose) or physical (pressure, mechanical stress, and cold) means were used to induce solation. Myosin was definitely required for contraction. During Ca++-or pH-elicited contraction: (a) actin, myosin, and a 95,000-dalton polypeptide were concentrated in the contracted extract; (b) the gelation activity was recovered in the material sqeezed out the contracting extract;(c) electron microscopy demonstrated that the number of free, recognizable F-actin filaments increased; (d) the actomyosin MgATPase activity was stimulated by 4- to 10-fold. In the absense of myosin the Dictyostelium extract did not contract, while gelation proceeded normally. During solation of the gel in the absense of myosin: (a) electron microscopy demonstrated that the number of free, recognizable F- actin filaments increased; (b) solation-dependent contraction of the extract and the Ca++-stimulated MgATPase activity were reconstituted by adding puried Dictyostelium myosin. Actin purified from the Dictyostelium extract did not gel (at 2 mg/ml), while low concentrations of actin (0.7-2 mg/ml) that contained several contaminating components underwent rapid Ca++ regulated gelation. These results indicated : (a) gelation in Dictyostelium extracts involves a specific Ca++-sensitive interaction between actin and several other components; (b) myosin is an absolute requirement for contraction of the extract; (c) actin-myosin interactions capable of producing force for movement are prevented in the gel, while solation of the gel by either physical or chemical means results in the release of F-actin capable of interaction with myosin and subsequent contraction. The effectiveness of physical agents in producting contraction suggests that the regulation of contraction by the gel is structural in nature.

The self-assembly of synthetic filaments of myosin isolated from Chaos carolinensis and Amoeba proteus

Synthetic myosin thick filaments were formed from preparations of electrophoretically homogeneous myosin isolated from Chaos carolinensis and Amoeba proteus when dialysed to physiological ionic strength and pH. Myosin dialysed directly against low ionic strength buffers formed native-like thick filaments in the presence and absence of exogenous divalent cations. The average dimensions of the synthetic filaments grown under these conditions were 455 nm long and 16 nm wide with a distinct bare central zone 174 nm long. Myosin predialysed against EGTA-EDTA solutions at high ionic strength and then dialysed to low ionic strength formed native-like filaments only in the presence of 1mM Mg2+. 1 mM Ca2+ could not be substituted for Mg2+ under these conditions to achieve native-like filaments. Filaments grown from predialysed myosin in the absence of Mg2+ resembled EGTA-dissociated myosin filaments observed in EGTA-treated cytoplasm and were highly branched, poorly formed filaments lacking a distinct bare central zone. The average dimensions of the filaments grown from predialysed myosin in the absence of Mg2+ were 328 nm long, 13 nm wide with a bare central zone 111 nm long. Under the conditions tested, myosin isolated from these amoebae did not demonstrate a divalent cation requirement for thick filament formation. The results obtained with myosin isolated from the 2 organisms were identical.

The contractile basis of amoeboid movement. I. The chemical control of motility in isolated cytoplasm

Cytoplasm has been isolated from single amoeba (Chaos carolinensis) in physiological solutions similar to rigor, contraction, and relaxation solutions designed to control the contractile state of vertebrate striated muscle. Contractions of the isolated cytoplasm are elicited by free calcium ion concentrations above ca. 7.0 x 10(-7) M. Amoeba cytoplasmic contractility has been cycled repeatedly through stabilized (rigor), contracted, and relaxed states by manipulating the exogenous free calcium and ATP concentrations. The transition from stabilized state to relaxed state was characterized by a loss of viscoelasticity which was monitored as changes in the capacity of the cytoplasm to exhibit strain birefringence when stretched. When the stabilized cytoplasm was stretched, birefringent fibrils were observed. Thin sections of those fibrils showed thick (150-250 A) and thin (70 A) filaments aligned parallel to the long axis of fibrils visible with the light microscope. Negatively stained cytoplasm treated with relaxation solution showed dissociated thick and thin filaments morphologically identical with myosin aggregates and purified actin, respectively, from vertebrate striated muscle. In the presence of threshold buffered free calcium, ATP, and magnesium ions, controlled localized contractions caused membrane-less pseudopodia to extend into the solution from the cytoplasmic mass. These experiments shed new light on the contractile basis of cytoplasmic streaming and pseudopod extension, the chemical control of contractility in the amoeba cytoplasm, the site of application of the motive force for amoeboid movement, and the nature of the rheological transformations associated with the circulation of cytoplasm in intact amoeba.