Background: The 4T1 mouse mammary tumor cell line is one of only a few breast cancer models with the capacity to metastasize efficiently to sites affected in human breast cancer. Here we describe two 4T1 cell lines modified to facilitate analysis of tumor growth and metastasis and evaluation of gene function in vivo. New information regarding the involvement of innate and acquired immunity in metastasis and other characteristics of the model relevant to its use in the study of late stage breast cancer are reported.

It has previously been shown that highly invasive MDA-MB231 human breast cancer cells express vacuolar protontranslocating ATPase (V-ATPases) at the cell surface, whereas the poorly invasive MCF7 cell line does not. Bafilomycin, a specific V-ATPase inhibitor, reduces the in vitro invasion of MB231 cells but not MCF7 cells. Targeting of V-ATPases to different cellular membranes is controlled by isoforms of subunit a. mRNA levels for a subunit isoforms were measured in MB231 and MCF7 cells using quantitative reverse transcription-PCR. The results show that although all four isoforms are detectable in both cell types, levels of a3 and a4 are much higher in MB231 than in MCF7 cells. Isoform-specific small interfering RNAs (siRNA) were employed to selectively reduce mRNA levels for each isoform in MB231 cells. V-ATPase function was assessed using the fluorescent indicators SNARF-1 and pyranine to monitor the pH of the cytosol and endosomal/lysosomal compartments, respectively. Cytosolic pH was decreased only on knockdown of a3, whereas endosome/lysosome pH was increased on knockdown of a1, a2, and a3. Treatment of cells with siRNA to a4 did not affect either cytosolic or endosome/lysosome pH. Measurement of invasion using an in vitro transwell assay revealed that siRNAs to both a3 and a4 significantly inhibited invasion of MB231 cells. Immunofluorescence staining of MB231 cells for V-ATPase distribution revealed extensive intracellular staining, with plasma membrane staining observed in ϳ18% of cells. Knockdown of a4 had the greatest effect on plasma membrane staining, leading to a 32% reduction. These results suggest that the a4 isoform may be responsible for targeting V-ATPases to the plasma membrane of MB231 cells and that cell surface V-ATPases play a significant role in invasion. However, other V-ATPases affecting the pH of the cytosol and intracellular compartments, particularly those containing a3, are also involved in invasion.The leading cause of mortality from cancer is metastasis, making inhibition of metastasis an important strategy in controlling cancer progression. The metastatic cascade involves a series of steps that include escape of cells from the site of the primary tumor into the circulation or lymphatic system and the extravasation of cells from the circulation or lymphatic system into secondary sites (1, 2). Both of these processes require the tumor cells to display an invasive phenotype in which they degrade extracellular matrix of the surrounding tissue. The vacuolar H ϩ -ATPases (or V-ATPases) 2 are a family of ATP-dependent proton pumps that have been implicated in tumor cell invasion (3). V-ATPases are up-regulated in tissue samples from highly invasive pancreatic carcinomas (4), and treatment of a human cancer cell line with antisense oligonucleotides to the V-ATPase c subunit decreased invasion in vitro (5). Inhibition of V-ATPase expression in hepatocellular carcinoma cells using siRNAs reduces invasiveness of these cells in vitro and metastasis in vivo (6). In addition, treatment of hig...

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A receptor that binds the phosphomannosyl recognition marker of bovine testicular P-galactosidase (13-D-galactoside galactohydrolase, EC 3.2.1.23) was isolated from bovine liver membranes. The receptor was extracted from crude plasma membrane preparations with Triton X-100 and immunoprecipitated as a receptor-f-galactosidase complex with anti-,B-galactosidase. The receptor was dissociated from the precipitate with mannose 6-phosphate, labeled with`RI, and purified on a (3-galactosidase-Sepharose 4B affinity matrix. A quantitative binding assay employing anti-(-galactosidase and IgGsorb (formalin-fixed Staphylococcus aureus) was devised to study the binding of`5I-labeled receptor to (3-galactosidase. Maximal binding of receptor to enzyme occurred at pH values between 5.7 and 6.5. Divalent cations were not required for binding. The values of the dissociation constant obtained for 83-galactosidase varied between 200 nM observed with "lower uptake" forms and 20 nM for "higher uptake" forms of the enzyme. A number of phosphorylated monosaccharides were tested as inhibitors of binding of enzyme to receptor; mannose 6-phosphate and fructose 1-phosphate served as inhibitors and exhibited 1I values of 0.064 mM and 0.24 mM, respectively. The receptor has a subunit molecular weight of 215,000. Similar receptors were also demonstrated in Triton X-100 extracts of human skin fibroblasts, Chinese hamster ovary cells, and rat hepatocytes. These cell types are known to assimilate lysosomal enzymes containing covalently bound mannose 6-phosphate residues.Certain lysosomal enzymes are selectively and efficiently taken up by cultured human fibroblasts (1). This process is thought to be mediated by a specific cell surface receptor that recognizes phosphomannosyl residues on oligosaccharide chains of the enzymes (2-5). Recognition of lysosomal enzymes by a phosphomannosyl receptor has been proposed as an essential step for the delivery of newly synthesized lysosomal enzymes to lysosomes (6-8). Direct evidence for the existence of phosphomannosyl receptors has been obtained by demonstration of the reversible binding of a-L-iduronidase to the cell surface ofhuman skin fibroblasts (9) and by the binding of P3-glucuronidase to fibroblast cell membranes (10).Phosphomannosyl receptors also occur in other mammalian cell types and tissues. Phosphomannosyl-dependent uptake or binding of lysosomal enzymes has been observed in Chinese hamster ovary cells (11), normal rat kidney cells (12), a rat liver epithelial cell line (13), rat hepatocytes (12,14), and Swarm rat chondrosarcoma (15). Binding studies using f-hexosaminidase suggest the presence of the phosphomannosyl receptor in all major tissues of the rat and in several rat liver subcellular fractions (16).In previous studies we demonstrated the presence of mannose 6-phosphate residues in f3-galactosidase (P-D-galactoside galactohydrolase, EC 3.2.1.23) (17,18) and showed that the enzyme is subject to endocytosis by the phosphomannosyl uptake system in human skin fibroblasts (5). We ...

Previous studies have shown that lysine residues on the surface of cathepsins and other lysosomal proteins are a shared component of the recognition structure involved in mannose phosphorylation. In this study, the involvement of specific lysine residues in mannose phosphorylation of cathepsin D was explored by site-directed mutagenesis. Mutation of two lysine residues in the mature portion of the protein, Lys-203 and Lys-293, cooperated to inhibit mannose phosphorylation by 70%. Other positively charged residues could not substitute for lysine at these positions, and comparison of thermal denaturation curves for the wild type and mutant proteins indicated that the inhibition could not be explained by alterations in protein folding. Structural comparisons of the two lysine residues with those required for phosphorylation of cathepsin L, using models generated from recently acquired crystal structures, revealed several relevant similarities. On both molecules, the lysine residues were positioned approximately 34 Å apart (34.06 Å for cathepsin D and 33.80 Å for cathepsin L). When the lysine pairs were superimposed, N-linked glycosylation sites on the two proteins were found to be oriented so that oligosaccharides extending out from the sites could share a common region of space. Further similarities in the local environments of the critical lysines were also observed. These results provide details for a common lysosomal targeting structure based on a specific arrangement of lysine residues with respect to each other and to glycosylation sites on the surface of lysosomal proteins.The mammalian lysosomal protein targeting system has the capability of recognizing and modifying lysosomal hydrolases and growth factors from a wide range of protein families with high specificity. The molecular basis for this selectivity is due to the activity of UDP-GlcNAc, lysosomal enzyme N-acetylglucosamine-1-phosphotransferase, which phosphorylates Nlinked oligosaccharides of lysosomal proteins by the addition of phospho-GlcNAc (1-5). This modification begins after lysosomal proteins are exported from the endoplasmic reticulum and is followed by removal of terminal GlcNAc moieties and binding to mannose 6-phosphate receptors as the proteins traverse the Golgi apparatus. The receptors mediate delivery of the phosphorylated proteins to the endosomal compartment, and from there the proteins are transported to lysosomes (for a review, see Ref. 1).Studies focusing on the molecular basis for mannose phosphorylation have shown that the phosphotransferase recognizes a protein-based structure or "signal" on lysosomal proteins and that this recognition is required for efficient phosphorylation in vivo and in vitro (5). Expression of this phosphorylation signal requires that the protein be in its native conformation (4 -6). Analysis of the cathepsin D signal, using chimeras of cathepsin D and the nonphosphorylated homologous aspartic protease pepsinogen, defined a "minimal" structure for phosphorylation involving Lys-203 and a segment of 27 ami...

Metastasis remains a leading cause of cancer mortality due to the lack of specific inhibitors against this complex process. To identify compounds selectively targeting the metastatic state, we used the perinuclear compartment (PNC), a complex nuclear structure associated with metastatic behaviors of cancer cells, as a phenotypic marker for a high-content screen of over 140,000 structurally diverse compounds. Metarrestin, obtained through optimization of a screening hit, disassembles PNCs in multiple cancer cell lines, inhibits invasion in vitro, blocks metastatic development in three mouse models of human cancer, and extends survival of mice in a metastatic pancreatic cancer xenograft model with no organ toxicity or discernable adverse effects. Metarrestin disrupts the nucleolar structure and inhibits RNA polymerase (Pol) I transcription, at least in part by interacting with the translation elongation factor eEF1A2. Altogether, metarrestin represents a potential therapeutic approach for the treatment of metastatic cancer.

The recognition of lysosomal enzymes by UDP-GlcNAc: lysosomal-enzyme GlcNAc-1-phosphotransferase (phosphotransferase) is mediated by a protein structure on lysosomal enzymes. It has been previously demonstrated that lysine residues are required for phosphorylation of procathepsin L and are a common feature of the site on many lysosomal proteins. In this work, the procathepsin L recognition structure was further defined by identification of the region of the protein containing the structure and the critical lysine residues involved. Removal of the cathepsin L propeptide by low pH-induced autocatalytic processing abolished phosphorylation. The addition of either the purified propeptide or a glutathione S-transferase-propeptide fusion protein to the processed protein restored phosphorylation. Mutagenesis of individual lysine residues demonstrated that two propeptide lysine residues (Lys-54 and Lys-99) were required for efficient phosphorylation of procathepsin L. By comparison of the phosphorylation rates of procathepsin L, lysine-modified procathepsin L, and the procathepsin L oligosaccharide, lysine residues were shown to account for most, if not all, of the protein-dependent interaction. On this basis, it is concluded that the proregion lysine residues are the major elements of the procathepsin L recognition site. In addition, lysine residues in cathepsin D were shown to be as important for phosphorylation as those in procathepsin L, supporting a general model of the recognition site as a specific three-dimensional arrangement of lysine residues exposed on the surface of lysosomal proteins.