Sphingosine-1-phosphate, a novel second messenger involved in cell growth regulation and signal transduction, affects growth and invasiveness of human breast cancer cells

This review will focus on the role of sphingosine and its phosphorylated derivative sphingosine-1-phosphate (SPP) in cell growth regulation and signal transduction. We will show that many of the effects attributed to sphingosine in quiescent Swiss 3T3 fibroblasts are mediated via its conversion to SPP. We propose that SPP has appropriate properties to function as an intracellular second messenger based on the following: it elicits diverse cellular responses; it is rapidly produced from sphingosine by a specific kinase and rapidly degraded by a specific lyase; its concentration is low in quiescent cells but increases rapidly and transiently in response to the growth factors, fetal calf serum (FCS) and platelet derived growth factor (PDGF); it releases Ca2+ from internal sources in an InsP3-independent manner; and finally, it may link sphingolipid signaling pathways to cellular ras-mediated signaling pathways by elevating phosphatidic acid levels. The effects of this novel second messenger on growth, differentiation and invasion of human breast cancer cells will be discussed. © 1994 Kluwer Academic Publishers.

Modulation of breast cancer progression and differentiation by the gp30/neregulin

In the last decade we have come to understand that the growth of cancer cells in general and of breast cancer in particular depends, in many cases, upon growth factors that will bind to and activate their receptors. One of these growth factor receptors is the erbB-2 protein which plays an important role in the prognosis of breast cancer and is overexpressed in nearly 30% of human breast cancer patients. While evidence accumulates to support the relationship between erbB-2 overexpression and poor overall survival in breast cancer, understanding of the biological consequence(s) of erbB-2 overexpression remains elusive. Our recent discovery of the gp30 has allowed us to identify a number of related but distinct biological endpoints which appear responsive to signal transduction through the erbB-2 receptor. These endpoints of growth, invasiveness, and differentiation have clear implications for the emergence, maintenance and/or control of malignancy, and represent established endpoints in the assessment of malignant progression in breast cancer. We have shown that gp30 induces a biphasic growth effect on cells with erbB-2 over-expression. We have recently determined the protein sequence of gp30 and cloned its full length cDNA sequence. We have also cloned two additional forms to the ligand, that are believed to be different isoforms. We are currently expressing the different forms in order to determine their biological effects. To elucidate the cellular mechanisms underlying cell growth inhibition by gp30, we tested the effect of this ligand on cell growth and differentiation of the human breast cancer cells which overexpress erbB-2 and cells which express low levels of this protooncogene. High concentrations of ligand induced differentiation of cells overexpressing erbB-2, as measured by inhibition of cell growth, and increased synthesis of milk components, and modulation of E-cadherin and up- regulation of c-jun and c-fos. These findings indicate that ligand-induced growth inhibition in cells overexpressing erbB-2 is associated with an apparent induction of differentiation. The availability of gp30 derived synthetic peptides and its full cDNAs provides tools necessary to acquire a better understanding of the mechanism of action of the this ligands and the erbB-2 receptor in breast cancer.

Collagen induced MMP-2 activation in human breast cancer

Matrix metalloproteinase-2 (MMP-2), a zymogen requiring proteolytic activation for catalytic activity, has been implicated broadly in the invasion and metastasis of many cancer model systems, including human breast cancer (HBC). MMP-2 has been immunolocalized to carcinomatous human breast, where the degree of activation of MMP-2 correlates well with tumor grade and patient prognosis. Using Matrigel assays, we have stratified HBC cell lines for invasiveness in vitro, and compared this to their potential for metastatic spread in nude mice. HBC cell lines expressing the mesenchymal marker protein vimentin were found to be highly invasive in vitro, and tended to form metastases in nude mice. We have further discovered that culture on collagen-I gels (Vitrogen(TM): Vg) induces MMP-2-activator in highly invasive but not poorly invasive HBC cell lines. As seen for other MMP-2-activator inducing regimens, this induction requires protein synthesis and an intact MMP-2 hemopexin-like domain, appears to be mediated by a cell surface activity, and can be inhibited by metalloproteinase inhibitors. The induction is highly specific to collagen I, and is not seen with thin coatings of collagen I, collagen IV, laminin, or fibronectin, or with 3-dimensional gels of laminin, Matrigel, or gelatin. This review focuses on collagen I and MMP- 2, their localization and source in HBC, and their relationship(s) to MMP-2 activation and HBC metastasis. The relevance of collagen I in activation of MMP-2 in vivo is discussed in terms of stromal cell: tumor cell interaction for collagen I deposition, MMP-2 production and MMP-2-activation. Such cooperativity may exist in vivo for MMP-2 participation in HBC dissemination. A more complete understanding of the regulation of MMP-2-activator by type I collagen may provide new avenues for improved diagnosis and prognosis of human breast cancer.

MCF7/LCC2: A 4-hydroxytamoxifen resistant human breast cancer variant that retains sensitivity to the steroidal antiestrogen ICI 182,780

The development of resistance to the antiestrogen tamoxifen occurs in a high percentage of initially responsive patients. We have developed a new model in which to investigate acquired resistance to triphenylethylenes. A stepwise in vitro selection of the hormone-independent human breast cancer variant MCF-7/LCC1 against 4-hydroxytamoxifen produced a stable resistant population designated MCF7/LCC2. MCF7/LCC2 cells retain levels of estrogen receptor expression comparable to the parental MCF7/LCC1 and MCF-7 cells. Progesterone receptor expression remains estrogen inducible in MCF7/LCC2 cells, although to levels significantly lower than observed in MCF-7 and MCF7/LCC1 cells. MCF7/ LCC2 cells form tumors in ovariectomized nude mice without estrogen supplementation, and these tumors are tamoxifen resistant but can be tstrogen stimulated. Significantly, MCF7/LCC2 cells have retained sensitivity to the steroidal antiestrogen ICI 182,780. These data suggest that some breast cancer patients who acquire resistance to tamoxifen may not develop cross-resistance to treatment with steroidal antiestrogens.

SPARC/osteonectin induces matrix metalloproteinase 2 activation in human breast cancer cell lines

Activation of the matrix metalloproteinase 2 (MMP-2) has been shown to play a major role in the proteolysis of extracellular matrix (ECM) associated with tumor invasion. Although the precise mechanism of this activation remains elusive, levels of the membrane type 1-MMP (MT1-MMP) at the cell surface and of the tissue inhibitor of MMP-2 (TIMP-2) appear to be two important determinants. Induction of MMP-2 activation in cells cultivated on collagen type I gels indicated that the ECM is important in the regulation of this process. In this study, we show that SPARC/osteonectin, a small ECM- associated matricellular glycoprotein, can induce MMP-2 activation in two invasive breast cancer cell lines (MDA-MB-231 and BT549) but not in a noninvasive counterpart (MCF7), which lacks MT1-MMP. Using a set of peptides from different regions of SPARC, we found that peptide 1.1 (corresponding to the NH2-terminal region of the protein) contained the activity that induced NIMP-2 activation. Despite the requirement for MT1-MMP, seen in MCF-7 cells transfected with MT1-MMP, the activation of MMP-2 by SPARC peptide 1.1 was not associated with increased steady-state levels of MT1-MMP mRNA or protein in either MT1-MMP-transfected MCF-7 cells or constitutively expressing MDA- MB-231 and BT549 cells. We did, however, detect decreased levels of TIMP-2 protein in the media of cells incubated with peptide 1.1 or recombinant SPARC; thus, the induction of MMP-2 activation by SPARC might be due in part to a diminution of TIMP-2 protein. We conclude that SPARC, and specifically its NH2-terminal domain, regulates the activation of MMP-2 at the cell surface and is therefore likely to contribute to the proteolytic pathways associated with tumor invasion.

The heat shock protein 90 inhibitor, 17-allylamino-17- demethoxygeldanamycin, enhances osteoclast formation and potentiates bone metastasis of a human breast cancer cell line

Breast cancer metastasis to the bone occurs frequently, causing numerous complications including severe pain, fracture, hypercalcemia, and paralysis. Despite its prevalence and severity, few effective therapies exist. To address this, we examined whether the heat shock protein 90 (Hsp90) inhibitor, 17-allylamino-17-demethoxygeldanamycin (17-AAG), would be efficacious in inhibiting breast cancer metastasis to bone. Utilizing the human breast cancer subline, MDA-MB-231SA, previously in vivo selected for its enhanced ability to generate osteolytic bone lesions, we determined that 17-AAG potently inhibited its in vitro proliferation and migration. Moreover, 17-AAG significantly reduced MDA-MB-231SA tumor growth in the mammary-fat pad of nude mice. Despite these findings, 17-AAG enhanced the incidence of bone metastasis and osteolytic lesions following intracardiac inoculation in the nude mouse. Consistent with these findings, 17-AAG enhanced osteoclast formation 2- to 4-fold in mouse bone marrow/osteoblast cocultures, receptor activator of nuclear factor κB ligand (BANKL)-stimulated bone marrow, and RAW264.7 cell models of in vitro osteoclastogenesis. Moreover, the drug enhanced osteoclastogenesis in human cord blood progenitor cells, demonstrating that its effects were not limited to mouse models. In addition to 17-AAG, other Hsp90 inhibitors, such as radicicol and herbimycin A, also enhanced osteoclastogenesis. A pro-osteolytic action of 17-AAG independent of tumor presence was also determined in vivo, in which 17-AAG-treated tumor-naive mice had reduced trabecular bone volume with an associated increase in osteoclast number. Thus, HSP90 inhibitors can stimulate osteoclast formation, which may underlie the increased incidence of osteolysis and skeletal tumor incidence causedby 17-AAG in vivo. These data suggest an important contraindication to the Hsp90 targeted cancer therapy currently undergoing clinical trial.

Epidermal growth factor promotes MDA-MB-231 breast cancer cell migration through a phosphatidylinositol 3'-kinase and phospholipase C-dependent mechanism

Epidermal growth factor receptor (EGFR) levels predict a poor outcome in human breast cancer and are most commonly associated with proliferative effects of epidermal growth factor (EGF), with little emphasis placed on motogenic responses to EGF. We found that MDA-MB-231 human breast cancer cells elicited a potent chemotactic response despite their complete lack of a proliferative response to EGF. Antagonists of EGFR ligation, the EGFR kinase, phosphatidylinositol 3'-kinase, and phospholipase C, but not the mitogen- activated protein kinases (extracellular signal-regulated protein kinase 1 and 2), blocked MDA-MB-231 chemotaxis. These findings suggest that EGF may influence human breast cancer progression via migratory pathways, the signaling for which appears to be dissociated, at least in part, from the proliferative pathways.

Loss of epithelial markers and acquisition of vimentin expression in adriamycin- and vinblastine-resistant human breast cancer cell lines

We have previously observed that breast cancer cell lines could exhibit either epithelial or fibroblastic phenotypes as reflected by their morphologies and intermediate filament protein expression (C. L. Sommers, D. Walker-Jones, S. E. Heckford, P. Worland, E. Valverius, R. Clark, M. Stampfer, and E. P. Gelmann, Cancer Res., 49: 4258-4263, 1989). Fibroblastoid, vimentin-expressing breast cancer cell lines are more invasive in vitro and in vivo (E. W. Thompson, S. Paik, N. Brunner, C. L. Sommers, G. Zugmaier, R. Clarke, T. B. Shima, J. Torri, S. Donahue, M. E. Lippman, G. R. Martin, and R. B. Dickson, J. Cell. Physiol., 150: 534-544, 1992). We hypothesized that a breast cancer cell with an epithelial phenotype could undergo a transition to a fibroblastic phenotype, possibly resulting in more invasive capacity. We now show that two Adriamycin-resistant MCF-7 cell lines and a vinblastine-resistant ZR-75-B cell line have undergone such a transition. Adriamycin-resistant MCF-7 cells express vimentin, have diminished keratin 19 expression, have lost cell adhesion molecule uvomorulin expression, and have reduced formation of desmosomes and tight junctions as determined by reduced immunodetection of their components desmoplakins I and II and zonula occludens (ZO)-1. Other MCF-7 cell lines selected for resistance to vinblastine and to Adriamycin and verapamil did not have these characteristics, indicating that drug selection does not invariably cause these phenotypic changes. In addition, to determine if vimentin expression in MCF-7 cells alone could manifest a fibroblastic phenotype, we transfected the full-length human vimentin complementary DNA into MCF-7 cells. Although vimentin expression was achieved in MCF-7 cells, it did not affect the phenotype of the cells in terms of the distribution of keratins, desmoplakins I and II, ZO-1, or uvomorulin or in terms of in vitro invasiveness. We conclude that vimentin expression is a marker for a fibroblastic and invasive phenotype in breast cancer cells but does not by itself give rise to this phenotype.

Pro-matrix metalloproteinase-2 transfection increases orthotopic primary growth and experimental metastasis of MDA-MB-231 human breast cancer cells in nude mice

The ability to activate pro-matrix metalloproteinase (pro-MMP)-2 via membrane type-MMP is a hallmark of human breast cancer cell lines that show increased invasiveness, suggesting that MMP-2 contributes to human breast cancer progression. To investigate this, we have stably transfected pro-MMP-2 into the human breast cancer cell line MDA-MB-231, which lacks MMP-2 expression but does express its cell surface activator, membrane type 1-MMP. Multiple clones were derived and shown to produce pro-MMP-2 and to activate it in response to concanavalin A. In vitro analysis showed that the pro-MMP-2-transfected clones exhibited an increased invasive potential in Boyden chamber and Matrigel outgrowth assays, compared with the parental cells or those transfected with vector only. When inoculated into the mammary fat pad of nude mice, each of the MMP-2-tranfected clones grew faster than each of the vector controls tested. After intracardiac inoculation into nude mice, pro-MMP-2-transfected clones showed a significant increase in the incidence of metastasis to brain, liver, bone, and kidney compared with the vector control clones but not lung. Increased tumor burden was seen in the primary site and in lung metastases, and a trend toward increased burden was seen in bone, however, no change was seen in brain, liver, or kidney. This data supports a role for MMP-2 in breast cancer progression, both in the growth of primary tumors and in their spread to distant organs. MMP-2 may be a useful target for breast cancer therapy when refinement of MMP inhibitors provides for MMP-specific agents.

Differential regulation of growth and invasiveness of MCF-7 breast cancer cells by antiestrogens

Estrogen increases the ability of the estrogen-dependent MCF-7 human breast cancer cell line to both proliferate and invade through an artificial basement membrane. In studying the response of MCF-7 cells to various antiestrogens, we found that 4-hydroxytamoxifen and tamoxifen inhibited cell proliferation but increased their invasiveness. In contrast, the structurally unrelated benzothiophene antiestrogens, LY117018 and LY156758, were potent antiproliferative agents which did not stimulate invasiveness. The differential effects of these antiestrogenic agents on invasion correlated with changes in production of collagenase IV, while no significant change was seen in the chemotactic activity of the cells. Invasiveness was increased by 17β-estradiol or 4-hydroxytamoxifen after a few hours of treatment and was rapidly lost when 17β-estradiol was withdrawn. Stimulation of invasiveness with 17β-estradiol was blocked by the antiestrogen, LY117018. Cells from the MDA-MB-231 line which lacks estrogen receptors were not affected by estrogen or antiestrogen in terms of proliferation or invasion. These studies indicate that the invasiveness of MCF-7 cells is regulated by antiestrogens through the estrogen receptor and may be mediated by collagenase IV activity. Antiestrogens which reduce both the proliferation and invasiveness of these cells may be interesting new candidates for clinical application.

Complex regulation of membrane-type matrix metalloproteinase expression and matrix metalloproteinase-2 activation by concanavalin A in MDA-MB-231 human breast cancer cells

Matrix Metalloproteinase-2 (MMP-2) is secreted as a zymogen, the activation of which has been associated with metastatic progression in human breast cancer (HBC). Concanavalin A (Con A) has been found to induce activation of MMP-2 in invasive HBC cell lines. Con A effects on the expression of mRNA for membrane-type matrix metalloproteinase (MT-MMP), a newly described cell surface-associated MMP, showed a close temporal correlation with induction of MMP-2 activation. It is surprising that MT-MMP mRNA is constitutively present in the uninduced MDA-MB-231 cell, despite a lack of MMP-2 activation. We have used actinomycin D to demonstrate a partial requirement for de novo gene expression in the induction of MMP-2 activation by Con A in MDA-MB-231 HBC cells. Furthermore, this transcriptional response to Con A appeared to require the continued presence of Con A for its manifestation. The nontranscriptional component of the Con A induction manifests rapidly, is quite substantial, and persists strongly despite actinomycin D abrogation of both constitutive and Con A-induced MT-MMP. Cycloheximide analyses suggest that protein synthesis may be involved in this rapid transcription-independent response. These studies suggest that Con A induces MMP-2-activation in part by up-regulation of MT-MMP expression but has a more complicated mode of action, involving additional nontranscriptional effects, which apparently require protein synthesis.

Expression of 67 kDa laminin receptor in human breast cancer cells : regulation by progestins

The level of 67 kDa laminin receptor (67LR) expression on breast and colon tumor cell surfaces was previously shown to be correlated with the capacity of tumor cells to metastasize. In the present work we investigate the effects of progestins and estrogen on the expression of 67LR in two sublines of the T47D human breast cancer cells: weakly tumorigenic, poorly invasive parental T47D cells and a highly tumorigenic, more invasive T47Dco subclone. Inmmunoblotting with an affinity purified antibody directed against a synthetic peptide recognizes the 67LR in these cells. 67LR expression in the T47Dco subclone is 5,5-fold higher than in their parental T47D cells. Treatment of T47D cells with 1 nM of the synthetic progestin R5020 results in a 4-fold increase in 67LR protein expression. Estrogen also induced 67LR expression, but only by 1.5-fold. The progestin-stimulated expression of the 67LR correlates with a 4.3-fold increase in attachment of T47D cells to laminin. A monoclonal antibody, mAb 13, directed against β1 integrin, completely blocks the attachment of T47D cells to fibronectin, only partially inhibits the attachment of T47D cells to laminin, and appears not to affect the progestin-stimulated laminin attachment of T47D cells. A new antiprogestin, ZK 112.993, significantly inhibits both progestin-stimulated 67LR expression and the increased attachment to laminin. These results suggest a possible role for progestin in mediating one of the multiple events thought to be important in metastasis of steroid receptor positive human breast cancer cells.

The invasive and metastatic properties of hormone-independent but hormone-responsive variants of MCF-7 human breast cancer cells

We have previously isolated a series of MCF-7 human breast cancer cell variants which no longer require estrogen-supplementation for tumor growth in nude mice (Clarke et al. Proc Natl Acad Sci USA 86: 3649-3653, 1989). We now report that these hormone-independent and hormone-responsive variants (MIII, MCF7/LCC1) can invade locally from solid mammary fat pad tumors, and produce primary extensions on the surface of intraperitoneal structures including liver, pancreas, and diaphragm. Both lymphatic and hematogenous dissemination are observed, resulting in the establishing of pulmonary, bone, and renal metastases. The pattern of metastasis by MIII and MCF7/LCC1 cells closely resembles that frequently observed in breast cancer patients, and provides the first evidence of metastasis from MCF-7 cells growing in vivo without supplementary estrogen. The interexperimental incidence of metastases, and the time from cell inoculation to the appearance of metastatic disease are variable. The increased metastatic potential is not associated with an increase in either the level of laminin attachment, laminin receptor mRNA expression, or secreted type IV collagenolytic activity. We also did not detect a significant decrease in the steady-state mRNA levels of the metastasis inhibitor nm23 gene. However, when growing without estrogen in vitro, MCF7/LCC1 cells produce elevated levels of the estrogen-inducible cathepsin D enzyme.

Epithelial mesenchymal transition traits in human breast cancer cell lines

Epithelial mesenchymal transition (EMT) has long been associated with breast cancer cell invasiveness and evidence of EMT processes in clinical samples is growing rapidly. Genome-wide transcriptional profiling of increasingly larger numbers of human breast cancer (HBC) cell lines have confirmed the existence of a subgroup of cell lines (termed Basal B/Mesenchymal) with enhanced invasive properties and a predominantly mesenchymal gene expression signature, distinct from subgroups with predominantly luminal (termed Luminal) or mixed basal/luminal (termed Basal A) features (Neve et al Cancer Cell 2006). Studies providing molecular and cellular analyses of EMT features in these cell lines are summarised, and the expression levels of EMT-associated factors in these cell lines are analysed. Recent clinical studies supporting the presence of EMT-like changes in vivo are summarised. Human breast cancer cell lines with mesenchymal properties continue to hold out the promise of directing us towards key mechanisms at play in the metastatic dissemination of breast cancer.

Human breast cancer cell metastasis to long bone and soft organs of nude mice : a quantitative assay

Bone is a common metastatic site in human breast cancer (HBC). Since bone metastasis occurs very rarely from current spontaneous or experimental metastasis models of HBC cells in nude mice, an arterial seeding model involving the direct injection of the cells into the left ventricle has been developed to better understand the mechanisms involved in this process. We present here a sensitive polymerase chain reaction (PCR) method to detect and quantitate bone and soft organ metastasis in nude mice which have been intracardially inoculated with Lac Z transduced HBC cells. Amplification of genomically incorporated Lac Z sequences in MDA-MB-231-BAG HBC cells enables us to specifically detect these cells in mouse organs and bones. We have also created a competitive template to use as an internal standard in the PCR reactions, allowing us to better quantitate levels of HBC metastasis. The results of this PCR detection method correlate well with cell culture detection from alternate long bones from the same mice, and are more sensitive than gross Lac Z staining with X-gal or routine histology. Comparable qualitative results were obtained with PCR and culture in a titration experiment in which mice were inoculated with increasing numbers of cells, but PCR is more quantifiable, less time consuming, and less expensive. This assay can be employed to study the molecular and cellular aspects of bone metastasis, and could easily be used in conjunction with RT-PCR-based analyses of gene products which may be involved with HBC metastasis.