Robert Wieder

Integrin α5β1 promotes survival of growth-arrested breast cancer cells: An in vitro paradigm for breast cancer dormancy in bone marrow

The mechanisms of long-term survival of occult breast cancer cells in the bone marrow microenvironment are not known. Using selected bone marrow stromal components with demonstrated roles in promoting growth arrest and survival of breast cancer cells, we reconstituted an in vitro model for dormancy of breast cancer cells in bone marrow. According to this model, basic fibroblast growth factor, a mammary differentiation factor abundant in the bone marrow stroma, induces growth arrest of relatively well-differentiated breast cancer cells, induces a spread appearance, and restricts their survival to fibronectin by up-regulating integrin α5β1. Most of the basic fibroblast growth factor-arrested cells fail to establish optimal ligation to fibronectin and undergo cell death. Cells that do attach to fibronectin, another major constituent of the bone marrow microenvironment, stay alive and growth-arrested for many weeks. Although capable of adhering to other stromal proteins collagen and laminin, dormant cells do not gain a survival advantage from these interactions. Using function-blocking peptides, we show a specific contribution of α5β1-fibronectin interaction in maintaining survival of growth-arrested cells, potentially by negatively modulating apoptotic response via signaling pathways. Blocking of phosphatidylinositol 3′-kinase and Akt inhibits survival of dormant clones, demonstrating this as one of those pathways. Experiments with human bone marrow stroma cocultures confirm the role of fibronectin ligation in maintaining survival of dormant clones.

1,25-dihydroxyvitamin D3 and retonic acid analogues induce differentiation in breast cancer cells with function- and cell-specific additive effects

Vitamin D3 derivatives and retinoids can induce cell cycle arrest, differentiation and cell death in many cell lines. These compounds can act cooperatively in some of their functions and may be of potential use either individually or in combination in the treatment of breast cancer. The effects of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), all-trans retinoic acid (ATRA) and several analogues were evaluated on malignant phenotypic traits of breast cancer cell lines MCF-7, T-47D and MDA-MB-231. Both 1,25(OH)2D3 and ATRA caused a decrease in anchorage independent colony formation in MCF-7 and T-47D cells in a dose-dependent manner. The effects of 1,25(OH)2D3 10−10 and 10−9M were synergistic with ATRA 10−8M in T-47D cells but were antagonistic in both MCF-7 and in T-47D cells at most concentrations. Both 1,25(OH)2D3 and ATRA individually induced an accumulation of MCF-7 cells in the G1 phase of the cell cycle and an associated increase in p21WAF1/Cip1, p27Kipl and a dephosphorylation of Rb but the effects were not additive. Both compounds inhibited the invasive capacity of MDA-MB-231 cells. 1,25(OH)2D3 but not ATRA caused an increase in E-cadherin levels in MDA-MB-231 cells. These two functions were not additive. The compounds 1,25(OH)2D3, a noncalcemic analogue 1,25(OH)2-16-ene-23-yne-D3, ATRA, AGN195183, an RARα-specific agonist, and AGN190168 (tazarotene), an RARβγ-selective agonist, induced differentiation as determined by measurements of lipid droplet formation. The individual effects of 1,25(OH)2-16-ene-23-yne-D3 combined with ATRA or with tazarotene at 10−9M each were additive in MCF-7 and MDA-MB-231 cells on lipid formation. The data demonstrate that both 1,25(OH)2D3, ATRA, and selected analogues induce a more differentiated phenotype in breast cancer cells with additive effects that are function- and cell-specific.

Overexpression of basic fibroblast growth factor (FGF-2) downregulates Bcl-2 and promotes apoptosis in MCF-7 human breast cancer cells.

Basic fibroblast growth factor (bFGF, FGF–2), a classical transforming factor, mitogen, and survival factor in multiple cell types, and has a paradoxic role in mammary epithelial cell transformation and proliferation. We have also demonstrated that recombinant FGF–2 uncharacteristically promotes cell death in MCF–7 human breast cancer cells. In this study, we investigated the effects of FGF–2 overexpression on survival in the same MCF–7 cells. In eight breast cancer cell lines and two nontransformed mammary epithelial cell lines, we demonstrated that high levels of Bcl–2 are only expressed in cells with undetectable levels of FGF–2 on western blot. In retrovirally transduced MCF–7 cells expressing both cytoplasm– and nucleus–localizing FGF–2 species and ones expressing only cytoplasm–localizing FGF–2 species, Bcl–2 levels were strongly decreased at both the mRNA and protein levels. Immunoprecipitation of Bax demonstrated a decreased association of Bax with Bcl–2 in these cells. Levels of Bax did not correlate with expression of FGF–2 in the 10 cell lines or in MCF–7 cells. The clonogenic potential of MCF–7 cells in tissue culture was decreased by the expression of FGF–2 and was additively suppressed by the chemotherapeutic agents etoposide and 5–fluorouracil in a dose and time dependent manner. MCF–7 cells overexpressing FGF–2 had a greater rate of programmed cell death at baseline and in response to etoposide and 5–fluorouracil in a TUNEL assay by immunofluorescent microphotography and by flow cytometric quantitation. The pro–apoptotic effect of FGF–2 overexpression on the chemosensitivity of these cells was confirmed by quantitative morphologic determination. These data demonstrate that the expression of FGF–2 downregulates Bcl–2 and promotes programmed cell death in MCF–7 human breast cancer cells.

Low Level Expression of Basic FGF Upregulates Bcl-2 and Delays Apoptosis, But High Intracellular Levels are Required to Induce Transformation in NIH 3T3 Cells

We investigated the roles of basic fibroblast growth factor (bFGF) in the transformation and survival of NIH 3T3 cells. We constructed NIH 3T3-derived cell lines expressing human bFGF using retroviral gene transfer with an N2-based vector. Clonally derived cell lines containing a single copy of the vector overexpress bFGF mRNA and produce more immunoreactive protein (0.407 +/- 0.010-3.028 +/- 0.087 ng bFGF/10(6) cells) which is biologically active than nontransduced (0.151 +/- 0.013 ng bFGF/10(6) cells) or N2-transduced (0.211 +/- 0.029 ng bFGF/10(6) cells) NIH 3T3 cells. All cells producing excess amounts of bFGF achieve greater density at confluence, show delayed apoptosis and increased survival and have elevated intracellular levels of Bcl-2. However, only cells expressing from 8-15 times background levels of bFGF are phenotypically transformed. The transformed cells form dense foci at confluence, have decreased adherence to tissue culture plates and grow colonies in soft agar. Exogenous bFGF induces higher Bcl-2 levels in a dose dependent manner and recapitulates the antiapoptotic effects of the overexpressed species but fails to induce changes associated with the transformed phenotype. In this study, we demonstrate a dissociation between phenotypic transformation secondary to bFGF overexpression and upregulation of cellular Bcl-2 that correlates with a delay in programmed cell death. Although low level expression of bFGF or exogenous bFGF is sufficient to upregulate Bcl-2 and delay apoptosis, high intracellular levels are required for cellular transformation. These data suggest that overexpression of bFGF modulates cellular transformation and Bcl-2-mediated inhibition of apoptosis through alternate molecular mechanisms.

Expression of FGF-2 alters focal adhesion dynamics in migration-restricted

Basic fibroblast growth factor (FGF-2) expression takes place during morphogenic differentiation of mammary ducts and is lost in breast cancer. Forced re-expression of FGF-2 in breast cancer cell lines induces a more differentiated phenotype and inhibits motility by unknown mechanisms. Here we demonstrate that MDA-MB-231 cells with encumbered motility due to forced re-expression of FGF-2 have activated focal complexes as determined by immunoprecipitation/western blotting and immunofluorescence staining with antibodies to FAK, p130Cas, paxillin, vinculin and phosphotyrosine. The activation of the focal adhesion complexes results in loss of stress fibers associated with malignant transformation of mammary epithelial cells and the formation of circumferentially-distributed actin bundles associated with non-transformed mammary epithelial cells. These effects require continuous FGF-2 expression, as the effects of exogenous recombinant FGF-2 are only small and transient. FGF-2 expression results in an increase in integrin α3 expression and decreases in integrin β1 and β4 expression. These changes, however, induce only a small decrease in adhesion to uncoated and fibronectin-coated tissue culture dishes suggesting that the primary cause of impaired motility is due to intrinsic signaling. These data suggest that FGF-2-inhibits motility in breast cancer cells by stabilization of focal complexes and induction of a more differentiated phenotype with disruption of stress fiber formation and a characteristic cortical actin distribution.

Flavopiridol Blocks Integrin-Mediated Survival in Dormant Breast Cancer Cells

Purpose: Breast cancer micrometastases in the bone marrow are resistant to chemotherapy. They can remain dormant for years before some begin to proliferate. We seek to understand survival mechanisms and develop targeted approaches to eliminating these cells. Experimental Design: In an in vitro model of dormancy, basic fibroblast growth factor 2 (FGF-2), abundant in the bone marrow, inhibits the growth of well-differentiated cells in the 2- to 10-cell stage and up-regulates integrin α5β1. Through this integrin, cells bind fibronectin, spread out, and acquire a survival advantage, partly through activation of the phosphatidylinositol 3-kinase/Akt pathway. We investigated the effects of Taxotere, flavopiridol, and mitogen-activated protein/extracellular signal-regulated kinase (ERK) kinase and p38 inhibitors on survival of dormant clones and that of flavopiridol on expression of integrins, adhesion strength, and phosphorylation of Akt, ERK 1/2, and p38. Results: Dormant MCF-7 and T-47D cell clones were resistant to Taxotere concentrations 10-fold higher than needed to eliminate growing clones but were almost completely eradicated by 200 nmol/L flavopiridol. Flavopiridol caused a decrease in FGF-2–induced expression of integrins, including α5 and β1, and decreased FGF-2–induced specific adhesion to fibronectin. It diminished Akt phosphorylation, but reexpression of active Akt was not sufficient to reverse dormant clone inhibition. Flavopiridol did not affect phosphorylation of ERK 1/2 and p38 but diminished total protein levels. Chemical inhibition of these pathways partially abrogated dormant clone survival. Conclusions: Flavopiridol has pleiotropic effects on key targets involved with survival of dormant breast cancer cells and may represent a useful approach to eliminating cells dependent on multiple signal pathways for survival.

Expression of FGF-2 alters focal adhesion dynamics in migration-restricted MDA-MB-231 breast cancer cells

Basic fibroblast growth factor (FGF-2) expression takes place during morphogenic differentiation of mammary ducts and is lost in breast cancer. Forced re-expression of FGF-2 in breast cancer cell lines induces a more differentiated phenotype and inhibits motility by unknown mechanisms. Here we demonstrate that MDA-MB-231 cells with encumbered motility due to forced re-expression of FGF-2 have activated focal complexes as determined by immunoprecipitation/western blotting and immunofluorescence staining with antibodies to FAK, p130Cas, paxillin, vinculin and phosphotyrosine. The activation of the focal adhesion complexes results in loss of stress fibers associated with malignant transformation of mammary epithelial cells and the formation of circumferentially-distributed actin bundles associated with non-transformed mammary epithelial cells. These effects require continuous FGF-2 expression, as the effects of exogenous recombinant FGF-2 are only small and transient. FGF-2 expression results in an increase in integrin alpha 3 expression and decreases in integrin beta 1 and beta 4 expression. These changes, however, induce only a small decrease in adhesion to uncoated and fibronectin-coated tissue culture dishes suggesting that the primary cause of impaired motility is due to intrinsic signaling. These data suggest that FGF-2-inhibits motility in breast cancer cells by stabilization of focal complexes and induction of a more differentiated phenotype with disruption of stress fiber formation and a characteristic cortical actin distribution.

Pharmacokinetics and safety of ILX23-7553, a non-calcemic-vitamin D3 analogue, in a phase I study of patients with advanced malignancies

Purpose: Differentiation therapy is an alternative to chemotherapy with potentially less toxicity, improved quality of life, and survival. We conducted a phase I trial of ILX23-7553, a formulation of 1,25-dihydroxy-16-ene-23-yne-vitamin D3, a 1,25-dihydroxyvitamin D3 analog with preclinically demonstrated antitumor and differentiating effects and diminished hypercalcemic effects. Patients and methods: The protocol consisted of five daily oral treatments during 14-day cycles at 15 dose levels from 1.3 to 45.0 μg/m2/day. We treated 42 heavily pretreated patients who had a variety of malignancies with 162 treatment cycles, and obtained pharmacokinetics from three patients at the two highest dose levels. Results: There were no grade 3 or 4 toxicities. Grade 1–2 toxicities included diarrhea, nausea, fatigue, constipation, and one grade 1 hypercalcemia. Average day 6 calcium was 9.26 ± 0.55 mg/dl in cycle 1 and 9.30 ± 0.67 mg/dl in cycle 2. Pharmacokinetics at dose levels 14 (40 μg/m2/day) (1 patient) and 15 (45 μg/m2/day) (2 patients) demonstrated an average Cmax of 30.4 ± 7.8 pg/ml (0.07 nM) and 104 ± 38.2 pg/ml (0.25 nM), and AUCs of 222.5 ± 225.2 pg·h/ml and 855 ± 536 pg h/ml, respectively. Eight patients (19%) had stable disease. While in vitro effects have been reported at these concentrations, they were at least 10-fold lower than ED50s, and the study was terminated before an MTD was reached. Conclusion: The drug is safe and has potential benefits at serum concentrations where effects begin to be noted in vitro. Further study is needed with a reformulated higher unit dose compound to determine the safety and efficacy of higher serum concentrations.

Dual FGF-2 and Intergrin α5β1 Signaling Mediate GRAF-Induced RhoA Inactivation in a Model of Breast Cancer Dormancy

Interactions with the bone marrow stroma regulate dormancy and survival of breast cancer micrometastases. In an in vitro model of dormancy in the bone marrow, we previously demonstrated that estrogen-dependent breast cancer cells are partially re-differentiated by FGF-2, re-express integrin α5β1 lost with malignant transformation and acquire an activated PI3K/Akt pathway. Ligation of integrin α5β1 by fibronectin and activation of the PI3K pathway both contribute to survival of these dormant cells. Here, we investigated mechanisms responsible for the dormant phenotype. Experiments demonstrate that integrin α5β1 controls de novo cytoskeletal rearrangements, cell spreading, focal adhesion kinase rearrangement to the cell perimeter and recruitment of a RhoA GAP known as GRAF. This results in the inactivation of RhoA, an effect which is necessary for the stabilization of cortical actin. Experiments also demonstrate that activation of the PI3K pathway by FGF-2 is independent of integrin α5β1 and is also required for cortical actin reorganization, GRAF membrane relocalization and RhoA inactivation. These data suggest that GRAF-mediated RhoA inactivation and consequent phenotypic changes of dormancy depend on dual signaling by FGF-2-initiated PI3K activation and through ligation of integrin α5β1 by fibronectin.

Overexpression of basic fibroblast growth factor in MCF-7 human breast cancer cells: Lack of correlation between inhibition of cell growth and MAP kinase activation

Basic fibroblast growth factor (bFGF, FGF‐2) is progressively lost from mammary epithelial cells as they become malignant. To investigate the effects of restoring the expression of bFGF in breast cancer cells, we constructed MCF‐7 cells that permanently overexpress 18‐kD cytoplasm‐localizing bFGF (MCF‐7/ΔAFGF(18) cells) and cells that express both the 18‐kD along with the 22‐ and 24‐kD nucleus‐localizing bFGF peptides (MCF‐7/NCFFGF(18,22,24) cells), using retroviral transduction. These stable cell constructs grew more slowly and had a larger fraction of their populations in the G0/G1 phase of the cell cycle than control cells. All forms of bFGF were eluted from MCF‐7/NCFFGF(18,22,24) cell monolayers with 2 M NaCl, in contrast to fibroblasts that were demonstrated to secrete only the 18‐kD bFGF isoform. High‐affinity binding of 18‐kD 125I‐bFGF to these cells was significantly decreased, probably because of competitive binding by the autocrine‐secreted bFGF. Recombinant 18‐kD bFGF that was previously demonstrated in our laboratory to inhibit proliferation, activate MAP kinase, and induce the cyclin‐dependent kinase inhibitor p21WAF1/CIP1 in MCF‐7 cells, further inhibited MCF‐7/ΔAFGF(18) cells but had no effect on MCF‐7/NCFFGF(18,22,24) cells. The total cellular content of the high‐affinity FGF receptors 1–3 was unchanged, but FGF receptor 4 was decreased in MCF‐7/NCFFGF(18,22,24) cells. Both cell types overexpressing bFGF isoforms had elevated levels of the cyclin‐dependent kinase inhibitor p27Kip1 but not that of p21WAF1/CIP1. In MCF‐7/ΔAFGF(18) cells, FGFR1 and MAP kinase were constitutively phosphorylated. Exogenous recombinant 18‐kD bFGF did not accentuate these effects but did induce an increase in the levels of p21WAF1/CIP1 corresponding to the further inhibition induced by exogenous bFGF in these cells. In MCF‐7/NCFFGF(18,22,24) cells, FGFR1 and MAP kinase were not phosphorylated at baseline nor upon stimulation with recombinant bFGF, and exogenous bFGF only had a minimal effect on low steady‐state p21WAF1/CIP1 levels. However, stimulation of these cells with phorbol ester or insulin did result in MAP kinase phosphorylation. While growth‐inhibited in the G1 phase of the cell cycle, MCF‐7/NCFFGF(18,22,24) cells retained active isoforms of cdk2 and the hyperphosphorylated form of Rb. These data suggest that high molecular weight forms of bFGF overexpressed in MCF‐7 cells do not activate the receptor‐mediated MAP kinase pathway, and do not induce p21WAF1/CIP1 in an autocrine manner, but inhibit proliferation through other, possibly direct nuclear signalling mechanisms. J. Cell. Physiol. 177:411–425, 1998.