Background

Metastatic disease develops in 40 % of early breast cancer patients [1] and rapidly metastasizes to bone in 85 % [2, 3]. Bone metastases are osteolytic [2], releasing factors from the matrix stimulating tumor growth and bone resorption: the “vicious cycle” [2]. Because inflammatory breast cancer (IBC) patients have lymph node involvement and distant metastases at diagnosis, IBC is similar to metastatic breast cancer [1].

Interaction of activated α5β1 integrin fibronectin receptors of metastatic or inflammatory breast cancer cells with the plasma fibronectin (pFn) PHSRN sequence induces constitutive invasion [4]. Moreover, α5β1 regulates invasion [5], angiogenesis [6], extravasation, and colonization by breast and other adenocarcinomas [7–13]. Radiation stimulates invasion by inducing surface upregulation of activated α5β1 integrin [14].

We devised a potent, targeted α5β1 integrin inhibitor, Ac-PHSCN-NH2 (PHSCN), as a validated lead compound to prevent metastatic [5] and angiogenic invasion [6]. In Phase I clinical trial, systemic PHSCN monotherapy prevented disease progression for up to 14 months [15]. Here, we report that Ac-PhScN-NH2—containing D-isomers of histidine (h) and cysteine (c)—is over 100,000-fold more potent than PHSCN at blocking basement membrane invasion by MDA-MB-231 and SUM149PT breast cancer cells. We also report that PhScN is 100- to 10,000-fold more potent at inhibiting lung extravasation, and 1000- to 10,000-fold more potent at reducing lung colonization in nude mice. Finally, we report that systemic 50 mg/kg PhScN monotherapy reduces established MDA-MB-231 intratibial colony progression by almost 80 %. Thus, PhScN is a highly potent, well-tolerated inhibitor of bone colony progression.

Materials and methods

Cell lines and cell culture

SUM149PT (Asterand USA, Detroit, MI), MDA-MB-231, and human microvascular endothelial cells (American Type Culture Collection, Manassas, VA) were cultured as recommended, and microscopic morphologies were routinely checked.

Peptide synthesis

N-terminal-acetylated, C-terminal-amidated PhScN, PHSCN, hSPNc, and HSPNC peptides, PhScNGGK-MAP poly-lysine dendrimer, and cysteine or biotinylated (-Bio) derivatives were synthesized and purified to 95 % by Peptide 2.0 (Chantilly, VA), Peptisyntha, Inc. (Torrance, CA) or the University of Michigan Peptide Synthesis Core.

In vitro invasion assays

In vitro invasion assays employing naturally serum-free (SF) basement membranes (SU-ECM) were performed and data analyzed as in [4–9, 14, 16, 17]. Peptides were prebound to serum-starved, suspended cells prior to placement on SU-ECM. Mean invasion percentages were analyzed using Prism software (GraphPad Software, San Diego, CA) as a function of log (inhibitor) versus normalized data, variable slope.

Determination of dissociation, K d, and inhibition constants, K i, for cell surface binding

PHSCN and PhScN dissociation constants were determined by centrifugation binding assays [18], using biotinylated, N-acetylated, C-amidated Ac-PHSCNGGK-Bio and Ac-PhScNGGK-Bio [9, 12]. Maximal α5β1 activation was promoted by manganese. Binding experiments were performed as pairs to ensure consistency. 150,000 cells, suspended in binding buffer (10 mM HEPES, pH 7.4, 150 mM NaCl, 0.1 % bovine calf serum, and 2 mM MnCl2) were incubated with varying concentrations of biotinylated peptides for 2 h at 4 °C. Washed cell pellets were incubated at 4 °C for 30 min with Streptavidin-peroxidase (Sigma) and o-phenylenediamine substrate, stabilized by the addition of HCl. Absorbances were recorded at 490 nM.

Competition assays were performed with a constant concentration of biotinylated peptide and varying amounts of unlabeled competitor. Equilibrium time was 3 h [9]. Binding data were analyzed, and curves were fit using nonlinear regression approaches [9, 19].

Fluorescent DiI labeling of MDA-MB-231 and SUM149PT cells

Washed, confluent cells were suspended and orange fluorescently labeled in SF medium with lipophilic carbocyanine vital dye DiI, 1,1′-dilinoleyl-3,3,3′3′-tetramethylindocarbocyanine perchlorate (Invitrogen) [7–9]. Labeled cells were pelleted, resuspended in 6 ml of medium, and cultured 72 h prior to use.

Mice

Female Foxn1nu athymic nude mice (Harlan) were housed according to the Association for the Assessment and Accreditation for Laboratory Animal Care guidelines. Studies were performed with approved institutional animal use protocols.

Lung extravasation and colonization by MDA-MB-231 or SUM149PT cells

Extravasation-inhibitory potencies of single systemic pretreatments of Ac-PhScN-NH2, Ac-PHSCN-NH2, or Ac-PhScNGGK-MAP were evaluated using DiI-labeled, peptide- or dendrimer-prebound cells [7–9]. Treated cells were prebound with 1, 10, or 100 ng/ml Ac-PhScN-NH2; 10, 100, or 1000 ng/ml Ac-PHSCN-NH2; 1000 ng/ml Ac-HSPNC-NH2; or 1000 ng/ml Ac-hSPNc-NH2, for 10 min at 37 °C. Groups of 10 nude mice received one systemic pretreatment with the appropriate peptide concentration by tail vein injection. Immediately thereafter, 10,000 cells, prebound with the appropriate peptide or with HBSS only were injected into tail veins in 0.1 ml HBSS. Mice received no other systemic treatments were euthanized 24 h later, and their lungs were removed. Frozen-tissue samples were prepared, and lung-extravasated cells were quantitated at 400-fold magnification by Zeiss Scanning Laser Confocal microscopy (LSM510). 25 10-µm sections, taken at intervals of 100 µm, were analyzed from one lung in each mouse. Data are presented as mean ± SEM. Mice evaluated for lung colonization were maintained without further treatment for 6 weeks prior to euthanization.

Western blot analysis of activated caspase-3 in SUM149PT cells

Adherent SUM149PT cells were serum-starved overnight, then incubated in serum-containing medium with 0, 10, 50, 100, 200, or 300 μg of Ac-PhScN-NH2, or 500 μg per 106 cells of Ac-hSPNc-NH2 for 1 h, prior to washing thrice with PBS. Anti-cleaved Caspase 3 MAb (Cell Signaling #9661) primary antibody and horseradish peroxidase HRP-linked anti-rabbit IgG (Cell Signaling, #7074) were used in Western blot analysis of cleaved Caspase-3, as instructed.

Intratibial injection of DiI-labeled MDA-MB-231 cells

Treatment groups of 10 mice were each injected in right tibial crest cortex with 10,000 DiI-labeled MDA-MB-231 cells, not pretreated with peptide [20]. Intratibial colonies progressed untreated for 2 weeks prior to initiation of thrice-weekly PhScN or PHSCN therapies, at 0.0, 5.0, and 50.0 mg/kg. Sterile peptide solutions were prepared in 50 mM HEPES, pH 7.4, and tail vein injected in 100 µl. Mice were euthanized 24 days later, after 10 systemic treatments. Tibia were fixed in 4.0 % paraformaldehyde at 4 °C for 3 days, rinsed in 1.0 % PBS for 2 days, and then stored in 20.0 % sucrose until transfer to a sucrose series in mounting medium, followed by embedding and staining with DAPI, conjugated with green fluorescence-labeled actin [7–9]. Twenty 10-micron sections, separated by 200 microns, were analyzed from each tibia.

Data analysis

Dose response data were analyzed as in [9] by Chou–Talalay Combination index (CI) [21], based on the multiple drug effect equation, where y = log(f a/f u), with respect to x = log(dose), defines the dose effect relationship without reaction rate constants. f a is the fraction of cells affected (invasion-inhibited), and f u is the fraction of cells unaffected (invaded). The x intercept represents the IC50 value. CI and DRI values were determined for Ac-PhScN-NH2, relative to Ac-PHSCN-NH2.

Results

Increased invasion-inhibitory potency of Ac-PhScN-NH2, and Ac-PhScNGGK-MAP

SF SU-ECM basement membranes [9] were utilized to evaluate invasion-inhibitory potencies of S-acetylated or S-methylated PHSCN derivatives [Ac-PHSC(S-OAc)N-NH2 and Ac-PHSC(S-Me)N-NH2], the D-His, D-Cys-containing PhScN peptide (Ac-PhScN-NH2), and the D-His, D-Cys-containing PhScNGGK poly-lysine multiantigenic peptide dendrimer (Ac-PhScNGGK-MAP) on α5β1-mediated, serum-induced, or SF PHSRN-induced invasion. Figure 1a shows Hill-Slope plots evaluating effects of varying concentrations of Ac-PHSC(S-OAc)N-NH2, Ac-PHSC(S-Me)N-NH2, and Ac-PhScN-NH2 on FCS-induced invasion, compared to Ac-PHSCN-NH2 (Fig. 1a). To demonstrate that PhScN targets α5β1 integrin-mediated invasion, the α5β1-specific Fn cell-binding domain ligand [22], Ac-PHSRN-NH2 (PHSRN), was used to induce SF invasion by MDA-MB-231 (Fig. 1b). The alternating L- and D-stereoisomer sequence of Ac-PhScN-NH2 produced an endoproteinase-resistant peptide with a 100,000-fold increased invasion-inhibitory potency, for serum-induced and SF PHSRN-induced invasion, similar to that achieved by covalent S-modification of the PHSCN cysteine residue. Similar Hill-Slope plots were obtained for SUM149PT (Table 1).

Fig. 1
figure 1

Hill-slope plots of increased invasion-inhibitory potency of Ac-PhScN-NH2 peptide, Ac-PhScNGGK-MAP dendrimer, and the cysteine-modified, methylated (Me), or acetylated (OAc) peptides, Ac-PHSC(Me)N-NH2, and Ac-PHSC(OAc)N-NH2 for MDA-MB-231 cells. a Effects on FBS-induced invasion by cysteine-modified Ac-PHSC(Me)N-NH2 or Ac-PHSC(OAc)N-NH2, and D-His, D-Cys containing Ac-PhScN-NH2 peptides. Symbols are denoted in the figure. X axis, log peptide concentration in pg per ml; Y axis, mean relative percentages of invaded cells (±SD). IC50 and DRI values are summarized in Table 1. b Increased potencies of PhScN peptide and PhScN dendrimer for both serum-induced and serum-free (SF), Ac-PHSRN-NH2 (1 μg/ml)-induced, α5β1-mediated invasion. Circles denote Ac-PhScN-NH2; squares denote Ac-PhScNGGK-MAP dendrimer; and triangles denote Ac-hSPNc-NH2. Closed symbols denote FBS-induced and open symbols denote Ac-PHSRN-NH2 (1 μg/ml) induced, SF invasion. X axis, log peptide concentration in pg/ml; Y axis, mean relative percentage of invaded cells (±SD). The solid and dashed lines represent published values for Ac-PHSCN-NH2 and Ac-PHSCNGGK-MAP, respectively [7]

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Table 1 Inhibition of α5β1-mediated invasion of basement membranes by SUM149PT and MDA-Mb-231 cells: IC50 and DRI values for PhScN and PHSCN derivatives
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Table 1 summarizes half maximal invasion-inhibitory concentration (IC50) and dose reduction index (DRI) values for PHSCN and S-methylated [C(Me)N] or S-acetylated [C(OAc)N] derivatives for MDA-MB-231 and SUM149PT. IC50 and DRI values based on pg/ml or molar (M) concentrations are presented for Ac-PhScN-NH2 (PhScN) and Ac-PhScNGGK-MAP dendrimer (cN-MAP) for serum- or SF Ac-PHSRN-NH2 (PHSRN)-induced invasion. Since PhScN is a highly potent inhibitor of SF PHSRN-induced invasion by MDA-MB-231 and SUM149PT, PhScN targets α5β1-mediated invasion in both cell lines.

PHSCN was suggested to inhibit invasion via disulfide bonding with α5β1 integrin [10]. However, increased-inhibitory potencies of the S-acetylated or S-methylated PHSCN derivatives for MDA MB231 and SUM149PT cells, and for prostate cancer [9], suggest that the productive mechanism is noncovalent.

PhScNGGK-MAP (7500 Da), containing 8 subunits of PhScN demonstrated a 107-fold increased invasion-inhibitory potency over monomeric PhScN (596 Da), significantly greater than the 1000-fold increase seen when comparing PHSCNGGK-MAP and PHSCN [7, 8] (Fig. 1b; Table 1).

Dissociation constants (K d) and competition binding assays for cell surface binding using biotinylated PhScN or PHSCN

D-amino acid substitutions change the orientation of the side chains on the PhScN peptide backbone, which could affect target binding and explain the 100,000-fold increase in PhScN invasion-inhibitory potency. A binding assay, developed to determine the dissociation constant (K d) of biotin-labeled PHSCN peptide to suspended cells [10], was utilized to compare the K d’s of biotinylated PhScN and PHSCN peptides. The invasion-inhibitory potencies of biotinylated PHSCN and PhScN were confirmed on SU-ECM. IC50’s of Ac-PhScNGGK-Bio and Ac-PHSCNGGK-Bio were similar to those of Ac-PhScN-NH2 and Ac-PHSCN-NH2, respectively, for suspended MDA-MB-231 (Fig. 2a; Table 2) and SUM149PT (Table 2).

Fig. 2
figure 2

a Hill-slope plots of invasion inhibition by biotinylated derivatives of PHSCN and PhScN for MDA-MB-231 cells induced by FBS. Specific agents (CN-Bio, Ac-PHSCNGGK-Bio; cN-Bio, Ac-PhScNGGK-Bio) are listed on the right. X axis, log peptide concentration in pg per ml; Y axis, mean relative percentages of invaded cells (±SD). IC50 and DRI values are summarized in Table 2. b K d binding assays for Ac-PHSCNGGK-Bio (CN-Bio) and Ac-PhScNGGK-Bio (cN-Bio) peptides with suspended MDA MB231 cells. Plots were fit using a total binding equation to account for nonspecific binding [19]. c Competition binding was determined by incubating suspended MDA-MB-231 cells with a constant concentration of 0.1 μM Ac-PHSCNGGK-Bio (labeled) and varying amounts of unlabeled Ac-PhScN-NH2. Plot was fitted using the Motulsky and Neubig competitive inhibition equation [19]

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Table 2 Invasion inhibition: IC50 and K d ‘s for biotinylated PhScN and PHSCN derivatives
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As shown in Fig. 2b and summarized in Table 2, K d’s for Ac-PHSCNGGK-Bio binding to suspended SUM149PT and MDA-MB-231 cells were 0.028 and 0.032 μM, respectively. K d values for Ac-PhScNGGK-Bio were 0.029 and 0.053 μM for suspended SUM 149PT and MDA-MB-231, respectively. Since PhScN and PHSCN K d values were similar, the improved invasion-inhibitory potency of PhScN (Table 1) is due to elimination of the nonproductive covalent side reaction by orienting the D-His and D-Cys side chains to the opposite side of the peptide ligand, rather than to tighter binding, as for prostate cancer [9]. Thus, noncovalent interaction is key to invasion-inhibitory potency.

Competition assays were also performed to confirm that PHSCN and PhScN interact with the same binding site on breast cancer cells, like prostate cancer [9]. Results of assays, in which suspended MDA-MB-231 cells were incubated with a constant concentration (0.1 μM) of biotinylated Ac-PHSCNGGK-Bio and varying concentrations of Ac-PhScN-NH2, are presented in Fig. 2c. They show that dissociation of Ac-PHSCNGGK-Bio with increasing concentrations of unlabeled Ac-PhScN-NH2 occurs over 2 orders of magnitude, demonstrating that PhScN and PHSCN compete for the same binding site on MDA-MB-231 [19], as for SUM149PT, not shown.

Increased lung extravasation inhibition by PhScN and PhScNGGK-MAP

Lung extravasation-inhibitory efficacies of PhScN and PhScNGGK-MAP were compared with PHSCN in nude mice. As shown in Fig. 3a, the mean number of extravasated MDA-MB-231 and SUM149PT cells per section was decreased by 100- to 1000-fold by Ac-PhScN-NH2, and by at least 100,000-fold by Ac-PhScNGGK-MAP, relative to Ac-PHSCN-NH2. The median-effect plot of Fig. 3b shows that increasing concentrations of PhScN or PHSCN, or PhScNGGK-MAP dendrimer, log-linearly decrease SUM149PT and MDA-MB-231 extravasation, with similar relative potencies. Pretreatment with 1 μg per ml Ac-hSPNc-NH2 scrambled sequence control, followed by a single systemic treatment at the equivalent dose, had no inhibitory effect on extravasation, not shown. As summarized in Table 3, Ac-PhScN-NH2 is 100- to 1000-fold more potent than Ac-PHSCN-NH2 at inhibiting SUM149PT and MDA-MB-231 lung extravasation, and Ac-PhScNGGK-MAP is 100,000- to 1000,000-fold more potent. These in vivo values correlate with those seen in SU-ECM invasion assays (Table 1), and are similar to those observed for metastatic prostate cancer [9]. Figure 3c, d shows typical examples of sectioned lung tissue for MDA-MB-231 and SUM149PT extravasation, obtained by confocal microscopic analysis after pretreatment with 100 ng/ml Ac-PhScN-NH2 or 100 ng/ml Ac-PHSCN-NH2, versus untreated or scrambled sequence controls.

Fig. 3
figure 3

Increased extravasation inhibition by Ac-PhScN-NH2 or Ac-PhScNGGK-MAP prebinding, relative to Ac-PHSCN-NH2 peptide. a Y axes, average cells extravasated/section; X axes, Con, control; 10, 100, 1000 ng/ml PHSCN; 1, 10, 100 ng/ml PhScN; 1, 10, 100 pg/ml PhScNGGK-MAP. b Median-effect plot for extravasation into lung after prebinding with PHSCN (circles), PhScN (squares) or PhScN-MAP (triangles); open symbols MDA-MB231 cells; closed symbols SUM 149 PT cells. X axes, log peptide concentration in ng/ml; Y axes, mean log fraction affected/fraction unaffected (f a/f u) ± SEM. IC50 and DRI values are summarized in Table 3. c Typical examples of sectioned lung tissue for MDA-MB231 extravasation analyzed by confocal microscopy after pretreatment with 100 ng/ml Ac-PhScN-NH2 or 100 ng/ml Ac-PHSCN-NH2, compared to untreated control. d Typical examples of sectioned lung tissue for SUM149PT extravasation analyzed by confocal microscopy after pretreatment with 100 ng/ml Ac-PhScN-NH2 or 1000 ng/ml Ac-PHSCN-NH2, compared to untreated control. Images represent the merged composite of DiI-labeled cells shown in orange; blue stained nuclei from DAPI Mounting Medium; green tissue from actin staining. (Color figure online)

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Table 3 Extravasation Inhibition, IC50’s and DRI values for PhScN, PHSCN, and the PhScNGGK-dendrimer, cN-MAP
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Increased potency of PhScN peptide as an inhibitor of lung colonization

Since extravasated cells may not establish a metastasis [23], effects of PhScN or PHSCN pretreatment on lung colonization were determined by allowing extravasated cells to grow 6 weeks without further treatment. Figure 4a shows the comparison of the effects of prebinding various concentrations of PhScN or PHSCN to suspended, DiI-labeled SUM149PT or MDA-MB-231 on lung colonization after tail vein injection. Dose response curves in Fig. 4b are median-effect plots, and the IC50 and DRI values are summarized in Table 4. Both PhScN and PHSCN pretreatment log-linearly decreased MDA-MB-231 and SUM149PT lung colonization. PhScN was 3850-fold more potent than PHSCN for reducing SUM149PT lung colony formation, and 32,143-fold more potent for MDA-MB-231. Furthermore, since all of the 120 control (untreated, Ac-HSPNC-NH2- or Ac-hSPNc-NH2-treated) mice, injected intravenously with suspended SUM149PT or MDA-MB-231 cells, had to be euthanized to prevent suffering due to respiratory distress, while none of the PhScN-treated mice did, systemic PhScN therapy may also be a more potent inhibitor of respiratory distress due to lung metastasis progression. Figure 4c shows the examples of DiI-labeled MDA-MB-231 colonies in fluorescent actin- and DAPI-stained lung tissue after no treatment, and after prebinding of the injected cells to 100 ng/ml of either PHSCN or PhScN. SUM149PT lung colonies had a similar appearance, data not shown.

Fig. 4
figure 4

Increased inhibition of lung colonization by Ac-PhScN-NH2 prebinding, relative to Ac-PHSCN-NH2. a Y axes, average colonies/section; X axes, Con, control; NC, 100 ng/ml HSPNC or hSPNc; 10, 100, 1000 ng/ml PHSCN; 1, 10, 100 ng/ml PhScN. b Median-effect plot for lung colony formation after prebinding to PHSCN (circles) or PhScN (squares). Closed symbols, dotted lines SUM149PT cells; open symbols, solid lines MDA-MB-231 cells. X axes, log peptide concentration in ng/ml. Y axes, mean log fraction affected/fraction unaffected (f a/f u) ± SEM. IC50 and DRI values are summarized in Table 4. c Typical examples of sectioned lung tissue for MDA-MB-231 colonization analyzed by confocal microscopy after pretreatment with 100 ng/ml Ac-PhScN-NH2 or 100 ng/ml Ac-PHSCN-NH2 compared to untreated control. Images represent the merged composite of DiI-labeled cells shown in orange; blue stained nuclei from DAPI Mounting Medium; green tissue from actin staining. d Example of a typical Western blot showing dose-dependent upregulation of activated Caspase-3 in adherent SUM149PT cells by a 1-h treatment with a range of Ac-PhScN-NH2 concentrations (10, 50, 100, 200, or 300 μg/106 cells). Micrograms of Ac-PhScN-NH2 per million SUM149PT cells are indicated. Lack of effects of an elevated concentration of a scrambled sequence control, 500 μg Ac-hSPNc-NH2 per 106 SUM149PT cells, is also shown. (Color figure online)

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Table 4 Effects of PHSCN or PhScN pretreatment on lung colonization
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Since stress activates α5β1 [24], and apoptosis induction could increase PhScN potency, we assessed apoptosis in vitro by examining effects of a 1-h treatment with a range of Ac-PhScN-NH2 concentrations. Figure 4d presents a typical Western blot indicating that PhScN rapidly induces dose-dependent upregulation of activated Caspase-3 in adherent SUM149PT. However, the concentrations are 1000- to 100,000-fold higher than those required for lung colonization inhibition. Lack of effect of an elevated concentration of a scrambled sequence control, 500 μg Ac-hSPNc-NH2 per 106 SUM149PT cells, is also shown. Lack of activated Caspase-3 upregulation in SUM149PT cells treated with hSPNc demonstrates that although elevated PhScN concentrations are required for activated Caspase-3 upregulation, the effect is sequence specific.

PhScN exhibits a similar increase in lung colonization-inhibitory potency, relative to PHSCN, for MDA-MB-231 and SUM149PT (1000- to 10,000-fold). However, the corresponding IC50 values are 10- to 100-fold higher for SUM149PT (Table 4)—perhaps due to differences between metastatic and inflammatory cancer cell lines—the DRI values suggest that PhScN is 1000- to 10,000-fold more potent at preventing lung colonization than PHSCN for both MDA-MB-231 and SUM149PT.

Inhibition of MDA-MB-231 bone metastasis progression by systemic PhScN

Breast cancer bone metastasis occurs earlier than lung colonization [3]. Interaction with surrounding bone marrow and matrix is described by the “vicious cycle” model [2], in which parathyroid hormone-related protein (PTHrP) expressed by breast cancer cells after bone colonization [25], stimulates progression and enhances osteoclastic bone resorption [24, 26–28]. Metastatic breast cancer cell signals induce osteoblasts to express osteoclast stimulatory factors [29], stimulating multinucleated osteoclasts to resorb mineralized bone matrix [30–32] and release sequestered growth factors that interact with their receptors to promote metastatic growth by stimulating angiogenesis [33–36]. Activated α5β1 receptors of microvascular endothelial cells interact with the pFn PHSRN sequence to induce angiogenic invasion [6]. Matrix metalloproteinase-1 (MMP1), induced by α5β1 FnR binding to PHSRN, mediates both angiogenic and metastatic invasion by breast, prostate, and pancreatic cancer cells [4–6, 14]. MMP-1 activates the protease activated receptor-1 (PAR-1) gene in both breast cancer [37] and endothelial cells [38], thereby promoting breast cancer invasion and angiogenesis. PTHrP stimulates osteoblasts to induce progenitor differentiation into active osteoclasts which mediate bone resorption, releasing epidermal growth factor (EGF)-like growth factors to further stimulate bone metastasis progression [39]. Overexpression of receptor tyrosine-protein kinase erbB-2 (HER2) induces α5β1-mediated, mammary epithelial cell invasion [40], due to surface downregulation of the invasion-inhibitory α4β1 integrin [41]. Furthermore, MDA-MB-231 colonies in the bones of athymic mice can induce angiogenesis in osteolytic metastases [36]. Identification of α5β1 as the primary integrin fibronectin receptor on human bone marrow stroma [42] suggests that PhScN could be effective in reducing bone metastases. These interactions and the potential inhibitory effects of PhScN therapy on α5β1-mediated breast cancer (BC) and angiogenic invasion are diagrammed in Scheme 1.

Scheme 1
scheme 1

Vicious cycle model of breast cancer bone metastasis. BC breast cancer, MMP-1 matrix metalloproteinase-1 interstitial collagenase, PTHrP parathyroid hormone-related protein, EGF-like GFs epidermal growth factor-like growth factors, EGFR epidermal growth factor receptor

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We evaluated microvascular endothelial cell (hmvec) invasion in vitro as described [6]; PHSCN and PhScN were confirmed to block invasion. The IC50 values were similar to those determined for MDA-MB-231 and SUM149PT. PhScN demonstrated a 6.3 × 104-fold increase in potency over PHSCN (PhScN IC50, 1.3 × 10−13 M; PHSCN IC50, 8.4 × 10−9 M).

To compare the effects of PHSCN and PhScN on established bone metastases, DiI-labeled, untreated MDA-MB-231 cells were injected intratibially and allowed to grow into intra-osteal colonies for 2 weeks before thrice-weekly systemic treatments of 5 or 50 mg/kg of PhScN or PHSCN were initiated. Systemic treatments were continued for 24 days. Each mouse received a total of 10 systemic treatments. As shown in Fig. 5a, 50 mg/kg systemic PhScN monotherapy reduced bone colony progression by 77 %, compared to 57 % for 50 mg/kg PHSCN. Median-effect analyses, Fig. 5b, indicate a dosage IC50 of 0.4 mg/kg for PhScN and 10 mg/kg for PHSCN. The DRI values indicate that PhScN is 25-fold more potent than PHSCN as a systemic inhibitor of bone metastasis progression. As seen in confocal microscopy examples, Fig. 5c, few cells remained in the bone marrow of 50 mg/kg PhScN-treated mice; significantly more were present in marrows of mice treated with 50 mg/kg PHSCN. Bone colonies appeared to be either extravascular, or closely associated with the vasculature.

Fig. 5
figure 5

Increased potency of PhScN as a systemic therapy to prevent breast cancer bone colony progression in athymic mice. a Anti-metastatic potencies of systemic PhScN (Ac-PhScN-NH2) versus PHSCN (Ac-PHSCN-NH2) for reducing MDA-MB-231 bone marrow progression in athymic mice. Y axes, average colonies per section; X axes, Con, untreated control; 5 and 50 mg/kg PhScN; 5 and 50 mg/kg PHSCN. b Median-effect plot for effects of PhScN versus PHSCN on bone marrow progression, X axis, log peptide dosage level (mg/kg); Y axis, mean log fraction affected/fraction unaffected (f a/f u) ± SEM. c Examples of bone marrow from untreated mice, and from mice treated with a total of 10 thrice-weekly tail vein injections of 50 mg/kg PhScN or 50 mg/kg PHSCN. Images represent the merged composite of DiI-labeled cells shown in orange; stained nuclei from DAPI Mounting Medium in blue; tissue from actin staining in green. Thus, DiI-labeled cells appear orange (Control) or green with orange inclusions (50 mg/kg PHSCN). No DiI-labeled cells appear in the image of sectioned bone from mice receiving 50 mg/kg PhScN. (Color figure online)

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Colocalization of Ac-PhScNGGK-Bio with DiI in extravasated breast cancer cells

Since nodal metastases occur in at least 20 % of patients undergoing completion axillary lymph node dissection (ALND) following a positive sentinel lymph node biopsy (SLNB), the use of SLNB alone for axilla staging underestimates nodal disease [43]. Hence, a more efficient means of detecting metastatic disease would be beneficial. Ac-PhScNGGK-Bio might be an efficient detection agent for extravasated breast cancer cells in sectioned biopsies. Thus, we assessed Ac-PhScNGGK-Bio/DiI colocalization in tail vein-injected, lung-extravasated SUM149PT cells. As shown in Fig. 6a, a range of Ac-PhScNGGK-Bio concentrations (1.0–50.0 μg per ml) labeled 89–90 % of extravasated cells 24 h after injection, suggesting that labeled PhScN is efficient agent for detecting extravasated breast cancer cells. Figure 6b shows representative images of extravasated DiI-labeled SUM149PT, with and without Ac-PhScNGGK-Bio.

Fig. 6
figure 6

Colocalization of Ac-PhScNGGK-Bio with DiI in lung-extravasated SUM149PT cells. a Percentage of DiI-labeled cells in lung tissue binding to different concentrations of Ac-PhScNGGK-Bio: 1, 5, or 50 μg per ml. X axis, binding agents: hSPNc, Ac-hSPNc-NH2 (a scrambled sequence control peptide); PhScN, Ac-PhScNGGK-Bio. Y axis, mean percentage of biotinylated, DiI-labeled SUM149PT cells (±SEM). b Examples of fluorescently stained lung tissue: control, no PhScNGGK-Bio; 50 μg PhScNGGK-Bio, 50 μg per ml Ac-PhScNGGK-Bio. DiI-labeled cells shown in orange; stained nuclei from DAPI Mounting Medium in blue; tissue from actin staining in green. (Color figure online)

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Discussion

Since recurrent or metastatic disease develops in 40 % of early breast cancer patients [1], with bone metastasis in most [2], there is an urgent need for effective therapy to prevent metastatic and angiogenic invasion. As summarized in Scheme 1, inhibition of activated α5β1 receptors on metastatic breast cancer and associated microvascular endothelial cells may form the basis of an effective targeted therapy to inhibit the vicious cycle of bone metastasis progression. Because overexpression of PTHrP in MDA-MB-231 cells induces a 10- to 25-fold upregulation of α5 integrin expression and increased α5β1 integrin [44], and because α5β1 mediates metastatic invasion, systemic dissemination [4, 5, 14, 16, 45], and microvascular endothelial cell invasion [6], α5β1 is a key therapeutic target [4–9, 14, 17].

Scheme 1 suggests that by targeting α5β1, PhScN may inhibit both angiogenesis and breast cancer bone invasion, as well as reducing bone resorption through its inhibitory effects on PTHrP- and growth factor-mediated signaling. We report that D-His, D-Cys-containing PhScN peptide is 105-fold more potent than PHSCN as an inhibitor of α5β1-mediated, basement membrane invasion, 100- to 1000-fold more potent as an inhibitor of lung extravasation, and 3800- to 32,000-fold more potent at reducing lung colonization, when the effects of a single exposure to each peptide were compared. Since all untreated, HSPNC- or hSPNc-treated mice had to be euthanized to prevent suffering due to respiratory distress from the effects of SUM149PT or MDA-MB-231 overgrowth of their lungs, systemic PhScN therapy might also be able to reduce suffering due to lung metastasis progression in breast cancer patients.

Results were also presented showing that the multivalent presentation of PhScN sequence increases its relative potency on a molar basis by an additional 108-fold as an in vitro invasion inhibitor, or by an additional 1000- to 10,000-fold as a lung extravasation inhibitor. Consistent with the key role of α5β1-mediated invasion in metastatic progression [4–6], we also report that systemic PhScN monotherapy was well-tolerated and reduced bone colony progression. Our results also suggested that labeled PhScN is an efficient agent for detecting extravasated, potentially metastatic breast cancer cells, due to surface-activated α5β1 integrin expression.

Advanced breast cancer patients are currently treated with Denosumab anti-receptor activator of nuclear factor kappa-B (RANK) ligand MAb or zoledronic acid to manage the bone metastasis symptoms and delay skeletal related events (SRE). However, neither treatment delays disease progression or increases overall survival [46–48]. In contrast, PhScN monotherapy demonstrated a nearly 80 % reduction of bone colony progression on established intra-osteal colonies. Thus, by targeting α5β1 receptors PhScN may offer the opportunity to slow disease progression and break the vicious cycle.