Systematic verification of upstream regulators of a computable cellular proliferation network model on non-diseased lung cells using a dedicated dataset

We recently constructed a computable cell proliferation network (CPN) model focused on lung tissue to unravel complex biological processes and their exposure-related perturbations from molecular profiling data. The CPN consists of edges and nodes representing upstream controllers of gene expression largely generated from transcriptomics datasets using Reverse Causal Reasoning (RCR). Here, we report an approach to biologically verify the correctness of upstream controller nodes using a specifically designed, independent lung cell proliferation dataset. Normal human bronchial epithelial cells were arrested at G1/S with a cell cycle inhibitor. Gene expression changes and cell proliferation were captured at different time points after release from inhibition. Gene set enrichment analysis demonstrated cell cycle response specificity via an overrepresentation of proliferation related gene sets. Coverage analysis of RCR-derived hypotheses returned statistical significance for cell cycle response specificity across the whole model as well as for the Growth Factor and Cell Cycle sub-network models.

A modular cell-type focused inflammatory process network model for non-diseased pulmonary tissue

Exposure to environmental stressors such as cigarette smoke (CS) elicits a variety of biological responses in humans, including the induction of inflammatory responses. These responses are especially pronounced in the lung, where pulmonary cells sit at the interface between the body’s internal and external environments. We combined a literature survey with a computational analysis of multiple transcriptomic data sets to construct a computable causal network model (the Inflammatory Process Network (IPN)) of the main pulmonary inflammatory processes. The IPN model predicted decreased epithelial cell barrier defenses and increased mucus hypersecretion in human bronchial epithelial cells, and an attenuated pro-inflammatory (M1) profile in alveolar macrophages following exposure to CS, consistent with prior results. The IPN provides a comprehensive framework of experimentally supported pathways related to CS-induced pulmonary inflammation. The IPN is freely available to the scientific community as a resource with broad applicability to study the pathogenesis of pulmonary disease.

Construction of a computable network model for DNA damage, autophagy, cell death, and senescence

Towards the development of a systems biology-based risk assessment approach for environmental toxicants, including tobacco products in a systems toxicology setting such as the “21st Century Toxicology”, we are building a series of computable biological network models specific to non-diseased pulmonary and cardiovascular cells/tissues which capture the molecular events that can be activated following exposure to environmental toxicants. Here we extend on previous work and report on the construction and evaluation of a mechanistic network model focused on DNA damage response and the four main cellular fates induced by stress: autophagy, apoptosis, necroptosis, and senescence. In total, the network consists of 34 sub-models containing 1052 unique nodes and 1538 unique edges which are supported by 1231 PubMed-referenced literature citations. Causal node-edge relationships are described using the Biological Expression Language (BEL), which allows for the semantic representation of life science relationships in a computable format. The Network is provided in .XGMML format and can be viewed using freely available network visualization software, such as Cytoscape.

Improved cellular pharmacokinetics and pharmacodynamics underlie the wide anticancer activity of sagopilone

Sagopilone (ZK-EPO) is the first fully synthetic epothilone undergoing clinical trials for the treatment of human tumors. Here, we investigate the cellular pathways by which sagopilone blocks tumor cell proliferation and compare the intracellular pharmacokinetics and the in vivo pharmacodynamics of sagopilone with other microtubule-stabilizing (or tubulin-polymerizing) agents. Cellular uptake and fractionation/localization studies revealed that sagopilone enters cells more efficiently, associates more tightly with the cytoskeleton, and polymerizes tubulin more potently than paclitaxel. Moreover, in contrast to paclitaxel and other epothilones [such as the natural product epothilone B (patupilone) or its partially synthetic analogue ixabepilone], sagopilone is not a substrate of the P-glycoprotein efflux pumps. Microtubule stabilization by sagopilone caused mitotic arrest, followed by transient multinucleation and activation of the mitochondrial apoptotic pathway. Profiling of the proapoptotic signal transduction pathway induced by sagopilone with a panel of small interfering RNAs revealed that sagopilone acts similarly to paclitaxel. In HCT 116 colon carcinoma cells, sagopilone-induced apoptosis was partly antagonized by the knockdown of proapoptotic members of the Bcl-2 family, including Bax, Bak, and Puma, whereas knockdown of Bcl-2, Bcl-X(L), or Chk1 sensitized cells to sagopilone-induced cell death. Related to its improved subcellular pharmacokinetics, however, sagopilone is more cytotoxic than other epothilones in a large panel of human cancer cell lines in vitro and in vivo. In particular, sagopilone is highly effective in reducing the growth of paclitaxel-resistant cancer cells. These results underline the processes behind the therapeutic efficacy of sagopilone, which is now evaluated in a broad phase II program.

Shift of syndecan-1 expression from epithelial to stromal cells during progression of solid tumours

Syndecan-1 (SDC-1), a protein found on cells and in the extracellular matrix, participates in cell proliferation, cell migration and cell-matrix interactions. SDC-1 expression correlates with the maintenance of epithelial morphology and inhibition of invasiveness. In the present study, a second SDC-1 mRNA isoform was identified and the expression of both transcripts was investigated in various normal and malignant tissues. Both transcripts were coexpressed at equal levels in all tissues and organs analysed. Cancer-profiling array (CPA) analysis of 241 non-enriched tumour and normal cDNAs revealed stronger upregulation of SDC-1 in tumour tissues as compared with oligonucleotide array-based expression analysis of SDC-1 in microdissected breast, prostate, lung, and colon carcinoma cells. With in situ hybridisation and immunohistochemistry it was demonstrated that this difference in SDC-1 expression originates from stromal cells present in tumour connective tissue. But only the cells in connective tissue surrounding breast, lung, colon and bladder carcinoma showed upregulation of SDC-1. These stromal cells were characterised as spindle cells with myofibroblastic differentiation and they may contribute to the dedifferentiation of tumour cells and the development of metastasis.

Estrogen/EGF receptor interactions in breast cancer: rationale for new therapeutic combination strategies

In the therapy of estrogen receptor (ER) positive human mammary carcinomas, the treatment with the antiestrogen tamoxifen has been well established. However, the development of hormone resistance is an important factor in breast cancer progression against endocrine therapy. The presence of the receptor for EGF (EGFR) correlates with lack of response towards antiestrogen therapy. The EGFR is not only involved in tumor cell growth, survival signaling, cell migration, metastasis formation and angiogenesis, but also seems to confer reduced responses of tumor cells towards anti-hormones. Concomitant inhibition of both, the receptors for estrogen and EGF may be necessary to improve breast cancer therapy.

Characterization of new estrogen receptor destabilizing compounds: effects on estrogen-sensitive and tamoxifen-resistant breast cancer

BACKGROUND

Antiestrogens of the selective estrogen receptor modulator (SERM) type, such as tamoxifen, have two major limitations: their mixed agonist and antagonist profile and the development of tumor resistance. We characterized two new pure antiestrogens-ZK-703 and ZK-253-that belong to the class of specific estrogen receptor destabilizers (SERDs), which includes fulvestrant, and compared their activity with that of fulvestrant and tamoxifen.

METHODS

Effects of antiestrogens on the growth of estrogen-dependent breast tumors in vivo were determined using several mouse xenograft models (including the tamoxifen-sensitive tumors MCF7, T47D, and MV3366 and the tamoxifen-resistant tumors ZR75-1 and MCF7/TAM) and chemically induced (nitrosomethyl urea [NMU] and dimethylbenzanthracene [DMBA]) rat breast cancer models (groups of 10 animals). We determined the initial response and effects on hormone receptor levels and the time to relapse after treatment (i.e., time to reach a predetermined tumor size threshold). Estrogen receptor (ER) levels were determined by immunoassay.

RESULTS

ZK-703 (administered subcutaneously) and ZK-253 (administered orally) were more effective than tamoxifen or fulvestrant at inhibiting the growth of ER-positive breast cancer in all xenograft models. For example, MCF7 tumors relapsed (i.e., reached the size threshold) in 10 weeks in mice treated with tamoxifen but in 30 weeks in mice treated with ZK-703. ZK-703 and ZK-253 also prevented further tumor progression in tamoxifen-resistant breast cancer models to a similar extent (more than 30 weeks in mice with ZR75-1 and MCF7/TAM tumors). In the chemically induced rat breast cancer models, orally administered ZK-703 and ZK-253 caused a nearly complete (>80%) inhibition of tumor growth. ER levels were dramatically reduced in MCF7 tumors after 5 weeks of ZK-703 treatment compared with ER levels in vehicle-treated tumors; by contrast, ER levels in tamoxifen-treated tumors were higher than those in control tumors.

CONCLUSION

ZK-703 and ZK-253 are potent, long-term inhibitors of growth in both tamoxifen-sensitive and tamoxifen-resistant breast cancer models.

Studies on the development of resistance to the pure antiestrogen Faslodex in three human breast cancer cell lines

In order to understand the mechanisms underlying the development of resistance to a pure antiestrogen we established three human breast carcinoma cell lines resistant to ZM 182780 (ZM) (Faslodex). Long-term cultivation of the ERalpha-positive, 17beta-estradiol (E(2))-responsive cell lines T47D, ZR-75-1, and MCF-7 with the pure antiestrogen ZM 182780 resulted in the T47D-r, ZR-75-1-r, and MCF-7-r cell lines, which proliferate continuously in the presence of 10(-6)M ZM 182780. The resulting antiestrogen-resistant cells grow equally well in medium with or without E(2) and in medium with or without ZM 182780 indicating that they are no longer estrogen-responsive. ERalpha expression was lost at the protein level in all three resistant cell lines. At the mRNA level, the ERalpha was only faintly detectable in T47D-r, whereas a weak signal was seen in ZR-75-1-r and MCF-7-r. By reverse transcription-polymerase chain reaction (RT-PCR) the ERbeta was detectable in the antiestrogen-sensitive and -resistant breast cancer cell lines, however, ZR75-1-r contained the smallest signal for ERbeta. In all three antiestrogen-resistant cells the PR was undetectable, whereas binding of epidermal growth factor (EGF) and protein expression of epidermal growth factor receptor (EGFR) were increased. To analyse alterations in the gene expression pattern in more detail Atlas arrays were hybridised with RNA isolated from T47D-r and T47D and the two Ca2+-binding proteins calgranulin A and B were found to be up-regulated in T47D-r compared to T47D. Calgranulin A and B were also both up-regulated in ZR-75-1-r and MCF-7-r compared to their antiestrogen-sensitive counterparts. Loss of ERalpha expression may be linked to the acquisition of antiestrogen resistance and enhanced expression of the EGFR and of proteins of the S100 family of Ca2+-binding proteins which may contribute to the outgrowth of resistant cells.

EGF receptor targeting in therapy-resistant human tumors

The development of resistance against cytotoxic or endocrine therapy limits the number of chemotherapeutic compounds used in the clinic. The receptor for EGF (EGFR) is not only involved in survival signaling, cell migration, metastasis formation and angiogenesis, but also confers reduced responses of tumor cells towards cytotoxic compounds or radiation. Clinical trials designed to combine EGFR inhibitors with standard chemo- or radiation therapy have been successful. Elucidation of some of the molecular mechanisms of EGFR-mediated chemoresistance may lead to novel treatment approaches against molecules linked to EGFR signal transduction. In human breast carcinomas, the presence of EGFR correlates with lack of response towards anti-estrogen therapy suggesting the concomitant inhibition of both the receptors for estrogen and EGF to improve breast cancer therapy.

Subcellular distribution of epothilones in human tumor cells

Epothilones are a new class of natural and potent antineoplastic agents that stabilize microtubules. Although 12,13-epoxide derivatives are potent antiproliferative agents, the activities of the corresponding 12,13-olefin analogs are significantly decreased. These data were confirmed for two new analogs, 6-propyl-EpoB (pEB) and 6-propyl-EpoD (pED), in comparison with the natural compounds EpoB/EpoD, by using human A431, MCF7, and MDR1-overexpressing NCI/Adr cells. By using tritiated pEB/pED, compound uptake, release, and nuclear accumulation were investigated in A431 and NCI/Adr cells. In these cells, epothilones can principally be recognized and exported by Verapamil-sensitive efflux pumps, which are not identical to MDR1. The degree of export depends on the structure, olefin vs. epoxide-analog, and also on the intracellular drug concentration. The accumulation of pED used at 3.5 or 70 nM, respectively, was increased in the presence of 10 microM Verapamil in both cell lines 2- to 8-fold. In contrast, the intracellular levels of pEB were affected by Verapamil only at 3.5 nM pEB in NCI/Adr (2-fold) and not in A431 cells. In addition, strong nuclear accumulation was observed for pEB (40-50%) but not paclitaxel or pED (5-15%) in both cell lines. Our study suggests that differences in growth inhibitory efficacy between epoxide and olefin analogs may be based on different mechanisms of drug accumulation and subcellular distribution.

Signaling-inactive epidermal growth factor receptor/ligand complexes in intact carcinoma cells by quinazoline tyrosine kinase inhibitors

Several inhibitors of EGF receptor (EGFR) tyrosine kinase activity have been developed that compete with ATP at its binding site such as the quinazolines PD 153035 and ZD 1839 or the 4,5-dianilino-phthalimides DAPH1 and DAPH2. When tested on human A431 cells, the quinazolines completely blocked EGF-induced receptor phosphorylation at 100 nM, whereas it was inhibited by DAPH1 and DAPH2 by only 20% at 3 microM. Quinazoline-treated A431 as well as tumor cells expressing less EGFR (A549, MDA MB 231, and T47D) bound 3- to 6-fold more (125)I-labeled EGF than untreated intact control cells. Scatchard analysis revealed the disappearance of low- and high-affinity EGFR on A431 cells upon PD 153035 treatment. A single receptor class of intermediate ligand binding affinity emerged and its number corresponded to the sum of the two classes. DAPH1 and DAPH2 did not change ligand binding properties of EGFR. PD 153035 exerted the most potent effects on EGF binding to A431 or on inhibiting EGF-stimulated growth of rat MTLn3 cells at low ligand concentrations. Cross-linking of EGFR on PD 153035-treated A431 cells indicated the formation of inactive dimers that further increased upon addition of EGF. Chemical cross-linking of (125)I-labeled EGF to PD 153035-treated A431 cells revealed increased binding to monomeric and dimeric EGFR. Thus, the quinazolines sequestered EGFR plus the ligand into inactive receptor/ligand complexes. This novel mode of action of quinazoline tyrosine kinase inhibitors may be the basis for their extraordinary potency especially in conditions when the ligand is present in limiting amounts.

Differential cross-talk of estrogen and growth factor receptors in two human mammary tumor cell lines

Cultured human mammary MCF7 and T47D tumor cell lines were used to test the interference of the partial antiestrogen 4'-hydroxytamoxifen (4-OH-TAM) and the pure antiestrogen ZM 182780 with growth factor (IGF-I, heregulin) signaling pathways. Growth of both cell lines was stimulated by IGF-I (20 ng/ml) or heregulin (3 nM). ZM 182780 effectively blocked growth factor induced as well as basal proliferation of MCF7 cells while the compound was ineffective in interfering with growth factor mitogenic activity in T47D cells. On both cell lines the IGF-I or heregulin- induced proliferation was enhanced further by 4-OH-TAM. This synergism could be inhibited dose-dependently by ZM 182780. When cells were grown in the presence of estradiol plus growth factors, the antiestrogenic potencies of both compounds and the efficacy of ZM 182780 were unaffected, while the efficacy of 4-OH-TAM was reduced. Our data show cell type specific cross-talk between the receptor for estrogen and that for IGF-I or heregulin, which is different in MCF7 and T47D cells, respectively. In MCF7 cells with demonstrable cross-talk, a clear superiority exists for a pure antiestrogen over a partial agonist in interfering with growth factor mitogenic activity.

Selective increase of alpha2-integrin sub-unit expression on human carcinoma cells upon EGF-receptor activation

The effects of chronic EGF exposure on expression of the alpha2beta1 collagen and alpha5beta1 fibronectin receptor in a pair of human carcinoma cell lines (A431 and A549) with differential responses to EGF in a short-term ECM-cell adhesion assay were investigated. Treatment with EGF at 10 ng/ml for 24 hr increased on both cell lines the expression of the alpha2- but not the beta1- or alpha5-integrin sub-units, and concomitantly cellular adhesion was increased on collagen IV but not on fibronectin. Increased collagen adhesion of A549 cells could be blocked by alpha2- and beta1-integrin-sub-unit antibodies down to control levels, while it was blocked by alpha2-integrin-sub-unit antibody only by 60% and completely by the beta1-integrin-sub-unit antibody on A431 cells. EGF induced disparate shifts in cell morphologies (dome-like structures, A431, vs. spindle-like fibroblastoid, A549) with concomitant opposite changes in the expression/localization of E-cadherin in cell-cell contacts. This could be taken as an indication for cell-type-specific differential changes in the ratio of cell-ECM vs. cell-cell contacts. The EGF-induced up-regulation of the alpha2beta1 integrin was instrumental in increasing collagen adhesion of A549 but only partly in the case of A431 cells, in which cells the alpha2beta1 integrin may have additional functions besides serving as cell-ECM receptor.

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.

Negative cooperativity between alpha 3 beta 1 and alpha 2 beta 1 integrins in human mammary carcinoma MDA MB 231 cells

The alpha 3 beta 1 integrin has been implicated as a receptor for several matrix components, including collagen, fibronectin, and laminins. The function of alpha 3 beta 1 seems to be very versatile involving cell adhesion to or migration on ECM, establishment of cell-cell contacts in aggregates, as well as linkage to intracellular tyrosine phosphorylation cascades. Here we report a strong induction of attachment of alpha 3 beta 1 integrin expressing human breast carcinoma cell line MDA MB 231 to matrix proteins by two alpha 3 integrin subunit function-blocking monoclonal antibodies (P1B5 and ASC-1). In contrast, stimulation of adhesion to ECM by inhibitory alpha 3 integrin-specific antibodies was not observed in the alpha 3 beta 1 integrin-expressing nonmalignant human mammary epithelial cell line MCF-10A or the human breast carcinoma cell line MDA MB 468 that expressed relatively low amounts of alpha 3 beta 1 integrin at the cell surface. This increase was specific for collagens and not observed on fibronectin or laminin. Physiological concentrations of bivalent cations were not required. MAb P1B5 did not induce homotypic aggregation of MDA MB 231 cells. The P1B5-induced increase in cell attachment to collagens could be prevented but not reduced below control levels by blocking mAb to the alpha 2 integrin subunit. Function blocking anti-alpha 5 integrin subunit mAb was without effect while anti-beta 1-mAb completely abolished adhesion. Our data indicate that negative cooperativity between integrins results in transdominant inhibition of alpha 2 beta 1 function by alpha 3 beta 1 in human MDA MB 231 but not MDA MB 468 tumor cells or nonmalignant MCF-10A cells.

Tumor metastasis inhibition with the prostacyclin analogue cicaprost depends on discontinuous plasma peak levels

Stable prostacyclin analogues exert a strong inhibitory effect on lymphogenous as well as haematogenous tumor metastasis in a series of tumor lines. The strong inhibition of metastasis was achieved by repeated once-daily i.g. applications. The mechanism of antimetastatic action is related to the expression of functional IP-receptors (PGI-receptors). As cellular assay systems indicated that the IP-receptor mediated signalling is down-regulated upon continuous exposure to prostacyclin or stable derivatives, it has been questioned whether a mode of drug application with constant plasma drug levels may potentially result in a decrease of the antimetastatic effect. We addressed this question using the stable prostacyclin analogue cicaprost in a disease model by comparing i.g. applications given once daily with a continuous administration of equivalent doses via drinking water. Very similar to our previous investigations in the 13762NF MTLn3 rat mammary carcinoma model, cicaprost administered by i.g. application strongly reduced lung and lymph node metastasis. In contrast, administration of equivalent doses via drinking water leading to lower but constant steady-state plasma levels failed to exert inhibitory effects. Plasma and urine levels of cicaprost were measured with a sensitive radioimmunoassay on the last treatment day. Pharmacokinetic evaluation demonstrated a similar bioavailability of cicaprost in both groups. This result first demonstrates a treatment failure of a prostacyclin derivative in a chronic disease model in association with a continuous drug administration leading to constant plasma levels. A desensitization of receptor signalling by constant plasma levels may be a possible mechanism for treatment failure.

Prevention of EGF-modulated adhesion of tumor cells to matrix proteins by specific EGF receptor inhibition

The adhesion of tumor cells to various extracellular matrix (ECM) proteins is influenced by epidermal growth factor (EGF). Maximal effects are obtained at low EGF concentrations, at which mostly the cytoskeleton-associated high-affinity EGF receptors (EGFRs) are saturated. Tumor cells expressing EGFR either endogenously (MDA MB 231, MTLn3) or, for the human EGFR, ectopically (MTC HER1/1) in intermediate amounts exhibited, upon EGF addition, increased cellular adhesion to various ECM proteins, such as fibronectin, collagens and vitronectin. In contrast, human A431 and MDA MB 468 cells, over-expressing EGFR, demonstrated reduced attachment in similar experimental conditions. Both increased as well as reduced EGF-dependent adhesion could be blocked using either ligand-blocking monoclonal antibody 14E1 or the potent EGFR tyrosine kinase inhibitor PD 153035. Our data indicate that signals downstream of EGFR activation are responsible for the opposing effects of EGF on cellular adhesion since both can be prevented by EGFR inhibition. Thus, the integration of EGFR- and integrin-dependent signals can be different in carcinoma cell lines and might be influenced by EGFR numbers.

Induction of apoptosis by EGF receptor in rat mammary adenocarcinoma cells coincides with enhanced spontaneous tumour metastasis

The low metastatic MTC (13762NF) rat mammary adenocarcinoma cell line is devoid of epidermal growth factor receptor (EGFR). To test for a link between expression of EGFR and the ability of tumour cells to metastasise from their orthotopic site (spontaneous metastasis), stable subclones of this line (S+) that had been retrovirally transduced to express an ectopic full length HER1 were established and characterised. Proliferation, survival, and response to TGF-alpha were investigated and related to the tumorigenic growth and metastatic properties of the cells. S+ clones responded in vitro to ligand stimulation by growth inhibition and apoptosis. Upon orthotopic inoculation into the mammary fat pad of nude (nu/nu) mice, S+ clones showed retarded growth and apoptosed in situ, while MTC cells or neoR control cells showed no signs of apoptosis. Yet, S+ cells exhibited more spontaneous metastasis than the MTC parental cells or neoR control cells. Spontaneous metastasis requires cellular detachment (primary site) as well as attachment (secondary site) and growth in target organs. Neither the HER1 mediated increased ECM adhesion nor its negative effect on growth potential explains the observed effect. This is the first direct demonstration of the potential of EGFR to promote spontaneous metastasis of mammary adenocarcinoma cells from their orthotopic site.

Signaling by epidermal growth factor differentially affects integrin-mediated adhesion of tumor cells to extracellular matrix proteins

The adhesion of different epidermal growth factor (EGF) receptor (EGFR) expressing cell lines to various extracellular matrix (ECM) proteins is influenced by EGF. To investigate a putative receptor crosstalk between EGFR and integrins we chose two cell lines for a more detailed analysis: the highly metastatic rat mammary carcinoma clone MTLn3 that showed increased adhesion to a panel of ECM proteins in the presence of 10 ng/ml EGF and the nonmetastatic human vulva carcinoma cell line A431 which showed a decreased adhesion under the same conditions. These EGF-mediated stimulatory or inhibitory effects on adhesion were observed within a few minutes. On human A431 cells the inhibitory effect was blocked by an EGFR specific antibody that interferes with ligand binding. In cell adhesion assays performed in the presence of divalent cations MTLn3 and A431 cells exhibited the typical behavior described for integrin-dependent matrix adhesion: Mn2+ enhanced binding to collagen IV and fibronectin whereas Ca2+ inhibited adhesion to collagen IV but not to fibronectin. Adhesion-inhibition assays with anti-human integrin antibodies revealed that A431 cells adhere to collagen via alpha 1 beta 1 and alpha 2 beta 1, and that adhesion to fibronectin is mediated predominantly through alpha 5 beta 1. The interaction of MTLn3 cells with fibronectin was in part RGD dependent, indicating the involvement of either alpha 3 beta 1 or alpha 5 beta 1. Addition of EGF in these assays showed that affecting the integrin extracellular domains by addition of either bivalent cations, RGD peptides, or function-blocking integrin antibodies did not prevent the effects mediated by EGF. We conclude that signals downstream of EGFR can modulate integrin-mediated adhesion to ECM proteins in both an inhibitory and a stimulatory manner.

Antimetastatic action of the prostacyclin analogue cicaprost in experimental mammary tumors

In breast cancer, the survival rate strongly depends on the number of lymph nodes involved. A drug with a specific inhibitory activity on lymph node and organ metastases would therefore be a candidate for adjuvant therapy after surgery. Prostacyclin and its stable analogues have been shown to interfere with certain steps of the metastatic cascade and to inhibit the number of lung colonies after i.v.-inoculation of various tumor cell lines. Our data reveal that cicaprost, a metabolically stable and orally active analogue of prostacyclin, has pronounced antimetastatic effects in a series of spontaneously metastasizing rodent tumors. In the SMT 2a and 13762 MTLn3 mammary carcinomas of the rat, cicaprost given daily from the day of tumor implantation strongly inhibits the number of lung metastases as well as lymph node weights without exerting an effect on the primary tumor. Even starting treatment when palpable primary tumors are present gives a pronounced antimetastatic activity. To demonstrate that cicaprost has an effect on metastases already settled in the respective organ, treatment was started after surgical removal of the primary tumor. In the SMT 2a tumor, a strong inhibition of the number of metastases was shown. Interestingly, a perioperative treatment schedule was also effective in both models used. As primary tumor growth in vivo or proliferation in vitro remained unchanged by cicaprost, its mode of action seems to be related to one or more mechanisms of the metastatic process. In tumor cell lines expressing a functional prostacyclin receptor, stimulated tumor cell migration is inhibited and changes of differentiation status are obvious. In conclusion, cicaprost strongly inhibits lymph node and organ metastases of spontaneously metastasizing rodent mammary tumors with a mode of action different from cytostatic or antihormonal drugs.

Ligand mediated activation of ectopic EGF receptor promotes matrix protein adhesion and lung colonization of rat mammary adenocarcinoma cells

Increased expression of EGF receptor (EGFR) in metastases of human mammary carcinoma as compared to cells of the primary cancer suggests a contribution of EGFR to mammary carcinoma metastasis. To test for a positive causative link, we investigated 13762NF rat mammary adenocarcinoma cloned tumor cell lines of high (MTLn3) or low (MTC) metastatic potential. While MTC cells expressed barely detectable amounts of EGFR, MTLn3 cells expressed readily detectable levels of functional receptors. A full length cDNA of the human EGFR (HER) was introduced by infection with a retroviral vector into MTC cells. Expression of HER was stable and receptors were functional with respect to surface expression, ligand binding and EGF-stimulated phosphorylation. Independent clones of the transfectants were isolated and characterized. Ligand stimulation of MTC HER cells and derived clones led to enhanced adhesion of cells to extracellular matrix proteins. Implantation of cells intravenously into female nu/nu mice revealed ligand-dependent enhancement of lung colonizing potential of EGFR-expressing cells.

The stable prostacyclin analogue Cicaprost inhibits metastasis to lungs and lymph nodes in the 13762NF MTLn3 rat mammary carcinoma

Prostacyclin and its stable analogues have been shown to interfere specifically with certain steps of the metastatic cascade. The antimetastatic activity of the stable prostacyclin analogue Cicaprost (Schering AG) on haematogenous metastasis in a series of tumours in rats and mice has been well established. In order to test the effect of Cicaprost on lymphogenous metastasis we chose the metastatic cell clone MTLn3 derived from the 13762NF rat mammary carcinoma. The effect of Cicaprost on prevention of lung metastasis, lymph node metastasis and primary tumour growth was investigated. Cicaprost given in daily doses of 0.01, 0.03 and 0.1 mg/kg orally, reduced the number of lung metastases in a dose-dependent manner. Whereas the median number of lung metastases in the controls was greater than 1000, Cicaprost at a dose of 0.1 mg/kg reduced the number of lung metastases to between 11 and 100. The weight of the ipsilateral axillary lymph nodes was diminished by Cicaprost to 30-50% of controls. Moreover, metastasis to the contralateral axillary lymph node was completely inhibited by Cicaprost at all three doses tested. Cicaprost did not influence the growth rate of the MTLn3 cell clone implanted into the mammary fat pad or the weight of the primary tumour at the end of treatment. In conclusion, in addition to its dose-dependent effect on haematogenous metastasis, Cicaprost strongly inhibits lymph node metastasis.

EGF receptor in neoplasia and metastasis

EGFR is a member of the tyrosine kinase family of cell surface receptors with a wide range of expression throughout development and in a variety of different cell types. The receptor can transmit signals to cells: i) upon interaction with ligands such as EGF, TGF alpha, amphiregulin or heparin binding EGF, ii) upon truncation or mutation of extracellular and/or intracellular domains, iii) upon amplification of a basal receptor activity (in the absence of ligand) through cooperation with other cellular signaling pathways or nuclear events (e.g. expression of v-erbA). The activated EGFR can exert pleiotropic functions on cells, depending on their tissue origin and state of differentiation. Under certain conditions it can also contribute to neoplasia and development of metastases. Such conditions can exist upon aberrant receptor/ligand expression and activation (e.g. in the wrong cell; at the wrong time; in the wrong amounts). Aberrant signalling can also occur through constitutive EGFR activation. Oncogenic potential of EGFR has been demonstrated in a wide range of experimental animals. EGFR is also implicated in human cancer, where it may contribute both to the initiation (glioblastoma) and progression (epithelial tumors) of the disease. EGFR may influence key steps in the processes of tumor invasion and dissemination. Involvement of EGFR in tumor spread may indicate a potential use of this receptor as a target for antimetastatic therapy.

A link between ras and metastatic behavior of tumor cells: ras induces CD44 promoter activity and leads to low-level expression of metastasis-specific variants of CD44 in CREF cells

The activated oncogene c-Ha-ras induces expression of the surface glycoprotein CD44 in cloned rat embryonic fibroblasts (CREF). Induction is transcriptional as shown by transient cotransfections of c-Ha-ras expression constructs and CD44 promoter reporter gene constructs and depends on the presence of an AP-1 binding site at position -110. Increased transcript levels for the standard isoform of CD44 (CD44s) are accompanied by the appearance of alternatively spliced RNAs and the synthesis of variants of CD44 (CD44v). These CD44v molecules differ from the standard type by the addition of sequences in the extracellular portion of the molecules. The occurrence of CD44v molecules in CREF cells upon induction of the CD44 promoter is probably due to leakiness of the splice control in these cells since stable transfection with c-Ha-ras does not alter the CD44v/total CD44 ratio. Upon ras overexpression, however, using an inducible mouse mammary tumor virus-ras construct, a transient increase of CD44v/total CD44 ratio of 3-4 has been determined suggesting that a burst of ras expression, in the genetic background of CREF cells, influences both promoter activity and splice control or accuracy. The expression of CD44v proteins is responsible for the metastatic potential in a variety of tumors (U. Günthert et al., Cell, 65: 13-24, 1991). Also in CREF cells expression of CD44v correlates with metastatic behavior, ras-transfected CREF cells are not only fully transformed but also give rise to metastatic spread as measured in the spontaneous metastasis assay. The adenoviral oncogene E1A counteracts ras-induced promoter function and, consequently, inhibits metastatic behavior without extinguishing transformation.

Rapid effects of EGF on cytoskeletal structures and adhesive properties of highly metastatic rat mammary adenocarcinoma cells

In the highly metastatic rat mammary adenocarcinoma cell clone MTLn3, EGF induced increased adhesion to fibronectin while in the human epidermoid carcinoma cell line A431 EGF induced diminished adhesive properties. Flattening of cells with extensive formation of filopodia was observed in MTLn3 cells within 5 min of EGF addition, while in A431 cells EGF induced rounding up and only occasional formation of filopodia. Immunofluorescent analysis revealed extension of microtubules (MT) into the filopodia and Western blot analysis demonstrated an EGF-induced 2- to 3-fold increase in the amount of assembled tubulin in MTLn3 but not in A431 cells. In MTLn3, but only marginally in A431 cells, EGF treatment resulted in phosphorylation of a 280 kD cytoskeleton-associated protein, which was rapid and dose-dependent. These results suggest differential signal transduction pathway of cytoskeleton-associated EGFRs in highly metastatic MTLn3 as compared with A431 cells.

Ligand-induced activation of epidermal growth factor receptor in intact rat mammary adenocarcinoma cells without detectable receptor phosphorylation

Expression and function of epidermal growth factor receptor (EGFR) was investigated in a metastatic cell clone (MTLn3) derived from the 13762NF rat mammary adenocarcinoma. No receptor phosphorylation could be identified in intact cells or in membrane preparations, while EGF-dependent phosphorylation of substrates occurred in intact cells. Indications for active suppression of receptor phosphorylation came from the fact that EGFRs bound in immunocomplexes or associated with the cytoskeleton of detergent treated cells were able to undergo basal and EGF-induced phosphorylation in vitro. Cross-linking experiments with 125I-EGF, as well as [35S]methionine labeling followed by immunoprecipitation with receptor specific antibodies readily detected in MTLn3 cells the expected 170-kDa EGFR protein. In addition, two proteins with molecular masses of 420-480 and 95 kDa specifically bound 125I-EGF on intact MTLn3 and sparse cultures of A431 cells. Phosphorylation of the 420-480 kDa molecule could be identified in immunocomplexes of EGFRs isolated from MTLn3 and sparse A431 cells, but the 95-kDa receptor molecule was never phosphorylated. While the presence of alternative forms of EGFR in the highly metastatic cell clone MTLn3 was unexpected, our observations of inefficient receptor autophosphorylation are in agreement with other recent reports and suggest that in MTLn3 cells EGFR-mediated signal transduction can be an event independent from receptor autophosphorylation.

Coexpression of cytokeratins characteristic for myoepithelial and luminal cell lineages in rat 13762NF mammary adenocarcinoma tumors and their spontaneous metastases

We used immunohistochemical procedures to study the cellular expression and distribution of cytokeratins (CKs) in rat 13762NF mammary adenocarcinoma cells growing at mammary fat pad sites and at spontaneous lymph node and lung sites. In order to establish CK distribution in normal rat mammary epithelia, immature, resting, and lactating rat mammary glands were probed with a panel of monospecific antibodies that recognize individual CKs. Basal/myepithelial cells were distinguished by expression of CKs 5 and 14 and coexpression of vimentin from luminal cells, which expressed CKs 8, 18, and 19. Antibody to CK 7 recognized luminal epithelium of immature and resting, but not lactating, mammary glands. Myoepithelial cells of lactating mammary gland were weakly recognized by antibodies to CKs 7 and 19. Tumors formed by cell lines and clones derived from parental 13762NF tumor (MTPa, MTC, MTA, and MTF7) were not recognized by any of the anti-CK antibodies. Only vimentin was expressed in these tumors and their metastases. In tumors and metastases generated from cell lines and clones derived from lymph node (MTLY) and lung metastases (MTLn2 and MTLn3) of the 13762NF tumor we observed heterogeneous CK phenotypes. Expression of CKs 5 and 18 was greatly reduced or lacking, while CK 14 was coexpressed with CKs 7, 8, and 19 with or without vimentin. Tumors from the highly metastatic clone MTLn3 had a dominant cellular phenotype, expressing CKs 7, 8, 14, and 19 and vimentin, a pattern that did not match normal mammary epithelia, whether luminal, basal/myoepithelial, or the dual-phenotype stem cell, in which CKs 5, 8, 14, and 18 were coexpressed. MTLn3 lymph node and lung metastases expressed the same cellular phenotype as the s.c. growing MTLn3 tumor. The results appear to contradict the belief that malignant mammary tumors may be distinguished from benign tumors or hyperplastic growths by the lack of basal/myoepithelial markers.

Cellular distribution and biological activity of epidermal growth factor receptors in A431 cells are influenced by cell-cell contact

The potential significance of cell-cell interactions on EGF receptor (EGFR) activity was investigated in cultured adherent A431 cells seeded as single-cell suspensions with different initial cell densities. In dense cultures, EGFRs were mainly localised at cell boundaries and in microvilli as shown by immunofluorescence analysis with an EGFR-specific antibody while in sparse cultures the distribution of EGFRs was more diffuse. Scatchard analysis showed that as cell density decreased the number of high-affinity receptors increased considerably. Upon treatment of adherent intact cells with EGF all cells in sparse cultures contained activated EGFRs as demonstrated by immunofluorescence analysis with a phosphotyrosine-specific antibody, while in dense cultures mainly cells at the periphery of a cluster and especially at their expanding borders exhibited activated EGFRs. EGF-induced phosphorylation in intact cells was greatly enhanced in sparse compared with dense cultures as demonstrated by immunoprecipitation with a phosphotyrosine-specific antibody. In contrast to intact cells, in cytoskeleton preparations, obtained after mild detergent treatment of adherent cells, EGFRs were able to undergo EGF-independent phosphorylation. Pretreatment of cells with EGF led to enhanced tyrosine phosphorylation of cytoskeletal-associated proteins. Our observations suggest that cell density has a considerable effect on the subcellular localisation as well as biological activity of the EGFR. Thus, in intact A431 cells growing with extensive cell-cell interactions some negative control mechanisms preventing EGFR activation may be exerted by adjacent cells.

The pyrimido-pyrimidine derivatives RA233 and RX-RA85 affect cell cycle distribution of two murine tumour cell lines

The pyrimido-pyrimidine derivatives RA233 and RX-RA85, which are potent inhibitors of platelet and tumour phosphodiesterases, were developed as antitumour agents. Clinical as well as animal studies suggest a tumour type specific, although moderate, antitumour activity for RA233. In our search for more potent congeners of RA233, we found that RX-RA85 was cytotoxic for cultured B16 melanoma and Lewis lung carcinoma cells at drug concentrations above 4 micrograms/ml whereas RA233 concentrations up to 400 micrograms/ml were tolerated. When tested for their effects on cell cycle distribution, RX-RA85 was 100-fold more potent than RA233 in producing an increase in the proportion of cells in S and G2 + M phase in 3LL cells. Progression of 3LL cells through the cell cycle was delayed for 5 h by RA233 treatment, whereas RX-RA85 was ineffective. In contrast, B16 cells responded poorly to either drug. The effects of both compounds were not only tumour cell specific but also dependent on the stage of tumour cell growth (drugs added to synchronously vs. asynchronously growing cultures). In the case of RX-RA85, the potency to affect tumour cell cycle distribution was highly dependent on tumour cell number, making the potential of this drug as an antitumour agent somewhat limited.

Effects of RA 233 treatment on the adhesive, invasive and metastatic properties of 13762NF rat mammary tumor cells

The pyrimido-pyrimidine analogue RA 233 has pleiotropic and differential effects on cultured tumor cell clones isolated from the 13762NF rat mammary adenocarcinoma. A nonresponsive clone of low metastatic potential (MTC) was not modified in its cell fragility or invasive, adhesive or lung-colonizing properties by RA 233 treatment. In contrast, a drug-responsive clone of high metastatic potential (MTLn3) was rendered less invasive and its cell fragility was decreased with RA 233 treatment, although its adhesiveness to lung microvascular endothelial cells and subendothelial matrix was unaffected by RA 233. Lung colonization of intravenously injected MTLn3 cells in syngeneic rats was significantly increased by RA 233 treatment, whereas spontaneous metastasis from the mammary fat pad to lung sites was decreased, although this decrease was not statistically significant.

Differential adhesion of metastatic rat mammary carcinoma cells to organ-derived microvessel endothelial cells and subendothelial matrix

The in vitro adhesion rates of rat 13762NF mammary adenocarcinoma cell clones of different spontaneous metastatic potentials to cloned microvessel endothelial cell monolayers and their subendothelial extracellular matrix were investigated. In this system, high rates of adhesion of the cloned tumor cell lines to syngeneic target (lung) organ-derived subendothelial matrix correlated with spontaneous metastatic potential, whereas adhesion to the lung microvessel endothelial cell apical surfaces occurred at lower rates and was not highly significantly different among the tumor cell lines. Adhesion rates to bovine aortic large vessel, and human brain and human meningeal microvessel endothelial cell monolayers were, in general, lower than those found with syngeneic lung microvessel endothelial cells, and did not correlate with spontaneous metastatic potential. Growth of endothelial cells in fetal bovine serum or platelet-poor horse serum did not affect the results, suggesting that in this system metastasis-associated organ-adhesive specificity is determined at the level of the subendothelial matrix.

Characterization of cytokeratins expressed in metastatic rat mammary adenocarcinoma cells

The expression of cytokeratins (CKs) was investigated in cell lines and clones established from the rat 13762NF mammary adenocarcinoma tumor and its spontaneous lymph node and lung metastases. Two-dimensional polyacrylamide gel electrophoresis of intermediate filament-enriched protein fractions from cultured cells revealed that clones established from spontaneous metastases contained three CKs (Mr approximately 54,000, approximately 52,000, and approximately 40,000) characteristic of simple epithelia and two CKs (Mr approximately 51,000 and approximately 47,000) characteristic of stratified epithelia. CK expression varied qualitatively and quantitatively between the different metastasis-derived cell clones. In contrast, cell clones established from the original mammary fat pad tumor expressed low or undetectable levels of CKs. Western blot analyses with a panel of anti-CK antibodies with defined specificities confirmed the observations. One-dimensional polyacrylamide gel electrophoresis of whole-cell lysates and intermediate filament-enriched extracts were transferred and probed with the panel of antibodies. The relative expression of individual CKs varied according to the cell line or clone examined and environmental conditions (low versus high passage and in vitro versus in vivo growth), whereas the amount of total CKs expressed relative to total cell protein varied according to cell line or clone and growth conditions.

Pyrimido-pyrimidine modulation of EGF growth-promoting activity and p21ras expression in rat mammary adenocarcinoma cells

RA 233, a pyrimido-pyrimidine analogue developed originally as an antiplatelet agent, has reduced the incidence of tumor metastases in clinical trials. However, in animal tumor models antimetastatic therapy using RA 233 has been inconsistent. We therefore tested RA 233 for additional effects, such as its direct action on tumor cells. Using the rat 13726NF mammary adenocarcinoma tumor system, low, nontoxic concentrations of RA 233 had pleiotropic and differential effects on two 13762NF tumor cell clones. The growth of MTC cells (low spontaneous metastatic potential) was not affected by low concentrations of RA 233 (50 microM) or epidermal growth factor (EGF) (up to 10 ng/ml) for 3 days in 0.5-10% fetal bovine serum. In contrast, MTLn3 (high spontaneous metastatic potential) cell cultures maintained for 3 days in low (0.5-1%) serum in the presence of 1.25-10 ng/ml EGF doubled in cell numbers compared with control cultures, and addition of 50 microM RA 233 abrogated the growth-stimulatory effect of EGF. The inhibitory effect of RA 233 on MTLn3 cells was dose dependent and not due to cell toxicity as determined by cell viability, cell growth, and colony formation properties after drug removal. In addition, incubation of MTLn3 cells with 50 microM RA 233 resulted in an increase of p21ras protein expression, whereas there was no effect on the level of p21ras in identically treated MTC cells or when either clone was treated with 10 ng/ml EGF. The results suggest that among the heterogeneous effects of RA 233 on tumor cells, modulation of growth factor responses and regulatory molecules may be important.

Cytoskeletal and junctional heterogeneity in mammary tumor cells and their possible significance in tumor progression

Malignant breast cancers appear to metastasize first via the lymphatics to colonize regional lymph nodes, and then via the blood circulation to colonize distant organs. Using a rat mammary tumor model based on the 13762NF adenocarcinoma, evidence is presented that malignant cell subpopulations spread lymphatically to regional lymph nodes, then become blood-borne and metastasize to lungs. Using chromosome and cytokeratin markers to identify specific tumor cell subpopulations, tumor progression in this system appears to be associated with the appearance of a highly specialized, metastatic cell subpopulation. This highly malignant cell subpopulation is completely uncoupled by gap junctions when examined for gap-junctional communication, in contrast with less malignant subpopulations that show varying degrees of cell communication through gap junctions. Loss of cell-cell communication may be one of the epigenetic events that leads to the generation of tumor cell diversification and heterogeneity. In concert with host selective pressures, this could result in the evolution of highly malignant cell subpopulations with unique characteristics.

Effects of the pyrimido-pyrimidine derivative RX-RA 85 on metastatic tumor cell-vascular endothelial cell interactions

An important step in the metastatic process is the interaction of blood-borne malignant cells with the vascular endothelium. Among the agents that may interfere with this process are pyrimido-pyrimidines, such as RX-RA 85, developed originally as an antiplatelet agent. Using an endothelial cell momolayer attachment assay we have investigated the effects of RX-RA 85 on tumor cell and endothelial cell properties. Exposure of bovine aortic endothelial cells for 3 h to greater than 4 micrograms/ml RX-RA 85 produced toxic effects, resulting in vacuole formation, retraction and finally rounding up of the cells. Endothelial cells derived from different sources behaved dissimilarly; human brain, human meninges, mouse brain, mouse lung and rat lung endothelial cells were less sensitive to drug treatment than bovine aortic endothelial cells. RX-RA 85 treatment of bovine aortic endothelial cells increased B16-F1 melanoma cell adhesion. When B16-F1 cells were exposed to 4-8 micrograms/ml RX-RA 85, increased adhesion to the subendothelial matrix occurred, whereas exposure to higher drug concentrations (8-16 micrograms/ml RX-RA 85) decreased adhesion. Indirect immunofluorescence staining of cytoskeletal structures in B16-F1 cells adhering to and spreading on matrix revealed that the differential effects of RX-RA 85 on the organization of microtubules and microfilaments might explain the dose-dependent differences in adhesion kinetics.

The pyrimido-pyrimidine derivatives RA 233 and RX-RA 85 affect growth and cytoskeletal organization of rat mammary adenocarcinoma cells

The pyrimido-pyrimidine derivatives RA 233 and RX-RA 85, which are potent inhibitors of platelet and tumor phosphodiesterase, were developed as antitumor agents. When tested by us, these drugs were cytostatic at low concentrations and produced dramatic changes in cell shape and organization of cytoskeletal structures in cultured MTF7 cells derived from the rat 13762NF mammary adenocarcinoma. At high concentrations (up to 600 micrograms/ml) RA 233 was cytostatic but not cytotoxic to MTF7 cells during a 24 hr incubation in vitro, whereas RX-RA 85 was cytotoxic at concentrations above 4 micrograms/ml. These drugs caused MTF7 cells to elongate and form numerous vacuoles, which surrounded the cell nucleus. Treatment of MTF7 cells with RA 233 or RX-RA 85 enhanced microtubular organization concomitant with a decrease in microfilament organization. In contrast, treatment of MTF7 cells with 1 mM dibutyryl cAMP resulted in an enhanced organization of microtubules but had no effect on microfilament organization. Previous studies suggested that RA 233 and RX-RA 85 increase cAMP levels in 2 other cell clones of rat 13762NF mammary adenocarcinoma by inhibiting phosphodiesterases. However, additional sites of drug action should also be considered based on the effects of these drugs on microfilament systems and cell vacuoles.

Direct effects of the pyrimido-pyrimidine derivative RA 233 (Rapenton) on rat 13762NF mammary tumor cell clones in vitro

Studies with the pyrimido-pyrimidine analogue RA 233 (Rapenton) suggest that its antimetastatic action may not be mediated entirely by inhibition of platelet function. Little is known about its direct effects on tumor cells. We investigated the in vitro effects of RA 233 on clones MTLn3 and MTC of differing metastatic potentials, isolated from the 13762NF rat mammary adenocarcinoma. The results indicated that RA 233 is cytostatic (EC50 of approximately 140 microM and approximately 180 microM for MTLn3 and MTC cells, respectively) rather than cytotoxic by determining changes in viable cell number, thymidine uptake, and incorporation of thymidine and methionine. In both clones RA 233 inhibited cAMP-dependent phosphodiesterase activity and affected cAMP accumulation in intact cells. In contrast, clonal heterogeneity in drug-induced morphological changes, such as vacuole formation and altered organization of cytoskeletal structures, as well as increased tumor cell growth at 50 microM RA 233 was observed between clones MTLn3 and MTC. These data could explain the conflicting results obtained with RA 233 when evaluated as an antimetastatic agent.

Antiplatelet pyrimido-pyrimidines and metastasis

The role of antiplatelet drugs in relation to their potential antimetastatic activities has been reviewed and the effects of two pyrimido-pyrimidine derivatives (RX-RA69 and RX-RA85) with strong antiplatelet activities investigated in metastasizing tumour models. The routes of administration and drug dosages were always chosen in such a way that good antiplatelet activities were obtained. RX-RA69 (20 mg/kg/day) given in the drinking water had no effect on spontaneous metastasis of Lewis lung carcinoma. RX-RA85 (20 mg/kg/day) did not influence spontaneous metastasis of B16 melanoma. On the other hand, giving RX-RA85 (8 mg/kg) daily i.p. to Lewis lung carcinoma bearing mice significantly increased the number of lung metastases but had no significant effect on primary tumour implant growth. Pretreating mice orally with 20 mg/kg RX-RA85 1 h before i.v. injection of B16 melanoma cells had no significant effect on lung colony number or distribution of extrapulmonary tumours while injecting the same dosage of RX-RA85 i.v. 1-2 h before tumour-cell injection decreased lung colony formation, but increased extrapulmonary tumour burden. This investigation like many others does not support the importance of platelets in metastasis formation.