PITX2 DNA-Methylation: Predictive versus Prognostic Value for Anthracycline-Based Chemotherapy in Triple-Negative Breast Cancer Patients

Background

PITX2 DNA methylation has been shown to predict outcomes in high-risk breast cancer patients after anthracycline-based chemotherapy. To determine its prognostic versus predictive value, the impact of PITX2 DNA methylation on outcomes was studied in an untreated cohort vs. an anthracycline-treated triple-negative breast cancer (TNBC) cohort.

Material and Methods

The percent DNA methylation ratio (PMR) of paired-like homeodomain transcription factor 2 (PITX2) was determined by a validated methylation-specific real-time PCR test. Patient samples of routinely collected archived formalin-fixed paraffin-embedded (FFPE) tissue and clinical data from 144 TNBC patients of 2 independent cohorts (i.e., 66 untreated patients and 78 patients treated with anthracycline-based chemotherapy) were analyzed.

Results

The risk of 5- and 10-year overall survival (OS) increased continuously with rising PITX2 DNA methylation in the anthracycline-treated population, but it increased only slightly during 10-year follow-up time in the untreated patient population. PITX2 DNA methylation with a PMR cutoff of 2 did not show significance for poor vs. good outcomes (OS) in the untreated patient cohort (HR = 1.55; p = 0.259). In contrast, the PITX2 PMR cutoff of 2 identified patients with poor (PMR >2) vs. good (PMR ≤2) outcomes (OS) with statistical significance in the anthracycline-treated cohort (HR = 3.96; p = 0.011). The results in the subgroup of patients who did receive anthracyclines only (no taxanes) confirmed this finding (HR = 5.71; p = 0.014).

Conclusion

In this hypothesis-generating study PITX2 DNA methylation demonstrated predominantly predictive value in anthracycline treatment in TNBC patients. The risk of poor outcome (OS) correlates with increasing PITX2 DNA methylation.

Abstract P3-08-65: PITX2 DNA methylation: A prognostic/predictive biomarker for anthracycline-based chemotherapy

Background: Published data on the basis of a research-use-only PITX2 methylation assay using fresh-frozen tissue showed that the PITX2 DNA methylation may be a predictive marker for response to (neo) adjuvant anthracycline-based chemotherapy in breast cancer patients, including high-risk lymph node positive, estrogen receptor-positive, HER2-negative breast cancer and triple-negative breast cancer (TNBC). A retrospective study showed that high-risk ER-positive patients with a high PITX2 methylation ratio (PMR>12) had a poor outcome after anthracycline-based chemotherapy [HR 2,28; 95%-Cl 1,49 - 3,49; p<0.001, median DFS 71 months] compared to patients with low PITX2 methylation ratios [PMR≤12, median DFS 105 months]. Nevertheless, the question regarding the prognostic versus predictive value of PITX2 methylation for anthracycline therapy remained unanswered so far. Within an exploratory study using archived tissue specimen, this question was addressed by comparing the impact of PITX2 methylation on outcome in an untreated versus treated TNBC population. Methods: Patient samples and clinical data from 144 TNBC patients of 2 cohorts were analyzed: 1) 66 untreated patients from the Karolinska Institute in Sweden (primary diagnosis 1971-1976) and 2) 78 patients treated with anthracycline-based chemotherapy from the Comprehensive Cancer Center TUM in Germany (primary diagnosis 1996 to 2014). The main eligibility criteria were as follows: ER-negative, PR-negative, HER2-negative breast cancer (TNBC); no endocrine therapy; untreated collective: no further therapy besides surgery and radiation therapy; treated collective: surgery followed by anthracycline based chemotherapy. Samples were assessed for PITX2 methylation using a CE marked PITX2 RGQ PCR Test. The study was designed to determine the PITX2 PMR cut-off value in the untreated population (n=66, prognostic value) and to subsequently determine if this cut-off provides statistical significance in the treated population (n=78, prognostic & predictive value) as well. If no statistically significant cut-off value can be determined in the untreated population, cut-off determination was performed in the treated population and the cut-off was applied in the untreated population. The primary clinical endpoint for all cut-offs was 10-years DFS, secondary clinical endpoints were DFS censored at 5-years and OS censored at 5- and 10-years. Results: The untreated TNBC study population demonstrated a higher event rate at 10-years DFS/OS compared to the anthracycline-treated population [36 DFS/28 OS events vs 28 DFS/14 OS events]. Anthracycline treatment improved significantly overall survival [HR=0.42; p=0.008]. Risk recurrence increased continuously over 5 year and 10 years DFS/OS with increasing PITX2 DNA methylation in the anthracycline-treated population but not in the untreated patients over 5 years DFS/OS and only marginally over 10 years DFS/OS. PITX2 methylation identifies at the cut-off of PMR 2 a patient population with poor vs good overall survival with statistical significance [HR=3.96, p=0.011] in the treated patient cohort but did not identify patients with poor vs good outcome in the untreated patient cohort [HR=1.55, p=0.259].

Summary: In this hypothesis generating study DNA-methylation of PITX2 identifies with high statistical significance anthracycline-treated patients with poor vs good survival. Recurrence risk increases with increasing PITX2 DNA-methylation. PITX2 DNA-methylation shows no prognostic, but significant predictive value for anthracycline treatment in TNBC patients in this study. To confirm conclusively the predictive value of PITX2 DNA-methylation, the results warrant a confirmatory study with tumor specimens from a prospective trial of anthracycline-treated patients vs. anthracycline-free treated patients.

Citation Format: Marion Kiechle, Gabriele Schricker, Rudolf Napieralski, Michaela Aubele, Gert Auer, Kurt Ulm, Jonathan Perkins, Stefan Paepke, Moritz Hamann, Olaf G. Wilhelm. PITX2 DNA methylation: A prognostic/predictive biomarker for anthracycline-based chemotherapy [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P3-08-65.

Clinical performance of an analytically validated assay in comparison to microarray technology to assess PITX2 DNA-methylation in breast cancer

Significant evidence has accumulated that DNA-methylation of the paired-like homeodomain transcription factor 2 (PITX2) gene can serve as a prognostic and predictive biomarker in breast cancer. PITX2 DNA-methylation data have been obtained so far from microarray and polymerase chain reaction (PCR)-based research tests. The availability of an analytically validated in vitro methylation-specific real-time PCR assay format (therascreen PITX2 RGQ PCR assay) intended for the determination of the percent methylation ratio (PMR) in the (PITX2) promoter 2 prompted us to investigate whether the clinical performance of these different assay systems generate comparable clinical outcome data. Mathematically converted microarray data of a previous breast cancer study (n = 204) into PMR values leads to a PITX2 cut-off value at PMR 14.73. Recalculation of the data to experimentally equivalent PMRs with the PCR PITX2 assay leads to a cut-off value at PMR 12 with the highest statistical significance. This cut-off predicts outcome of high-risk breast cancer patients to adjuvant anthracycline-based chemotherapy (n = 204; Hazard Ratio 2.48; p < 0.001) comparable to microarray generated results (n = 204; Hazard ratio 2.32; p < 0.0001). The therascreen PITX2 RGQ PCR assay is an analytically validated test with high reliability and robustness and predicts outcome of high-risk breast cancer patients to anthracycline-based chemotherapy.

Clinical Validation of PITX2 DNA Methylation to Predict Outcome in High-Risk Breast Cancer Patients Treated with Anthracycline-Based Chemotherapy

Background: Breast cancer patients at high risk for recurrence are treated with anthracycline-based chemotherapy, but not all patients do equally benefit from such a regimen. To further improve therapy decision-making, biomarkers predicting outcome are of high unmet medical need. Methods: The percent DNA methylation ratio (PMR) of the promoter gene coding for the Paired-like homeodomain transcription factor 2 (PITX2) was determined by a validated methylation-specific real-time polymerase chain reaction (PCR) test. The multicenter study was conducted in routinely collected archived formalin-fixed paraffin-embedded (FFPE) tissue from 205 lymph node-positive breast cancer patients treated with adjuvant anthracycline-based chemotherapy. Results: The cut-off for the PITX2 methylation status (PMR = 12) was confirmed in a randomly selected cohort (n = 60) and validated (n = 145) prospectively with disease-free survival (DFS) at the 10-year follow-up. DFS was significantly different between the PMR ≤ 12 versus the PMR > 12 group with a hazard ratio (HR) of 2.74 (p < 0.001) in the validation cohort and also for the patient subgroup treated additionally with endocrine therapy (HR 2.47; p = 0.001). Conclusions: Early-stage lymph node-positive breast cancer patients with low PITX2 methylation do benefit from adjuvant anthracycline-based chemotherapy. Patients with a high PITX2 DNA methylation ratio, approximately 30%, show poor outcome and should thus be considered for alternative chemotherapy regimens.

The Predictive Value of PITX2 DNA Methylation for High-Risk Breast Cancer Therapy: Current Guidelines, Medical Needs, and Challenges

High-risk breast cancer comprises distinct tumor entities such as triple-negative breast cancer (TNBC) which is characterized by lack of estrogen (ER) and progesterone (PR) and the HER2 receptor and breast malignancies which have spread to more than three lymph nodes. For such patients, current (inter)national guidelines recommend anthracycline-based chemotherapy as the standard of care, but not all patients do equally benefit from such a chemotherapy. To further improve therapy decision-making, predictive biomarkers are of high, so far unmet, medical need. In this respect, predictive biomarkers would permit patient selection for a particular kind of chemotherapy and, by this, guide physicians to optimize the treatment plan for each patient individually. Besides DNA mutations, DNA methylation as a patient selection marker has received increasing clinical attention. For instance, significant evidence has accumulated that methylation of the PITX2 (paired-like homeodomain transcription factor 2) gene might serve as a novel predictive and prognostic biomarker, for a variety of cancer diseases. This review highlights the current understanding of treatment modalities of high-risk breast cancer patients with a focus on recommended treatment options, with special attention on the future clinical application of PITX2 as a predictive biomarker to personalize breast cancer management.

Synthesis, solution structure, and biological evaluation of urokinase type plasminogen activator (uPA)-derived receptor binding domain mimetics

Tumor cell migration and metastasis in cancer are facilitated by interaction of the serine protease urokinase type plasminogen activator (uPA) with its receptor uPAR (CD 87). Overexpression of uPA and uPAR in cancer tissues is associated with a high incidence of disease recurrence and early death. In agreement with these findings, disruption of the protein-protein interaction between uPAR present on tumor cells and its ligand uPA evolved as an attractive intervention strategy to impair tumor growth and metastasis. For this, the uPAR antagonist cyclo[19,31][D-Cys(19)]-uPA(19)(-)(31) was optimized to efficiently interrupt binding of uPA to cellular uPAR. First, the disulfide bridge of this lead compound was shifted and then the modified peptide was shortened from the amino and carboxy terminus to generate cyclo[21,29][Cys(21,29)]-uPA(21)(-)(30). Next, cyclo[21,29][D-Cys(21)Cys(29)]-uPA(21)(-)(30) was yielded by changing the chirality of Cys(21) to D-Cys(21). For analysis of uPAR binding activity, we employed competitive flow cytofluorometric receptor binding assays, using FITC-uPA as the ligand and U937 promyeloid leukemia cells as the cellular source of uPAR. As demonstrated for cyclo[21,29][D-Cys(21)Cys(29)]-uPA(21)(-)(30), the achieved peptide modifications maintained receptor binding activity (IC(50) = 0.04 microM), which is close in order to that of the parent protein ligand, uPA (IC(50) = 0.01 microM). A detailed NMR analysis with restrained and free molecular dynamics calculations in explicit H(2)O exhibits a well-defined structure with characteristic features such as an omega-loop with two betaI-turns about Lys(3), Tyr(4), Ser(6), and Asn(7). Hydrophobic clustering of the side chains of Tyr(4), Phe(5), Ile(8), and Trp(10) is observed. Side chain mobility is analyzed with time-dependent distance restraints. The NMR structure of cyclo[21,29][D-Cys(21)Cys(29)]-uPA(21)(-)(30) is very similar to the previously reported structure of the amino terminal fragment of uPA. Systematic point mutations led to cyclo[21,29][D-Cys(21)Nle(23)Cys(29)]-uPA(21)(-)(30), which still binds to uPAR but is resistant to proteolytic cleavage, e.g., by the tumor-associated serine proteases uPA and plasmin, and is stable in blood serum or plasma. In conclusion, small cyclic peptides were created, which mimic the structure and activity of the binding epitope of uPA to uPAR and which may serve as novel therapeutic agents in cancer metastasis.

Interaction of plasminogen activator inhibitor type-1 (PAI-1) with vitronectin

The serpin plasminogen activator inhibitor type 1 (PAI-1) plays an important role in physiological processes such as thrombolysis and fibrinolysis, as well as pathophysiological processes such as thrombosis, tumor invasion and metastasis. In addition to inhibiting serine proteases, mainly tissue-type (tPA) and urokinase-type (uPA) plasminogen activators, PAI-1 interacts with different components of the extracellular matrix, i.e. fibrin, heparin (Hep) and vitronectin (Vn). PAI-1 binding to Vn facilitates migration and invasion of tumor cells. The most important determinants of the Vn-binding site of PAI-1 appear to reside between amino acids 110-147, which includes alpha helix E (hE, amino acids 109-118). Ten different PAI-1 variants (mostly harboring modifications in hE) as well as wild-type PAI-1, the previously described PAI-1 mutant Q123K, and another serpin, PAI-2, were recombinantly produced in Escherichia coli containing a His(6) tag and purified by affinity chromatography. As shown in microtiter plate-based binding assays, surface plasmon resonance and thrombin inhibition experiments, all of the newly generated mutants which retained inhibitory activity against uPA still bound to Vn. Mutant A114-118, in which all amino-acids at positions 114-118 of PAI-1 were exchanged for alanine, displayed a reduced affinity to Vn as compared to wild-type PAI-1. Mutants lacking inhibitory activity towards uPA did not bind to Vn. Q123K, which inhibits uPA but does not bind to Vn, served as a control. In contrast to other active PAI-1 mutants, the inhibitory properties of A114-118 towards thrombin as well as uPA were significantly reduced in the presence of Hep. Our results demonstrate that the wild-type sequence of the region around hE in PAI-1 is not a prerequisite for binding to Vn.

The uPA/uPA receptor system as a target for tumor therapy

Invasiveness of a variety of tumors depends on the regulated expression of proteolytic enzymes that degrade the surrounding extracellular matrix and dissociate cell-cell and/or cell-matrix attachments. The tumor cell surface-associated urokinase-type plasminogen activator (uPA) system plays an especially important role in tumor cell invasion and metastasis. It consists of the serine protease uPA, its membrane-bound receptor (uPAR, CD87) and one of the natural inhibitors PAI-1 or PAI-2. There are strong indications based on animal experiments that interference with this system by inhibiting the enzymatic activity of uPA and/or antagonizing its binding to the receptor is of therapeutic relevance. With the recent solution of various X-ray structures of uPA/inhibitor complexes, structural information is available for optimizing existing lead compounds in their affinity and selectivity for uPA. Furthermore, peptide compounds capable of mimicking the structural epitope of uPA responsible for binding to the receptor efficiently antagonize this recognition process. Thus, both approaches prove to be well suited for the design of highly promising drugs in human medicine.

Cyclo19,31[D-Cys19]-uPA19-31 is a potent competitive antagonist of the interaction of urokinase-type plasminogen activator with its receptor (CD87)

Urokinase-type plasminogen activator (uPA) represents a central molecule in pericellular proteolysis and is implicated in a variety of physiological and pathophysiological processes such as tissue remodelling, wound healing, tumor invasion, and metastasis. uPA binds with high affinity to a specific cell surface receptor, uPAR (CD87), via a well defined sequence within the N-terminal region of uPA (uPA19-31). This interaction directs the proteolytic activity of uPA to the cell surface which represents an important step in tumor cell proliferation, invasion, and metastasis. Due to its fundamental role in these processes, the uPA/uPAR-system has emerged as a novel target for tumor therapy. Previously, we have identified a synthetic, cyclic, uPA-derived peptide, cyclo19,31uPA19-31, as a lead structure for the development of low molecular weight uPA-analogues, capable of blocking uPA/uPAR-interaction [Burgle et al., Biol. Chem. 378 (1997), 231-237]. We now searched for peptide variants of cyclo19,31uPA19-31 with elevated affinities for uPAR binding. Among other tasks, we performed a systematic D-amino acid scan of uPA19-31, in which each of the 13 L-amino acids was individually substituted by the corresponding D-amino acid. This led to the identification of cyclo19,31[D-Cys19]-uPA19-31 as a potent inhibitor of uPA/uPAR-interaction, displaying only a 20 to 40-fold lower binding capacity as compared to the naturally occurring uPAR-ligands uPA and its amino-terminal fragment. Cyclo19,31[D-Cys19]-uPA19-31 not only blocks binding of uPA to uPAR but is also capable of efficiently displacing uPAR-bound uPA from the cell surface and to inhibit uPA-mediated, tumor cell-associated plasminogen activation and fibrin degradation. Thus, cyclo19,31[D-Cys19]-uPA19-31 represents a promising therapeutic agent to significantly affect the tumor-associated uPA/uPAR-system.

High level synthesis of recombinant soluble urokinase receptor (CD87) by ovarian cancer cells reduces intraperitoneal tumor growth and spread in nude mice

Focussing of the serine protease urokinase (uPA) to the tumor cell surface via interaction with its receptor (uPAR) is an important step in tumor invasion and metastasis. The human ovarian cancer cell line OV-MZ-6#8 was stably transfected with expression plasmids either encoding cell-associated uPAR (GPI-uPAR) or a soluble form of uPAR (suPAR) lacking its glycan lipid anchor. In vitro, high level synthesis of functionally active recombinant suPAR inhibited cell proliferation and led to reduced cell-associated fibrin matrix degradation, whereas fibrinolytic activity was increased in OV-MZ-6#8 cells overexpressing GPI-uPAR. Both OV-MZ-6#8-derived clones were inoculated into the peritoneum of nude mice and tested for tumor growth and spread. High level synthesis of recombinant suPAR (without altering the physiological expression levels of GPI-uPAR and uPA in these cells) resulted in a significant reduction of tumor burden (up to 86%) in the xenogeneic mouse model. In contrast, overexpression of GPI-uPAR in tumor cells did not affect tumor growth. Our results demonstrate that high levels of suPAR in the ovarian cancer cell vicinity can act as a potent scavenger for uPA, thereby significantly reducing tumor cell growth and cancer progression in vivo.

(4-aminomethyl)phenylguanidine derivatives as nonpeptidic highly selective inhibitors of human urokinase

Increased expression of the serine protease urokinase-type plasminogen activator (uPA) in tumor tissues is highly correlated with tumor cell migration, invasion, proliferation, progression, and metastasis. Thus inhibition of uPA activity represents a promising target for antimetastatic therapy. So far, only the x-ray crystal structure of uPA inactivated by H-Glu-Gly-Arg-chloromethylketone has been reported, thus limited data are available for a rational structure-based design of uPA inhibitors. Taking into account the trypsin-like arginine specificity of uPA, (4-aminomethyl)phenylguanidine was selected as a potential P1 residue and iterative derivatization of its amino group with various hydrophobic residues, and structure-activity relationship-based optimization of the spacer in terms of hydrogen bond acceptor/donor properties led to N-(1-adamantyl)-N'-(4-guanidinobenzyl)urea as a highly selective nonpeptidic uPA inhibitor. The x-ray crystal structure of the uPA B-chain complexed with this inhibitor revealed a surprising binding mode consisting of the expected insertion of the phenylguanidine moiety into the S1 pocket, but with the adamantyl residue protruding toward the hydrophobic S1' enzyme subsite, thus exposing the ureido group to hydrogen-bonding interactions. Although in this enzyme-bound state the inhibitor is crossing the active site, interactions with the catalytic residues Ser-195 and His-57 are not observed, but their side chains are spatially displaced for steric reasons. Compared with other trypsin-like serine proteases, the S2 and S3/S4 pockets of uPA are reduced in size because of the 99-insertion loop. Therefore, the peculiar binding mode of the new type of uPA inhibitors offers the possibility of exploiting optimized interactions at the S1'/S2' subsites to further enhance selectivity and potency. Because crystals of the uPA/benzamidine complex allow inhibitor exchange by soaking procedures, the structure-based design of new generations of uPA inhibitors can rely on the assistance of x-ray analysis.

Epitope mapping of monoclonal antibodies directed to PAI-1 using PAI-1/PAI-2 chimera and PAI-1-derived synthetic peptides

Plasminogen activator inhibitor type-1 is a key regulatory protein of the fibrinolytic system that is involved in a variety of physiological and pathophysiological processes. A panel of 14 monoclonal antibodies directed against plasminogen activator inhibitor type-1 was analyzed regarding epitope specificity on plasminogen activator inhibitor type-1. For this purpose, chimera consisting of plasminogen activator inhibitor type-1 and another plasminogen activator inhibitor, plasminogen activator inhibitor type-2, with different portions of the respective wild-type proteins, were generated and plasminogen activator inhibitor type-1-derived 20-mer and 10-mer linear peptides were synthesized. Nine of the 14 monoclonal antibodies recognized an epitope located in the region between amino acid 76-188 of plasminogen activator inhibitor type-1, which encompasses the binding sites for vitronectin, heparin, and part of the fibrin binding region. Of these nine monoclonal antibodies, six reacted with a quadruple plasminogen activator inhibitor type-1 mutant (N152H, K156T, Q321L, M356I), and seven detected a plasminogen activator inhibitor type-1 deletion mutant (DeltaF111-H114). Two of the remaining five monoclonal antibodies recognized epitopes located between amino acid 209-227 and amino acid 352-371, respectively, while the other three antibodies reacted with wild-type plasminogen activator inhibitor type-1, only. Additional experiments revealed that two of the 14 mAbs neutralized and one monoclonal antibodies increased plasminogen activator inhibitor type-1 activity toward urokinase-type plasminogen activator, one of its target proteases.

Reduction of breast carcinoma tumor growth and lung colonization by overexpression of the soluble urokinase-type plasminogen activator receptor (CD87)

The serine protease urokinase-type plasminogen activator, uPA, when bound to its specific receptor, uPAR (CD87), plays a significant role in tumor cell invasion and metastasis. In breast cancer, enhanced uPA antigen in the primary tumor is correlated with poor prognosis of the patient. In an in vivo nude mouse model, we tested tumor growth and metastasis of human breast carcinoma cells that had been transfected with an expression plasmid encoding a soluble form of uPAR (suPAR). We explored, whether suPAR/uPA interaction reduces the binding of uPA to cell surface-associated uPAR, and, as a consequence, could suppress tumor growth and metastasis of the human breast cancer cell line MDA-MB-231 BAG. Overexpressed, secreted suPAR was shown to bind and thus scavenge the uPA secreted by the transfected lines suPAR3 and suPAR10. In vitro, an overexpression of suPAR did not alter the proliferation rate of the transfected tumor cells, nor did it affect the expression of uPA. Overexpression of suPAR led to a reduction in the plasminogen activation-related proteolytic activity of breast carcinoma cells. Primary tumor growth in the mammary fat pad of nude mice was followed up for 52 days. Overexpression of suPAR correlated with a reduction in tumor growth (from day 21, reaching 30% by day 34) as well as lung colonization (lung metastasis-positive mice in suPAR3: 4 of 17; suPAR10: 3 of 10; parental MDA-MB-231 BAG: 13 of 18). We conclude that suPAR overexpression leading to effective scavenge of uPA impairs proteolysis as well as the tumor growth and metastatic potential of breast carcinoma cells in vivo.

Cellular glycosylphosphatidylinositol-specific phospholipase D regulates urokinase receptor shedding and cell surface expression

The glycosylphosphatidylinositol (GPI)-anchored, multifunctional receptor for the serine proteinase, urokinase plasminogen activator (uPAR, CD87), regulates plasminogen activation and cell migration, adhesion, and proliferation. uPAR occurs in functionally distinct, membrane-anchored and soluble isoforms (s-uPAR) in vitro and in vivo. Recent evidence indicates that s-uPAR present in the circulation of cancer patients correlates with tumor malignancy and represents a valuable prognostic marker in certain types of cancer. We have therefore analyzed the mechanism of uPAR shedding in vitro. We present evidence that uPAR is actively released from ovarian cancer cells since the rate of receptor shedding did not correlate with uPAR expression. While s-uPAR was derived from the cell surface, it lacked the hydrophobic portion of the GPI moiety indicating anchor cleavage. We show that uPAR release is catalyzed by cellular GPI-specific phospholipase D (GPI-PLD), an enzyme cleaving the GPI anchor of the receptor. Thus, recombinant GPI-PLD expression increased receptor release up to fourfold. Conversely, a 40% reduction in GPI-PLD activity by GPI-PLD antisense mRNA expression inhibited uPAR release by more than 60%. We found that GPI-PLD also regulated uPAR expression, possibly by releasing a GPI-anchored growth factor. Our data suggest that cellular GPI-PLD might be involved in the generation of circulating prognostic markers in cancer and possibly regulate the function of GPI-anchored proteins by generating functionally distinct, soluble counterparts.

Thrombomodulin, a receptor for the serine protease thrombin, is decreased in primary tumors and metastases but increased in ascitic fluids of patients with advanced ovarian cancer FIGO IIIc

The human ovarian cancer cell line OV-MZ-19, established from a patient with cystadenocarcinoma of the ovary, expressing thrombomodulin (TM), a cell surface receptor for the serine protease thrombin, interacts with monoclonal and polyclonal antibodies having different specificity for TM. These antibodies detect TM antigen by means of flow cytofluorometry, laser scanning microscopy, immunocytochemistry, and ELISA. Therefore a highly sensitive ELISA for TM antigen was established using two different monoclonal antibodies to quantify TM in tissue extracts and biological fluids, e.g. peritoneal malignant ascites. Primary malignant ovarian tumors and metastases of the omentum and intestine contain TM antigen as determined by ELISA but in significantly lower concentrations than benign ovarian tumors (p=0.0056). In contrast, malignant ascitic fluid of patients with advanced ovarian cancer (FIGO IIIc) contain significantly elevated concentrations of soluble TM than benign peritoneal exudates (p=0.0003). Immunoaffinity purified ascites-derived TM efficiently activates protein C. Protein C activation of ascites-derived TM as well as TM expressed by the tumor cells is inhibited by the monoclonal antibodies. TM abrogates the procoagulant activity of thrombin, reduces pericellular thrombin via internalization, accelerates the thrombin-mediated inactivation of pro-uPA, and the EGF domains of TM exhibit mitogenic activity towards fibroblasts and tumor cells. Both, thrombin and pro-uPA play important roles in tumor invasion and metastasis. Therefore, downregulation and/or release of TM into ascitic fluid may play an important role in the malignant behavior of tumor cells.

The urokinase receptor is a major vitronectin-binding protein on endothelial cells

We have previously demonstrated that vitronectin (VN), a morphoregulatory protein in the vessel wall, is internalized and translocated to the subendothelial matrix by an integrin-independent mechanism (J. Histochem. Cytochem. 41, 1823-1832, 1993). The cell surface component which mediates the initial contact of VN with endothelial cells is defined here. The specific binding of VN to endothelial cells demonstrated the following properties: a threefold increase after phorbol ester treatment; 85% inhibition by pretreatment of cells with phosphatidylinositol-phospholipase C to release glycolipid-anchored surface proteins; a 90% inhibition by urokinase (u-PA) receptor blocking antibody. u-PA increased VN binding to cells due to an eightfold increase in the affinity of VN for the u-PA receptor. Structure-function studies showed that the amino-terminal fragment of u-PA, devoid of any proteolytic activity, mediated this effect. Active plasminogen activator inhibitor-1 (PAI-1), but not inactivated PAI-1, inhibited VN binding to cells and displaced VN that was prebound to endothelial cell monolayers. Similarly, VN binding to purified (immobilized) u-PA receptor, but not to integrin, was enhanced by u-PA and inhibited by PAI-1. Hence, the binding of soluble VN to endothelial cell surfaces is mediated by the u-PA receptor, and the relative concentrations of u-PA and PAI-1 are able to regulate the strength of this interaction. Endothelial cell adhesion to immobilized VN was found to be integrin-mediated without any involvement of the VN-uPA-receptor system. Hence, the interaction of VN with the u-PA receptor may be involved in the regulation of cellular processes necessary for endothelial cell invasion and migration at VN-rich extracellular matrix sites.

Disulfide pairing of the recombinant kringle-2 domain of tissue plasminogen activator produced in Escherichia coli

The kringle-2 domain of tissue plasminogen activator, cloned and expressed in Escherichia coli (Wilhelm, O.G., Jaskunas, S.R., Vlahos, C.J., and Bang, N.U. (1990) J. Biol. Chem. 265, 14606-14611), was internally radiolabeled using [35S]methionine-cysteine. Following refolding and isolation, the labeled polypeptide was further purified by reverse-phase high performance liquid chromatography. The purified kringle-2 domain was digested with thermolysin, and the resulting peptides were purified by high performance liquid chromatography. Five major peptides containing 35S were obtained. Amino acid sequence analysis showed that these peptides represented various cleavage products containing one or more of the following disulfides: Cys180-Cys261, Cys201-Cys243, Cys232-Cys256 (sequence numbering based on Pennica et al. (Pennica, D., Holmes, W.E., Kohr, W.J., Hakins, R.N., Vehar, G. A., Ward, C.A., Bennett, W.F., Yelverton E., Seeburg, P.H., Heynecker, H.L., Goeddel, E.V., and Collen, D. (1983) Nature 301, 214-221)). These results confirm that the refolding methodology used produced kringle-2 with the predicted disulfide linkage and, thus, yielded material suitable for structural and functional studies.

Functional properties of the recombinant kringle-2 domain of tissue plasminogen activator produced in Escherichia coli

The kringle-2 domain (residues 176-262) of tissue-type plasminogen activator (t-PA) was cloned and expressed in Escherichia coli. The recombinant peptide, which concentrated in cytoplasmic inclusion bodies, was isolated, solubilized, chemically refolded, and purified by affinity chromatography on lysine-Sepharose to apparent homogeneity. [35S]Cysteine-methionine-labeled polypeptide was used to study the interactions of kringle-2 with lysine, fibrin, and plasminogen activator inhibitor-1. The kringle-2 domain bound to lysine-Sepharose and to preformed fibrin with a Kd = 104 +/- 6.2 microM (0.86 +/- 0.012 binding site) and a Kd = 4.2 +/- 1.05 microM (0.80 +/- 0.081 binding site), respectively. Competition experiments and direct binding studies showed that the kringle-2 domain is required for the formation of the ternary t-PA-plasminogen-intact fibrin complex and that the association between the t-PA kringle-2 domain and fibrin does not require plasmin degradation of fibrin and exposure of new COOH-terminal lysine residues. We also observed that kringle-2 forms a complex with highly purified guanidine-activated plasminogen activator inhibitor-1, dissociable by 0.2 M epsilon-aminocaproic acid. The kringle-2 polypeptide significantly inhibited tissue plasminogen activator/plasminogen activator inhibitor-1 interaction. The kringle-2 domain bound to plasminogen activator inhibitor-1 in a specific and saturable manner with a Kd = 0.51 +/- 0.055 microM (0.35 +/- 0.026 binding site). Therefore, the t-PA kringle-2 domain is important for the interaction of t-PA not only with fibrin, but also with plasminogen activator inhibitor-1 and thus represents a key structure in the regulation of fibrinolysis.

After coronary thrombolysis and reperfusion, what next?

Thrombolytic therapy for the removal of intravascular thrombi was introduced when streptokinase was first given to humans 40 years ago, the same year the American College of Cardiology was founded. Streptokinase was first administered to patients with acute myocardial infarction in 1959. Today, thrombolytic therapy has been established to offer significant benefits to patients with acute myocardial infarction provided they are brought to medical attention early enough after the onset of symptoms. The two major agents, streptokinase and recombinant tissue-type plasminogen activator (rt-PA), have been shown to result in reperfusion of infarct-related arteries, to salvage ischemic myocardium, to improve myocardial performance and to reduce mortality. In spite of these impressive gains, this novel therapy has shortcomings. The interval from the start of thrombolytic treatment to coronary reperfusion varies significantly from patient to patient and may, at times, be too long to produce a real benefit in terms of salvage of ischemic myocardium. The rate of reocclusion lies somewhere between 10% and 20% and appears not to be influenced by concomitant heparin anticoagulation. The rate of bleeding complications even with the "fibrin-specific" rt-PA is higher than anticipated and may range from 10% to 30%. As a consequence, intensive efforts are being directed at the development of improved thrombolytic agents and for adjunctive therapy evaluating better anticoagulants than heparin and better antiplatelet agents than aspirin. This review is a status report summarizing where we are in thrombolytic therapy in acute myocardial infarction, where we need to improve treatment results and what is being done mainly at the preclinical level to bring about such improvements.