Milk Fat Globule – EGF – factor VIII (MFGE8), also called lactadherin, is a secreted protein, which binds extracellularly to phosphatidylserine and to αvβ3 and αvβ5 integrins. On human and mouse cells expressing these integrins, such as endothelial cells, phagocytes and some tumors, MFGE8/lactadherin has been shown to promote survival, epithelial to mesenchymal transition and phagocytosis. A protumoral function of MFGE8 has consequently been documented for a few types of human cancers, including melanoma, a subtype of breast cancers, and bladder carcinoma. Inhibiting the functions of MFGE8 could thus represent a new type of therapy for human cancers. Here, we show by immunohistochemistry on a collection of human ovarian cancers that MFGE8 is overexpressed in 45% of these tumors, and we confirm that it is specifically overexpressed in the triple-negative subtype of human breast cancers. We have established new in vitro assays to measure the effect of MFGE8 on survival, adhesion and migration of human ovarian and triple-negative breast cancer cell lines. Using these assays, we could identify new MFGE8-specific monoclonal antibodies, which efficiently blocked these three tumor-promoting effects of MFGE8. Our results suggest future use of MFGE8-blocking antibodies as new anti-cancer therapeutics in subgroups of ovarian carcinoma, and triple-negative breast carcinoma patients.

SummaryIncreased levels of plasminogen activator inhibitor-1 (PAI-1), the main physiological inhibitor of tissue-type plasminogen activator (t-PA) in plasma, are a known risk factor for thromboembolic and cardiovascular diseases. The elucidation of the binding site of inhibitory monoclonal antibodies may contribute to the rational design of PAI-1 modulating therapeutics. In this study, homolog-scanning mutagenesis was used to identify the binding region of a variety of human PAI-1 inhibitory antibodies, lacking cross-reactivity with rat PAI-1. Therefore, eight chimeric human/rat PAI-1 variants, containing rat PAI-1 substitutions at the N-terminal or C-terminal end with splicing sites at positions 26, 81, 187, 277 or 327, were generated and purified. Biochemical characterization revealed that all chimeras were folded properly. Subsequently, surface plasmon resonance was used to determine the affinity of various monoclonal antibodies for these chimera. Comparative analysis of the affinity and ELISA data allowed the identification of the major binding region of the inhibitory antibodies MA-8H9D4, MA-33B8F7, MA-44E4, MA-42A2F6 and MA-56A7C10. Thus, three segments in human PAI-1 containing each at least one site involved in the neutralization of PAI-1 activity could be identified, i.e. (1) the segment from residue 81 to residue 187 (comprising -helices hD, hE and hF, -strands s4C, s3A, s2A and s1A and the loops connecting these elements), (2) the segment between residues 277 and 327 (hI, thIs5A, s5A and s6A) and (3) the region C-terminal from amino acid 327, including the reactive site loop. The current data, together with previous data, indicate that PAI-1 contains at least four different regions that could be considered as putative targets to modulate its activity.

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SummaryTwo enzyme-linked immunosorbent assays (ELISAs) for the quantitation of rat plasminogen activator inhibitor-1 (PAI-1) antigen and activity, respectively, in biological fluids were developed using monoclonal antibodies raised against recombinant rat PAI-1. These assays had a lower limit of sensitivity in plasma of 0.3 and 0.15 ng/ml, respectively. The intra-assay, inter-assay and inter-dilution coefficients of variation were 9, 14 and 9%, respectively, for the antigen assay and 8, 17 and 13%, respectively for the activity assay. Assay recoveries of recombinant rat PAI-1 (5 to 20 ng/ml) added to plasma were 73 to 88% and 89 to 93% for the antigen and the activity assay, respectively. The level of PAI-1 antigen in rat plasma was 1.8 ± 0.9 ng/ml (mean ± SD, n = 18), with a corresponding value of 1.0 ± 0.5 ng/ml for PAI-1 activity. In lysed platelet-rich rat plasma PAI-1 antigen was 6.0 ± 1.0 ng/ml (n = 8) and PAI-1 activity was 2.3 ± 0.4 ng/ml (n = 8). Endotoxin injection (0.5 mg/kg) induced a time-dependent increase of both PAI-1 antigen and PAI-1 activity levels in rat plasma, eventually resulting in a 100- to 200-fold increase (p <0.0001 vs. baseline). A linear correlation was found between PAI-1 antigen and activity levels in normal plasma (r = 0.63, n = 18, p <0.01) and in plasma from endotoxin-treated rats (r = 0.90, n = 35, p <0.001). Application of these assays for the analysis of gel filtration experiments of plasma from endotoxin-treated rats demonstrated that PAI-1 antigen eluted as two peaks (with corresponding Mr of ~430 kDa and 61 kDa) whereas PAI-1 activity eluted as a single peak corresponding with the high molecular weight antigen form.Thus, these unique assays allowing the specific determination of rat PAI-1 antigen and rat PAI-1 activity may constitute important tools for further investigations on the pathophysiological role of PAI-1 in a variety of experimental rat models.

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