Dormancy of micrometastases: Balanced proliferation and apoptosis in the presence of angiogenesis suppression

Programmed cell death (PCD) plays a key role in developmental biology and in maintenance of the steady state in continuously renewing tissues. Currently, its existence is inferred mainly from gel electrophoresis of a pooled DNA extract as PCD was shown to be associated with DNA fragmentation. Based on this observation, we describe here the development of a method for the in situ visualization of PCD at the single-cell level, while preserving tissue architecture. Conventional histological sections, pretreated with protease, were nick end labeled with biotinylated poly dU, introduced by terminal deoxy- transferase, and then stained using avidin-conjugated peroxidase. The reaction is specific, only nuclei located at positions where PCD is expected are stained. The initial screening includes: small and large intestine, epidermis, lymphoid tissues, ovary, and other organs. A detailed analysis revealed that the process is initiated at the nuclear periphery, it is relatively short (1-3 h from initiation to cell elimination) and that PCD appears in tissues in clusters. The extent of tissue-PCD revealed by this method is considerably greater than apoptosis detected by nuclear morphology, and thus opens the way for a variety of studies.

The phenomenon of inhibition of tumor growth by tumor mass has been repeatedly studied, but without elucidation of a satisfactory mechanism. In our animal model, a primary tumor inhibits its remote metastases. After tumor removal, metastases neovascularize and grow. When the primary tumor is present, metastatic growth is suppressed by a circulating angiogenesis inhibitor. Serum and urine from tumor-bearing mice, but not from controls, specifically inhibit endothelial cell proliferation. The activity copurifies with a 38 kDa plasminogen fragment that we have sequenced and named angiostatin. A corresponding fragment of human plasminogen has similar activity. Systemic administration of angiostatin, but not intact plasminogen, potently blocks neovascularization and growth of metastases. We here show that the inhibition of metastases by a primary mouse tumor is mediated, at least in part, by angiostatin.

Monoclonal antibodies specific for 5-bromodeoxyuridine have been produced and applied in detecting low levels of DNA replication on a cell-by-cell basis in vitro. The immunoglobulin-producing hybridomas were derived from spleen cells of mice immunized with a conjugate of iodouridine and ovalbumin. The cells were fused with the plasmacytoma line SP2/0Ag14. The antibodies produced are highly specific for bromodeoxyuridine and iododeoxyuridine and do not cross-react with thymidine. DNA synthesis in cultured cells exposed to bromodeoxyuridine for as short a time as 6 minutes can be detected easily and rapidly by an immunofluorescent staining method and quantitated by flow cytometry.