[Mechanisms of metastasis and molecular markers of malignant tumor progression. I. Colorectal cancer].

The ability of neoplastic cells to dissemination from a primary tumor to lymphatic nodes and to adjacent and distant tissues and organs is an inseparable feature of malignant tumors and the main cause of failure in their treatment. Metastasis formation is a multistage process which includes proteolysis, the motility and migration of cells, proliferation, and neoangiogenesis. In the first step, the cells released from the primary tumor have to penetrate to the blood or lymphatic vessels (intravasation), the road which dissemination follows. Circulating cells can then migrate through the walls of vessels to surrounding tissues (extravasation) where they settle, proliferate, and induce angiogenesis, creating metastases. Indispensable in the process of intra- and extravasation is the activation of proteolytic enzymes capable of degrading the extracellular matrix (ECM) surrounding the endothelium or creating the basement membrane of epithelial tissue in different organs. In this stage, the activation of proteolytic enzymes, such as proteinases of the plasmin system, serine proteinases, and matrix metalloproteinases (MMPs), is necessary. Simultaneously, changes occur in the expression of many superficial glycoproteins and factors responsible for cell adhesion (integrins) and intercellular communication (cadherins). Neoangiogenesis is connected with the expression of many markers of this process, among them vascular endothelial growth factor (VEGF), endoglin (CD105), a transmembranous glycoprotein which is a component of the receptor for transforming growth factor beta (TGFbeta), as well as neuropilin (NRP), the co-receptor for VEGF. Conventionally, the prognosis of neoplastic disease and its treatment are based mainly on exact clinical and histopathological staging. This prognosis could, however, be improved by measuring the molecular and cellular markers which play key roles in tumor progression. Understanding the cellular processes responsible for tumor dissemination can be useful not only in the diagnosis and prognosis of treatment results, but also in developing targeted drugs, selectively directed towards those factors responsible for tumor invasiveness, as well as in creating new therapeutic strategies permitting the use of such drugs. In the present review the authors concentrate mainly on one tumor type, colorectal carcinoma, in which distant metastases, predominantly to the liver, are the main cause of failure, in spite of surgical curing of the primary tumor.