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Metastasis is the major cause of morbidity and death for cancer patients. Treatment modalities such as surgery, chemo therapy, and radiotherapy can now cure approximately 50% of the patients who develop a malignant tumor. The majority of the patients in the treatment failure group succumb to the direct effects of the mé tastasesor to complications associated with treatment of mé tastases (1-5). The dispersed anatomic location of mé tastasesand their heterogeneous cell composition prevent surgical removal and limit the response to systemic anticancer agents. Consequently a major challenge to cancer scientists is the development of improved methods to predict the metastatic aggressiveness of a patient's individual tumor, prevent local invasion, and identify and treat clinically silent micrometastases. Many laboratories are studying the fundamental mechanisms of invasion and mé tastases, with the hope of identifying specific biochemical factors which can be the basis for such diagnostic or therapeutic strategies. Over the last several years, significant progress has been made toward this goal. The complexity of the metastatic process has forced investi gators to focus on one step at a time in order to reduce the number of variables to a reasonable level. In recent years, our laboratory has focused on the interaction of metastatic tumor cells with the extracellular matrix. The metastasizing tumor cell must interact with the matrix at many stages of tumor invasion and mé tastases. One particular type of matrix, the basement membrane, appears to play a crucial role during the progression of invasive tumors and during hematogenous …