An in vitro study of human melanoma tumor cell metastasis: Cytological and molecular events during extravasation.

Abstract

In order to study the process by which human melanoma cells achieve invasion of basement membranes, a modification of the Membrane Invasion Culture System was developed to allow the in vitro collection of invasive tumor cells from heterogeneous tumor cell populations. A significant increase in the number of double minute chromosomes was observed in metaphase nuclei of the low metastatic A375P human melanoma cells which had invaded 2 consecutive amniotic membranes over that of cells in the control groups. After 25 days in culture, the incidence of double minutes had dropped below the control range. These data indicate that an unstable gene amplification event may be part of the process by which melanoma cells execute invasion through basement membranes. A375P cells which had invaded 1, 3 and 5 consecutive basement membrane-coated filters were established and compared with the parental cell line and a highly metastatic subclonal line for the following characteristics: (a) in vitro invasive potential, (b) mRNA expression of several oncogenes, (c) expression of laminin receptor, at the (cell surface) protein and mRNA levels, and, (d) secretion of endogenous laminin. There was a progressive increase in invasive potential and expression of endogenous laminin and laminin receptor which correlated with the number of membrane-coated filters through which the A375P cells had been selected. There were significant increases in the steady-state mRNA expression of c-myc and c-fos, a decrease in c-jun, and no change in Ha-ras, that correlated with increases in the invasive and metastatic potential of the cells. A novel in vitro adhesion assay was developed to study the interaction of tumor cells with lymphatic endothelium, the first step of extravasation from the lymphatic vessel. Human tumor cells from: one primary Ewing sarcoma, two melanoma, two colon and two breast carcinomas were assayed for their ability to attach to monolayers of lymphatic endothelium. There was a clear positive correlation between the metastatic potential and attachment potential of the melanoma cell lines. Overall, these data suggested that highly fibroblastic established tumor cell lines were more adaptive in rapid adhesion than primary tumor cell cultures with a more rounded morphology.