Effects of gamma irradiation on cultured rat and mouse microvessel endothelial cells: metastatic tumor cell adhesion, subendothelial matrix degradation, and secretion of tumor cell growth factors.

The effects of gamma-irradiation on the properties of microvessel endothelial cells were studied in vitro. After incubating confluent endothelial cell monolayers in low serum-containing medium for 24 h, the monolayers were irradiated with 137Cs. Survival of rat lung microvessel endothelial (RLE) and mouse brain microvessel endothelial (MBE) cells were similar after irradiation (Do = 2.17 and 1.75 Gy, Dq = 4.44 and 5.67 Gy, and n = 7.8 and 25 for RLE and MBE cells, respectively). We examined the effects of gamma-irradiation on endothelial cell morphology, adhesion of syngeneic rat lung or mouse brain metastasizing tumor cells, release of the subendothelial matrix-degrading enzyme heparanase, and secretion of soluble mitogenic factors that stimulated the growth of syngeneic metastatic tumor cells. The effects of gamma-irradiation were not apparent until several hours after irradiation, and by 24 h doses of greater than or equal to 10 Gy caused limited endothelial cell retraction and reorganization of the endothelial monolayer. By 24 h after irradiation there was also increased adhesion of metastatic tumor cells to RLE but not MBE cells. We also examined the effects of gamma-irradiation on the release from endothelial cells of enzymes that solubilize the subendothelial matrix. Radiation resulted in a significant increase in the release of matrix-degrading enzyme (heparanase) that solubilized [35S]-labeled heparan sulfate from subendothelial matrix. This was most pronounced in the 24 h sample from gamma-irradiated endothelial cells. Finally, we examined the gamma-irradiation-induced release of mitogenic factors from endothelial cells that could stimulate the growth of metastatic cells in serum-limiting medium. The medium from RLE but not MBE cells stimulated the growth of a rat mammary carcinoma cell line. The results suggest that gamma-irradiation of microvessel endothelial cells can affect the interactions of tumor cells with endothelial cells and their subendothelial matrix; these processes could facilitate metastasis formation in irradiated tissues such as the lung.