A transforming growth factor beta1 (TGF beta1) antisense expression plasmid under constitutive control of the Rous sarcoma virus promoter was introduced into the highly tumorigenic and invasive colon carcinoma U9A cell line, which uses its autocrine TGF beta1 as a growth-stimulating factor. Stable transfectants were infrequent, and only the K6 transfectant exhibited 39 and 33%, respectively, of the levels of TGF beta1 mRNA and active, secreted TGF beta1 protein of the parental line. K6 exhibited no change in TGF beta2 expression, and TGF beta3 expression was not detected in either parental or transfectant cells. Compared to the parental line, the K6 antisense transfectant exhibited a 3-fold increase in lag time in anchorage-dependent colony formation. The parental line was 44 times as invasive through a collagen l-coated polycarbonate membrane in vitro as K6 cells and, after s.c. injection at low-cell inocula, U9A cells induced tumors 75 times as large in vivo as did the K6 antisense transfectant. The decreases in in vitro invasion and anchorage-dependent colony formation seen in K6 cells were largely reversed by the addition of TGF beta1. Tumors that did arise from the K6 antisense transfectant cells had lost antisense TGF beta1 expression and expressed the same TGF beta1 mRNA levels as controls. U9A cells were more metastatic to the liver after intrasplenic injection than K6 cells. These findings demonstrate a role for autocrine TGE beta1 in colon cancer tumorigenicity and invasion. They also show that a relatively small decrease in TGF beta1 levels was enough to markedly decrease colon carcinoma cell aggressiveness. This is not unprecedented, as we had found in an earlier study that a small, 2-4-fold increase in TGF beta1 protein levels in human colon cancers correlated with disease progression to metastases (E. Friedman et al., Cancer Epidemiol, Biomarkers & Prev., 4:549-554, 1995).