Reduced expression of transforming growth factor beta type I receptor contributes to the malignancy of human colon carcinoma cells

Transforming growth factor beta (TGF beta) type I (RI) and type II (RII) receptors are essential for TGF beta signal transduction, A human colon carcinoma cell line, designated GEO, is marginally responsive to TGF beta and expresses a low level of RI mRNA relative to colon carcinoma cells, which are highly responsive to TGF beta, Hence, the role of RI as a limiting factor for TGF beta sensitivity and the contribution of low RI levels to the malignant phenotype of GEO cells were examined, Stable transfection of a tetracycline-regulatable rat RI cDNA increased TGF beta(1) binding to RI and resulted in increased growth inhibition by exogenous TGF beta(1). In contrast, although stable transfection of an RII expression vector into the same GEO cells increased TGF beta(1) binding to RII, growth inhibition by exogenous TGF beta(1) was not altered, This indicated that the low level of RI is a limiting factor for the growth-inhibitory effects of TGF beta in GEO cells, RI-transfected cells were growth-arrested at a lower saturation density than GEO control cells. They also showed reduced growth and clonogenicity in plating efficiency and soft agarose assays, whereas RII-transfected cells did not show any differences from the NEO control cells in these assays, Tetracycline repressed RI expression in transfected cells and reversed the reduction in plating efficiency of RI-transfected clones, confirming that growth effects were due to increased RI expression in transfected cells. TGF beta(1) neutralizing antibody stimulated the proliferation of RI-transfected cells but had little effect on GEO control cells, indicating that increased autocrine-negative TGF beta activity also resulted from increased RI expression. Tumorigenicity in athymic nude mice was significantly delayed in RI-transfected cells, These results indicate that low RI expression can be a limiting factor for response to exogenous TGF beta, as well as TGF beta autocrine-negative activity, and that reduction of RI expression can contribute to malignant progression.