Overexpression of transforming growth factor beta type I receptor abolishes malignant phenotype of a rat bladder carcinoma cell line.

Transforming growth factor beta1 (TGF-beta1), a potent growth inhibitor of bladder carcinoma cells, elicits its effects by binding to cell surface receptors. LMC19, a highly invasive and metastatic rat bladder carcinoma cell line, was insensitive to the growth-suppressive effect of TGF-beta1, and it expressed undetectable levels of TGF-beta type I receptor (TbetaRI) mRNA by reverse transcription-PCR and its protein by Western blot analysis. To evaluate the effect of TbetaRI in reducing the malignant phenotype, we transfected LMC19 with an expression vector containing human TbetaRI cDNA. Stable transfection with the expression vector yielded five transfectants that expressed the introduced TbetaRI mRNA. The binding activity of TGF-beta1 to TbetaRI was restored in all of the transfectants. The growth of the transfectants on a plastic surface was markedly inhibited in the presence of TGF-beta1 in the culture medium (P < 0.001), whereas the control cells (parental and transfectant with only neo gene) remained TGF-beta1 insensitive. The colony-forming efficiency of the transfectants was strongly reduced in soft agar medium containing 5% FCS (P < 0.001) and was restored by the addition of a neutralizing anti-TGF-beta antibody. Furthermore, none of the transfectants tested formed tumors in athymic nude mice, whereas the control cells did so in all mice tested. These findings indicate that introduction of TbetaRI can revert a malignant phenotype to a less aggressive (even benign) phenotype in a rat bladder carcinoma cell line that lacks TbetaRI, and that reduced expression of TbetaRI may be associated with the development and progression of bladder carcinomas.