Transforming growth factor beta as a potent promoter in two-stage BALB/c 3T3 cell transformation.

We have tested transforming growth factor beta (TGF beta) in the two-stage BALB/c 3T3 cell transformation assay for possible tumor-promoting activity, since it has several effects similar to those of tumor-promoting phorbol esters. After initiation of BALB/c 3T3 cells with 3-methylchol-anthrene, treatment with TGF beta at 1 ng/ml alone or in combination with epidermal growth factor (EGF) for 4 weeks enhanced the number of transformed foci by 5- to 6-fold in comparison with uninitiated cells. Initiation treatment alone induced no or very few transformed foci in several assays. Treatment with phorbol-12,13-didecanoate (PDD) at 100 ng/ml for 4 weeks enhanced the number of transformed foci in initiated BALB/c 3T3 cells by 4- to 5-fold in comparison with uninitiated cells. Thus, TGF beta at 1 ng/ml is as potent as PDD at 100 ng/ml for tumor-promoting activity in the two-stage BALB/c 3T3 cell transformation assay. The enhancing effect of TGF beta was dose-related in the dose range tested (0.03-1 ng/ml) and was not reversible. Some of the foci induced by combined MCA-TGF beta-EGF treatment were cloned, and eight out of nine clones tested produced tumors in nude mice. TGF beta (1 ng/ml) plus EGF (2 ng/ml) increased the saturation density to a similar extent as PDD (100 ng/ml) but did not affect the growth of BALB/c 3T3 cells. We observed no change in junctional intercellular communication, as measured by the dye transfer method, when cells were treated with TGF beta during the two-stage BALB/c 3T3 cell transformation assay. Nevertheless, there was selective communication between transformed and surrounding nontransformed cells; MCA-TGF beta transformed cells intercommunicated among themselves but not with surrounding nontransformed cells. Our results indicate that TGF beta has potent tumor-promoting activity in vitro, but that this activity is not mediated by a complete blockage of intercellular communication, as is suggested for phorbol ester tumor promoters.