Restored expression of transforming growth factor β type II receptor in kras‐transformed thyroid cells, TGFβ‐resistant, reverts their malignant phenotype

Abstract

Transforming growth factor β1 (TGFβ1) inhibits the growth of normal rat epithelial thyroid cells (FRTL-5 strain) by counteracting thyrotropin (TSH)-stimulated DNA synthesis and by slowing the cells in the G1 phase of the cell cycle. Here, we have studied two clones of FRTL-5 thyroid cell line transformed by the wild type (wt) v-k-ras oncogene (K.M.A1, K.M.A2) and one clone (A6) transformed by a temperature-sensitive (ts) v-k-ras mutant. Anchorage-dependent as well as anchorage-independent growth of these k-ras-transformed cells was not inhibited by TGFβ1. TGFβ1 resistance appeared to be dependent by a functional p21 k-ras, because A6 cell growth was partially inhibited at the nonpermissive temperature (39°C). To determine the basis for TGFβ1 resistance in k-ras-transformed thyroid cells, we looked for possible defects in the expression of type I (TβR-I/ALK5) and type II TGFβ receptors (TβR-II). Lower levels of type II receptors were present in all of the k-ras-transformed clones, as revealed by both Northern blot and cross-linking experiments. A partial reversion of the malignant phenotype of the wt k-ras-transformed clone was obtained in two clones isolated after transfection of the malignant thyroid cells (K.M.A1) with a TβR-II expression vector. These two clones also showed restored levels of exogenous TβR-II mRNA and protein, and both clones showed a partially reacquired sensitivity to TGFβ1. Similarly, the reversion of the malignant phenotype of the A6 clone grown at the nonpermissive temperature was accompanied by a restored expression of the TβR-II receptors. These data indicate that active k-ras oncogene can induce TGFβ1 resistance in rat thyroid cells and suggest that one of the possible mechanisms of escape from TGFβ1 growth control in k-ras-induced thyroid carcinogenesis involves a reduced expression of TβR-II receptors. J. Cell. Physiol. 172:200–208, 1997. © 1997 Wiley-Liss, Inc.