HEREGULIN-DEPENDENT AUTOCRINE LOOP REGULATES GROWTH OF K-RAS BUT NOT ERBB-2 TRANSFORMED RAT-THYROID EPITHELIAL-CELLS

The ECF-like family of proteins, such as epidermal growth factor (ECF) , transforming growth factor alpha (TGF alpha), amphiregulin (AR), bet acellulin (BTC), cripto-1 (CR-1), and heregulin (HRC), plays an import ant role in the pathogenesis of several human carcinomas as autocrine growth factors. Differentiation and proliferation of rat thyroid cells in culture (FRTL-5 cells) are regulated by thyrotropin (TSH); withdra wal of TSH from culture medium produces growth arrest, whereas its add ition to quiescent cells stimulates cell entry into S phase. Instead, transformed thyroid cell lines as FRTL-5H2 cell line, overexpressing e rbB-2, Kimol cells, transformed by the wild-type K-ras and A6 clone, t ransformed by a temperature sensitive K-ras mutant, can grow without a ddition of TSH to the culture medium. In order to identify whether ECF -like growth factors and corresponding receptors (erbB-2, erbB-3, and erbB-4) could be involved in the autonomous growth of these transforme d rat thyroid epithelial cells, Northern blot for mRNA analysis and We stern blot for protein expression were performed. In contrast to norma l control FRTL-5 cells, both K-ras and erbB-2-transformed cells expres sed elevated levels of erbB-2 receptor. Moreover, both K-ras transform ed cells, Kimol and A6 cells, but no FRTL-5H2 cells, were found able t o express also high levels of erbB-4 receptor and HRG/NDF ligand. Trea tment of K-ras transformed thyroid cells with neutralizing antibody ag ainst HRG/NDF reduced by 50% cell proliferation. These data indicate t hat unlike the erbB-2 overexpressing FRTL-5 cells, in K-ras rat thyroi d epithelial cells, the growth factor heregulin signals through the he terodimer erbB-2/erbB-4 receptors in an autocrine fashion. (C) 1998 Wi ley-Liss, Inc.