Potentiation of the malignant phenotype of the undifferentiated ARO thyroid cell line by insertion of thebcl-2 gene

Abstract

We have reported that bcl‐2 is expressed in normal human thyroid epithelium and that its expression is down‐regulated in undifferentiated thyroid tumors. Production of IL‐6 was concomitantly down‐regulated in these forms. Based on these observations, we analyzed whether insertion of bcl‐2 would reverse the highly malignant phenotype of a thyroid cell line (ARO) derived from an undifferentiated carcinoma. This cell line fails to produce Bcl‐2 and IL‐6. By infection with a bcl‐2 retroviral vector, ARO cells expressing bcl‐2 (ARObcl‐2) were obtained. Compared with parental cells, expression of bcl‐2 was associated with enhancement of growth potential (DNA synthesis, in vitro proliferation rate, anchorage‐independent growth in semi‐solid media). Chemotaxis and invasive potential in Boyden chambers were also increased. bcl‐2‐expressing cells showed a reduced response to apoptotic stimuli (low‐serum conditions or anti‐neoplastic drugs). Large branched colonies were formed in Matrigel from ARObcl‐2 cells but not from parental cells. Finally, ARObcl‐2 cells showed a decreased latency of tumor appearance when injected into immunodeficient mice. Potentiation of the malignant phenotype of ARO cells by bcl‐2 was not ascribed to altered expression of (i) cytokine/growth factors (IL‐4, IL‐6, IL‐8, IL‐10, IL‐12, TGF‐α, TGF‐β), (ii) thyroid‐specific transcripts (TG, TPO, TSH‐R, PIGF, PAX‐8) or (iii) genes influencing tumor aggressiveness [VEGF, HMGI (Y), HMGI‐C]. Our data indicate that bcl‐2 potentiates the malignant phenotype of ARO cells not only by limiting the response to apoptotic stimuli but also by enhancing proliferation and tumor aggressiveness. Int. J. Cancer81:956–962, 1999.