The Autocrine Loop of Epidermal Growth Factor Receptor‐Epidermal Growth Factor/Transforming Growth Factor‐α in Malignant Rhabdoid Tumor Cell Lines: Heterogeneity of Autocrine Mechanism in TTC549

To investigate the effects of the autocrine loop of epidermal growth factor receptor (EGFR)‐epidermal growth factor (EGF)/transforming growth factor‐α (TGF‐α) on the proliferation and differentiation of malignant rhabdoid tumor (MRT), we used five MRT cell lines, TM87‐16, STM91‐01, TTC549, TTC642, and YAM‐RTK1. RT‐PCR analyses revealed expression of EGFR mRNA in all MRT cell lines. In contrast, the expression of either EGF or TGF‐α mRNA was detected in all MRT cell lines. Expression of EGF, TGF‐α, and EGFR as determined by immunocytochemical staining and in situ hybridization, correlated with the results of RT‐PCR. Upon differentiation‐induction with 12‐O‐tetradecanoylphorbol‐13‐acetate (TPA), in TTC549, showing an expression of TGF‐α but not EGF initially, de novo expression of EGF mRNA appeared abruptly on day 2 of TPA treatment. To confirm the EGFR‐EGF/TGF‐α autocrine loop, we used TGF‐α, EGF, and their antibodies in the cultures. Monoclonal antibody (mAb) to EGFR alone significantly inhibited the growth of cell line TTC549. However, mAb to EGF or TGF‐α could inhibit proliferation of this cell line only when administrated together. Our findings would suggest that growth of the TTC549 cell line is constitutionally regulated by TGF‐α/EGFR, but that inhibition of this autocrine mechanism results in transient activation of an autocrine loop involving EGF/EGFR. Our results may indicate the presence of two different autocrine loops of EGFR‐EGF and/or EGFR‐TGF‐α in MRT cell lines. The heterogeneity of autocrine mechanisms found in MRT cell lines would be consistent with the multiphenotypic diversity and aggressive characteristics of this enigmatic tumor.