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.