Amphiregulin (AR), a new member of the EGF family of ligand, is a glycoprotein containing a 78 or 84 amino acid core polypeptide that was originally purified from the conditioned medium of the breast carcinoma cell line MCF-7 after treatment with phorbol 12-myristate 13-acetate. The aim of the present study was to determine whether, like EGF, TGF alpha, heparin binding EGF-related growth factor (HB-EGF) and betacellulin (ETC), the recombinant 78 amino acid form of mature human AR transmits its biological effects following binding to the EGF receptor (EGFR). We show that unlike EGF, TGF alpha, HB-EGF and BTC, the mature AR is not effective in blocking the binding of I-125-EGF or the iodinated anti-EGFR antibodies (mAbs) I-125-ICR62 and I-125-ICR80 to the external domain of the EGF receptor on EJ cells. Again, in contrast to other EGF ligands, AR is not effective in enhancing the binding of another anti-EGFR mAb ICR9 to the EGFR on EJ cells. Like EGF, TGF alpha and HB-EGF, AR could inhibit the growth in culture of EGFR overexpressing tumour cell lines, namely HN5, HSC-1 and MDA-MB468 cells, and again compared to other ligands AR was moderately effective at low concentration. Despite these differences, we show that like EGF, AR could induce the tyrosine phosphorylation of the 170 kDa EGF receptor on HN5 cells and that this effect could be blocked in the presence of anti-EGFR mAbs ICR62 and ICR80. Moreover, like EGF, the AR-induced growth inhibition of MDA-MB468 cells could also be reversed in the presence of anti-EGFR mAbs ICR62 and ICR80. On the basis of our results we conclude that, unlike the EGF, TGF alpha, HB-EGF and BTC, the AR-induced activation of the EGFR may involve another receptor.

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