Heregulin Degradation in the Absence of Rapid Receptor-Mediated Internalization

Abstract

Heregulin receptors are unable to mediate the rapid internalization of bound ligand as demonstrated in cells transfected with chimeric or wild-type ErbB-2, -3, or -4 receptors (Baulidaet al.,1996,J. Biol. Chem.271, 5251–5257; Pinkas-Kramanskiet al.,1996,EMBO J.15, 2452–2467). This observation is now extended to include mammary carcinoma cell lines (SK-BR-3 and MDA-543) which express endogenous ErbB-2 and ErbB-3 receptors. Also, the fate of receptor-bound heregulin is examined. While receptor-bound heregulin is not rapidly internalized, the ligand is subject to a slow process of inactivation and degradation, which requires heregulin incubation at 37°C with cells that express heregulin receptors. The degradation of heregulin is blocked to a significant extent by chloroquine, an inhibitor of endosome fusion with lysosomes, indicating that heregulin is slowly internalized and degraded. However, this process is not sufficiently rapid to produce ligand-dependent down-regulation of heregulin receptors.