Nuclear HER4 mediates acquired resistance to trastuzumab and is associated with poor outcome in HER2 positive breast cancer
Heregulin was shown to promote the proteolytic cleavage of its receptor, ErbB-4, in several cell lines. The growth factor also rapidly promoted the transient translocation of ErbB-4 to a detergent-insoluble fraction, in which the receptor was hyper-tyrosine-phosphorylated compared with the receptor present in the detergent-soluble pool. However, an 80-kDa proteolytic fragment of ErbB-4 was found in the detergent-soluble fraction, but not in the detergent-insoluble fraction. Although the heregulin-induced cleavage of ErbB-4 produced a fragment of ErbB-4 very similar to that induced by 12-O-tetradecanoylphorbol-13-acetate or pervanadate (each of which is blocked by metalloprotease inhibitors), the growth factor-induced cleavage was not sensitive to these inhibitors under the same conditions. The heregulin-induced cleavage of ErbB-4 could be blocked by conditions that prevent clathrin-coated pit formation, suggesting that heregulin-mediated ErbB-4 cleavage occurs subsequent to internalization. When reagents that prevent acidification of endosomes were employed, heregulin-induced ErbB-4 cleavage was sensitive to metalloprotease inhibitors. The results imply that during ligand-dependent receptor trafficking, activated ErbB-4 receptors are subject to proteolytic cleavage involving an intracellular metalloprotease.