Signalling-competent truncated forms of ErbB2 in breast cancer cells: differential regulation by protein kinase C and phosphatidylinositol 3-kinase

Alterations that affect the ectodomain of receptor tyrosine kinases are oft en associated with constitutive activation of the enzymic activity of the m utant cell-associated receptor. Since the ectodomain of the ErbB2 receptor tyrosine kinase has been detected as a soluble fragment in the culture supe rnatant of cells and serum from patients with advanced breast cancer, the p ossible presence of cell-associated truncated forms of ErbB2 in cancer cell s was investigated. Several cell-bound N-terminal truncated forms of ErbB2 were identified in breast cancer cells overexpressing this receptor. The pr esence of the truncated fragments was independent of lysosomal/proteasomal activity, indicating that classical receptor tyrosine kinase degradation sy stems were not involved in the N-terminal cleavages. The presence of these truncated forms of ErbB2 was up-regulated by protein kinase C and neureguli n; and down-regulated by phosphatidylinositol 3-kinase, and monoclonal anti bodies that target the ectodomain of ErbB2, indicating that N-terminal clea vages of ErbB2 were regulated by multiple mechanisms. The truncated fragmen ts were tyrosine-phosphorylated under resting conditions, and associated wi th the signalling intermediates Shc and Grb2. It is therefore likely that t hese truncated forms may be endowed with constitutive activity that allows them to permanently signal.