Secretase-dependent tyrosine phosphorylation of Mdm2 by the ErbB-4 intracellular domain fragment.

Heregulin activation of the endogenous receptor tyrosine kinase ErbB-4 in ZR-75-1 breast cancer cells provokes tyrosine phosphorylation of Hdm2 in a manner that is sensitive to inhibition of alpha- or gamma-secretase activity, indicating that liberation of the tyrosine kinase intracellular domain (ICD) fragment is required. Similar results are obtained when Erbb-4 is exogenously expressed in 32D cells, which do not otherwise express any ErbB family members. Expression of the ErbB-4 ICD fragment leads to its constitutive association with Mdm2 and tyrosine phosphorylation of Mdm2, a protein that is predominantly localized in the nucleus and that regulates p53 levels. When the ErbB-4 ICD fragment was expressed in H1299 cells, it promoted Hdm2 ubiquitination and increased the levels of p53 and p21, a transcriptional target of p53. In addition, expression of the ICD fragment increased p53 activity toward the p21 promoter in a luciferase reporter assay.