title: Both t-Darpp and DARPP-32 can cause resistance to trastuzumab in breast cancer cells and are frequently expressed in primary breast cancers.
abstract: The clinical use of trastuzumab (Herceptin), a humanized antibody against the HER2 growth factor receptor, has improved survival in patients with breast tumors with ERBB2 amplification and/or over-expression. However, most patients with advanced ERBB2 amplified breast cancers whose tumors initially respond to trastuzumab develop resistance to the drug, leading to tumor progression. To identify factors responsible for acquired resistance to trastuzumab, gene expression profiling was performed on subclones of an ERBB2 amplified breast cancer cell line, BT474, which had acquired resistance to trastuzumab. The most overexpressed gene in these subclones was PPP1R1B, encoding the DARPP-32 phosphatase inhibitor. Western analysis revealed that only the truncated isoform of the DARPP-32 protein, t-Darpp, was overexpressed in the trastuzumab resistant cells. Using gene silencing experiments, we confirmed that t-Darpp over-expression was required for trastuzumab resistance in these cells. Furthermore, transfecting t-Darpp in parental BT-474 cells conferred resistance to trastuzumab, suggesting that t-Darpp expression was sufficient for trastuzumab resistance. We also found that t-Darpp over-expression was associated with Akt activation and that the T75 residue in t-Darpp was required for both Akt activation and trastuzumab resistance. Finally, we found that full-length DARPP-32 and t-Darpp are expressed in a majority of primary breast tumors. Over-expression of full-length DARPP-32 can also confer resistance to trastuzumab and, moreover, is associated with a poor prognostic value in breast cancers. Thus, t-Darpp and DARPP-32 expression are novel prognostic and predictive biomarkers in breast cancer.
authors.affiliation: Department of Oncology and Surgery, Segal Cancer Center, Sir Mortimer B. Davis Jewish General Hospital, 3755 Cote Ste Catherine, Montreal, QC, H3T 1E2, Canada.
authors.forename: Sophie
authors.initials: S
authors.lastname: Hamel
authors.forename: Amélie
authors.initials: A
authors.lastname: Bouchard
authors.forename: Cristiano
authors.initials: C
authors.lastname: Ferrario
authors.forename: Saima
authors.initials: S
authors.lastname: Hassan
authors.forename: Adriana
authors.initials: A
authors.lastname: Aguilar-Mahecha
authors.forename: Marguerite
authors.initials: M
authors.lastname: Buchanan
authors.forename: Louise
authors.initials: L
authors.lastname: Quenneville
authors.forename: Wilson
authors.initials: W
authors.lastname: Miller
authors.forename: Mark
authors.initials: M
authors.lastname: Basik
mesh_terms: D000911:Antibodies, Monoclonal / Q000627:therapeutic use*; D061067:Antibodies, Monoclonal, Humanized; D000970:Antineoplastic Agents / Q000627:therapeutic use*; D014408:Biomarkers, Tumor / Q000032:analysis / Q000235:genetics; D015153:Blotting, Western; D001943:Breast Neoplasms / Q000188:drug therapy / Q000235:genetics*; D055028:Comparative Genomic Hybridization; D051937:Dopamine and cAMP-Regulated Phosphoprotein 32 / Q000096:biosynthesis / Q000235:genetics*; D019008:Drug Resistance, Neoplasm / Q000235:genetics*; D004789:Enzyme Activation / Q000235:genetics; D005260:Female; D020869:Gene Expression Profiling; D018734:Genes, erbB-2 / Q000235:genetics; D006801:Humans; D007150:Immunohistochemistry; D011379:Prognosis; D020033:Protein Isoforms / Q000096:biosynthesis / Q000235:genetics; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D014162:Transfection; D000068878:Trastuzumab
chemical_list: D000911:Antibodies, Monoclonal; D061067:Antibodies, Monoclonal, Humanized; D000970:Antineoplastic Agents; D014408:Biomarkers, Tumor; D051937:Dopamine and cAMP-Regulated Phosphoprotein 32; D020033:Protein Isoforms; D051057:Proto-Oncogene Proteins c-akt; D000068878:Trastuzumab