title: t-Darpp Promotes Enhanced EGFR Activation and New Drug Synergies in Her2-Positive Breast Cancer Cells.
abstract: Trastuzumab has led to improved survival rates of HER2+ breast cancer patients. However, acquired resistance remains a problem in the majority of cases. t-Darpp is over-expressed in trastuzumab-resistant cell lines and its over-expression is sufficient for conferring the resistance phenotype. Although its mechanism of action is unknown, t-Darpp has been shown to increase cellular proliferation and inhibit apoptosis. We have reported that trastuzumab-resistant BT.HerR cells that over-express endogenous t-Darpp are sensitized to EGFR inhibition in the presence (but not the absence) of trastuzumab. The purpose of the current study was to determine if t-Darpp might modulate sensitivity to EGFR inhibitors in trastuzumab-resistant cells. Using EGFR tyrosine kinase inhibitors AG1478, gefitinib and erlotinib, we found that trastuzumab-resistant SK.HerR cells were sensitized to EGFR inhibition, compared to SK-Br-3 controls, even in the absence of trastuzumab. t-Darpp knock-down in SK.HerR cells reversed their sensitivity to EGFR inhibition. Increased EGFR sensitivity was also noted in SK.tDp cells that stably over-express t-Darpp. High levels of synergy between trastuzumab and the EGFR inhibitors were observed in all cell lines with high t-Darpp expression. These cells also demonstrated more robust activation of EGFR signaling and showed greater EGFR stability than parental cells. The T75A phosphorylation mutant of t-Darpp did not confer sensitivity to EGFR inhibition nor activation of EGFR signaling. The over-expression of t-Darpp might facilitate enhanced EGFR signaling as part of the trastuzumab resistance phenotype. This study suggests that the presence of t-Darpp in HER2+ cancers might predict the enhanced response to dual HER2/EGFR targeting.
authors.affiliation: Department of Cancer Biology, Beckman Research Institute at City of Hope, Duarte, California, United States of America.
authors.forename: Erin C
authors.initials: EC
authors.lastname: Denny
authors.affiliation: Department of Cancer Biology, Beckman Research Institute at City of Hope, Duarte, California, United States of America.
authors.forename: Susan E
authors.initials: SE
authors.lastname: Kane
mesh_terms: D000970:Antineoplastic Agents / Q000494:pharmacology*; D001943:Breast Neoplasms / Q000378:metabolism*; D045744:Cell Line, Tumor; D051937:Dopamine and cAMP-Regulated Phosphoprotein 32 / Q000235:genetics / Q000378:metabolism*; D019008:Drug Resistance, Neoplasm / Q000235:genetics; D066246:ErbB Receptors / Q000378:metabolism*; D005260:Female; D000077156:Gefitinib; D006801:Humans; D011799:Quinazolines / Q000494:pharmacology; D018719:Erb-b2 Receptor Tyrosine Kinases / Q000235:genetics / Q000378:metabolism*; D000068878:Trastuzumab / Q000494:pharmacology; D020032:Tyrphostins / Q000494:pharmacology
chemical_list: D000970:Antineoplastic Agents; D051937:Dopamine and cAMP-Regulated Phosphoprotein 32; D066246:ErbB Receptors; D000077156:Gefitinib; D011799:Quinazolines; D018719:Erb-b2 Receptor Tyrosine Kinases; D000068878:Trastuzumab; D020032:Tyrphostins; C101044:RTKI cpd; C508053:ERBB2 protein, human