Darpp-32 and its truncated variant t-Darpp have antagonistic effects on breast cancer cell growth and herceptin resistance.
BACKGROUND
Herceptin (trastuzumab) is a humanized monoclonal antibody that is approved for the treatment of metastatic breast cancer patients whose tumors overexpress Her2 (erbB2/neu). Up to 70% of Her2-positive breast cancers demonstrate a response to Herceptin-based therapies, but resistance almost inevitably arises within a year of the initial response. To help understand the mechanism of Herceptin resistance, we isolated clonal variants of Her2-positive BT474 human breast cancer cells (BT/Her(R)) that are highly resistant to Herceptin. These cell lines exhibit sustained PI3K/Akt signaling as an essential component of Herceptin-resistant proliferation. Several genes in the protein kinase A (PKA) signaling network have altered expression in BT/Her(R) cells, including PPP1R1B, which encodes a 32 kDa protein known as Darpp-32 and its amino-terminal truncated variant, t-Darpp. The purpose of the current work was to determine the role of Darpp-32 and t-Darpp in Herceptin resistance.


RESULTS
We determined expression of Darpp-32 and t-Darpp in BT/Her(R) cells selected for resistance to Herceptin. Subsequently, cDNAs encoding the two isoforms of Darpp-32 were transfected, separately and together, into Her2-positive SK-Br-3 breast cancer cells. Transfected cells were tested for resistance to Herceptin and Herceptin-mediated dephosphorylation of Akt. DNA binding activity by the cAMP response element binding protein (CREB) was also measured. We found that BT/Her(R) cells overexpressed t-Darpp but not Darpp-32. Moreover, t-Darpp overexpression in SK-Br-3 cells was sufficient for conferring resistance to Herceptin and Herceptin-mediated dephosphorylation of Akt. Darpp-32 co-expression reversed t-Darpp's effects on Herceptin resistance and Akt phosphorylation. t-Darpp overexpression led to increased CREB binding activity, which was also reversible by Darpp-32.


CONCLUSIONS
t-Darpp and Darpp-32 appear to have antagonistic effects on Herceptin resistance. We present a unified model by which these effects might be mediated via the PKA regulatory network.
Gu
Long
L
Division of Tumor Cell Biology, Beckman Research Institute of City of Hope, Duarte, CA, USA.
Waliany
Sarah
S
Kane
Susan E
SE
D000911:Antibodies, Monoclonal / Q000627:therapeutic use*; D061067:Antibodies, Monoclonal, Humanized; D000970:Antineoplastic Agents / Q000627:therapeutic use*; D001943:Breast Neoplasms / Q000188:drug therapy / Q000473:pathology*; D045744:Cell Line, Tumor; D051937:Dopamine and cAMP-Regulated Phosphoprotein 32 / Q000235:genetics / Q000502:physiology*; D019008:Drug Resistance, Neoplasm; D024202:Electrophoretic Mobility Shift Assay; D005260:Female; D006801:Humans; D020033:Protein Isoforms / Q000235:genetics / Q000502:physiology*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D000068878:Trastuzumab
D000911:Antibodies, Monoclonal; D061067:Antibodies, Monoclonal, Humanized; D000970:Antineoplastic Agents; D051937:Dopamine and cAMP-Regulated Phosphoprotein 32; C497928:PPP1R1B protein, human; D020033:Protein Isoforms; D000068878:Trastuzumab