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.
METHODOLOGY AND 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.