t-Darpp overexpression in HER2-positive breast cancer confers a survival advantage in lapatinib

Drug resistance is a major barrier to successful cancer treatment. For patients with HER2-positive breast cancer who initially respond to therapy, the majority develop resistance within one year of treatment. Patient outcomes could improve significantly if we can find and exploit common mechanisms of acquired resistance to different targeted therapies. Overexpression of t-Darpp, a truncated form of the dual kinase/phosphatase inhibitor Darpp-32, has been linked to acquired resistance to trastuzumab, a front-line therapy for HER2-positive breast cancer. Darpp-32 reverses t-Darpp's effect on trastuzumab resistance. In this study, we examined whether t-Darpp could be involved in resistance to lapatinib, another HER2-targeted therapeutic. Lapatinib-resistant SKBR3 cells (SK/LapR) showed a marked change in the Darpp-32:t-Darpp ratio toward a predominance of t-Darpp. Overexpression of t-Darpp alone was not sufficient to confer lapatinib resistance, but cells that overexpress t-Darpp partially mimicked the molecular resistance phenotype observed in SK/LapR cells exposed to lapatinib. SK/LapR cells failed to down-regulate Survivin and failed to induce BIM accumulation in response to lapatinib; cells overexpressing t-Darpp exhibited only the failed BIM accumulation. t-Darpp knock-down reversed this phenotype. Using a fluorescence-based co-culture system, we found that cells overexpressing t-Darpp formed colonies in lapatinib within 3-4 weeks, whereas parental cells in the same co-culture did not. Overall, t-Darpp appears to mediate a survival advantage in lapatinib, possibly linked to failed lapatinib-induced BIM accumulation. t-Darpp might also be relevant to acquired resistance to other cancer drugs that rely on BIM accumulation to induce apoptosis.