t-Darpp activates IGF-1R signaling to regulate glucose metabolism in trastuzumab-resistant breast cancer cells.

Purpose: Increased glycolysis and glucose dependence is a hallmark of malignancy that enables tumors to maximize cell proliferation. In HER2(+) cancers, an increase in glycolytic capacity is associated with trastuzumab resistance. IGF-1R activation and t-Darpp overexpression both confer trastuzumab resistance in breast cancer. We therefore investigated a role for IGF-1R and t-Darpp in regulating glycolytic capacity in HER2(+) breast cancers. Experimental Design: We examined the relationship between t-Darpp and IGF-1R expression in breast tumors and their respective relationships with patient survival. To assess t-Darpp's metabolic effects, we used the Seahorse flux analyzer to measure glucose metabolism in trastuzumab-resistant SK-BR-3 cells (SK.Her(R)) that have high endogenous t-Darpp levels and SK.tDrp cells that stably overexpress exogenous t-Darpp. To investigate t-Darpp's mechanism of action, we evaluated t-Darpp: IGF-1R complexes by coimmunoprecipitation and proximity ligation assays. We used path-way-specific inhibitors to study the dependence of t-Darpp effects on IGF-1R signaling. We used siRNA knockdown to determine whether glucose reliance in SK. Her(R) cells was mediated by t-Darpp. Results: In breast tumors, PPP1R1B mRNA levels were inversely correlated with IGF-1R mRNA levels and directly associated with shorter overall survival. t-Darpp overexpression was sufficient to increase glucose metabolism in SK. tDrp cells and essential for the glycolytic phenotype of SK. Her(R) cells. Recombinant t-Darpp stimulated glucose uptake, glycolysis, and IGF-1R-Akt signaling in SK-BR-3 cells. Finally, t-Darpp stimulated IGF-1R heterodimerization with ErbB receptors and required IGF-1R signaling to confer its metabolic effects. Conclusions: t-Darpp activates IGF-1R signaling through heterodimerization with EGFR and HER2 to stimulate glycolysis and confer trastuzumab resistance.