Regulatory Feedback Programs in the Apoptotic Decision-Making of Her2-Positive Breast Cancer Cells in Response to HER2 Inhibition

Abstract

HER2 overexpression drives tumorigenesis in a large subset of breast cancers and pharmacological inhibition of this receptor tyrosine kinase is currently used in the clinic for these patients. While all of the cancer cells within the tumor are dependent on HER2 signaling, treatment with HER2 inhibitors results in the establishment of two separate populations; cells that undergo apoptosis and cells that survive. The mechanisms underlying the decision-making processes are not well understood.In this study, HER2 positive breast cancer cell lines were utilized to model the signaling and gene expression changes in distinct populations of inhibitor-treated breast cancer cells. Previous studies have demonstrated that lapatinib induces a gene expression program that is comprised of both anti-apoptotic and pro-apoptotic genes. This provides the possibility that differential gene expression in the apoptotic and surviving populations may account for the commitment to cell fate. To directly test this, BT474 cells (a cell line derived from the tumor of a HER2-positive breast cancer patient) were treated with the small-molecule HER2 inhibitor, lapatinib (TykerbTM), for 24 hours, labeled with AnnexinV-FITC (apoptotic marker), and sorted using FACS. Differential gene expression analysis demonstrated that the surviving population had dramatic increases in the levels of several genes including irs2. IRS2 is an adaptor protein that may be able to bypass HER2 and signal to the major growth pathways. Ongoing experiments are further characterizing the temporal profile of this induction in the two populations. Experiments using siRNA to silence FOXO3a have demonstrated that the induction of irs2 is mediated by this Forkhead-family transcription factor. These results provide the basis for a model where HER2 inhibition differentially modulates gene expression in a cell population, thereby driving distinct cell fates.