S100β as a serum marker in endocrine resistant breast cancer

Background: Endocrine therapy is standard treatment for estrogen receptor (ER)-positive breast cancer. However, its efficacy is limited by intrinsic and acquired resistance. Here the potential of S100 beta as a biomarker and inhibition of its signaling network as a therapeutic strategy in endocrine treated patients was investigated. Methods: The expression of S100 beta in tissue and serum was assessed by immunohistochemistry and an enzyme-linked immunosorbent assay, respectively. The S100 beta signaling network was investigated in cell line models of endocrine resistance by western blot, PCR, immunoprecipitation, and chromatin-immunoprecipitation. Endocrine resistant xenografts and tumor explants from patients with resistant tumors were treated with endocrine therapy in the presence and absence of the p-Src kinase inhibitor, dasatinib. Results: Tissue and serum levels of S100 beta were found to predict poor disease-free survival in endocrine-treated patients (n = 509, HR 2.32, 95% CI is 1.58-3.40, p < 0.0001 and n = 187, HR 4.009, 95% CI is 1.66-9.68, p = 0.002, respectively). Moreover, elevated levels of serum S100 beta detected during routine surveillance over the patient treatment period significantly associated with subsequent clinically confirmed disease recurrence (p = 0.019). In vivo studies demonstrated that endocrine treatment induced transcriptional regulation of S100 beta which was successfully disrupted with tyrosine kinase inhibition. In endocrine resistant xenografts and tumor explants from patients with endocrine resistant breast cancer, combined endocrine and dasatinib treatment reduced tumor proliferation and down-regulated S100 beta protein expression in comparison to endocrine treatment alone. Conclusions: S100 beta has potential as a new surveillance tool for patients with ER-positive breast cancer to monitor ongoing response to endocrine therapy. Moreover, endocrine resistant breast cancer patients with elevated S100 beta may benefit from combined endocrine and tyrosine-kinase inhibitor treatment.