Development of the biomarker S100β in metastatic endocrine resistant breast cancer
Introduction: Many ER positive breast cancers develop resistance to endocrine therapy which is often concurrent with the generation of distant metastasis. Very little is known regarding the biological processes involved in the generation of metastasis in endocrine related breast cancer as no models are yet available demonstrating distant metastasis of ER positive cells originating from the primary tumour. The development of one such model would be of value in the generation of novel therapeutic strategies to treat the disease. Endocrine resistant cells utilise growth factor pathways to evade endocrine therapy. The tyrosine kinase src has been implicated in initiating growth factor signalling. In endocrine resistant cells, growth factor signalling contributes to the activity of SRC-1 which has previously been shown to regulate the transcription of the secreted calcium binding protein S100β via interaction with the transcription factor HOXC11. Targeting this endocrine driven signalling network to S100β using tyrosine kinase inhibitors, may produce a therapeutic response in endocrine resistant breast cancer by inhibiting the activity of SRC-1, which may be marked with a corresponding decrease in S100β.
Results: Recent advances in xenograft imaging allow for the detection of as little as 600 cells by stable transfection of luciferase reporter genes. The tamoxifen resistant cell line LY2 has been characterised as a highly motile estrogen responsive cell population. In this study a tamoxifen resistant xenograft model was established using the tamoxifen resistant cell line LY2 which was stably transfected with a luciferase construct to allow monitoring of disease progression. LY2 tumours treated with tamoxifen formed distant metastasis to the lung, liver and the bone. LY2 tumours in the absence of tamoxifen displayed metastatic signals when analysed ex vivo, however at a significantly lower quantity than that of the tamoxifen treated model. Tumour tissue and metastatic tissue of the lung and bone of the tamoxifen treated xenograft model expressed p-src tyrosine kinase which was initiated by endocrine treatment. The expression of p-src associated with the expression of both Her2 and SRC-1 in a patient population of 436. The proteins involved in regulating the biomarker S100β were elevated in endocrine resistant cell lines, and the serum levels of were S100β also elevated in the endocrine resistant xenograft model. Targeting this pathway using dasatinib significantly inhibited the progression of endocrine resistant disease when used in combination with endocrine therapy. This finding was reproduced using an ex vivo patient tissue explants model, where the combination of dasatinib and letrozole inhibited Ki67 staining while inhibiting the expression of S100β.
Conclusion: Patients expressing elevated levels of S100β in response to endocrine therapy indicate a poor response to treatment. Introducing TKI treatment in combination to endocrine therapy inhibits the agonistic affect tamoxifen is displaying and results in delays disease progression. A positive response to this therapeutic intervention results in a corresponding decrease in S100β expression. Data from this study aided in the acquisition of a patent to use S100β as a biomarker in breast cancer.