Endocrine Abstracts

In breast cancer associations between p160 co-activator proteins and the development of resistance to endocrine treatment have been shown. We hypothesized that nuclear co-regulatory proteins may interact with non-steroid receptors. We investigated the effect of silencing the co-activator, SRC-1, on tumour cell growth in vitro. We also examined the MAPK activated transcription factors, Ets, as possible interaction proteins of the co-activator SRC-1 in human breast cancer. The effect of SRC-1 silencing on the Ets target genes was also investigated. SiRNA technology was used to inhibit estrogen induced cell growth of breast cancer cells in vitro. Protein-protein interactions between SRC-1 and Ets-2 were assessed using co-immunoprecipitation. It was found that Ets-2 interacted with SRC-1 under basal conditions and that the addition of growth factors further increased the level of interaction. Recruitment of SRC-1 to the Ets response element was demonstrated in primary breast tumour cell cultures and in the SKBR3 cell line using electromobilty shift assay. It was shown that growth factors induced interaction between Ets and their DNA response element and stimulated recruitment of co-activators to the transcription factor-DNA complex. Silencing of SRC-1 was found to down-regulate expression of the Ets target gene, c-myc.

Expression and co-expression of Ets and the co-regulatory protein SRC-1 was investigated using immunohistochemistry and immunofluorescence in a cohort of breast tumour patients (n=132). Ets-2 was found to be associated with reduced disease-free survival (p<0.0001), as was expression of SRC-1 (p<0.0001). Co-expression of Ets-2 and SRC-1 significantly reduced the period of disease-free survival (p<0.0001).

These data describing associations and interactions between non-steroid transcription factors and co-regulatory proteins may provide the basis for a new model of co-activator mediated endocrine resistance in breast cancer.