Activated c-Src (Y215) kinase expression predicts for early relapse on tamoxifen in human breast cancer.

Abstract

Abstract Abstract #3025 Recent studies have demonstrated activation of the non-receptor tyrosine kinase c-Src in tamoxifen resistant breast cancer cell lines, which when treated with Src inhibitors demonstrate a reduction in motility and invasiveness. However, there are little published translational studies confirming a role for c-Src in tamoxifen resistance in vivo. We aim to determine if activated c-Src expression correlates clinically with increased recurrence on tamoxifen.
 Methods
 Tissue microarrays were constructed from a patient cohort of 402 Tamoxifen treated ER positive patients. Immunohistochemistry was performed using commercially available antibodies to Total Src as well as to activated (phosphorylated) c-Src at 2 sites (Y416 and Y215) (pSrc416 and pSrc215 respectively). Expression was assessed by 2 independent scorers (weighted histoscore method). HER1-3 status and expression of membranous phosphorylated ER (sites serine 118 and 167) had previously been determined. All statistical calculations were performed using SPSS 15, including Kaplan-Meier life table analysis with log rank testing of differences in breast cancer related relapse whilst on tamoxifen.
 Results
 Membranous pSrc215 was observed in 20.3% of the cases but was more frequently observed in the cytoplasm (85.9%) and nucleus (90.5%). Overexpression of cytoplasmic pSrc215 was associated with an earlier time to recurrence (p = 0.037).
 
 Cox regression analysis confirmed this to be independent of grade, nodal and HER1-3 status (p=0.028, HR 3.17 (1.14–8.85)). Cytoplasmic pSrc215 was also strongly correlated with membranous phospho-ER, EGFR and HER3 status (Mann Whitney; p<0.001, p=0.026, p<0.001 respectively). Membranous and nuclear pSrc215 staining were not associated with time to relapse.
 Over expression of Total Src and pSrc416 did not predict for relapse.
 Discussion
 Our findings provide support to in vitro studies linking c-Src with tamoxifen resistance. The phosphorylation site pSrc215 is a putative site of activation by HER2 and PGFR resulting in up to 50 fold activation of Src in vitro. The correlations demonstrated with HER status and membranous ER lead us to speculate that this interplay between non genomic ER and Src activation may provide a mechanism for the lack of response to tamoxifen in these patients.
 Further studies are required to determine if activated pSrc215 represents activated c-Src alone or may also represent activation of other Src family members as the Y416 and Y215 sequences are highly conserved amongst the Src kinases with homologous activation sites. If confirmed, future use of novel combination therapies with Src inhibitors may have an important role in combating tamoxifen resistance. Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 3025.