Our data support a role for Pak1, particular Pak1 localized to the nucleus, in ERalpha signaling and in tamoxifen resistance.

Here we define a function of metastasis-associated protein 1 (MTA1), a presumed corepressor of estrogen receptor α (ERα), as a transcriptional activator of Breast Cancer Amplified Sequence 3 (BCAS3), a gene amplified and overexpressed in breast cancers. We identified BCAS3 as a MTA1 chromatin target in a functional genomic screen. MTA1 stimulation of BCAS3 transcription required ERα and involved a functional ERE half-site in BCAS3 . Furthermore, we discovered that MTA1 is acetylated on lysine 626, and that this acetylation is necessary for a productive transcriptional recruitment of RNA polymerase II complex to the BCAS3 enhancer sequence. BCAS3 expression was elevated in mammary tumors from MTA1 transgenic mice and 60% of the human breast tumors, and correlated with the coexpression of MTA1 as well as with tumor grade and proliferation of primary breast tumor samples. These findings reveal a previously unrecognized function of MTA1 in stimulating BCAS3 expression and suggest an important role for MTA1-BCAS3 pathway in promoting cancerous phenotypes in breast tumor cells.

Phosphorylation of oestrogen receptor α at serine 305 (ERαS305-P) induces tamoxifen resistance in experimental studies, but does not influence response to other endocrine agents, such as fulvestrant. We evaluated ERαS305-P using immunohistochemistry in 377 breast carcinomas from premenopausal participants of a randomized trial (n = 248) and patients with advanced disease (n = 129). Among the premenopausal patients, adjuvant tamoxifen improved recurrence-free survival (RFS) for ERαS305-P-negative tumours (multivariate HR = 0.53, 95% CI 0.32-0.86, p = 0.010), but not for ERαS305-P-positive tumours (multivariate HR = 1.01, 95% CI 0.33-3.05, p = 0.99) (interaction p = 0.131). Notably, ERαS305-P was not significantly associated with RFS in patients not treated with tamoxifen (multivariate HR = 0.64, 95% CI 0.30-1.37, p = 0.248), indicating that ERαS305-P is a marker for treatment outcome rather than tumour progression. Given the direct experimental link between ERαS305-P and tamoxifen resistance and these first clinical data suggesting that premenopausal patients with ERαS305-P-positive breast cancer are resistant to adjuvant tamoxifen, further research is encouraged to study whether alternative endocrine treatment should be considered for this subgroup.

Purpose: Tamoxifen is one of many standard therapeutic options currently available for estrogen receptor-α-positive breast cancer patients. Emerging data have suggested that levels of estrogen receptor coregulatory proteins play a significant role in acquiring resistance to antiestrogen action. It has been suggested that high levels of estrogen receptor coactivators and its mislocalization may enhance the estrogen agonist activity of tamoxifen and contribute to tamoxifen resistance. Experimental Design: In an effort to understand the impact of nongenomic signaling and its contribution to hormone resistance in a whole-animal setting, we generated a transgenic mouse expressing a cytoplasmic version of proline-, glutamic acid-, and leucine-rich protein-1 (PELP1) mutant defective in its nuclear translocation (PELP1-cyto) and implanted these mice with tamoxifen pellets to assess its responsiveness. Results: We show that mammary glands from these mice developed widespread hyperplasia with increased cell proliferation and enhanced activation of mitogen-activated protein kinase and AKT as early as 12 weeks of age. Treatment with tamoxifen did not inhibit this hyperplasia; instead, such treatment exaggerated hyperplasia with an enhanced degree of alteration, indicative of hypersensitivity to tamoxifen. Analysis of molecular markers in the transgenic mammary glands from the tamoxifen-treated transgenic mice showed higher levels of proliferation markers proliferating cell nuclear antigen and activated mitogen-activated protein kinase than in untreated PELP1-cyto cell-derived mice. We also found that nude mice with MCF-7/PELP1-cyto cell-derived tumor xenografts did not respond to tamoxifen. Using immunohistochemical analysis, we found that 43% of human breast tumor samples had high levels of cytoplasmic PELP1, which shows a positive correlation between tumor grade and proliferation. Patients whose tumors had high levels of cytoplasmic PELP1 exhibited a tendency to respond poorly to tamoxifen compared with patients whose tumors had low levels of cytoplasmic PELP1. Conclusions: These findings suggest that PELP1 localization could be used as a determinant of hormone sensitivity or vulnerability. The establishment of the PELP1-cyto transgenic mouse model is expected to facilitate the development of preclinical approaches for effective intervention of breast tumors using cytoplasmic coregulators and active nongenomic signaling.The steroid hormone 17-β estradiol is known to exert its cellular effects in tissues by binding to its major target, the estrogen receptor-α. Estrogen receptor, a ligand-dependent transcription factor, has been implicated in the progression of breast cancer, as evidenced by the fact that almost 70% of breast tumors are estrogen receptor-positive at the time of early diagnosis (1, 2). Currently, tamoxifen represent one of many available standard treatments for estrogen receptor-positive breast cancer patients (3), although most patients who respond to tamoxifen eventually acquire tamoxifen resistan...

MethodsUsing immunohistochemistry, a score including PAK1 and co-expression of PKA and ER S305-P (PKA/ ER S305-P) was developed on a training set consisting of 103 patients treated with tamoxifen for metastatic disease, and validated on 231 patients randomized between adjuvant tamoxifen or no treatment. ResultsIn the training set, PAK1 levels were associated with tumor progression after tamoxifen (HR 1.57, 95% CI 0.99-2.48), as was co-expression of PKA and ER S305-P (HR 2.00, 95% CI 1.14-3.52). In the validation set, a significant tamoxifen benefit was found among the 73% patients negative for PAK1 and PKA/ER S305-P (HR 0.54, 95% CI 0.34-0.87), while others (27%) were likely to have no benefit from tamoxifen (HR 0.88, 95% 0.42-1.82). The test for interaction showed a significant difference in recurrence-free survival between groups defined by PAK1 and PKA/ER S305-P (p=0.037). Elevated PAK1 and PKA/ ER S305-P appeared to influence tamoxifen sensitivity. Conclusion 3Both PAK1 and PKA/ER S305-P levels were associated with sensitivity to tamoxifen in breast tumors and the combination of these variables should be considered in predicting tamoxifen benefit.