Raptor localization predicts prognosis and tamoxifen response in estrogen receptor-positive breast cancer

Abstract

Purpose: Deregulated PI3K/mTOR signals can promote the growth of breast cancer and contribute to endocrine treatment resistance. This report aims to investigate raptor and its intracellular localization to further understand its role in ER-positive breast cancer.

Methods: Raptor protein expression was evaluated by immunohistochemistry in 756 primary breast tumors from postmenopausal patients randomized to tamoxifen or no tamoxifen. In vitro, the MCF7 breast cancer cell line and tamoxifen-resistant MCF7 cells were studied to track the raptor signaling changes upon resistance, and raptor localization in ERα-positive cell lines was compared with that in ERα-negative cell lines.

Results: Raptor protein expression in the nucleus was high in ER/PgR-positive and HER2-negative tumors with low grade, features associated with the luminal A subtype. Presence of raptor in the nucleus was connected with ERα signaling, here shown by a coupled increase of ERα phosphorylation at S167 and S305 with accumulation of nuclear raptor. In addition, the expression of ERα-activated gene products correlated with nuclear raptor. Similarly, in vitro we observed raptor in the nucleus of ERα-positive, but not of ER-negative cells. Interestingly, raptor localized to the nucleus could still be seen in tamoxifen-resistant MCF7 cells. The clinical benefit from tamoxifen was inversely associated with an increase of nuclear raptor. High cytoplasmic raptor expression indicated worse prognosis on long-term follow-up.

Conclusion: We present a connection between raptor localization to the nucleus and ERα-positive breast cancer, suggesting raptor as a player in stimulating the growth of the luminal A subtype and a possible target along with endocrine treatment.

Keywords: Endocrine resistance; Estrogen receptor (ER) α; Luminal A; Tamoxifen; mTOR.

Fig. 1

Raptor nuclear histological score (0, 2–6) correlated strongly with ER phosphorylations pER-S167 (a) and pER-S305 (b). Mean and 95% CI (confidence interval) plots of p-ER index grouped by raptor nuclear score. The p values refer to the Kruskal–Wallis H test

Fig. 2

Raptor expression in ERα-positive, ERα-negative, and tamoxifen-resistant breast cancer cells. Raptor expression was higher in ERα-positive than in ERα-negative cell lines by Western blot analysis (a). Comparing ERα-positive with ERα-negative cells shows raptor in the ERα-positive but not in the ERα-negative nuclei as observed by in situ immunofluorescence (b). Raptor presence increased in the nucleus (N) in MCF7 tamoxifen-resistant cells (Tam^R) compared with MCF7 parental cells (Control), and changes in mTORC1 and Akt signaling were observed (c)

Fig. 3

Tamoxifen response is inversely associated with increased nuclear raptor score. Tamoxifen is beneficial for patients with low score of nuclear raptor in luminal A-like graded tumors (a). The tamoxifen response is sequentially reduced with increased score of nuclear raptor, showing a trend toward benefit in the group of medium score (b), and no significant benefit in the group of high score (c). The interaction between tamoxifen response and raptor showed a significant decrease in benefit with higher score (p = 0.036). Distant metastasis-free survival (DMFS)

Fig. 4

Raptor as a prognostic marker shows diverse outcomes depending on localization. a High intensity of raptor in the cytoplasm indicated a significantly worse prognosis. This was not evident during the first 5 years after randomization. For the group of patients that had no distant recurrences within the first 5 years, the raptor intensity in the cytoplasm had significant impact on the distant metastasis rate. All patients were included in the analysis. b High intensity of raptor in the cytoplasm tended to be significantly worse for systemically untreated patients. No difference was observed for the time periods before and after 5 years from randomization. c Low, medium, and high scores of raptor in the nucleus of all patients, p = 0.031. d Low, medium, and high scores of raptor in the nucleus of systemically untreated patients, p = 0.13