Abstract P3-04-02: Differential turnover of estrogen receptor alpha in invasive lobular carcinoma

Abstract

Abstract Background: Invasive lobular carcinoma (ILC) accounts for 10-15% of invasive breast cancers diagnosed annually. There is increasing evidence that endocrine treatment response might differ between Invasive Ductal Carcinoma (IDC) and ILC, and that patients with ILC have worse long-term survival when other prognostic factors are taken into account. One such factor is ER status, which is more likely to be positive in ILC (90-95%) compared to IDC (60-70%). There are few studies that have directly compared mRNA and protein levels between ER+ ILC and ER+ IDC. Hypothesis: Differences in ER protein steady state levels, and/or turn-over rates contribute to differences in endocrine treatment response between patients with ILC vs IDC. Methods: We utilized publicly available TCGA data to compare ER mRNA and protein levels between ER+ ILC (n=184) and IDC (n=534). Correlation analysis with Spearman's rank order coefficient (ρ) was used to study the relationship between mRNA and protein levels. METABRIC data were analyzed to compare ER mRNA levels between ER+ ILC (n=130) and IDC (n=1152). ER H-scores and mRNA levels were also analyzed from patients with ER+ ILC (n=180) and IDC (n=1183) seen at our local UPMC Magee Womens Hospital. Finally, ER mRNA and total protein levels, and RNA and protein turn-over rates were determined in 2 IDC and 2 ILC breast cancer cell lines, using qRT-PCR and immunoblots analysis. Results: Analysis of ESR1 gene expression in the TCGA database revealed significantly lower levels of ER mRNA (Mann-Whitney, p<.0005) in ER+ ILC compared to IDC, whereas ER protein levels were similar in the two histological subtypes. The correlation between ER mRNA and protein levels is weaker in ER+ ILC (ρ=0.60) compared to ER+ IDC (ρ=0.69) tumors, though not statistically significant. The weaker correlation between mRNA and protein expression in ILC is more clear when analyzing all 130 RNA and protein pairs with available RRPA data, (ILC median ρ=0.28; IDC median ρ=0.34, p<.0005). In the METABRIC dataset, ESR1 mRNA levels were also found to be lower in ER+ ILC tumor samples compared to IDCs (Mann-Whitney, p<0.005). In concordance with these observations, the study of patients seen at our local hospital showed similar ER IHC H-scores for ER+ ILCs (H-score = 244) and IDCs (H Score = 248), despite there being significantly lower ESR1 mRNA in ILC (p<0.005). Finally, our in vitro data showed that rate of estrogen-mediated turn-over of ER protein was significantly lower in the ILC cell lines compared to the IDC cell lines, which might explain the lack of lower ER protein levels despite lower ER mRNA levels. We are currently confirming these findings in additional cell lines, and deciphering the mechanisms through the study of ER ubiquitin-modification and proteasome machinery comparing ILC and IDC. Conclusion: We have provided functional and in silico data that collectively suggest altered ER protein turn-over in ILC compared to IDC. We are currently testing if and how this affects sensitivity of ILC cells to SERDs, and underlying mechanisms. Citation Format: Sreekumar S, Levine K, Sikora MJ, Boone D, Dabbs DJ, Lee AV, Jankowitz RC, Oesterreich S. Differential turnover of estrogen receptor alpha in invasive lobular carcinoma [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P3-04-02.