LXR/RXR pathway signaling associated with triple-negative breast cancer in African American women

Abstract

Background: Triple-negative breast cancer (TNBC) is more prevalent in African and African American (AA) women compared to European American (EA) women. African and AA women diagnosed with TNBC experience high frequencies of metastases and less favorable outcomes. Emerging evidence indicates that this disparity may in fact be the result of the uniquely aggressive biology of African and AA disease.

Purpose: To understand the reasons for TNBC in AA aggressive biology, we designed the present study to examine the proteomic profiles of TNBC and luminal A (LA) breast cancer within and across patients' racial demographic groups in order to identify proteins or molecular pathways altered in TNBC that offer some explanation for its aggressiveness and potential targets for treatment.

Materials and methods: Proteomic profiles of TNBC, LA tumors, and their adjacent normal tissues from AA and EA women were obtained using 2-dimensional gel electrophoresis and bioinformatics, and differentially expressed proteins were validated by Western blot and immunohistochemistry. Our data showed that a number of proteins have significantly altered in expression in LA tumors compared to TNBC, both within and across patients' racial demographic groups. The differentially overexpressed proteins in TNBC (compared to LA) of AA samples were distinct from those in TNBC (compared to LA) of EA women samples. Among the signaling pathways altered in AA TNBC compared to EA TNBC are innate immune signaling, calpain protease, and pyrimidine de novo synthesis pathways. Furthermore, liver LXR/RXR signaling pathway was altered between LA and TNBC in AA women and may be due to the deficiency of the CYP7B1 enzyme responsible for cholesterol degradation.

Conclusion: These findings suggest that TNBC in AA women enriched in signaling pathways that are different from TNBC in EA women. Our study draws a link between LXR/RXR expression, cholesterol, obesity, and the TNBC in AA women.

Keywords: African American; European American; breast; luminal; luminal breast cancer; triple-negative breast cancer.

Figure 1

Representative 2-DE gel images of protein profiles of invasive breast carcinoma. Notes: (A) TNBC vs LA in African American women, (B) African American vs European American women breast cancer samples, (C) TNBC in African American vs European American women, (D) TNBC vs LA in European American women, (E) TNBC vs LA regardless of race, and (F) LA in African American women vs LA in European American women. Proteins were separated by IEF as the first dimension, using 24 cm tube gels (pH 4–8), and linear gradient gel (11–19%) as the second dimension. The protein spots were cut from the gel, tryptically digested, and identified via MALDI–MS. Significantly expressed spots are posted in Table 2 along with their individual PDQuest spot number assignment and other data. Abbreviations: DE, 2-dimensional gel electrophoresis; LA, luminal A; MALDI-MS, matrix assisted laser desorption/ionisation - mass spectrometry; TNBC, triple-negative breast cancer.

Figure 2

Differentially expressed proteins in breast cancer tissues from AA compared to European women regardless of hormonal status. Note: Selected area of the spots showing intensity differences between AA breast and Caucasian women breast tissues was amplified and is indicated by circles. Abbreviations: AA, African American; EA, European American;

Figure 3

Differentially expressed proteins in (A) Luminal A breast cancer vs TNBC in AA women, (B) LA vs TNBC in European American women, and (C) TNBC in AA women vs TNBC in European American women. Abbreviations: AA, African American; EA, European American; ER, estrogen receptors; LA, luminal A; TNBC, triple-negative breast cancer.

Figure 4

IPA of canonical pathways of differentially altered protein expressed in breast carcinoma (A) LA vs TNBC in African American women; (B) LA vs TNBC in European American women; (C) TNBC in African American women vs TNBC in European American women. Abbreviations: IPA, ingenuity pathway analysis; LA, luminal A; TNBC, triple-negative breast cancer.

Figure 5

Western blot (A and E) and immunohistochemistry (C) validation of selected shown proteins. Bar graph of the Western blotting assay of all proteins (B) and CYP7B1 (F). Stain intensity of IHC for each protein tested is shown in (D). Each bar represents the relative value of the protein relative to β-actin. For each data point, samples were tested in triplicate; the graph represents the mean ± SD. Asterisks denote significance: *significant at 0.05 and **significant at <0.005. Abbreviations: LA, luminal A; TNBC, triple-negative breast cancer.