Bromodomain inhibition shows antitumoral activity in mice and human luminal breast cancer

Abstract

BET bromodomain inhibitors, which have an antitumoral effect against various solid cancer tumor types, have not been studied in detail in luminal breast cancer, despite the prevalence of this subtype of mammary malignancy. Here we demonstrate that the BET bromodomain inhibitor JQ1 exerts growth-inhibitory activity in human luminal breast cancer cell lines associated with a depletion of the C-MYC oncogene, but does not alter the expression levels of the BRD4 bromodomain protein. Interestingly, expression microarray analyses indicate that, upon JQ1 administration, the antitumoral phenotype also involves downregulation of relevant breast cancer oncogenes such as the Breast Carcinoma-Amplified Sequence 1 (BCAS1) and the PDZ Domain-Containing 1 (PDZK1). We have also applied these in vitro findings in an in vivo model by studying a transgenic mouse model representing the luminal B subtype of breast cancer, the MMTV-PyMT, in which the mouse mammary tumor virus promoter is used to drive the expression of the polyoma virus middle T-antigen to the mammary gland. We have observed that the use of the BET bromodomain inhibitor for the treatment of established breast neoplasms developed in the MMTV-PyMT model shows antitumor potential. Most importantly, if JQ1 is given before the expected time of tumor detection in the MMTV-PyMT mice, it retards the onset of the disease and increases the survival of these animals. Thus, our findings indicate that the use of bromodomain inhibitors is of great potential in the treatment of luminal breast cancer and merits further investigation.

Keywords: C-MYC; JQ1; bromodomain inhibitor; luminal breast cancer; mice model.

Figure 1. JQ1 treatment of human luminal breast cancer cell lines reduces cell viability and downregulates C-MYC

(A) Effect of JQ1 on cell viability determined by the MTT assay in MCF7 and T47D cells. The corresponding half-maximal inhibitory concentration (IC50) values are shown for each cell line. (B) Downregulation of C-MYC upon JQ1 treatment (1 μM) at 6, 12, 24 and 48 h in breast cancer cell lines, as determined by Western blot (left) and qRT-PCR (right). BRD4 expression levels do not change. *P<0.05; ***P<0.001; ns: non-significant.

Figure 2. Expression microarray analyses identify PDZK1 and BCAS1 as breast cancer oncoproteins downregulated by JQ1 treatment in human luminal breast cancer cell lines

(A) Flowchart of the expression microarray experiment. (B) qRT-PCR validation of the microarray results for PDZK1 and BCAS1. (C) Downregulation of PDZK1 and BCAS1, determined by qRT-PCR, upon JQ1 use at different times. *P<0.05; **P<0.01; ***P<0.001.

Figure 3. JQ1 treatment inhibits growth and prevents formation of luminal breast tumors in the MMTV-PyMT mouse model

(A) Design of the “curative treatment experiment”. (B) Tumor volume monitored over time in vehicle- and JQ1-treated MMTV-PyMT mice. **P<0.01. (C) Design of the “prevention treatment experiment”. (D) Tumor volume monitored over time in vehicle- and JQ1-treated MMTV-PyMT mice. *P<0.05. (E) Kaplan-Meier survival curves for vehicle- and JQ1-treated MMTV-PyMT mice. Statistical differences tested with Log Rank (Mantel-Cox) test.