The role of maintenance proteins in the preservation of epithelial cell identity during mammary gland remodeling and breast cancer initiation

Abstract

During normal postnatal mammary gland development and adult remodeling related to the menstrual cycle, pregnancy, and lactation, ovarian hormones and peptide growth factors contribute to the delineation of a definite epithelial cell identity. This identity is maintained during cell replication in a heritable but DNA-independent manner. The preservation of cell identity is fundamental, especially when cells must undergo changes in response to intrinsic and extrinsic signals. The maintenance proteins, which are required for cell identity preservation, act epigenetically by regulating gene expression through DNA methylation, histone modification, and chromatin remodeling. Among the maintenance proteins, the Trithorax (TrxG) and Polycomb (PcG) group proteins are the best characterized. In this review, we summarize the structures and activities of the TrxG and PcG complexes and describe their pivotal roles in nuclear estrogen receptor activity. In addition, we provide evidence that perturbations in these epigenetic regulators are involved in disrupting epithelial cell identity, mammary gland remodeling, and breast cancer initiation.

Figure 1.. The estradiol (E)-estrogen receptor (ER) complex may activate gene transcription directly by binding to specific sequences (estrogen response elements, EREs) within the promoter/enhancer regions of target genes or indirectly by physical interaction with other transcription factors, such as activating protein 1 (AP1).

CBP, histone acetyltransferase CREB-binding protein; SRC-1, steroid receptor coactivator-1; AIB1, amplified in breast cancer 1.

Figure 2.. Forkhead box A1 (FoxA1) is a pioneer factor belonging to a special class of proteins that are able to interact directly with condensed chromatin prior to any other transcription factors.

Upon chromatin binding, FoxA1 induces a nucleosomal rearrangement that results in a more relaxed chromatin structure, which is accessible to other transcription regulators, including ERs. Green diamonds represent epigenetic histone modifications (dimethylation of histone H3 at lysine 4, H3K4me2).

Figure 3.. Brahma-related gene 1 (BRG1)-containing chromatin-remodeling complexes.

In addition to the switching-defective (SWI)/sucrose non-fermenting (SNF)-BRG1-associated factors (BAF) and SWI/SNF-polybromo-associated BAF (PBAF) complexes, the catalytic subunit BRG1 (or, less frequently, BRM) can associate with several chromatin-remodeling complexes, including transcription coactivators and corepressors. BRG1 can be found in complexes with transcription coactivators and histone-modifying enzymes such as Williams syndrome transcription factor (WSTF)-including nucleosome assembly complex (WINAC) and nucleosomal methylation activation complex (NUMAC). Conversely, BRG1 can be assembled in complexes known to repress transcription and induce gene silencing, such as the nuclear receptor corepressor (N-CoR) and mSin3A/histone deacetylase (HDAC) complexes. Orange: BRG1 subunit; gold: essential subunits; green: nuclear receptor-associated factors; red: DNA replication factors; yellow: actin-related complexes; blue: factors with other functions.

Figure 4.. Mixed lineage leukemia proteins (MLLs) are the catalytic subunits of covalent histone-modifying complexes and have H3K4-specific methyltransferase activity.

According to a proposed working model, Menin interacts directly with ER through its activation function 2 (AF2) domain, recruiting the MLL complex to the ERE of the target gene. In multi-protein complexes that include MLL3 or MLL4 as the catalytic subunit, the activating signal cointegrator-2 (ASC-2) protein acts as a coactivator in the transactivation of the nuclear estrogen receptor. This crosstalk functions at least in part by mutually facilitating the recruitment of the MLL and ER complexes to ER target genes.

Figure 5.. Polycomb repressive complex (PRC)-1 and PRC-2 work cooperatively.

Enhancer of zeste homolog 2 (EZH2), the catalytic subunit of PRC-2 with methyltransferase activity, mediates the covalent trimethylation of histone H3 at lysine 27 (H3K27me3). This specific histone modification serves as the docking site for proteins harboring a chromobox domain (especially Cbx4 and Cbx8) that aid in the recruitment of PRC-1 to chromatin through their interactions with H3K27me3. Thereafter, PRC-1 catalyzes the monoubiquitilation of histone H2A at lysine 119 through the enzymatic action of RING1/RING1b and blocks transcriptional elongation by RNA polymerase II.