BRD4 in Inflammation and Immune Response A Molecular Orchestrator

What is BRD4？

BRD4 is a transcription and epigenetic genetic regulatory factor, which plays a key role in the occurrence of embryo and cancer development. As members of the bromine domain and end -end -end domain (BET) family, BRD4 is characterized by two series bromine domains (BD1, BD2). BDS combines acetyl lyricine residues on target protein (including histone), so it has higher affinity with protein with multiple acetylidized residues. BRD4 interacts with the protein area of the ultracel group in chromatin. It accumulates on the transcriptional activity regulatory component and promotes gene transcription in the start and extension steps.

The Structure and Function of BRD4

Brd4, originally identified as a unique chromatin binding factor retaining its connection to chromosomes during mitosis[1], belongs to the BET (bromodomain and extra-terminal domain) family of chromatin-binding proteins. Consequently, its name was changed from MCAP (mitotic chromosome-associated protein) to bromodomain containing protein 4 (Brd4)[2]. Brd4, an indispensable protein[3], is ubiquitous in rapidly dividing cells. It's distinguished by its tandem bromodomains, which interact with acetylated tails of H3 and H4 histones[4]. Notably, Brd4 plays a critical role as a mitotic bookmark, marking genes that become active shortly after mitotic exit[5]. By decompacting chromatin and recruiting transcriptional initiation and elongation factors, Brd4 rapidly activates early G1 genes post-mitosis and during interphase[6]. Moreover, Brd4 recruits the transcriptional elongation factor p-TEFb to promoters, enhancing the phosphorylation of the C-terminal tail (CTD) of RNA polymerase II promoters to stimulate transcription[7]. Additionally, Brd4 directly phosphorylates the RNA polymerase II CTD, further promoting transcription[8]. These pivotal roles in transcriptional regulation position Brd4 at the center of a multitude of diverse biological activities.

The Brd4 gene encodes two proteins: the shorter form, containing the two bromodomains and the extra-terminal (ET) region (crucial for numerous protein-protein interactions), and the longer form, which includes an extended, unique C-terminal region. The structures of both bromodomains and ET domains have been elucidated[9]. While the bromodomains bind specifically to acetylated lysines on H3 and H4, BD2 (bromodomain 2) can also interact with acetylated residues in other proteins, such as cyclin T1 (a p-TEFb subunit) and the relA subunit of NFκB[10].

The extra-terminal domain, comprising three alpha-helices, appears to serve as a hotspot for protein-protein interactions[11]. It binds to proteins involved in transcriptional regulation, including NSD3 (also known as WHSC1L1), a histone methyltransferase, JMJD6, a histone demethylase, and CHD4, a component of the NuRD (nuclear remodeling and deacetylase) repressor complex. This versatility allows Brd4 to potentially assemble multifaceted positive and negative regulatory complexes on promoters[12].

Although the function of the C-terminal region remains largely uncharacterized, the last 100 amino acids are known to be crucial for interacting with the papillomavirus E2 protein and with p-TEFb. Consequently, p-TEFb engages with two distinct regions of the Brd4 protein[13].

Disease-Related Functions of BRD4

BRD4 plays a pivotal role in the regulation of various cellular processes in normal mammalian cells, including cell proliferation, apoptosis, and transcription[15]. When a BRD4-specific antibody is microinjected into the nuclei of HeLa cells, it results in cell cycle arrest, highlighting BRD4's necessity for the G2-M phase transition[16]. Furthermore, BRD4 is indispensable for the expression of Aurora B kinase, a key player in chromosome separation and cytokinesis during mitosis. BRD4 achieves this by recruiting transcriptional regulatory complexes to chromatin through a series of protein-protein interactions, facilitated by its bromodomains, extra-terminal (ET), and C-terminal (CTD) domains. Notably, BET (bromodomain and extra-terminal domain) inhibition has shown promising therapeutic potential across various pathologies, particularly in cancer and inflammation models[17].

Through its interactions with cyclin T1 and CDK9, BRD4 recruits P-TEFb to mitotic chromosomes, resulting in increased expression of growth-promoting genes[18]. Chromosomal translocation of BRD4 to the nuclear protein in the testis (NUT) locus creates a BRD4-NUT fusion protein, leading to c-MYC overexpression and NUT midline carcinoma (NMC), an aggressive and treatment-resistant squamous cell malignancy. BET inhibition effectively downregulates MYC transcription and its downstream target genes, making it a promising strategy for cancer treatment. The BRD4 inhibitor (+)-JQ1 has demonstrated high efficacy in reducing NMC tumor growth in xenografted mice[19]. Additionally, BRD4 interacts physically with the androgen receptor (AR), and BET inhibitors disrupt this interaction, preventing BRD4 localization to AR target loci and AR-mediated gene transcription. Interestingly, BET inhibition has proven more effective in reducing tumor size in xenograft mouse models of castration-resistant prostate cancer (CRPC) than direct AR antagonism. Moreover, BRD4 plays a role in the activation of WNT5A expression, crucial for the invasion and maintenance of cancer stem cell-like properties in basal-like breast cancer (BLBC). BRD4 is also amplified and overexpressed in a subset of melanoma specimens and cell lines, and treatment with compound 7 has demonstrated efficacy in inhibiting melanoma proliferation in vitro and impairing melanoma tumor growth in vivo. RNAi screens have identified BRD4 as a therapeutic target in acute myeloid leukemia (AML) and ovarian carcinoma. Additionally, BRD4 is implicated in the proliferation of various other cancer types, including the activated B-cell-like subtype of diffuse large B-cell lymphoma (DLBCL), neuroblastoma, and lung adenocarcinoma[20].

BRD4 is also involved in the transcriptional coactivation of NF-κB, regulating the transcription of P-TEFb-dependent proinflammatory target genes. Specifically, BRD4 binds acetylated lysine-310 of RelA, facilitating the recruitment of NF-κB to these genes. Moreover, BRD4 is highly enriched at enhancers associated with genes involved in profibrotic pathways, often colocalized with profibrotic transcription factors. BRD4 inhibitors not only prevent cytokine-induced activation of hepatic stellate cells but also reverse fibrotic responses in mouse models of carbon tetrachloride-induced fibrosis. BRD4 inhibition has also shown promise in attenuating experimental lung fibrosis induced by repetitive TGF-β challenge, primarily through the NF-κB/RelA signaling pathway. Furthermore, as a synthetic histone mimic, compound 2 has been found to reduce inflammation in an LPS-induced C57BL mouse model, suggesting a novel approach to treating inflammatory conditions by interfering with BET inhibitors' recognition of acetylated histones. Knockdown of BRD4 using siRNA leads to upregulation of apolipoprotein A (ApoA1), which is protective against atherosclerosis and other inflammatory processes. In human airway epithelial cells, BRD4 is essential for IL-1β-induced inflammation, and BRD4 knockdown significantly reduces the release of IL-6 and CXCL8, making it a promising target for chronic obstructive pulmonary disease. Pharmacological BRD4 inhibition can also mitigate enhanced migration, proliferation, and IL-6 release in lung fibroblasts from patients with rapidly progressing idiopathic pulmonary fibrosis. Compound 7, for example, has been effective in suppressing bleomycin-induced lung fibrosis in mice, suggesting that BRD4 inhibitors hold potential for the treatment of this condition[21].

Furthermore, BRD4's involvement extends to the transcriptional regulation of viruses such as HIV, Epstein–Barr virus (EBV), and degradation of human papillomavirus (HPV). The bovine papillomavirus E2 protein binds to BRD4's CTD, anchoring viral DNA to host mitotic chromosomes. Disrupting the E2/BRD4 interaction could inhibit viral transformation, offering a novel approach for treating or preventing HPV infections and related diseases. Bromodomains play diverse roles in the HIV life cycle, including transcription and integration. Binding of the BRD4 CTD domain with P-TEFb interferes with the interaction between the HIV transactivator Tat and P-TEFb, suppressing Tat's ability to transactivate the HIV promoter[25]. BRD4, as a negative regulator of HIV-1 replication, enhances proviral transcriptional elongation and reduces HIV-1 latency in cell-line models. Compounds 3 and 7 have been reported to reactivate latent HIV, indicating their potential in eliminating latent HIV-1 reservoirs, a significant challenge in achieving a complete cure for AIDS[26].

Signaling Pathway of BRD4

BRD4 participates in a variety of signal pathways and biological processes in cells. The following is a detailed explanation of the main signal channels participating in BRD4 and its role:

1. Transcription regulatory signal channel:

BRD4 interacts with acetylatic modified groups of protein, especially binding to acetylatic modification sites of histone H3 and H4. This interaction allows BRD4 to recruit transcription regulatory factor and RNA polymerase to a promoter area from specific genes to promote gene transcription.

BRD4 participates in regulatory gene expression, especially in cancer cells. It can affect the expression of tumor -related genes and promote the growth and proliferation of tumor cells.

1. Cell cycle regulatory signal pathway:

BRD4 plays a key role in the regulation of the cell cycle, especially in the G1/S transition period. It interacts with cell cycle protein and regulatory factor, helps cells enter the DNA replication stage.

1. Inflammation and immune signal pathway:

The role of BRD4 in inflammation and immunity has attracted much attention. It can affect the expression of inflammatory genes, especially by regulating the NF-κB pathway to promote the activation of immune cells.

The abnormal activation of BRD4 is related to the development of inflammatory diseases (such as rheumatoid arthritis, inflammatory bowel disease, etc.). Its inhibitors are studied as potential strategies for treating these diseases.

1. Signal conduction channel:

BRD4 interacts with the molecular components of a variety of signal conduction channels, including MAPK, PI3K/AKT and WNT. These interactions can affect the activity of these signal pathways, thereby affecting the biological behavior of cells.

1. Cancer signal channel:

BRD4 is related to a variety of cancer -related signal pathways, such as MyC and P53. The abnormal activation of BRD4 is related to the proliferation and metastasis of tumor cells, so it is regarded as a potential anti -cancer drug target.

In short, BRD4 participates in multiple signal pathways and biological processes, including transcriptional regulation, cyclical regulation, inflammation and immune, signal conduction and cancer. Its diversified function makes it an important focus in biomedical research, and also provides potential opportunities for development of the treatment strategies related to BRD4.

BRD4 Protein

Recombinant Human Brd4 Protein

Click here hor more BRD4

Synonym : Brd4 BRD4-NUT FUSION BRD4-NUT fusion oncoprotein BRD4_HUMAN Bromodomain containing 4 bromodomain containing protein 4 Bromodomain-containing protein 4 CAP chromosome associated protein HUNK1 HUNKI MCAP Mitotic chromosome-associated protein Protein HUNK1

References:

 

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