The complexity of TRIM28 contribution to cancer

Abstract

Since the first discovery in 1996, the engagement of TRIM28 in distinct aspects of cellular biology has been extensively studied resulting in identification of a complex nature of TRIM28 protein. In this review, we summarize core biological functions of TRIM28 that emerge from TRIM28 multi-domain structure and possessed enzymatic activities. Moreover, we will discuss whether the complexity of TRIM28 engagement in cancer biology makes TRIM28 a possible candidate for targeted anti-cancer therapy. Briefly, we will demonstrate the role of TRIM28 in regulation of target gene transcription, response to DNA damage, downregulation of p53 activity, stimulation of epithelial-to-mesenchymal transition, stemness sustainability, induction of autophagy and regulation of retrotransposition, to provide the answer whether TRIM28 functions as a stimulator or inhibitor of tumorigenesis. To date, number of studies demonstrate significant upregulation of TRIM28 expression in cancer tissues which correlates with worse overall patient survival, suggesting that TRIM28 supports cancer progression. Here, we present distinct aspects of TRIM28 involvement in regulation of cancer cell homeostasis which collectively imply pro-tumorigenic character of TRIM28. Thorough analyses are further needed to verify whether TRIM28 possess the potential to become a new anti-cancer target.

Keywords: Autophagy; Cancer; Cancer stem cells; EMT; KAP1; TRIM28; Transcriptional co-repressor.

Fig. 1

TRIM28 multi-domain protein contributes to the regulation of a variety of intracellular processes. Core biological functions of TRIM28 including inhibition and degradation of p53 tumor suppressor, regulation of retrotransposition, regulation of gene expression through heterochromatization, mediation of DNA damage response, stimulation of EMT, maintenance of stem cell pluripotency and stimulation of autophagophore formation (induction of autophagy) are frequently intercepted by cancer cells to promote the proliferation and acquire resistance to stress inducing agents

Fig. 2

Structure and post-translational modifications of TRIM28 protein. RING – Really Interesting New Gene; zinc finger type domain which contains a C₃HC₄ amino acid motif that binds two zinc cations; has an intrinsic E3 Ubiquitin ligase activity; B1, B2 – B-box type 1 and B-box type zinc finger type domains of around 40 amino acid residues in length; CC – Coiled Coil, a structural motif in which 2 alpha-helices are coiled together (like the strands of a rope); RBCC – RING domain followed by B-boxes and CC domain, RBCC domain is responsible for interaction with KRAB domain of KRAB-ZFPs; TSS – TRIM Specific Sequence; HP1BD – HP1 protein binding domain, contains a consensus PxVxL motif; PHD – Plant Homeodomain, zinc finger type domain which contains a C₄HC₃ amino acid motif that binds two zinc cations, has an intrinsic E3 SUMO ligase activity; BROMO – Bromodomain, together with PHD are responsible for NuRD/SETDB1 recruitment and binding

Fig. 3

TRIM28 is highly expressed in many types of tumors (The Human Protein Atlas data). Immunohistochemistry was performed with rabbit polyclonal antibody HPA064033 (Sigma Aldrich). Strong nuclear staining is observed in almost all types of cancers presented in The Human Protein Atlas database. Only lymphoma and liver and kidney tumors present medium intensity for TRIM28 staining. Normal tissues showed moderate to strong nuclear positivity with HPA064033 antibody as presented at the bottom, however in most cancer types the presence of TRIM28 in cancer cell nuclei is more pronounced than in adjacent normal tissues

Fig. 4

TRIM28 is recruited to the genome by KRAB-ZNF transcription factors to facilitate chromatin compaction-mediated gene repression. a TRIM28 binds to KRAB domain of KRAB-ZNF proteins tethered to specific sequences within the genome using N-terminal RING-B-boxes-Coiled-Coil (RBCC) domain and catalyzes auto-SUMOylation of Bromodomain (utilizing Ubc9 as an E2 SUMO ligase). b SUMOylated TRIM28 recruits SETDB1 and NuRD complex proteins leading to the creation of the H3K9me3 mark on nearby nucleosomes together with deacetylation of histone proteins. HP1 binding further stabilizes the TRIM28-containing complex bound to the KRAB-ZNF

Fig. 5

ATM-mediated phosphorylation of TRIM28 upon DNA double-strand break leads to destabilization of heterochromatin and enhances the accession of DNA repair machinery to the damaged site. In response to DNA damage, PI3-Kinase family members, like ATM, phosphorylate TRIM28 at serine 824 (near BROMO domain) and serine 473 (near HP1 binding domain). Serine 824 phosphorylation leads to abrogated NuRD and SETDB1 recruitment, and therefore halts heterochromatin compaction [47, 51]. Phosphorylation of TRIM28 at serine 473 attenuates its binding to the HP1 family proteins, and inhibits its transcriptional repression activity to KRAB-ZNFs target genes. Altogether, phosphorylation of TRIM28 results in chromatin relaxation that enhances the accession of DNA repair machinery (BRCA1 and 53BP1 proteins) to the damaged site [50]. BRCA1 – Breast Cancer 1; 53BP1 – p53 Binding Protein 1

Fig. 6

TRIM28 together with CBF-A bound to FTS-1 sequence forms a proximal activator of transcription in epithelial-mesenchymal transition, enhancing the expression of EMT transcriptome. CBF-A/TRIM28 complex binds to FTS-1 (fibroblast transcription site-1) response elements that exist in the promoters of genes modulating EMT transcriptome (including promoters of TWIST1, SNAI1, HMGA2, LEF1, ETS1, etc.) leading to acquisition of mesenchymal phenotype and loss of epithelial-related gene expression. Formation of CBF-A/TRIM28/FTS-1 is sufficient for the induction of epithelial-mesenchymal transition

Fig. 7

TRIM28 possesses an intrinsic E3 ubiquitin ligase activity (RING domain) and subsequently directs p53 for proteasomal degradation. TRIM28 directly interacts with MDM2 through its’ coiled-coil (CC) region within RBCC domain and cooperates with MDM2 to promote p53 polyubiquitination and degradation in the proteasome. MDM2 - E3 ubiquitin-protein ligase, major regulator of the p53 tumor suppressor; p53 – tumor suppressor, guardian of the genome, tetrameric in active conformation

Fig. 8

TRIM28-PHD-mediated SUMOylation of VPS34 protein in response to stress conditions stimulates the phagophore formation. Acetylated HSP70 recruits TRIM28 E3 ligase for SUMOylation of VPS34 at Lys840 increasing VPS34 activity bound to BECLIN1. Consequently, this complex promotes phagophore formation