Members of the tripartite motif (TRIM) protein family are characterized by a shared N-terminal structure consisting of a RING finger, 2 B-box domains, a coiled-coil domain, and for most TRIM proteins, an E3 ubiquitin ligase activity. TRIM33 is part of a TRIM subfamily characterized by a C-terminal plant homeodomain (PHD) juxtaposed to a bromodomain. Members of this TRIM subfamily, including TRIM33, do not bind DNA directly but can be recruited by DNA-binding proteins to act as transcriptional repressors (summary by Ferri et al., 2015).

Differential control of gene expression by nuclear receptors (NRs) that are ligand-dependent transregulators may be mediated by several interacting proteins. Transcriptional intermediary factor-1-alpha (TIF1A; 603406) interacts specifically in a ligand-dependent manner with the ligand-binding domain of several NRs. TIF1-beta (TIF1B; 601742) also represses transcription but is not an interacting partner for NRs. Using TIF1A to screen a liver cDNA library, Venturini et al. (1999) identified a cDNA encoding TIF1-gamma (TIF1G). The predicted 1,120-amino acid protein, which is 48% identical to TIF1A and 32% identical to TIF1B, contains an N-terminal RING finger followed by 2 B-box-type fingers, an alpha-helical coiled-coil domain forming a tripartite motif (designated RBCC), and a C-terminal bromodomain preceded by a C4HC3 zinc finger motif, or PHD/TTC finger. In addition, Venturini et al. (1999) identified a 25-amino acid stretch rich in tryptophan and phenylalanine located downstream of the coiled-coil motif. This 25-amino acid stretch is highly conserved between the 3 TIF1 proteins, and the authors termed this the TIF1 signature sequence, or TSS. Northern blot analysis revealed variable expression of 1 to 5 TIF1G transcripts, ranging from 2.5 to 8.8 kb, in all tissues tested.

Using immunofluorescence and immunoblot analyses, He et al. (2006) detected TIF1-gamma in all cell types tested, including primary human hematopoietic progenitors, mouse embryo fibroblasts, and mouse T cells, as well as cell lines from normal tissues and tumors. Immunofluorescence analysis revealed that TIF1-gamma localized to the nucleus, outside of nucleoli, in HaCaT human keratinocytes and all other cell types tested. A punctate pattern coexisted with a more diffuse nuclear distribution. Immunohistochemical analysis of mouse embryo sections showed Tif1-gamma nuclear staining of most tissues throughout gestation, with pronounced staining in round hematopoietic cells in yolk sac blood islands at embryonic day 8.5. Strong nuclear staining was also observed in CD34 (142230)-positive hematopoietic stem/progenitor cells from human umbilical cord blood.

Venturini et al. (1999) mapped the TRIM33 gene to chromosome 1p13 by FISH.

Using luciferase analysis, Venturini et al. (1999) found that TIF1G, like TIF1A and TIF1B, repressed transcription by binding through the TSS to promoter regions. In contrast to TIF1A, TIF1G did not interfere with the nuclear retinoic acid receptor (RAR; 180240). Yeast 2-hybrid and in vitro binding analyses showed that TIF1G did not interact with NRs, nor, unlike TIF1A and TIF1B, did it interact with HP1-alpha (CBX5; 604478), HP1-beta (CBX1; 604511), HP1-gamma (CBX3; 604477), or the KRAB domain of KOX1 (194538).

Dupont et al. (2005) showed that Xenopus Trim33, which they called ectodermin, functioned as a ubiquitin ligase. By ubiquitinating Smad4 (600993), ectodermin restricted the mesoderm-inducing activity of Tgf-beta (TGFB1; 190180), favored neural induction, and was essential for specification of the ectoderm germ layer in early Xenopus embryos. Depletion of ectodermin in several human cell lines inhibited cell proliferation, and this inhibition was dependent on expression of functional SMAD4. Dupont et al. (2005) concluded that ectodermin is a negative regulator of TGF-beta signaling during early embryonic development and cell proliferation.

Formation of transcription regulatory complexes by the association of SMAD4 with receptor-phosphorylated SMAD2 (601366) and SMAD3 (603109) is a central event in the canonical TGF-beta pathway. He et al. (2006) found that TIF1-gamma competed with SMAD4 for selective binding of receptor-phosphorylated SMAD2 and SMAD3 in human cells. Domain deletion experiments showed that the activated MH2 domains of SMAD2 and SMAD3 interacted directly with the middle region of TIF1-gamma. TGF-beta induced formation of endogenous SMAD2/3-TIF1-gamma and SMAD2/3-SMAD4 complexes in human and other mammalian hematopoietic, mesenchymal, and epithelial cells. In human CD34-positive hematopoietic stem/progenitor cells, where TGF-beta inhibits proliferation and stimulates erythroid differentiation, TIF1-gamma mediated the differentiation response, whereas SMAD4 mediated the antiproliferative response, with SMAD2 and SMAD3 participating in both responses. He et al. (2006) concluded that SMAD2/3-TIF1-gamma and SMAD2/3-SMAD4 function as complementary effector arms in the control of hematopoietic cell fate by the TGF-beta/SMAD pathway.

Ferri et al. (2015) found that Trim33 deficiency was associated with increased Ifnb1 (147640) mRNA levels and increased IFN-beta secretion during the late stages of lipopolysaccharide (LPS) activation of mouse bone-marrow-derived macrophages (BMDMs). The coiled-coil domain of Trim33 was required for Ifnb1 regulation, as Trim33 lacking the coiled-coil domain failed to restore Ifnb1 expression to normal in activated Trim33 -/- cells. Chromatin immunoprecipitation-sequencing analysis revealed that Trim33 bound to a distal Ifnb1 gene regulatory element (ICE) in mouse macrophages. ICE functioned as a cis-acting transcriptional repressor element of Ifnb1 activation in macrophages. Binding of Trim33 and Pu.1 (165170) to ICE appeared to play an important role in repressing Ifnb1 transcription during the late phase of macrophage activation. ICE exhibited a promoter-like chromatin signature established early during myeloid differentiation. ICE interacted with the Ifnb1 proximal region in a constitutive and Trim33-independent manner, and this interaction was strengthened following LPS stimulation. Further investigation revealed that Trim33 regulated Ifnb1 expression by inhibiting Cbp (CREBBP; 600140)/p300 (EP300; 602700) recruitment, as enhanced CBP/p300 recruitment and activity at late times of activation were required for sustained Ifnb1 expression in Trim33 -/- BMDMs. The authors concluded that TRIM33 regulates IFNB1 expression at the late phase of macrophage activation by preventing recruitment of CBP/p300.

Using pooled short hairpin RNA (shRNA) screening of human RKO colorectal cancer cells, Shi et al. (2016) identified TRIM33 as a negative regulator of resistance to bromodomain and extraterminal domain (BET) protein inhibitors (BETi). Evaluation of TRIM33-knockdown RKO cells confirmed that TRIM33 promoted sensitivity to BET bromodomain inhibition. RNA sequencing analysis demonstrated that TRIM33 knockdown maintained MYC (190080) expression following BETi treatment, and TRIM33 associated with the MYC promoter in BETi-treated RKO cells. Further analysis found that the TGF-beta (TGFB1; 190180) signaling pathway also contributed to BETi resistance, as TRIM33 knockdown potentiated TGF-beta signaling, and inhibition of the TGF-beta pathway increased BETi sensitivity in TRIM33-knockdown RKO cells.

Tanaka et al. (2018) showed that conditional knockout of Trim33 in T cells of mice resulted in decreased Il17 (603149) and Ccr6 (601835) expression, but enhanced Il10 (124092) production, leading to protection against experimental autoimmune encephalomyelitis. Further examination revealed that Trim33 played a crucial role in differentiation of Th17 cells, but not inducible regulatory T (Treg) cells. Microarray and real-time RT-PCR analyses confirmed downregulation of Il17 and Ccr6 and upregulation of Il10 in the absence of Trim33. Il17 and Ccr6 downregulation was not due to enhanced Il10 expression, as Il10 blockade did not restore Il17 and Ccr6 expression in Trim33-knockout T cells. Genomewide analysis of Trim33-bound genes showed that Il17 and Il10 were collaboratively regulated by Trim33 and Ror-gamma (RORC; 602943) at the transcriptional level. Trim33 controlled Il17 and Il10 expression at the chromatin level through regulation of histone modifications. Smad2 was crucial for binding of Trim33 to Il17 and Il10 loci, and the Trim33/Smad2/Ror-gamma complex was necessary for optimal expression of Il17 and repression of Il10 in Th17 cells. Moreover, enhanced expression of Il10 in Trim33-knockout T cells was almost completely suppressed by deletion of Smad4, indicating that Trim33 suppresses Il10 expression by reduction of Smad4 protein in T cells. The authors concluded that TRIM33 promotes the proinflammatory function of Th17 cells by inducing IL17 and suppressing IL10 expression.

The predominant molecular lesions in papillary thyroid carcinomas (PTC; see 188550), which were particularly prevalent after the Chernobyl nuclear reactor accident, are rearrangements of the RET receptor tyrosine kinase (164761). Klugbauer and Rabes (1999) identified 2 novel types of RET rearrangements, which they termed PTC6 and PTC7. In PTC6, RET is fused to the N-terminal part of TIF1A, and in PTC7, RET is fused to a C-terminal part of TIF1G, which the authors termed RFG7.

Ferri et al. (2015) found that Trim33 -/- mice were healthy with no developmental abnormalities.

Tanaka et al. (2018) showed that conditional knockout of Trim33 in T cells of mice resulted in decreased Il17 and Ccr6 expression, but enhanced Il10 (124092) production, leading to protection against experimental autoimmune encephalomyelitis.