Ret Finger Protein: An E3 Ubiquitin Ligase Juxtaposed to the XY Body in Meiosis

Emerging roles of TRIM27 in cancer and other human diseases

As a member of the TRIM protein family, TRIM27 is a RING-mediated E3 ubiquitin ligase that can mark other proteins for degradation. Its ubiquitination targets include PTEN, IκBα and p53, which allows it to regulate many signaling pathways to exert its functions under both physiological and pathological conditions, such as cell proliferation, differentiation and apoptosis. During the past decades, TRIM27 was reported to be involved in many diseases, including cancer, lupus nephritis, ischemia-reperfusion injury and Parkinson's disease. Although the research interest in TRIM27 is increasing, there are few reviews about the diverse roles of this protein. Here, we systematically review the roles of TRIM27 in cancer and other human diseases. Firstly, we introduce the biological functions of TRIM27. Next, we focus on the roles of TRIM27 in cancer, including ovarian cancer, breast cancer and lung cancer. At the same time, we also describe the roles of TRIM27 in other human diseases, such as lupus nephritis, ischemia-reperfusion injury and Parkinson's disease. Finally, we discuss the future directions of TRIM27 research, especially its potential roles in tumor immunity.

Ubiquitin-Proteasome System-Regulated Protein Degradation in Spermatogenesis

Spermatogenesis is a prolonged and highly ordered physiological process that produces haploid male germ cells through more than 40 steps and experiences dramatic morphological and cellular transformations. The ubiquitin proteasome system (UPS) plays central roles in the precise control of protein homeostasis to ensure the effectiveness of certain protein groups at a given stage and the inactivation of them after this stage. Many UPS components have been demonstrated to regulate the progression of spermatogenesis at different levels. Especially in recent years, novel testis-specific proteasome isoforms have been identified to be essential and unique for spermatogenesis. In this review, we set out to discuss our current knowledge in functions of diverse USP components in mammalian spermatogenesis through: (1) the composition of proteasome isoforms at each stage of spermatogenesis; (2) the specificity of each proteasome isoform and the associated degradation events; (3) the E3 ubiquitin ligases mediating protein ubiquitination in male germ cells; and (4) the deubiquitinases involved in spermatogenesis and male fertility. Exploring the functions of UPS machineries in spermatogenesis provides a global picture of the proteome dynamics during male germ cell production and shed light on the etiology and pathogenesis of human male infertility.

Crosstalk Between SUMO and Ubiquitin-Like Proteins: Implication for Antiviral Defense

Interferon (IFN) is a crucial first line of defense against viral infection. This cytokine induces the expression of several IFN-Stimulated Genes (ISGs), some of which act as restriction factors. Upon IFN stimulation, cells also express ISG15 and SUMO, two key ubiquitin-like (Ubl) modifiers that play important roles in the antiviral response. IFN itself increases the global cellular SUMOylation in a PML-dependent manner. Mass spectrometry-based proteomics enables the large-scale identification of Ubl protein conjugates to determine the sites of modification and the quantitative changes in protein abundance. Importantly, a key difference amongst SUMO paralogs is the ability of SUMO2/3 to form poly-SUMO chains that recruit SUMO ubiquitin ligases such RING finger protein RNF4 and RNF111, thus resulting in the proteasomal degradation of conjugated substrates. Crosstalk between poly-SUMOylation and ISG15 has been reported recently, where increased poly-SUMOylation in response to IFN enhances IFN-induced ISGylation, stabilizes several ISG products in a TRIM25-dependent fashion, and results in enhanced IFN-induced antiviral activities. This contribution will highlight the relevance of the global SUMO proteome and the crosstalk between SUMO, ubiquitin and ISG15 in controlling both the stability and function of specific restriction factors that mediate IFN antiviral defense.

Identification of TRIM27 as a novel degradation target of herpes simplex virus 1 ICP0

The herpes simplex virus 1 (HSV-1) immediate early protein ICP0 performs many functions during infection, including transactivation of viral gene expression, suppression of innate immune responses, and modification and eviction of histones from viral chromatin. Although these functions of ICP0 have been characterized, the detailed mechanisms underlying ICP0's complex role during infection warrant further investigation. We thus undertook an unbiased proteomic approach to identifying viral and cellular proteins that interact with ICP0 in the infected cell. Cellular candidates resulting from our analysis included the ubiquitin-specific protease USP7, the transcriptional repressor TRIM27, DNA repair proteins NBN and MRE11A, regulators of apoptosis, including BIRC6, and the proteasome. We also identified two HSV-1 early proteins involved in nucleotide metabolism, UL39 and UL50, as novel candidate interactors of ICP0. Because TRIM27 was the most statistically significant cellular candidate, we investigated the relationship between TRIM27 and ICP0. We observed rapid, ICP0-dependent loss of TRIM27 during HSV-1 infection. TRIM27 protein levels were restored by disrupting the RING domain of ICP0 or by inhibiting the proteasome, arguing that TRIM27 is a novel degradation target of ICP0. A mutant ICP0 lacking E3 ligase activity interacted with endogenous TRIM27 during infection as demonstrated by reciprocal coimmunoprecipitation and supported by immunofluorescence data. Surprisingly, ICP0-null mutant virus yields decreased upon TRIM27 depletion, arguing that TRIM27 has a positive effect on infection despite being targeted for degradation. These results illustrate a complex interaction between TRIM27 and viral infection with potential positive or negative effects of TRIM27 on HSV under different infection conditions.

IMPORTANCE

During productive infection, a virus must simultaneously redirect multiple cellular pathways to replicate itself while evading detection by the host's defenses. To orchestrate such complex regulation, viruses, including herpes simplex virus 1 (HSV-1), rely on multifunctional proteins such as the E3 ubiquitin ligase ICP0. This protein regulates various cellular pathways concurrently by targeting a diverse set of cellular factors for degradation. While some of these targets have been previously identified and characterized, we undertook a proteomic screen to identify additional targets of this activity to further characterize ICP0's role during infection. We describe a set of candidate interacting proteins of ICP0 identified through this approach and our characterization of the most statistically significant result, the cellular transcriptional repressor TRIM27. We present TRIM27 as a novel degradation target of ICP0 and describe the relationship of these two proteins during infection.

TRIM27/MRTF-B-dependent integrin β1 expression defines leading cells in cancer cell collectives

For collective invasion, cancer cells form cohesive groups comprised of leading cells (LCs) at the forefront and following cells (FCs) at the rear. However, the molecular mechanisms that define LCs and FCs remain elusive. Here, we demonstrated that LCs, but not FCs, upregulated the expression of integrin β1 after the loss of intercellular adhesion. The LC-specific expression of integrin β1 was posttranscriptionally regulated by the TRIM27/MRTF-B complex in response to the loss of intercellular adhesion, thereby regulating the stability and translation of integrin β1 mRNA via microRNA-124 in LCs. Accordingly, depletion of TRIM27 and MRTF-B abrogated the upregulation of integrin β1 in LCs and blocked the invasion of cancer cell groups in vitro and in vivo. Therefore, our findings revealed that the specific function of LCs was defined by intrinsic mechanisms related to the presence of the cell's free surface, providing insights into the regulation of intratumor heterogeneity.

Ubiquitin-proteasome system in spermatogenesis

Spermatogenesis represents a complex succession of cell division and differentiation events resulting in the continuous formation of spermatozoa. Such a complex program requires precise expression of enzymes and structural proteins which is effected not only by regulation of gene transcription and translation, but also by targeted protein degradation. In this chapter, we review current knowledge about the role of the ubiquitin-proteasome system in spermatogenesis, describing both proteolytic and non-proteolytic functions of ubiquitination. Ubiquitination plays essential roles in the establishment of both spermatogonial stem cells and differentiating spermatogonia from gonocytes. It also plays critical roles in several key processes during meiosis such as genetic recombination and sex chromosome silencing. Finally, in spermiogenesis, we summarize current knowledge of the role of the ubiquitin-proteasome system in nucleosome removal and establishment of key structures in the mature spermatid. Many mechanisms remain to be precisely defined, but present knowledge indicates that research in this area has significant potential to translate into benefits that will address problems in both human and animal reproduction.

Role of the tripartite motif protein 27 in cancer development

BACKGROUND

The tripartite motif family protein 27 (TRIM27) is a transcriptional repressor that interacts with, and attenuates senescence induction by, the retinoblastoma-associated protein (RB1). High expression of TRIM27 was noted in several human cancer types including breast and endometrial cancer, where elevated TRIM27 expression predicts poor prognosis. Here, we investigated the role of TRIM27 expression in cancer development.

METHODS

We assessed TRIM27 expression in human cancer using cancer profiling arrays containing paired tumor and normal cRNA (n = 261) as well as in murine skin cancer induced by 7, 12-dimethylbenzanthracene (DMBA)/12-O-tetradecanoylphorbol-13-acetate (TPA). We generated mice with disrupted expression of murine TRIM27 (Trim27(-/-)) and assessed their susceptibility to DMBA/TPA-induced skin tumor development compared with isogenic littermates (n = 26 mice per group). We assessed the effect of Trim27 loss on senescence propensity in mouse embryonic fibroblasts (MEFs) by quantifying cell proliferation alongside senescence markers (senescence-associated β-galactosidase [SA-β-gal] activity and hypertrophic cell morphology). The contribution of RB1 on senescence and cancer susceptibility (n > 20 mice per group) in Trim27(-/-) backgrounds was also assessed. Data were analyzed using the Student's t, χ(2), or log-rank test as indicated. All statistical tests were two-sided.

RESULTS

TRIM27 transcript levels are statistically significantly increased in common human cancers, including colon and lung, vs normal tissues (TRIM27 expression relative to ubiquitin: cancers vs normal tissues, mean = 0.59, 95% confidence interval [CI] = 0.55 to 0.63 vs mean = 0.46, 95% CI =0.43 to 0.49, P < .001) as well as in chemically induced mouse skin cancer compared with matched normal tissue (Trim27 expression relative to Gapdh control: tumor vs normal skin, mean = 4.2, 95% CI = 3.97 to 4.43 vs mean = 0.96, 95% CI = 0.69 to 1.2, P < .001). Trim27(-/-) mice (n = 14) were resistant to chemically induced skin cancer development (eight [57.2%] of 14 mice were tumor free) compared with Trim27(+/+) wild-type littermates (n = 13) (one [7.7%] of 13 mice was tumor free). Trim27(-/-) MEFs show enhanced senescence propensity in response to replicative (percentage of SA-β-gal-positive cells: Trim27(+/+) MEFs vs Trim27(-/-) MEFs, mean = 14.2%, 95% CI = 11.1% to 17.4% vs mean = 53.3%, 95% CI = 48.7% to 57.9%, P < .001) or oncogenic stress (percentage of SA-β-gal-positive cells: Trim27(+/+) MEFs + Ras vs Trim27(-/-) MEFs + Ras, mean = 24.0%, 95% CI = 19.9% to 28.1% vs mean = 37.3%, 95% CI = 32.2% to 42.4%, P < .05) compared with Trim27(+/+) MEFs. These responses were alleviated following inactivation of murine RB1 (Rb1). Furthermore, Trim27(-/-) mice are not protected from cancers arising as a consequence of Rb1 deletion (median survival: Trim27(-/-)Rb(+/-) vs Trim27(+/+)Rb(+/-), 14 vs 13 months; difference = 1.0 month, 95% CI = 0.5 to 1.6 months, P = .14).

CONCLUSION

TRIM27 expression is a modifier of disease incidence and progression relevant to the development of common human cancers and is a potential target for intervention in cancer.

SUMO E3 ligase activity of TRIM proteins

SUMOylation governs numerous cellular processes and is essential to most eukaryotic life. Despite increasing recognition of the importance of this process, an extremely limited number of small ubiquitin-like modifier (SUMO) protein ligases (E3s) have been identified. Here we show that at least some members of the functionally diverse tripartite motif (TRIM) superfamily are SUMO E3s. These TRIM proteins bind both the SUMO-conjugating enzyme Ubc9 and substrates and strongly enhance transfer of SUMOs from Ubc9 to these substrates. Among the substrates of TRIM SUMO E3s are the tumor suppressor p53 and its principal antagonist Mdm2. The E3 activity depends on the TRIM motif, suggesting it to be the first widespread SUMO E3 motif. Given the large number of TRIM proteins, our results may greatly expand the identified SUMO E3s. Furthermore, TRIM E3 activity may be an important contributor to SUMOylation specificity and the versatile functions of TRIM proteins.

Functional interactions between ubiquitin E2 enzymes and TRIM proteins

The TRIM (tripartite motif) family of proteins is characterized by the presence of the tripartite motif module, composed of a RING domain, one or two B-box domains and a coiled-coil region. TRIM proteins are involved in many cellular processes and represent the largest subfamily of RING-containing putative ubiquitin E3 ligases. Whereas their role as E3 ubiquitin ligases has been presumed, and in several cases established, little is known about their specific interactions with the ubiquitin-conjugating E2 enzymes or UBE2s. In the present paper, we report a thorough screening of interactions between the TRIM and UBE2 families. We found a general preference of the TRIM proteins for the D and E classes of UBE2 enzymes, but we also revealed very specific interactions between TRIM9 and UBE2G2, and TRIM32 and UBE2V1/2. Furthermore, we demonstrated that the TRIM E3 activity is only manifest with the UBE2 with which they interact. For most specific interactions, we could also observe subcellular co-localization of the TRIM involved and its cognate UBE2 enzyme, suggesting that the specific selection of TRIM-UBE2 pairs has physiological relevance. Our findings represent the basis for future studies on the specific reactions catalysed by the TRIM E3 ligases to determine the fate of their targets.