RING protein Trim32 associated with skin carcinogenesis has anti-apoptotic and E3-ubiquitin ligase properties

E3 ubiquitin ligase TRIM32 negatively regulates tumor suppressor p53 to promote tumorigenesis

Tumor suppressor p53 has a key role in maintaining genomic stability and preventing tumorigenesis through its regulation of cellular stress responses, including apoptosis, cell cycle arrest and senescence. To ensure its proper levels and functions in cells, p53 is tightly regulated mainly through post-translational modifications, such as ubiquitination. Here, we identified E3 ubiquitin ligase TRIM32 as a novel p53 target gene and negative regulator to regulate p53-mediated stress responses. In response to stress, such as DNA damage, p53 binds to the p53 responsive element in the promoter of the TRIM32 gene and transcriptionally induces the expression of TRIM32 in cells. In turn, TRIM32 interacts with p53 and promotes p53 degradation through ubiquitination. Thus, TRIM32 negatively regulates p53-mediated apoptosis, cell cycle arrest and senescence in response to stress. TRIM32 is frequently overexpressed in different types of human tumors. TRIM32 overexpression promotes cell oncogenic transformation and tumorigenesis in mice in a largely p53-dependent manner. Taken together, our results demonstrated that as a novel p53 target and a novel negative regulator for p53, TRIM32 has an important role in regulation of p53 and p53-mediated cellular stress responses. Furthermore, our results also revealed that impairing p53 function is a novel mechanism for TRIM32 in tumorigenesis.

Loss of TRIM62 expression is an independent adverse prognostic factor in acute myeloid leukemia

BACKGROUND

Tripartite motif (TRIM)-62 is a putative tumor suppressor gene whose role in leukemia is unknown.

MATERIALS AND METHODS

We evaluated the effect of TRIM62 protein expression in patients with acute myeloid leukemia (AML). We used reverse-phase protein array methodology to determine TRIM62 levels in leukemia-enriched protein samples from 511 patients newly diagnosed with AML.

RESULTS

TRIM62 levels in AML cells were significantly lower than in normal CD34-positive cells, suggesting that TRIM62 loss might be involved in leukemogenesis, but was not associated with specific karyotypic abnormalities or Nucleophosmin (NPM1), Fms-like Tyrosine Kinase-3 (FLT3), or rat sarcoma viral oncogene (RAS) mutational status. Low TRIM62 levels were associated with shorter complete remission duration and significantly shorter event-free and overall survival rates, particularly among patients with intermediate-risk cytogenetics. In that AML subgroup, age and TRIM62 levels were the most powerful independent prognostic factors for survival. TRIM62 protein levels further refined the risk associated with NPM1 and FLT3 mutational status. TRIM62 loss was associated with altered expression of proteins involved in leukemia stem cell homeostasis (β-catenin and Notch), cell motility, and adhesion (integrin-β3, ras-related C3 botulinum toxin substrate [RAC], and fibronectin), hypoxia (Hypoxia-inducible factor 1-alpha [HIF1α], egl-9 family hypoxia-inducible factor 1 [Egln1], and glucose-regulated protein, 78 kDa [GRP78]), and apoptosis (B-cell lymphoma-extra large (BclXL) and caspase 9).

CONCLUSION

Low TRIM62 levels, consistent with a tumor suppressor role, represent an independent adverse prognostic factor in AML.

Trim32 reduces PI3K-Akt-FoxO signaling in muscle atrophy by promoting plakoglobin-PI3K dissociation

By promoting dissociation of the desmosomal component plakoglobin from PI3K, the ubiquitin ligase Trim32 reduces PI3K–Akt–FoxO signaling in normal and atrophying muscle, potentially contributing to insulin resistance and catabolic disorders.

Activation of the PI3K–Akt–FoxO pathway induces cell growth, whereas its inhibition reduces cell survival and, in muscle, causes atrophy. Here, we report a novel mechanism that suppresses PI3K–Akt–FoxO signaling. Although skeletal muscle lacks desmosomes, it contains multiple desmosomal components, including plakoglobin. In normal muscle plakoglobin binds the insulin receptor and PI3K subunit p85 and promotes PI3K–Akt–FoxO signaling. During atrophy, however, its interaction with PI3K–p85 is reduced by the ubiquitin ligase Trim32 (tripartite motif containing protein 32). Inhibition of Trim32 enhanced plakoglobin binding to PI3K–p85 and promoted PI3K–Akt–FoxO signaling. Surprisingly, plakoglobin overexpression alone enhanced PI3K–Akt–FoxO signaling. Furthermore, Trim32 inhibition in normal muscle increased PI3K–Akt–FoxO signaling, enhanced glucose uptake, and induced fiber growth, whereas plakoglobin down-regulation reduced PI3K–Akt–FoxO signaling, decreased glucose uptake, and caused atrophy. Thus, by promoting plakoglobin–PI3K dissociation, Trim32 reduces PI3K–Akt–FoxO signaling in normal and atrophying muscle. This mechanism probably contributes to insulin resistance during fasting and catabolic diseases and perhaps to the myopathies and cardiomyopathies seen with Trim32 and plakoglobin mutations.

Intracellular antibody immunity and the cytosolic Fc receptor TRIM21

Until recently, it was thought that antibody effector mechanisms were mediated purely by Fc receptors expressed on professional cells, following capture of immune complexes in the extracellular space. Recently a new Fc receptor, TRIM21, was discovered that is expressed by cells of all histogenetic lineages and which mediates immune responses intracellularly. This new receptor possesses many unique structural and functional properties. TRIM21 binds both IgG and IgM, interacts primarily with the CH3 rather than CH2 domain and engages two heavy chains simultaneously. This latter property allows TRIM21 to bind antibodies with a higher affinity than any other Fc receptor. TRIM21 is cytosolic, has both effector and signalling functions and is exquisitely conserved in mammals. The discovery of this missing part of humoral immunity has important implications for where and how antibodies work.

Regulatory feedback loop between TP73 and TRIM32

The p73 transcription factor is one of the members of the p53 family of tumor suppressors with unique biological functions in processes like neurogenesis, embryonic development and differentiation. For this reason, p73 activity is tightly regulated by multiple mechanisms, including transcription and post-translational modifications. Here, we identified a novel regulatory loop between TAp73 and the E3 ubiquitin ligase tripartite motif protein 32 (TRIM32). TRIM32, a new direct p73 transcriptional target in the context of neural progenitor cells, is differentially regulated by p73. Although TAp73 binds to the TRIM32 promoter and activates its expression, TAp73-induced TRIM32 expression is efficiently repressed by DNp73. TRIM32 in turn physically interacts with TAp73 and promotes its ubiquitination and degradation, impairing p73-dependent transcriptional activity. This mutual regulation between p73 and TRIM32 constitutes a novel feedback loop, which might have important implications in central nervous system development as well as relevance in oncogenesis, and thus emerges as a possible therapeutic target.

14-3-3 proteins sequester a pool of soluble TRIM32 ubiquitin ligase to repress autoubiquitylation and cytoplasmic body formation

Deregulated expression of tripartite motif-containing protein 32 (TRIM32, an E3 ubiquitin-protein ligase) contributes to various diseases. Here we report, using quantitative proteomics and biochemistry, that 14-3-3 proteins bind to phosphorylated TRIM32 and prevent TRIM32 autoubiquitylation and the formation of TRIM32-containing cytoplasmic bodies, which are potential autoregulatory mechanisms that can reduce the concentration of soluble free TRIM32. The 14-3-3-TRIM32 interaction is dependent on protein-kinase-A-catalyzed phosphorylation of TRIM32 at Ser651. We found that the inhibitory effect of 14-3-3 is, in part, a consequence of disrupting the propensity of TRIM32 to undergo higher-order self-association without affecting its dimerization. Consequently, dimerized TRIM32 bound to 14-3-3 was sequestered in a distinct cytoplasmic pool away from the microtubule network, whereas a TRIM32 mutant that cannot bind 14-3-3 underwent multimerization and was unavailable to facilitate cell growth. Our results reveal a novel connection between ubiquitylation and phosphorylation pathways, which could modulate a variety of cell events by stimulating the formation of the 14-3-3-TRIM32 signaling complex.

Tripartite motif ligases catalyze polyubiquitin chain formation through a cooperative allosteric mechanism

Ligation of polyubiquitin chains to proteins is a fundamental post-translational modification, often resulting in targeted degradation of conjugated proteins. Attachment of polyubiquitin chains requires the activities of an E1 activating enzyme, an E2 carrier protein, and an E3 ligase. The mechanism by which polyubiquitin chains are formed remains largely speculative, especially for RING-based ligases. The tripartite motif (TRIM) superfamily of ligases functions in many cellular processes including innate immunity, cellular localization, development and differentiation, signaling, and cancer progression. The present results show that TRIM ligases catalyze polyubiquitin chain formation in the absence of substrate, the rates of which can be used as a functional readout of enzyme function. Initial rate studies under biochemically defined conditions show that TRIM32 and TRIM25 are specific for the Ubc5 family of E2-conjugating proteins and, along with TRIM5α, exhibit cooperative kinetics with respect to Ubc5 concentration, with submicromolar [S]0.5 and Hill coefficients of 3-5, suggesting they possess multiple binding sites for their cognate E2-ubiquitin thioester. Mutation studies reveal a second, non-canonical binding site encompassing the C-terminal Ubc5α-helix. Polyubiquitin chain formation requires TRIM subunit oligomerization through the conserved coiled-coil domain, but can be partially replaced by fusing the catalytic domain to GST to promote dimerization. Other results suggest that TRIM32 assembles polyubiquitin chains as a Ubc5-linked thioester intermediate. These results represent the first detailed mechanistic study of TRIM ligase activity and provide a functional context for oligomerization observed in the superfamily.

Ubiquitylation by Trim32 causes coupled loss of desmin, Z-bands, and thin filaments in muscle atrophy

During muscle atrophy, myofibrillar proteins are degraded in an ordered process in which MuRF1 catalyzes ubiquitylation of thick filament components (Cohen et al. 2009. J. Cell Biol. http://dx.doi.org/10.1083/jcb.200901052). Here, we show that another ubiquitin ligase, Trim32, ubiquitylates thin filament (actin, tropomyosin, troponins) and Z-band (α-actinin) components and promotes their degradation. Down-regulation of Trim32 during fasting reduced fiber atrophy and the rapid loss of thin filaments. Desmin filaments were proposed to maintain the integrity of thin filaments. Accordingly, we find that the rapid destruction of thin filament proteins upon fasting was accompanied by increased phosphorylation of desmin filaments, which promoted desmin ubiquitylation by Trim32 and degradation. Reducing Trim32 levels prevented the loss of both desmin and thin filament proteins. Furthermore, overexpression of an inhibitor of desmin polymerization induced disassembly of desmin filaments and destruction of thin filament components. Thus, during fasting, desmin phosphorylation increases and enhances Trim32-mediated degradation of the desmin cytoskeleton, which appears to facilitate the breakdown of Z-bands and thin filaments.

The p53 target gene TRIM22 directly or indirectly interacts with the translation initiation factor eIF4E and inhibits the binding of eIF4E to eIF4G

Background information

The interferon (IFN)‐inducible protein TRIM22 (Staf50) is a member of the tripartite motif protein family and has been suggested a role in the regulation of viral replication as well as of protein ubiquitylation. In addition, we have previously shown that TRIM22 is a direct target gene for the tumour suppressor p53. Consistently, over‐expression of TRIM22 inhibits the clonogenic growth of monoblastic U937 cells, suggesting anti‐proliferative or cell death‐inducing effects.

Results

Here, we demonstrate that TRIM22 directly or indirectly interacts with the eukaryotic translation initiation factor (eIF)4E, and inhibits the binding of eIF4E to eIF4G, thus disturbing the assembly of the eIF4F complex, which is necessary for cap‐dependent translation. Furthermore, TRIM22 exerts a repressive effect on luciferase reporter protein levels and to some extent on radiolabelled methionine incorporation. Even though all nuclear mRNAs are capped, some are more dependent on eIF4F than others for translation. The translation of one of these mRNAs, IRF‐7C, was indeed found to be repressed in the presence of TRIM22.

Conclusions

Our data suggest TRIM22 to repress protein translation preferably of some specific mRNAs. Taken together, we show that TRIM22 represses translation by inhibiting the binding of eIF4E to eIF4G, suggesting a mechanism for regulation of protein translation, which may be of importance in response to p53 and/or IFN signalling.

Characterization and biological function analysis of the trim3a gene from zebrafish (Danio rerio)

The biological significance of tripartite motif (TRIM) proteins is increasingly being appreciated due to their roles in a broad range of biological processes that associated with innate immunity. In this study, we have described the structural and functional analysis of TRIM3a from zebrafish. Annotation of domain architectures found that the TRIM3a fulfills the TRIM-NHL rule of domain composition with a Filamin/ABP280 domain and NHL repeats at its C-terminal region. In addition, the mRNA expression level of TRIM3a was the highest in brain, and with a relatively higher level in spleen, liver, and gill. A strong expression starting at 36 h post fertilization (hpf) was observed by real-time PCR and could be detected in brain by in situ hybridization, suggesting that TRIM3a protein might play an important role in brain development in zebrafish. Considering that TRIM3a has a RING finger domain, we expressed and purified the TRIM3a protein and performed ubiquitylation assays, our results showed that TRIM3a underwent self-polyubiquitylation in combination with E1, UbcH5c, biotin-ubiquitin in vitro. Meanwhile, TRIM3a-R without the RING domain was expressed and purified as well, in vitro ubiquitylation assays showed that the self-ubiquitylation of TRIM3a was dependent on its RING domain, suggesting that TRIM3a might function as a RING finger E3 ubiquitin ligase.

Satellite cell senescence underlies myopathy in a mouse model of limb-girdle muscular dystrophy 2H

Mutations in the E3 ubiquitin ligase tripartite motif-containing 32 (TRIM32) are responsible for the disease limb-girdle muscular dystrophy 2H (LGMD2H). Previously, we generated Trim32 knockout mice (Trim32-/- mice) and showed that they display a myopathic phenotype accompanied by neurogenic features. Here, we used these mice to investigate the muscle-specific defects arising from the absence of TRIM32, which underlie the myopathic phenotype. Using 2 models of induced atrophy, we showed that TRIM32 is dispensable for muscle atrophy. Conversely, TRIM32 was necessary for muscle regrowth after atrophy. Furthermore, TRIM32-deficient primary myoblasts underwent premature senescence and impaired myogenesis due to accumulation of PIAS4, an E3 SUMO ligase and TRIM32 substrate that was previously shown to be associated with senescence. Premature senescence of myoblasts was also observed in vivo in an atrophy/regrowth model. Trim32-/- muscles had substantially fewer activated satellite cells, increased PIAS4 levels, and growth failure compared with wild-type muscles. Moreover, Trim32-/- muscles exhibited features of premature sarcopenia, such as selective type II fast fiber atrophy. These results imply that premature senescence of muscle satellite cells is an underlying pathogenic feature of LGMD2H and reveal what we believe to be a new mechanism of muscular dystrophy associated with reductions in available satellite cells and premature sarcopenia.

TRIM proteins in cancer

Some members of the tripartite motif (TRIM/RBCC) protein family are thought to be important regulators of carcinogenesis. This is not surprising as the TRIM proteins are involved in several biological processes, such as cell growth, development and cellular differentiation and alteration of these proteins can affect transcriptional regulation, cell proliferation and apoptosis. In particular, four TRIM family genes are frequently translocated to other genes, generating fusion proteins implicated in cancer initiation and progression. Among these the most famous is the promyelocytic leukaemia gene PML, which encodes the protein TRIM19. PML is involved in the t(15;17) translocation that specifically occurs in Acute Promyelocytic Leukaemia (APL), resulting in a PML-retinoic acid receptor-alpha (PML-RARalpha) fusion protein. Other members of the TRIM family are linked to cancer development without being involved in chromosomal re-arrangements, possibly through ubiquitination or loss of tumour suppression functions. This chapter discusses the biological functions of TRIM proteins in cancer.

TRIM proteins and cancer

Emerging clinical evidence shows that the deregulation of ubiquitin-mediated degradation of oncogene products or tumour suppressors is likely to be involved in the aetiology of carcinomas and leukaemias. Recent studies have indicated that some members of the tripartite motif (TRIM) proteins (one of the subfamilies of the RING type E3 ubiquitin ligases) function as important regulators for carcinogenesis. This Review focuses on TRIM proteins that are involved in tumour development and progression.

The common missense mutation D489N in TRIM32 causing limb girdle muscular dystrophy 2H leads to loss of the mutated protein in knock-in mice resulting in a Trim32-null phenotype

Mutations in tripartite motif protein 32 (TRIM32) are responsible for several hereditary disorders that include limb girdle muscular dystrophy type 2H (LGMD2H), sarcotubular myopathy (STM) and Bardet Biedl syndrome. Most LGMD2H mutations in TRIM32 are clustered in the NHL β-propeller domain at the C-terminus and are predicted to interfere with homodimerization. To get insight into TRIM32's role in the pathogenesis of LGMD2H and to create an accurate model of disease, we have generated a knock-in mouse (T32KI) carrying the c.1465G > A (p.D489N) mutation in murine Trim32 corresponding to the human LGMD2H/STM pathogenic mutation c.1459G > A (p.D487N). Our data indicate that T32KI mice have both a myopathic and a neurogenic phenotype, very similar to the one described in the Trim32-null mice that we created previously. Analysis of Trim32 gene expression in T32KI mice revealed normal mRNA levels, but a severe reduction in mutant TRIM32 (D489N) at the protein level. Our results suggest that the D489N pathogenic mutation destabilizes the protein, leading to its degradation, and results in the same mild myopathic and neurogenic phenotype as that found in Trim32-null mice. Thus, one potential mechanism of LGMD2H might be destabilization of mutated TRIM32 protein leading to a null phenotype.

TRIM32 protein sensitizes cells to tumor necrosis factor (TNFα)-induced apoptosis via its RING domain-dependent E3 ligase activity against X-linked inhibitor of apoptosis (XIAP)

TRIM32, which belongs to the tripartite motif (TRIM) protein family, has the RING finger, B-box, and coiled-coil domain structures common to this protein family, along with an additional NHL domain at the C terminus. TRIM32 reportedly functions as an E3 ligase for actin, a protein inhibitor of activated STAT y (PIASy), dysbindin, and c-Myc, and it has been associated with diseases such as muscular dystrophy and epithelial carcinogenesis. Here, we identify a new substrate of TRIM32 and propose a mechanism through which TRIM32 might regulate apoptosis. Our overexpression and knockdown experiments demonstrate that TRIM32 sensitizes cells to TNFα-induced apoptosis. The RING domain is necessary for this pro-apoptotic function of TRM32 as well as being responsible for its E3 ligase activity. TRIM32 colocalizes and directly interacts with X-linked inhibitor of apoptosis (XIAP), a well known cancer therapeutic target, through its coiled-coil and NHL domains. TRIM32 overexpression enhances XIAP ubiquitination and subsequent proteasome-mediated degradation, whereas TRIM32 knockdown has the opposite effect, indicating that XIAP is a substrate of TRIM32. In vitro reconstitution assay reveals that XIAP is directly ubiquitinated by TRIM32. Our novel results collectively suggest that TRIM32 sensitizes TNFα-induced apoptosis by antagonizing XIAP, an anti-apoptotic downstream effector of TNFα signaling. This function may be associated with TRIM32-mediated tumor suppressive mechanism.

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.

Limb-girdle muscular dystrophy 2H and the role of TRIM32

Limb-girdle muscular dystrophy (LGMD) 2H is a slowly progressive condition characterized by proximal weakness, atrophy, and mildly to moderately raised levels of creatine kinase. Facial weakness, scapular winging, hypertrophied calves, and Achilles tendon contractions are not uncommon and the age of onset ranges between the first and fourth decade. LGMD2H was originally described in the Hutterite population that resides in central Canada and the Dakotas of the USA. LGMD2H was mapped to a specific mutation in the TRIM32 gene and it has subsequently been shown that the same mutation also results in the "sarcotubular myopathy" syndrome, which was described histopathologically. TRIM32 appears to be an E3 ubiquitin ligase, containing the tripartite motif common to this family of proteins (RING finger, B-box, coiled-coil). A few substrates have been identified, including actin and dysbindin. Recent studies have identified additional mutations in the C-terminal region of TRIM32 that result in a dystrophic myopathy. Although TRIM32 appears to be expressed ubiquitously, it is still not clear why certain mutations of TRIM32 would result in a phenotype relatively restricted to skeletal muscle. A mutation in the B-box region of TRIM32 has also been shown to result in a more pleiotropic disorder, Bardet-Biedl Syndrome (BBS11). This disorder is associated with obesity, retinopathy, diabetes, polydactyly, renal abnormalities, learning disability, and hypogenitalism. It is likely that C-terminal mutations in TRIM32 affect the ability of muscle proteins to be degraded by the ubiquitin-proteasome pathway.

Roles of E3 ubiquitin ligases in cell adhesion and migration

Recent studies have demonstrated that a number of E3 ubiquitin ligases, including Cbl, Smurf1, Smurf2, HDM2, BCA2, SCF(beta-TRCP) and XRNF185, play important roles in cell adhesion and migration. Cbl negatively regulates cell adhesion via alpha integrin and Rap1 and inhibits actin polymerization by ubiquitinating mDab1 and WAVE2. Smurf1 regulates cell migration through ubiquitination of RhoA, talin head domain and hPEM2, while Smurf2 ubiquitinates Smurf1, TGFbeta type I receptor and RaplB to modulate cell migration and adhesion. HDM2 negatively regulates cell migration by targeting NFAT (a transcription factor) for ubiquitination and degradation, while SCF(beta-TRCP) ubiquitinates Snail (a transcriptional repressor of E-cadherin) to inhibit cell migration. TRIM32 promotes cell migration through ubiquitination of Abl interactor 2 (Abi2), a tumor suppressor. RNF5 and XRNF185 modulate cell migration by ubiquitinating paxillin. Thus, these E3 ubiquitin ligases regulate cell adhesion and (or) migration through ubiquitination of their specific substrates.

Regulation of the psoriatic chemokine CCL20 by E3 ligases Trim32 and Piasy in keratinocytes

Psoriasis is an inflammatory skin disorder with aberrant regulation of keratinocytes and immunocytes. Although it is well known that uncontrolled keratinocyte proliferation is largely driven by proinflammatory cytokines from the immunocytes, the functional role of keratinocytes in the regulation of immunocytes is poorly understood. Recently, we found that tripartite motif-containing protein 32 (Trim32), an E3-ubiquitin ligase, is elevated in the epidermal lesions of human psoriasis. We previously showed that Trim32 binds to the protein inhibitor of activated STAT-Y (Piasy) and mediates its degradation through ubiquitination. Interestingly, the Piasy gene is localized in the PSORS6 susceptibility locus on chromosome 19p13, and Piasy negatively regulates the activities of several transcription factors, including NF-kappaB, STAT, and SMADs, that are implicated in the pathogenesis of psoriasis. In this study, we show that Trim32 activates, and Piasy inhibits, keratinocyte production of CC chemokine ligand 20 (CCL20), a psoriatic chemokine essential for recruitment of DCs and T helper (Th)17 cells to the skin. Further, Trim32/Piasy regulation of CCL20 is mediated through Piasy interaction with the RelA/p65 subunit of NF-kappaB. As CCL20 is activated by Th17 cytokines, the upregulation of CCL20 production by Trim32 provides a positive feedback loop of CCL20 and Th17 activation in the self-perpetuating cycle of psoriasis.

Lessons from development: A role for asymmetric stem cell division in cancer

Asymmetric stem cell division has emerged as a major regulatory mechanism for physiologic control of stem cell numbers. Reinvigoration of the cancer stem cell theory suggests that tumorigenesis may be regulated by maintaining the balance between asymmetric and symmetric cell division. Therefore, mutations affecting this balance could result in aberrant expansion of stem cells. Although a number of molecules have been implicated in regulation of asymmetric stem cell division, here, we highlight known tumor suppressors with established roles in this process. While a subset of these tumor suppressors were originally defined in developmental contexts, recent investigations reveal they are also lost or mutated in human cancers. Mutations in tumor suppressors involved in asymmetric stem cell division provide mechanisms by which cancer stem cells can hyperproliferate and offer an intriguing new focus for understanding cancer biology. Our discussion of this emerging research area derives insight from a frontier area of basic science and links these discoveries to human tumorigenesis. This highlights an important new focus for understanding the mechanism underlying expansion of cancer stem cells in driving tumorigenesis.

TRIM32 is an E3 ubiquitin ligase for dysbindin

Mutations in the gene encoding tripartite motif protein 32 (TRIM32) cause two seemingly diverse diseases: limb-girdle muscular dystrophy type 2H (LGMD2H) or sarcotubular myopathy (STM) and Bardet–Biedl syndrome type 11(BBS11). Although TRIM32 is involved in protein ubiquitination, its substrates and the molecular consequences of disease-causing mutations are poorly understood. In this paper, we show that TRIM32 is a widely expressed ubiquitin ligase that is localized to the Z-line in skeletal muscle. Using the yeast two-hybrid system, we found that TRIM32 binds and ubiquitinates dysbindin, a protein implicated in the genetic aetiology of schizophrenia, augmenting its degradation. Small-interfering RNA-mediated knock-down of TRIM32 in myoblasts resulted in elevated levels of dysbindin. Importantly, the LGMD2H/STM-associated TRIM32 mutations, D487N and R394H impair ubiquitin ligase activity towards dysbindin and were mislocalized in heterologous cells. These mutants were able to self-associate and also co-immunoprecipitated with wild-type TRIM32 in transfected cells. Furthermore, the D487N mutant could bind to both dysbindin and its E2 enzyme but was defective in monoubiquitination. In contrast, the BBS11 mutant P130S did not show any biochemical differences compared with the wild-type protein. Our data identify TRIM32 as a regulator of dysbindin and demonstrate that the LGMD2H/STM mutations may impair substrate ubiquitination.

Characterization of TRIM31, upregulated in gastric adenocarcinoma, as a novel RBCC protein

To explore the molecules associated with gastric adenocarcinoma, we used the gene expression profile database of various human tissues and identified TRIM31 upregulated in both patients with chronic gastritis and stomach cancer. TRIM31 is a new member of RBCC proteins composed of RING finger, B-box and coiled-coil domains. We characterized TRIM31 biochemically and found it possess properties in common with other RBCC proteins, such as occurrence of alternative splicing transcripts, in vitro autoubiquitylating activity and a tendency to homo-oligomerize. The primary localization site of TRIM31 is the cytoplasm but some fraction is potentially associated with the mitochondria. TRIM31 overexpression suppresses colony formation of HCT116 cells while knockdown of its expression with short interfering RNAs (siRNAs) consistently tends to enhance growth of AsPC-1 cells slightly. Thus, TRIM31 is a characteristic RBCC protein with the ability to regulate cell proliferation negatively and may be a potential biomarker of gastric cancer as it is overexpressed from the early stage of gastric carcinogenesis.

TRIM family proteins and their emerging roles in innate immunity

The superfamily of tripartite motif-containing (TRIM) proteins is conserved throughout the metazoan kingdom and has expanded rapidly during vertebrate evolution; there are now more than 60 TRIM proteins known in humans and mice. Many TRIM proteins are induced by type I and type II interferons, which are crucial for many aspects of resistance to pathogens, and several are known to be required for the restriction of infection by lentiviruses. In this Review, we describe recent data that reveal broader antiviral and antimicrobial activities of TRIM proteins and discuss their involvement in the regulation of pathogen-recognition and transcriptional pathways in host defence.

Tripartite motif protein 32 facilitates cell growth and migration via degradation of Abl-interactor 2

Tripartite motif protein 32 (TRIM32) mRNA has been reported to be highly expressed in human head and neck squamous cell carcinoma, but the involvement of TRIM32 in carcinogenesis has not been fully elucidated. In this study, we found by using yeast two-hybrid screening that TRIM32 binds to Abl-interactor 2 (Abi2), which is known as a tumor suppressor and a cell migration inhibitor, and we showed that TRIM32 mediates the ubiquitination of Abi2. Overexpression of TRIM32 promoted degradation of Abi2, resulting in enhancement of cell growth, transforming activity, and cell motility, whereas a dominant-negative mutant of TRIM32 lacking the RING domain inhibited the degradation of Abi2. In addition, we found that TRIM32 suppresses apoptosis induced by cis-diamminedichloroplatinum (II) in HEp2 cell lines. These findings suggest that TRIM32 is a novel oncogene that promotes tumor growth, metastasis, and resistance to anticancer drugs.

Regulation of the Drosophila lin-41 homologue dappled by let-7 reveals conservation of a regulatory mechanism within the LIN-41 subclade

Drosophila Dappled (DPLD) is a member of the RBCC/TRIM superfamily, a protein family involved in numerous diverse processes such as developmental timing and asymmetric cell divisions. DPLD belongs to the LIN-41 subclade, several members of which are micro RNA (miRNA) regulated. We re-examined the LIN-41 subclade members and their relation to other RBCC/TRIMs and dpld paralogs, and identified a new Drosophila muscle specific RBCC/TRIM: Another B-Box Affiliate, ABBA. In silico predictions of candidate miRNA regulators of dpld identified let-7 as the strongest candidate. Overexpression of dpld led to abnormal eye development, indicating that strict regulation of dpld mRNA levels is crucial for normal eye development. This phenotype was sensitive to let-7 dosage, suggesting let-7 regulation of dpld in the eye disc. A cell-based assay verified let-7 miRNA down-regulation of dpld expression by means of its 3'-untranslated region. Thus, dpld seems also to be miRNA regulated, suggesting that miRNAs represent an ancient mechanism of LIN-41 regulation.

Williams-Beuren syndrome TRIM50 encodes an E3 ubiquitin ligase

Williams-Beuren syndrome (WBS) is a neurodevelopmental and multisystemic disease that results from hemizygosity of approximately 25 genes mapping to chromosomal region 7q11.23. We report here the preliminary description of eight novel genes mapping within the WBS critical region and/or its syntenic mouse region. Three of these genes, TRIM50, TRIM73 and TRIM74, belong to the TRIpartite motif gene family, members of which were shown to be associated to several human genetic diseases. We describe the preliminary functional characterization of these genes and show that Trim50 encodes an E3 ubiquitin ligase, opening the interesting hypothesis that the ubiquitin-mediated proteasome pathway might be involved in the WBS phenotype.

The interferon response inhibits HIV particle production by induction of TRIM22

Treatment of human cells with Type 1 interferons restricts HIV replication. Here we report that the tripartite motif protein TRIM22 is a key mediator. We used transcriptional profiling to identify cellular genes that were induced by interferon treatment and identified TRIM22 as one of the most strongly up-regulated genes. We confirmed, as in previous studies, that TRIM22 over-expression inhibited HIV replication. To assess the role of TRIM22 expressed under natural inducing conditions, we compared the effects of interferon in cells depleted for TRIM22 using RNAi and found that HIV particle release was significantly increased in the knockdown, implying that TRIM22 acts as a natural antiviral effector. Further studies showed that TRIM22 inhibited budding of virus-like particles containing Gag only, indicating that Gag was the target of TRIM22. TRIM22 did not block the release of MLV or EIAV Gag particles. Inhibition was associated with diffuse cytoplasmic staining of HIV Gag rather than accumulation at the plasma membrane, suggesting TRIM22 disrupts proper trafficking. Mutational analyses of TRIM22 showed that the catalytic amino acids Cys15 and Cys18 of the RING domain are required for TRIM22 antiviral activity. These data disclose a pathway by which Type 1 interferons obstruct HIV replication.

Interferons are produced by cells in response to challenge by foreign pathogens such as viruses. The molecular mechanisms by which Type I interferons (e.g., IFNβ) inhibit the replication of HIV-1 are not fully clarified. We identified a gene called TRIM22 that belongs to the tripartite motif (TRIM) family that was strongly induced by IFNβ. Using RNA interference to reduce the expression of TRIM22, we showed that TRIM22 is a key mediator of the IFNβ response when expressed at natural levels. We demonstrate that TRIM22 blocks the intracellular trafficking of the viral structural protein Gag to the surface of the cell, and that the antiviral activity of TRIM22 is dependent on two cysteine residues (Cys15 and Cys18) that are critical for the E3 ligase activity of RING-containing proteins. This report describes a mechanism by which Type I interferons block HIV-1 replication.

Functional interplay between the B-box 2 and the B30.2(SPRY) domains of TRIM5alpha

The retroviral restriction factors, TRIM5alpha and TRIMCyp, consist of RING and B-box 2 domains separated by a coiled coil from carboxy-terminal domains. These carboxy-terminal domains (the B30.2(SPRY) domain in TRIM5alpha and the cyclophilin A domain in TRIMCyp) recognize the retroviral capsid. Here we show that some B-box 2 changes in TRIM5alpha, but not in TRIMCyp, resulted in decreased human immunodeficiency virus (HIV-1) capsid binding. The phenotypic effects of these B-box 2 changes on the restriction of retroviral infection depended on the potency of restriction and the affinity of the TRIM5alpha interaction with the viral capsid, two properties specified by the B30.2(SPRY) domain. Thus, some alterations in the TRIM5alpha B-box 2 domain apparently affect the orientation or conformation of the B30.2(SPRY) domain, influencing capsid recognition.

Novel initiation genes in squamous cell carcinomagenesis: a role for substrate-specific ubiquitylation in the control of cell survival

The study of experimental epidermal carcinogenesis offers several advantages over other epithelial carcinogenesis models, including easy accessibility and a database of research findings spanning over a century. Our studies make use of a clonal in vitro/in vivo keratinocyte carcinogenesis model with low frequency of ras mutation and derivative clonal-initiated lineages with distinct tumor fate. Analysis of this model has yielded candidate genes involved in the stages of initiation and tumorigenic progression, and has revealed novel roles for ubiquitylation in transcriptional control of survival and apoptotic pathways during the early stages of carcinogenesis. The expression of a recently described E3-ubiquitin ligase, Trim32, is elevated during initiation, and ectopic expression of Trim32 confers extended survival in response to terminal differentiation and ultraviolet light (UV) B/TNF-alpha death signals. Trim32 binds and ubiquitylates Piasy, controlling its stability and accumulation. Piasy is a SUMOylation factor involved in the control of apoptosis, senescence, and NF-kappaB activation. NF-kappaB is a survival factor for keratinocytes in response to UV irradiation, the main carcinogenic stimulus for the epidermis. Piasy inhibits NF-kappaB activity, and promotes keratinocyte apoptosis in response to UV and TNF-alpha. In human skin squamous cell carcinoma (SCC) samples, we found an inverse correlation between Trim32 and Piasy expression supporting a role for Trim32-Piasy interaction in human epidermal carcinogenesis. Our hypothesis is that increased expression of Trim32 may enhance epidermal carcinogenesis, by increasing the threshold of NF-kappaB activity through Piasy downmodulation.

The RBCC gene RFP2 (Leu5) encodes a novel transmembrane E3 ubiquitin ligase involved in ERAD

RFP2, a gene frequently lost in various malignancies, encodes a protein with RING finger, B-box, and coiled-coil domains that belongs to the RBCC/TRIM family of proteins. Here we demonstrate that Rfp2 is an unstable protein with auto-polyubiquitination activity in vivo and in vitro, implying that Rfp2 acts as a RING E3 ubiquitin ligase. Consequently, Rfp2 ubiquitin ligase activity is dependent on an intact RING domain, as RING deficient mutants fail to drive polyubiquitination in vitro and are stabilized in vivo. Immunopurification and tandem mass spectrometry enabled the identification of several putative Rfp2 interacting proteins localized to the endoplasmic reticulum (ER), including valosin-containing protein (VCP), a protein indispensable for ER-associated degradation (ERAD). Importantly, we also show that Rfp2 regulates the degradation of the known ER proteolytic substrate CD3-delta, but not the N-end rule substrate Ub-R-YFP (yellow fluorescent protein), establishing Rfp2 as a novel E3 ligase involved in ERAD. Finally, we show that Rfp2 contains a C-terminal transmembrane domain indispensable for its localization to the ER and that Rfp2 colocalizes with several ER-resident proteins as analyzed by high-resolution immunostaining. In summary, these data are all consistent with a function for Rfp2 as an ERAD E3 ubiquitin ligase.

Ins and outs of therapy in limb girdle muscular dystrophies

Muscular dystrophies are hereditary degenerative muscle diseases that cause life-long disability in patients. They comprise the well-known Duchenne Muscular Dystrophy (DMD) but also the group of Limb Girdle Muscular Dystrophies (LGMD) which account for a third to a fourth of DMD cases. From the clinical point of view, LGMD are characterised by predominant effects on the proximal limb muscles. The LGMD group is still growing today and consists of 19 autosomal dominant and recessive forms (LGMD1A to LGMD1G and LGMD2A to LGMD2M). The proteins involved are very diverse and include sarcomeric, sarcolemmal and enzymatic proteins. With respect to this variability and in line with the intense search for a potent therapeutic approach for DMD, many different strategies have been tested in rodent models. These include replacing the lost function by gene transfer or stem cell transplantation, using a related protein for functional substitution, increasing muscle mass, or blocking the molecular pathological mechanisms by pharmacological means to alleviate the symptoms. The purpose of this review is to summarize current data arising from these preclinical studies and to examine the potential of the tested strategies to lead to clinical applications.

TRIM/RBCC, a novel class of 'single protein RING finger' E3 ubiquitin ligases

The TRIM/RBCC proteins are defined by the presence of the tripartite motif composed of a RING domain, one or two B-box motifs and a coiled-coil region. These proteins are involved in a plethora of cellular processes such as apoptosis, cell cycle regulation and viral response. Consistently, their alteration results in many diverse pathological conditions. The highly conserved modular structure of these proteins suggests that a common biochemical function may underlie their assorted cellular roles. Here, we review recent data indicating that some TRIM/RBCC proteins are implicated in ubiquitination and propose that this large protein family represents a novel class of 'single protein RING finger' ubiquitin E3 ligases.

Brain site-specific gene expression analysis in Alzheimer's disease patients

BACKGROUND

Alzheimer's disease (AD) is an age-related neurodegenerative disorder that is characterized by a progressive loss of higher cognitive functions. The brain of an individual with AD exhibits extracellular senile plaques (SPs) of aggregated amyloid-beta peptide (Abeta) and intracellular neurofibrillary tangles (NFTs). Given the critical role of neuronal transport of both proteins and organelles, it is not surprising that perturbation of microtubule-based transport may play a major role in the pathogenesis of AD.

MATERIALS AND METHODS

We used the cDNA subtraction methodology and in vitro neural cell culture analyses to study the meaning of the brain site-specific gene expression pattern in cerebral tissue obtained from AD patients and also from control subjects at autopsy.

RESULTS

We observed that cytoskeleton-associated proteins were down-regulated in AD subjects. We also noted an altered expression of the microtubule-associated protein 1B (MAP1B), the heat-shock protein (HSP)-90 (a key chaperone molecule), the tripartite motif-containing proteins (TRIM)-32/37 (an anti apoptotic enzyme with ubiquitin-protein ligase activity) and the Reticulon-3 (a modulator of the amyloid-precursor-protein (APP) cleavage) in AD brains. Additional molecular- and cell-biological studies revealed that small interfering RNA (siRNA)-mediated down-regulation of MAP1B expression leads to neuronal cell death in vitro.

CONCLUSION

Altered expression of MAP1B, HSP90, TRIM32/37 and Reticulon-3 provides new clues by which the ubiquitin-proteasome-, the protein-chaperon- and the APP-processing systems are disturbed in AD, thus, leading to neuritic amyloid plaques and neurofibrillary tangles.

The interaction of Piasy with Trim32, an E3-ubiquitin ligase mutated in limb-girdle muscular dystrophy type 2H, promotes Piasy degradation and regulates UVB-induced keratinocyte apoptosis through NFkappaB

Protein inhibitors of activated STATs (PIAS) family members are ubiquitin-protein isopeptide ligase-small ubiquitin-like modifier ligases for diverse transcription factors. However, the regulation of PIAS protein activity in cells is poorly understood. Previously, we reported that expression of Trim32, a RING domain ubiquitin-protein isopeptide ligase-ubiquitin ligase mutated in human limb-girdle muscular dystrophy type 2H (LGMD2H) and Bardet-Biedl syndrome, is elevated during mouse skin carcinogenesis, protecting keratinocytes from apoptosis induced by UVB and tumor necrosis factor-alpha (TNFalpha). Here we report that Trim32 interacts with Piasy and promotes Piasy ubiquitination and degradation. Ubiquitination of Piasy by Trim32 could be reproduced in vitro using purified components. Their interaction was induced by treatment with UVB/TNFalpha and involved redistribution of Piasy from the nucleus to the cytoplasm, where it accumulated in cytoplasmic granules that colocalized with Trim32. Piasy destabilization and ubiquitination required an intact RING domain in Trim32. The LGMD2H-associated missense point mutation prevented Trim32 binding to Piasy, and human Piasy failed to colocalize with human Trim32 in fibroblasts isolated from an LGMD2H patient. Trim32 expression increased the transcriptional activity of NFkappaB in epidermal keratinocytes, both under basal treatment and after UVB/TNFalpha treatment. Conversely, Piasy inhibited NFkappaB activity under the same conditions and sensitized keratinocytes to apoptosis induced by TNFalpha and UVB. Our results indicate that, by controlling Piasy stability, Trim32 regulates UVB-induced keratinocyte apoptosis through induction of NFkappaB and suggests loss of function of Trim32 in LGMD2H.

The Sjögren’s Syndrome-Associated Autoantigen Ro52 Is an E3 Ligase That Regulates Proliferation and Cell Death

Patients affected by Sjögren's syndrome and systemic lupus erythematosus (SLE) carry autoantibodies to an intracellular protein denoted Ro52. Although the serologic presence of Ro52 autoantibodies is used clinically for diagnostic purposes, the function of the protein or why it is targeted as an autoantigen in several rheumatic conditions has not been elucidated. In this study, we show that the expression of Ro52 is significantly increased in PBMC of patients with Sjögren's syndrome and SLE, and demonstrate that Ro52 is a RING-dependent E3 ligase involved in ubiquitination. Overexpression of Ro52, but not of Ro52 lacking the RING domain, in a mouse B cell line lead to decreased growth in steady state and increased cell death after activation via the CD40 pathway. The role of Ro52 in activation-mediated cell death was further confirmed as a reduction in Ro52 expression restored cell viability. These findings suggest that the increased expression of the Ro52 autoantigen in patients may be directly involved in the reduced cellular proliferation and increased apoptotic cell death observed in Sjögren's syndrome and SLE, and may thus contribute to the autoantigenic load and induction of autoimmune B and T cell responses observed in rheumatic patients.

Homozygosity mapping with SNP arrays identifies TRIM32, an E3 ubiquitin ligase, as a Bardet-Biedl syndrome gene (BBS11)

The identification of mutations in genes that cause human diseases has largely been accomplished through the use of positional cloning, which relies on linkage mapping. In studies of rare diseases, the resolution of linkage mapping is limited by the number of available meioses and informative marker density. One recent advance is the development of high-density SNP microarrays for genotyping. The SNP arrays overcome low marker informativity by using a large number of markers to achieve greater coverage at finer resolution. We used SNP microarray genotyping for homozygosity mapping in a small consanguineous Israeli Bedouin family with autosomal recessive Bardet-Biedl syndrome (BBS; obesity, pigmentary retinopathy, polydactyly, hypogonadism, renal and cardiac abnormalities, and cognitive impairment) in which previous linkage studies using short tandem repeat polymorphisms failed to identify a disease locus. SNP genotyping revealed a homozygous candidate region. Mutation analysis in the region of homozygosity identified a conserved homozygous missense mutation in the TRIM32 gene, a gene coding for an E3 ubiquitin ligase. Functional analysis of this gene in zebrafish and expression correlation analyses among other BBS genes in an expression quantitative trait loci data set demonstrate that TRIM32 is a BBS gene. This study shows the value of high-density SNP genotyping for homozygosity mapping and the use of expression correlation data for evaluation of candidate genes and identifies the proteasome degradation pathway as a pathway involved in BBS.

The most common mutation in FKRP causing limb girdle muscular dystrophy type 2I (LGMD2I) may have occurred only once and is present in Hutterites and other populations

Limb girdle muscular dystrophy (LGMD) is common in the Hutterite population of North America. We previously identified a mutation in the TRIM32 gene in chromosome region 9q32, causing LGMD2H in approximately two-thirds of the 60 Hutterite LGMD patients studied to date. A genomewide scan was undertaken in five families who did not show linkage to the LGMD2H locus on chromosome 9. A second LGMD locus, LGMD2I, was identified in chromosome region 19q13.3, and the causative mutation was identified as c.826C>A (L276I), a missense mutation in the FKRP gene. A comparison of the clinical characteristics of the two LGMD patient groups in this population reveals some differences. LGMD2I patients generally have an earlier age at diagnosis, a more severe course, and higher serum creatine kinase (CK) levels. In addition, some of these patients show calf hypertrophy, cardiac symptoms, and severe reactions to general anesthesia. None of these features are present among LGMD2H patients. A single common haplotype surrounding the FKRP gene was identified in the Hutterite LGMD2I patients. An identical core haplotype was also identified in 19 other non-Hutterite LGMD2I patients from Europe, Canada, and Brazil. The occurrence of this mutation on a common core haplotype suggests that L276I is a founder mutation that is dispersed among populations of European origin.

Hutterite brothers both affected with two forms of limb girdle muscular dystrophy: LGMD2H and LGMD2I

Limb girdle muscular dystrophy (LGMD) is very common in the Hutterite population of the North American Prairies. We have recently reported the homozygous c.1459G>A mutation in TRIM32 associated with LGMD2H. We have also identified Hutterite patients with LGMD2I, homozygous for the common c.826C>A mutation in FKRP. To date, all Hutterites with LGMD have been shown to be homozygous for either the TRIM32 or FKRP mutation. We now report a Hutterite family in which both parents and five sons were all found to be homozygous for the TRIM32 mutation. The father had slowly progressive proximal muscle weakness, whereas three sons and their mother, all currently asymptomatic, had normal physical examinations. The remaining two sons (7 and 10 years old), presented with mild decrease in stamina, had normal neuromuscular examinations and were found to be homozygous for the FKRP mutation in addition to the TRIM32 mutation. These two boys do not differ in age at or mode of presentation, physical findings, or serum CK levels compared to age-matched individuals affected with LGMD2I alone. This suggests that the effects of these two mutations are not acting synergistically at this time. It remains to be seen whether there will be signs of interaction between these two mutations as the patients get older.

TRIM family proteins: retroviral restriction and antiviral defence

Members of the tripartite motif (TRIM) protein family are involved in various cellular processes, including cell proliferation, differentiation, development, oncogenesis and apoptosis. Some TRIM proteins display antiviral properties, targeting retroviruses in particular. The potential activity of TRIM19, better known as promyelocytic leukaemia protein, against several viruses has been well documented and, recently, TRIM5alpha has been identified as the factor responsible for the previously described Lv1 and Ref1 antiretroviral activities. There is also evidence indicating that other TRIM proteins can influence viral replication. These findings are reviewed here, and the possibility that TRIMs represent a new and widespread class of antiviral proteins involved in innate immunity is also considered.

Melanocyte and keratinocyte carcinogenesis: p53 family protein activities and intersecting mRNA expression profiles

Melanocytes and keratinocytes were analyzed for potential roles of p53, p73, and p63 tumor suppressor family proteins and of malignancy-specific gene expression changes in the etiology of multi-step cancer. Melanocytes expressed deltaNp73alpha, two p63 isoforms and p53. Although p21 and Noxa mRNA levels increased following DNA damage, p53 family member binding to p21 and Noxa DNA probes was undetectable, suggesting p53 family-independent responses. In contrast, keratinocytes expressed multiple isoforms each of p73 and p63 that were induced to bind p21 and Noxa DNA probes after ionizing (IR) or after ultraviolet B (UVB) irradiation, correlating with p21 and Noxa mRNA induction and with apoptosis. Interestingly, IR-resistant malignant melanocytes and keratinocytes both exhibited Noxa mRNA induction after UVB treatment, correlating with DNA binding of p53 family proteins to the Noxa probe only in keratinocytes. To uncover other malignancy-specific events, we queried mouse initiated keratinocyte clones for early changes that were exacerbated in malignant derivatives and also differentially expressed in human advanced melanoma versus normal melanocytes. Using a new method for ranking and normalization of microarray data for 5000 probe sets, 27 upregulated and 13 downregulated genes satisfied our query. Of these, the majority was associated with late-stage human cancers and six were novel genes. Thus, clonal lineage mouse models representing early through late cancer progression stages may inform the focus on early, potentially causal events from microarray studies of human cancers, facilitating prognosis and molecular therapy.

Trim32 is a Ubiquitin Ligase Mutated in Limb Girdle Muscular Dystrophy Type 2H that Binds to Skeletal Muscle Myosin and Ubiquitinates Actin

Trim32 belongs to the tripartite motif (TRIM) protein family, which is characterized by a common domain structure composed of a RING-finger, a B-box, and a coiled-coil motif. In addition to these motifs, Trim32 possesses six C-terminal NHL-domains. A point mutation in one NHL domain (D487N) has been linked to two forms of muscular dystrophy called limb girdle muscular dystrophy type 2H and sarcotubular myopathy. In the present study we demonstrate that Trim32 is an E3 ubiquitin ligase that acts in conjunction with ubiquitin-conjugating enzymes UbcH5a, UbcH5c, and UbcH6. Western blot analysis showed that Trim32 is expressed primarily in skeletal muscle, and revealed its differential expression from one muscle to another. The level of Trim32 expression was elevated significantly in muscle undergoing remodeling due to changes in weight bearing. Furthermore, expression of Trim32 was induced in myogenic differentiation. Thus, variability in Trim32 expression in different skeletal muscles could be due to induction of Trim32 expression upon changes in physiological conditions. We show that Trim32 associates with skeletal muscle thick filaments, interacting directly with the head and neck region of myosin. Our data indicate that myosin is not a substrate of Trim32; however, Trim32 was found to ubiquitinate actin in vitro and to cause a decrease in the level of endogenous actin when transfected into HEK293 cells. In conclusion, our results demonstrate that Trim32 is a ubiquitin ligase that is expressed in skeletal muscle, can be induced upon muscle unloading and reloading, associates with myofibrils and is able to ubiquitinate actin, suggesting its likely participation in myofibrillar protein turnover, especially during muscle adaptation.

Commonality ofTRIM32mutation in causing sarcotubular myopathy and LGMD2H

Sarcotubular myopathy (OMIM 268950) is a rare autosomal recessive myopathy first described in two Hutterite brothers from South Dakota and in two non-Hutterite brothers from Germany. We report that sarcotubular myopathy (STM) is caused by mutation in TRIM32, the gene encoding the tripartite motif-containing protein 32. TRIM32 was found to be the gene mutated in limb girdle muscular dystrophy type 2H (LGMD2H [OMIM 254110]), a disorder that has been confined to the Hutterite population. The TRIM32 mutation found in the STM patients is identical to the causative mutation for LGMD2H (D487N), Haplotype analysis shows that the disease chromosomes share common ancestry.

Expression of proopiomelanocortin, corticotropin-releasing hormone (CRH), and CRH receptor in melanoma cells, nevus cells, and normal human melanocytes

Proopiomelanocortin (POMC) is a 31 kDa prohormone that is processed to various bioactive peptides, including adrenocorticotropin (ACTH), melanotropins (alpha, beta, gamma-MSH), lipotropins, and endorphins. POMC is expressed not only in the pituitary gland but also in a variety of nonpituitary organs and tumors, including melanomas. We previously showed that normal human melanocytes produce and secrete alpha-MSH and ACTH, and furthermore, that advanced melanoma cells generally produce higher amounts of POMC peptides that correlate with tumor progression. To elucidate the mechanism of this upregulation, the expression of genes encoding corticotropin-releasing hormone (CRH) and its receptor, CRH-R, as well as POMC and the MSH receptor (MC1-R), was evaluated by reverse transcriptase-polymerase chain reaction using cultured human melanoma cells, nevus cells, and normal melanocytes. Our results show that all melanocytic cells express CRH, CRH-R, POMC, and MC1-R, with highest intensities in melanoma cells. Furthermore, immunohistochemistry shows that CRH as well as POMC is strongly expressed in advanced melanomas, such as vertically growing lesions of acral lentiginous, nodular and metastatic melanomas, in contrast to negative expression in nevus cells. These results indicate that tumor progression accentuates CRH, CRH-R, and POMC expression by melanoma cells.