|Note|| RhoBTB1 is one of the three members of the RhoBTB family in vertebrates. The RhoBTB family was identified during the study of the genes encoding Rho-related proteins in the lower eukaryote Dictyostelium discoideum (Rivero et al., 2002). All three RhoBTB proteins may be implicated in tumorigenesis.|
| || Architecture of RhoBTB proteins. The figure shows the three human (Hs) RhoBTB subfamily members as well as the Drosophila (Dm) and Dictyostelium (Dd) orthologues. The simplified phylogenetic tree on the left illustrates the relationship among the proteins (overall percentage similarity between branches). The different domains are indicated with colours.|
|Description|| RhoBTB1 is 696 amino acids long. All RhoBTB proteins share the same domain architecture: a GTPase domain is followed by a proline-rich region, a tandem of two BTB domains and a C-terminal region (Figure 2). The GTPase domain is Rho-related and contains a Rho insert that is longer than usual, two insertions and one deletion, as well as a few deviations from the GTPase consensus of most Rho GTPases. In analogy to RhoBTB2, the GTPase domain of RhoBTB1 is likely to bind GTP (Manjarrez et al., 2014). |
The proline-rich region links the GTPase to the first BTB domain. This region could act as a SH3 domain-binding site.
The BTB domain (broad complex, tramtract and bric-a-brac) is an evolutionary conserved protein-protein interaction domain that participates in homomeric and heteromeric associations with other BTB domains. The BTB domain was also identified as a component of multimeric cullin3-dependent ubiquitin ligase complexes. The first BTB domain is bipartite, being interrupted by an insertion of unknown function. The BTB domains of RhoBTB allow the formation of homodimers and of heterodimers with other proteins of the RhoBTB family (Berthold et al., 2008).
The C-terminus is a region conserved in all members of the RhoBTB subfamily. It predictably folds as 4 consecutive alpha-helices and one beta-strand and may constitute a RING finger domain (Manjarrez et al., 2014). Many RING finger domains function as ubiquitin ligases. RhoBTB1 does not bear a CAAX motif that is typical for classical Rho GTPases and serves for localization of the protein to membranes.
|Expression|| RHOBTB1 is ubiquitously expressed, with high levels detected in skeletal muscle, placenta, stomach, kidney, testis, ovary, uterus and adrenal gland. The gene is also expressed in foetal tissues (Ramos et al., 2002; Nagase et al., 1998). |
Expression of RHOBTB1 has been found decreased in kidney, breast and stomach tumors in a cancer profiling array (Berthold et al., 2008), in 37% of 46 head and neck squamous cell carcinomas (Beder et al., 2006) and in colon cancer tissues (Xu et al., 2013).
RHOBTB1 is a target of the microRNA MIR31- (Alder et al., 2012; Xu et al., 2013).
|Localisation|| The localisation of endogenous RhoBTB1 has not been investigated. In cells expressing RhoBTB1 ectopically the protein tends to form aggregates in the cytoplasm (Aspenström et al., 2004).|
|Function|| RHOBTB1 has been proposed as a candidate tumour suppressor gene (Beder et al., 2006). The mechanisms by which RhoBTB proteins in general exert this and other roles remain speculative. Much of what we know about RhoBTB2 may be made extensive to RhoBTB1 because of their similarity.|
RhoBTB1 binds to cullin3 and by analogy to RhoBTB2 and RhoBTB3 may constitute ubiquitin ligase complexes. RhoBTB proteins appear to exist in an inactive state through an intramolecular interaction of the BTB domain region with the GTPase domain (Berthold et al., 2008). This model has been refined recently for RhoBTB2 to show that the HSP90AA1 (Hsp90) chaperone machinery unlocks RhoBTB, enabling GTP binding and interaction with Cullin 3 and the COPS8 (COP9) signalosome. COP9 deneddylates Cullin 3 and stabilizes the complex (Manjarrez et al. 2014). Considering the high degree of similarity between RhoBTB2 and RhoBTB1, this mechanism is very likely to apply to RhoBTB1 too.
RHOBTB1 has been identified as a target gene of the nuclear hormone receptor PPARG. RhoBTB1 mRNA and protein levels are decreased in the aorta of mice expressing a dominant negative PPARG. These mice also present a concomitant decrease in Cullin 3, and it has been proposed that RhoBTB1 regulates Cullin 3 levels or activity, which in turn regulates RHOA. turnover in smooth muscle. RhoBTB1 emerges as a component of a signaling mechanism that regulates vascular function and blood pressure (Pelham et al., 2012).
RhoBTB1, like RhoBTB2, is required for expression of the chemokine CXCL14 in keratinocytes independently of Cullin3-mediated protein degradation (McKinnon et al., 2008).
RhoBTB1 displays only a moderate influence on the morphology and actin organisation of porcine aortic endothelial cells upon ectopic expression. It does not interact with the GTPase-binding domain of WASP, PAK1 or RTKN (Rhotekin), which are well-known effectors of many typical Rho GTPases (Aspenstrom et al., 2004).
RhoBTB1, like RhoBTB2 and RhoBTB3, interacts with LLRC41 (leucine rich repeat containing 41, MUF1). MUF-1 is a nuclear protein and carries a BC-box that functions as a linker in multicomponent Cullin 5-dependent ubiquitin ligase complexes (Schenková et al., 2012). MUF1 may be a substrate for RhoBTB-Cullin 3 ubiquitin ligase complexes. The function of MUF1 is unknown, but it is suspected to be involved in the DNA damage response
|Homology|| There are three RhoBTB proteins in vertebrates: RhoBTB1, RhoBTB2 and RhoBTB3 (Figure 2). RhoBTB1 is very similar to RhoBTB2, while RhoBTB3 displays very low similarity to these. Orthologues have been found in amoebae and in insects but they are absent in plants and fungi.|
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|Entity|| Head and neck squamous cell carcinoma|
|Note|| RHOBTB1 resides in a hotspot region with high frequency of allelic loss in HNSCCs. Loss of heterozygosity at the RHOBTB1 locus was identified in 12 of 52 tumour samples. RHOBTB1 showed decreased expression levels in 37% of 46 tumour samples and all informative low-expression samples displayed loss of heterozygosity. Three silent nucleotide changes were found in a mutation analysis (Beder et al., 2006).|
|Oncogenesis|| RhoBTB1, along with RhoBTB2, seems to be required for expression of the chemokine CXCL14 (McKinnon et al., 2008). CXCL14 controls leukocyte migration and angiogenesis and its expression is frequently lost in diverse epithelial tumours, including most HNSCCs.|
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|Entity|| Colon cancer|
|Note|| RhoBTB1 levels were found decreased in colon cancer specimens using immunohistochemistry and Western blot. RHOBTB1 was identified as a target of miR-31 using bioinformatics tools, and the effect of miR-31 was verified experimentally. Downregulation of RHOBTB1 is responsible for the tumor promoting effects of miR-31. Silencing of RHOBTB1 in the colon cancer cell line HT29 increases cell proliferation and promotes cell clonal growth, mimicking the effects of increased miR-31 expression (Xu et al., 2013).|
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|Entity|| Esophageal cancer|
|Note|| Overexpression of miR-31 in esophagus caused by zinc deficiency was found associated to down-regulation of RHOBTB1, which was identified as a target gene of miR-31. Zinc deficiency is a risk factor for the development of esophageal squamous cell carcinoma (Alder et al., 2012).|
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|Entity|| Various cancers including breast, kidney and stomach|
|Note|| Expression of RHOBTB1 was found moderately but significantly decreased in kidney, breast and stomach tumour samples in a cancer profiling array. The decrease affected to 80% of kidney and to 58% of breast cancer samples. The expression changes correlated with those of CUL3 in the same samples (Berthold et al., 2008)|
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| Dysregulation of miR-31 and miR-21 induced by zinc deficiency promotes esophageal cancer.|
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| Rho GTPases have diverse effects on the organization of the actin filament system.|
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| Identification of a candidate tumor supressor gene RHOBTB1 located at a novel allelic loss region 10q21 in head and neck cancer.|
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| Manjarrez JR, Sun L, Prince T, Matts RL.|
| PLoS One 2014; 9: e90054|
| The atypical Rho GTPase RhoBTB2 is required for expression of the chemokine CXCL14 in normal and cancerous epithelial cells.|
| McKinnon CM, Lygoe KA, Skelton L, Mitter R, Mellor H.|
| Oncogene 2008; 27: 6856-6865.|
| Prediction of the coding sequences of unidentified human genes. XI. The complete sequences of 100 new cDNA clones from brain which code for large proteins in vitro.|
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| DNA Res 1999; 5: 277-286.|
| Cullin-3 regulates vascular smooth muscle function and arterial blood pressure via PPAR? and RhoA/Rho-kinase.|
| Pelham CJ, Ketsawatsomkron P, Groh S, Grobe JL, de Lange WJ, Ibeawuchi S-RC, Keen HL, Weatherford ET, Faraci FM, Sigmund CD.|
| Cell Metab 2012; 16: 462-472.|
| Genomic organization and expression profile of the small GTPases of the RhoBTB family in human and mouse.|
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| Gene 2002? 298: 147-157.|
| The Dictyostelium discoideum family of Rho-related proteins.|
| Rivero F, Dislich H, Glöckner G, Noegel AA.|
| Nucleic Acids Res 2001; 29: 1068-1079.|
| MUF1/Leucine-rich repeat containing 41 (LRRC41), a substrate of RhoBTB-dependent Cullin 3 ubiquitin ligase complexes, is a predominantly nuclear dimeric protein.|
| Schenková K, Lutz J, Kopp M, Ramos S, Rivero F.|
| J Mol Biol 2012; 422: 659 - 673.|
| The tumor suppressor gene RhoBTB1 is a novel target of miR-31 in human colon cancer.|
| Xu RS, Wu XD, Zhang SQ, Li CF, Yang L, Li DD, Zhang BG, Zhang Y, Jin JP, Zhang B.|
| Int J Oncol 2013; 42: 676-682.|