RhoU and RhoV are members of the Rho family of small GTPases that comprise their own subfamily. RhoUV GTPases are classified as atypical due to the kinetics of their GTP/GDP binding cycles. They also possess unique N- and C-termini that regulate their subcellular localization and activity. RhoU and RhoV have been linked to cytoskeletal regulation, cell adhesion, and cell migration. They each exhibit distinct expression patterns during embryonic development and diseases such as cancer metastasis, suggesting they have specialized functions. In this review, we will discuss the known functions of RhoU and RhoV, with a focus on their roles in early development, organogenesis, and disease.

RhoU is an atypical member of the Rho family of small G-proteins, which has N- and C-terminal extensions compared to the classic Rho GTPases RhoA, Rac1 and Cdc42, and associates with membranes through C-terminal palmitoylation rather than prenylation. RhoU mRNA expression is upregulated in prostate cancer and is considered a marker for disease progression. Here, we show that RhoU overexpression in prostate cancer cells increases cell migration and invasion. To identify RhoU targets that contribute to its function, we found that RhoU homodimerizes in cells. We map the region involved in this interaction to the C-terminal extension and show that C-terminal palmitoylation is required for self-association. Expression of the isolated C-terminal extension reduces RhoU-induced activation of p21-activated kinases (PAKs), which are known downstream targets for RhoU, and induces cell morphological changes consistent with inhibiting RhoU function. Our results show for the first time that the activity of a Rho family member is stimulated by self-association, and this is important for its activity.

BACKGROUND: Numerous studies have revealed that long noncoding RNAs (lncRNAs) are closely related to the development of many diseases and carcinogenesis. However, their specific biological function and molecular mechanism in oesophageal squamous cell carcinoma (ESCC) remains unclear.
METHODS: RNA-Seq was performed to determine the differential expressions of lncRNAs in ESCC, and the level of SNHG16 expression was detected in ESCC and intraepithelial neoplasia (IEN) samples. In vitro and in vivo experiments were performed to explore the role of SNHG16 and the interaction of EIF4A3 and Ras homologue family member U (RhoU) signalling.
RESULTS: One hundred and seventy-five upregulated and 134 downregulated lncRNAs were identified by RNA-Seq. SNHG16 was highly expressed in ESCC and intraepithelial neoplasia (IEN) samples, and its expression level was correlated with tumour differentiation and T stage. Overexpression of SNHG16 can facilitate ESCC cell proliferation and metastasis. Mechanistically, we noticed that SNHG16 could bind RNA binding protein (RBP)-eukaryotic translation initiation factor (EIF4A3) and interact with it to form a complex. Importantly, the coalition of SNHG16 and EIF4A3 ultimately regulated Ras homologue family member U (RhoU). SNHG16 modulated RhoU expression by recruiting EIF4A3 to regulate the stability of RhoU mRNA. Knockdown of RhoU further alleviated the effect of the SNHG16 oncogene in ESCC cells.
CONCLUSIONS: The newly identified SNHG16-EIF4A3-RhoU signalling pathway directly coordinates the response in ESCC pathogenesis and suggests that SNHG16 is a promising target for potential ESCC treatment.

Fatty acid synthase (FASN) is commonly overexpressed in prostate cancer and associated with tumour progression. FASN is responsible for de novo synthesis of the fatty acid palmitate; the building block for protein palmitoylation. A functional role for FASN in regulating cell proliferation is widely accepted. We recently reported that FASN activity can also mediate prostate cancer HGF-mediated cell motility. Moreover, we found that modulation of FASN expression specifically impacts on the palmitoylation of RhoU. Findings we will describe here. We now report that loss of FASN expression also impairs HGF-mediated cell dissociation responses. Taken together our results provide compelling evidence that FASN activity directly promotes cell migration and supports FASN as a potential therapeutic target in metastatic prostate cancer.

Despite much progress toward understanding how epithelial morphogenesis is shaped by intra-epithelial processes including contractility, polarity, and adhesion, much less is known regarding how such cellular processes are coordinated by extra-epithelial signaling. During embryogenesis, the coelomic epithelia on the two sides of the chick embryo undergo symmetrical lengthening and thinning, converging medially to generate and position the dorsal mesentery (DM) in the embryonic midline. We find that Hedgehog signaling, acting through downstream effectors Sec5 (ExoC2), an exocyst complex component, and RhoU (Wrch-1), a small GTPase, regulates coelomic epithelium morphogenesis to guide DM midline positioning. These effects are accompanied by changes in epithelial cell-cell alignment and N-cadherin and laminin distribution, suggesting Hedgehog regulation of cell organization within the coelomic epithelium. These results indicate a role for Hedgehog signaling in regulating epithelial morphology and provide an example of how transcellular signaling can modulate specific cellular processes to shape tissue morphogenesis.

Breast cancer is a heterogeneous disease whose clinical management is very challenging. Although specific molecular features characterize breast cancer subtypes with different prognosis, the identification of specific markers predicting disease outcome within the single subtypes still lags behind. Both the non-canonical Wingless-type MMTV Integration site (WNT) and the Signal Transducer and Activator of Transcription (STAT)3 pathways are often constitutively activated in breast tumors, and both can induce the small GTPase Ras Homolog Family Member U RhoU. Here we show that RhoU transcription can be triggered by both canonical and non-canonical WNT ligands via the activation of c-JUN N-terminal kinase (JNK) and the recruitment of the Specificity Protein 1 (SP1) transcription factor to the RhoU promoter, identifying for the first time SP1 as a JNK-dependent mediator of WNT signaling. RhoU down-regulation by silencing or treatment with JNK, SP1 or STAT3 inhibitors leads to impaired migration and invasion in basal-like MDA-MB-231 and BT-549 cells, suggesting that STAT3 and SP1 can cooperate to induce high RhoU expression and enhance breast cancer cells migration. Moreover, in vivo concomitant binding of STAT3 and SP1 defines a subclass of genes belonging to the non-canonical WNT and the Interleukin (IL)-6/STAT3 pathways and contributing to breast cancer aggressiveness, suggesting the relevance of developing novel targeted therapies combining inhibitors of the STAT3 and WNT pathways or of their downstream mediators.

Rho GTPases play central roles in a wide variety of cellular processes, including cytoskeletal dynamics, cell adhesion and cell polarity. RhoU and RhoV are Rho GTPases that have some atypical properties compared with classical Rho family members, such as the presence of N- and C-terminal extension regions, unusual GDP/GTP cycling and post-translational modification by palmitoylation but not prenylation. Their activity and localization is regulated by the N-terminal and C-terminal regions, and so far no GEFs or GAPs have been identified for them. Similar to Rac and Cdc42, they interact with PAK serine/threonine kinases, and in the case of PAK4, this interaction leads to RhoU protein stabilization. In cells, RhoU and RhoV alter cell shape and cell adhesion, which probably underlies some of the phenotypes reported for these proteins in vivo, for example in heart development and epithelial morphogenesis. However, the molecular basis for these functions of RhoU and RhoV remains to be characterized.

The Rho GTPases were discovered more than 30 years ago, and they were for a long time considered to follow simple cycling between GDP-bound and GTP-bound conformations, as for the Ras subfamily of small GTPases. The Rho GTPases consist of 20 members, but at least 10 of these do not follow this classical GTPase cycle. Thus, based on their kinetic properties, these Rho GTPases can instead be classified as atypical. Some of these atypical Rho GTPases do not hydrolyze GTP, and some have significantly increased intrinsic GDP/GTP exchange activity. This review focuses on this latter category of atypical Rho GTPases, the so-called \'fast-cycling\' Rho GTPases. The different members of these fast-cycling atypical Rho GTPases are described in more detail here, along with their potential regulatory mechanisms. Finally, some insights are provided into the involvement of the atypical Rho GTPases in human pathologies.

This review addresses the developmental roles of 2 GTPases of the Rho family, RhoV/Chp and RhoU/Wrch. These two GTPases form a distinct subfamily related to Rac and Cdc42 proteins and were detected in a screen for Rho members that are particularly expressed in the neural crest, an embryonic tissue peculiar to vertebrates. The neural crest represents a physiological model of normal epithelial to mesenchymal transition (EMT), in which epithelial cells at the border of neural and non-neural ectoderm differentiate, lose their intercellular connections and migrate throughout the embryo. We showed that RhoV, transiently induced by the canonical Wnt pathway, is required for the full differentiation of neural crest cells, while RhoU, induced later by the non-canonical Wnt pathway, is necessary for the migration process. These two GTPases, which are highly conserved across vertebrates, are thus tightly functionally linked to Wnt signaling, whose implication in embryonic development and cancer progression is well established. In the light of the recent literature, we discuss how RhoV and RhoU may achieve their physiological functions.

Bone is a dynamic tissue constantly renewed through a regulated balance between bone formation and resorption. Excessive bone degradation by osteoclasts leads to pathological decreased bone density characteristic of osteolytic diseases such as post-menopausal osteoporosis or bone metastasis. Osteoclasts are multinucleated cells derived from hematopoietic stem cells via a complex differentiation process. Their unique ability to resorb bone is dependent on the formation of the actin-rich sealing zone. Within this adhesion structure, the plasma membrane differentiates into the ruffled border where protons and proteases are secreted to demineralize and degrade bone, respectively. On the bone surface, mature osteoclasts alternate between stationary resorptive and migratory phases. These are associated with profound actin cytoskeleton reorganization, until osteoclasts die of apoptosis. In this review, we highlight the role of Rho GTPases in all the steps of osteoclasts differentiation, function, and death and conclude on their interest as targets for treatment of osteolytic pathologies.