The RND1 Small GTPase: Main Functions and Emerging Role in Oncogenesis

Abstract

The Rho GTPase family can be classified into classic and atypical members. Classic members cycle between an inactive Guanosine DiPhosphate -bound state and an active Guanosine TriPhosphate-bound state. Atypical Rho GTPases, such as RND1, are predominantly in an active GTP-bound conformation. The role of classic members in oncogenesis has been the subject of numerous studies, while that of atypical members has been less explored. Besides the roles of RND1 in healthy tissues, recent data suggest that RND1 is involved in oncogenesis and response to cancer therapeutics. Here, we present the current knowledge on RND1 expression, subcellular localization, and functions in healthy tissues. Then, we review data showing that RND1 expression is dysregulated in tumors, the molecular mechanisms involved in this deregulation, and the role of RND1 in oncogenesis. For several aggressive tumors, RND1 presents the features of a tumor suppressor gene. In these tumors, low expression of RND1 is associated with a bad prognosis for the patients. Finally, we highlight that RND1 expression is induced by anticancer agents and modulates their response. Of note, RND1 mRNA levels in tumors could be used as a predictive marker of both patient prognosis and response to anticancer agents.

Keywords: RND; Rho GTPase; migration; oncogenesis; prognosis factor; response to anticancer agents.

Figure 1

Rho GTPase family. The phylogenetic tree of the Rho GTPase family was obtained from the alignment of the amino-acid sequences of the 20 Rho GTPases using the Clustal Omega program. The tree was generated using the iTOL software. Rho members are structured into eight subfamilies, including the Rho subfamily (RHOA, RHOB, and RHOC), the RAC subfamily (RAC1, RAC2, RAC3, and RHOG), the CDC42 subfamily (CDC42, RHOQ and RHOJ), the RND subfamily (RND1, RND2 and RND3), the RHOU/V family (RHOU and RHOV), the RHOD/F subfamily (RHOD and RHOF), the RHOBTB subfamily (RHOBTB1 and RHOBTB2) and RHOH. Classic Rho GTPases comprise the RHO, RAC, CDC42, and RHOD/F subfamilies. Atypical Rho GTPases include the RND, RHOU/V, RHOBTB, and RHOH subfamilies. Rho GTPases, which have a knockout (KO) mouse model, are annotated with a triangle. Percentage of amino-acid-sequences identity between RND1 and other Rho GTPases members were determined with the Clustal Omega tool.

Figure 2

Structure of RND1 gene and mutational pattern in human tumors. The orange box is the lipid raft motif; the yellow boxes are the guanosine nucleotide binding sites; the pink box is Rho insert domain; the blue boxes are the switch domains. SI: Switch I; SII: Switch II. Mutation types and corresponding color codes are as follows: Black represents the missense mutations and green represents the nonsense mutations. The stars indicate the mutations for which the impact on the RND1 function has been studied [10].

Figure 3

Regulation of RND1 protein in healthy and cancer cells. 1: RND1 transcription is regulated by diverse mechanisms. In healthy cells, the transcription factor NFATc1 increases RND1 transcription (marked as “+” in the figure). In cancer cells, the transcription factor Oct4, the lncRNA AGAP2-AS1 and epigenetic mechanisms, including DNA methylation and histone acetylation, act as repressors of RND1 transcription (marked as “−” in the figure) whereas PARP-1 stimulates RND1 transcription (marked as “+” in the figure). 2: RND1 expression is inhibited by the miRNAs miR-199a-5p and miR-603. The 3′UTR of RND1 mRNA has one or more specific binding sites for miR-199a-5p whereas the binding sites for miR-603 have not yet been identified. 3: RND1 protein is stabilized when it associates with its effectors p190RhoGAP and Syx. When RND1 is phosphorylated, it binds to the 14-3-3 protein, which induces the translocation of RND1 from the plasma membrane to the cytoplasm.

Figure 4

RND1 is involved in the formation of nerve connections. (A) Plexin dependent mechanisms. 1: The interaction of RND1 with plexin A1 stimulates its R-Ras GTPase activity, leading to the growth cone collapse. This effect occurs even in the absence of a semaphorin signal. This interaction can be inhibited by RhoD, which binds the plexin to the same binding site as RND1. 2: After stimulation by semaphorin 4D (Sema4D), the interaction of RND1 with plexin B1 stimulates its R-Ras GTPase activity and its binding with PDZ-RhoGEF, resulting in the inhibition of R-Ras and the activation of RhoA, respectively. These mechanisms both contribute to growth cone collapse and neurite retraction. (B) Plexin independent mechanisms. The interaction of RND1 with STI1 inhibits growth cone collapse inhibition induced by the RND1-Plexin A1 association and increases neurite extension. Binding of RND1 with SCG10 induces microtubule depolymerization and axon extension.

Figure 5

RND1 is involved in adhesion and migration during embryogenesis. In Xenopus embryos, RND1 can bind both the FLRT3 transmembrane protein and the netrin receptor, Unc5B. The association between RND1 and FLRT3 decreases the level of C-cadherin present on the cell surface through internalization of C-cadherin into endocytic vesicles in the cytoplasm, resulting in cell adhesion inhibition. The effect of the interaction with Unc5B on the internalization of C-cadherin has not yet been studied. RND1 also allows endo-mesodermal cells to migrate during embryogenesis, which leads to gastrulation.

Figure 6

RND1 limits angiogenesis. Vascular endothelial growth factor (VEGF) activates calcineurin-NFATc1 signaling axis. Nuclear localized NFATc1 binds to the RND1 promoter and stimulates its transcription. The increase of RND1 decreases RHOA activation resulting in the inhibition of angiogenesis.

Figure 7

Correlation between RND1 expression and sensitivity to anticancer agents in hematological and lymphoid tumor cells. Using the CCLE database, correlations between quantitative data were determined as in Table 2. Two-sided p-values of less than 0.05 were considered statistically significant. All statistical analyses were performed using STATA 12.0 software.