Expression of RhoB in the developing Xenopus laevis embryo

Mechanical Tensions Regulate Gene Expression in the Xenopus laevis Axial Tissues

During gastrulation and neurulation, the chordamesoderm and overlying neuroectoderm of vertebrate embryos converge under the control of a specific genetic programme to the dorsal midline, simultaneously extending along it. However, whether mechanical tensions resulting from these morphogenetic movements play a role in long-range feedback signaling that in turn regulates gene expression in the chordamesoderm and neuroectoderm is unclear. In the present work, by using a model of artificially stretched explants of Xenopus midgastrula embryos and full-transcriptome sequencing, we identified genes with altered expression in response to external mechanical stretching. Importantly, mechanically activated genes appeared to be expressed during normal development in the trunk, i.e., in the stretched region only. By contrast, genes inhibited by mechanical stretching were normally expressed in the anterior neuroectoderm, where mechanical stress is low. These results indicate that mechanical tensions may play the role of a long-range signaling factor that regulates patterning of the embryo, serving as a link coupling morphogenesis and cell differentiation.

The RhoB small GTPase in physiology and disease

RhoB is a Rho family GTPase that is highly similar to RhoA and RhoC, yet has distinct functions in cells. Its unique C-terminal region is subject to specific post-translational modifications that confer different localization and functions to RhoB. Apart from the common role with RhoA and RhoC in actin organization and cell migration, RhoB is also implicated in a variety of other cellular processes including membrane trafficking, cell proliferation, DNA-repair and apoptosis. RhoB is not an essential gene in mice, but it is implicated in several physiological and pathological processes. Its multiple roles will be discussed in this review.

RhoB determines tumor aggressiveness in a murine EGFRL858R-induced adenocarcinoma model and is a potential prognostic biomarker for Lepidic lung cancer

PURPOSE

A crucial event in lung adenocarcinoma progression is the switch from an aerogenous spread toward an infiltrating tumor. Loss of RhoB expression has been suggested to be critical for lung cancer invasion. Here, we tested RhoB expression as a prognostic biomarker in non-small cell lung cancer (NSCLC) with a special focus on lepidic pattern.

EXPERIMENTAL DESIGN

We analyzed RhoB expression using both IHC and RT-qPCR in two series of operated patients (n = 100 and 48, respectively) and in a series of advanced lepidic adenocarcinoma (n = 31) from different hospitals. Next, we examined the role of RhoB in lung cancer progression in transgenic mice that express inducible EGFR(L858R) crossed with Rhob null mice.

RESULTS

We identified that loss of RhoB expression was strongly associated with worse survival (P = 0.0001) and progression-free survival (P < 0.001) in the first series. We then confirmed these results after multivariate analyses of the second series. In the series of adenocarcinoma with lepidic features issued from a clinical trial (IFCT-0401), we showed that loss of RhoB expression was associated with higher aggressiveness of stage IV. Finally, we showed that EGFR(L858R)/Rhob(+/+) mice developed mainly diffuse lung tumors with a lepidic pattern, whereas EGFR(L858R)/Rhob(+/-) and EGFR(L858R)/Rhob(-/-) developed a greater number of tumors, and aggressive adenocarcinomas with invasive properties.

CONCLUSIONS

We showed that RhoB is not only a strong prognostic factor in NSCLC but it is also critical for the acquisition of an aggressive phenotype of adenocarcinoma.

PleiotRHOpic: Rho pathways are essential for all stages of Neural Crest development

Neural Crest (NC) cells are a multipotent migratory stem cell population unique to vertebrates, which contributes extensively to the formation of a wide array of neural and non-neural structures in the embryo. NC cells originate in the ectoderm at the border of the neural tube, undergo an epithelial-mesenchymal transition and acquire outstanding individual and collective migratory properties that allow them to disseminate and differentiate to different parts of the body. This exquisite capacity to switch from an epithelium to motile cells represents both a puzzling biological issue and an attractive model to address the basic mechanisms of cell migration and their alteration during cancer progression. Here we review how signaling pathways controlled by Rho GTPases, key players in cell adhesion, contraction, migration and polarity, contribute to the control the different phases of NC development.

JNK-mediated transcriptional upregulation of RhoB is critical for apoptosis of HCT-116 colon cancer cells by a novel diarylsulfonylurea derivative

Diarylsulfonylureas are potent antitumor agents that have been tested in clinical trials. However, detailed mechanisms of their apoptotic activity remain unclear. Here, we report a new diarylsulfonylurea derivative, LB2A, that upregulates RhoB, thereby inducing potent apoptosis in HCT-116 human colon cancer cells independently of p53 status. LB2A decreased procaspase-3, increased phospho-JNK, and cleaved PARP, leading to apoptosis of HCT-116 cells. Prior treatment of HCT-116 cells with the JNK inhibitor SP600125 and the RNA synthesis inhibitor DRB blocked apoptosis, implying that JNK activation and mRNA production are important for apoptosis by LB2A. Western blotting, RT-PCR, and RhoB-promoter luciferase reporter assays revealed that LB2A increased RhoB via JNK-mediated transcriptional activation. LB2A decreased HDAC1 and increased acetyl-H3, both of which activate the RhoB promoter and were blocked by SP600125. Ectopic expression of RhoB induced apoptosis of HCT-116 cells, suggesting that RhoB is critical for the anti-cancer activity of LB2A in human colon cancer cells. LB2A also exhibited potent tumor growth inhibition of HCT-116 cells in vivo using a mouse xenograft assay. Taken together, these results show that LB2A induces apoptosis of HCT-116 cells via JNK-mediated transcriptional upregulation of RhoB and may therefore provide a potential therapy for human colon cancer.

Activity of the RhoU/Wrch1 GTPase is critical for cranial neural crest cell migration

The neural crest (NC) is a stem cell-like population that arises at the border of neural and non-neural ectoderm. During development, NC undergoes an epithelio-mesenchymal transition (EMT), i.e. loss of epithelial junctions and acquisition of pro-migratory properties, invades the entire embryo and differentiates into a wide diversity of terminal tissues. We have studied the implication of Rho pathways in NC development and previously showed that RhoV is required for cranial neural crest (CNC) cell specification. We show here that the non-canonical Wnt response rhoU/wrch1 gene, closely related to rhoV, is also expressed in CNC cells but at later stages. Using both gain- and loss-of-function experiments, we demonstrate that the level of RhoU expression is critical for CNC cell migration and subsequent differentiation into craniofacial cartilages. In in vitro cultures, RhoU activates pathways that cooperate with PAK1 and Rac1 in epithelial adhesion, cell spreading and directional cell migration. These data support the conclusion that RhoU is an essential regulator of CNC cell migration.

Loss of RhoB expression promotes migration and invasion of human bronchial cells via activation of AKT1

Lung cancer is the leading cause of cancer-related death worldwide, mainly due to its highly metastatic properties. Previously, we reported an inverse correlation between RhoB expression and the progression of the lung cancer, occurring between preinvasive and invasive tumors. Herein, we mimicked the loss of RhoB observed throughout lung oncogenesis with RNA interference in nontumoral bronchial cell lines and analyzed the consequences on both cell transformation and invasion. Down-regulation of RhoB did not modify the cell growth properties but did promote migration and invasiveness. Furthermore, RhoB depletion was accompanied by modifications of actin and cell adhesion. The specific activation of the Akt1 isoform and Rac1 was found to be critical for this RhoB-mediated regulation of migration. Lastly, we showed that RhoB down-regulation consecutive to K-RasV12 cell transformation is critical for cell motility but not for cell proliferation. We propose that RhoB loss during lung cancer progression relates to the acquisition of invasiveness mediated by the phosphatidylinositol 3-kinase (PI3K)/AKT and Rac1 pathways rather than to tumor initiation.

A negative modulatory role for rho and rho-associated kinase signaling in delamination of neural crest cells

Background

Neural crest progenitors arise as epithelial cells and then undergo a process of epithelial to mesenchymal transition that precedes the generation of cellular motility and subsequent migration. We aim at understanding the underlying molecular network. Along this line, possible roles of Rho GTPases that act as molecular switches to control a variety of signal transduction pathways remain virtually unexplored, as are putative interactions between Rho proteins and additional known components of this cascade.

Results

We investigated the role of Rho/Rock signaling in neural crest delamination. Active RhoA and RhoB are expressed in the membrane of epithelial progenitors and are downregulated upon delamination. In vivo loss-of-function of RhoA or RhoB or of overall Rho signaling by C3 transferase enhanced and/or triggered premature crest delamination yet had no effect on cell specification. Consistently, treatment of explanted neural primordia with membrane-permeable C3 or with the Rock inhibitor Y27632 both accelerated and enhanced crest emigration without affecting cell proliferation. These treatments altered neural crest morphology by reducing stress fibers, focal adhesions and downregulating membrane-bound N-cadherin. Reciprocally, activation of endogenous Rho by lysophosphatidic acid inhibited emigration while enhancing the above. Since delamination is triggered by BMP and requires G1/S transition, we examined their relationship with Rho. Blocking Rho/Rock function rescued crest emigration upon treatment with noggin or with the G1/S inhibitor mimosine. In the latter condition, cells emigrated while arrested at G1. Conversely, BMP4 was unable to rescue cell emigration when endogenous Rho activity was enhanced by lysophosphatidic acid.

Conclusion

Rho-GTPases, through Rock, act downstream of BMP and of G1/S transition to negatively regulate crest delamination by modifying cytoskeleton assembly and intercellular adhesion.

The small GTPase RhoV is an essential regulator of neural crest induction in Xenopus

In vertebrates, the Rho family of GTPases is made of 20 members which regulate a variety of cellular functions, including actin cytoskeleton dynamics, cell adhesion and motility, cell growth and survival, gene transcription and membrane trafficking. To get a comprehensive view of Rho implication in physiological epithelial-mesenchymal transition, we carried out an in situ hybridization-based screen to identify Rho members expressed in Xenopus neural crest cells, in which we previously reported RhoB expression at the migrating stage. In the present study, we identify RhoV as an early expressed neural crest marker and provide evidence that its activity is essential for neural crest cell induction. RhoV mRNA is maternally expressed and accumulates shortly after gastrulation in the neural crest forming region. Using antisense morpholino injection, we show that at neurula stages, RhoV depletion impairs expression of the neural crest markers Sox9, Slug or Twist but has no effect on Snail induction. At the tailbud stage, RhoV knockdown causes a dramatic loss of cranial neural crest derived structures. All these defects are rescued by ectopic wild-type RhoV, whose overexpression on its own expands the neural crest territory. Our findings disclose an unprecedented Rho function in pathways that control neural crest cells specification.

Regulation ofXSnail2 expression by Rho GTPases

We analyzed the effects of Rho GTPases on XSnail2 expression during neural crest (NC) ontogeny in Xenopus laevis embryos. The ectopic expression of both dominant-negative (N-) and constitutively active (V-) Rho GTPase mutants after RNA or DNA microinjection disrupted the endogenous expression of XSnail2, XFoxD3, and XSnail1. V14RhoA and N17Rac1 were inhibitory, whereas N19RhoA and V12Rac1 increased NC marker gene expression. In reporter assays using a XSnail2 promoter-green fluorescent protein (GFP) construct (alpha700BA-GFP), the ectopic expression of V14RhoA, N17Rac1, or the Rac1 inhibitor NSC 23766 decreased reporter expression in NC-neural plate, whereas N19RhoA or the RhoA inhibitor Y27632 and V12Rac1 enhanced it. Similarly, transgenic embryos expressing Rho GTPase mutants and GFP under control of the alpha700BA promoter displayed variations similar to those observed for ectopic RNA and DNA expression. These results show that Rho GTPases can regulate the expression of XSnail2 during NC ontogeny.