Localization and translocation of RhoA protein in the human gastric cancer cell line SGC-7901

Ran promotes membrane targeting and stabilization of RhoA to orchestrate ovarian cancer cell invasion

Ran is a nucleocytoplasmic shuttle protein that is involved in cell cycle regulation, nuclear-cytoplasmic transport, and cell transformation. Ran plays an important role in cancer cell survival and cancer progression. Here, we show that, in addition to the nucleocytoplasmic localization of Ran, this GTPase is specifically associated with the plasma membrane/ruffles of ovarian cancer cells. Ran depletion has a drastic effect on RhoA stability and inhibits RhoA localization to the plasma membrane/ruffles and RhoA activity. We further demonstrate that the DEDDDL domain of Ran is required for the interaction with serine 188 of RhoA, which prevents RhoA degradation by the proteasome pathway. Moreover, the knockdown of Ran leads to a reduction of ovarian cancer cell invasion by impairing RhoA signalling. Our findings provide advanced insights into the mode of action of the Ran-RhoA signalling axis and may represent a potential therapeutic avenue for drug development to prevent ovarian tumour metastasis.

Ran, a nucleus-cytoplasm shuttle protein, is implicated in cancer development and survival. Here, the authors show that Ran binds RhoA to impair its degradation and allow its localisation to the plasma membrane of ovarian cancer cells for tumour invasion.

Post-Translational Modification and Subcellular Distribution of Rac1: An Update

Rac1 is a small GTPase that belongs to the Rho family. The Rho family of small GTPases is a subfamily of the Ras superfamily. The Rho family of GTPases mediate a plethora of cellular effects, including regulation of cytoarchitecture, cell size, cell adhesion, cell polarity, cell motility, proliferation, apoptosis/survival, and membrane trafficking. The cycling of Rac1 between the GTP (guanosine triphosphate)- and GDP (guanosine diphosphate)-bound states is essential for effective signal flow to elicit downstream biological functions. The cycle between inactive and active forms is controlled by three classes of regulatory proteins: Guanine nucleotide exchange factors (GEFs), GTPase-activating proteins (GAPs), and guanine-nucleotide-dissociation inhibitors (GDIs). Other modifications include RNA splicing and microRNAs; various post-translational modifications have also been shown to regulate the activity and function of Rac1. The reported post-translational modifications include lipidation, ubiquitination, phosphorylation, and adenylylation, which have all been shown to play important roles in the regulation of Rac1 and other Rho GTPases. Moreover, the Rac1 activity and function are regulated by its subcellular distribution and translocation. This review focused on the most recent progress in Rac1 research, especially in the area of post-translational modification and subcellular distribution and translocation.

Phosphorylation and Activation of RhoA by ERK in Response to Epidermal Growth Factor Stimulation

The small GTPase RhoA has been implicated in various cellular activities, including the formation of stress fibers, cell motility, and cytokinesis. In addition to the canonical GTPase cycle, recent findings have suggested that phosphorylation further contributes to the tight regulation of Rho GTPases. Indeed, RhoA is phosphorylated on serine 188 (188S) by a number of protein kinases. We have recently reported that Rac1 is phosphorylated on threonine 108 (108T) by extracellular signal-regulated kinases (ERK) in response to epidermal growth factor (EGF) stimulation. Here, we provide evidence that RhoA is phosphorylated by ERK on 88S and 100T in response to EGF stimulation. We show that ERK interacts with RhoA and that this interaction is dependent on the ERK docking site (D-site) at the C-terminus of RhoA. EGF stimulation enhanced the activation of the endogenous RhoA. The phosphomimetic mutant, GFP-RhoA S88E/T100E, when transiently expressed in COS-7 cells, displayed higher GTP-binding than wild type RhoA. Moreover, the expression of GFP-RhoA S88E/T100E increased actin stress fiber formation in COS-7 cells, which is consistent with its higher activity. In contrast to Rac1, phosphorylation of RhoA by ERK does not target RhoA to the nucleus. Finally, we show that regardless of the phosphorylation status of RhoA and Rac1, substitution of the RhoA PBR with the Rac1 PBR targets RhoA to the nucleus and substitution of Rac1 PBR with RhoA PBR significantly reduces the nuclear localization of Rac1. In conclusion, ERK phosphorylates RhoA on 88S and 100T in response to EGF, which upregulates RhoA activity.

Effect of RhoA gene silencing on proliferation and migration of gastric MGC-803 cells

In this study, the expression of silencing RhoA gene in gastric MGC-803 Cells was investigated, in order to discuss the effect of RhoA gene on cell proliferation, cell cycles and tumor migration. SiRNA sequence of RhoA gene was designed and synthesized; MGC-803 cells were transfected by Lipofectamine(TM2000). The expression of RhoA gene in mRNA and protein after interference was detected by RT-PCR and Western blot; flow cytometry was used to detect the cell cycle; cell proliferation was detected by CCK-8 assay and cell migration was detected by scratch healing assay. RhoA expression in mRNA and protein of the experimental group was significantly lower than that of the control group and blank group, and the difference was statistically significant (P < 0.05). The growth rate significantly slowed down in experimental group; the cell cycle was arrested in the G0/G1 phase and the number of cells in S-phase reduced; there was a statistically significant difference (P < 0.05). Scratch healing assay showed that cell migration of the experimental group was significantly decreased, with a statistically significant difference (P < 0.05). Specific interference on RhoA gene expression could inhibit the proliferation and migration of MGC-803 cells; therefore, siRNA sequences of RhoA gene may be an effective target for the treatment of gastric cancer.

The flavonoid isoquercitrin promotes neurite elongation by reducing RhoA activity

BACKGROUND

Neurite formation and synaptic patterning are fundamental to the development of a functional nervous system. Flavonoids are natural molecules known for having beneficial effects on brain health through diverse molecular pathways. Cytoskeletal changes occurring during neuritogenesis and synapse formation often involve Rho GTPases. Here we hypothesized that the flavonoid isoquercitrin promotes neuronal differentiation through Rho signalling.

METHODOLOGY/PRINCIPAL FINDINGS

We performed time lapse imaging of NG108-15 cells during incubation with/without isoquercitrin. Isoquercitrin stimulated extensive neurites enriched in the synaptic vesicle protein synaptotagmin-1. Neurite extension was augmented by the ROCK inhibitor Y-27632 suggesting an inactivation of RhoA/Rho kinase as the mechanism. To test this, we first measured the dose-dependent effect of isoquercitrin on RhoA activity and found a 47% reduction in RhoA activity at concentrations which induced neurites (≥40 µM). Secondly, we tested the ability of isoquercitrin to rescue the neural phenotype in a model of RhoA-induced neurite retraction and found that 40 µM isoquercitrin added to cultures previously treated with the RhoA activator calpeptin produced significantly more neurite length/cell than calpeptin alone. Finally, we tested the hypothesis that isoquercitrin may affect RhoA localization preventing the translocation to the plasma membrane. Unexpectedly, immunolocalization studies showed that RhoA was present in nuclear compartments of control NG108-cells, but underwent translocation to the cytoplasm upon treatment with isoquercitrin. DNA microarrays and reverse transcription - quantitative PCR (RT-qPCR) revealed differences in global gene expression of Rho GTPase family members. These data taken together indicate that isoquercitrin is a potential stimulator of neuronal differentiation, through multiple Rho GTPase mediated mechanisms.

CONCLUSIONS/SIGNIFICANCE

As several members of the Rho GTPase family are implicated in human neurological disorders/injuries, our results suggest that isoquercitrin could be used in the treatment of these pathological states through its effect on this family of molecular switches.

LPS-induced nuclear translocation of RhoA is dependent on NF-κB in the human lung cancer cell line A549

RhoA, an extensively studied member of the Rho GTPase family, has been identified as a mediator of pro-inflammatory responses and aggressive carcinogenesis. Bacterial lipopolysaccharide (LPS) is known to be a potent stimulator of inflammatory cytokine production. LPS is able to alter the activity of RhoA and the subcellular distribution of RhoA is altered according to its activity. In this study, we investigated a possible link between RhoA and the LPS/nuclear factor (NF)-κB signaling pathway. In the present study, western blotting and pull-down and immunofluorescence assays were performed to investigate the activity of RhoA in A549 cells following LPS stimulation. The results showed that LPS was able to activate RhoA. Furthermore, western blotting and an immunofluorescence assay were carried out to investigate the nuclear expression of RhoA in A549 cells following LPS stimulation. The results indicated that LPS triggers the nuclear translocation of RhoA. Furthermore, western blotting, NF-κB small interfering RNA (siRNA) transfection and an immunofluorescence assay were performed to investigate the role of NF-κB in LPS-induced RhoA nuclear translocation in A549 cells. The results showed that LPS-induced RhoA nuclear translocation was inhibited by NF-κB depletion in A549 cells. RhoA and NF-κB siRNA transfection, western blotting and ELISA were carried out to investigate the role of RhoA in the LPS-induced secretion of interleukin (IL)-6 and IL-8 in A549 cells. The depletion of RhoA using RhoA siRNA decreased the LPS-induced secretion of IL-6 and IL-8, similar to the effect of NF-κB depletion. These results demonstrate that LPS is able to activate RhoA and trigger its nuclear translocation, which is dependent on NF-κB, and that RhoA plays a significant role in the LPS/NF-κB signaling pathway.

Requirement of Osteopontin in the migration and protection against Taxol-induced apoptosis via the ATX-LPA axis in SGC7901 cells

Background

Autotaxin (ATX) possesses lysophospholipase D (lyso PLD) activity, which converts lysophosphatidylcholine (LPC) into lysophosphatidic acid (LPA). The ATX-LPA signaling axis has been implicated in angiogenesis, chronic inflammation and tumor progression. Osteopontin (OPN) is an important chemokine involved in the survival, proliferation, migration, invasion and metastasis of gastric cancer cells. The focus of the present study was to investigate the relationship between the ATX-LPA axis and OPN.

Results

In comparison with non-treated cells, we found that the ATX-LPA axis up-regulated OPN expression by 1.92-fold in protein levels and 1.3-fold in mRNA levels. The ATX-LPA axis activates LPA2, Akt, ERK and ELK-1 and also protects SGC7901 cells from apoptosis induced by Taxol treatment.

Conclusions

This study provides the first evidence that expression of OPN induced by ATX-LPA axis is mediated by the activation of Akt and MAPK/ERK pathways through the LPA2 receptor. In addition, OPN is required for the protective effects of ATX-LPA against Taxol-induced apoptosis and ATX-LPA-induced migration of SGC7901 cells.

Engineering Ca2+/calmodulin-mediated modulation of protein translocation by overlapping binding and signaling peptide sequences

Protein translocation is used by cells to regulate protein activity in time and space. Synthetic systems have studied the effect of second messengers and exogenous chemicals on translocation, and have used translocation-based sensors to monitor unrelated pathways such as caspase activity. We have created a synthetic Ca2+-inducible protein using calmodulin binding peptides that selectively reveal nuclear localization and export signals in low Ca2+ (0 microM) and high Ca2+ (10 microM) environments, respectively. Experiments in live cells showed that our construct translocates between the nucleolus and plasma membrane with time constants of approximately 2 h. Further, a single amino acid mutation (Cys20Ala) in our construct prevented translocation to the plasma membrane and instead targeted it the mitochondria as predicted by bioinformatic analysis. Lastly, we studied the effect of cell line, Ca2+ concentration, chemical inhibitors, and cell morphology on translocation and found these conditions affected the rate, extent and direction of translocation. Our work demonstrates the feasibility of engineering Ca2+/calmodulin-mediated modulation of protein translocation and suggests that more natural analogs may exist.

Recent advances in understanding the role of lysophosphatidic acid in the pathogenesis of gastrointestinal tumors

Lysophosphatidic acid is an important intercellular lipid mediator that acts through G-protein coupled receptors to mediate the development and progression of a variety of tumors. In this article, we will summarize the structure and physiological function of lysophosphatidic acid and review its role in the pathogenesis of colon cancer, gastric cancer, liver cancer and other gastrointestinal tumors.

The long journey of actin and actin-associated proteins from genes to polysomes

During gene expression, multiple regulatory steps make sure that alterations of chromatin structure are synchronized with RNA synthesis, co-transcriptional assembly of ribonucleoprotein complexes, transport to the cytoplasm and localized translation. These events are controlled by large multiprotein complexes commonly referred to as molecular machines, which are specialized and at the same time display a highly dynamic protein composition. The crosstalk between these molecular machines is essential for efficient RNA biogenesis. Actin has been recently proposed to be an important factor throughout the entire RNA biogenesis pathway as a component of chromatin remodeling complexes, associated with all eukaryotic RNA polymerases as well as precursor and mature ribonucleoprotein complexes. The aim of this review is to present evidence on the involvement of actin and actin-associated proteins in RNA biogenesis and propose integrative models supporting the view that actin facilitates coordination of the different steps in gene expression.