RhoE enhances multidrug resistance of gastric cancer cells by suppressing Bax

CircRNA-ABCB10 promotes gastric cancer progression by sponging miR-1915-3p to upregulate RaC1

BACKGROUND

Gastric cancer (GC) is a common malignant tumor of the digestive system. Increasing reports have demonstrated the crucial roles of circRNAs in tumorigenesis and progression of GC.

METHODS

The relative expression of circ-ABCB10 in GC tissues and cell lines was detected by qRT-PCR. A series of in vitro assays and a xenograft model in vivo were applied to explore the function of circ-ABCB10 in GC cells.

RESULTS

Circ-ABCB10 expression was upregulated in GC tissues and cell lines and positively correlated with poor survival of GC patients. Circ-ABCB10 downregulation decreased cell viability, inhibited cell growth, invasion, and migration, while promoted cell apoptosis of GC cell lines SGC-7901 and MKN-48. Circ-ABCB10 could upregulate Rac1 expression by directly sponging miR-1915-3p. Rescue experiments revealed that miR-1915-3p inhibitor obviously reversed the inhibitory effect of si-circ-ABCB10, and Rac1 overexpression obviously reversed the inhibitory effect of miR-1915-3p mimics on cell growth, invasion, migration, apoptosis, and cell cycle progression. Moreover, si-circ-ABCB10 effectively inhibited tumor growth in a xenograft model.

CONCLUSIONS

Our study revealed that circ-ABCB10 promoted GC progression via targeting the miR-1915-3p/Rac1 axis, and circ-ABCB10 might be a potential target for GC diagnosis and treatment.

Diagnostic, Prognostic, and Therapeutic Value of miR-148b in Human Cancers

MicroRNAs (miRs) is a class of conserved, small, noncoding RNA molecules that modulate gene expression post-transcriptionally. miR-148b is a member of miR- 148/152 family generally known to be a tumor suppressor via its effect on different signaling pathways and regulatory genes. Aberrant expression of miR-148b has recently been shown to be responsible for tumorigenesis of several different cancer types. This review discusses the current evidence regarding the involvement of miR-148b expression in human cancers and its potential clinical importance for tumor diagnosis, prognosis, and therapeutics.

Identification of potential novel drug resistance mechanisms by genomic and transcriptomic profiling of colon cancer cells with p53 deletion

TP53 (p53) is a pivotal player in tumor suppression with fifty percent of all invasive tumors displaying mutations in the TP53 gene. In the present study, we characterized colon cancer cells (HCT116 p53 ^−/−) with TP53 deletion, a sub-line derived from HCT116-p53 ^+/+ cells. RNA sequencing and network analyses were performed to identify novel drug resistance mechanisms. Chromosomal aberrations were identified by multicolor fluorescence in situ hybridization (mFISH) and array comparative genomic hybridization (aCGH). Numerous genes were overexpressed in HCT116 p53 ^−/− cells: RND3/RhoE (235.6-fold up-regulated), DCLK1 (60.2-fold up-regulated), LBH (31.9-fold up-regulated), MYB (28.9-fold up-regulated), TACSTD2 (110.1-fold down-regulated), NRIP1 (81.5-fold down-regulated) and HLA-DMB (69.7-fold down-regulated) are among the identified genes with potential influence on multidrug resistance (MDR) and they are associated with cancer progression and tumorigenesis, according to previously published studies. Probably due to TP53 deletion, disturbances in DNA repair and apoptosis are leading to aberrancies in cellular and organismal organization, ultimately increasing tumorigenesis and cancer progression potential. With NFκB, PI3K and HSP70, being at the center of merged protein network, and TH1-2 pathways, being among the influenced pathways, it can be speculated that the inflammatory pathway contributes to a resistance phenotype together with cell cycle regulation and heat-shock response. HCT116-p53 ^−/− cells have more chromosomal aberrations, gains and losses in copy numbers than HCT116-p53 ^+/+ cells. In conclusion, numerous genomic aberrations, which might be associated with yet unknown drug resistance mechanisms, were identified. This may have important implications for future treatment strategies.

Dysregulation of Rho GTPases in Human Cancers

Rho GTPases play central roles in numerous cellular processes, including cell motility, cell polarity, and cell cycle progression, by regulating actin cytoskeletal dynamics and cell adhesion. Dysregulation of Rho GTPase signaling is observed in a broad range of human cancers, and is associated with cancer development and malignant phenotypes, including metastasis and chemoresistance. Rho GTPase activity is precisely controlled by guanine nucleotide exchange factors, GTPase-activating proteins, and guanine nucleotide dissociation inhibitors. Recent evidence demonstrates that it is also regulated by post-translational modifications, such as phosphorylation, ubiquitination, and sumoylation. Here, we review the current knowledge on the role of Rho GTPases, and the precise mechanisms controlling their activity in the regulation of cancer progression. In addition, we discuss targeting strategies for the development of new drugs to improve cancer therapy.

Rho GTPases: Promising candidates for overcoming chemotherapeutic resistance

Despite therapeutic advances, resistance to chemotherapy remains a major challenge to patients with malignancies. Rho GTPases are essential for the development and progression of various diseases including cancer, and a vast number of studies have linked Rho GTPases to chemoresistance. Therefore, understanding the underlying mechanisms can expound the effects of Rho GTPases towards chemotherapeutic agents, and targeting Rho GTPases is a promising strategy to downregulate the chemo-protective pathways and overcome chemoresistance. Importantly, exceptions in certain biological conditions and interactions among the members of Rho GTPases should be noted. In this review, we focus on the role of Rho GTPases, particularly Rac1, in regulating chemoresistance and provide an overview of their related mechanisms and available inhibitors, which may offer novel options for future targeted cancer therapy.

miR-148b-3p inhibits gastric cancer metastasis by inhibiting the Dock6/Rac1/Cdc42 axis

Background

Our previous work showed that some Rho GTPases, including Rho, Rac1 and Cdc42, play critical roles in gastric cancer (GC); however, how they are regulated in GC remains largely unknown. In this study, we aimed to investigate the roles and molecular mechanisms of Dock6, an atypical Rho guanine nucleotide exchange factor (GEF), in GC metastasis.

Methods

The expression levels of Dock6 and miR-148b-3p in GC tissues and paired nontumor tissues were determined by immunohistochemistry (IHC) and in situ hybridization (ISH), respectively. The correlation between Dock6/miR-148b-3p expression and the overall survival of GC patients was calculated by the Kaplan-Meier method and log-rank test. The roles of Dock6 and miR-148b-3p in GC were investigated by in vitro and in vivo functional studies. Rac1 and Cdc42 activation was investigated by GST pull-down assays. The inhibition of Dock6 transcription by miR-148b-3p was determined by luciferase reporter assays.

Results

A significant increase in Dock6 expression was found in GC tissues compared with nontumor tissues, and its positive expression was associated with lymph node metastasis and a higher TNM stage. Patients with positive Dock6 expression exhibited shorter overall survival periods than patients with negative Dock6 expression. Dock6 promoted GC migration and invasion by increasing the activation of Rac1 and Cdc42. miR-148b-3p expression was negatively correlated with Dock6 expression in GC, and it decreased the motility of GC cells by inhibiting the Dock6/Rac1/Cdc42 axis.

Conclusions

Dock6 was over-expressed in GC tissues, and its positive expression was associated with GC metastasis and indicated poor prognosis of GC patients. Targeting of Dock6 by miR-148b-3p could activate Rac1 and Cdc42, directly affecting the motility of GC cells. Targeting the Dock6-Rac1/Cdc42 axis could serve as a new therapeutic strategy for GC treatment.

Electronic supplementary material

The online version of this article (10.1186/s13046-018-0729-z) contains supplementary material, which is available to authorized users.

Rho GTPases: Regulation and roles in cancer cell biology

Rho GTPases are well known for their roles in regulating cell migration, and also contribute to a variety of other cellular responses. They are subdivided into 2 groups: typical and atypical. The typical Rho family members, including RhoA, Rac1 and Cdc42, cycle between an active GTP-bound and inactive GDP-bound conformation, and are regulated by GEFs, GAPs and GDIs, whereas atypical Rho family members have amino acid substitutions that alter their ability to interact with GTP/GDP and hence are regulated by different mechanisms. Both typical and atypical Rho GTPases contribute to cancer progression. In a few cancers, RhoA or Rac1 are mutated, but in most cancers expression levels and/or activity of Rho GTPases is altered. Rho GTPase signaling could therefore be therapeutically targeted in cancer treatment.

Rnd3 in Cancer: A Review of the Evidence for Tumor Promoter or Suppressor

Rho-GTPases are members of the Ras superfamily of small GTPases and are general modulators of important cellular processes in tumor biology such as migration and proliferation. Among these proteins, Rnd3/RhoE, an atypical Rho-GTPase devoid of GTP hydrolytic activity, has recently been studied for its putative role in tumorigenesis. Indeed, Rnd3 is implicated in processes, such as proliferation and migration, whose deregulation is linked to cancer development and metastasis. The aim of this review is to provide an overview of the data surrounding Rnd3 deregulation in cancers, its origin, and consequences. Presented here is a comprehensive account of the expression status and biological output obtained in prostate, liver, stomach, colon, lung, and brain cancers as well as in melanoma and squamous cell carcinoma. Although there appears to be no general consensus about Rnd3 expression in cancers as this protein is differently altered according to the tumor context, these alterations overwhelmingly favor a protumorigenic role. Thus, depending on the tumor type, it may behave either as a tumor suppressor or as a tumor promoter. Importantly, the deregulation of Rnd3, in most cases, is linked to patient poor outcome.

IMPLICATIONS

Rnd3 has prognostic marker potential as exemplified in lung cancers and Rnd3 or Rnd3-associated signaling pathways may represent a new putative therapeutic target. Mol Cancer Res; 14(11); 1033-44. ©2016 AACR.

The c-Jun/RHOB/AKT pathway confers resistance of BRAF-mutant melanoma cells to MAPK inhibitors

The response of BRAF-mutant melanoma patients to BRAF inhibitors is dramatically impaired by secondary resistances and rapid relapse. So far, the molecular mechanisms driving these resistances are not completely understood. Here, we show that, in BRAF-mutant melanoma cells, inhibition of BRAF or its target MEK induces RHOB expression by a mechanism that depends on the transcription factor c-Jun. In those cells, RHOB deficiency causes hypersensitivity to BRAF and MEK inhibitors-induced apoptosis. Supporting these results, loss of RHOB expression in metastatic melanoma tissues is associated with an increased progression-free survival of BRAF-mutant patients treated with vemurafenib. Following BRAF inhibition, RHOB activates AKT whose inhibition causes hypersensitivity of BRAF-mutant melanoma cells to BRAF inhibitors. In mice, AKT inhibition synergizes with vemurafenib to block tumor growth of BRAF-mutant metastatic melanoma. Our findings reveal that BRAF inhibition activates a c-Jun/RHOB/AKT pathway that promotes tumor cell survival and further support a role of this pathway in the resistance of melanoma to vemurafenib. Our data also highlight the importance of using RHOB tumor levels as a biomarker to predict vemurafenib patient's response and to select those that would benefit of the combination with AKT inhibitors.

Pathophysiological Functions of Rnd3/RhoE

Rnd3, also known as RhoE, belongs to the Rnd subclass of the Rho family of small guanosine triphosphate (GTP)-binding proteins. Rnd proteins are unique due to their inability to switch from a GTP-bound to GDP-bound conformation. Even though studies of the biological function of Rnd3 are far from being concluded, information is available regarding its expression pattern, cellular localization, and its activity, which can be altered depending on the conditions. The compiled data from these studies implies that Rnd3 may not be a traditional small GTPase. The basic role of Rnd3 is to report as an endogenous antagonist of RhoA signaling-mediated actin cytoskeleton dynamics, which specifically contributes to cell migration and neuron polarity. In addition, Rnd3 also plays a critical role in arresting cell cycle distribution, inhibiting cell growth, and inducing apoptosis and differentiation. Increasing data have shown that aberrant Rnd3 expression may be the leading cause of some systemic diseases; particularly highlighted in apoptotic cardiomyopathy, developmental arrhythmogenesis and heart failure, hydrocephalus, as well as tumor metastasis and chemotherapy resistance. Therefore, a better understanding of the function of Rnd3 under different physiological and pathological conditions, through the use of suitable models, would provide a novel insight into the origin and treatment of multiple human diseases.

The Effects and Molecular Mechanisms of MiR-106a in Multidrug Resistance Reversal in Human Glioma U87/DDP and U251/G Cell Lines

Chemotherapy resistance is one of the major obstacles to effective glioma therapy. Currently, the mechanism underlying chemotherapy resistance is unclear. A recent study showed that miR-106a is an important molecule involved in chemotherapy resistance. To explore the effects and mechanisms of miR-106a on multidrug resistance reversal in human glioma cells, we silenced miR-106a expression in the cisplatin-resistant U87 (U87/DDP) and the gefitinib-resistant U251 (U251/G) glioma cell lines and measured the resulting drug sensitivity, cell apoptosis rate and rhodamine 123 content. In addition, we detected decreased expression of P-glycoprotein, MDR1, MRP1, GST-π, CDX2, ERCC1, RhoE, Bcl-2, Survivin and Topo-II, as well as reduced production of IL-6, IL-8 and TGF-β in these cell lines. Furthermore, we found decreased expression of p-AKT and transcriptional activation of NF-κB, Twist, AP-1 and Snail in these cell lines. These results suggest that miR-106a is a promising therapeutic target for the treatment of human multidrug resistant glioma.

Inhibition of Vav3 could reverse the drug resistance of gastric cancer cells by downregulating JNK signaling pathway

This study aims to investigate the effect and mechanism of Vav3 on the multidrug resistance of gastric cancer. Fluorescence quantitative RT-PCR and western blot assay were used to detect Vav3 and drug resistance genes in gastric cancer tissues as well as gastric cell lines such as SGC7901, SGC7901/adriamycin (ADR) and GES-1. Besides, Vav3-specific small interfering RNA (Vav3-siRNA) was applied to inhibit Vav3 in SGC7901/ADR, and SRB assay was used to determine chemosensitivity. After that, drug resistance genes and proteins in MAPK and PI3K/AKT signaling pathway were detected after Vav3-siRNA transfection. The results showed that overexpressed Vav3 was found in gastric cancer tissues and SGC7901 and SGC7901/ADR cells. Activity of SGC7901/ADR cells transfected with Vav3-siRNA combined with 5-fluorouracil/oxaliplatin was much lower than that of control groups, and MDR1/P-gp, GST-π and Bcl-2, Bax genes were significantly downregulated in Vav3-siRNA transfection group. AKT, ERK and p38 total protein and their phosphorylation levels showed no significant change in Vav3-siRNA-transfected SGC7901/ADR cells, whereas the ratio of C-Jun phosphorylation levels to total C-Jun protein was significantly downregulated. The results suggested that Vav3 may play a role in drug resistance of gastric cancer by inhibiting drug resistance genes MDR1/P-gp, GST-π and Bcl-2 through regulating the JNK signaling pathway.

Reversing effect of sorcin in the drug resistance of human nasopharyngeal carcinoma

Nasopharyngeal carcinoma (NPC) is one of the most common cancers originating in the nasopharynx, and chemoresistance is an essential aspect of NPC chemotherapy failure. Sorcin has been implicated in multidrug resistance (MDR) of many types of human tumor. However, the effect and mechanism of Sorcin in MDR of human NPC is not fully clear. In this report, we silenced Sorcin in human NPC CNE2/DDP cells, and explored the role of Sorcin in MDR reversal. The results showed an increased cytotoxicity of cisplatin and intracellular accumulation of Rhodamine-123 and glutathione depletion in Sorcin silencing CNE2/DDP cells. We also found a decreased messenger RNA and protein expression of multidrug resistance gene (MDR1), multidrug resistance-associated protein (MRP1), excision repair cross-complementing gene 1 (ERCC1), glutathione S-transferase-π (GST-n), RhoE, Bcl-2, and Survivin in Sorcin silencing CNE2/DDP cells. The increased expression of PTEN and decreased expression of p-Akt and NF-κB suggested that the key cellular signaling pathways were triggered by Sorcin silencing. We concluded that Sorcin silencing would contribute to establish a potent target point for MDR reversal.

RhoE promotes metastasis in gastric cancer through a mechanism dependent on enhanced expression of CXCR4

RhoE, a novel member of the Rho protein family, is a key regulator of the cytoskeleton and cell migration. Our group has previously shown that RhoE as a direct target for HIF-1α and mediates hypoxia-induced epithelial to mesenchymal transition in gastric cancer cells. Therefore, we assumed that RhoE might play an important role in gastric cancer metastasis. In the present study, we have explored the role of RhoE expression in gastric cancer, cell invasion and metastasis, and the influence of RhoE on regulating the potential expression of down-stream genes. RhoE expression was elevated in gastric cancer tissues as compared with normal gastric tissues. We also found a close correlation between the histological grade and the diagnosis of the patient. Up-regulation of RhoE significantly enhanced the migratory and invasive abilities of gastric cancer cells both in vitro and in vivo. Moreover, down-regulation of RhoE diminished the metastatic potential of cancer cells. PCR array and subsequent transwell assay showed that the regulation of gastric cancer metastasis by RhoE was partially mediated by CXCR4. This observation suggested that CXCR4 might be a downstream effector for RhoE. In summary, our study identified RhoE as a novel prognostic biomarker and metastatic-promoting gene of gastric cancer.

Adenovirus-mediated IL-24 expression enhances the chemosensitivity of multidrug-resistantgastric cancer cells to cisplatin

Chemotherapy is one of the commonly used strategies in gastric cancer, especially for unresectable patients, but it becomes insensitive to repeated administration of even the most effective chemotherapeutic agents, such as cisplatin. Given this, there is an urgent need for developing chemosensitizers to overcome acquired resistance to chemotherapeutic agents. Interleukin-24 (IL-24), a cytokine-tumor suppressor, shows broad-spectrum and tumor-specific antitumor properties, and studies have demonstrated that IL-24 could conspicuously restore the chemosensitivity of MDR cancer cells. Herein, we developed a human MDR gastric cancer cell subline, SGC7901/CDDP, by repeated selection of resistant clones of parental sensitive cells, and further investigated the chemosensitizing effects and the underlying mechanisms of adenovirus-mediated IL-24 (Ad-IL-24) gene therapy plus CDDP for the human MDR gastric cancer cells SGC7901/CDDP in vitro and in vivo. The results demonstrated that the expression of IL-24 mRNA and protein was profoundly downregulated in SGC7901/CDDP cells by RT-PCR and western blot analysis. In addition, the cell viability assay showed that the IC50 of SGC7901/CDDP cells to CDDP, 5-FU, ADM and MTX was significantly enhanced compared to parental sensitive SGC7901 cells. Ad-IL-24-induced IL-24 overexpression decreased the IC50 of the above agents (not MTX), induced G2/M cell cycle arrest, and Ad-IL-24 plus CDDP elicited significant apoptosis and tumor suppression of SGC7901/CDDP cells in vitro and SGC7901/CDDP cell xenograft tumors in vivo, respectively. Moreover, our results demonstrated that the mechanisms of Ad-IL-24-elicited chemosensitizing effects were closely associated with a substantial upregulation of Bax and downregulation of P-gp and Bcl-2 in SGC7901/CDDP cells in vitro and SGC7901/CDDP xenograft tissues in vivo. Thus, this study indicates that overexpression of IL-24 gene can significantly promote chemosensitivity in MDR phenotype SGC7901/CDDP gastric cancer cells.

MicroRNA-200c regulates the sensitivity of chemotherapy of gastric cancer SGC7901/DDP cells by directly targeting RhoE

Gastric cancer remains a worldwide burden as the second leading cause of cancer-related death. Drug resistance of chemotherapy looms as a major clinical obstacle to successful treatment. Recent evidence indicated that miRNA-200c can restore the sensitivity of NSCLC cells to cisplatin and cetuximab. The expression of miRNA-200c and RhoE were investigated in gastric cancer tissues and cells (SGC7901 and SGC7901/DDP) by qRT-PCR. A luciferase reporter assay was done to understand the potential correlation between miRNA-200c and RhoE. Pre-miR-200c was transfected in SGC7901/DDP cells to confirm whether miRNA-200c could regulate RhoE expression. RhoE was knocked down to explore the role of RhoE on sensitivity of chemotherapy in gastric cancer by MTT. Western blot analysis was performed to further explore the mechanism of RhoE in regulating drug resistance. The results showed that miRNA-200c was significantly lower in cancerous tissues than those in the paired normal tissues, whereas the expression of RhoE was just the opposite. The significant difference of miRNA-200c and RhoE were observed between SGC7901 cells and SGC7901/DDP cells. miRNA-200c has target sites in the 3'-UTR of RhoE mRNA by luciferase reporter assay. Transfection of pre-miR-200c reduces RhoE expression. Meanwhile, the knockdown of RhoE enhanced the sensitivity of SGC7901/DDP cells and changed expression of some genes. These suggested that miRNA-200c regulated the sensitivity of chemotherapy to cisplatin (DDP) in gastric cancer by possibly targeting RhoE.

In-depth research of multidrug resistance related cell surface glycoproteome in gastric cancer

Human gastric cancer is a big public health problem. Multidrug resistance is a main obstacle to successful chemotherapeutic treatment in gastric cancers and the underlying mechanism is not clear. Glycosylation, one of the most important post translational modifications of proteins, plays a vital role in diverse aspects of tumor progression. In the present study, we applied two multidrug resistance cell lines and their parental drug sensitive gastric cancer cell line to a modified cell surface capturing strategy with triplex labeling to characterize MDR related cell surface glycoproteome. Finally, 56 cell membrane glycoproteins were successfully identified via combination of identification by glycopeptides and quantitation by non-glycopeptides, and 11 of them were found to be differentially expressed with the same trend in both drug resistant cell lines compared with that in sensitive cell line. The further analysis by western blot and in vitro drug sensitivity assay demonstrated that our approach is reliable and accurate and suggested that these glycoproteins may represent as biomarkers for multidrug resistance in gastric cancer.

BIOLOGICAL SIGNIFICANCE

In this study, we performed a cell surface glycoproteomics research of multidrug resistance in gastric cancer using a modified CSC approach. Totally we identified and quantified 11 membrane N-glycoproteins which were significantly changed in MDR gastric cancer cells. These glycoproteins are quite possible to be biomarkers for predicting MDR or key regulators for targeted therapy, and are also helpful for better interpreting the sophisticated mechanisms of MDR in gastric cancer. In addition to that, this approach used in this study can be well applied to screen aberrantly glycosylated biomarkers associated with other malignant phenotypes of various kinds of cancers.

RhoE functions as a tumor suppressor in esophageal squamous cell carcinoma and modulates the PTEN/PI3K/Akt signaling pathway

Emerging evidence indicates that RhoE as novel member of the Rho GTPases family plays an essential role in carcinogenesis and tumor progression of human various tumors, but the functional significance of RhoE in human esophageal squamous cell carcinoma (ESCC) is still unclear. In the current study, RhoE expression in ESCC tissues and cells was examined, and the biological functions of RhoE in ESCC cells were determined. The results revealed that RhoE expression at mRNA and protein levels was significantly downregulated in ESCC tissues and cell lines (P < 0.05). RhoE expression was tightly associated with differentiation degree, clinical staging, and lymph node metastasis of the patients with ESCC (P < 0.05), but no significant correlations were found between RhoE expression and gender or age of the patients with ESCC (P > 0.05). Additionally, we found that downregulation of RhoE expression in ESCC cells promoted cell proliferation, cell cycle progression, as well as cell invasion in vitro, and inhibited cell apoptosis. Conversely, upregulation of RhoE expression in ESCC cells inhibited cell proliferation, arrested cell cycle at G0/G1 phase, reduced cell invasion, and promoted cell apoptosis. Furthermore, the downregulation of RhoE expression significantly reduced PTEN level and enhanced pAkt level; however, elevation of RhoE expression markedly increased PTEN level and decreased pAkt level. Stepwise investigations demonstrated that overexpression of RhoE in ESCC cells increased the expressions of p27 and bax proteins but decreased the expressions of cyclin D1 and bcl-2 proteins. These data demonstrate that RhoE may play a driving role in the development and progression of ESCC, and targeting the RhoE may be an effective and feasible approach for treatment of ESCC.

miR-138 might reverse multidrug resistance of leukemia cells

Here we firstly investigated the role of miR-138 in multidrug resistance of leukemia cells. miR-138 was found up-regulated in the vincristine-induced multidrug resistance (MDR) leukemia cell line HL-60/VCR as compared with HL-60 cells. Up-regulation of miR-138 could reverse resistance of both P-glycoprotein-related and P-glycoprotein-non-related drugs on HL-60/VCR cells, and promote adriamycin-induced apoptosis, accompanied by increased accumulation and decreased releasing amount of adriamycin. miR-138 could significantly down-regulate the expression of P-glycoprotein, Bcl-2, and the transcription of the multidrug resistance gene 1. Further study of the biological functions of miR-138 might be helpful for developing possible strategies to treat leukemia.

Rho GTPase function in tumorigenesis

Malignant tumor cells display uncontrolled proliferation, loss of epithelial cell polarity, altered interactions with neighboring cells and the surrounding extracellular matrix, and enhanced migratory properties. Proteins of the Rho GTPase family regulate all these processes in cell culture and, for that reason, Rho GTPases, their regulators, and their effectors have been suggested to control tumor formation and progression in humans. However, while the tumor-relevant functions of Rho GTPases are very well documented in vitro, we are only now beginning to assess their contribution to cancer in human patients and in animal models. This review will give a very brief overview of Rho GTPase function in general and then focus on in vivo evidence for a role of Rho GTPases in malignant tumors, both in human patients and in genetically modified mice.