The endocytic catalysts, Rab5a and Rab7, are tandem regulators of thyroid hormone production

Perturbation of endoplasmic reticulum proteostasis triggers tissue injury in the thyroid gland

Defects in endoplasmic reticulum (ER) proteostasis have been linked to diseases in multiple organ systems. Here we examined the impact of perturbation of ER proteostasis in mice bearing thyrocyte-specific knockout of either HRD1 (to disable ER-associated protein degradation [ERAD]) or ATG7 (to disable autophagy) in the absence or presence of heterozygous expression of misfolded mutant thyroglobulin (the most highly expressed thyroid gene product, synthesized in the ER). Misfolding-inducing thyroglobulin mutations are common in humans but are said to yield only autosomal-recessive disease - perhaps because misfolded thyroglobulin protein might undergo disposal by ERAD or ER macroautophagy. We find that as single defects, neither ERAD, nor autophagy, nor heterozygous thyroglobulin misfolding altered circulating thyroxine levels, and neither defective ERAD nor defective autophagy caused any gross morphological change in an otherwise WT thyroid gland. However, heterozygous expression of misfolded thyroglobulin itself triggered significant ER stress and individual thyrocyte death while maintaining integrity of the surrounding thyroid epithelium. In this context, deficiency of ERAD (but not autophagy) resulted in patchy whole-follicle death with follicular collapse and degeneration, accompanied by infiltration of bone marrow-derived macrophages. Perturbation of thyrocyte ER proteostasis is thus a risk factor for both cell death and follicular demise.

High RAS-related protein Rab-7a (RAB7A) expression is a poor prognostic factor in pancreatic adenocarcinoma

Pancreatic adenocarcinoma (PAAD) is a frequent type of cancer in adults worldwide, and the search for better biomarkers is one of the current challenges. Although RAB7A is associated with tumour progression in multiple tumour types, there are only a few reports in PAAD. Therefore, in this paper, RNA sequencing data were obtained from TCGA(The Cancer Genome Atlas) and GTEx to analyse RAB7A expression and differentially expressed genes (DEGs) in PAAD. The functional enrichment of RAB7A-associated DEGs was analysed by protein‒protein interaction (PPI) networks, immune cell infiltration analysis and GO/KEGG analyses. Additionally, Kaplan‒Meier and Cox regression analyses were used to determine the clinical significance of RAB7A in PAAD. High RAB7A expression was associated with poor prognosis in 182 PAAD specimens, including subgroups of patients aged ≤ 65 years, with male sex, not receiving radiotherapy, and with a history of previous alcohol consumption (P < 0.05). Cox regression analysis showed that elevated RAB7A was an independent prognostic factor, and the prognostic nomogram model included radiotherapy status, presence of postoperative tumour residual and histologic grade. Overall, RAB7A overexpression may serve as a biomarker for poor outcome in pancreatic cancer. The DEGs and pathways revealed in this work provide a tentative molecular mechanism for the pathogenesis and progression of PAAD.

Linking Late Endosomal Cholesterol with Cancer Progression and Anticancer Drug Resistance

Cancer cells undergo drastic metabolic adaptions to cover increased bioenergetic needs, contributing to resistance to therapies. This includes a higher demand for cholesterol, which often coincides with elevated cholesterol uptake from low-density lipoproteins (LDL) and overexpression of the LDL receptor in many cancers. This implies the need for cancer cells to accommodate an increased delivery of LDL along the endocytic pathway to late endosomes/lysosomes (LE/Lys), providing a rapid and effective distribution of LDL-derived cholesterol from LE/Lys to other organelles for cholesterol to foster cancer growth and spread. LDL-cholesterol exported from LE/Lys is facilitated by Niemann–Pick Type C1/2 (NPC1/2) proteins, members of the steroidogenic acute regulatory-related lipid transfer domain (StARD) and oxysterol-binding protein (OSBP) families. In addition, lysosomal membrane proteins, small Rab GTPases as well as scaffolding proteins, including annexin A6 (AnxA6), contribute to regulating cholesterol egress from LE/Lys. Here, we summarize current knowledge that links upregulated activity and expression of cholesterol transporters and related proteins in LE/Lys with cancer growth, progression and treatment outcomes. Several mechanisms on how cellular distribution of LDL-derived cholesterol from LE/Lys influences cancer cell behavior are reviewed, some of those providing opportunities for treatment strategies to reduce cancer progression and anticancer drug resistance.

Rab7 Is Associated with Poor Prognosis of Gastric Cancer and Promotes Proliferation, Invasion, and Migration of Gastric Cancer Cells

BACKGROUND Rab7 belongs to the Ras oncogene family. Many studies have shown that its dysfunction is associated with many types of malignant tumors, but its effect on the pathogenesis of gastric cancer (GC) is still unknown. Therefore, we investigated the effect and mechanism of Rab7 in GC. MATERIAL AND METHODS The expression of Rab7 in GC and adjacent tissues was detected by immunohistochemistry, Western blot analysis, and qRT-PCR. The relationship of Rab7 with clinicopathological parameters and prognosis was analyzed. The expressions of Rab7, PI3K, and AKT in GC cells were assessed by Western blot. Overexpressed and silenced GC cell lines were constructed and AGS cells were treated with LY294002. The proliferation capacity of GC cells was detected by CCK8 assay, cell cycle changes were detected by flow cytometry, and the invasion and migration abilities of GC cells were assessed by transwell assay. RESULTS The expression of Rab7 was upregulated in the samples and cells, and was positively correlated with lymph node metastasis but negatively correlated with histological differentiation and clinical prognosis. In cell function experiments, overexpression of Rab7 induced the transition from S phase to G2 phase and promoted the proliferation, invasion, and migration of GC cells. Our assessment of the molecular mechanism showed that Rab7 promoted the phosphorylation of PI3K and AKT in GC cells. Incubation with the PI3K inhibitor Ly294002 impaired the enhanced effect of Rab7 overexpression on proliferation, migration, and invasion abilities of GC cells. These results show that the Rab7 affects GC cell progression by modulating the PI3K/AKT pathway. CONCLUSIONS Rab7 could be a prognostic biomarker and therapeutic target of the PI3K/AKT pathway in GC.

Downregulation of Rab17 promotes cell proliferation and invasion in non-small cell lung cancer through STAT3/HIF-1α/VEGF signaling

BACKGROUND

Rab GTPases play a key role in regulating intercellular vesicle trafficking in both exo- and endocytic pathways. Recent studies have reported that Rab small GTPases and the associated regulatory proteins and effectors are involved in many cancers. The purpose of this study was to investigate the biological role of Rab17 in non-small cell lung cancer (NSCLC) and the relative mechanism.

METHODS

Rab17 expression in human NSCLC cell lines and tissues was evaluated using real-time PCR (RT-PCR), western blot and immunohistochemical (IHC) staining. NSCLC cell lines with RAB17 stable knockdown were generated to explore its function in vitro and in vivo. Additionally, we investigated the potential mechanism of Rab17 by identifying the expression levels of STAT3/HIF-1α/VEGF pathway using western blot analysis.

RESULTS

Decreased Rab17 expression was correlated with poor overall survival in NSCLC patients. The functional assays showed that knockdown of Rab17 could promote tumorigenic properties of NSCLC cells in vitro and in vivo, including enhanced cell proliferation, colony formation, invasion and migration, angiogenesis and tumor xenograft growth, and suppressed apoptosis. Moreover, Rab17 downregulation decreased epithelial marker E-cadherin and increased mesenchymal markers Vimentin and β-catenin, suggesting knockdown of Rab17 induced epithelial-mesenchymal transition (EMT).

CONCLUSION

Downregulation of Rab17 promotes cell invasion and enhances tumorigenicity in part through the STAT3/HIF-1α/VEGF pathway, which may represent a novel potential therapeutic target.

Role of the RAB7 Protein in Tumor Progression and Cisplatin Chemoresistance

RAB7 is a small guanosine triphosphatase (GTPase) extensively studied as regulator of vesicular trafficking. Indeed, its role is fundamental in several steps of the late endocytic pathway, including endosome maturation, transport from early endosomes to late endosomes and lysosomes, clustering and fusion of late endosomes and lysosomes in the perinuclear region and lysosomal biogenesis. Besides endocytosis, RAB7 is important for a number of other cellular processes among which, autophagy, apoptosis, signaling, and cell migration. Given the importance of RAB7 in these cellular processes, the interest to study the role of RAB7 in cancer progression is widely grown. Here, we describe the current understanding of oncogenic and oncosuppressor functions of RAB7 analyzing cellular context and other environmental factors in which it elicits pro and/or antitumorigenic effects. We also discuss the role of RAB7 in cisplatin resistance associated with its ability to regulate the late endosomal pathway, lysosomal biogenesis and extracellular vesicle secretion. Finally, we examined the potential cancer therapeutic strategies targeting the different molecular events in which RAB7 is involved.

The role of thyroglobulin in thyroid hormonogenesis

In humans, the thyroid hormones T₃ and T₄ are synthesized in the thyroid gland in a process that crucially involves the iodoglycoprotein thyroglobulin. The overall structure of thyroglobulin is conserved in all vertebrates. Upon thyroglobulin delivery from thyrocytes to the follicular lumen of the thyroid gland via the secretory pathway, multiple tyrosine residues can become iodinated to form mono-iodotyrosine (MIT) and/or di-iodotyrosine (DIT); however, selective tyrosine residues lead to preferential formation of T₄ and T₃ at distinct sites. T₄ formation involves oxidative coupling between two DIT side chains, and de novo T₃ formation involves coupling between an MIT donor and a DIT acceptor. Thyroid hormone synthesis is stimulated by TSH activating its receptor (TSHR), which upregulates the activity of many thyroid gene products involved in hormonogenesis. Additionally, TSH regulates post-translational changes in thyroglobulin that selectively enhance its capacity for T₃ formation - this process is important in iodide deficiency and in Graves disease. 167 different mutations, many of which are newly discovered, are now known to exist in TG (encoding human thyroglobulin) that can lead to defective thyroid hormone synthesis, resulting in congenital hypothyroidism.

Coordination between Rac1 and Rab Proteins: Functional Implications in Health and Disease

The small GTPases of the Rho family regulate many aspects of actin dynamics, but are functionally connected to many other cellular processes. Rac1, a member of this family, besides its known function in the regulation of actin cytoskeleton, plays a key role in the production of reactive oxygen species, in gene transcription, in DNA repair, and also has been proven to have specific roles in neurons. This review focuses on the cooperation between Rac1 and Rab proteins, analyzing how the coordination between these GTPases impact on cells and how alterations of their functions lead to disease.

Disruption of endocytic trafficking protein Rab7 impairs invasiveness of cholangiocarcinoma cells

BACKGROUND

Alterations and mutations of endo-lysosomal trafficking proteins have been associated with cancer progression. Identification and characterization of endo-lysosomal trafficking proteins in invasive cholangiocarcinoma (CCA) cells may benefit prognosis and drug design for CCA.

OBJECTIVE

To identify and characterize endo-lysosomal trafficking proteins in invasive CCA.

METHODS

A lysosomal-enriched fraction was isolated from a TNF-α induced invasive CCA cell line (KKU-100) and uninduced control cells and protein identification was performed with nano-LC MS/MS. Novel lysosomal proteins that were upregulated in invasive CCA cells were validated by real-time RT-PCR. We selected Rab7 for further studies of protein level using western blotting and subcellular localization using immunofluorescence. The role of Rab7 in CCA invasion was determined by siRNA gene knockdown and matrigel transwell assay.

RESULTS

Rab7 mRNA and protein were upregulated in invasive CCA cells compared with non-treated controls. Immunofluorescence studies demonstrated that Rab7 was expressed predominantly in invasive CCA cells and was localized in the cytoplasm and lysosomes. Suppression of Rab7 translation significantly inhibited TNF-α-induced cell invasion compared to non-treated control (p= 0.044).

CONCLUSIONS

Overexpression of Rab7 in CCA cells was associated with cell invasion, supporting Rab7 as a novel candidate for the development of diagnostic and therapeutic strategies for CCA.

Rab25 GTPase: Functional roles in cancer

Rab25, a small GTPase belongs to the Rab protein family, has a pivotal role in cancer pathophysiology. Rab25 governs cell-surface receptors recycling and cellular signaling pathways activation, allowing it to control a diverse range of cellular functions, including cell proliferation, cell motility and cell death. Aberrant expression of Rab25 was linked to cancer development. Majority of research findings revealed that Rab25 is an oncogene. Elevated expression of Rab25 was correlated with poor prognosis and aggressiveness of renal, lung, breast, ovarian and other cancers. However, tumor suppressor function of Rab25 was reported in several cancers, such as colorectal cancer, indicating the tumor type-specific function of Rab25. In this review, we recapitulate the current knowledge of Rab25 in cancer development and therapy.

Rab5-regulated endocytosis plays a crucial role in apical extrusion of transformed cells

Newly emerging transformed cells are often eliminated from epithelial tissues. Recent studies have revealed that this cancer-preventive process involves the interaction with the surrounding normal epithelial cells; however, the molecular mechanisms underlying this phenomenon remain largely unknown. In this study, using mammalian cell culture and zebrafish embryo systems, we have elucidated the functional involvement of endocytosis in the elimination of RasV12-transformed cells. First, we show that Rab5, a crucial regulator of endocytosis, is accumulated in RasV12-transformed cells that are surrounded by normal epithelial cells, which is accompanied by up-regulation of clathrin-dependent endocytosis. Addition of chlorpromazine or coexpression of a dominant-negative mutant of Rab5 suppresses apical extrusion of RasV12 cells from the epithelium. We also show in zebrafish embryos that Rab5 plays an important role in the elimination of transformed cells from the enveloping layer epithelium. In addition, Rab5-mediated endocytosis of E-cadherin is enhanced at the boundary between normal and RasV12 cells. Rab5 functions upstream of epithelial protein lost in neoplasm (EPLIN), which plays a positive role in apical extrusion of RasV12 cells by regulating protein kinase A. Furthermore, we have revealed that epithelial defense against cancer (EDAC) from normal epithelial cells substantially impacts on Rab5 accumulation in the neighboring transformed cells. This report demonstrates that Rab5-mediated endocytosis is a crucial regulator for the competitive interaction between normal and transformed epithelial cells in mammals.

Predominant Rab-GTPase amplicons contributing to oral squamous cell carcinoma progression to metastasis

Metastatic oral squamous cell carcinoma (OSCC) is frequently associated with recurrent gene abnormalities at specific chromosomal loci. Here, we utilized array comparative genomic hybridization and genome-wide screening of metastatic and non-metastatic tongue tumors to investigate genes potentially contributing to OSCC progression to metastasis. We identified predominant amplifications of chromosomal regions that encompass the RAB5, RAB7 and RAB11 genes (3p24-p22, 3q21.3 and 8p11-12, respectively) in metastatic OSCC. The expression of these Rab GTPases was confirmed by immunohistochemistry in OSCC tissues from a cohort of patients with a follow-up of 10 years. A significant overexpression of Rab5, Rab7 and Rab11 was observed in advanced OSCC cases and co-overexpression of these Rabs was predictive of poor survival (log-rank test, P = 0.006). We generated a Rab interaction network and identified central Rab interactions of relevance to metastasis signaling, including focal adhesion proteins. In preclinical models, mRNA and protein expression levels of these Rab members were elevated in a panel of invasive OSCC cell lines, and their down-regulation prevented cell invasion at least in part via inhibition of focal adhesion disassembly. In summary, our results provide insights into the cooperative role of Rab gene amplifications in OSCC progression and support their potential utility as prognostic markers and therapeutic approach for advanced OSCC.

RAB7 counteracts PI3K-driven macropinocytosis activated at early stages of melanoma development

Derailed endolysosomal trafficking is emerging as a widespread feature of aggressive neoplasms. However, the oncogenic signals that alter membrane homeostasis and their specific contribution to cancer progression remain unclear. Understanding the upstream drivers and downstream regulators of aberrant vesicular trafficking is distinctly important in melanoma. This disease is notorious for its inter- and intra-tumoral heterogeneity. Nevertheless, melanomas uniformly overexpress a cluster of endolysosomal genes, being particularly addicted to the membrane traffic regulator RAB7. Still, the underlying mechanisms and temporal determinants of this dependency have yet to be defined. Here we addressed these questions by combining electron microscopy, real time imaging and mechanistic analyses of vesicular trafficking in normal and malignant human melanocytic cells. This strategy revealed Class I PI3K as the key trigger of a hyperactive influx of macropinosomes that melanoma cells counteract via RAB7-mediated lysosomal degradation. In addition, gain- and loss-of-function in vitro studies followed by histopathological validation in clinical biopsies and genetically-engineered mouse models, traced back the requirement of RAB7 to the suppression of premature cellular senescence traits elicited in melanocytes by PI3K-inducing oncogenes. Together, these results provide new insight into the regulators and modes of action of RAB7, broadening the impact of endosomal fitness on melanoma development.

Rab7 Regulates CDH1 Endocytosis, Circular Dorsal Ruffles Genesis, and Thyroglobulin Internalization in a Thyroid Cell Line

Rab7 regulates the biogenesis of late endosomes, lysosomes, and autophagosomes. It has been proposed that a functional and physical interaction exists between Rab7 and Rac1 GTPases in CDH1 endocytosis and ruffled border formation. In FRT cells over-expressing Rab7, increased expression and activity of Rac1 was observed, whereas a reduction of Rab7 expression by RNAi resulted in reduced Rac1 activity, as measured by PAK1 phosphorylation. We found that CDH1 endocytosis was extremely reduced only in Rab7 over-expressing cells but was unchanged in Rab7 silenced cells. In Rab7 under or over-expressing cells, Rab7 and LC3B-II co-localized and co-localization in large circular structures occurred only in Rab7 over-expressing cells. These large circular structures occurred in about 10% of the cell population; some of them (61%) showed co-localization of Rab7 with cortactin and f-actin and were identified as circular dorsal ruffles (CDRs), the others as mature autophagosomes. We propose that the over-expression of Rab7 is sufficient to induce CDRs. Furthermore, in FRT cells, we found that the expression of the insoluble/active form of Rab7, rather than Rab5, or Rab8, was inducible by cAMP and that cAMP-stimulated FRT cells showed increased PAK1 phosphorylation and were no longer able to endocytose CDH1. Finally, we demonstrated that Rab7 over-expressing cells are able to endocytose exogenous thyroglobulin via pinocytosis/CDRs more efficiently than control cells. We propose that the major thyroglobulin endocytosis described in thyroid autonomous adenomas due to Rab7 increased expression, occurs via CDRs. J. Cell. Physiol. 231: 1695-1708, 2016. © 2015 Wiley Periodicals, Inc.

A mouse model suggests two mechanisms for thyroid alterations in infantile cystinosis: decreased thyroglobulin synthesis due to endoplasmic reticulum stress/unfolded protein response and impaired lysosomal processing

Thyroid hormones are released from thyroglobulin (Tg) in lysosomes, which are impaired in infantile/nephropathic cystinosis. Cystinosis is a lysosomal cystine storage disease due to defective cystine exporter, cystinosin. Cystinotic children develop subclinical and then overt hypothyroidism. Why hypothyroidism is the most frequent and earliest endocrine complication of cystinosis is unknown. We here defined early alterations in Ctns(-/-) mice thyroid and identified subcellular and molecular mechanisms. At 9 months, T4 and T3 plasma levels were normal and TSH was moderately increased (∼4-fold). By histology, hyperplasia and hypertrophy of most follicles preceded colloid exhaustion. Increased immunolabeling for thyrocyte proliferation and apoptotic shedding indicated accelerated cell turnover. Electron microscopy revealed endoplasmic reticulum (ER) dilation, apical lamellipodia indicating macropinocytic colloid uptake, and lysosomal cystine crystals. Tg accumulation in dilated ER contrasted with mRNA down-regulation. Increased expression of ER chaperones, glucose-regulated protein of 78 kDa and protein disulfide isomerase, associated with alternative X-box binding protein-1 splicing, revealed unfolded protein response (UPR) activation by ER stress. Decreased Tg mRNA and ER stress suggested reduced Tg synthesis. Coordinated increase of UPR markers, activating transcription factor-4 and C/EBP homologous protein, linked ER stress to apoptosis. Hormonogenic cathepsins were not altered, but lysosome-associated membrane protein-1 immunolabeling disclosed enlarged vesicles containing iodo-Tg and impaired lysosomal fusion. Isopycnic fractionation showed iodo-Tg accumulation in denser lysosomes, suggesting defective lysosomal processing and hormone release. In conclusion, Ctns(-/-) mice showed the following alterations: 1) compensated primary hypothyroidism and accelerated thyrocyte turnover; 2) impaired Tg production linked to ER stress/UPR response; and 3) altered endolysosomal trafficking and iodo-Tg processing. The Ctns(-/-) thyroid is useful to study disease progression and evaluate novel therapies.

Prognostic impact of expression and methylation status of DENN/MADD domain-containing protein 2D in gastric cancer

BACKGROUND

Patients with advanced gastric cancer (GC) have an adverse prognosis even after curative resection. Development of novel diagnostic and therapeutic approaches for GC is urgently required.

METHODS

The expression and methylation status of DENN/MADD domain-containing protein 2D (DENND2D), a member of the membrane trafficking proteins, were evaluated in 12 GC cell lines and 112 pairs of surgical specimens. Subgroup analysis based on tumor differentiation, location, and morphology was also performed. Expression and distribution of DENND2D protein were determined by immunohistochemistry.

RESULTS

The majority of GC cell lines (75%) and tissues (79%) showed reduced expression of DENND2D mRNA compared with noncancerous gastric tissues. GC tissues showed a significantly lower mean expression level of mRNA and a higher frequency of promoter hypermethylation of DENND2D than corresponding noncancerous tissues. No significant differences in DENND2D mRNA expression and methylation status were found between GC subtypes categorized by tumor differentiation, location, and morphology. The expression patterns of DENND2D protein were confirmed to be consistent with those of DENND2D mRNA. Downregulation of DENND2D mRNA in GC tissues was significantly associated with factors related to more advanced GC and subsequent adverse prognosis. Among 72 patients who underwent R0 resection, downregulation of DENND2D mRNA in GC tissues was an independent prognostic factor and associated with early recurrence.

CONCLUSIONS

Our results suggested that DENND2D is a putative tumor suppressor gene regulated by promoter hypermethylation in GC. Downregulation of DENND2D can serve as a novel tumor biomarker to predict progression and early recurrence of all types of GC.

Rab11 in disease progression

Membrane/protein trafficking in the secretory/biosynthetic and endocytic pathways is mediated by vesicles. Vesicle trafficking in eukaryotes is regulated by a class of small monomeric GTPases: the Rab protein family. Rab proteins represent the largest branch of the Ras superfamily GTPases, and have been concerned in a variety of intracellular vesicle trafficking and different intracellular signalling pathways. Rab11 (a subfamily of the Ypt/Rab gene family), an evolutionarily conserved ubiquitously expressed subfamily of Rab GTPases, has been implicated in regulating vesicular trafficking through the recycling of endosomes. Rabs have been grouped into different subfamilies based on the distinct unambiguous sequence motifs. Three members: Rab11a, Rab11b and Rab25 make up the Rab11 GTPase subfamily. In this review article, we describe an overview over Rab11 subfamily with a brief structural aspect and its roles in implicating different disease progression.

The role of the small GTPase Rab31 in cancer

Members of the small GTPase family Rab are emerging as potentially important factors in cancer development and progression. A good number of Rabs have been implicated or associated with various human cancers, and much recent excitement has been associated with the roles of the Rab11 subfamily member Rab25 and its effector, the Rab coupling protein (RCP), in tumourigenesis and metastasis. In this review, we focus on a Rab5 subfamily member, Rab31, and its implicated role in cancer. Well recognized as a breast cancer marker with good prognostic value, recent findings have provided some insights as to the mechanism underlying Rab31's influence on oncogenesis. Levels of Oestrogen Receptor α (ERα)- responsive Rab31 could be elevated through stabilization of its transcript by the RNA binding protein HuR, or though activation by the oncoprotein mucin1-C (MUC1-C), which forms a transcriptional complex with ERα. Elevated Rab31 stabilizes MUC1-C levels in an auto-inductive loop that could lead to aberrant signalling and gene expression associated with cancer progression. Rab31 and its guanine nucleotide exchange factor GAPex-5 have, however, also been shown to enhance early endosome-late endosome transport and degradation of the epidermal growth factor receptor (EGFR). The multifaceted action and influences of Rab31 in cancer is discussed in the light of its new interacting partners and pathways.

Rab proteins: the key regulators of intracellular vesicle transport

Vesicular/membrane trafficking essentially regulates the compartmentalization and abundance of proteins within the cells and contributes in many signalling pathways. This membrane transport in eukaryotic cells is a complex process regulated by a large and diverse array of proteins. A large group of monomeric small GTPases; the Rabs are essential components of this membrane trafficking route. Most of the Rabs are ubiquitously expressed proteins and have been implicated in vesicle formation, vesicle motility/delivery along cytoskeleton elements and docking/fusion at target membranes through the recruitment of effectors. Functional impairments of Rabs affecting transport pathways manifest different diseases. Rab functions are accompanied by cyclical activation and inactivation of GTP-bound and GDP-bound forms between the cytosol and membranes which is regulated by upstream regulators. Rab proteins are characterized by their distinct sub-cellular localization and regulate a wide variety of endocytic, transcytic and exocytic transport pathways. Mutations of Rabs affect cell growth, motility and other biological processes.

Supporting a role for the GTPase Rab7 in prostate cancer progression

Invasion and subsequent metastasis is the major cause of death from most cancers including prostate cancer. Herein we report on the potential tumor suppressive properties of Rab7, a GTPase that regulates trafficking of lysosomes. The movement of lysosomes to the cell surface in response to environmental cues increases the secretion of proteinases and cell invasion. We determined that Troglitazone and other members of the Thiazolidinedione family inhibit cell-surface directed lysosome trafficking and cathepsin B secretion through a Rab7-dependent mechanism. Moreover, Rab7 shRNA expressing cells were found to be more invasive in vitro and in vivo. Increased invasiveness was accompanied by elevated expression of the c-Met receptor and prolonged downstream signaling, thereby supporting a role for Rab7 as a mediator of signaling down-regulation. Taken together, these results suggested that Rab7 acts as a negative regulator of prostate tumor growth and invasion, providing further evidence for its potential as a tumor suppressor.

Reduced expression of DENND2D through promoter hypermethylation is an adverse prognostic factor in squamous cell carcinoma of the esophagus

Esophageal cancer ranks sixth in cancer mortality worldwide and patients with esophageal squamous cell carcinoma (ESCC) have a poor prognosis with a 5-year survival rate of less than 10%. Elucidation of the mechanisms of carcinogenesis and tumor progression in esophageal cancer is urgently required to develop targets for therapy and prognostic biomarkers. In the present study, the expression and regulatory mechanism of the differentially expressed in normal and neoplastic cells domain containing 2D (DENND2D), which is a regulator of Rab GTPases, were investigated to explore its potential as a tumor suppressor gene for ESCC. The level of DENND2D mRNA expression in ESCC cell lines and surgical specimens was determined using a quantitative real-time reverse transcription-polymerase chain reaction assay, and the relationship between the expression levels of DENND2D mRNA and clinicopathological factors was evaluated. The expression and distribution of DENND2D were determined using immunohistochemistry. DNA methylation analysis was performed to determine the regulatory mechanism of DENND2D expression in ESCC. The level of DENND2D mRNA expression was reduced in 8/9 ESCC cell lines and in 59/65 surgical specimens, and the mean expression levels were significantly lower in cancerous tissues compared to corresponding normal tissues (p<0.001). The expression pattern of DENND2D protein and mRNA was consistent. Downregulation of DENND2D mRNA in ESCC tissues was identified as an independent prognostic factor in multivariate analysis (hazard ratio, 2.194; p=0.039). The DENND2D promoter was methylated in 5/9 ESCC cell lines, and DNA demethylation reactivated DENND2D mRNA expression. Hypermethylation of DENND2D was frequently detected in ESCC tissues (64.6%) and was significantly associated with downregulation of DENND2D mRNA expression (P=0.008). Taken together, our data suggest that DENND2D is a candidate tumor suppressor gene that was inactivated by promoter hypermethylation in patients with ESCC and may serve as a novel biomarker of ESCC.

Rab25 regulates invasion and metastasis in head and neck cancer

PURPOSE

Head and neck squamous cell carcinoma (HNSCC) is one of the 10 most common cancers with a 50% five-year survival rate, which has remained unchanged for the past three decades. One of the major reasons for the aggressiveness of this cancer is that HNSCCs readily metastasize to cervical lymph nodes that are abundant in the head and neck region. Hence, discovering new molecules controlling the metastatic process as well as understanding their regulation at the molecular level are essential for effective therapeutic strategies.

EXPERIMENTAL DESIGN

Rab25 expression level was analyzed in HNSCC tissue microarray. We used a combination of intravital microscopy in live animals and immunofluorescence in an in vitro invasion assay to study the role of Rab25 in tumor cell migration and invasion.

RESULTS

In this study, we identified the small GTPase Rab25 as a key regulator of HNSCC metastasis. We observed that Rab25 is downregulated in HNSCC patients. Next, we determined that reexpression of Rab25 in a metastatic cell line is sufficient to block invasion in a three-dimensional collagen matrix and metastasis to cervical lymph nodes in a mouse model for oral cancer. Specifically, Rab25 affects the organization of F-actin at the cell surface, rather than cell proliferation, apoptosis, or tumor angiogenesis.

CONCLUSION

These findings suggest that Rab25 plays an important role in tumor migration and metastasis, and that understanding its function may lead to the development of new strategies to prevent metastasis in oral cancer patients.

Rab1b overexpression modifies Golgi size and gene expression in HeLa cells and modulates the thyrotrophin response in thyroid cells in culture

An increase in Rab1b levels induces changes in Golgi size and in gene expression. These Rab1b-dependent changes require the activity of p38 mitogen-activated protein kinase and the cAMP-responsive element binding protein consensus binding. The results show a Rab1b increase in secretory cells after stimulation and suggest that this increase is required to elicit a secretory response.

Rab1b belongs to the Rab-GTPase family that regulates membrane trafficking and signal transduction systems able to control diverse cellular activities, including gene expression. Rab1b is essential for endoplasmic reticulum–Golgi transport. Although it is ubiquitously expressed, its mRNA levels vary among different tissues. This work aims to characterize the role of the high Rab1b levels detected in some secretory tissues. We report that, in HeLa cells, an increase in Rab1b levels induces changes in Golgi size and gene expression. Significantly, analyses applied to selected genes, KDELR3, GM130 (involved in membrane transport), and the proto-oncogene JUN, indicate that the Rab1b increase acts as a molecular switch to control the expression of these genes at the transcriptional level, resulting in changes at the protein level. These Rab1b-dependent changes require the activity of p38 mitogen-activated protein kinase and the cAMP-responsive element-binding protein consensus binding site in those target promoter regions. Moreover, our results reveal that, in a secretory thyroid cell line (FRTL5), Rab1b expression increases in response to thyroid-stimulating hormone (TSH). Additionally, changes in Rab1b expression in FRTL5 cells modify the specific TSH response. Our results show, for the first time, that changes in Rab1b levels modulate gene transcription and strongly suggest that a Rab1b increase is required to elicit a secretory response.

Prenyltransferase Inhibitors: Treating Human Ailments from Cancer to Parasitic Infections

The posttranslational modification of protein prenylation is a covalent lipid modification on the C-terminus of substrate proteins that serves to enhance membrane affinity. Oncogenic proteins such as Ras have this modification and significant effort has been placed into developing inhibitors of the prenyltransferase enzymes for clinical therapy. In addition to cancer therapy, prenyltransferase inhibitors have begun to find important therapeutic uses in other diseases, including progeria, hepatitis C and D, parasitic infections, and other maladies. This review will trace the evolution of prenyltransferase inhibitors from their initial use as cancer therapeutics to their expanded applications for other diseases.

Deregulation of Rab and Rab effector genes in bladder cancer

Growing evidence indicates that Rab GTPases, key regulators of intracellular transport in eukaryotic cells, play an important role in cancer. We analysed the deregulation at the transcriptional level of the genes encoding Rab proteins and Rab-interacting proteins in bladder cancer pathogenesis, distinguishing between the two main progression pathways so far identified in bladder cancer: the Ta pathway characterized by a high frequency of FGFR3 mutation and the carcinoma in situ pathway where no or infrequent FGFR3 mutations have been identified. A systematic literature search identified 61 genes encoding Rab proteins and 223 genes encoding Rab-interacting proteins. Transcriptomic data were obtained for normal urothelium samples and for two independent bladder cancer data sets corresponding to 152 and 75 tumors. Gene deregulation was analysed with the SAM (significant analysis of microarray) test or the binomial test. Overall, 30 genes were down-regulated, and 13 were up-regulated in the tumor samples. Five of these deregulated genes (LEPRE1, MICAL2, RAB23, STXBP1, SYTL1) were specifically deregulated in FGFR3-non-mutated muscle-invasive tumors. No gene encoding a Rab or Rab-interacting protein was found to be specifically deregulated in FGFR3-mutated tumors. Cluster analysis showed that the RAB27 gene cluster (comprising the genes encoding RAB27 and its interacting partners) was deregulated and that this deregulation was associated with both pathways of bladder cancer pathogenesis. Finally, we found that the expression of KIF20A and ZWINT was associated with that of proliferation markers and that the expression of MLPH, MYO5B, RAB11A, RAB11FIP1, RAB20 and SYTL2 was associated with that of urothelial cell differentiation markers. This systematic analysis of Rab and Rab effector gene deregulation in bladder cancer, taking relevant tumor subgroups into account, provides insight into the possible roles of Rab proteins and their effectors in bladder cancer pathogenesis. This approach is applicable to other group of genes and types of cancer.

A competitive nucleotide binding inhibitor: in vitro characterization of Rab7 GTPase inhibition

Mapping the functionality of GTPases through small molecule inhibitors represents an underexplored area in large part due to the lack of suitable compounds. Here we report on the small chemical molecule 2-(benzoylcarbamothioylamino)-5,5-dimethyl-4,7-dihydrothieno[2,3-c]pyran-3-carboxylic acid (PubChem CID 1067700) as an inhibitor of nucleotide binding by Ras-related GTPases. The mechanism of action of this pan-GTPase inhibitor was characterized in the context of the Rab7 GTPase as there are no known inhibitors of Rab GTPases. Bead-based flow cytometry established that CID 1067700 has significant inhibitory potency on Rab7 nucleotide binding with nanomolar inhibitor (K(i)) values and an inhibitory response of ≥97% for BODIPY-GTP and BODIPY-GDP binding. Other tested GTPases exhibited significantly lower responses. The compound behaves as a competitive inhibitor of Rab7 nucleotide binding based on both equilibrium binding and dissociation assays. Molecular docking analyses are compatible with CID 1067700 fitting into the nucleotide binding pocket of the GTP-conformer of Rab7. On the GDP-conformer, the molecule has greater solvent exposure and significantly less protein interaction relative to GDP, offering a molecular rationale for the experimental results. Structural features pertinent to CID 1067700 inhibitory activity have been identified through initial structure-activity analyses and identified a molecular scaffold that may serve in the generation of more selective probes for Rab7 and other GTPases. Taken together, our study has identified the first competitive GTPase inhibitor and demonstrated the potential utility of the compound for dissecting the enzymology of the Rab7 GTPase, as well as serving as a model for other small molecular weight GTPase inhibitors.

Sorting nexin 5 of a new diagnostic marker of papillary thyroid carcinoma regulates Caspase-2

Papillary thyroid carcinoma (PTC) is a well-differentiated endocrine malignant tumor that develops from thyroid follicular epithelium. The tumor represents the most common type of endocrine malignancy; however, its tumorigenesis is not fully elucidated. The aim of this study was to address the functional role of the sorting nexin (SNX) family in PTC because of recent experimental evidence suggesting that the SNX family members actively control endocytotic transportation as well as cell fate. Expression profiles of SNX family members of PTC showed a significant quantity of transcripts of SNX5. Further immunohistochemical analysis with an SNX5-specific monoclonal antibody established in this study consistently demonstrated the preferential expression of SNX5 in PTC (94.2%, 113/120 cases) as indicated by studies on 440 cases of various tumors. In contrast, other major carcinomas originating from the lung (2.6%, 1/38 cases), breast (5.1%, 2/39 cases), and intestine (4.2%, 1/24 cases) scarcely expressed SNX5. When we investigated models of murine thyroid tumors induced by the administration of carcinogens, high expression of Snx5 was also observed in well-differentiated thyroid tumors, further implying that the tumorigenesis of the thyroid gland was tightly associated with the abundance of SNX5/Snx5. Moreover epithelial cells expressing excess SNX5 showed high levels of Caspase-2 of an initiator caspase. Collectively these findings suggest that the evaluation of SNX5 expression would support pathological diagnosis of primary and secondary PTC.

Small G proteins and their regulators in cellular signalling

Small molecular weight GTPases (small G proteins) are essential in the transduction of signals from different plasma membrane receptors. Due to their endogenous GTP-hydrolyzing activity, these proteins function as time-dependent biological switches controlling diverse cellular functions including cell shape and migration, cell proliferation, gene transcription, vesicular transport and membrane-trafficking. This review focuses on endocrine diseases linked to small G proteins. We provide examples for the regulation of the activity of small G proteins by various mechanisms such as posttranslational modifications, guanine nucleotide exchange factors (GEFs), GTPase activating proteins (GAPs) or guanine nucleotide dissociation inhibitors (GDIs). Finally we summarize endocrine diseases where small G proteins or their regulatory proteins have been revealed as the cause.

Differential expression of Rab27A/B correlates with clinical outcome in hepatocellular carcinoma

AIM: To investigate the association of Rab27A and Rab27B expression with clinicopathological characteristics and prognosis of hepatocellular carcinoma (HCC).

METHODS: We used reverse transcription polymerase chain reaction (RT-PCR), real-time PCR, and Western blotting to detect Rab27A and Rab27B mRNA and protein expression in 5 human HCC lines and the immortalized hepatic HL-7702 cell line. We further examined 148 primary HCC samples matched with adjacent normal tissue and 80 non-HCC specimens by immunohistochemistry to evaluate the correlation of Rab27A and Rab27B expression with clinicopathological features and prognosis.

RESULTS: Our data showed that Rab27A and Rab27B were differentially expressed in cell lines and primary HCC tumors. Rab27A mRNA and protein were detected in 67% (4/6) of human cell lines and 80% (4/5) of HCC cell lines, while Rab27B was found in 50% (3/6) of human lines and 40% (2/5) of HCC lines. Rab27A expression was higher in primary HCC (46.2%, 66/143) than in matched adjacent tissue (24.3%, 33/136, P < 0.001), whereas immunopositivity for Rab27B was lower in primary HCC (57.4%, 81/141) than in matched adjacent tissue (87.5%, 119/136, P < 0.001). Analysis of clinicopathological characteristics of 148 HCC specimens revealed significant correlations between Rab27A and Rab27B expression and tumor tumor-node-metastasis (TNM) classification (P = 0.046 and P = 0.027, respectively), and between strong Rab27A expression and tumor differentiation grade (P = 0.008). Survival analyses revealed that patients with Rab27A⁺ or Rab27B⁺ tumors had significantly reduced overall survival compared with that of patients with Rab27A^- or Rab27B^- tumors (P = 0.015 and P = 0.005, respectively). Risk analyses revealed that Rab27B⁺ and TNM III-IV were independent poor prognosis factors associated with a 3.36- and 3.37- fold higher relative risk of death, respectively.

CONCLUSION: Rab27A and Rab27B expression were closely correlated with tumor progression and can be valuable prognostic indicators for HCC patients.

Different endocytic functions of AGEF-1 in C. elegans coelomocytes

BACKGROUND

ADP-ribosylation factors (ARFs) are a family of small GTP-binding proteins that play roles in membrane dynamics and vesicle trafficking. AGEF-1, which is thought to act as a guanine nucleotide exchange factor of class I ARFs, is required for caveolin-1 body formation and receptor-mediated endocytosis in oocytes of Caenorhabditis elegans. This study explores additional roles of AGEF-1 in endocytic transport.

METHODS

agef-1 expression was knocked down by using RNAi in C. elegans. Markers that allow analysis of endocytic transport in scavenger cells were investigated for studying the effect of AGEF-1 on different steps of membrane transport.

RESULTS

Knockdown of AGEF-1 levels results in two apparent trafficking defects in coelomocytes of C. elegans. First, there is a delay in the uptake of solutes from the extracellular medium. Second, there is a dramatic enlargement of the sizes of lysosomes, even though lysosomal acidification is normal and degradation still occurs.

CONCLUSION

Our results suggest that AGEF-1 regulates endosome/lysosome fusion or fission events, in addition to earlier steps in endocytic transport.

GENERAL SIGNIFICANCE

AGEF-1 is the first identified GTPase regulator that functions at the lysosome fusion or fission stage of the endocytic pathway. Our study provides insight into lysosome dynamics in C. elegans.

Proteomic profiling of the effect of metabolic acidosis on the apical membrane of the proximal convoluted tubule

The physiological response to the onset of metabolic acidosis requires pronounced changes in renal gene expression. Adaptations within the proximal convoluted tubule support the increased extraction of plasma glutamine and the increased synthesis and transport of glucose and of NH(4)(+) and HCO(3)(-) ions. Many of these adaptations involve proteins associated with the apical membrane. To quantify the temporal changes in these proteins, proteomic profiling was performed using brush-border membrane vesicles isolated from proximal convoluted tubules (BBMV(PCT)) that were purified from normal and acidotic rats. This preparation is essentially free of contaminating apical membranes from other renal cortical cells. The analysis identified 298 proteins, 26% of which contained one or more transmembrane domains. Spectral counts were used to assess changes in protein abundance. The onset of acidosis produced a twofold, but transient, increase in the Na(+)-dependent glucose transporter and a more gradual, but sustained, increase (3-fold) in the Na(+)-dependent lactate transporter. These changes were associated with the loss of glycolytic and gluconeogenic enzymes that are contained in the BBMV(PCT) isolated from normal rats. In addition, the levels of γ-glutamyltranspeptidase increased twofold, while transporters that participate in the uptake of neutral amino acids, including glutamine, were decreased. These changes could facilitate the deamidation of glutamine within the tubular lumen. Finally, pronounced increases were also observed in the levels of DAB2 (3-fold) and myosin 9 (7-fold), proteins that may participate in endocytosis of apical membrane proteins. Western blot analysis and accurate mass and time analyses were used to validate the spectral counting.

Pharmacological actions of statins: a critical appraisal in the management of cancer

Statins, among the most commonly prescribed drugs worldwide, are cholesterol-lowering agents used to manage and prevent cardiovascular and coronary heart diseases. Recently, a multifaceted action in different physiological and pathological conditions has been also proposed for statins, beyond anti-inflammation and neuroprotection. Statins have been shown to act through cholesterol-dependent and -independent mechanisms and are able to affect several tissue functions and modulate specific signal transduction pathways that could account for statin pleiotropic effects. Typically, statins are prescribed in middle-aged or elderly patients in a therapeutic regimen covering a long life span during which metabolic processes, aging, and concomitant novel diseases, including cancer, could occur. In this context, safety, toxicity, interaction with other drugs, and the state of health have to be taken into account in subjects treated with statins. Some evidence has shown a dichotomous effect of statins with either cancer-inhibiting or -promoting effects. To date, clinical trials failed to demonstrate a reduced cancer occurrence in statin users and no sufficient data are available to define the long-term effects of statin use over a period of 10 years. Moreover, results from clinical trials performed to evaluate the therapeutic efficacy of statins in cancer did not suggest statin use as chemotherapeutic or adjuvant agents. Here, we reviewed the pharmacology of the statins, providing a comprehensive update of the current knowledge of their effects on tissues, biological processes, and pathological conditions, and we dissected the disappointing evidence on the possible future use of statin-based drugs in cancer therapy.

microRNA-101 is a potent inhibitor of autophagy

Autophagy is an evolutionarily conserved mechanism of cellular self-digestion in which proteins and organelles are degraded through delivery to lysosomes. Defects in this process are implicated in numerous human diseases including cancer. To further elucidate regulatory mechanisms of autophagy, we performed a functional screen in search of microRNAs (miRNAs), which regulate the autophagic flux in breast cancer cells. In this study, we identified the tumour suppressive miRNA, miR-101, as a potent inhibitor of basal, etoposide- and rapamycin-induced autophagy. Through transcriptome profiling, we identified three novel miR-101 targets, STMN1, RAB5A and ATG4D. siRNA-mediated depletion of these genes phenocopied the effect of miR-101 overexpression, demonstrating their importance in autophagy regulation. Importantly, overexpression of STMN1 could partially rescue cells from miR-101-mediated inhibition of autophagy, indicating a functional importance for this target. Finally, we show that miR-101-mediated inhibition of autophagy can sensitize breast cancer cells to 4-hydroxytamoxifen (4-OHT)-mediated cell death. Collectively, these data establish a novel link between two highly important and rapidly growing research fields and present a new role for miR-101 as a key regulator of autophagy.

Interaction of activated Rab5 with actin-bundling proteins, L- and T-plastin and its relevance to endocytic functions in mammalian cells

Rab5 is a GTP-binding protein that is crucial for endocytic machinery functions. We previously identified L-plastin as a binding protein for Rab5, using an affinity column with constitutively active Rab5. L- and T-plastin are isoforms of a plastin protein family belonging to actin-bundling proteins that are implicated in the regulation of cell morphology, lamellipodium protrusion, bacterial invasion and tumor progression. However, the physiological relevance of Rab5 binding to plastin has remained unclear. Here, we show that L- and T-plastin interacted only with activated Rab5 and that they co-localized with Rab5 on the plasma membrane and endosome. Rab5 activity was also higher in both L- and T-plastin over-expressing Cos-1 cells. Furthermore, expression of L- and T-plastin increased the rate of fluid-phase endocytosis. These findings imply that the Rab5 is either activated or the activity is sustained by interaction with plastin, and that this interaction influences endocytic activity.

Role of Rab GTPases in membrane traffic and cell physiology

Intracellular membrane traffic defines a complex network of pathways that connects many of the membrane-bound organelles of eukaryotic cells. Although each pathway is governed by its own set of factors, they all contain Rab GTPases that serve as master regulators. In this review, we discuss how Rabs can regulate virtually all steps of membrane traffic from the formation of the transport vesicle at the donor membrane to its fusion at the target membrane. Some of the many regulatory functions performed by Rabs include interacting with diverse effector proteins that select cargo, promoting vesicle movement, and verifying the correct site of fusion. We describe cascade mechanisms that may define directionality in traffic and ensure that different Rabs do not overlap in the pathways that they regulate. Throughout this review we highlight how Rab dysfunction leads to a variety of disease states ranging from infectious diseases to cancer.

Pulmonary surfactant protein A enhances endolysosomal trafficking in alveolar macrophages through regulation of Rab7

Surfactant protein A (SP-A), the most abundant pulmonary soluble collectin, modulates innate and adaptive immunity of the lung, partially via its direct effects on alveolar macrophages (AM), the most predominant intra-alveolar cells under physiological conditions. Enhanced phagocytosis and endocytosis are key functional consequences of AM/SP-A interaction, suggesting a SP-A-mediated modulation of small Rab (Ras related in brain) GTPases that are pivotal membrane organizers in both processes. In this article, we show that SP-A specifically and transiently enhances the protein expression of endogenous Rab7 and Rab7b, but not Rab5 and Rab11, in primary AM from rats and mice. SP-A-enhanced GTPases are functionally active as determined by increased interaction of Rab7 with its downstream effector Rab7 interacting lysosomal protein (RILP) and enhanced maturation of cathepsin-D, a function of Rab7b. In AM and RAW264.7 macrophages, the SP-A-enhanced lysosomal delivery of GFP-Escherichia coli is abolished by the inhibition of Rab7 and Rab7 small interfering RNA transfection, respectively. The constitutive expression of Rab7 in AM from SP-A(-/-) mice is significantly reduced compared with SP-A(+/+) mice and is restored by SP-A. Rab7 blocking peptides antagonize SP-A-rescued lysosomal delivery of GFP-E. coli in AM from SP-A(-/-) mice. Activation of Rab7, but not Rab7b, by SP-A depends on the PI3K/Akt/protein kinase Cζ (PKCζ) signal transduction pathway in AM and RAW264.7 macrophages. SP-A induces a Rab7/PKCζ interaction in these cells, and the disruption of PKCζ by small interfering RNA knockdown abolishes the effect of SP-A on Rab7. The data demonstrate a novel role for SP-A in modulating endolysosomal trafficking via Rab7 in primary AM and define biochemical pathways involved.

Transformation of rat intestinal epithelial cells by overexpression of Rab25 is microtubule dependent

Little research has addressed the role of membrane trafficking and recycling in the regulation of the transformed phenotype of neoplastic cells. The small GTPase Rab25 is an epithelial-specific modulator of membrane recycling. Recent studies have demonstrated that Rab25 expression is up-regulated in a number of epithelial cancers and overexpression may increase the aggressive phenotype of certain cancers. We have utilized the nontransformed RIE cell line to examine the influence of Rab25 on transformation. Overexpression of Rab25 in RIE cells leads to morphological transformation as well as growth in soft agar, tumor formation in nude mice, disruption of integrin-based focal adhesions, and alteration in modified microtubule subsets. Although the predominance of recent cancer research has focused on the manipulation of the actin-based cytoskeleton, recycling trafficking relies on microtubules. Transformation of RIE cells through overexpression of Rab25, but not with H-Ras(V12) , was reversed by inhibitors of microtubule polymerization. These results suggest that up-regulation of Rab25 in RIE cells leads to microtubule-dependent transformation. Thus, depolymerization of microtubules may be a potent therapeutic target for cancer therapy through the reversal of the invasive phenotype of certain cancer cells.

Integrin-mediated function of Rab GTPases in cancer progression

The RAS (rat sarcoma) superfamily of small GTPases is broadly subdivided into five groups: Ras, Rho, Rab, Ran, and Arf. Rab family proteins are important in regulating signal transduction and cellular processes such as differentiation, proliferation, vesicle transport, nuclear assembly, and cytoskeleton formation. However, some Rab proteins have been reported to be necessary for the adhesion and migration of cancer cells. Although Ras and Rho family members have been strongly implicated in cancer progression, knowledge of Rabs action in this regard is limited. Some reports have also linked Rab GTPases with cancer cell migration and invasiveness. This review discusses the implications of the involvement of Rabs in malignant transformation and cancer therapy through integrin-mediated signaling events, with particular emphasis on breast cancer.

Quantitative proteomics analysis of inborn errors of cholesterol synthesis: identification of altered metabolic pathways in DHCR7 and SC5D deficiency

Smith-Lemli-Opitz syndrome (SLOS) and lathosterolosis are malformation syndromes with cognitive deficits caused by mutations of 7-dehydrocholesterol reductase (DHCR7) and lathosterol 5-desaturase (SC5D), respectively. DHCR7 encodes the last enzyme in the Kandutsch-Russel cholesterol biosynthetic pathway, and impaired DHCR7 activity leads to a deficiency of cholesterol and an accumulation of 7-dehydrocholesterol. SC5D catalyzes the synthesis of 7-dehydrocholesterol from lathosterol. Impaired SC5D activity leads to a similar deficiency of cholesterol but an accumulation of lathosterol. Although the genetic and biochemical causes underlying both syndromes are known, the pathophysiological processes leading to the developmental defects remain unclear. To study the pathophysiological mechanisms underlying SLOS and lathosterolosis neurological symptoms, we performed quantitative proteomics analysis of SLOS and lathosterolosis mouse brain tissue and identified multiple biological pathways affected in Dhcr7(Delta3-5/Delta3-5) and Sc5d(-/-) E18.5 embryos. These include alterations in mevalonate metabolism, apoptosis, glycolysis, oxidative stress, protein biosynthesis, intracellular trafficking, and cytoskeleton. Comparison of proteome alterations in both Dhcr7(Delta3-5/Delta3-5) and Sc5d(-/-) brain tissues helps elucidate whether perturbed protein expression was due to decreased cholesterol or a toxic effect of sterol precursors. Validation of the proteomics results confirmed increased expression of isoprenoid and cholesterol synthetic enzymes. This alteration of isoprenoid synthesis may underlie the altered posttranslational modification of Rab7, a small GTPase that is functionally dependent on prenylation with geranylgeranyl, that we identified and validated in this study. These data suggested that although cholesterol synthesis is impaired in both Dhcr7(Delta3-5/Delta3-5) and Sc5d(-/-) embryonic brain tissues the synthesis of nonsterol isoprenoids may be increased and thus contribute to SLOS and lathosterolosis pathology. This proteomics study has provided insight into the pathophysiological mechanisms of SLOS and lathosterolosis, and understanding these pathophysiological changes will help guide clinical therapy for SLOS and lathosterolosis.

Armus is a Rac1 effector that inactivates Rab7 and regulates E-cadherin degradation

BACKGROUND

Cell-cell adhesion and intracellular trafficking are regulated by signaling pathways from small GTPases of the Rho, Arf, and Rab subfamilies. How signaling from distinct small GTPases are integrated in a given process is poorly understood.

RESULTS

We find that a TBC/RabGAP protein, Armus, integrates signaling between Arf6, Rac1, and Rab7 during junction disassembly. Armus binds specifically to activated Rac1 and its C-terminal TBC/RabGAP domain inactivates Rab7. Thus, Armus is a novel Rac1 effector and a bona fide GAP for Rab7 in vitro and in vivo, a unique and previously unreported combination. Arf6 activation efficiently disrupts cell-cell contacts and is known to activate Rac1 and Rab7. Arf6-induced E-cadherin degradation is efficiently blocked by expression of Armus C-terminal domain or after Armus RNAi. Coexpression of Arf6 with dominant-negative Rab7 or Rac1 also inhibits junction disassembly. Importantly, Armus RabGAP expression also prevents EGF-induced scattering in keratinocytes, a process shown here to require Arf6, Rac1, and Rab7 function. To our knowledge, this is the first report to demonstrate a molecular and functional link between Rac1 and Rab7.

CONCLUSIONS

Our data indicate that active Rac1 recruits Armus to locally inactivate Rab7 and facilitate E-cadherin degradation in lysosomes. Thus, the integration of Rac1 and Rab7 activities by Armus provides an important regulatory node for E-cadherin turnover and stability of cell-cell contacts.

Rab7: roles in membrane trafficking and disease

The endocytosis pathway controls multiple cellular and physiological events. The lysosome is the destination of newly synthesized lysosomal hydrolytic enzymes. Internalized molecules or particles are delivered to the lysosome for degradation through sequential transport along the endocytic pathway. The endocytic pathway is also emerging as a signalling platform, in addition to the well-known role of the plasma membrane for signalling. Rab7 is a late endosome-/lysosome-associated small GTPase, perhaps the only lysosomal Rab protein identified to date. Rab7 plays critical roles in the endocytic processes. Through interaction with its partners (including upstream regulators and downstream effectors), Rab7 participates in multiple regulation mechanisms in endosomal sorting, biogenesis of lysosome [or LRO (lysosome-related organelle)] and phagocytosis. These processes are closely related to substrates degradation, antigen presentation, cell signalling, cell survival and microbial pathogen infection. Consistently, mutations or dysfunctions of Rab7 result in traffic disorders, which cause various diseases, such as neuropathy, cancer and lipid metabolism disease. Rab7 also plays important roles in microbial pathogen infection and survival, as well as in participating in the life cycle of viruses. Here, we give a brief review on the central role of Rab7 in endosomal traffic and summarize the studies focusing on the participation of Rab7 in disease pathogenesis. The underlying mechanism governed by Rab7 and its partners will also be discussed.

Emerging roles for Rab family GTPases in human cancer

Member of the Ras-associated binding (Rab) family of small GTPases function as molecular switches regulating vesicular transport in eukaryotes cells. Their pathophysiological roles in human malignancies are less well-known compared to members of Ras and Rho families. Several members of the Rab family have, however, been shown to be aberrantly expressed in various cancer tissues. Recent findings have also revealed , in particular, Rab25 as a determinant of tumor progression and aggressiveness of epithelial cancers. Rab25 associates with alpha5beta1 integrin, and enhances tumor cell invasion by directing the localization of integrin-containing vesicles to the leading edge of matrix invading pseudopodia. We summarized here recent integrin on Rab25 and other Rabs implicated to be involved in a variety of human cancers, and discussed plausible mechanisms of how dysregulation of Rab expression could be tumorigenic or tumor suppressive.

Rab proteins and Rab-associated proteins: major actors in the mechanism of protein-trafficking disorders

Ras-associated binding (Rab) proteins and Rab-associated proteins are key regulators of vesicle transport, which is essential for the delivery of proteins to specific intracellular locations. More than 60 human Rab proteins have been identified, and their function has been shown to depend on their interaction with different Rab-associated proteins regulating Rab activation, post-translational modification and intracellular localization. The number of known inherited disorders of vesicle trafficking due to Rab cycle defects has increased substantially during the past decade. This review describes the important role played by Rab proteins in a number of rare monogenic diseases as well as common multifactorial human ones. Although the clinical phenotype in these monogenic inherited diseases is highly variable and dependent on the type of tissue in which the defective Rab or its associated protein is expressed, frequent features are hypopigmentation (Griscelli syndrome), eye defects (Choroideremia, Warburg Micro syndrome and Martsolf syndrome), disturbed immune function (Griscelli syndrome and Charcot-Marie-Tooth disease) and neurological dysfunction (X-linked non-specific mental retardation, Charcot-Marie-Tooth disease, Warburg Micro syndrome and Martsolf syndrome). There is also evidence that alterations in Rab function play an important role in the progression of multifactorial human diseases, such as infectious diseases and type 2 diabetes. Rab proteins must not only be bound to GTP, but they need also to be 'prenylated'-i.e. bound to the cell membranes by isoprenes, which are intermediaries in the synthesis of cholesterol (e.g. geranyl geranyl or farnesyl compounds). This means that isoprenylation can be influenced by drugs such as statins, which inhibit isoprenylation, or biphosphonates, which inhibit that farnesyl pyrophosphate synthase necessary for Rab GTPase activity.

CONCLUSION

Although protein-trafficking disorders are clinically heterogeneous and represented in almost every subspeciality of pediatrics, the identification of common pathogenic mechanisms may provide a better diagnosis and management of patients with still unknown Rab cycle defects and stimulate the development of therapeutic agents.