Simultaneous loss of the DLC1 and PTEN tumor suppressors enhances breast cancer cell migration

Spatiotemporal distribution of PTEN before directed cell migration in monolayers

The intracellular distribution of phosphatase and tensin homolog (PTEN) is closely related to directed cell migration. In single cells, PTEN accumulates at the rear of the cell before and during directed migration; however, the spatiotemporal distribution of PTEN in confluent cell monolayers, particularly before directed migration, remains unclear. In this study, we wounded a cell in confluent fetal rat skin keratinocytes (FRSKs) and examined the dynamics of PTEN in the cells adjacent to the wounded cell. In contrast to single-cell migration, we found that PTEN translocated to the nucleus before the beginning of directed migration. This nuclear translocation of PTEN did not occur in disconnected cells, and it was also suppressed by importin-β inhibitor and actin inhibitor. When the nuclear localization of PTEN was inhibited by an importin-β inhibitor, cell elongation in the direction of migration was also significantly inhibited. Our results indicate that PTEN translocation is induced by the disruption of cell-cell adhesion and requires the involvement of importin-β and actin cytoskeleton signaling. In addition, phosphatidylinositol 3,4,5-triphosphate (PIP3) may regulate PTEN distribution through its localized accumulation at the cell edge. Our findings suggest that the translocation of PTEN is crucial for directed cell migration and for responding to mechanical environmental changes in confluent cell monolayers.

Human mesenchymal stem cells increase LLC metastasis and stimulate or decelerate tumor development depending on injection method and cell amount

Mesenchymal stem cells (MSCs) being injected into the body can stimulate or decelerate carcinogenesis. Here, the direction of influence of human placenta-derived MSCs (P-MSCs) on the Lewis lung carcinoma (LLC) tumor development and metastatic potential is investigated in C57BL/6 mice depending on the injection method. After intramuscular co-inoculation of LLC and P-MSCs (LLC + P-MSCs), the growth of primary tumor and angiogenesis are slowed down compared to the control LLC on the 15th day. This is explained by the fact of a decrease in the secretion of proangiogenic factors during in vitro co-cultivation of an equal amount of LLC and P-MSCs. When P-MSCs are intravenously (i.v.) injected in the mice with developing LLC (LLC + P-MSCs(i.v.)), the tumor growth and angiogenesis are stimulated on the 15th day. A highly activated secretion of proangiogenic factors by P-MSCs in a similar in vitro model can explain this. In both the models compared to the control on the 23rd day, there is no significant difference in the tumor growth, while angiogenesis remains correspondingly decelerated or stimulated. However, in both the models, the total volume and number of lung metastases constantly increase compared to the control: it is mainly due to small-size metastases for LLC + P-MSCs(i.v.) and larger ones for LLC + P-MSCs. The increase in the rate of LLC cell dissemination after the injection of P-MSCs is explained by the disordered polyploidy and chromosomal instability, leading to an increase in migration and invasion of cancer cells. After LLC + P-MSCs co-inoculation, the tumor cell karyotype has the most complex and heterogeneous chromosomal structure. These findings indicate a bidirectional effect of P-MSCs on the growth of LLC in the early periods after injection, depending on the injection method, and, correspondingly, the number of contacting cells. However, regardless of the injection method, P-MSCs are shown to increase LLC aggressiveness related to cancer-associated angiogenesis and metastasis activation in the long term.

Ras superfamily GTPase activating proteins in cancer: Potential therapeutic targets?

To search more therapeutic strategies for Ras-mutant tumors, regulators of the Ras superfamily involved in the GTP/GDP (guanosine triphosphate/guanosine diphosphate) cycle have been well concerned for their anti-tumor potentials. GTPase activating proteins (GAPs) provide the catalytic group necessary for the hydrolysis of GTPs, which accelerate the switch by cycling between GTP-bound active and GDP-bound inactive forms. Inactivated GAPs lose their function in activating GTPase, leading to the continuous activation of downstream signaling pathways, uncontrolled cell proliferation, and eventually carcinogenesis. A growing number of evidence has shown the close link between GAPs and human tumors, and as a result, GAPs are believed as potential anti-tumor targets. The present review mainly summarizes the critically important role of GAPs in human tumors by introducing the classification, function and regulatory mechanism. Moreover, we comprehensively describe the relationship between dysregulated GAPs and the certain type of tumor. Finally, the current status, research progress, and clinical value of GAPs as therapeutic targets are also discussed, as well as the challenges and future direction in the cancer therapy.

DLC1 inhibits colon adenocarcinoma cell migration by promoting secretion of the neurotrophic factor MANF

DLC1 (deleted in liver cancer-1) is downregulated or deleted in colorectal cancer (CRC) tissues and functions as a potent tumor suppressor, but the underlying molecular mechanism remains elusive. We found that the conditioned medium (CM) collected from DLC1-overexpressed SW1116 cells inhibited the migration of colon adenocarcinoma cells HCT116 and SW1116, but had no effect on proliferation, which suggested DLC1-mediated secretory components containing a specific inhibitor for colon adenocarcinoma cell migration. Analysis by mass spectrometry identified mesencephalic astrocyte-derived neurotrophic factor (MANF) as a candidate. More importantly, exogenous MANF significantly inhibited the migration of colon adenocarcinoma cells HCT116 and SW1116, but did not affect proliferation. Mechanistically, DLC1 reduced the retention of MANF in ER by competing the interaction between MANF and GRP78. Taken together, these data provided new insights into the suppressive effects of DLC1 on CRC, and revealed the potential of MANF in the treatment of CRC.

Insights into the Steps of Breast Cancer-Brain Metastases Development: Tumor Cell Interactions with the Blood-Brain Barrier

Brain metastases (BM) represent a growing problem for breast cancer (BC) patients. Recent studies have demonstrated a strong impact of the BC molecular subtype on the incidence of BM development. This study explores the interaction between BC cells of different molecular subtypes and the blood–brain barrier (BBB). We compared the ability of BC cells of different molecular subtypes to overcome several steps (adhesion to the brain endothelium, disruption of the BBB, and invasion through the endothelial layer) during cerebral metastases formation, in vitro as well as in vivo. Further, the impact of these cells on the BBB was deciphered at the molecular level by transcriptome analysis of the triple-negative (TNBC) cells themselves as well as of hBMECs after cocultivation with BC cell secretomes. Compared to luminal BC cells, TNBC cells have a greater ability to influence the BBB in vitro and consequently develop BM in vivo. The brain-seeking subline and parental TNBC cells behaved similarly in terms of adhesion, whereas the first showed a stronger impact on the brain endothelium integrity and increased invasive ability. The comparative transcriptome revealed potential brain-metastatic-specific key regulators involved in the aforementioned processes, e.g., the angiogenesis-related factors TNXIP and CXCL1. In addition, the transcriptomes of the two TNBC cell lines strongly differed in certain angiogenesis-associated factors and in several genes related to cell migration and invasion. Based on the present study, we hypothesize that the tumor cell’s ability to disrupt the BBB via angiogenesis activation, together with increased cellular motility, is required for BC cells to overcome the BBB and develop brain metastases.

Tumor suppressor gene DLC1: Its modifications, interactive molecules, and potential prospects for clinical cancer application

Deleted in liver cancer 1 (DLC1) is a recognized tumor suppressor gene that negatively regulates Rho family proteins by hydrolyzing the active GTP-bound state to its inactive GDP-bound state. Active Rho proteins play a positive role in tumorigenesis. Numerous in vitro and in vivo experiments have shown that DLC1 is downregulated or inactivated in various solid tumors, which may be due to the following five reasons: genomic deletion, epigenetic modification and ubiquitin-dependent proteasomal degradation may cause DLC1 underexpression; phosphorylation at the post-translation level may cause DLC1 inactivation; and failure to localize at focal adhesions (FAs) may prevent DLC1 from exerting full activity. All of the causes could be attributed to molecular binding. Experimental evidence suggests that direct or indirect targeting of DLC1 is feasible for cancer treatment. Therefore, elucidating the interaction of DLC1 with its binding partners might provide novel targeted therapies for cancer. In this review, we summarized the binding partners of DLC1 at both the gene and protein levels and expounded a variety of anticancer drugs targeting DLC1 to provide information about DLC1 as a cancer diagnostic indicator or therapeutic target.

Mutational drivers of cancer cell migration and invasion

Genomic instability and mutations underlie the hallmarks of cancer-genetic alterations determine cancer cell fate by affecting cell proliferation, apoptosis and immune response, and increasing data show that mutations are involved in metastasis, a crucial event in cancer progression and a life-threatening problem in cancer patients. Invasion is the first step in the metastatic cascade, when tumour cells acquire the ability to move, penetrate into the surrounding tissue and enter lymphatic and blood vessels in order to disseminate. A role for genetic alterations in invasion is not universally accepted, with sceptics arguing that cellular motility is related only to external factors such as hypoxia, chemoattractants and the rigidity of the extracellular matrix. However, increasing evidence shows that mutations might trigger and accelerate the migration and invasion of different types of cancer cells. In this review, we summarise data from published literature on the effect of chromosomal instability and genetic mutations on cancer cell migration and invasion.

DLC1 is a direct target of activated YAP/TAZ that drives collective migration and sprouting angiogenesis

Endothelial YAP/TAZ (YAP is also known as YAP1, and TAZ as WWTR1) signaling is crucial for sprouting angiogenesis and vascular homeostasis. However, the underlying molecular mechanisms that explain how YAP/TAZ control the vasculature remain unclear. This study reveals that the focal adhesion protein deleted-in-liver-cancer 1 (DLC1) is a direct transcriptional target of the activated YAP/TAZ-TEAD complex. We find that substrate stiffening and VEGF stimuli promote expression of DLC1 in endothelial cells. In turn, DLC1 expression levels are YAP and TAZ dependent, and constitutive activation of YAP is sufficient to drive DLC1 expression. DLC1 is needed to limit F-actin fiber formation, integrin-based focal adhesion lifetime and integrin-mediated traction forces. Depletion of endothelial DLC1 strongly perturbs cell polarization in directed collective migration and inhibits the formation of angiogenic sprouts. Importantly, ectopic expression of DLC1 is sufficient to restore migration and angiogenic sprouting in YAP-depleted cells. Together, these findings point towards a crucial and prominent role for DLC1 in YAP/TAZ-driven endothelial adhesion remodeling and collective migration during angiogenesis.This article has an associated First Person interview with the first author of the paper.

Combination of dihydroartemisinin and resveratrol effectively inhibits cancer cell migrationviaregulation of the DLC1/TCTP/Cdc42 pathway

Mechanism of DHA combined with RES in inhibition of cancer cell migration by DLC1/TCTP/Cdc42 signaling.

DLC1 SAM domain-binding peptides inhibit cancer cell growth and migration by inactivating RhoA

Deleted-in-liver cancer 1 (DLC1) exerts its tumor suppressive function mainly through the Rho-GTPase-activating protein (RhoGAP) domain. When activated, the domain promotes the hydrolysis of RhoA-GTP, leading to reduced cell migration. DLC1 is kept in an inactive state by an intramolecular interaction between its RhoGAP domain and the DLC1 sterile α motif (SAM) domain. We have shown previously that this autoinhibited state of DLC1 may be alleviated by tensin-3 (TNS3) or PTEN. We show here that the TNS3/PTEN-DLC1 interactions are mediated by the C2 domains of the former and the SAM domain of the latter. Intriguingly, the DLC1 SAM domain was capable of binding to specific peptide motifs within the C2 domains. Indeed, peptides containing the binding motifs were highly effective in blocking the C2-SAM domain-domain interaction. Importantly, when fused to the tat protein-transduction sequence and subsequently introduced into cells, the C2 peptides potently promoted the RhoGAP function in DLC1, leading to decreased RhoA activation and reduced tumor cell growth in soft agar and migration in response to growth factor stimulation. To facilitate the development of the C2 peptides as potential therapeutic agents, we created a cyclic version of the TNS3 C2 domain-derived peptide and showed that this peptide readily entered the MDA-MB-231 breast cancer cells and effectively inhibited their migration. Our work shows, for the first time, that the SAM domain is a peptide-binding module and establishes the framework on which to explore DLC1 SAM domain-binding peptides as potential therapeutic agents for cancer treatment.

Decreased expression of ARHGAP15 promotes the development of colorectal cancer through PTEN/AKT/FOXO1 axis

Copious evidence demonstrates the crucial role of Rho GTPase-activating proteins in human malignancies. The downregulation of Rho GTPase-activating protein 15 (ARHGAP15), a Rac1-specific GAP, has been observed in glioma and pancreatic ductal adenocarcinoma. The present study explored the expression in colorectal cancer (CRC) by quantitative real-time PCR and immunohistochemistry analysis. The possible function of ARHGAP15 in CRC was investegated in vitro and in vivo. We found that ARHGAP15 expression was obviously lower in CRC specimens than in normal colonic mucosa. ARHGAP15 expression was significantly correlated with clinical stage, tumor size metastasis, vital status, and overall survival of CRC patients. ARHGAP15 overexpression inhibited cell growth, migration, and invasion of HT29 and RKO cells in vitro, whereas opposite results were observed in ARHGAP15-silenced LoVo cells. Mechanically, we found that PTEN (phosphatase and tensin homology deleted on chromosome 10) signaling pathway was closely correlated with ARHGAP15 expression by Gene set enrichment analysis with The Cancer Genome Atlas CRC data set. Increased PTEN and Forkhead box protein O1 (FOXO1, a downstream transcription factor of AKT), and decreased phosphorylation of AKT were observed in ARHGAP15-overexpressed HT29 and RKO cells. In addition, ARHGAP15 overexpression increased p21, which was responsible for the accelerated cell growth and S phase arrest, but decreased the protein levels of MMP-2 and MMP-9, which were stimuli for cell metastasis. Notably, upregulating PTEN expression, FOXO1 overexpression and interdicting the activation of AKT pathway with MK2206 suppressed the proliferation and the metastatic ability of ARHGAP15-silenced LoVo cells. In addition, FOXO1 overexpression markedly enhanced the expression and the promoter activity of ARHGAP15. Furthermore, ARHGAP15 overexpression significantly decelerated the pace of tumor growth and metastasis in the lung in vivo. In summary, these results suggest that ARHGAP15 might serve as a tumor suppressor during CRC progression and metastasis through PTEN/AKT/FOXO1-signaling pathway.

FKBP51 regulates cell motility and invasion via RhoA signaling

FK506 binding protein 51 (FKBP51), a member of the immunophilin family, is involved in multiple signaling pathways, tumorigenesis, and chemoresistance. FKBP51 expression correlates with metastatic potential in melanoma and prostate cancer. However, the functions of FKBP51, particularly involving the regulation of cell motility and invasion, are not fully understood. We discovered two novel interacting partner proteins of FKBP51, i.e., deleted in liver cancer 1 (DLC1) and deleted in liver cancer 2 (DLC2), using immunoprecipitation and mass spectrometry. DLC1 and DLC2 are Rho GTPase‐activating proteins that are frequently downregulated in various cancers. Next, we demonstrated that overexpression of FKBP51 enhances cell motility and invasion of U2OS cells via upregulation of RhoA activity and enhanced Rho‐ROCK signaling. Moreover, FKBP51‐depleted cells displayed a cortical distribution of actin filaments and decreased cell motility and invasion. Consistent with this phenotype, FKBP51 depletion caused a downregulation of RhoA activity. Considered together, our results demonstrate that FKBP51 positively controls cell motility by promoting RhoA and ROCK activation; thus, we have revealed a novel role for FKBP51 in cytoskeletal rearrangement and cell migration and invasion.

Clinicopathological Significance of DLC-1 Expression in Cancer: a Meta-Analysis

BACKGROUND

Recent reports have shown that DLC-1 is widely expressed in normal tissues and is down- regulated in a wide range of human tumors, suggesting it may act as a tumor suppressor gene. We conducted a meta-analysis to determine the correlation between DLC-1 expression and clinicopathological characteristics in cancers.

MATERIALS AND METHODS

A detailed literature search was made for relevant publications from PubMed, EMBASE, Cochrane library databases, Web of Science, CNKI. The methodological quality of the studies was also evaluated. Analyses of pooled data were performed and odds ratios (ORs) were calculated and summarized.

RESULTS

Final analysis was performed of 1,815 cancer patients from 19 eligible studies. We observed that DLC- 1 expression was significantly lower in cancers than in normal tissues. DLC-1 expression was not found to be associated with tumor differentiation status. However, DLC-1 expression was obviously lower in advance stage than in early-stage cancers and was more down-regulated in metastatic than non-metastatic cancers.

CONCLUSIONS

The results of our meta-analysis suggested that DLC-1 expression is significantly lower in cancers than in normal tissues. Aberrant DLC-1 expression may play an important role in cancer genesis and metastasis.

The Antitumor Effect of Metformin Is Mediated by miR-26a in Breast Cancer

Metformin, a drug approved for diabetes type II treatment, has been associated with a reduction in the incidence of breast cancer and metastasis and increased survival in diabetic breast cancer patients. High levels of miR-26a expression have been proposed as one of the possible mechanisms for this effect; likewise, this miRNA has also been associated with survival/apoptosis processes in breast cancer. Our aim was to evaluate if miR-26a and some of its targets could mediate the effect of metformin in breast cancer. The viability of MDA-MB-231, MDA-MB-468, and MCF-7 breast cancer cell lines was evaluated with an MTT assay after ectopic overexpression and/or downregulation of miR-26a. Similarly, the expression levels of the miR-26a targets CASP3, CCNE2, ABL2, APAF1, XIAP, BCL-2, PTEN, p53, E2F3, CDC25A, BCL2L1, MCL-1, EZH2, and MTDH were assessed by quantitative polymerase chain reaction (PCR). The effect of metformin treatment on breast cancer cell viability and miR-26a, BCL-2, PTEN, MCL-1, EZH2, and MTDH modulation were evaluated. Wound healing experiments were performed to analyze the effect of miR-26a and metformin treatment on cell migration. MiR-26a overexpression resulted in a reduction in cell viability that was partially recovered by inhibiting it. E2F3, MCL-1, EZH2, MTDH, and PTEN were downregulated by miR-26a and the PTEN (phosphatase and tensin homolog) protein was also reduced after miR-26a overexpression. Metformin treatment reduced breast cancer cell viability, increased miR-26a expression, and led to a reduction in BCL-2, EZH2, and PTEN expression. miR-26a inhibition partly prevents the metformin viability effect and the PTEN and EZH2 expression reduction. Our results indicate that metformin effectively reduces breast cancer cell viability and suggests that the effects of the drug are mediated by an increase in miR-26a expression and a reduction of its targets, PTEN and EHZ2 Thus, the use of metformin in breast cancer treatment constitutes a promising potential breast cancer therapy.

Dlc1 interaction with non-muscle myosin heavy chain II-A (Myh9) and Rac1 activation

The Deleted in liver cancer 1 (Dlc1) gene codes for a Rho GTPase-activating protein that also acts as a tumour suppressor gene. Several studies have consistently found that overexpression leads to excessive cell elongation, cytoskeleton changes and subsequent cell death. However, none of these studies have been able to satisfactorily explain the Dlc1-induced cell morphological phenotypes and the function of the different Dlc1 isoforms. Therefore, we have studied the interacting proteins associated with the three major Dlc1 transcriptional isoforms using a mass spectrometric approach in Dlc1 overexpressing cells. We have found and validated novel interacting partners in constitutive Dlc1-expressing cells. Our study has shown that Dlc1 interacts with non-muscle myosin heavy chain II-A (Myh9), plectin and spectrin proteins in different multiprotein complexes. Overexpression of Dlc1 led to increased phosphorylation of Myh9 protein and activation of Rac1 GTPase. These data support a role for Dlc1 in induced cell elongation morphology and provide some molecular targets for further analysis of this phenotype.

Tensin1 positively regulates RhoA activity through its interaction with DLC1

DLC1 is a RhoGAP-containing tumor suppressor and many of DLC1's functions are absolutely dependent on its RhoGAP activity. Through its RhoGAP domain, DLC1 inhibits the activity of RhoA GTPase, which regulates actin cytoskeleton networks and dis/assembly of focal adhesions. Tensin1 (TNS1) is a focal adhesion molecule that links the actin cytoskeleton to integrins and forms signaling complexes through its multiple binding domains. Here, we report that TNS1 enhances RhoA activity in a DLC1-dependent manner. This is accomplished by binding to DLC1 through TNS1's C2, SH2, and PTB domains. Point mutations at these three sites disrupt TNS1's interaction with DLC1 as well as its effect on RhoA activity. The biological relevance of this TNS1-DLC1-RhoA signaling axis is investigated in TNS1 knockout (KO) cells and mice. Endothelial cells isolated from TNS1 KO mice or those silenced with TNS1 siRNA show significant reduction in proliferation, migration, and tube formation activities. Concomitantly, the RhoA activity is down-regulated in TNS1 KO cells and this reduction is restored by further silencing of DLC1. Furthermore, the angiogenic process is compromised in TNS1 KO mice. These studies demonstrate that TNS1 binds to DLC1 and fine-tunes its RhoGAP activity toward RhoA and that the TNS1-DLC1-RhoA signaling axis is critical in regulating cellular functions that lead to angiogenesis.

Low expression of DLC1 is predictive of poor therapeutic efficiency of fluoropyrimidine and oxaliplatin as adjuvant chemotherapy in gastric cancer

The Rho-GTPase-activating protein, deleted in liver cancer-1 (DLC1), has been reported to be a tumor suppressor. However, the prognostic value of DLC1 in gastric cancer (GC) remains to be fully elucidated. Fluoropyrimidine-oxaliplatin (FP-LOHP) combination therapy has been widely used for the adjuvant chemotherapy of GC, however, no reliable marker has been identified to determine its efficiency. Thus, the present study performed a retrospective investigation involving 251 patients with stage IB-III GC, who received adjuvant chemotherapy following radical resection and 37 patients with stage IV GC, who underwent palliative resection. The expression of DLC1 was found to be reduced in the majority of GC samples (212/288 pairs of samples), compared with normal mucosa, in immunohistochemical analyses. Lower expression levels of DLC1 indicated a more advanced tumor-node-metastasis stage, increased lymph node metastasis, deeper tumor invasion, increased tumor size and a higher rate of distant metastasis. By contrast, relatively increased expression levels of DLC1 indicated a longer time to recurrence (TTR) [hazard ratio (HR), 2.232; P=0.004] and overall survival (OS) rate (HR, 2.910; P=0.001). Patients receiving FP-LOHP adjuvant chemotherapy were significantly less likely to suffer GC recurrence (P=0.001) and succumb to mortality (P=0.004), compared with those who received alternative chemotherapies. However, only the patients with DLC1-positive GC receiving FP-LOHP [DLC1 (+)/FP-LOHP (+)] exhibited a more favorable TTR and OS, compared with the patients with DLC1 (+)/FP-LOHP (−) (TTR, P=0.001; OS, P=0.020). No significant improvement in clinical outcome was observed in GC patients with low DLC1 receiving FP-LOHP treatment (TTR, P=0.270; OS, P=0.197). In conclusion, low expression of DLC1 correlated with GC progression and is predictive of higher rates of recurrence and mortality. Only patients with DLC1-positive GC may have an improved treatment outcome from the use of FP-LOHP as adjuvant chemotherapy.

A phosphorylation switch controls the spatiotemporal activation of Rho GTPases in directional cell migration

Although cell migration plays a central role in development and disease, the underlying molecular mechanism is not fully understood. Here we report that a phosphorylation-mediated molecular switch comprising deleted in liver cancer 1 (DLC1), tensin-3 (TNS3), phosphatase and tensin homologue (PTEN) and phosphoinositide-3-kinase (PI3K) controls the spatiotemporal activation of the small GTPases, Rac1 and RhoA, thereby initiating directional cell migration induced by growth factors. On epidermal growth factor (EGF) or platelet-derived growth factor (PDGF) stimulation, TNS3 and PTEN are phosphorylated at specific Thr residues, which trigger the rearrangement of the TNS3–DLC1 and PTEN–PI3K complexes into the TNS3–PI3K and PTEN–DLC1 complexes. Subsequently, the TNS3–PI3K complex translocates to the leading edge of a migrating cell to promote Rac1 activation, whereas PTEN–DLC1 translocates to the posterior for localized RhoA activation. Our work identifies a core signalling mechanism by which an external motility stimulus is coupled to the spatiotemporal activation of Rac1 and RhoA to drive directional cell migration.

Directed cell migration requires spatially regulated activity of GTPases Rac1 and RhoA. Here Cao et al. show that growth factor stimulation promotes phosphorylation of tensin-3 and phosphatase and tensin homologue (PTEN) and their association with PI 3-kinase and deleted in liver cancer 1 (DLC1) to regulate GTPase activity.

GAP-independent functions of DLC1 in metastasis

Metastases are responsible for most cancer-related deaths. One of the hallmarks of metastatic cells is increased motility and migration through extracellular matrixes. These processes rely on specific small GTPases, in particular those of the Rho family. Deleted in liver cancer-1 (DLC1) is a tumor suppressor that bears a RhoGAP activity. This protein is lost in most cancers, allowing malignant cells to proliferate and disseminate in a Rho-dependent manner. However, DLC1 is also a scaffold protein involved in alternative pathways leading to tumor and metastasis suppressor activities. Recently, substantial information has been gathered on these mechanisms and this review is aiming at describing the potential and known alternative GAP-independent mechanisms allowing DLC1 to impair migration, invasion, and metastasis formation.

The Rho-specific GAP protein DLC3 coordinates endocytic membrane trafficking

Membrane trafficking is known to be coordinated by small GTPases, but the identity of their regulators, the guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs) that ensure balanced GTPase activation at different subcellular sites is largely elusive. Here, we show in living cells that deleted in liver cancer 3 (DLC3, also known as STARD8) is a functional Rho-specific GAP protein, the loss of which enhances perinuclear RhoA activity. DLC3 is recruited to Rab8-positive membrane tubules and is required for the integrity of the Rab8 and Golgi compartments. Depletion of DLC3 impairs the transport of internalized transferrin to the endocytic recycling compartment (ERC), which is restored by the simultaneous downregulation of RhoA and RhoB. We further demonstrate that DLC3 loss interferes with epidermal growth factor receptor (EGFR) degradation associated with prolonged receptor signaling. Taken together, these findings identify DLC3 as a novel component of the endocytic trafficking machinery, wherein it maintains organelle integrity and regulates membrane transport through the control of Rho activity.

Deleted in liver cancer-1 (DLC1): an emerging metastasis suppressor gene

While significant progress continues to be made in the early detection and therapeutic management of primary tumors, the incidence of metastatic disease remains the major cause of mortality. Accordingly, the development of novel effective therapies that can ameliorate dissemination and secondary tumor growth are a clinical priority. The identification of genetic and functional alterations in cancer cells that affect factors implicated in the metastatic process is critical for designing preventive and therapeutic strategies. Evidence implicating the protein deleted in liver cancer-1 (DLC1), a Rho GTPase activator, in metastasis has accumulated to a point where DLC1 may be considered as a metastasis suppressor gene. This review presents evidence supporting an anti-metastatic role for DLC1 in several human cancers and discusses the mechanisms contributing to its inhibitory effects. In addition, promising opportunities for therapeutic interventions based on DLC1 function and downstream pathways involved in the metastatic process are considered.

PTEN redundancy: overexpressing lpten, a homolog of Dictyostelium discoideum ptenA, the ortholog of human PTEN, rescues all behavioral defects of the mutant ptenA-

Mutations in the tumor suppressor gene PTEN are associated with a significant proportion of human cancers. Because the human genome also contains several homologs of PTEN, we considered the hypothesis that if a homolog, functionally redundant with PTEN, can be overexpressed, it may rescue the defects of a PTEN mutant. We have performed an initial test of this hypothesis in the model system Dictyostelium discoideum, which contains an ortholog of human PTEN, ptenA. Deletion of ptenA results in defects in motility, chemotaxis, aggregation and multicellular morphogenesis. D. discoideum also contains lpten, a newly discovered homolog of ptenA. Overexpressing lpten completely rescues all developmental and behavioral defects of the D. discoideum mutant ptenA⁻. This hypothesis must now be tested in human cells.

Deleted in liver cancer 1 expression and localization in hepatocellular carcinoma tissue sections

The deleted in liver cancer (DLC) protein family is composed of proteins that are hypothesized to function predominantly by regulating the activity of the small GTPases. The aim of the present study was to determine the expression and exact localization of DLC1 in hepatocellular carcinoma (HCC) tissue sections. In two types of HCC tissues, typical and fibrolamellar, immunohistochemical and immunofluorescent analysis were performed to assess DLC1 immunoreactivity. Additionally, the DLC1 gene status was determined by the fluorescence in situ hybridization. According to the observations, DLC1 is often lost in cancer cells; however, it can remain within the stromal component of tumor sections. The DLC1 immunoreactivity was particularly noticeable within the capsules surrounding the tumor masses. It was found that the DLC1 gene was deleted in 52% of HCC cases. In addition, the hemizygous deletion was observed to be independent of the HCC subtype. The results indicate that although the loss of DLC1 is a common step during hepatocarcinogenesis, this protein may be present in the tumor microenvironment.

Role of oxidative stress and the microenvironment in breast cancer development and progression

Breast cancer is a highly complex tissue composed of neoplastic and stromal cells. Carcinoma-associated fibroblasts (CAFs) are commonly found in the cancer stroma, where they promote tumor growth and enhance vascularity in the microenvironment. Upon exposure to oxidative stress, fibroblasts undergo activation to become myofibroblasts. These cells are highly mobile and contractile and often express numerous mesenchymal markers. CAF activation is irreversible, making them incapable of being removed by nemosis. In breast cancer, almost 80% of stromal fibroblasts acquire an activated phenotype that manifests by secretion of elevated levels of growth factors, cytokines, and metalloproteinases. They also produce hydrogen peroxide, which induces the generation of subsequent sets of activated fibroblasts and tumorigenic alterations in epithelial cells. While under oxidative stress, the tumor stroma releases high energy nutrients that fuel cancer cells and facilitate their growth and survival. This review describes how breast cancer progression is dependent upon oxidative stress activated stroma and proposes potential new therapeutic avenues.

Loss of DLC1 is an independent prognostic factor in patients with oral squamous cell carcinoma

Deleted in liver cancer (DLC1), a Rho GTPase-activating protein, was observed to be differentially expressed in oral squamous cell carcinoma in comparison with normal tissues using tissue proteomics. In the current study, we investigated the clinical significance of loss of DLC1 expression in different stages of development of oral squamous cell carcinoma to determine its potential as a biomarker for oral dysplasia and prognosis of oral squamous cell carcinoma. Immunohistochemical analysis of DLC1 expression was carried out in oral squamous cell carcinoma patients (n=214), dysplasia (n=51), hyperplastic squamous mucosa (n=45), and histologically normal oral tissues (n=80), and correlated with clinicopathological parameters and disease prognosis over 91 months for oral squamous cell carcinomas. Loss of DLC1 expression was observed in oral squamous cell carcinoma (64%), oral dysplasia (31%), hyperplastic squamous mucosa (22%), and normal mucosa (16%). Significant loss of DLC1 expression was observed in oral squamous cell carcinomas as compared with dysplasia (P<0.001, odds ratio=3.8, 95% CI=2.0-7.3), suggesting it may be an important event involved in cancer progression. Among oral squamous cell carcinomas, the loss of DLC1 expression was significantly associated with poor prognosis (P=0.021, hazards ratio (HR)=1.8, 95% CI=1.1-2.9). Multivariate analysis revealed loss of DLC1 (P=0.023, HR=2.1, 95% CI=1.2-3.9) and histopathological grade (P=0.015, HR=1.7, 95% CI=1.1-2.7) to be independent predictors for disease-free survival in oral squamous cell carcinoma patients in comparison with known prognostic factors, viz. tumor stage, nodal status, and overall stage. Loss of DLC1 expression emerged as an important biomarker for predicting patients diagnosed with oral dysplasia at high risk of transformation upon future validation in longitudinal studies. Loss of DLC1 expression is a poor prognostic marker for oral squamous cell carcinoma patients.

Silencing of DLC1 upregulates PAI-1 expression and reduces migration in normal prostate cells

Deleted in liver cancer 1 (DLC1) is a GTPase-activating protein (GAP) domain containing tumor suppressor that localizes to focal adhesions. In cancer cells, loss of DLC1 is known to enhance cancer cell migration. However, the role of DLC1 in normal cell migration has not been well studied. Here, we show that silencing of DLC1 (shDLC1) in normal prostate epithelial cells reduces cell migration in both Transwell and wound-healing assays. This migration defect is mainly due to upregulation of plasminogen activator inhibitor 1 (PAI-1). Silencing of PAI-1 rescues the shDLC1-reduced migration phenotype. Reexpression of DLC1 suppresses PAI-1 and restores the migration defect as well. In contrast, DLC1-K714E (GAP inactive) mutant neither decreases the PAI-1 level nor rescues the shDLC1 migration defect. Interestingly, DLC1-Y442F (tensin-binding and focal adhesion-localizing defective) mutant is able to suppress PAI-1 expression but does not restore the migration defect. Furthermore, PAI-1 upregulation in shDLC1 cells is EGFR-MEK pathway dependent and is able to promote in vitro angiogenesis. Together, our results show that at least the following two new mechanisms are involved in DLC1-mediated normal cell migration: (i) DLC1 modulates the expression of PAI-1, which is a negative regulator for cell migration, in a GAP domain and EGFR-MEK-dependent manner and (ii) Independent of PAI-1, the interaction of DLC1 with tensin members positively regulates cell migration.

Loss of phosphatase and tensin homolog enhances cell invasion and migration through AKT/Sp-1 transcription factor/matrix metalloproteinase 2 activation in hepatocellular carcinoma and has clinicopathologic significance

Phosphatase and tensin homolog (PTEN) is frequently inactivated in cancers and is associated with advanced stages of cancers or metastasis. However, the molecular mechanism of PTEN in hepatocellular carcinoma (HCC) metastasis is unclear. In this study, we found frequent (47.5%, n = 40) protein underexpression of PTEN in human HCCs compared with their corresponding nontumorous livers. Significantly, PTEN underexpression was associated with larger tumor size (P = 0.021), tumor microsatellite formation (P = 0.027), and shorter overall survival of patients (P = 0.035). Using different cell models, we observed that PTEN-knockdown HCC cells and PTEN-knockout mouse embryonic fibroblasts (MEFs) had enhanced cell migratory and invasive abilities. In addition to activation of AKT, there was up-regulation of the Sp1 transcription factor (SP1) and matrix metalloproteinase 2 (MMP2), as well as MMP2 activation in PTEN-knockdown HCC cells and PTEN(-/-) MEFs. With dual luciferase reporter assay, exogenous expression of SP1 in HCC cells led to enhanced MMP2 promoter activity by up to 74%, whereas deletion of the putative SP1 binding site on the MMP2 promoter led to reduced promoter activity by up to 65%. Using chromatin immunoprecipitation assay, we documented increased binding of SP1 to the MMP2 promoter in PTEN-knockdown HCC cells. Overexpression of SP1 and MMP2 was significantly but negatively associated with PTEN underexpression in human HCCs.

CONCLUSION

Our results show that PTEN was underexpressed in HCCs, and this underexpression was associated with more aggressive biological behavior and poorer patient survival. We have provided the first evidence that MMP2 up-regulation upon PTEN loss is SP1-dependent. Our findings indicate that PTEN plays a significant role in down-regulating HCC cell invasion via the AKT/SP1/MMP2 pathway.

Dual interference with novel genes mfgl2 and mTNFR1 ameliorates murine hepatitis virus type 3-induced fulminant hepatitis in BALB/cJ mice

Our studies and those of many others have implicated hepatocyte necrosis and apoptosis mediated by fibrinogen-like protein-2 (fgl2) prothrombinase and tumor necrosis factor receptor (TNFR) in the development of fulminant viral hepatitis, a disease with a mortality rate greater than 80% in cases lacking immediate organ transplantation. This study was designed to explore the efficacy of dual short hairpin RNA (shRNA) interference with fgl2 and TNFR1 in the treatment of murine hepatitis virus strain 3 (MHV-3)-induced fulminant hepatitis in mice. Plasmids p-mfgl2shRNA and p-mTNFR1shRNA, complementary to the sequences for mfgl2 and mTNFR1, were constructed. Plasmids pEGFP-mfgl2 and pEGFP-mTNFR1 expressing mfgl2-EGFP (enhanced green fluorescent protein) and mTNFR1-EGFP fusion proteins were also constructed to screen the inhibitory effect of p-mfgl2shRNA and p-mTNFR1shRNA on mfgl2 and mTNFR1 expression. Cotransfection of individual shRNA plasmids and pcDNA3.0-mfgl2 and pcDNA3.0-mTNFR1 expression constructs into Chinese hamster ovary (CHO) cells significantly inhibited mfgl2 and mTNFR1 gene expression, as evidenced by fluorescence microscopy, reverse transcription-polymerase chain reaction, and Western blotting. In vivo hydrodynamic delivery of dual-interference shRNA plasmids for mfgl2 and mTNFR1 significantly decreased mfgl2 and mTNFR1 expression; markedly ameliorated fibrin deposition, hepatocyte necrosis, and apoptosis; and prolonged survival against fulminant viral hepatitis induced by MHV-3 in BALB/cJ mice compared with mfgl2 or TNFR1 single-gene interference. These results indicate that in vivo interference with genes for more than one key target provides superior treatment efficacy compared with single-gene interference.

The cooked meat-derived mammary carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine promotes invasive behaviour of breast cancer cells

The cooked meat derived genotoxic carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) induces cancer of the colon, prostate and mammary gland when fed to rats. Epidemiology studies link these tumours to a Western diet and exposure to heterocyclic amines such as PhIP. We have shown that PhIP is also potently estrogenic and have proposed that this hormonal activity contributes to its target site carcinogenicity. We now postulate that the estrogenic properties of PhIP influence metastatic potential. We have used an in vitro assay for cell invasion based upon digestion and migration through a reconstituted basement membrane model. Zymography and immunoblotting were used to confirm PhIP-mediated changes associated with induction of the invasive phenotype. Treatment of the mammary cancer cell lines MCF-7 and T47D with PhIP induces cells to digest and migrate through a reconstituted basement membrane. The response was dose dependent, observed at sub-nanomolar concentrations of PhIP and was inhibited by the antiestrogen ICI 182,780. The PhIP-induced invasive phenotype was associated with expression of cathepsin D, cyclooxygenase-2 and matrix metalloproteinase activity. These findings emphasise the range and potency of the biological activities associated with this cooked meat product and mechanistically support the tissue-specific carcinogenicity of the chemical.

Akt phosphorylation of deleted in liver cancer 1 abrogates its suppression of liver cancer tumorigenesis and metastasis

BACKGROUND & AIMS

Deleted in liver cancer 1 (DLC1), which encodes a Rho GTPase activating protein, is a bona fide tumor suppressor in hepatocellular carcinoma. Underexpression of DLC1 in cancer has been attributed to genomic deletion and epigenetic silencing. However, the regulatory mechanism of the tumor suppressive activity of DLC1 remains elusive. In this study, we elucidated a novel post-translational modification by which the activity of DLC1 is functionally regulated.

METHODS

Molecular and biochemical approaches were employed to study Akt phosphorylation of DLC1. In vitro and in vivo functional assays were performed to elucidate the functional significance of Akt phosphorylation of DLC1.

RESULTS

Phosphorylation of ectopically expressed and endogenous DLC1 was enhanced upon insulin induction or with Akt expression in liver cancer cell lines. Conversely, addition of a phosphatidylinositol 3-kinase/Akt pathway inhibitor or silencing of Akt attenuated the phosphorylation level of DLC1. Site-directed mutagenesis was employed to replace the serine residue of the consensus Akt substrate motifs of DLC1 with alanine. S567 of DLC1 was identified as the only target of Akt phosphorylation. S567 is well conserved in all DLC family members. DLC2 was phosphorylated by Akt at the corresponding residue. Functional assays demonstrated that the S567D phosphomimetic DLC1 mutant lost its inhibitory activities in tumorigenesis and metastasis of oncogenically transformed hepatoblasts in a mouse model.

CONCLUSIONS

This study has revealed a novel post-translational modification that functionally deregulates the biologic activities of DLC1. Phosphorylation of DLC1 and DLC2 by Akt at the conserved residue points to a common regulatory mechanism of the DLC tumor suppressor family.

DLC1 activation requires lipid interaction through a polybasic region preceding the RhoGAP domain

Deleted in Liver Cancer 1 (DLC1) is a GTPase-activating protein (GAP) with specificity for RhoA, RhoB, and RhoC that is frequently deleted in various tumor types. By inactivating these small GTPases, DLC1 controls actin cytoskeletal remodeling and biological processes such as cell migration and proliferation. Here we provide evidence that DLC1 binds to phosphatidylinositol-4,5-bisphosphate (PI(4,5)P(2)) through a previously unrecognized polybasic region (PBR) adjacent to its RhoGAP domain. Importantly, PI(4,5)P(2)-containing membranes are shown to stimulate DLC1 GAP activity in vitro. In living cells, a DLC1 mutant lacking an intact PBR inactivated Rho signaling less efficiently and was severely compromised in suppressing cell spreading, directed migration, and proliferation. We therefore propose that PI(4,5)P(2) is an important cofactor in DLC1 regulation in vivo and that the PBR is essential for the cellular functions of the protein.