FBX4 mediates rapid cyclin D1 proteolysis upon DNA damage in immortalized esophageal epithelial cells.

It has been implied that deregulation of cyclin D1 turnover under stresses can facilitate genomic instability and trigger tumorigenesis. Much focus has been placed on identifying the E3 ligases responsible for mediating cyclin D1 degradation. However, the findings were quite controversial and cell type-dependent. Little is known about how cyclin D1 is regulated in precancerous cells upon DNA damage and which E3 ligases mediate the effects. Here we found cyclin D1 reduction is an early response to DNA damage in immortalized esophageal epithelial cells, with expression dropping to a low level within 1 h after γ-irradiation. Comparison of temporal expression of cyclin D1 upon DNA damage between immortalized NE083-hTERT and NE083-E6E7, the latter being p53/p21-defective, showed that DNA damage-induced rapid cyclin D1 reduction was p53-independent and occurred before p21 accumulation. Overexpression of cyclin D1 in NE083-E6E7 cells could attenuate G0/G1 cell cycle arrest at 1 h after irradiation. Furthermore, rapid reduction of cyclin D1 upon DNA damage was attributed to proteasomal degradation, as evidenced by data showing that proteasomal inhibition by MG132 blocked cyclin D1 reduction while cycloheximide facilitated it. Inhibition of ATM activation and knockdown of E3 ligase adaptor FBX4 reversed cyclin D1 turnover in immortalized NE083-hTERT cells. Further study showed that knockdown of FBX4 facilitated DNA breaks, as indicated by an increase in γ-H2AX foci in esophageal cancer cells. Taken together, the results substantiated a pivotal role of ATM and FBX4 in cyclin D1 proteolysis upon DNA damage in precancerous esophageal epithelial cells, implying that deregulation of the process may contribute to carcinogenesis of esophageal squamous cell carcinoma.

Transport Vesicle Tethering at the Trans Golgi Network: Coiled Coil Proteins in Action.

The Golgi complex is decorated with so-called Golgin proteins that share a common feature: a large proportion of their amino acid sequences are predicted to form coiled-coil structures. The possible presence of extensive coiled coils implies that these proteins are highly elongated molecules that can extend a significant distance from the Golgi surface. This property would help them to capture or trap inbound transport vesicles and to tether Golgi mini-stacks together. This review will summarize our current understanding of coiled coil tethers that are needed for the receipt of transport vesicles at the trans Golgi network (TGN). How do long tethering proteins actually catch vesicles? Golgi-associated, coiled coil tethers contain numerous binding sites for small GTPases, SNARE proteins, and vesicle coat proteins. How are these interactions coordinated and are any or all of them important for the tethering process? Progress toward understanding these questions and remaining, unresolved mysteries will be discussed.

Protein flexibility is required for vesicle tethering at the Golgi.

The Golgi is decorated with coiled-coil proteins that may extend long distances to help vesicles find their targets. GCC185 is a trans Golgi-associated protein that captures vesicles inbound from late endosomes. Although predicted to be relatively rigid and highly extended, we show that flexibility in a central region is required for GCC185's ability to function in a vesicle tethering cycle. Proximity ligation experiments show that that GCC185's N-and C-termini are within <40 nm of each other on the Golgi. In physiological buffers without fixatives, atomic force microscopy reveals that GCC185 is shorter than predicted, and its flexibility is due to a central bubble that represents local unwinding of specific sequences. Moreover, 85% of the N-termini are splayed, and the splayed N-terminus can capture transport vesicles in vitro. These unexpected features support a model in which GCC185 collapses onto the Golgi surface, perhaps by binding to Rab GTPases, to mediate vesicle tethering.

Molecular and cellular characterization of GCC185: a tethering protein of the trans-Golgi network.

Transport vesicle tethers are proteins that link partner membranes together to permit subsequent SNARE protein pairing and fusion. Despite the identification of a relatively large number of tethering proteins, little is known about the precise mechanisms by which they act. Biochemical isolation of tethers permits direct analysis of their physical characteristics and molecular interactions. Here, we describe the expression and purification of GCC185, a trans-Golgi-localized, 190-kDa coiled-coil tethering protein. In addition, we present a gene rescue approach to analyze the function of this tether after its depletion from cells using siRNA.

Expression of Epstein-Barr virus-encoded LMP1 and hTERT extends the life span and immortalizes primary cultures of nasopharyngeal epithelial cells.

Cell immortalization is regarded as an early and pre-requisite step in tumor development. Defining the specific genetic events involved in cell immortalization may provide insights into the early events of carcinogenesis. Nasopharyngeal carcinoma is common among the Southern Chinese population. Epstein-Barr virus (EBV) infection is associated closely with nasopharyngeal carcinoma. The involvement of LMP1 (an EBV-encoded oncogene) has been implicated in the pathogenesis of nasopharyngeal carcinoma. In this study, LMP1 expression, in combination with ectopic expression of hTERT (catalytic unit of human telomerase), was shown to extend the life span of primary cultures of nasopharyngeal epithelial cells and facilitate the immortalization of one of the cell lines (NP446). This is the first report on the successful immortalization of nasopharyngeal epithelial cells involving LMP1. The events associated with the immortalization of nasopharyngeal epithelial cells by LMP1/hTERT were characterized. Expression of c-Myc, Bmi-1, and Id-1 were upregulated at an early stage of immortalization. At a later stage of immortalization, downregulation of p21 and p16 expression were observed. Upregulation of EGFR expression and activation of MAPK signaling pathway were observed in LMP1/hTERT-immortalized nasopharyngeal epithelial cells. The LMP1/hTERT-immortalized NP446 cells were non-tumorigenic in immunosuppressed nude mice and retained anchorage-dependent growth, suggesting that additional events are required for tumorigenic transformation. The ability of the EBV-encoded LMP1, in the presence of hTERT expression, to extend the life span and immortalize primary cultures of nasopharyngeal epithelial cells supports the involvement of EBV infection and its viral products in the early stage of pathogenesis of nasopharyngeal carcinoma.

Genetic alterations in a telomerase-immortalized human esophageal epithelial cell line: implications for carcinogenesis.

Ectopic expression of viral oncoproteins disrupts cellular functions and limits the value of many existing immortalization models as models for carcinogenesis, especially for cancers without definitive viral etiology. Our newly established telomerase-immortalized human esophageal epithelial cell line, NE2-hTERT, retained nearly-diploid and non-tumorigenic characteristics, but exhibited genetic and genomic alterations commonly found in esophageal cancer, including progressive loss of the p16(INK4a) alleles, upregulation of anti-apoptotic proteins, epithelial-mesenchymal transition, whole-chromosome 7 gain and duplicated 5q arm. Our data also revealed a novel positive regulation of p16(INK4a) on cyclin D1. These findings probably represent early crucial events and mechanisms in esophageal carcinogenesis.

Berberine inhibits Rho GTPases and cell migration at low doses but induces G2 arrest and apoptosis at high doses in human cancer cells.

Berberine is an active ingredient extracted from Coptidis rhizoma which has been used for centuries as a traditional Chinese medicine for treatment of inflammatory diseases. Recent studies have indicated that berberine has anticancer properties. Berberine arrested cell growth and inhibited cell migration in various cancer cell lines. In this study, we examined the effects of berberine on HONE1 cells, which have been commonly used as a cell model for nasopharyngeal carcinoma. We observed the inhibitory effects of berberine on HONE1 cells at a high dosage (>150 microM). Berberine effectively induced the mitotic arrest of HONE1 cells at 300 microM which was associated with apoptosis. Berberine had differential intracellular localization at low and high doses. At a low dose (50 microM), berberine was localized in the mitochondria while at a high dose (300 microM), berberine was localized in the nucleus which may have induced mitotic arrest. Berberine effectively inhibited cell migration and invasion at low doses. Using a specific GST pull-down assay of activated Rho GTPases, we demonstrated that berberine suppressed the activation of Rho GTPases including RhoA, Cdc42 and Rac1. This indicates a novel function of berberine in the suppression of Rho GTPase signaling to mediate its inhibitory action on cell migration and motility. The potential of berberine to inhibit cancer metastasis in cancer warrants further investigation.

Id-1 activation of PI3K/Akt/NFkappaB signaling pathway and its significance in promoting survival of esophageal cancer cells.

Inhibitor of differentiation or DNA binding (Id-1) is a helix-loop-helix protein that is over-expressed in many types of cancer including esophageal cancer. This study aims to investigate its effects on the phosphatidylinositol-3-kinase (PI3K)/Akt/ nuclear factor kappa B (NFkappaB) signaling pathway and the significance in protecting esophageal cancer cells against apoptosis. We found elevated expression of phosphorylated forms of Akt, glycogen synthase kinase 3beta and inhibitor of kappa B, as well as increased nuclear translocation of NFkappaB subunit p65 and NFkappaB DNA-binding activity, in esophageal cancer cells with stable ectopic Id-1 expression. Transient transfection of Id-1 into HEK293 cells confirmed activation of PI3K/Akt/NFkappaB signaling and the effects were counteracted by the PI3K inhibitor LY294002. Treatment with tumor necrosis factor-alpha (TNF-alpha) elicited a significantly weaker apoptotic response, following a marked and sustained activation of Akt and NFkappaB in the Id-1-over-expressing cells, compared with the vector control. The effects of Id-1 on the PI3K/Akt/NFkappaB signaling pathway and apoptosis were reversed in esophageal cancer cells transfected with siRNA against Id-1. In addition, inhibition of PI3K or NFkappaB signaling using the PI3K inhibitor LY294002 or the NFkappaB inhibitor Bay11-7082 increased the sensitivity of Id-1-over-expressing esophageal cancer cells to TNF-alpha-induced apoptosis. Our results provide the first evidence that Id-1 induces the activation of PI3K/Akt/NFkappaB signaling pathway, and protects esophageal cancer cells from TNF-alpha-induced apoptosis in vitro. Inactivation of Id-1 may provide us with a novel strategy to improve the treatment and survival of patients with esophageal cancer.

Id-1 promotes proliferation of p53-deficient esophageal cancer cells.

The helix-loop-helix protein inhibitor of differentiation and DNA binding (Id-1) is known to promote cellular proliferation in several types of human cancer. Although it has been reported that Id-1 is over-expressed in esophageal squamous cell carcinoma (ESCC), its function and signaling pathways in esophageal cancer are unknown. In our study, we investigated the direct effects of Id-1 on esophageal cancer cell growth by transfecting an Id-1 expression vector into an ESCC cell line (HKESC-3), which showed serum-dependent Id-1 expression. Ectopic Id-1 expression resulted in increased serum-independent cell growth and G1-S phase transition, as well as up-regulation of mouse double minute 2 (MDM2) and down-regulation of p21Waf1/Cip1 protein expressions in the transfectant clones in a p53-independent manner. However, overexpression of Id-1 had no effect on the pRB, CDK4 and p16INK4A expressions. Stable transfection of Id-1 antisense expression vector to inhibit the expression of endogenous Id-1 in another ESCC cell line (HKESC-1) reversed the effects on MDM2 and p21Waf1/Cip1. In addition, Id-1 expression protected ESCC cells from Tumor Necrosis Factor (TNF)-alpha-induced apoptosis by up-regulating and activating Bcl-2. In conclusion, our study provides evidence for the first time that Id-1 plays a role in both proliferation and survival of esophageal cancer cells. Our findings also suggest that unlike prostate, hepatocellular and nasopharyngeal carcinomas in which Id-1 induces cell proliferation through inactivation of p16INK4A/RB pathway, the increased cell proliferation observed in ESCC cells may be mediated through a different mechanism.