Cyclin G2 is a centrosome-associated nucleocytoplasmic shuttling protein that influences microtubule stability and induces a p53-dependent cell cycle arrest

beta-tting on p63 as a metastatic suppressor

Although much is known about the genes that promote metastasis, few suppressors of metastasis have been found. Adorno et al. (2009) now identify p63 as a potent suppressor of metastasis and uncover an intricate mechanism for the inactivation of metastasis in cancer cells in response to transforming growth factor beta.

Estradiol downregulation of the tumor suppressor gene BTG2 requires estrogen receptor-alpha and the REA corepressor

B-cell Translocation Gene 2 (BTG2/TIS21/PC3) is an anti-proliferative tumor suppressor gene whose expression is significantly reduced in breast carcinomas, and in MCF-7 and T-47D breast cancer cell lines treated with estradiol (E2). In this study the mechanisms involved in E2 down regulation of BTG2 gene expression were examined. Depletion of ERalpha by siRNA indicated that the receptor is required for E2 down regulation of BTG2 mRNA levels, and cycloheximide experiments indicated that the effect of E2 on BTG2 expression was independent of intermediary protein synthesis. Chromatin immunoprecipitation analyses revealed that ERalpha interacts with the BTG2 promoter in a ligand-independent fashion whereas transfection experiments indicated that ERalpha's DNA and ligand binding domains are required for E2 repression of BTG promoter activity. Surprisingly, histone deacetylase (HDACs) activity is essential for basal expression as evidenced by trichostatin A inhibition of BTG2 mRNA levels. Estradiol treatment did not alter histone H3 acetylation although it did induce displacement of RNA polymerase II from the BTG2 gene. Depletion of the ER specific corepressor REA (Repressor of Estrogen Receptor Activity) significantly abrogated E2-mediated BTG2 repression. Taken together, our results reveal a requirement of HDAC activity for basal BTG2 expression and the ERalpha-REA interaction for estrogen repression of the BTG2 gene. The ability of E2-bound ERalpha and REA to suppress BTG2 expression indicates a positive role for this corepressor in regulation of breast cancer cell proliferation.

Evidence that human blastomere cleavage is under unique cell cycle control

Purpose

To understand the molecular pathways that control early human embryo development.

Methods

Improved methods of linear amplification of mRNAs and whole human genome microarray analyses were utilized to characterize gene expression in normal appearing 8-Cell human embryos, in comparison with published microarrays of human fibroblasts and pluripotent stem cells.

Results

Many genes involved in circadian rhythm and cell division were over-expressed in the 8-Cells. The cell cycle checkpoints, RB and WEE1, were silent on the 8-Cell arrays, whereas the recently described tumor suppressor, UHRF2, was up-regulated >10-fold, and the proto-oncogene, MYC, and the core element of circadian rhythm, CLOCK, were elevated up to >50-fold on the 8-Cell arrays.

Conclusions

The canonical G1 and G2 cell cycle checkpoints are not active in totipotent human blastomeres, perhaps replaced by UHRF2, MYC, and intracellular circadian pathways, which may play important roles in early human development.

Electronic supplementary material

The online version of this article (doi:10.1007/s10815-009-9306-x) contains supplementary material, which is available to authorized users.

Large-scale detection of ubiquitination substrates using cell extracts and protein microarrays

Identification of protein targets of post-translational modification is an important analytical problem in biology. Protein microarrays exposed to cellular extracts could offer a rapid and convenient means of identifying modified proteins, but this kind of biochemical assay, unlike DNA microarrays, depends on a faithful reconstruction of in vivo conditions. Over several years, concentrated cellular extracts have been developed, principally for cell cycle studies that reproduce very complex cellular states. We have used extracts that replicate the mitotic checkpoint and anaphase release to identify differentially regulated poyubiquitination. Protein microarrays were exposed to these complex extracts, and the polyubiquitinated products were detected by specific antibodies. We expected that among the substrates revealed by the microarray should be substrates of the anaphase promoting complex (APC). Among 8,000 proteins on the chip, 10% were polyubiquitinated. Among those, we found 11 known APC substrates (out of 16 present on the chip) to be polyubiquitinated. Interestingly, only 1.5% of the proteins were differentially ubiquitinated on exit from the checkpoint. When we arbitrarily chose 6 proteins thought to be involved in mitosis from the group of differentially modified proteins, all registered as putative substrates of the APC, and among 4 arbitrarily chosen non-mitotic proteins picked from the same list, 2 were ubiquitinated in an APC-dependent manner. The striking yield of potential APC substrates from a simple assay with concentrated cell extracts suggests that combining microarray analysis of the products of post-translational modifications with extracts that preserve the physiological state of the cell can yield information on protein modification under various in vivo conditions.

HIF-1alpha and HIF-2alpha have divergent roles in colon cancer

Hypoxia-inducible factor (HIF)-1 and HIF-2 are heterodimeric transcription factors that mediate the cellular response to hypoxia. Their key regulatory subunits, HIF-1alpha and HIF-2alpha, are induced similarly by hypoxia, but their functional roles in cancer may be distinct and isoform-specific. SW480 colon cancer cells with stable expression of siRNA to HIF-1alpha or HIF-2alpha or both were established. HIF-1alpha-deficient cells displayed lower rates of proliferation and migration, but HIF-2alpha-deficient cells exhibited enhanced anchorage independent growth in a soft agar assay. Xenograft studies revealed that HIF-1alpha deficiency inhibited overall tumor growth, whereas deficiency of HIF-2alpha stimulated tumor growth. In human colon cancer tissues, expression of HIF-1alpha and to a lesser extent, HIF-2alpha, was linked to upregulation of VEGF and tumor angiogenesis. However, loss of expression of HIF-2alpha but not HIF-1alpha was strongly correlated with advanced tumor stage. DNA microarray analysis identified distinct sets of HIF-1alpha and HIF-2alpha target genes that may explain these phenotypic differences. Collectively, these findings suggest that HIF isoforms may have differing cellular functions in colon cancer. In particular, HIF-1alpha promoted the growth of SW480 colon cancer cells but HIF-2alpha appeared to restrain growth. Consequently, therapeutic approaches that target HIF may need to consider these isoform-specific properties.

Dynamic subcellular localization of the Ewing sarcoma proto-oncoprotein and its association with and stabilization of microtubules

The Ewing sarcoma (EWS) protein is a member of a large family of RNA-binding proteins. Chimeric EWS oncoproteins generated by chromosomal translocations between the EWS protein and several transcription factors cause various malignant tumors. Due to its multifunctional properties, the EWS protein is involved in such processes as meiotic DNA pairing/recombination, cellular senescence, gene expression, RNA processing and transport, and cell signaling. The EWS protein is predominantly located in the nucleus. It was found in the cytoplasm and associated with the cell membrane. In this study, analysis of the localization of endogenous and fluorescently labeled recombinant EWS protein in different phases of the cell cycle in different cell lines revealed a very dynamic subcellular distribution of the EWS protein. In Cos7 and HeLa cells, an association of the EWS protein with the centrosomal compartments was shown. Furthermore, in HEK (human embryonic kidney)-293 (T) cells, an interaction of the overexpressed recombinant EWS-yellow fluorescent protein fusion protein with microtubules, leading to their stabilization and cell cycle arrest, was demonstrated. As an outlook, the present findings provide an important insight into temporally and spatially regulated functions of the EWS protein and, particularly, into its role in the regulation of the cell cycle and possibly cell differentiation.

Cyclin G2 is degraded through the ubiquitin-proteasome pathway and mediates the antiproliferative effect of activin receptor-like kinase 7

We have previously reported that Nodal, a member of the TGF-beta superfamily, acts through activin receptor-like kinase 7 (ALK7) to inhibit ovarian cancer cell proliferation. To determine the mechanism underlying their effects, a cell cycle gene array was performed and cyclin G2 mRNA was found to be strongly up-regulated by Nodal and ALK7. To study the function and regulation of cyclin G2 in ovarian cancer cells, expression constructs were generated. We found that cyclin G2 protein level decreased rapidly after transfection, and this decrease was prevented by 26S proteasome inhibitors. Immunoprecipitation and pull-down studies showed that ubiquitin, Skp1, and Skp2 formed complexes with cyclin G2. Knockdown of Skp2 by siRNA increased, whereas overexpression of Skp2 decreased cyclin G2 levels. Nodal and ALK7 decreased the expression of Skp1 and Skp2 and increased cyclin G2 levels. Overexpression of cyclin G2 inhibited cell proliferation whereas cyclin G2-siRNA reduced the antiproliferative effect of Nodal and ALK7. Taken together, these findings provide strong evidence that cyclin G2 is degraded by the ubiquitin-proteasome pathway and that Skp2 plays a role in regulating cyclin G2 levels. Furthermore, our results also demonstrate that the antiproliferative effect of Nodal/ALK7 on ovarian cancer cells is in part mediated by cyclin G2.

Cotylenin A, a new differentiation inducer, and rapamycin cooperatively inhibit growth of cancer cells through induction of cyclin G2

Cotylenin A, a plant growth regulator, and rapamycin, an inhibitor of mammalian target of rapamycin (mTOR), are potent inducers of differentiation of myeloid leukemia cells. Recently, we found that cotylenin A and rapamycin effectively inhibited the proliferation of several human breast cancer cell lines including MCF-7. Herein, we demonstrate that cotylenin A and rapamycin rapidly and markedly induced the cyclin G2 gene expression in several cancer cells including MCF-7 cells. The growth arrest of the MCF-7 cells at the G1 phase, induced by the treatment with cotylenin A and rapamycin or the culture in low serum medium, markedly induced the cyclin G2 gene expression. Anticancer drugs including doxorubicin, etoposide and 5-fluorouracil also induced cyclin G2 expression during induction of growth arrest of the MCF-7 cell at the G1 phase or G2/M phase. Ectopically inducible cyclin G2 expression potently inhibited the proliferation of MCF-7 cells. Furthermore, cyclin G2 knockdown induced by cyclin G2 small interfering RNA markedly reduced the potency of cotylenin A plus rapamycin to induce growth inhibition. Taken together, our results suggest that cotylenin A and rapamycin induce inhibition of cancer cell growth through the induction of cyclin G2.

Activation of the insulin receptor (IR) by insulin and a synthetic peptide has different effects on gene expression in IR-transfected L6 myoblasts

Single-chain peptides have been recently produced that display either mimetic or antagonistic properties against the insulin and IGF-1 (insulin-like growth factor 1) receptors. We have shown previously that the insulin mimetic peptide S597 leads to significant differences in receptor activation and initiation of downstream signalling cascades despite similar binding affinity and in vivo hypoglycaemic potency. It is still unclear how two ligands can initiate different signalling responses through the IR (insulin receptor). To investigate further how the activation of the IR by insulin and S597 differentially activates post-receptor signalling, we studied the gene expression profile in response to IR activation by either insulin or S597 using microarray technology. We found striking differences between the patterns induced by these two ligands. Most remarkable was that almost half of the genes differentially regulated by insulin and S597 were involved in cell proliferation and growth. Insulin either selectively regulated the expression of these genes or was a more potent regulator. Furthermore, we found that half of the differentially regulated genes interact with the genes involved with the MAPK (mitogen-activated protein kinase) pathway. These findings support our signalling results obtained previously and confirm that the main difference between S597 and insulin stimulation resides in the activation of the MAPK pathway. In conclusion, we show that insulin and S597 acting via the same receptor differentially affect gene expression in cells, resulting in a different mitogenicity of the two ligands, a finding which has critical therapeutic implications.

Roles of human epidermal growth factor receptor 2, c-jun NH2-terminal kinase, phosphoinositide 3-kinase, and p70 S6 kinase pathways in regulation of cyclin G2 expression in human breast cancer cells

The CCNG2 gene that encodes the unconventional cyclin G2 was one of the few genes up-regulated on anti-human epidermal growth factor receptor 2 (HER2) antibody-mediated inhibition of HER2 signaling. The purpose of this study was to explore how HER2 signaling modulates cyclin G2 expression and the effect of elevated cyclin G2 on breast cancer cell growth. Treatment of breast cancer cells that overexpress HER2 (BT474, SKBr3, and MDAMB453) with the anti-HER2 antibody trastuzumab or its precursor 4D5 markedly up-regulated cyclin G2 mRNA in vitro and in vivo, as shown by real-time PCR. Immunoblot and immunofluorescence analysis with specific antibodies against cyclin G2 showed that anti-HER2 antibody significantly increased cyclin G2 protein expression and translocated the protein to the nucleus. Trastuzumab was not able to induce cyclin G2 expression in cells weakly expressing HER2 (MCF7) or in cells that had developed resistance to trastuzumab. Enforced expression of HER2 in T47D and MDAMB435 breast cancer cells reduced cyclin G2 levels. Collectively, these data suggest that HER2-mediated signaling negatively regulates cyclin G2 expression. Inhibition of phosphoinositide 3-kinase (LY294002), c-jun NH(2)-terminal kinase (SP600125), and mammalian target of rapamycin (mTOR)/p70 S6 kinase (p70S6K; rapamycin) increased cyclin G2 expression. In contrast, treatment with inhibitors of p38 mitogen-activated protein kinase (SB203580), mitogen-activated protein kinase/extracellular signal-regulated kinase kinase 1/2 (U0126), or phospholipase Cgamma (U73122) did not affect cyclin G2 expression. Anti-HER2 antibody in combination with LY294002, rapamycin, or SP600125 induced greater cyclin G2 expression than either agent alone. Ectopic expression of cyclin G2 inhibited cyclin-dependent kinase 2 activity, Rb phosphorylation, cell cycle progression, and cellular proliferation without affecting p27(Kip1) expression. Thus, cyclin G2 expression is modulated by HER2 signaling through multiple pathways including phosphoinositide 3-kinase, c-jun NH(2)-terminal kinase, and mTOR signaling. The negative effects of cyclin G2 on cell cycle and cell proliferation, which occur without altering p27(Kip1) levels, may contribute to the ability of trastuzumab to inhibit breast cancer cell growth.