The phenotype of a “Cdc2 kinase target site-deficient” mutant of oncoprotein 18 reveals a role of this protein in cell cycle control

Stathmin 1 deficiency induces erythro-megakaryocytic defects leading to macrocytic anemia and thrombocythemia in Stathmin 1 knock out mice

Stathmin 1 (STMN1) is a cytosolic phosphoprotein that was discovered as a result of its high level of expression in leukemic cells. It plays an important role in the regulation of mitosis by promoting depolymerization of the microtubules that make up the mitotic spindle and, aging has been shown to impair STMN1 levels and change microtubule stability. We have previously demonstrated that a high level of STMN1 expression during early megakaryopoiesis is necessary for proliferation of megakaryocyte progenitors and that down-regulation of STMN1 expression during late megakaryopoiesis is important for megakaryocyte maturation and platelet production. In this report, we examined the effects of STMN1 deficiency on erythroid and megakaryocytic lineages in the mouse. Our studies show that STMN1 deficiency results in mild thrombocytopenia in young animals which converts into profound thrombocythemia as the mice age. STMN1 deficiency also lead to macrocytic changes in both erythrocytes and megakaryocytes that persisted throughout the life of STMN1 knock-out mice. Furthermore, STMN1 knock-out mice displayed a lower number of erythroid and megakaryocytic progenitor cells and had delayed recovery of their blood counts after chemotherapy. These studies show an important role for STMN1 in normal erythro-megakaryopoietic development and suggests potential implications for disorders affecting these hematopoietic lineages.

During mitosis ZEB1 "switches" from being a chromatin-bound epithelial gene repressor, to become a microtubule-associated protein

Microtubules are polymers of α/β-tubulin, with microtubule organization being regulated by microtubule-associated proteins (MAPs). Herein, we describe a novel role for the epithelial gene repressor, zinc finger E-box-binding homeobox 1 (ZEB1), that "switches" from a chromatin-associated protein during interphase, to a MAP that associates with α-, β- and γ-tubulin during mitosis. Additionally, ZEB1 was also demonstrated to associate with γ-tubulin at the microtubule organizing center (MTOC). Using confocal microscopy, ZEB1 localization was predominantly nuclear during interphase, with α/β-tubulin being primarily cytoplasmic and the association between these proteins being minimal. However, during the stages of mitosis, ZEB1 co-localization with α-, β-, and γ-tubulin was significantly increased, with the association commonly peaking during metaphase in multiple tumor cell-types. ZEB1 was also observed to accumulate in the cleavage furrow during cytokinesis. The increased interaction between ZEB1 and α-tubulin during mitosis was also confirmed using the proximity ligation assay. In contrast to ZEB1, its paralog ZEB2, was mainly perinuclear and cytoplasmic during interphase, showing some co-localization with α-tubulin during mitosis. Considering the association between ZEB1 with α/β/γ-tubulin during mitosis, studies investigated ZEB1's role in the cell cycle. Silencing ZEB1 resulted in a G₂-M arrest, which could be mediated by the up-regulation of p21^Waf1/Cip1 and p27^Kip1 that are known downstream targets repressed by ZEB1. However, it cannot be excluded the G₂/M arrest observed after ZEB1 silencing is not due to its roles as a MAP. Collectively, ZEB1 plays a role as a MAP during mitosis and could be functionally involved in this process.

Thyroid hormone suppresses expression of stathmin and associated tumor growth in hepatocellular carcinoma

Stathmin (STMN1), a recognized oncoprotein upregulated in various solid tumors, promotes microtubule disassembly and modulates tumor growth and migration activity. However, the mechanisms underlying the genetic regulation of STMN1 have yet to be elucidated. In the current study, we report that thyroid hormone receptor (THR) expression is negatively correlated with STMN1 expression in a subset of clinical hepatocellular carcinoma (HCC) specimens. We further identified the STMN1 gene as a target of thyroid hormone (T₃) in the HepG2 hepatoma cell line. An analysis of STMN1 expression profile and mechanism of transcriptional regulation revealed that T₃ significantly suppressed STMN1 mRNA and protein expression, and further showed that THR directly targeted the STMN1 upstream element to regulate STMN1 transcriptional activity. Specific knockdown of STMN1 suppressed cell proliferation and xenograft tumor growth in mice. In addition, T₃ regulation of cell growth arrest and cell cycle distribution were attenuated by overexpression of STMN1. Our results suggest that the oncogene STMN1 is transcriptionally downregulated by T₃ in the liver. This T₃-mediated suppression of STMN1 supports the theory that T₃ plays an inhibitory role in HCC tumor growth, and suggests that the lack of normal THR function leads to elevated STMN1 expression and malignant growth.

Cell cycle inhibition therapy that targets stathmin in in vitro and in vivo models of breast cancer

Stathmin is the founding member of a family of microtubule-destabilizing proteins that have a critical role in the regulation of mitosis. Stathmin is expressed at high levels in breast cancer and its overexpression is linked to disease progression. Although there is a large body of evidence to support a role for stathmin in breast cancer progression, the validity of stathmin as a viable therapeutic target for breast cancer has not been investigated. Here, we used a bicistronic adenoviral vector that co-expresses green fluorescent protein and a ribozyme that targets stathmin messenger RNA in preclinical breast cancer models with different estrogen receptor (ER) status. We examined the effects of anti-stathmin ribozyme on the malignant phenotype of breast cancer cells in vitro and in xenograft models in vivo both as a single agent and in combination with chemotherapeutic agents. Adenovirus-mediated gene transfer of anti-stathmin ribozyme resulted in a dose-dependent inhibition of proliferation and clonogenicity associated with a G2/M arrest and increase in apoptosis in both ER-positive and ER-negative breast cancer cell lines. This inhibition was markedly enhanced when stathmin-inhibited breast cancer cells were exposed to low concentrations of taxol, which resulted in virtually complete loss of the malignant phenotype. Interestingly, breast cancer xenografts treated with low doses of anti-stathmin therapy and taxol showed regression in a majority of tumors, while some tumors stopped growing completely. In contrast, combination of anti-stathmin ribozyme and adriamycin resulted in only a modest inhibition of growth in vitro and in breast cancer xenografts in vivo. Although inhibition of tumor growth was observed in both the combination treatment groups compared with groups treated with single agent alone, combination of anti-stathmin therapy and taxol had a more profound inhibition of tumorigenicity, as both agents target the microtubule pathway. Clinically, these findings are highly relevant because taxol is one of the most active chemotherapeutic agents in breast cancer. These studies provide the proof-of-principle that stathmin provides an attractive molecular target, which could serve as a primary focus of novel approaches to breast cancer.

The E2F transcription factor 1 transactives stathmin 1 in hepatocellular carcinoma

BACKGROUND

Through data mining the Stanford Microarray Database, the stathmin 1 (STMN1) transcript was found to be frequently upregulated in the hepatocellular carcinoma (HCC) with low alpha-fetoprotein level. The molecular mechanism of STMN1 upregulation in HCCs remained unclear.

METHODS

Quantitative RT-PCR, immunoblotting, immunohistochemistry, and transfection of expression or small hairpin RNA interference plasmids, chromatin immunoprecipitation (ChIP), and quantitative ChIP assays were performed in HCC specimens or 2 distinct HCC-derived cell lines. Dual luciferase assay and site-directed mutagenesis were applied to analyze the activities of STMN1 proximal promoter region.

RESULTS

STMN1 mRNA and proteins were significantly associated with several clinicopathological features. High STMN1 or E2F1 immunoexpression was predictive of poor overall survival (OS) rate (P < .01). In HCC-derived cell lines, E2F1 was elevated before STMN1 mRNA during the cell cycle. Exogenous expression of E2F1 or both transcription factor DP-1 (TFDP1) and E2F1 genes induced E2F1 and STMN1 mRNA (P < .01). Knockdown of the E2F1 gene suppressed E2F1 and STMN1 mRNA and E2F1 and STMN1 protein levels (P < .05). The promoter activity of STMN1 gene increased with overexpression of both E2F1 and TFDP1 genes (P < .05); however, it decreased when mutations were introduced in the E2F1-binding sites (P < .05).

CONCLUSIONS

Upregulation of E2F1 and STMN1 proteins associate with worse outcomes in patients with HCC. E2F1 significantly correlates with STMN1 protein level in HCC lesions and in vitro transactivation assays, suggesting that STMN1 gene is transactivated by the E2F1 protein.

Down-regulation of stathmin expression is required for megakaryocyte maturation and platelet production

The final stages of of megakaryocyte (MK) maturation involve a series of steps, including polyploidization and proplatelet formation. Although these processes are highly dependent on dynamic changes in the microtubule (MT) cytoskeleton, the mechanisms responsible for regulation of MTs in MKs remain poorly defined. Stathmin is a highly conserved MT-regulatory protein that has been suggested to play a role in MK differentiation of human leukemic cell lines. However, previous studies defining this relationship have reached contradictory conclusions. In this study, we addressed this controversy and investigated the role of stathmin in primary human MKs. To explore the importance of stathmin down-regulation during megakaryocytopoiesis, we used a lentiviral-mediated gene delivery system to prevent physiologic down-regulation of stathmin in primary MKs. We demonstrated that sustained expression of constitutively active stathmin delayed cytoplasmic maturation (ie, glycoprotein GPIb and platelet factor 4 expression) and reduced the ability of MKs to achieve high levels of ploidy. Moreover, platelet production was impaired in MKs in which down-regulation of stathmin expression was prevented. These studies indicate that suppression of stathmin is biologically important for MK maturation and platelet production and support the importance of MT regulation during the final stages of thrombopoiesis.

Quantitative phosphoproteomic analysis of prion-infected neuronal cells

Prion diseases or transmissible spongiform encephalopathies (TSEs) are fatal diseases associated with the conversion of the cellular prion protein (PrP^C) to the abnormal prion protein (PrP^Sc). Since the molecular mechanisms in pathogenesis are widely unclear, we analyzed the global phospho-proteome and detected a differential pattern of tyrosine- and threonine phosphorylated proteins in PrP^Sc-replicating and pentosan polysulfate (PPS)-rescued N2a cells in two-dimensional gel electrophoresis. To quantify phosphorylated proteins, we performed a SILAC (stable isotope labeling by amino acids in cell culture) analysis and identified 105 proteins, which showed a regulated phosphorylation upon PrP^Sc infection. Among those proteins, we validated the dephosphorylation of stathmin and Cdc2 and the induced phosphorylation of cofilin in PrP^Sc-infected N2a cells in Western blot analyses. Our analysis showed for the first time a differentially regulated phospho-proteome in PrP^Sc infection, which could contribute to the establishment of novel protein markers and to the development of novel therapeutic intervention strategies in targeting prion-associated disease.

Characterization and detection of cellular and proteomic alterations in stable stathmin-overexpressing, taxol-resistant BT549 breast cancer cells using offgel IEF/PAGE difference gel electrophoresis

Stathmin/oncoprotein 18, a protein that regulates microtubule dynamics, is highly expressed in a number of tumors including leukemia, lymphoma, neuroblastoma, breast, ovarian, and prostate cancers. High stathmin levels have been associated with the development of resistance to the widely used anti-cancer drug taxol ((®)Taxol, paclitaxel). The mechanisms of stathmin-mediated taxol resistance are not well-understood at the molecular level. To better understand the role of stathmin in taxol resistance, we stably overexpressed stathmin twofold in BT549 human breast cancer cells and characterized several cell processes involved in the mechanism of action of taxol. After stable overexpression of stathmin, neither the cell doubling time nor the mitotic index was altered and the microtubule polymer mass was reduced only modestly (by 18%). Unexpectedly, microtubule dynamicity was reduced by 29% after stathmin overexpression, resulting primarily from reduction in the catastrophe frequency. Sensitivity to taxol was reduced significantly (by 44%) in a clonogenic assay, and stathmin appeared to protect the cells from the spindle-damaging effects of taxol. The results suggest that in the stably stathmin-overexpressing clones, compensatory gene expression occurred that resulted in normal rates of cell proliferation and prevented the increase in catastrophe frequency expected in response to stathmin. Stathmin overexpression protected the cells from taxol-induced abnormal mitoses, and thus induced taxol resistance. Using offgel IEF/PAGE difference gel electrophoresis, we identified a number of proteins whose expression is reduced in the taxol-resistant stathmin-overexpressing cell lines, including proteins involved in the cytoskeleton and cell structure, the stress response, protein folding, glycolysis, and catalysis.

p27kip1 controls cell morphology and motility by regulating microtubule-dependent lipid raft recycling

p27(kip1) (p27) is an inhibitor of cyclin/cyclin-dependent kinase complexes, whose nuclear loss indicates a poor prognosis in various solid tumors. When located in the cytoplasm, p27 binds Op18/stathmin (stathmin), a microtubule (MT)-destabilizing protein, and restrains its activity. This leads to MT stabilization, which negatively affects cell migration. Here, we demonstrate that this p27 function also influences morphology and motility of cells immersed in three-dimensional (3D)matrices. Cells lacking p27 display a decrease in MT stability, a rounded shape when immersed in 3D environments, and a mesenchymal-amoeboid conversion in their motility mode. Upon cell contact to extracellular matrix, the decreased MT stability observed in p27 null cells results in accelerated lipid raft trafficking and increased RhoA activity. Importantly, cell morphology, motility, MT network composition, and distribution of p27 null cells were rescued by the concomitant genetic ablation of Stathmin, implicating that the balanced expression of p27 and stathmin represents a crucial determinant for cytoskeletal organization and cellular behavior in 3D contexts.

Regulation of microtubule dynamics through phosphorylation on stathmin by Epstein-Barr virus kinase BGLF4

Stathmin is an important microtubule (MT)-destabilizing protein, and its activity is differently attenuated by phosphorylation at one or more of its four phosphorylatable serine residues (Ser-16, Ser-25, Ser-38, and Ser-63). This phosphorylation of stathmin plays important roles in mitotic spindle formation. We observed increasing levels of phosphorylated stathmin in Epstein-Barr virus (EBV)-harboring lymphoblastoid cell lines (LCLs) and nasopharyngeal carcinoma (NPC) cell lines during the EBV lytic cycle. These suggest that EBV lytic products may be involved in the regulation of stathmin phosphorylation. BGLF4 is an EBV-encoded kinase and has similar kinase activity to cdc2, an important kinase that phosphorylates serine residues 25 and 38 of stathmin during mitosis. Using an siRNA approach, we demonstrated that BGLF4 contributes to the phosphorylation of stathmin in EBV-harboring NPC. Moreover, we confirmed that BGLF4 interacts with and phosphorylates stathmin using an in vitro kinase assay and an in vivo two-dimensional electrophoresis assay. Interestingly, unlike cdc2, BGLF4 was shown to phosphorylate non-proline directed serine residues of stathmin (Ser-16) and it mediated phosphorylation of stathmin predominantly at serines 16, 25, and 38, indicating that BGLF4 can down-regulate the activity of stathmin. Finally, we demonstrated that the pattern of MT organization was changed in BGLF4-expressing cells, possibly through phosphorylation of stathmin. In conclusion, we have shown that a viral Ser/Thr kinase can directly modulate the activity of stathmin and this contributes to alteration of cellular MT dynamics and then may modulate the associated cellular processes.

Regulation of microtubule dynamic instability

Proper regulation of MT (microtubule) dynamics is essential for various vital processes, including the segregation of chromosomes, directional cell migration and differentiation. MT assembly and disassembly is modulated by a complex network of intracellular factors that co-operate or antagonize each other, are highly regulated in space and time and are thus attuned to the cell cycle and differentiation processes. While we only begin to appreciate how the concerted action of MT stabilizers and destabilizers shapes different MT patterns, a clear picture of how individual factors affect the MT structure is emerging. In this paper, we review the current knowledge about proteins that modulate MT dynamic instability.

Stathmin, a microtubule regulatory protein, is associated with hypoxia-inducible factor-1alpha levels in human endometrial and endothelial cells

Local hypoxia that occurs during menstruation triggers angiogenesis that is crucial for cyclical remodeling of the endometrium during the menstrual cycle. Hypoxia is thought to be important for the expression of vascular endothelial growth factor (VEGF) via its transcriptional factor, hypoxia inducible factor (HIF)-1alpha, in the endometrium. The activation of the phosphatidylinositol-3-kinase (PI3K)/Akt signaling pathway may modulate HIF-1alpha protein levels. Stathmin, a microtubule regulatory protein, was expressed in the stroma, glandular epithelium, and vascular endothelium in human endometrium. In this study, we examined a possible role of stathmin in hypoxia-induced HIF-1alpha and VEGF expression in primary isolated and immortalized human endometrial stromal cells, glandular epithelial cells, and human umbilical venous endothelial cells (HUVEC). Knocking down stathmin expression using small interfering RNA caused microtubule stabilization and inhibited hypoxia-induced VEGF mRNA expression via the reduction of HIF-1alpha protein levels in endometrial cells and HUVEC. Treatment of the cells with a PI3K inhibitor, wortmannin, inhibited the expression of VEGF mRNA and the accumulation of HIF-1alpha protein. Silencing of stathmin expression repressed the activation (phosphorylation) of Akt in endometrial cells and HUVEC. These results suggest that endometrial stathmin is linked to HIF-1alpha protein accumulation and VEGF expression through the PI3K/Akt signaling pathway and may be involved in regeneration of the endometrium during the menstrual cycle in human uterine cells.

EBV-encoded LMP1 regulates Op18/stathmin signaling pathway by cdc2 mediation in nasopharyngeal carcinoma cells

Oncoprotein 18/stathmin (Op18/stathmin) plays a crucial role in maintaining cell biological characteristics by regulating microtubule dynamics, especially entry into mitosis; phosphorylated Op18/stathmin promotes microtubule polymerization to form the mitotic spindle, which is essential for chromosome segregation and cell division. Cdc2 is a critical kinase in starting M phase events in cell-cycle progression and is a positive regulator of the cell cycle. Latent membrane protein 1 (LMP1) is an Epstein-Barr virus (EBV)-encoded oncogenic protein that is able to induce carcinogenesis via various signaling pathways. This study focused on regulation by LMP1 of Op18/stathmin signaling in nasopharyngeal carcinoma (NPC) cells and showed that LMP1 regulates Op18/stathmin signaling through cdc2 mediation, LMP1 upregulates cdc2 kinase activity, and Op18/stathmin phosphorylation promotes the interaction of cdc2 with Op18/stathmin and microtubule polymerization during mitosis, and inhibition of LMP1 expression attenuates the interaction of cdc2 and Op18/stathmin and promotes microtubule depolymerization. These results reveal a new pathway via which LMP1 regulates Op18/stathmin signaling by cdc2 mediation; this new signaling pathway not only perfects the LMP1 regulation network but also elucidates the molecular mechanism of LMP1 that leads to carcinogenesis.

Stathmin 1: a novel therapeutic target for anticancer activity

Stathmin 1 (STMN1), also known as p17, p18, p19, 19K, metablastin, oncoprotein 18, LAP 18 and Op18, is a 19 kDa cytosolic protein. It was the first discovered member of a family of phylogenetically related microtubule-destabilizing phosphoproteins critically involved in the construction and function of the mitotic spindle. A threshold level of STMN1 is required for orderly progression through mitosis in a variety of cell types. STMN1 is overexpressed across a broad range of human malignancies (leukemia, lymphoma, neuroblastoma; ovarian, prostatic, breast and lung cancers and mesothelioma). It is also upregulated in normally proliferating cell lines but is only rarely upregulated in nonproliferating cell lines with the exception of neurons, anterior pituitary cells and glial cells. Its expression is also upregulated in hepatocytes during regeneration and in lymphoid cells when they are signaled to proliferate. In this review, we summarize available data as rationale for the therapeutic manipulation of STMN1 in cancer patients.

Expression of stathmin/op18 as a significant prognostic factor for cervical carcinoma patients

PURPOSE

Stathmin (Oncoprotein18), a ubiquitous and highly conserved 19-kDa cytosolic phosphoprotein, has been reported to play a critical role in mitosis and possibly other cellular processes, which is associated with tumor carcinogenesis and development. The purpose of this study was to examine the involvement of stathmin in human cervical carcinogenesis and to evaluate its prognostic significance in human cervical carcinoma.

METHODS

Using semiquantitative RT-PCR and Western blotting, we detected the expression of stathmin in human normal cervical epithelial cell line, immortalized cervical epithelial cell lines, and cervical carcinoma cell lines. Additionally, we also detected the expression of stathmin protein in 15 cases of cervical carcinoma tissues and adjacent non-carcinomous margin tissues. Furthermore, specimens from 148 patients with different grade and stage cervical carcinoma were investigated by immunohistochemistry for stathmin expression. Correlations between the expression of stathmin and various clinicopathological factors were studied, while statistical analyses were performed to evaluate prognostic and diagnostic associations.

RESULTS

The levels of stathmin mRNA and protein expression were significantly higher in cervical carcinoma cells and immortalized cervical epithelial cells than in normal cervical epithelial cells (P < 0.05). Moreover, Western blotting revealed high stathmin protein expression in 73.3% (11/15) cervical carcinoma tissues, while stathmin were overexpressed in tumor tissues as compared with adjacent non-carcinomous margin samples (P = 0.017). In addition, immunohistochemical staining revealed stathmin immunoreactivity in 81.1% (120/148) of cervical carcinoma tissues and high stathmin expression was significantly correlated with clinical stage (P = 0.006), T classification (P = 0.012), regional lymph node metastasis (P = 0.005) and hematogenous metastasis (P = 0.021). Kaplan-Meier analysis showed that high stathmin positivity was significantly associated with a shorter survival time (P < 0.001). Clinical stage (P = 0.0022), T classification (P = 0.0035), regional lymph node (P = 0.0008) or hematogenous metastasis (P = 0.0015) were also associated with survival time. Furthermore, by Cox multivariate analysis, only lymph node (P = 0.0052) or hematogenous metastasis (P = 0.0046) maintained their significance as independent prognostic factors, although stathmin was not an independent prognostic factor (risk ratio: 1.45; P = 0.0624).

CONCLUSIONS

Stathmin expression correlates with cervical carcinogenesis and tumor progression. This molecule is a valuable prognostic marker in patients with cervical carcinoma.

KIS protects against adverse vascular remodeling by opposing stathmin-mediated VSMC migration in mice

Vascular proliferative diseases are characterized by VSMC proliferation and migration. Kinase interacting with stathmin (KIS) targets 2 key regulators of cell proliferation and migration, the cyclin-dependent kinase inhibitor p27Kip1 and the microtubule-destabilizing protein stathmin. Phosphorylation of p27Kip1 by KIS leads to cell-cycle progression, whereas the target sequence and the physiological relevance of KIS-mediated stathmin phosphorylation in VSMCs are unknown. Here we demonstrated that vascular wound repair in KIS-/- mice resulted in accelerated formation of neointima, which is composed predominantly of VSMCs. Deletion of KIS increased VSMC migratory activity and cytoplasmic tubulin destabilizing activity, but abolished VSMC proliferation through the delayed nuclear export and degradation of p27Kip1. This promigratory phenotype resulted from increased stathmin protein levels, caused by a lack of KIS-mediated stathmin phosphorylation at serine 38 and diminished stathmin protein degradation. Downregulation of stathmin in KIS-/- VSMCs fully restored the phenotype, and stathmin-deficient mice demonstrated reduced lesion formation in response to vascular injury. These data suggest that KIS protects against excessive neointima formation by opposing stathmin-mediated VSMC migration and that VSMC migration represents a major mechanism of vascular wound repair, constituting a relevant target and mechanism for therapeutic interventions.

Global regulation of the interphase microtubule system by abundantly expressed Op18/stathmin

Op18/stathmin (Op18), a conserved microtubule-depolymerizing and tubulin heterodimer-binding protein, is a major interphase regulator of tubulin monomer-polymer partitioning in diverse cell types in which Op18 is abundant. Here, we addressed the question of whether the microtubule regulatory function of Op18 includes regulation of tubulin heterodimer synthesis. We used two human cell model systems, K562 and Jurkat, combined with strategies for regulatable overexpression or depletion of Op18. Although Op18 depletion caused extensive overpolymerization and increased microtubule content in both cell types, we did not detect any alteration in polymer stability. Interestingly, however, we found that Op18 mediates positive regulation of tubulin heterodimer content in Jurkat cells, which was not observed in K562 cells. By analysis of cells treated with microtubule-poisoning drugs, we found that Jurkat cells regulate tubulin mRNA levels by a posttranscriptional mechanism similarly to normal primary cells, whereas this mechanism is nonfunctional in K562 cells. We present evidence that Op18 mediates posttranscriptional regulation of tubulin mRNA in Jurkat cells through the same basic autoregulatory mechanism as microtubule-poisoning drugs. This, combined with potent regulation of tubulin monomer-polymer partitioning, enables Op18 to exert global regulation of the microtubule system.

Op18/Stathmin counteracts the activity of overexpressed tubulin-disrupting proteins in a human leukemia cell line

Op18/stathmin (Op18) is a phosphorylation-regulated and differentially expressed microtubule-destabilizing protein in animal cells. Op18 regulates tubulin monomer-polymer partitioning of the interphase microtubule system and forms complexes with tubulin heterodimers. Recent reports have shown that specific tubulin-folding cofactors and related proteins may disrupt tubulin heterodimers. We therefore investigated whether Op18 protects unpolymerized tubulin from such disruptive activities. Our approach was based on inducible overexpression of two tubulin-disrupting proteins, namely TBCE, which is required for tubulin biogenesis, and E-like, which has been proposed to regulate tubulin turnover and microtubule stability. Expression of either of these proteins was found to cause a rapid degradation of both alpha-tubulin and beta-tubulin subunits of unpolymerized, but not polymeric, tubulin heterodimers. We found that depletion of Op18 by means of RNA interference increased the susceptibility of tubulin to TBCE or E-like mediated disruption, while overexpressed Op18 exerted a tubulin-protective effect. Tubulin protection was shown to depend on Op18 levels, binding affinity, and the partitioning between tubulin monomers and polymers. Hence, the present study reveals that Op18 at physiologically relevant levels functions to preserve the integrity of tubulin heterodimers, which may serve to regulate tubulin turnover rates.

Prognostic significance of stathmin expression in correlation with metastasis and clinicopathological characteristics in human ovarian carcinoma

Stathmin, also called oncoprotein 18, is a founding member of the family of microtubule-destabilizing proteins that play a critical role in the regulation of mitosis. Stathmin is non-expressed in normal tissues, but stathmin gene is expressed at high levels in many human malignancies and the relationships between the levels of this gene expression in tumors and prognosis of the patients have been addressed. In this report, we explored the relationships between stathmin mRNA expression in ovarian carcinoma tissues and clinicopathological parameters. We collected and analyzed paraffin wax-embedded ovarian tumor biopsy tissues from 42 ovarian cancer patients in our hospital. We employed RT-PCR method and performed a densitometric analysis to determine the ratio of stathmin relative to beta-actin as an internal marker. Results showed that the stathmin mRNA expression was detected in all the ovarian carcinoma tissue samples and those samples with metastasis had higher levels of stathmin mRNA expression in initial biopsy specimens (P<0.05). Moreover, the levels of stathmin mRNA expression between samples with and without metastasis showed a statistically significant difference (P<0.05).

The Schistosoma mansoni protein Sm16/SmSLP/SmSPO-1 is a membrane-binding protein that lacks the proposed microtubule-regulatory activity

Sm16/SmSLP/SPO-1 (Sm16) has been identified as a developmentally regulated protein that is released from specific glands of the Schistosoma mansoni parasite during skin penetration. Sm16 has been ascribed both anti-inflammatory activities and a functional similarity with the conserved cytosolic tubulin-binding protein stathmin/Op18. Here we used a cell line to confirm signal peptide-dependent secretion and to define the secreted form of Sm16 for production in E. coli. We present evidence from both in vitro experiments and studies on transfected human cells that refute any functional similarity with stathmin/Op18. Instead of an Op18-like activity, we found that targeting of Sm16 to the cytosol of human cells, which was achieved by ectopic expression of Sm16 lacking the signal peptide, results in a caspase-dependent apoptotic response. Interestingly, by analysis of recombinant preparations we found that the secreted form of Sm16 is a lipid bilayer-binding protein that efficiently binds to the surface of diverse cell types by a polyanion-independent mechanism, which results in uptake by endocytosis. While the significance of the pro-apoptotic activity exerted by cytosolic Sm16 remains unclear, the present findings on cell-surface-binding properties of Sm16 seems likely to be of functional relevance during skin penetration of the parasite.

Microtubule-associated proteins as targets in cancer chemotherapy

Natural and synthetic compounds that disrupt microtubule dynamics are among the most successful and widely used cancer chemotherapeutic agents. However, lack of reliable markers that predict sensitivity of cancers to these agents and development of resistance remain vexing issues. There is accumulating evidence that a family of cellular proteins that are associated with and alter the dynamics of microtubules can determine sensitivity of cancer cells to microtubule-targeting agents and play a role in tumor cell resistance to these agents. This growing family of microtubule-associated proteins (MAP) includes products of oncogenes, tumor suppressors, and apoptosis regulators, suggesting that alteration of microtubule dynamics may be one of the critical events in tumorigenesis and tumor progression. The objective of this review is to integrate the knowledge on these seemingly unrelated proteins that share a common function and examine their relevance to microtubule-targeting therapies and highlight MAPs-tubulin-drug interactions as a novel avenue for new drug discovery. Based on the available evidence, we propose that rational microtubule-targeting cancer therapeutic approaches should ideally include proteomic profiling of tumor MAPs before administration of microtubule-stabilizing/destabilizing agents preferentially in combination with agents that modulate the expression of relevant MAPs.

Increased expression and differential phosphorylation of stathmin may promote prostate cancer progression

BACKGROUND

Proteins which regulate normal development may promote tumorigenesis, tumor progression, or metastasis through dysregulation of these functions. We postulate that proteins, which regulate prostate growth also promote prostate cancer (PCa) progression.

METHODS

Two Dimensional Gel Electrophoresis was utilized to compare patterns of protein expression in 12T-7f prostates (LPB-Tag mouse model for PCa) during tumor development and progression with those of normal developing and adult wild type CD-1 prostates. Stathmin expression and phosphorylation patterns were analyzed in mouse and human PCa cell lines as well as in human PCa tissue arrays.

RESULTS

Stathmin was identified by two-dimensional gel electrophoresis and mass spectrometry. Stathmin levels increase early during normal mouse prostate development and again during prostate tumor development and progression. In human prostate adenocarcinoma, stathmin increases in Gleason pattern 5. Further, stathmin is differentially phosphorylated in androgen-dependent LNCaP cells compared to androgen-independent PC-3 and DU145 cells. This differential phosphorylation is modulated by androgen and anti-androgen treatment.

CONCLUSION

Stathmin expression is highest when the prostate is undergoing morphogenesis or tumorigenesis and these processes may be regulated through differential phosphorylation. Furthermore, modulation of stathmin phosphorylation may correlate with the development of androgen-independent PCa.

Effects of Helicobacter hepaticus on the proteome of HEp-2 cells

Helicobacter hepaticus infects the bowel and biliary tree of several animals, producing inflammation. Colonisation of mouse livers can induce hepatocellular carcinomas. The effects of H. hepaticus on the proliferation and global protein expression of human HEp-2 cells were studied by examining the changes in the protein profiles of cells exposed to the bacterium. HEp-2 cells were grown for four days under a microaerobic atmosphere or under the same conditions in co-cultures with H. hepaticus at various inoculum densities. Enlargement, distension and elongation of HEp-2 cells were observed in co-cultures with H. hepaticus. The number of live cells declined by only an order of magnitude at bacterial inocula of approximately 10(9)cfu/ml, but were reduced to less than 10(3)cells/ml at approximately 10(10)cfu/ml bacteria inocula. Protein expression by HEp-2 cells was investigated employing two-dimensional gel electrophoresis. In cells grown with or without bacteria, 17 differentially expressed proteins were identified by tandem mass spectrometry. These proteins participated in several biological functions including amino acid metabolism, cell growth and proliferation, stress response, protein translation and modification, etc. The onset of a catastrophic killing of HEp-2 cells at a bacterial density of approximately 10(9)cfu/ml suggested a multimodal action for H. hepaticus infection, and the modulation of the expression of proteins involved in different biological functions showed that the presence of H. hepaticus has broad effects on the physiology of HEp-2 cells.

Interphase-specific phosphorylation-mediated regulation of tubulin dimer partitioning in human cells

The microtubule cytoskeleton is differentially regulated by a diverse array of proteins during interphase and mitosis. Op18/stathmin (Op18) and microtubule-associated protein (MAP)4 have been ascribed opposite general microtubule-directed activities, namely, microtubule destabilization and stabilization, respectively, both of which can be inhibited by phosphorylation. Here, using three human cell models, we depleted cells of Op18 and/or MAP4 by expression of interfering hairpin RNAs and we analyzed the resulting phenotypes. We found that the endogenous levels of Op18 and MAP4 have opposite and counteractive activities that largely govern the partitioning of tubulin dimers in the microtubule array at interphase. Op18 and MAP4 were also found to be the downstream targets of Ca(2+)- and calmodulin-dependent protein kinase IV and PAR-1/MARK2 kinase, respectively, that control the demonstrated counteractive phosphorylation-mediated regulation of tubulin dimer partitioning. Furthermore, to address mechanisms regulating microtubule polymerization in response to cell signals, we developed a system for inducible gene product replacement. This approach revealed that site-specific phosphorylation of Op18 is both necessary and sufficient for polymerization of microtubules in response to the multifaceted signaling event of stimulation of the T cell antigen receptor complex, which activates several signal transduction pathways.

Stathmin overexpression cooperates with p53 mutation and osteopontin overexpression, and is associated with tumour progression, early recurrence, and poor prognosis in hepatocellular carcinoma

Stathmin, a major microtubule-depolymerizing protein, is involved in cell cycle progression and cell motility. This study aimed to elucidate its role in the progression, early tumour recurrence (ETR), and prognosis of hepatocellular carcinoma (HCC). Stathmin mRNA was overexpressed in 88/156 (56%) resected, unifocal, primary HCCs, while p53 mutation was present in 72 (46%) and osteopontin mRNA overexpression in 79 (51%). Stathmin mRNA expression exhibited high concordance (93%) with protein expression in 107 cases examined by immunohistochemistry. Stathmin overexpression correlated with high alpha-fetoprotein (>200 ng/ml, p = 0.02), larger tumour size (>5 cm, p = 0.012), high tumour grade (p < 0.0002), high tumour stage (stage IIIA-IV) with vascular invasion and various degrees of intrahepatic metastasis (p < 1 x 10(-8)), ETR (p = 0.003), and lower 5-year survival (p = 0.0007). Stathmin protein expression was often more intense in the peripheral regions of tumour trabeculae, tumour borders, and portal vein tumour thrombi. Stathmin overexpression correlated with p53 mutation (p = 0.017) and osteopontin overexpression (p = 1 x 10(-8)), both of which were associated with vascular invasion (both p < 0.0001) and poorer prognosis (p < 0.0004 and p = 0.0004, respectively). Regardless of the status of p53 mutation or osteopontin expression, stathmin overexpression was associated with higher vascular invasion (all p < 0.0001). Approximately 90% of HCCs harbouring stathmin overexpression with concomitant p53 mutation or osteopontin overexpression exhibited vascular invasion, and hence the lowest 5-year survival, p = 0.00018 and p = 0.0009, respectively. However, we did not find that stathmin overexpression exerted prognostic impact independent of tumour stage. In conclusion, stathmin expression correlates with metastatic potential, is an important prognostic factor for HCC, and may serve as a useful marker to predict ETR.

Aneugenic activity of Op18/stathmin is potentiated by the somatic Q18-->e mutation in leukemic cells

Op18/stathmin (Op18) is a phosphorylation-regulated microtubule destabilizer that is frequently overexpressed in tumors. The importance of Op18 in malignancy was recently suggested by identification of a somatic Q18-->E mutation of Op18 in an adenocarcinoma. We addressed the functional consequences of aberrant Op18 expression in leukemias by analyzing the cell cycle of K562 cells either depleted of Op18 by expression of interfering hairpin RNA or induced to express wild-type or Q18E substituted Op18. We show here that although Op18 depletion increases microtubule density during interphase, the density of mitotic spindles is essentially unaltered and cells divide normally. This is consistent with phosphorylation-inactivation of Op18 during mitosis. Overexpression of wild-type Op18 results in aneugenic activities, manifest as aberrant mitosis, polyploidization, and chromosome loss. One particularly significant finding was that the aneugenic activity of Op18 was dramatically increased by the Q18-->E mutation. The hyperactivity of mutant Op18 is apparent in its unphosphorylated state, and this mutation also suppresses phosphorylation-inactivation of the microtubule-destabilizing activity of Op18 without any apparent effect on its phosphorylation status. Thus, although Op18 is dispensable for mitosis, the hyperactive Q18-->E mutant, or overexpressed wild-type Op18, exerts aneugenic effects that are likely to contribute to chromosomal instability in tumors.

Targeting stathmin in prostate cancer

Stathmin is the founding member of a family of microtubule-destabilizing proteins that regulate the dynamics of microtubule polymerization and depolymerization. Stathmin is expressed at high levels in a variety of human cancers and provides an attractive molecule to target in cancer therapies that disrupt the mitotic apparatus. We developed replication-deficient bicistronic adenoviral vectors that coexpress green fluorescent protein and ribozymes that target stathmin mRNA. The therapeutic potential of these recombinant adenoviruses was tested in an experimental androgen-independent LNCaP prostate cancer model. Adenovirus-mediated transfer of anti-stathmin ribozymes resulted in efficient transduction and marked inhibition of stathmin expression in these cells. Cells that were transduced with the anti-stathmin adenoviruses showed a dramatic dose-dependent growth inhibition. This was associated with accumulation of LNCaP cells in the G2-M phases of the cell cycle. A similar dose-dependent inhibition of clonogenic potential was also observed in cells infected with anti-stathmin adenoviruses. Morphologic and biochemical analysis of infected cells showed a marked increase in apoptosis characterized by detachment of the cells, increased chromatin condensation, activation of caspase-3, and fragmentation of internucleosomal DNA. If these findings are confirmed in vivo, it may provide an effective approach for the treatment of prostate cancer.

Stathmin-deficient mice develop fibrosis and show delayed recovery from ischemic-reperfusion injury

In kidneys subjected to ischemic reperfusion injury (IRI) stathmin, a tubulin-binding protein involved in the regulation of mitosis, is expressed in dedifferentiated and proliferating renal tubule cells during the recovery phase. To ascertain the role of stathmin in the recovery from ischemic kidney injury, stathmin-deficient (OP18-/-) and wild-type (WT) animals were subjected to experimental IRI. At 3, 7, and 14 days after reperfusion serum samples and kidneys were collected for the examination of parameters of renal function, morphology, and recovery. Our studies indicate that on day 14 after reperfusion OP18-/- mice have significant renal failure, whereas the creatinine levels of WT animals have returned to baseline. Compared with WT animals OP18-/- mice had more extensive tubular fibrosis. The examination of proliferating cell nuclear antigen expression indicated that OP18-/- animals have increased proliferative or DNA repair activity for a more prolonged duration. The OP18-/- animals also had an increased number of tubules with apoptotic cells. These results suggest that in stathmin-deficient mice subjected to IRI, the aberrant regulation of cell cycle progression, not observed under normal conditions, impairs or at least delays the process of tubular repair and recovery after acute renal injury.

Stat3 regulates microtubules by antagonizing the depolymerization activity of stathmin

Stat3 is a member of the signal transducer and activator of transcription family, which is important in cytokine signaling. Gene ablation studies have revealed a requirement for Stat3 in diverse biological processes (Akira, S. 2000. Oncogene. 19: 2607-2611; Levy, D.E., and C.K. Lee. 2002. J. Clin. Invest. 109:1143-1148). Previously, the function of Stat3 had been attributed exclusively to its transcriptional activity in the nucleus. In this study, we reveal an interaction between Stat3 and the microtubule (MT)-destabilizing protein stathmin. Stathmin did not overtly affect ligand-stimulated Stat3 activation. In contrast, the expression of Stat3 is required for the stabilization of MTs and cell migration. We further demonstrate that Stat3-containing cells are resistant to the MT-destabilizing effect of stathmin overexpression. In addition, down-regulation of stathmin protein levels in Stat3-deficient cells partially reversed the MT and migration deficiencies. Recombinant Stat3 was also capable of reversing stathmin inhibition of tubulin polymerization in vitro. Our results indicate that Stat3 modulates the MT network by binding to the COOH-terminal tubulin-interacting domain of stathmin and antagonizing its MT destabilization activity.

Distinct and sequential upregulation of genes regulating cell growth and cell cycle progression during hepatic ischemia-reperfusion injury

Ischemia-reperfusion injury (IRI) in liver and other organs is manifested as an injury phase followed by recovery and resolution. Control of cell growth and proliferation is essential for recovery from the injury. We examined the expression of three related regulators of cell cycle progression in liver IRI: spermidine/spermine N-acetyltransferase (SSAT), p21 (a cyclin-dependent kinase inhibitor), and stathmin. Mice were subjected to hepatic IRI, and liver tissues were harvested at timed intervals. The expression of SSAT, the rate-limiting enzyme in the polyamine catabolic pathway, had increased fivefold 6 h after IRI and correlated with increased putrescine levels in the liver, consistent with increased SSAT enzymatic activity in IRI. The expression of p21, which is transactivated by p53, was undetectable in sham-operated animals but was heavily induced at 12 and 24 h of reperfusion and declined to undetectable baseline levels at 72 h of reperfusion. The interaction of the polyamine pathway with the p53-p21 pathway was shown in vitro, where activation of SSAT with polyamine analog or the addition of putrescine to cultured hepatocytes induced the expression of p53 and p21 and decreased cell viability. The expression of stathmin, which is under negative transcriptional regulation by p21 and controls cell proliferation and progression through mitosis, remained undetectable at 6, 12, and 24 h of reperfusion and was progressively and heavily induced at 48 and 72 h of reperfusion. Double-immunofluorescence labeling with antibodies against stathmin and PCNA, a marker of cell proliferation, demonstrated colocalization of stathmin and PCNA at 48 and 72 h of reperfusion in hepatocytes, indicating the initiation of cell proliferation. The distinct and sequential upregulation of SSAT, p21, and stathmin, along with biochemical activation of the polyamine catabolic pathway in IRI in vivo and the demonstration of p53-p21 upregulation by SSAT and putrescine in vitro, points to the important role of regulators of cell growth and cell cycle progression in the pathophysiology and/or recovery in liver IRI. The data further suggest that SSAT may play a role in the initiation of injury, whereas p21 and stathmin may be involved in the resolution and recovery after liver IRI.

Microtubule-associated proteins and their essential roles during mitosis

Microtubules play essential roles during mitosis, including chromosome capture, congression, and segregation. In addition, microtubules are also required for successful cytokinesis. At the heart of these processes is the ability of microtubules to do work, a property that derives from their intrinsic dynamic behavior. However, if microtubule dynamics were not properly regulated, it is certain that microtubules alone could not accomplish any of these tasks. In vivo, the regulation of microtubule dynamics is the responsibility of microtubule-associated proteins. Among these, we can distinguish several classes according to their function: (1) promotion and stabilization of microtubule polymerization, (2) destabilization or severance of microtubules, (3) functioning as linkers between various structures, or (4) motility-related functions. Here we discuss how the various properties of microtubule-associated proteins can be used to assemble an efficient mitotic apparatus capable of ensuring the bona fide transmission of the genetic information in animal cells.

The role of stathmin in the regulation of the cell cycle

Stathmin is the founding member of a family of proteins that play critically important roles in the regulation of the microtubule cytoskeleton. Stathmin regulates microtubule dynamics by promoting depolymerization of microtubules and/or preventing polymerization of tubulin heterodimers. Upon entry into mitosis, microtubules polymerize to form the mitotic spindle, a cellular structure that is essential for accurate chromosome segregation and cell division. The microtubule-depolymerizing activity of stathmin is switched off at the onset of mitosis by phosphorylation to allow microtubule polymerization and assembly of the mitotic spindle. Phosphorylated stathmin has to be reactivated by dephosphorylation before cells exit mitosis and enter a new interphase. Interfering with stathmin function by forced expression or inhibition of expression results in reduced cellular proliferation and accumulation of cells in the G2/M phases of the cell cycle. Forced expression of stathmin leads to abnormalities in or a total lack of mitotic spindle assembly and arrest of cells in the early stages of mitosis. On the other hand, inhibition of stathmin expression leads to accumulation of cells in the G2/M phases and is associated with severe mitotic spindle abnormalities and difficulty in the exit from mitosis. Thus, stathmin is critically important not only for the formation of a normal mitotic spindle upon entry into mitosis but also for the regulation of the function of the mitotic spindle in the later stages of mitosis and for the timely exit from mitosis. In this review, we summarize the early studies that led to the identification of the important mitotic function of stathmin and discuss the present understanding of its role in the regulation of microtubules dynamics during cell-cycle progression. We also describe briefly other less mature avenues of investigation which suggest that stathmin may participate in other important biological functions and speculate about the future directions that research in this rapidly developing field may take.

Stathmin and its phosphoprotein family: general properties, biochemical and functional interaction with tubulin

Stathmin, also referred to as Op18, is a ubiquitous cytosolic phosphoprotein, proposed to be a small regulatory protein and a relay integrating diverse intracellular signaling pathways involved in the control of cell proliferation, differentiation and activities. It interacts with several putative downstream target and/or partner proteins. One major action of stathmin is to interfere with microtubule dynamics, by inhibiting the formation of microtubules and/or favoring their depolymerization. Stathmin (S) interacts directly with soluble tubulin (T), which results in the formation of a T2S complex which sequesters free tubulin and therefore impedes microtubule formation. However, it has been also proposed that stathmin's action on microtubules might result from the direct promotion of catastrophes, which is still controversial. Phosphorylation of stathmin regulates its biological actions: it reduces its affinity for tubulin and hence its action on microtubule dynamics, which allows for example progression of cells through mitosis. Stathmin is also the generic element of a protein family including the neural proteins SCG10, SCLIP and RB3/RB3'/RB3". Interestingly, the stathmin-like domains of these proteins also possess a tubulin binding activity in vitro. In vivo, the transient expression of neural phosphoproteins of the stathmin family leads to their localization at Golgi membranes and, as previously described for stathmin and SCG10, to the depolymerization of interphasic microtubules. Altogether, the same mechanism for microtubule destabilization, that implies tubulin sequestration, is a common feature likely involved in the specific biological roles of each member of the stathmin family.

Proteomic analysis of the TGF-beta signaling pathway in pancreatic carcinoma cells using stable RNA interference to silence Smad4 expression

Smad4 is a tumor-suppressor gene that is lost or mutated in 50% of pancreatic carcinomas. Smad4 is also an intracellular transmitter of transforming growth factor-beta (TGF-beta) signals. Although its tumor-suppressor function is presumed to reside in its capacity to mediate TGF-beta-induced growth inhibition, there seems to be a Smad4-independent TGF-beta signaling pathway. Here, we succeeded in establishing Smad4 knockdown (S4KD) pancreatic cancer cell lines using stable RNA interference. Smad4 protein expression and TGF-beta-Smad4 signaling were impaired in S4KD cells, and we compared the proteomic changes with TGF-beta stimulation using two-dimensional gel electrophoresis (2-DE) and mass spectrometry. We identified five proteins that were up-regulated and seven proteins that were down-regulated; 10 of them were novel targets for TGF-beta. These proteins function in processes such as cytoskeletal regulation, cell cycle, and oxidative stress. Introducing siRNA-mediated gene silencing into proteomics revealed a novel TGF-beta signal pathway that did not involve Smad4.

Altered levels and regulation of stathmin in paclitaxel-resistant ovarian cancer cells

Two paclitaxel(Ptx)-resistant ovarian cancer cell lines, 1A9/Ptx-10 and 1A9/Ptx-22, isolated from the 1A9 cell line (a clone of the A2780 line) by continuous exposure to Ptx and verapamil, have point mutations in their major beta-tubulin gene and in one or both alleles of their TP53 gene. These cells were examined for alterations in cell cycle regulators and the tubulin-binding protein stathmin. Unlike parental cells, neither 1A9/Ptx-10 nor 1A9/Ptx-22 expressed detectable levels of p21(WAF1/Cip1), a putative transcriptional regulator of stathmin, but did overexpress stathmin and Bcl2. No differences were noted in the expression levels of proliferative cell nuclear antigen or tyrosine-phosphorylated p34Cdc2. Ptx treatment altered little the expression of stathmin in the parental cell line, although it increased p21(WAF1/Cip1) levels several-fold. Infection of Ptx-resistant lines with a wild-type TP53-bearing adenovirus (AdWTp53) changed cell cycle distribution and increased the levels of p21(WAF1/Cip1), but caused no changes in stathmin levels. Microtubule drug resistance in ovarian carcinoma may be associated with altered p53/21(WAF1/Cip1) regulatory pathways for stathmin expression and function.

Sorting nexin 9 participates in clathrin-mediated endocytosis through interactions with the core components

Sorting nexin 9 (SNX9) belongs to a family of proteins, the sorting nexins, that are characterized by the presence of a subclass of the phosphoinositide-binding phox domain. SNX9 has in its amino terminus a Src homology 3 domain and a region with predicted low complexity followed by a carboxyl-terminal part containing the phox domain. We previously found that SNX9 is one of the major proteins in hematopoietic cells that binds to the alpha and beta2-appendages of adaptor protein complex 2 (AP-2), a protein with a critical role in the formation of clathrin-coated vesicles at the plasma membrane. In the present study we show that clathrin and dynamin-2, two other essential molecules in the endocytic process, also interact with SNX9. We found that both AP-2 and clathrin bind to the low complexity region in SNX9 in a cooperative manner, whereas dynamin-2 binds to the Src homology 3 domain. In the cytosol, SNX9 is present in a 14.5 S complex containing dynamin-2 and an unidentified 41-kDa protein. In HeLa cells, SNX9 co-localized with both AP-2 and dynamin-2 at the plasma membrane or on vesicular structures derived from it but not with the early endosomal marker EEA1 or with AP-1. The results suggest that SNX9 may be recruited together with dynamin-2 and become co-assembled with AP-2 and clathrin at the plasma membrane. Overexpression in both K562 and HeLa cells of truncated forms of SNX9 interfered with the uptake of transferrin, consistent with a role of SNX9 in endocytosis.

Deciphering the cellular functions of the Op18/Stathmin family of microtubule-regulators by plasma membrane-targeted localization

The Op18/stathmin family of microtubule regulators includes the ubiquitous cytosolic Op18/stathmin (Op18) and the neuronal, primarily Golgi-associated proteins SCG10 and RB3, which all form ternary complexes with two head-to-tail-aligned tubulin heterodimers. To understand the physiological significance of previously observed differences in ternary complex stability, we have fused each of the heterodimer-binding regions of these three proteins with the CD2 cell surface protein to generate confined plasma membrane localization of the resulting CD2 chimeras. Herein, we show that, in contrast to constitutively active CD2-Op18-tetraA, both the CD2-SCG10 and CD2-RB3 chimeras sequestered tubulin at the plasma membrane, which results in >35% reduction of cytosolic tubulin heterodimer levels and consequent delayed formation of mitotic spindles. However, all three CD2 chimeras, including the tubulin sequestration-incompetent CD2-Op18-tetraA, destabilize interphase microtubules. Given that microtubules are in extensive contact with the plasma membrane during interphase, but not during mitosis, these findings indicate that Op18-like proteins have the potential to destabilize microtubules by both sequestration and direct interaction with microtubules. However, the differences in tubulin binding observed in cells also indicate conceptual differences between the functions of low-abundance neural family members, which will accumulate tubulin at specific cellular compartments, and the abundant cytosolic Op18 protein, which will not.

Stathmin expression and megakaryocyte differentiation: a potential role in polyploidy

OBJECTIVE

Megakaryopoiesis is characterized by two major processes, acquisition of lineage-specific markers and polyploidization. Polyploidy is a result of endomitosis, a process that is characterized by continued DNA replication in the presence of abortive mitosis. Stathmin is a major microtubule-regulatory protein that plays an important role in the regulation of the mitotic spindle. Our previous studies had shown that inhibition of stathmin expression in human leukemia cells results in the assembly of atypical mitotic spindles and abnormal exit from mitosis. We hypothesized that the absence of stathmin expression in megakaryocytes might be important for their abortive mitosis.

MATERIALS AND METHODS

The experimental models that we used were human K562 and HEL cell lines that can be induced to undergo megakaryocytic differentiation and primary murine megakaryocytes generated by in vitro culture of bone marrow cells. The megakaryocytic phenotype was evaluated by flow cytometry and light microscopy. The DNA content (ploidy) was analyzed by flow cytometry. Stathmin expression was analyzed by Western and Northern blotting and by RT-PCR.

RESULTS

Our studies showed an inverse correlation between the level of ploidy and the level of stathmin expression in megakaryocytic cell lines and in primary cells. More importantly, inhibition of stathmin expression in K562 cells enhanced the propensity of these cells to undergo endomitosis and to become polyploid upon induction of megakaryocytic differentiation. In contrast, inhibition of stathmin expression interfered with the ability of the cells to acquire megakaryocyte-specific markers of differentiation.

CONCLUSION

Based on these observations, we propose a model of megakaryopoiesis in which stathmin expression is necessary for the proliferation and differentiation of early megakaryoblasts and its suppression in the later stages of megakaryocytic maturation is necessary for polyploidization.

E2F mediates sustained G2 arrest and down-regulation of Stathmin and AIM-1 expression in response to genotoxic stress

Exposure of cells to genotoxic agents results in activation of checkpoint pathways leading to cell cycle arrest. These arrest pathways allow repair of damaged DNA before its replication and segregation, thus preventing accumulation of mutations. The tumor suppressor retinoblastoma (RB) is required for the G(1)/S checkpoint function. In addition, regulation of the G(2) checkpoint by the tumor suppressor p53 is RB-dependent. However, the molecular mechanism underlying the involvement of RB and its related proteins p107 and p130 in the G(2) checkpoint is not fully understood. We show here that sustained G(2)/M arrest induced by the genotoxic agent doxorubicin is E2F-dependent and involves a decrease in expression of two mitotic regulators, Stathmin and AIM-1. Abrogation of E2F function by dominant negative E2F abolishes the doxorubicin-induced down-regulation of Stathmin and AIM-1 and leads to premature exit from G(2). Expression of the E7 papilloma virus protein, which dissociates complexes containing E2F and RB family members, also prevents the down-regulation of these mitotic genes and leads to premature exit from G(2) after genotoxic stress. Furthermore, genotoxic stress increases the levels of nuclear E2F-4 and p130 as well as their in vivo binding to the Stathmin promoter. Thus, functional complexes containing E2F and RB family members appear to be essential for repressing expression of critical mitotic regulators and maintaining the G(2)/M checkpoint.

Autonomous and phosphorylation-responsive microtubule-regulating activities of the N-terminus of Op18/stathmin

Op18 is the prototypical member of a family of phosphorylation-responsive regulators of microtubule (MT) dynamics. Previous dissection of Op18 has suggested that it has a functional dichotomy in which an intact N-terminus is required for catastrophe promotion (i.e. transition from growing to shrinking MTs), whereas an intact C-terminus is required for efficient ternary Op18-tubulin complex formation and the resultant tubulin-sequestering activity. Here we have expressed and functionally analyzed the properties of the N-terminus of Op18. The data show that the N-terminal 57 residues are sufficient for low-affinity tubulin interactions, as shown by inhibition of basal GTP hydrolysis of soluble heterodimers. In addition, high concentrations of the Op18 N-terminal portion increased the catastrophe rate during MT assembly in vitro. Overexpression of the N-terminus in a human cell line results in MT destabilization in interphase and phosphorylation-modulated accumulation of metaphase-arrested cells with dense short MTs. These results demonstrate that the N-terminus of Op18 has autonomous activity. Evidently, this activity is enhanced by the increase in tubulin affinity that is provided by the extended alpha-helical portion of native Op18.

Regulation of microtubule-associated proteins

Microtubule-associated proteins (MAPs) function to regulate the assembly dynamics and organization of microtubule polymers. Upstream regulation of MAP activities is the major mechanism used by cells to modify and control microtubule assembly and organization. This review summarizes the functional activities of MAPs found in animal cells and discusses how these MAPs are regulated. Mechanisms controlling gene expression, isoform-specific expression, protein localization, phosphorylation, and degradation are discussed. Additional regulatory mechanisms include synergy or competition between MAPs and the activities of cofactors or binding partners. For each MAP it is likely that regulation in vivo reflects a composite of multiple regulatory mechanisms.

Cloning and characterization of a proliferation-associated cytokine-inducible protein, CIP29

We identified a novel erythropoietin (Epo)-induced protein (CIP29) in lysates of human UT-7/Epo leukemia cells using two-dimensional gel analysis and cloned its full-length cDNA. CIP29 contains 210 amino acids with a predicted MW of 24 kDa, and has a N-terminal SAP DNA-binding motif. CIP29 expression was higher in cancer and fetal tissues than in normal adult tissues. CIP29 mRNA expression is cytokine regulated in hematopoietic cells, being up-regulated by Epo in UT7/Epo cells, and by thrombopoietin (Tpo), FLT3 ligand (FL) and stem cell factor (SCF) in primary human CD34(+) cells. Up-regulation of CIP29 in UT7/Epo cells by Epo was associated with cell cycle progression but not with antiapoptosis. Epo withdrawal reduced CIP29 expression concomitant with cell cycle arrest. Overexpression of CIP29-GFP in HEK293 cells enhances cell cycle progression. CIP29 appears to be a new cytokine regulated protein involved in normal and cancer cell proliferation.

Inhibition of heat-induced phosphorylation of stathmin by the bioflavonoid quercetin

Effects of quercetin on heat-induced phosphorylation of stathmin in JURKAT cells were examined. Two-dimensional electrophoresis of stathmin showed that heat shock increases mono- and diphosphorylation of stathmin. Monophosphorylation induced by heat shock was inhibited by the presence of 0.1 mM quercetin, but not by the presence of 0.1 microM staurosporine. Immunoblot analysis of phosphorylated stathmin showed that heat-induced phosphorylation at Ser-38 was inhibited by quercetin but not by staurosporine. Quercetin enhanced heat-induced tyrosine phosphorylation of MAP kinase. These observations indicate that quercetin inhibits heat-induced phosphorylation at Ser-38 of stathmin but mitogen-activated protein (MAP) kinase is not involved in its phosphorylation.

Stathmin-deficient mice develop an age-dependent axonopathy of the central and peripheral nervous systems

Stathmin is a cytosolic protein that binds tubulin and destabilizes cellular microtubules, an activity regulated by phosphorylation. Despite its abundant expression in the developing mammalian nervous system and despite its high degree of evolutionary conservation, stathmin-deficient mice do not exhibit a developmental phenotype.(1) Here we report that aging stathmin(-/-) mice develop an axonopathy of the central and peripheral nervous systems. The pathological hallmark of the early axonal lesions was a highly irregular axoplasm predominantly affecting large, heavily myelinated axons in motor tracts. As the lesions progressed, degeneration of axons, dysmyelination, and an unusual glial reaction were observed. At the functional level, electrophysiology recordings demonstrated a significant reduction of motor nerve conduction velocity in stathmin(-/-) mice. At the molecular level, increased gene expression of SCG 10-like protein, a stathmin-related gene with microtubule destabilizing activity, was detected in the central nervous system of aging stathmin(-/-) mice. Together, these findings suggest that stathmin plays an essential role in the maintenance of axonal integrity.

Expression of SCG10 and stathmin proteins in the rat olfactory system during development and axonal regeneration

The membrane-associated protein SCG10 is expressed specifically by neuronal cells. Recent experiments have suggested that it promotes neurite outgrowth by increasing microtubule dynamics in growth cones. SCG10 is related to the ubiquitous but neuron-enriched cytosolic protein stathmin. To better understand the role played by SCG10 and stathmin in vivo, we have analyzed the expression and localization of these proteins in both the olfactory epithelium and the olfactory bulb in developing and adult rats, as well as in adult bulbectomized rats. The olfactory epithelium is exceptional in that olfactory receptor neurons constantly regenerate and reinnervate the olfactory bulb throughout animal life-span. SCG10 and stathmin expression in the olfactory receptor neurons was found to be regulated during embryonic and postnatal development and to correlate with neuronal maturation. Whereas SCG10 expression was restricted to immature olfactory receptor neurons (GAP-43-positive, olfactory marker protein-negative), stathmin was also expressed by the basal cells. In the olfactory bulb of postnatal and adult rats, a moderate to strong SCG10 immunoreactivity was present in the olfactory nerve layer, whereas no labeling was detected in the glomerular layer. Olfactory glomeruli also showed no apparent immunoreactivity for several cytoskeletal proteins such as tubulin and microtubule-associated proteins. In unilaterally bulbectomized rats, SCG10 and stathmin were seen to be up-regulated in the regenerating olfactory epithelium at postsurgery stages corresponding to olfactory axon regeneration. Our data strongly suggest that, in vivo, both SCG10 and stathmin may play a role in axonal outgrowth during ontogenesis as well as during axonal regeneration.

The chicken stathmin gene and its expression in the embryo

Stathmin, which functions as an intracellular relay in signal transduction pathways, has been suggested as a potential indicator of pluripotent cells in the early mouse embryo. In this study, chicken stathmin cDNA and genomic DNA were analyzed. In mammals stathmin consists of five exons and four introns; exons 3, 4, and 5 in the mammalian stathmin gene are equivalent to one relatively large exon in the chicken stathmin gene. Introns equivalent to introns 3 and 4 in the mammalian stathmin gene are not present in the counterpart gene in chickens and, although intron 2 was shown to be present in both mammals and birds, it is smaller in the chicken stathmin gene. Despite differences in the genomic organization of the gene and its smaller size in chickens compared with that in humans and mice, similarities in the coding sequences and in the expression of the chicken and mouse stathmin genes at certain stages of embryo development, as determined by whole-mount in situ hybridization experiments, suggest that their products are functional homologues. The argument is thus substantiated for further investigations into the use of regulatory regions of the stathmin gene in a system for the establishment of long-term cultures of germline competent chicken embryonic stem (ES) cells by the selective ablation of differentiated cells in culture using drug selection.

Effects of stathmin inhibition on the mitotic spindle

Stathmin is a major cytosolic phosphoprotein that plays an important role in the regulation of microtubule dynamics during cell cycle progression. It has recently been proposed that the major function of stathmin is to promote depolymerization of the microtubules that make up the mitotic spindle. In this report, we tested the prediction that a deficiency in stathmin expression would result in constitutive stabilization of microtubules and lead to abnormalities in the organization of the mitotic spindle. Our studies demonstrate that antisense inhibition of stathmin expression in K562 erythroleukemic cells results in increased ratio of polymerized to depolymerized tubulin. These changes are associated with phenotypic abnormalities of the mitotic spindle and difficulty in completing mitosis. These studies also showed that inhibition of stathmin expression results in increased susceptibility of K562 leukemic cells to the pharmacological agents, like taxol, which are known to stabilize the mitotic spindle. In contrast, stathmin inhibition results in decreased sensitivity to vinblastine, an agent that destabilizes the mitotic spindle. Thus, our experimental findings are supportive of the model that stathmin is a microtubule-destabilizing factor that plays an important role in the regulation of the mitotic spindle. We also suggest a potential therapeutic approach for cancer based on the combination of stathmin inhibition with pharmacologic agents that stabilize the mitotic spindle.

Development of ribozymes that target stathmin, a major regulator of the mitotic spindle

Stathmin is a major cytosolic phosphoprotein that plays an important role in the control of cellular proliferation by regulating the dynamics of the microtubules that make up the mitotic spindle. Because stathmin is expressed at high levels in all human cancers, it is an attractive molecular target for anticancer interventions. We had shown previously that antisense stathmin inhibition results in marked abrogation of the transformed phenotype of leukemic cells in vitro and in vivo. Unlike the antisense approach, ribozymes can catalytically cleave several molecules of target RNA. This may provide a more efficient strategy for downregulating genes, such as stathmin, that are expressed at very high levels in cancer cells. We designed several antistathmin hammerhead ribozymes and tested their cleavage activity against short synthetic stathmin RNA substrates. In vitro cleavage studies demonstrated site-specific cleavage of stathmin RNA that was dependent on ribozyme concentration and duration of exposure to ribozyme. The most active antistathmin ribozyme was capable of cleaving >90% stathmin RNA in a catalytic manner, cleaving multiple substrate molecules per ribozyme molecule. We also demonstrated that the designed antistathmin ribozymes are capable of selectively cleaving native stathmin RNA in a mixture of total RNA isolated from leukemic cells. These antistathmin ribozymes may provide a novel and effective form of gene therapy that may be applicable to a wide variety of human cancers.