Multiple signal transduction pathways induce phosphorylation of serines 16, 25, and 38 of oncoprotein 18 in T lymphocytes

A strategy to disentangle direct and indirect effects on (de)phosphorylation by chemical modulators of the phosphatase PP1 in complex cellular contexts

Chemical activators and inhibitors are useful probes to identify substrates and downstream effects of enzymes; however, due to the complex signaling environment within cells, it is challenging to distinguish between direct and indirect effects. This is particularly the case for phosphorylation, where a single (de)phosphorylation event can trigger rapid changes in many other phosphorylation sites. An additional complication arises when a single catalytic entity, which acts in the form of many different holoenzymes with different substrates, is activated or inhibited, as it is unclear which holoenzymes are affected, and in turn which of their substrates are (de)phosphorylated. Direct target engaging MS-based technologies to study targets of drugs do not address these challenges. Here, we tackle this by studying the modulation of protein phosphatase-1 (PP1) activity by PP1-disrupting peptides (PDPs), as well as their selectivity toward PP1, by using a combination of mass spectrometry-based experiments. By combining cellular treatment with the PDP with in vitro dephosphorylation by the enzyme, we identify high confidence substrate candidates and begin to separate direct and indirect effects. Together with experiments analyzing which holoenzymes are particularly susceptible to this treatment, we obtain insights into the effect of the modulator on the complex network of protein (de)phosphorylation. This strategy holds promise for enhancing our understanding of PP1 in particular and, due to the broad applicability of the workflow and the MS-based read-out, of chemical modulators with complex mode of action in general.

The Global Phosphorylation Landscape of SARS-CoV-2 Infection

The causative agent of the coronavirus disease 2019 (COVID-19) pandemic, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has infected millions and killed hundreds of thousands of people worldwide, highlighting an urgent need to develop antiviral therapies. Here we present a quantitative mass spectrometry-based phosphoproteomics survey of SARS-CoV-2 infection in Vero E6 cells, revealing dramatic rewiring of phosphorylation on host and viral proteins. SARS-CoV-2 infection promoted casein kinase II (CK2) and p38 MAPK activation, production of diverse cytokines, and shutdown of mitotic kinases, resulting in cell cycle arrest. Infection also stimulated a marked induction of CK2-containing filopodial protrusions possessing budding viral particles. Eighty-seven drugs and compounds were identified by mapping global phosphorylation profiles to dysregulated kinases and pathways. We found pharmacologic inhibition of the p38, CK2, CDK, AXL, and PIKFYVE kinases to possess antiviral efficacy, representing potential COVID-19 therapies.

Sulforaphane metabolites cause apoptosis via microtubule disruption in cancer

Sulforaphane (SFN) inhibited growth in many cancers, but its half-life is 2 h in circulation. However, its metabolites, sulforaphane-cysteine (SFN-Cys) and sulforaphane-N-acetyl-cysteine (SFN-NAC) had longer half-lives and decreased the cell viability in both dose- and time-dependent manners in human prostate cancer. Flow cytometry assay revealed that these two SFN metabolites induced apoptosis with the features such as vacuolization, disappeared nuclear envelope, nuclear agglutination and fragmentation via transmission electron microscopy observation. Western blot showed that the sustained phosphorylation of ERK1/2 mediated by SFN metabolites caused activation and upregulation of cleaved Caspase 3 and downregulation of α-tubulin. High expression of α-tubulin was demonstrated to be positively correlated with cancer pathological grading. Both co-immunoprecipitation and immunofluorescence staining implicated the interaction between SFN metabolite-induced phosphorylated ERK1/2 and α-tubulin, and Caspase 3 cleavage assay showed that α-tubulin might be the substrate for cleaved Caspase 3. More, the SFN metabolite-mediated reduction of α-tubulin increased the depolymerization and instability of microtubules by microtubule polymerization assay. Reversely, microtubule-associated protein Stathmin-1 phosphorylation was increased via phosphorylated ERK1/2 and total Stathmin-1 was reduced, which might promote over-stability of microtubules. Immunofluorescence staining also showed that SFN metabolites induced the 'nest-like' structures of microtubule distribution resulting from the disrupted and aggregated microtubules, and abnormal nuclear division, suggesting that the disturbance of spindle formation and mitosis turned up. Thus, SFN-Cys and SFN-NAC triggered the dynamic imbalance of microtubules, microtubule disruption leading to cell apoptosis. These findings provided a novel insight into the chemotherapy of human prostate cancer.

miR-34a Regulates Expression of the Stathmin-1 Oncoprotein and Prostate Cancer Progression

In aggressive prostate cancers, the oncoprotein STMN1 (also known as stathmin 1 and oncoprotein 18) is often overexpressed. STMN1 is involved in various cellular processes, including cell proliferation, motility, and tumor metastasis. Here, it was found that the expression of STMN1 RNA and protein is elevated in metastatic prostate cancers. Knockdown of STMN1 resulted in reduced proliferation and invasion of cells and tumor growth and metastasis in vivo Furthermore, miR-34a downregulated STMN1 by directly binding to its 3'-UTR. Overexpression of miR-34a in prostate cancer cells reduced proliferation and colony formation, suggesting that it is a tumor suppressor. The transcriptional corepressor C-terminal binding protein 1 (CtBP1) negatively regulated expression of miR-34a. Furthermore, gene expression profiling of STMN1-modulated prostate cancer cells revealed molecular alterations, including elevated expression of growth differentiation factor 15 (GDF15), which is involved in cancer progression and potentially in STMN1-mediated oncogenesis. Thus, in prostate cancer, CtBP1-regulated miR-34a modulates STMN1 expression and is involved in cancer progression through the CtBP1\miR-34a\STMN1\GDF15 axis.Implications: The CtBP1\miR-34a\STMN1\GDF15 axis is a potential therapeutic target for treatment of aggressive prostate cancer. Mol Cancer Res; 16(7); 1125-37. ©2017 AACR.

Stathmin and phospho-stathmin protein signature is associated with survival outcomes of breast cancer patients

Currently, Stathmin1 (STMN1) and phospho-STMN1 levels in breast cancers and their clinical implications are unknown. We examined the expression of STMN1 and its serine phospho-site (Ser16, Ser25, Ser38, and Ser63) status by immunohistochemistry. Using Cox regression analysis, a STMN1 expression signature and phosphorylation profile plus clinicopathological characteristics (STMN1-E/P/C) was developed in the training set (n = 204) and applied to the validation set (n = 106). This tool enabled us to separate breast cancer patients into high- and low-risk groups with significantly different disease-free survival (DFS) rates (P < 0.001). Importantly, this STMN1-E/P/C model had a greater prognostic value than the traditional TNM classifier, especially in luminal subtype breast cancer (P = 0.002). Further analysis showed that patients in the low-risk group would benefit more from adjuvant paclitaxel-based chemotherapy (P = 0.002). In conclusion, the STMN1-E/P/C signature is a reliable prognostic indicator for luminal subtype breast cancer and may predict the therapeutic response to paclitaxel-based treatments, potentially facilitating individualized management.

The phosphorylation-specific association of STMN1 with GRP78 promotes breast cancer metastasis

Metastasis is a major cause of death in patients with breast cancer. Stathmin1 (STMN1) is a phosphoprotein associated with cancer metastasis. It exhibits a complicated phosphorylation pattern in response to various extracellular signals, but its signaling mechanism is poorly understood. In this study, we report that phosphorylation of STMN1 at Ser25 and Ser38 is necessary to maintain cell migration capabilities and is associated with shorter disease-free survival (DFS) in breast cancer. In addition, we report that glucose-regulated protein of molecular mass 78 (GRP78) is a novel phospho-STMN1 binding protein upon STMN1 Ser25/Ser38 phosphorylation. This phosphorylation-dependent interaction is regulated by MEK kinase and is required for STMN1-GRP78 complex stability and STMN1-mediated migration. We also propose a prognostic model based on phospho-STMN1 and GRP78 to assess metastatic risk in breast cancer patients.

Comparative proteomic and phosphoproteomic analysis of the silkworm (Bombyx mori) posterior silk gland under high temperature treatment

The proteins from the posterior silk gland of silkworm hybrids and their parents reared under high temperatures were studied by using comparative proteomic and phosphoproteomic analysis. A total of 82.07, 6.17 and 11.76 % protein spots showed additivity, overdominance and underdominance patterns, respectively. Fifteen differentially expressed protein spots were identified by peptide mass fingerprinting. Among these, four spots, including sHSPs and prohibitin protein that were directly relevant to heat response, were identified. Eleven protein spots were found to play an important role in silk synthesis, and nine protein spots expressed phosphorylation states. According to Gene ontology and KEGG pathway analysis, these nine spots played an important role in stress-induced signal transduction. Expression of most silk synthesis-related proteins was reduced, whereas stress-responsive proteins increased with heat exposure time in three breeds. Furthermore, most proteins showed under- or overdominance in the hybrids compared to the parents. The results suggested that high temperature could alter the expression of proteins related to silk synthesis and heat response in silkworm. Moreover, differentially expressed proteins occurring in the hybrid and its parents may be the main explanation of the observed heterosis.

Involvement of stathmin 1 in the neurotrophic effects of PACAP in PC12 cells

Rat pheochromocytoma PC12 cells have been widely used to investigate the neurotrophic activities of pituitary adenylate cyclase-activating polypeptide (PACAP). In particular, PACAP has been shown to promote differentiation and to inhibit apoptosis of PC12 cells. In order to identify the mechanisms mediating these effects, we sought for proteins that are phosphorylated upon PACAP treatment. High-performance liquid chromatography and 2D gel electrophoresis analysis, coupled with mass spectrometry, revealed that stathmin 1 is strongly phosphorylated within only 5 min of exposure to PACAP. Western blot experiments confirmed that PACAP induced a robust phosphorylation of stathmin 1 in a time-dependent manner. On the other hand, PACAP decreased stathmin 1 gene expression. Investigations of the signaling mechanisms known to be activated by PACAP revealed that phosphorylation of stathmin 1 was mainly mediated through the protein kinase A and mitogen-activated protein kinase pathways. Blockage of stathmin 1 expression with small interfering RNA did not affect PC12 cell differentiation induced by PACAP but reduced the ability of the peptide to inhibit caspase 3 activity and significantly decreased its neuroprotective action. Taken together, these data demonstrate that stathmin 1 is involved in the neurotrophic effect of PACAP in PC12 cells.

The p38/MAPK pathway regulates microtubule polymerization through phosphorylation of MAP4 and Op18 in hypoxic cells

In both cardiomyocytes and HeLa cells, hypoxia (1% O(2)) quickly leads to microtubule disruption, but little is known about how microtubule dynamics change during the early stages of hypoxia. We demonstrate that microtubule associated protein 4 (MAP4) phosphorylation increases while oncoprotein 18/stathmin (Op18) phosphorylation decreases after hypoxia, but their protein levels do not change. p38/MAPK activity increases quickly after hypoxia concomitant with MAP4 phosphorylation, and the activated p38/MAPK signaling leads to MAP4 phosphorylation and to Op18 dephosphorylation, both of which induce microtubule disruption. We confirmed the interaction between phospho-p38 and MAP4 using immunoprecipitation and found that SB203580, a p38/MAPK inhibitor, increases and MKK6(Glu) overexpression decreases hypoxic cell viability. Our results demonstrate that hypoxia induces microtubule depolymerization and decreased cell viability via the activation of the p38/MAPK signaling pathway and changes the phosphorylation levels of its downstream effectors, MAP4 and Op18.

Regulation of microtubule dynamic instability in vitro by differentially phosphorylated stathmin

Stathmin is an important regulator of microtubule polymerization and dynamics. When unphosphorylated it destabilizes microtubules in two ways, by reducing the microtubule polymer mass through sequestration of soluble tubulin into an assembly-incompetent T2S complex (two alpha:beta tubulin dimers per molecule of stathmin), and by increasing the switching frequency (catastrophe frequency) from growth to shortening at plus and minus ends by binding directly to the microtubules. Phosphorylation of stathmin on one or more of its four serine residues (Ser(16), Ser(25), Ser(38), and Ser(63)) reduces its microtubule-destabilizing activity. However, the effects of phosphorylation of the individual serine residues of stathmin on microtubule dynamic instability have not been investigated systematically. Here we analyzed the effects of stathmin singly phosphorylated at Ser(16) or Ser(63), and doubly phosphorylated at Ser(25) and Ser(38), on its ability to modulate microtubule dynamic instability at steady-state in vitro. Phosphorylation at either Ser(16) or Ser(63) strongly reduced or abolished the ability of stathmin to bind to and sequester soluble tubulin and its ability to act as a catastrophe factor by directly binding to the microtubules. In contrast, double phosphorylation of Ser(25) and Ser(38) did not affect the binding of stathmin to tubulin or microtubules or its catastrophe-promoting activity. Our results indicate that the effects of stathmin on dynamic instability are strongly but differently attenuated by phosphorylation at Ser(16) and Ser(63) and support the hypothesis that selective targeting by Ser(16)-specific or Ser(63)-specific kinases provides complimentary mechanisms for regulating microtubule function.

Measurements of stathmin-tubulin interaction in solution

Stathmin is an important phosphorylation-controlled regulator of microtubule dynamics and plays a crucial role in cell division and cell proliferation. In its non-phosphorylated form, stathmin is the protein that interacts the most tightly with tubulin, in a 2:1 tubulin-stathmin (T2S) complex that does not participate in microtubule assembly. The importance of stathmin at different levels of phosphorylation in different steps of mitosis This article is a short overview of the different methods that have been or could be used to monitor the kinetic and thermodynamic parameters of tubulin-stathmin interaction and to evaluate the effects of phosphorylation. The author has tried to emphasize how hydrodynamic and spectroscopic methods measuring direct binding of stathmin to tubulin can be complemented by methods that make use of linked functions, measuring how the change in a functional property of tubulin upon binding stathmin provides information on binding parameters.

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.

Identifying novel substrates for mouse Cdk5 kinase using the yeast Saccharomyces cerevisiae

Among the mammalian Cdk family members, Cdk5, activated by the binding of p35, plays an important role in the control of neurogenesis, and its deregulation is thought to be one of the causes of neurodegenerative diseases. Overproduction of Cdk5 and p35 in yeast cells causes growth arrest, probably because of hyperphosphorylation of yeast proteins. We screened mouse brain cDNA that could recover the growth of yeast cells overproducing Cdk5 and p35, hoping that such cDNA encodes a substrate or inhibitor of Cdk5. Mouse brain cDNA library was introduced into a yeast strain in which Cdk5, p35 and mouse cDNA were over-expressed under the control of the GAL promoter, and cDNA plasmids were isolated from the transformants that recovered growth on galactose medium. The analysis of those plasmids revealed that they harbored cDNA that encodes neuronal proteins including SCLIP and CRMP-1, and those with unknown function. We found that Cdk5 could phosphorylate SCLIP and CRMP-1 in vitro and the two proteins in cultured cells showed a mobility shift depending on Cdk5 activity and the presence of specific Ser or Thr residues, indicating that SCLIP and CRMP-1 are likely substrates for Cdk5 in vitro and in cultured cells. Further screening with these systems will enable us to identify more novel substrates and regulators of Cdk5/p35, which will lead to the exploration of Cdk5 function in diverse cellular systems.

Analysis on heat stress-induced hyperphosphorylation of stathmin at serine 37 in Jurkat cells by means of two-dimensional gel electrophoresis and tandem mass spectrometry

Two-dimensional gel electrophoresis (2-DE) and tandem mass spectrometry were successfully used for determination of a phosphorylation site of stathmin induced by heat stress to Jurkat cells of a human T lymphoblastic cell line. The cells were incubated for 30 min at 41 degrees C up to 45 degrees C in a serum free 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) buffered culture medium. The intracellular soluble proteins were separated by 2-DE, and some of the proteins increased their abundance by heat stress. Those proteins were identified to be calmodulin, protein kinase C substrate, thymosin beta-4 and F-actin capping protein beta-subunit by peptide mass fingerprinting (PMF) with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). On the contrary, protein phosphatase 2C gamma-isoform, nucleophosmin, translationally controlled tumor protein, Rho GDP-dissociation inhibitor-1, eukaryotic translation initiation factors 5A and 3A subunit 2, ubiquitin-like protein SMT 3B and chloride intracellular channel protein-1 were decreased their abundance. A protein spot of M(r) 18,000 and pI 5.9 was markedly increased at temperatures higher than 43 degrees C at which the cells were led to apoptosis. The spot was identified to be stathmin of a signal relay protein which has a function of sequestering microtubule. MALDI-quadrupole ion trap (QIT)-TOF-MS/MS and immunoblotting with a monoclonal antibody specific for a phosphorylation site of stathmin showed that the spot was a phosphorylated stathmin at serine 37 (Ser 37). The phosphorylation was suppressed by treatment of cells with olomoucine of an inhibitor specific for cyclin dependent kinase (Cdk-1). These results strongly suggest that heat stress activates Cdk-1 which phosphorylates Ser 37 on the stathmin molecule. The phosphorylation may cause the functional loss of stathmin for dynamic microtubule assembly and leads Jurkat cells to cell cycle arrest and apoptosis.

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.

Differential phosphoprotein labeling (DIPPL), a method for comparing live cell phosphoproteomes using simultaneous analysis of (33)P- and (32)P-labeled proteins

We developed a differential method to reveal kinase-specific phosphorylation events in live cells. In this method, cells in which the specified kinase is inactive are labeled with (32)Pi, whereas cells in which the kinase is active are labeled with (33)Pi. The two cell extracts are then mixed, and proteins are separated on a single two-dimensional gel. The dried gel is exposed twice. The first exposure reveals both (32)P- and (33)P-labeled proteins; the kinase-specific spots are revealed because of (33)P labeling. The second exposure is conducted with two acetate sheets intervening between the gel and the detection plate. This maneuver screens out the less energetic (33)P-labeled proteins while allowing the more energetic (32)P-labeled proteins to be detected, thus leaving only those spots that were phosphorylated independently of the specified kinase. We demonstrate the utility of this method for detecting kinase substrates in rare tissue by focusing on extracellular signal-regulated kinase-specific phosphorylation of stathmin/OP18 in primary rat sympathetic neurons.

Inhibiting MAP kinase activity prevents calcium transients and mitosis entry in early sea urchin embryos

A transient calcium increase triggers nuclear envelope breakdown (mitosis entry) in sea urchin embryos. Cdk1/cyclin B kinase activation is also known to be required for mitosis entry. More recently, MAP kinase activity has also been shown to increase during mitosis. In sea urchin embryos, both kinases show a similar activation profile, peaking at the time of mitosis entry. We tested whether the activity of both kinases is required for mitosis entry and whether either kinase controls mitotic calcium signals. We found that reducing the activity of either mitotic kinase prevents nuclear envelope breakdown, despite the presence of a calcium transient, when cdk1/cyclin B kinase activity is alone inhibited. When MAP kinase activity alone was inhibited, the calcium signal was absent, suggesting that MAP kinase activity is required to generate the calcium transient that triggers nuclear envelope breakdown. However, increasing intracellular free calcium by microinjection of calcium buffers or InsP(3) while MAP kinase was inhibited did not itself induce nuclear envelope breakdown, indicating that additional MAP kinase-regulated events are necessary. After MAP kinase inhibition early in the cell cycle, the early events of the cell cycle (pronuclear migration/fusion and DNA synthesis) were unaffected, but chromosome condensation and spindle assembly are prevented. These data indicate that in sea urchin embryos, MAP kinase activity is part of a signaling complex alongside two components previously shown to be essential for entry into mitosis: the calcium transient and the increase in cdk1/cyclinB kinase activity.

Identification of stathmin as a novel marker of cell proliferation in the recovery phase of acute ischemic renal failure

Ischemic renal injury can be classified into the initiation and extension phase followed by the recovery phase. The recovery phase is characterized by increased dedifferentiated and mitotic cells in the damaged tubules. Suppression subtractive hybridization was performed by using RNA from normal and ischemic kidneys to identify the genes involved in the physiological response to ischemia-reperfusion injury (IRI). The expression of stathmin mRNA increased by fourfold at 24 h of reperfusion. The stathmin mRNA did not increase in sodium-depleted animals or in animals with active, persistent injury secondary to cis-platinum. Immunofluorescent labeling demonstrated that the expression of stathmin increased dramatically at 48 h of reperfusion. Labeling with antibodies to stathmin and proliferating cell nuclear antigen (PCNA) indicates that the expression of stathmin was induced before the upregulation of PCNA and that all PCNA-positive cells expressed stathmin. Double immunofluorescent labeling demonstrated the colocalization of stathmin with vimentin, a marker of dedifferentiated cells. Stathmin expression was also significantly enhanced in acute tubular necrosis in humans. On the basis of its induction profile in IRI, the data indicating its enhanced expression in proliferating cells and regenerating organs, we propose that stathmin is a marker of dedifferentiated, mitotically active epithelial cells that may contribute to tubular regeneration and could prove useful in distinguishing the injury phase from recovery phase in IRI.

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.

The oncoprotein 18/stathmin family of microtubule destabilizers

The past several years have seen major advances in our understanding of the mechanisms of microtubule destabilization by oncoprotein18/stathmin (Op18/stathmin) and related proteins. New structural information has clearly shown how members of the Op18/stathmin protein family bind tubulin dimers and suggests models for how these proteins stimulate catastrophe, the transition from microtubule growth to shortening. Regulation of Op18/stathmin by phosphorylation continues to capture much attention. Studies suggest that phosphorylation occurs in a localized fashion, resulting in decreased microtubule destabilizing activity near chromatin or microtubule polymer. A spatial gradient of inactive Op18/stathmin associated with chromatin or microtubules could contribute significantly to mitotic spindle assembly.

Differences in phosphorylation of human and chicken stathmin by MAP kinase

Stathmin/Op18 is a highly conserved 19 kDa cytosolic phosphoprotein. Human and chicken stathmin share 93% identity with only 11 amino acid substitutions. One of the substituted amino acids is serine 25, which is a glycine in chicken stathmin. In human stathmin, serine 25 is the main phosphorylation site for MAP kinase. In this study, we have compared the phosphorylation of human and chicken stathmin. The proteins were expressed in Sf9 cells using the baculovirus expression system and purified for in vitro phosphorylation assays. Phosphorylation with MAP kinase showed that chicken stathmin was phosphorylated 10 times less than human stathmin. To identify the phosphorylation sites we used liquid chromatography/mass spectrometry (LC/MS/MS). The only amino acid found phosphorylated was serine 38, which corresponds to the minor phosphorylation site in human stathmin. Phosphorylation with p34(cdc2)- and cGMP-dependent protein kinases gave almost identical phosphorylation levels in the two stathmins.

Model for stathmin/OP18 binding to tubulin

Stathmin/OP18 is a regulatory phosphoprotein that controls microtubule (MT) dynamics. The protein does not have a defined three-dimensional structure, although it contains three distinct regions (an unstructured N-terminus, N: 1-44; a region with high helix propensity, H 1: 44-89; and a region with low helix propensity, H 2: 90-142). The full protein and a combination of H 1 and H 2 inhibits tubulin polymerization, while the combination of H 1 and the N-terminus is less efficient. None of the individual three regions alone are functional in this respect. However, all of them cross-link to alpha-tubulin, but only full-length stathmin produces high-molecular-weight products. Mass spectrometry analysis of alpha-tubulin-stathmin/OP18 and its truncation products shows that full-length stathmin/OP18 binds to the region around helix 10 of alpha-tubulin, a region that is involved in longitudinal interactions in the MT, sequestering the dimer and possibly linking two tubulin heterodimers. In the absence of the N-terminus, stathmin/OP18 binds to only one molecule of alpha-tubulin, at the top of the free tubulin heterodimer, preventing polymerization.

Cellular and subcellular localization of stathmin during oocyte and preimplantation embryo development

Stathmin is a 19 kDa cytosolic phosphoprotein, proposed to act as a relay integrating diverse intracellular signaling pathways involved in regulation of cell proliferation, differentiation, and function. To gain further information about its significance during early development, we analyzed stathmin expression and subcellular localization in mouse oocytes and preimplantation embryos. RT-PCR analysis revealed a low expression of stathmin mRNA in unfertilized oocytes and a higher expression at the blastocyst stage. A fine cytoplasmic punctuate fluorescent immunoreactive stathmin pattern was detected in the oocyte, while it evolved toward an increasingly speckled pattern in the two-cell and later four- to eight-cell embryo, with even larger speckles at the morula stage. In blastocysts, stathmin immunoreactivity was fine and intense in inner cell mass cells, whereas it was low and variable in trophectodermal cells. Electron microscopic analysis allowed visualization with more detail of two types of stathmin immunolocalization: small clusters in the cytoplasm of oocytes and blastocyst cells, together with loosely arranged clusters around the outer membrane of cytoplasmic vesicles, corresponding to the immunofluorescent speckles in embryos until the morula stage. In conclusion, it appears from our results that maternal stathmin is accumulated in the oocyte and is relocalized within the oocyte and early preimplantation embryonic cell cytoplasm to interact with specific cytoplasmic membrane formations. Probably newly synthesized, embryonic stathmin is expressed in the blastocyst, where it is localized more uniformly in the cytoplasm mostly of inner cell mass (ICM) cells. These expression and localization patterns are probably related to the particular roles of stathmin at the successive steps of oocyte maturation and early embryonic development. They further support the proposed physiologic importance of stathmin in essential biologic regulation.

The Ca-calmodulin-dependent protein kinase cascade

The Ca2+-calmodulin-dependent protein kinase (CaM kinase) cascade includes three kinases: CaM-kinase kinase (CaMKK); and the CaM kinases CaMKI and CaMKIV, which are phosphorylated and activated by CaMKK. Members of this cascade respond to elevation of intracellular Ca2+ levels and are particularly abundant in brain and in T cells. CaMKK and CaMKIV localize both to the nucleus and to the cytoplasm, whereas CaMKI is only cytosolic. Nuclear CaMKIV regulates transcription through phosphorylation of several transcription factors, including CREB. In the cytoplasm, there is extensive cross-talk between CaMKK, CaMKIV and other signaling cascades, including those that involve the cAMP-dependent kinase (PKA), MAP kinases and protein kinase B (PKB; also known as Akt). Activation of PKB by CaMKK appears to be important in protection of neurons from programmed cell death during development.

Mass spectrometric characterization of stathmin isoforms separated by 2D PAGE

In proteome analysis, the determination of the phosphorylation status of proteins and protein isoforms, which have been separated by two-dimensional polyacrylamide gel electrophoresis (2D PAGE), is of prime importance in addition to their identification. In this study, the extent to which such information can be directly extracted from the mass spectrometric data used for identification was evaluated. By searching for metastable peaks which are characteristic for loss of phosphoric acid, the Ser-phosphorylated peptides were identified with a high success rate in reflector matrix-assisted laser desorption/ionization (MALDI) mass maps of in-gel digested proteins. Furthermore, by employing a double enzymatic strategy using trypsin and Glu-C in parallel, improved sequence coverage and additional separation of the potential phosphorylation sites of the isoforms were achieved. The precise location of the modified sites within an identified phosphopeptide was obtained by submitting the corresponding molecular ions directly to nano-electrospray tandem mass spectrometric analysis. In this way the detailed phosphorylation status of six isomers of stathmin separated by 2D PAGE was determined. Two of these six isomers were phosphorylated at all four known sites (serines 15, 24, 37 and 62) and were probably derived from the previously reported alpha and beta forms, which differ by a yet unknown modification. In addition, isomers phosphorylated at serines 15, 24 and 37, serines 24, 37 and 62, serines 24 and 37 and serine 37 only were characterized.

Expression of a new isoform of the tumor susceptibility TSG101 protein lacking a leucine zipper domain in Burkitt lymphoma cell lines

The tumor susceptibility gene, TSG101, has been identified as a candidate tumor suppressor gene. We have examined the expression of TSG101 in Burkitt lymphoma cell lines. Several aberrant messages were detected in all cell lines. Aberrant splice donor sites are located within exon 1 at positions 132, 154, 172 and 284. Splice acceptors are located at positions 847 and 1054 within exon 5. The aberrant messages are coexpressed with a normal message and could be the result of additional splicing reactions of the mature message that behaves as an intermediate. The normal message codes for 46 kDa protein (TSG101A). One aberrant message joins in frame nucleotides 283-1055 and codes for a protein isoform of 17 kDa (TSG101B), as demonstrated by in vitro translation assays. The TSG101B isoform lacks the leucine zipper near the C-terminus, a transcriptional repressor domain, and retains most of the N-terminal region which has homology to E2 ubiquitin regulatory enzymes and the CROC-1 transcriptional regulator. The TSG101B isoform was detected in sixteen out of twenty-two (72%) BL cell lines, but not in normal lymphoid populations. The presence of two TSG101 isoforms with different dimerization potential opens up a new level of regulation of the TSG101 proteins possibly affecting cell cycle regulation.

Analysis of aberrant transcription of TSG101 in hepatocellular carcinomas

A variety of studies suggest that tumour suppressor loci on chromosome 11p are important in various forms of human neoplasia. Recently, a gene located at the chromosome 11p 15.1-15.2 region called TSG101 was discovered and proposed as a candidate tumour suppressor gene in breast cancers. We evaluated the TSG101 gene in a panel of liver cancer cell lines and paired tumours and non-malignant tissues. In this study, four of the seven (57%) cell lines, eight of the 18 (44%) tumours and four of the 18 (22%) non-malignant liver tissues exhibited aberrant TSG101 transcripts by nested reverse transcription-polymerase chain reaction (RT-PCR) analysis. However, a normal-sized transcript without sequence abnormalities verified by single-stranded conformation polymorphism (SSCP) analysis was expressed at robust levels in all the cell lines and most of the tissue samples tested. In addition, Southern blot analysis could identify no genomic abnormalities of the gene. Our results suggest either that the TSG101 gene may not be involved in hepatocarcinogenesis or that it plays a role in the development and/or progress of hepatocellular carcinomas through an unusual mechanism.

Analysis of heat shock-induced monophosphorylation of stathmin in human T lymphoblastic cell line JURKAT by two-dimensional gel electrophoresis

Two-dimensional gel electrophoresis (2-DE) was used to study alterations in intracellular proteins of the human T lymphoblastic cell line JURKAT by heat shock at 45 degrees C for 30 min. The 2-DE patterns indicated an increase in the amount of a spot of molecular weight (M(r)) 18,500 and isoelectric point (pI) 5.6, which was a monophosphorylated form of stathmin. Stathmin is a major substrate for a proline-rich peptide-specific serine protein kinase, a mitogen-activated protein kinase, however, the enzyme was not activated by the heat shock. Further examinations of the effects of cAMP, phorbol myristate acetate, cyclosporin A, and staurosporine on phosphorylation suggest that cyclin-dependent kinases might be responsible for the heat shock-induced monophosphorylation of stathmin.

Identification of stathmin as a novel substrate for p38 delta

p38 mitogen-activated protein kinases (MAPK) are a family of kinases that are activated by cellular stresses and inflammatory cytokines. Although there are many similarities shared by the isoforms of p38 (alpha, beta, gamma, and delta), p38 delta differs from the others in some respects such as inhibitor sensitivity and substrate specificity. Utilizing in a solution kinase assay, we identified a novel p38 delta substrate as stathmin. Stathmin is a cytoplasmic protein that was previously reported to be a substrate of several intracellular signaling kinases and has recently been linked to regulation of microtubule dynamics. p38 delta has significantly higher in vitro phosphorylating activity against stathmin than other p38 isoforms or related MAPKs. In transient expression studies, we found that in addition to different stimuli osmotic stress activates p38 delta to phosphorylate stathmin. The sites of phosphorylation were mapped to Ser-25 and Ser-38, both in vitro and in cells.

Serine 16 of Stathmin as a Cytosolic Target for Ca2+/Calmodulin-Dependent Kinase II After CD2 Triggering of Human T Lymphocytes

We investigated specific signaling events initiated after T cell triggering through the costimulatory surface receptors CD2 and CD28 as compared with activation via the Ag receptor (TCR/CD3). We therefore followed the phosphorylation of stathmin, a ubiquitous cytoplasmic phosphoprotein proposed as a general relay integrating diverse intracellular signaling pathways through the combinatorial phosphorylation of serines 16, 25, 38, and 63, the likely physiologic substrates for Ca2+/calmodulin (CaM)-dependent kinases, mitogen-activated protein (MAP) kinase, cyclin-dependent kinases (cdks), and protein kinase A, respectively. We addressed the specific protein kinase systems involved in the CD2 pathway of T cell activation through the analysis of stathmin phosphorylation patterns in exponentially growing Jurkat T cells, as revealed by phosphopeptide mapping. Stimulation via CD2 activated multiple signal transduction pathways, resulting in phosphorylation of distinct sites of stathmin, the combination of which only partially overlaps the CD3- and CD28-induced patterns. The partial redundancy of the three T cell activation pathways was evidenced by the phosphorylation of Ser25 and Ser38, substrates of MAP kinases and of the cdk family kinase(s), respectively. Conversely, the phosphorylation of Ser16 of stathmin was observed in response to both CD2 and CD28 triggering, but not CD3 triggering, with a kinetics compatible with the lasting activation of CaM kinase II in response to CD2 triggering. In vitro, Ser16 of recombinant human stathmin was phosphorylated also by purified CaM kinase II, and in vivo, CaM kinase II activity was indeed stimulated in CD2-triggered Jurkat cells. Altogether, our results favor an association of CaM kinase II activity with costimulatory signals of T lymphocyte activation and phosphorylation of stathmin on Ser16.

A novel SCG10-related gene uniquely expressed in the nervous system

We have isolated a novel cDNA (HiAT3: hippocampus abundant transcript 3) in the course of screening for genes that are preferentially expressed in neonatal mouse hippocampus using random 3'-directed cDNA sequencing approach. It encodes a 180-aa protein that has high similarity to SCG10, a neuron-specific negative regulator of microtubule dynamics during neurite outgrowth. The expression of HiAT3 is limited to neurons and peaks about 1 week after birth. The identification of HiAT3 suggests that there may be an elaborate destabilizing regulation for microtubule dynamics in neurons in addition to the stabilizing effect of multiple microtubule-associated proteins (MAPs).

Characterization of rat brain stathmin isoforms by two-dimensional gel electrophoresis-matrix assisted laser desorption/ionization and electrospray ionization-ion trap mass spectrometry

Stathmin is a regulatory phosphoprotein that is a target for both cell cycle and cell surface receptor-regulated phosphorylation events. There are at least 14 isoforms of stathmin that migrate on two-dimensional electrophoresis (2-DE): two unphosphorylated, and 12 increasingly phosphorylated proteins. Following extracellular stimuli, stathmin is phosphorylated on four serines (Ser16, Ser25, Ser38, and Ser63) by several kinases, such as mitogen-activated protein (MAP), cdc2 kinase, protein kinase A, and Ca2+/calmodulin-dependent kinase-Gr. While all forms of stathmin are derived from the same protein encoded by a single mRNA, the precise nature of the post-translational modifications has not been clear. In this study we have characterized three rat brain stathmin isoforms, #1, #3 and #4, which electrophorese on 2-DE with apparent molecular weight (Mr)/isoelectric point (pI) values of 15,500/6.2, 15,000/6.1, and 15,000/6.0, respectively. The phosphorylation status of these isoforms was determined using a combination of peptide mapping, matrix-assisted laser desorption/ionization mass spectrometry and electrospray-ionization ion trap mass spectrometry. Stathmin isoform #1 was not phosphorylated, stathmin isoform #3 was phosphorylated on Ser38 only, and stathmin isoform #4 was phosphorylated on Ser38; however, the phosphorylation status of Ser63 could not be determined. In addition, three proteins which electrophorese near stathmin were identified in order to more accurately define the Mr/pI locus of this region of the 2-DE gel map. These include: phosphatidyl ethanolamine binding protein (Mr approximately 18,000/pI 6.0), synuclein forms 2 and 3 (Mr approximately 14,000/pI 5.4), and synuclein form 2 (Mr approximately 15,000/pI 5.0).

A putative tumor suppressor, TSG101, acts as a transcriptional suppressor through its coiled-coil domain

TSG101 is thought as a putative tumor suppressor gene, and mutations of this gene were recently found in 7 of 15 breast cancer patients, though the physiological function remains to be elucidated. In this report, we showed that TSG101 protein acts as a transcriptional suppressor for estrogen receptor (ER) as well as other members of the nuclear hormone receptor super-family, VP16, and on its own. The basal promoter activity was also inhibited by TSG101. The suppression of transcription by TSG101 protein required its coiled-coil domain, which is also shown to be required for the tumor suppressive function. Expressed TSG101 protein did not have any histone acetylase nor deacetylase activity, which certain transcriptional co-factors have. The requirement of the same domain in the TSG101 protein for transcriptional suppression and in the tumor suppression indicates a possibility that transcriptional suppression of TSG101 is related to its tumor suppression.

Cell cycle-dependent subcellular localization of the TSG101 protein and mitotic and nuclear abnormalities associated with TSG101 deficiency

TSG101 is a recently discovered tumor susceptibility gene whose functional inactivation in mouse fibroblasts results in cell transformation and the ability to form metastatic tumors in nude mice. Although restoration of TSG101 activity reverses tumorigenesis, neoplasia is irreversible in some cells, suggesting that permanent genetic alteration can occur during TSG101 inactivation. Here we describe studies that support this notion. We find that localization of TSG101 is cell cycle-dependent, occurring in the nucleus and Golgi complex during interphase, and in mitotic spindles and centrosomes during mitosis; cells made neoplastic by a deficiency in TSG101 expression show a series of mitosis-related abnormalities, including multiple microtubule organizing centers, aberrant mitotic spindles, abnormal distribution of metaphase chromatin, aneuploidy, and nuclear anomalies. Our findings suggest that TSG101 deficiency may lead to genome instability in addition to previously reported reversible neoplastic transformation.

Regulation of microtubule dynamics by extracellular signals: cAMP-dependent protein kinase switches off the activity of oncoprotein 18 in intact cells

Oncoprotein 18 (Op18, also termed p19, 19K, metablastin, stathmin, and prosolin) is a recently identified regulator of microtubule (MT) dynamics. Op18 is a target for both cell cycle and cell surface receptor-coupled kinase systems, and phosphorylation of Op18 on specific combinations of sites has been shown to switch off its MT-destabilizing activity. Here we show that induced expression of the catalytic subunit of cAMP-dependent protein kinase (PKA) results in a dramatic increase in cellular MT polymer content concomitant with phosphorylation and partial degradation of Op18. That PKA may regulate the MT system by downregulation of Op18 activity was evaluated by a genetic system allowing conditional co-expression of PKA and a series of kinase target site-deficient mutants of Op18. The results show that phosphorylation of Op18 on two specific sites, Ser-16 and Ser-63, is necessary and sufficient for PKA to switch off Op18 activity in intact cells. The regulatory importance of dual phosphorylation on Ser-16 and Ser-63 of Op18 was reproduced by in vitro assays. These results suggest a simple model where PKA phosphorylation downregulates the MT-destabilizing activity of Op18, which in turn promotes increased tubulin polymerization. Hence, the present study shows that Op18 has the potential to regulate the MT system in response to external signals such as cAMP-linked agonists.

Activation and signal transduction via mitogen-activated protein (MAP) kinases in T lymphocytes

The various mitogen-activated protein (MAP) kinases have central roles in the signalling pathways of T lymphocytes. Their activation is uniquely dependent on dual phosphorylation of a serine/threonine and a tyrosine residue and is regulated by several levels of kinases in parallel cascades. In addition, both the MAP kinases and their upstream, activating kinases are regulated by several phosphatases. Although each of the MAP kinases have many cytoplasmic substrates, their ability to translocate to the nucleus means that they can transmit signals from the cytoplasm directly to transcription factors, which are sometimes nuclear bound. The MAP kinase cascades are activated in T lymphocytes by a variety of different external stimuli. They play an important role in transducing both the signal from T cell receptor and costimulatory molecules, on the T cell surface, and are able to regulate several of the transcription factors controlling the expression of critical genes, including that for IL-2. This review examines how the activation of several MAP kinases is regulated, their role in signal transduction initiated by a variety of stimuli, and how this may lead to different cellular responses.

Brain-derived neurotrophic factor stimulates phosphorylation of stathmin in cortical neurons

We have identified by two-dimensional polyacrylamide gel electrophoresis a protein known as stathmin which is phosphorylated in a time- and concentration-dependent manner in response to brain-derived neurotrophic factor (BDNF) in primary cultures of cortical neurons. We show that stathmin phosphorylation is preceded by the activation of mitogen-activated protein kinase (MAPK) isoforms p44 and p42. Moreover, the MAPK kinase inhibitor PD 098059, which inhibits MAPK activation, also markedly reduces BDNF-stimulated phosphorylation of stathmin, therefore suggesting that phosphorylation of stathmin is triggered by the activation of MAPK. Phosphorylation of stathmin is specific for BDNF since nerve growth factor does not stimulate MAPK and stathmin phosphorylation in cultured cortical neurons. Taken together, these results identify stathmin as a new target protein of BDNF, possibly involved in the development of cortical neurons.

KIS is a protein kinase with an RNA recognition motif

Protein phosphorylation is involved at multiple steps of RNA processing and in the regulation of protein expression. We present here the first identification of a serine/threonine kinase that possesses an RNP-type RNA recognition motif: KIS. We originally isolated KIS in a two-hybrid screen through its interaction with stathmin, a small phosphoprotein proposed to play a general role in the relay and integration of diverse intracellular signaling pathways. Determination of the primary sequence of KIS shows that it is formed by the juxtaposition of a kinase core with little homology to known kinases and a C-terminal domain that contains a characteristic RNA recognition motif with an intriguing homology to the C-terminal motif of the splicing factor U2AF. KIS produced in bacteria has an autophosphorylating activity and phosphorylates stathmin on serine residues. It also phosphorylates in vitro other classical substrates such as myelin basic protein and synapsin but not histones that inhibit its autophosphorylating activity. Immunofluorescence and biochemical analyses indicate that KIS overexpressed in HEK293 fibroblastic cells is partly targetted to the nucleus. Altogether, these results suggest the implication of KIS in the control of trafficking and/or splicing of RNAs probably through phosphorylation of associated factors.

The TSG101 tumor susceptibility gene is located in chromosome 11 band p15 and is mutated in human breast cancer

Recent work has identified a mouse gene (tsg101) whose inactivation in fibroblasts results in cellular transformation and the ability to produce metastatic tumors in nude mice. Here, we report that the human homolog, TSG101, which we isolated and mapped to chromosome 11, bands 15.1-15.2, a region proposed to contain tumor suppressor gene(s), is mutated at high frequency in human breast cancer. In 7 of 15 uncultured primary human breast carcinomas, intragenic deletions were shown in TSG101 genomic DNA and transcripts by gel and sequence analysis, and mutations affecting two TSG101 alleles were identified in four of these cancers. No TSG101 defects were found in matched normal breast tissue from the breast cancer patients. These findings strongly implicate TSG101 mutations in human breast cancer.

Effects of phorbol ester on phospholipase D and mitogen-activated protein kinase activities in T-lymphocyte cell lines

The effects of phorbol 12-myristate 13-acetate (PMA) on the activities of phospholipase D (PLD3), mitogen-activated protein kinase (ERK), and c-Jun N-terminal kinase (JNK) were studied in Jurkat, a human T cell line, and EL4, a murine T-cell line. PMA treatment rapidly activated PLD in Jurkat, as detected either in intact or broken cells. In contrast, PMA did not stimulate PLD activity in EL4 cells. PLD activity was not detected in membranes prepared from EL4 cells. Jurkat, but not EL4, expresses a 120-kDa protein recognized by an anti-PLD antibody. In both Jurkat and EL4 cells, PMA caused activation of ERKs. Incubation of EL4 cells with bacterial PLD increased phosphatidic acid levels, but did not activate ERK. In both EL4 and Jurkat cells, co-stimulation with PMA and ionomycin stimulated JNK activity. These results show that activation of PLD is not required for activation of ERKs or JNKs by PMA in T-cell lines. Thus, while PLD activity is expressed in some T-cell lines, the role of this enzyme and its products in T-cell activation remain to be elucidated.

Normal development of mice lacking metablastin (P19), a phosphoprotein implicated in cell cycle regulation

Metablastin, also called P19, stathmin, prosolin, Lap18, and oncoprotein18, is a highly conserved cytosolic protein that undergoes extracellular factor- and cell cycle-regulated serine phosphorylation and developmentally regulated expression in mammals. It has been implicated in a variety of cellular functions including growth and differentiation, and recent evidence suggests an involvement in cell cycle control. To explore its potential role in mammalian development, we have disrupted the gene encoding metablastin by gene targeting in mice. The metablastin null mutants have no overt phenotype regarding development, growth rate, behavior, T cell maturation, or fertility and do not exhibit an increased predisposition to tumors. SCG10, a protein closely related in structure to metablastin, shows no compensatory up-regulation in metablastin-/- mice. Although the data suggest that metablastin is not essential for mammalian development, the knockout mice should prove valuable in exploring the role of this protein in cell cycle regulation.

Tsg101: a novel tumor susceptibility gene isolated by controlled homozygous functional knockout of allelic loci in mammalian cells

Using a novel strategy that enables the isolation of previously unknown genes encoding selectable recessive phenotypes, we identified a gene (tsg101) whose homozygous functional disruption produces cell transformation. Antisense RNA from a transactivated promoter introduced randomly into transcribed genes throughout the genome of mouse 3T3 fibroblasts was used to knock out alleles of chromosomal genes adjacent to promoter inserts, generating clones that grew in 0.5% agar and formed metastatic tumors in nude mice. Removal of the transactivator restored normal growth. The protein encoded by tsg101 cDNA encodes a coiled-coil domain that interacts with stathmin, a cytosolic phosphoprotein implicated previously in tumorigenesis. Overexpression of tsg101 antisense transcripts in naive 3T3 cells resulted in cell transformation and increased stathmin-specific mRNA.

Differentiation-stage specific expression of oncoprotein 18 in human and rat prostatic adenocarcinoma

Oncoprotein 18 (Op18) is an intracellular phosphoprotein that has been shown to be overexpression in a number of human malignancies. In the present report we have studied the pattern of Op18 expression on normal, hyperplastic, and malignant prostatic tissue as well as in rat prostatic tumor lines. One of the objectives of the present work was to establish whether the level of Op18 expression can be used as a prognostic marker in human prostatic adenocarcinoma. To that end, sections from normal, hyperplastic, and malignant human prostatic tissue were examined by immunohistochemistry for expression of Op18. In the normal and hyperplastic prostate, Op18 expression was observed in basal glandular epithelial cells, whereas the columnar luminal epithelial cells were not stained by the anti Op18 antibodies. In highly differentiated prostatic cancers occasional epithelial cells were stained, while in poorly differentiated tumors most of the epithelial cells contained Op18 immunoreactivity. The staining pattern was similar in the primary prostatic tumor and in the regional lymph node metastases. Most importantly, a limited survey of prostatic cancer patient samples (n = 40) showed a significant correlation between the fraction of Op18 immunoreactive cells and survival. Studies of a rat prostatic tumor model, showed that only a few cells were stained in the highly differentiated Dunning R3327PAP tumor, while most cells were stained in the anaplastic AT1 rat prostatic tumor. Interestingly, castration of rats resulted in an increased Op18 immunoreactivity, within 14 days, in the highly differentiated rat R3327PAP prostatic tumor. In conclusion, the level of Op18 expression seems to be related to cellular differentiation, histological grade, and survival in prostatic cancers. These findings show that Op18 immunoreactivity may be useful as a prognostic marker in prostatic cancer. In addition it may help in the differentiation between highly differentiated prostatic tumors and non-malignant conditions.

Differential expression of nucleophosmin and stathmin in human T lymphoblastic cell lines, CCRF-CEM and JURKAT analyzed by two-dimensional gel electrophoresis

Two-dimensional gel electrophoresis was used to study the expression of intracellular proteins in adherent cells of human T lymphoblastic cell line, CCRF-CEM. The adherent cells grown in monolayer on a culture plate decreased the amount of proteins of M(r) 37,000 and pI 4.7-4.9, and of 17,000 and pI 5.7. The proteins were identified to be nucleophosmin for the 37,000 protein and stathmin for the 17,000 protein by microsequencing their CNBr fragments. The amount of proteins was increased in CCRF-CEM cells grown in floating mass to a comparable level of JURKAT cells which grew in floating mass throughout the culture. The adherent cells decreased their growth rate as compared with the cells in the floating mass. These results suggest that the adhesion of human T lymphoblastic cells modulates their morphology and proliferation via a concomitant decrease in the amount of nucleophosmin and stathmin.

G2/M transition requires multisite phosphorylation of oncoprotein 18 by two distinct protein kinase systems

Oncoprotein 18 (Op18) is a conserved cytosolic protein that is a target for both cell cycle and cell surface receptor-regulated phosphorylation events. The four residues Ser16, Ser25, Ser38, and Ser63 are all subject to cell cycle-regulated phosphorylation. Ser25 and Ser38 are targets for cyclin dependent kinases (CDKs), while Ser16 and Ser63 are phosphorylated by an unidentified protein kinase. We have recently shown that induced expression of a CDK target site-deficient mutant, Op18-S25A,S38A, blocks human cell lines during G2/M transition. In the present report we show that mitosis is associated with complete phosphorylation of the two Op18 CDK target sites Ser25 and Ser38 and that Ser16 and Ser63 are also phosphorylated to a high stoichiometry. To evaluate the function of multisite phosphorylation of Op18, we expressed and analyzed the cell cycle phenotype of different kinase target site-deficient mutants. The data showed that induced expression of the S16A,S63A, S25A,S38A, and S16A,S25A,S38A,S63A mutants all resulted in an indistinguishable phenotype, i.e. immediate G2/M block and subsequent endoreduplication, a given fraction of G2 versus M-phase blocked cells, and a characteristic nuclear morphology of M-blocked cells. This result was unexpected; however, a likely explanation was provided by analysis of Op18 phosphoisomers, which revealed that mutations of the CDK sites interfere with phosphorylation of Ser16 and Ser63. The simplest interpretation of our results is that phosphorylation of Ser16 and Ser63 is essential during G2/M transition and that the phenotype of the S25A,S38A mutant is mediated by the observed block of Ser16/Ser63 phosphorylation.