Presence of the betaII isotype of tubulin in the nuclei of cultured mesangial cells from rat kidney

Nuclear βII-Tubulin and its Possible Utility in Cancer Diagnosis, Prognosis and Treatment

Microtubules are organelles that usually occur only in the cytosol. Walss et al. (1999) discovered the βII isotype of tubulin, complexed with α, in the nuclei of certain cultured cells, in non-microtubule form. When fluorescently labeled tubulins were microinjected into the cells, only αβII appeared in the nucleus, and only after one cycle of nuclear disassembly and reassembly. It appeared as if αβII does not cross the nuclear envelope but is trapped in the nucleus by the re-forming nuclear envelope in whose reassembly βII may be involved. βII is present in the cytoplasm and nuclei of many tumor cells. With some exceptions, normal tissues that expressed βII rarely had βII in their nuclei. It is possible that βII is involved in nuclear reassembly and then disappears from the nucleus. Ruksha et al. (2019) observed that patients whose colon cancer cells in the invasive front showed no βII had a median survival of about 5.5 years, which was more than halved if they had cytosolic βII and further lessened if they had nuclear βII, suggesting that the presence and location of βII in biopsies could be a useful prognostic indicator and also that βII may be involved in cancer progression. Yeh and Ludueña. (2004) observed that many tumors were surrounded by non-cancerous cells exhibiting cytosolic and nuclear βII, suggesting a signaling pathway that causes βII to be synthesized in nearby cells and localized to their nuclei. βII could be useful in cancer diagnosis, since the presence of βII in non-cancerous cells could indicate a nearby tumor. Investigation of this pathway might reveal novel targets for chemotherapy. Another possibility would be to combine αβII with CRISPR-Cas9. This complex would likely enter the nucleus of a cancer cell and, if guided to the appropriate gene, might destroy the cancer cell or make it less aggressive; possible targets will be discussed here. The possibilities raised here about the utility of βII in cancer diagnosis, prognosis, biology and therapy may repay further investigation.

TUBULIN ROLE IN CANCER DEVELOPMENT AND TREATMENT

This review work is done to show a significance of tubulin in cancer development. Within last decades there are a lot of studies have performed in this area. Now it is clear that there are an enormous number of functions in cell performing by microtubules, a structure unit of which is tubulin. Now it used widely as a predictive factor of tumor aggressiveness, but increasingly it becomes a target for studying and treatment elaboration, since it is well-known that to nowadays tubulin-targeted medicines, such as taxanes or vinca-alkaloids, resistance develops rather quickly, so it consists a large problem in oncology. This work reveals basic microtubule functions, violations that it may undergo and consequences of these. Also it is described here the main modern tendencies in creation of remedy which will make it possible breakthrough treatment resistance barrier.

Over-Expression of βII-Tubulin and Especially Its Localization in Cell Nuclei Correlates with Poorer Outcomes in Colorectal Cancer

Tubulin is a heterodimer of α and β subunits, both existing as isotypes differing in amino acid sequence encoded by different genes. Specific isotypes of tubulin have associations with cancer that are not well understood. Previous studies found that βII-tubulin is expressed in a number of transformed cells and that this isotype is found in cell nuclei in non-microtubule form. The association of βII expression and its nuclear localization with cancer progression has not previously been addressed. We here used a monoclonal antibody to βII to examine patients with colorectal cancer and found that patients whose tumors over-express βII have a greatly decreased life expectancy which is even shorter in those patients with nuclear βII. Our results suggest that βII-tubulin may facilitate cancer growth and metastasis and, to accomplish this, may not need to be in microtubule form. Furthermore, βII expression and localization could be a useful prognostic marker. We also found that βII appears in the nuclei of otherwise normal cells adjacent to the tumor. It is possible therefore that cancer cells expressing βII influence nearby cells to do the same and to localize βII in their nuclei by an as yet uncharacterized regulatory pathway.

Nuclear LC3 Associates with Slowly Diffusing Complexes that Survey the Nucleolus

MAP1LC3B (microtubule-associated protein 1 light chain 3, LC3) is a key component of the autophagy pathway, contributing to both cargo selection and autophagosome formation in the cytoplasm. Emerging evidence suggests that nuclear forms of LC3 are also functionally important; however, the mechanisms that facilitate the nuclear targeting and trafficking of LC3 between the nucleus and cytoplasm under steady-state conditions are poorly understood. In this study, we examine how residues known to regulate the interactions between LC3 and other proteins or RNA (F52 L53, R68-R70 and G120) contribute to its nuclear targeting, nucleocytoplasmic transport and association with nucleoli and other nuclear components. We find that residues F52 L53 and R68-70, but not G120, regulate targeting of LC3 to the nucleus, its rates of nucleocytoplasmic transport and the apparent sizes of LC3-associated complexes in the nucleus inferred from fluorescence recovery after photobleaching (FRAP) measurements. We also show that LC3 is enriched in nucleoli and its triple arginine motif is especially important for nucleolar targeting. Finally, we identify a series of candidate nuclear LC3-interacting proteins using mass spectrometry, including MAP1B, tubulin and several 40S ribosomal proteins. These findings suggest LC3 is retained in the nucleus in association with high-molecular weight complexes that continuously scan the nucleolus.

Yessotoxin activates cell death pathways independent of Protein Kinase C in K-562 human leukemic cell line

Protein Kinase C (PKC) is a group of enzymes involved in pro-survival or pro-apoptotic events depending on the cellular model. Moreover, Yessotoxin (YTX) modulates its expression and activates different cell death pathways. In K-562 tumor cell line, YTX induces apoptosis and autophagy after 24 and 48 h of incubation, respectively, and the toxin carries out its action through the phosphodiesterase 4A (PDE4A). Therefore, the levels of two subtypes of PKC, conventional (cPKC) and δ isotype of novel PKC (PKCδ) were studied at these times after YTX incubation. Also their involvement in the cell death activated by the toxin and their relationship with PDE4A was checked. The expression of cPKC and PKCδ in cytosol, plasma membrane and nucleus was studied in normal and PDE4A-silenced cells. Furthermore, cell viability of normal cells, as well as cPKC-, PKCδ- and PDE4A-silenced cells was tested by Lactate Dehydrogenase (LDH) assay. As a result, PKCδ showed a key role in K-562 cell survive, since without this protein, K-562 cell decreased their viability. Furthermore, modulation of PKCs by YTX treatment was observed, however, the changes in the expression of these proteins are independent of cell death activated by the toxin. In addition, the modulation of PKCs detected is PDE4A-dependent, since the silencing of this protein change PKC expression pattern.

Key role of phosphodiesterase 4A (PDE4A) in autophagy triggered by yessotoxin

Understanding the mechanism of action of the yessotoxin (YTX) is crucial since this drug has potential pharmacological effects in allergic processes, tumor proliferation and neurodegenerative diseases. It has been described that YTX activates apoptosis after 24h of treatment, while after 48 h of incubation with the toxin a decrease in cell viability corresponding to cellular differentiation or non-apoptotic cell death was observed. In this paper, these processes were extensively studied by using the erythroleukemia K-562 cell line. On one hand, events of K-562 cell differentiation into erythrocytes after YTX treatment were studied using hemin as positive control of cell differentiation. Cell differentiation was studied through the cyclic nucleotide response element binding (phospho-CREB) and the transferrin receptor (TfR) expression. On the other hand, using rapamycin as positive control, autophagic hallmarks, as non-apoptotic cell death, were studied after toxin exposure. In this case, the mechanistic target of rapamycin (mTOR) and light chain 3B (LC3B) levels were measured to check autophagy activation. The results showed that cell differentiation was not occurring after 48 h of toxin incubation while at this time the autophagy was triggered. Furthermore after 24h of toxin treatment none of these processes were activated. In addition, the role of the type 4A phosphodiesterase (PDE4A), the intracellular target of YTX, was checked. PDE4A-silencing experiments showed different regulation steps of PDE4A in the autophagic processes triggered either by traditional compounds or YTX. In summary, after 48 h YTX treatment PDE4A-dependent autophagy, as non-apoptotic programmed cell death, is activated.

Role of AKAP 149-PKA-PDE4A complex in cell survival and cell differentiation processes

The cellular localization of A-kinase anchoring proteins (AKAPs), protein kinase A (PKAs) and phosphodiesterases (PDEs) is a key step to the spatiotemporal regulation of the second messenger adenosine 3',5'-cyclic monophosphate (cAMP). In this paper the cellular distribution of the mitochondrial AKAP 149-PKA-PDE4A complex and its implications in the cell death induced by YTX treatment, a known PDE modulator, was studied. K-562 cell line was incubated with YTX for 24 or 48 h. Under these conditions AKAP 149, PKA and type-4A PDE (PDE4A) levels were measured in the cytosol, in the plasma membrane and in the nucleus. Apoptotic hallmarks were also measured after the same conditions. In addition, YTX effect on cell viability was checked after AKAP 149 and PDE4A silencing. The results obtained show a decrease in AKAP 149-PKA-PDE4A levels in cytosol after YTX exposure. 24h after the toxin addition, the complex expression increased in the plasma membrane and after 48 h in the nucleus domain. Furthermore Bcl-2 levels were decreased and the expression of caspase 3 together with caspase 8 activity were increased after 24h of toxin incubation but not after 48 h. These results suggest apoptotic cell death at 24h and a non-apoptotic cell death after 48 h. When AKAP 149 and PDE4A were silenced YTX did not induce cellular death. In summary, AKAP 149-PKA-PDE4A complex localization is related with YTX effect in K-562 cell line. When this complex is mainly located in the plasma membrane apoptosis is activated while when the complex is in the nuclear domain non-apoptotic cellular death or cellular differentiation is activated. Therefore AKAP 149-PKA-PDE4A distribution and integrity have a key role in cellular survival.

Influenza virus A/Beijing/501/2009(H1N1) NS1 interacts with β-tubulin and induces disruption of the microtubule network and apoptosis on A549 cells

NS1 of influenza A virus is a key multifunctional protein that plays various roles in regulating viral replication mechanisms, host innate/adaptive immune responses, and cellular signalling pathways. These functions rely on its ability to participate in a multitude of protein-protein and protein-RNA interactions. To gain further insight into the role of NS1, a tandem affinity purification (TAP) method was utilized to find unknown interaction partner of NS1. The protein complexes of NS1 and its interacting partner were purified from A549 cell using TAP-tagged NS1 as bait, and co-purified cellular factors were identified by mass spectrometry (MS). We identified cellular β-tubulin as a novel interaction partner of NS1. The RNA-binding domain of NS1 interacts with β-tubulin through its RNA-binding domain, as judged by a glutathione S-transferase (GST) pull-down assay with the GST-fused functional domains of NS1. Immunofluorescence analysis further revealed that NS1 with β-tubulin co-localized in the nucleus. In addition, the disruption of the microtubule network and apoptosis were also observed on NS1-transfected A549 cells. Our findings suggest that influenza A virus may utilize its NS1 protein to interact with cellular β-tubulin to further disrupt normal cell division and induce apoptosis. Future work will illustrate whether this interaction is uniquely specific to the 2009 pandemic H1N1 virus.

Identification of two novel synaptic γ-secretase associated proteins that affect amyloid β-peptide levels without altering Notch processing

Synaptic degeneration is one of the earliest hallmarks of Alzheimer disease (AD) and results in loss of cognitive function. One of the causative agents for the synaptic degeneration is the amyloid β-peptide (Aβ), which is formed from its precursor protein by two sequential cleavages mediated by β- and γ-secretase. We have earlier shown that γ-secretase activity is enriched in synaptic compartments, suggesting that the synaptotoxic Aβ is produced locally. Proteins that interact with γ-secretase at the synapse and regulate the production of Aβ can therefore be potential therapeutic targets. We used a recently developed affinity purification approach to identify γ-secretase associated proteins (GSAPs) in synaptic membranes and synaptic vesicles prepared from rat brain. Liquid chromatography-tandem mass spectrometry analysis of the affinity purified samples revealed the known γ-secretase components presenilin-1, nicastrin and Aph-1b along with a number of novel potential GSAPs. To investigate the effect of these GSAPs on APP processing, we performed siRNA experiments to knock down the expression of the GSAPs and measured the Aβ levels. Silencing of NADH dehydrogenase [ubiquinone] iron-sulfur protein 7 (NDUFS7) resulted in a decrease in Aβ levels whereas silencing of tubulin polymerization promoting protein (TPPP) resulted in an increase in Aβ levels. Treatment with γ-secretase inhibitors often results in Notch-related side effects and therefore we also studied the effect of the siRNAs on Notch processing. Interestingly, silencing of TPPP or NDUFS7 did not affect cleavage of Notch. We also studied the expression of TPPP and NDUFS7 in control and AD brain and found NDUFS7 to be highly expressed in vulnerable neurons such as pyramidal neurons in the hippocampus, whereas TPPP was found to accumulate in intraneuronal granules and fibrous structures in hippocampus from AD cases. In summary, we here report on two proteins, TPPP and NDUFS7, which interact with γ-secretase and alter the Aβ levels without affecting Notch cleavage.

Nucleocytoplasmic shuttling of cytoskeletal proteins: molecular mechanism and biological significance

Various nuclear functional complexes contain cytoskeletal proteins as regulatory subunits; for example, nuclear actin participates in transcriptional complexes, and actin-related proteins are integral to chromatin remodeling complexes. Nuclear complexes such as these are involved in both basal and adaptive nuclear functions. In addition to nuclear import via classical nuclear transport pathways or passive diffusion, some large cytoskeletal proteins spontaneously migrate into the nucleus in a karyopherin-independent manner. The balance of nucleocytoplasmic distribution of such proteins can be altered by several factors, such as import versus export, or capture and release by complexes. The resulting accumulation or depletion of the nuclear populations thereby enhances or attenuates their nuclear functions. We propose that such molecular dynamics constitute a form of cytoskeleton-modulated regulation of nuclear functions which is mediated by the translocation of cytoskeletal components in and out of the nucleus.

The beta isotypes of tubulin in neuronal differentiation

The differences among the vertebrate beta isotypes of tubulin are highly conserved in evolution, suggesting that they have functional significance. To address this, we have used differentiating neuroblastoma cells as a model system. These cells express the betaI, betaII, and betaIII isotypes. Although there is no difference prior to differentiation, a striking difference is seen after differentiation. Both betaI and betaIII occur in cell bodies and neurites, while betaII occurs mostly in neurites. Knocking down betaI causes a large decrease in cell viability while silencing betaII and betaIII does not. Knocking down betaII causes a large decrease in neurite outgrowth without affecting viability. Knocking down betaIII has little effect on neurite outgrowth and only decreases viability if cells are treated with glutamate and glycine, a combination known to generate free radicals and reactive oxygen species. It appears, therefore, that betaI is required for cell viability, betaII for neurite outgrowth and betaIII for protection against free radicals and reactive oxygen species.

Intracellular signaling pathways regulating net protein balance following diaphragm muscle denervation

Unilateral denervation (DNV) of rat diaphragm muscle increases protein synthesis at 3 days after DNV (DNV-3D) and degradation at DNV-5D, such that net protein breakdown is evident by DNV-5D. On the basis of existing models of protein balance, we examined DNV-induced changes in Akt, AMP-activated protein kinase (AMPK), and ERK&frac12; activation, which can lead to increased protein synthesis via mammalian target of rapamycin (mTOR)/p70S6 kinase (p70S6K), glycogen synthase kinase-3β (GSK3β), or eukaryotic initiation factor 4E (eIF4E), and increased protein degradation via forkhead box protein O (FoxO). Protein phosphorylation was measured using Western analyses through DNV-5D. Akt phosphorylation decreased at 1 h and 6 h after DNV compared with sham despite decreased AMPK phosphorylation. Both Akt and AMPK phosphorylation returned to sham levels by DNV-1D. Phosphorylation of their downstream effector mTOR (Ser2481) did not change at any time point after DNV, and phosphorylated p70S6K and eIF4E-binding protein 1 (4EBP1) increased only by DNV-5D. In contrast, ERK&frac12; phosphorylation and its downstream effector eIF4E increased 1.7-fold at DNV-1D and phosphorylated GSK3β increased 1.5-fold at DNV-3D (P < 0.05 for both comparisons). Thus, following DNV there are differential effects on protein synthetic pathways with preferential activation of GSK3β and eIF4E over p70S6K. FoxO1 nuclear translocation occurred by DNV-1D, consistent with its role in increasing expression of atrogenes necessary for subsequent ubiquitin-proteasome activation evident by DNV-5D. On the basis of our results, increased protein synthesis following DNV is associated with changes in ERK&frac12;-dependent pathways, but protein degradation results from downregulation of Akt and nuclear translocation of FoxO1. No single trigger is responsible for protein balance following DNV. Protein balance in skeletal muscle depends on multiple synthetic/degradation pathways that should be studied in concert.

p150/glued modifies nuclear estrogen receptor function

Estrogen modulates gene expression through interactions with estrogen receptors (ERs) that bind chromosomal target genes. Recent studies have suggested an interaction between the cytoskeletal system and estrogen signaling; these have implicated a role of cytoplasmic microtubules in scaffolding ERalpha and enhancing nongenomic function; in addition, other experiments demonstrate that dynein light chain 1 may chaperone ERalpha to the nucleus, indirectly increasing transcriptional potency. Actin/myosin and dynein light chain 1 are also required for estrogen-mediated chromosomal movement that is required for transcriptional up-regulation of ERalpha targets. We present evidence that the dynactin component, p150/glued, directly influences the potency of nuclear ER function. Increasing the stoichiometric ratio of p150/glued and ERalpha by overexpression enhances estrogen responses. ERalpha enhancement by p150/glued does not appear to be influenced by shifts in subcellular localization because microtubule disruption fails to increase nuclear ERalpha. Rather, we find that modest amounts of p150/glued reside in the nucleus of cells, suggesting that it plays a direct role in nuclear transcription. Notably, p150/glued is recruited to the pS2 promoter in the presence of hormone, and, in MCF-7 cells, knockdown of p150/glued levels reduces estrogen-dependent transcription. Our results suggest that p150/glued modulates estrogen sensitivity in cells through nuclear mechanisms.

Inhibition of hepatic tumor cell proliferation in vitro and tumor growth in vivo by taltobulin, a synthetic analogue of the tripeptide hemiasterlin

AIM: To investigate the inhibitory effects of taltobulin (HTI-286), a synthetic analogue of natural hemiasterlin derived from marine sponges, on hepatic tumor growth in vitro and in vivo.

METHODS: The potential anti-proliferative effects of HTI-286 on different hepatic tumor cell lines in vitro and in vivo were examined.

RESULTS: HTI-286 significantly inhibited proliferation of all three hepatic tumor cell lines (mean IC₅₀ = 2 nmol/L ± 1 nmol/L) in vitro. Interestingly, no decrease in viable primary human hepatocytes (PHH) was detected under HTI-286 exposure. Moreover, intravenous administration of HTI-286 significantly inhibited tumor growth in vivo (rat allograft model).

CONCLUSION: HTI-286 might be considered a potent promising drug in treatment of liver malignancies. HTI-286 is currently undergoing clinical evaluation in cancer patients.

Dynamics and anchoring of heterochromatic loci during development

Positioning a euchromatic gene near heterochromatin can influence its expression. To better understand expression-relevant changes in locus positioning, we monitored in vivo movement of centromeres and a euchromatic locus (with and without a nearby insertion of heterochromatin) in developing Drosophila tissue. In most undifferentiated nuclei, the rate of diffusion and step size of the locus is unaffected by the heterochromatic insertion. Interestingly, although the movement observed here is non directional, the heterochromatic insertion allows the flanking euchromatic region to enter and move within the heterochromatic compartment. This study also finds that a constraint on chromatin movement is imposed which is a factor of distance from the centric heterochromatic compartment. This restraint prevents the heterochromatic locus from moving away from the centric heterochromatin compartment. Therefore, because of the constraint, even distinct and non-random nuclear organizations can be attained from random chromatin movements. We also find a general constraint on chromatin movement is imposed during differentiation, which stabilizes changes in nuclear organization in differentiated nuclei.

Intranuclear accumulation of plant tubulin in response to low temperature

Concurrently with cold-induced disintegration of microtubular structures in the cytoplasm, gradual tubulin accumulation was observed in a progressively growing proportion of interphase nuclei in tobacco BY-2 cells. This intranuclear tubulin disappeared upon rewarming. Simultaneously, new microtubules rapidly emerged from the nuclear periphery and reconstituted new cortical arrays, as was shown by immunofluorescence. A rapid exclusion of tubulin from the nucleus during rewarming was also observed in vivo in cells expressing GFP-tubulin. Nuclei were purified from cells that expressed GFP fused to an endoplasmic-reticulum retention signal (BY-2-mGFP5-ER), and green-fluorescent protein was used as a diagnostic marker to confirm that the nuclear fraction was not contaminated by nuclear-envelope proteins. These purified, GFP-free nuclei contained tubulin when isolated from cold-treated cells, whereas control nuclei were void of tubulin. Furthermore, highly conserved putative nuclear-export sequences were identified in tubulin sequences. These results led us to interpret the accumulation of tubulin in interphasic nuclei, as well as its rapid nuclear export, in the context of ancient intranuclear tubulin function during the cell cycle progression.

Intranuclear microtubules are hallmarks of an unusual form of cell death in cisplatin-treated C6 glioma cells

We describe an unusual form of non-accidental cell death marked by ectopic microtubules in the nucleus of a subpopulation of cisplatin-treated C6 glioma astrocytes in culture. At electron microscopy, the perinuclear condensed chromatin did not completely adhere to the nuclear envelope of these cells being separated by single or loosely bundled 20-nm-thick microtubules located in an electron-lucid slit-like zone; the presence of alpha-tubulin lining the inner membrane of the nuclear envelope was confirmed by immunolabeling at confocal microscopy. Since tufts of microfilaments-like fibers also occurred in their central nuclear areas, these cells are referred to as CIMMs (Cells with Intranuclear Microtubules and Microfilaments). The nuclear reorganization of CIMMs also involved nucleolar segregation and formation of heterogeneous ectopic ribonucleoprotein (RNP)-derived structures, indicating disruption of the RNP-based transcription machinery. The cytoplasmic organelles of CIMMs were structurally intact, and propidium iodide did not accumulate intracellularly under vital conditions while the plasma membrane was often Annexin V-positive. All these findings suggest that CIMMs were lethally damaged and committed to an atypical programmed cell death resembling early apoptosis (this is also supported by the presence of a limited number of TUNEL-positive CIMMs). CIMMs appeared well before the main cisplatin-induced cycling arrest of the cell population (G2/M block at 72 h) and had mostly G1 DNA content: this suggests that they may represent the cohort of cells which passed cisplatin-altered mitoses with intranuclear retention of microtubules from an incompletely disassembled mitotic spindle.

Review: tubulin function, action of antitubulin drugs, and new drug development

Anticancer agents that interfere with microtubulin function are in widespread use in man and have a broad spectrum of activity against both hematological malignancies and solid tumors. The mechanisms of actions of these agents have been better defined during the past decade, indicating that there are distinct binding sites for these agents and that they interfere with microtubulin dynamics (growth and shortening of tubules) at low concentrations and only evoke microtubulin aggregation or dissociation at high concentrations. Tubulin has been recently described in the nucleus of cells and in mitochondria. Downstream events from tubulin binding are believed to be critical events for the generation of apoptosis in the malignant cell. The effects of vinca alkaloids and taxanes are distinct, suggesting that the interference with the tubulin cap by high-affinity binding of effective agents is not the only mechanism of cytotoxic effect, and the low-affinity binding of drug, which distorts microtubulin function, may also be important. The epothilones share some of the binding characteristics of the taxanes and are in clinical trials because of cytoxic activity in taxane resistant cells. Tubulin has additional target sites for anticancer drugs including interference with the binding and function of microtubule associated proteins and interference with motor proteins which are essential for the transport of substances within the cell. Because many of these microtubule associated proteins have an ATP binding site, both computer-aided design and combinatorial chemistry techniques can be used to make agents to interfere with their function analogous to imatinib mesylate (Gleevec). Agents that interfere with the motor protein kinesin are entering clinical trials.

Upregulation of class II beta-tubulin expression in differentiating keratinocytes

The diverse functions of microtubules (MT) in different cells and tissues may be facilitated by compositional changes in tubulin isotypes. We obtained partial cDNA clones of class II beta-tubulin from a library of differentiating normal human epidermal keratinocytes (NHEK) cells, whereas screening via subtractive hybridization for genes involved in calcium-induced keratinocyte differentiation. Analysis of the isotypic composition of beta-tubulin from NHEK cells revealed elevations in class II beta-tubulin concentrations at both protein and message levels during cell differentiation, resulting in increased rates of incorporation of class II beta-tubulin into MT. Immunohistochemistry of normal and pathologic skin tissues showed that class II beta-tubulin occurred in the granular layer of the epidermis and in differentiated areas of carcinomas. Class II beta-tubulin was, however, not observed in the uppermost granular and cornified layers of normal epidermis. Further experiments showed that MT were likely to decay in the final stage of terminal differentiation during formation of the cornified envelope. Our results suggest that there is differential modulation of MT composition and stability during keratinocyte differentiation.

The pre-mRNA-splicing factor SF3a66 functions as a microtubule-binding and -bundling protein

SF3a (splicing factor 3a) complex is an essential component of U2 snRNPs (small nuclear ribonucleoprotein particles), which are involved in pre-mRNA splicing. This complex consists of three subunits: SF3a60, SF3a66 and SF3a120. Here, we report a possible non-canonical function of a well-characterized RNA-splicing factor, SF3a66. Ectopic expression experiments using each SF3a subunit in N1E 115 neuroblastoma cells reveals that SF3a66 alone can induce neurite extension, suggesting that SF3a66 functions in the regulation of cell morphology. A screen for proteins that bind to SF3a66 clarifies that SF3a66 binds to beta-tubulin, and also to microtubules, with high affinity, indicating that SF3a66 is a novel MAP (microtubule-associated protein). Electron microscopy experiments show that SF3a66 can bundle microtubules, and that bundling of microtubules is due to cross-bridging of microtubules by high-molecular-mass complexes of oligomerized SF3a66. These results indicate that SF3a66 is likely to be a novel MAP, and can function as a microtubule-bundling protein independently of RNA splicing.

Nuclear βII-Tubulin Associates with the Activated Notch Receptor to Modulate Notch Signaling

The Notch signal pathway plays important roles in proliferation, apoptosis, and differentiation. Abnormalities in Notch signaling are linked to many human diseases. After ligand binding, Notch signaling is activated through the cleavage of Notch receptors to release and translocate the Notch intracellular domain into the nucleus. The Notch1 receptor intracellular domain (N1IC), the activated form of the Notch1 receptor, can modulate downstream target genes via C promoter-binding factor 1-dependent and -independent pathways. To further dissect the Notch1 signaling pathway, we screened the N1IC-associated proteins using a yeast two-hybrid system and identified nuclear beta(II)-tubulin as a candidate for the N1IC-associated proteins. It was suggested that the presence of beta(II)-tubulin in nuclei might be correlated with the cancerous state of cells. However, the function of beta(II)-tubulin locating in the nucleus still is unknown. Herein, we show that the complex of alpha- and beta(II)-tubulin is associated with N1IC in cancer cells by a coimmunoprecipitation analysis. The ankyrin domain of the Notch1 receptor alone was sufficient to associate with beta(II)-tubulin. Furthermore, alpha- and beta(II)-tubulin were localized in the nucleus and formed a complex with N1IC. Treatment with Taxol increased the amounts of nuclear alpha- and beta(II)-tubulin in K562 and HeLa cells and promoted the C promoter-binding factor 1-dependent transactivation activity of N1IC. We also show that nuclear beta(II)-tubulin was bound on the C promoter-binding factor 1 response elements via the association with N1IC. These results suggest that nuclear beta(II)-tubulin can modulate Notch signaling through interaction with N1IC in cancer cells.

The βIIisotype of tubulin is present in the cell nuclei of a variety of cancers

Tubulin, the subunit protein of microtubules, has generally been thought to be exclusively a cytoplasmic protein in higher eukaryotes. We have previously shown that cultured rat kidney mesangial cells contain the betaII isotype of tubulin in their nuclei in the form of an alphabetaII dimer [Walss et al., 1999: Cell Motil. Cytoskeleton 42:274-284, 1999]. More recently, we examined a variety of cancerous and non-cancerous cell lines and found betaII in the nuclei of all of the former and only a few of the latter (Walss-Bass et al., 2002: Cell Tissue Res. 308:215-223]. In order to determine if betaII-tubulin occurs in the nuclei of actual cancers as well as in cancer cell lines, we used the immunoperoxidase method to look for nuclear betaII in a variety of tumors excised from 201 patients. We found that 75% of these tumors contain betaII in their nuclei. Distribution of nuclear betaII was highly dependent on the type of cancer, with 100% of the colon and prostate cancers, but only 19% of the skin tumors, having nuclear betaII. Nuclear betaII was particularly marked in tumors of epithelial origin, of which 83% showed nuclear betaII, in contrast to 54% in tumors of non-epithelial origin. In many cases, betaII staining occurred very strongly in the nuclei and not in the cytoplasm; in other cases, betaII was present in both. In many cases, particularly metastases, otherwise normal cells adjacent to the tumor also showed nuclear betaII, suggesting that cancer cells may influence nearby cells to synthesize betaII and localize it to their nuclei. Our results have implications for the diagnosis, biology, and chemotherapy of cancer.

Subunits of the chaperonin CCT are associated with Tetrahymena microtubule structures and are involved in cilia biogenesis

The cytosolic chaperonin CCT is a heterooligomeric complex of about 900 kDa that mediates the folding of cytoskeletal proteins. We observed by indirect immunofluorescence that the Tetrahymena TpCCTalpha, TpCCTdelta, TpCCTepsilon, and TpCCTeta-subunits colocalize with tubulin in cilia, basal bodies, oral apparatus, and contractile vacuole pores. TpCCT-subunits localization was affected during reciliation. These findings combined with atomic force microscopy measurements in reciliating cells indicate that these proteins play a role during cilia biogenesis related to microtubule nucleation, tubulin transport, and/or axoneme assembly. The TpCCT-subunits were also found to be associated with cortex and cytoplasmic microtubules suggesting that they can act as microtubule-associated proteins. The TpCCTdelta being the only subunit found associated with the macronuclear envelope indicates that it has functions outside of the 900 kDa complex. Tetrahymena cytoplasm contains granular/globular-structures of TpCCT-subunits in close association with microtubule arrays. Studies of reciliation and with cycloheximide suggest that these structures may be sites of translation and folding. Combined biochemical techniques revealed that reciliation affects the oligomeric state of TpCCT-subunits being tubulin preferentially associated with smaller CCT oligomeric species in early stages of reciliation. Collectively, these findings indicate that the oligomeric state of CCT-subunits reflects the translation capacity of the cell and microtubules integrity.

Multiple effects of paclitaxel are modulated by a high c-myc amplification level

Paclitaxel affects microtubule stability by binding to beta-tubulin, thus leading to cell accumulation in the G(2)/M phase, polyploidization, and apoptosis. Because both cell proliferation and apoptosis could be somehow regulated by the protooncogene c-myc, in this work we have investigated whether the c-myc amplification level could modulate the multiple effects of paclitaxel. To this aim, paclitaxel was administered to SW613-12A1 and -B3 human colon carcinoma cell lines (which are characterized by a high and low c-myc endogenous amplification level, respectively), and to the B3mycC5 cell line, with an enforced exogenous expression of c-myc copies. In this experimental system, we previously demonstrated that a high endogenous/exogenous level of amplification of c-myc enhances serum deprivation- and DNA damage-induced apoptosis. Accordingly, the present results indicate that a high c-myc amplification level potentiates paclitaxel cytotoxicity, confers a multinucleated phenotype, and promotes apoptosis to a great extent, thus suggesting that c-myc expression level is relevant in modulating the cellular responses to paclitaxel. We have recently shown in HeLa cells that the phosphorylated form of c-Myc accumulates in the nucleus, as distinct nucleolar and extranucleolar spots; here, we demonstrated that, after the treatment with paclitaxel, phosphorylated c-Myc undergoes redistribution, becoming diffused in the nucleoplasm.

Effect of the antitumor drug vinblastine on nuclear betaII-tubulin in cultured rat kidney mesangial cells

Tubulin, the main component of microtubules, is a major target for antitumor drugs such as vinblastine. We have recently discovered that the betaII isotype of tubulin is present in the nuclei of cultured rat kidney mesangial cells, smooth-muscle-like cells present in the renal glomerular mesangium (Walss C, Kreisberg JI, Ludueña RF: Cell Motil Cytoskeleton 42: 274-284, 1999). Here, we have investigated the effect of vinblastine on nuclear betaII-tubulin in these cells. We have found that, at concentrations of 15 nM and higher, vinblastine caused a reversible loss of betaII-tubulin from the nucleus. Our results raise the possibility that nuclear betaII-tubulin constitutes a population of tubulin that could be a novel target for antitumor drugs such as vinblastine.

Tubulin is an inherent component of mitochondrial membranes that interacts with the voltage-dependent anion channel

We have previously reported that anti-tubulin agents induce the release of cytochrome c from isolated mitochondria. In this study, we show that tubulin is present in mitochondria isolated from different human cancerous and non-cancerous cell lines. The absence of polymerized microtubules and cytosolic proteins was checked to ensure that this tubulin is an inherent component of the mitochondria. In addition, a salt wash did not release the tubulin from the mitochondria. By using electron microscopy, we then showed that tubulin is localized in the mitochondrial membranes. As compared with cellular tubulin, mitochondrial tubulin is enriched in acetylated and tyrosinated alpha-tubulin and is also enriched in the class III beta-tubulin isotype but contains very little of the class IV beta-tubulin isotype. The mitochondrial tubulin is likely to be organized in alpha/beta dimers and represents 2.2 +/- 0.5% of total cellular tubulin. Lastly, we showed by immunoprecipitation experiments that the mitochondrial tubulin is specifically associated with the voltage-dependent anion channel, the main component of the permeability transition pore. Thus, tubulin is an inherent component of mitochondrial membranes, and it could play a role in apoptosis via interaction with the permeability transition pore.

Characterization of nuclear betaII-tubulin in tumor cells: a possible novel target for taxol

As the subunits of microtubules, alpha- and beta-tubulins have been thought to only exist in the cytoplasm where they are incorporated into microtubules. However, the beta(II) isotype of tubulin has recently been observed in the nuclei of rat kidney mesangial cells [Walss et al., 1999: Cell Motil. Cytoskeleton 42:274-284]. In this study, we detected nuclear beta(II)-tubulin in rat C6 glioma cells, human T98G glioma cells, human MCF-7 breast carcinoma cells, human MDA-MB-435 breast carcinoma cells, and human Hela cervix carcinoma cells. In addition, nuclear beta(II)-tubulin in these cells was found to exist as alphabeta(II) dimers instead of assembled microtubules and appeared to be particularly concentrated in the nucleoli. Several anti-tubulin drugs were used to treat C6 cells to determine their influence on nuclear beta(II)-tubulin. Taxol, a tubulin drug with higher specificity for beta(II)-tubulin than for other beta-tubulin isotypes, irreversibly decreased nuclear beta(II) content in a concentration-dependent manner in C6 cells. Meanwhile, cells were found to be apoptotic as was suggested by the presence of multiple micronuclei and DNA fragmentation. On the other hand, no depletion of nuclear beta(II)-tubulin was observed when C6 cells were incubated with colchicine or nocodazole, two anti-tubulin drugs with higher specificity for the alphabeta(IV) isotype, supporting the hypothesis that drugs with higher specificity for beta(II)-tubulin deplete nuclear beta(II)-tubulin.

Subcellular distribution of epothilones in human tumor cells

Epothilones are a new class of natural and potent antineoplastic agents that stabilize microtubules. Although 12,13-epoxide derivatives are potent antiproliferative agents, the activities of the corresponding 12,13-olefin analogs are significantly decreased. These data were confirmed for two new analogs, 6-propyl-EpoB (pEB) and 6-propyl-EpoD (pED), in comparison with the natural compounds EpoB/EpoD, by using human A431, MCF7, and MDR1-overexpressing NCI/Adr cells. By using tritiated pEB/pED, compound uptake, release, and nuclear accumulation were investigated in A431 and NCI/Adr cells. In these cells, epothilones can principally be recognized and exported by Verapamil-sensitive efflux pumps, which are not identical to MDR1. The degree of export depends on the structure, olefin vs. epoxide-analog, and also on the intracellular drug concentration. The accumulation of pED used at 3.5 or 70 nM, respectively, was increased in the presence of 10 microM Verapamil in both cell lines 2- to 8-fold. In contrast, the intracellular levels of pEB were affected by Verapamil only at 3.5 nM pEB in NCI/Adr (2-fold) and not in A431 cells. In addition, strong nuclear accumulation was observed for pEB (40-50%) but not paclitaxel or pED (5-15%) in both cell lines. Our study suggests that differences in growth inhibitory efficacy between epoxide and olefin analogs may be based on different mechanisms of drug accumulation and subcellular distribution.

Interaction of the betaIV-tubulin isotype with actin stress fibers in cultured rat kidney mesangial cells

Microtubules and actin filaments are two of the major components of the cytoskeleton. There is accumulating evidence for interaction between the two networks. Both the alpha- and beta-subunits of tubulin exist as numerous isotypes, some of which have been highly conserved in evolution. In an effort to better understand the functional significance of tubulin isotypes, we used a double immunofluorescence labeling technique to investigate the interactions between the tubulin beta-isotypes and the actin stress fiber network in cultured rat kidney mesangial cells, smooth-muscle-like cells from the renal glomerulus. Removal of the soluble cytoplasmic and nucleoplasmic proteins by detergent extraction caused the microtubule network to disappear while the stress fiber network was still present. In these extracted cells, the betaI- and betaII-tubulin isotypes were no longer present in the cytoplasm while the betaIV-isotype co-localized with actin stress fibers. Co-localization between betaIV-tubulin and actin stress fibers was also observed when the microtubule network was disrupted by the anti-tubulin drug colchicine and also by microinjection of the betaIV-tubulin antibody. Our results suggest that the betaIV isotype of tubulin may be involved in interactions between microtubules and actin.

Mechanism of localization of betaII-tubulin in the nuclei of cultured rat kidney mesangial cells

Tubulin is an alphabeta heterodimer. Both the alpha and beta polypeptides exist as multiple isotypes. Although tubulin was generally thought to exist only in the cytoplasm, we have previously reported the presence of the betaII isotype of tubulin in the nuclei of cultured rat kidney mesangial cells, smooth-muscle-like cells that reside in the glomerular mesangium; nuclear betaII exists as an alphabetaII dimer, capable of binding to colchicine, but in non-microtubule form [Walss et al., 1999: Cell Motil. Cytoskeleton 42:274-284]. We have now investigated the nature of the process by which alphabetaII enters the nuclei of these cells. By micro-injecting fluorescently labeled alphabetaII into mesangial cells, we found that alphabetaII was present in the nuclei of cells only if they were allowed to go through mitosis. In contrast, there were no circumstances in which microinjected fluorescently labeled abetaII or alphabetaIV dimers entered the nuclei. These findings, together with the absence of any nuclear localization signal in alphabetaII, strongly favor the model that alphabetaII, rather than being transported into the intact nucleus, co-assembles with the nucleus at the end of mitosis. Our results also indicate that the nuclear localization mechanism is specific for alphabetaII. This result raises the possibility that alphabetaII may have a specific function that requires its presence in the nuclei of cultured rat kidney mesangial cells.

Tissue-specific expression and subcellular localisation of mammalian delta-tubulin

The properties of the microtubule network are regulated at various levels including tissue-dependent isotype switching, post-translational modification of alpha- and beta-tubulin, and by a variety of microtubule-associated molecules (for reviews, see [1-3]). Microtubule nucleation is attributed to gamma-tubulin, which is present in protein complexes at the centrosome and in the cytoplasm [4,5]. A screen for flagellar mutants in the green alga Chlamydomonas reinhardtii has led to the identification of a fourth member of the tubulin gene superfamily, delta-tubulin. In this unicellular organism, the lack of a functional delta-tubulin gene copy causes aberrant numbers of flagella, depending on the age of the corresponding basal bodies; mutants also show abnormal ultrastructure of the basal bodies and a misplacement of the cleavage furrow at mitosis [6]. Here, we report the isolation of the mouse delta-tubulin homologue and show that the gene is highly expressed in testis. In the elongating spermatid, delta-tubulin associated with the manchette, a specialised microtubule system present during reshaping of the sperm head. The protein specifically localised at the perinuclear ring of the manchette, at the centriolar vaults and along the principal piece of the sperm flagellum. In somatic cell lines, unlike most other tubulins, mammalian delta-tubulin was both cytoplasmic and nuclear and did not colocalise with microtubules. The protein was enriched at the spindle poles during mitosis and we found that gamma-tubulin coimmunoprecipitated with delta-tubulin. Together, the data indicate a specialised role for mammalian delta-tubulin that is distinct from other known tubulins.

C-terminal domain of the mitotic apparatus protein p62 targets the protein to the nucleolus during interphase

Mitotic apparatuses from sea urchin embryos contain a protein (p62), previously shown to be required for mitotic progression. This protein localizes to the mitotic apparatus during cell division in urchin embryos and mammalian tissue culture cells. We show here by immunofluorescence that p62 is localized to the nucleus of mammalian cells during interphase and is highly concentrated in nucleoli. In addition, a fusion protein composed of full-length p62 and green fluorescent protein also localizes to nucleoli when expressed in COS-7 cells in culture. Analysis of the primary sequence of p62 reveals three distinct domains of the protein based on amino acid charge distribution: the acidic N-terminal domain, the basic C-terminal domain, and the central, M-domain, which contains alternating subdomains of clusters of acidic and basic residues. To identify the domain important for nucleolar localization during interphase, specific domains of p62 alone, or in combination with each other or with beta-galactosidase were fused to green fluorescent protein. Following confirmation of the fusion constructs by sequence analysis, the constructs were expressed in mammalian cells, expression was confirmed by immunoblotting, and the fusion proteins were localized via fluorescence microscopy. The data demonstrate that the C-terminal domain of p62 is both necessary and sufficient for the nuclear localization and nucleolar binding of p62 that is observed during interphase.