The presence of neuron-associated microtubule proteins in the human U-251 MG cell line. A comparative immunoblot and immunohistochemical study

Novel Functions of the Neurodegenerative-Related Gene Tau in Cancer

The analysis of global and comparative genomics between different diseases allows us to understand the key biological processes that explain the etiology of these pathologies. We have used this type of approach to evaluate the expression of several neurodegeneration-related genes on the development of tumors, particularly brain tumors of glial origin (gliomas), which are an aggressive and incurable type of cancer. We have observed that genes involved in Amyotrophic lateral sclerosis (ALS), as well as in Alzheimer’s and Parkinson’s diseases, correlate with better prognosis of gliomas. Within these genes, high Tau/MAPT expression shows the strongest correlation with several indicators of prolonged survival on glioma patients. Tau protein regulates microtubule stability and dynamics in neurons, although there have been reports of its expression in glial cells and also in gliomas. However, little is known about the regulation of Tau/MAPT transcription in tumors. Moreover, our in silico analysis indicates that this gene is also expressed in a variety of tumors, showing a general correlation with survival, although its function in cancer has not yet been addressed. Another remarkable aspect of Tau is its involvement in resistance to taxanes in various tumors types such as breast, ovarian and gastric carcinomas. This is due to the fact that taxanes have the same tubulin-binding site as Tau. In the present work we review the main knowledge about Tau function and expression in tumors, with a special focus on brain cancer. We will also speculate with the therapeutic implications of these findings.

Tubulin targets in the pathobiology and therapy of glioblastoma multiforme. I. Class III beta-tubulin

Glioblastoma multiforme (GBM) is the most common and deadliest form of primary brain cancer in adults. Despite advances in molecular biology and genetics of gliomas currently there is no effective treatment or promising molecularly targeted experimental therapeutic strategies for these tumors. In previous studies we have shown aberrant overexpression of the class III beta-tubulin isotype (betaIII-tubulin) in GBM and have proposed that this change may reflect perturbations in microtubule dynamics associated with glioma tumorigenesis, tumor progression and malignant transformation into GBM. This minireview focuses on microtubules and tubulin as emerging targets in potential therapy of GBM using a new class of betaIII-tubulin-targeted drugs in the light of recent developments concerning the function and potential role of this isotype in clinically aggressive tumor behavior, cancer stem cells, tumor hypoxia and chemoresistance to tubulin binding agents, principally taxanes.

Class III β-Tubulin and γ-Tubulin are Co-expressed and Form Complexes in Human Glioblastoma Cells

We have previously shown that the neuronal-associated class III beta-tubulin isotype and the centrosome-associated gamma-tubulin are aberrantly expressed in astrocytic gliomas (Cell Motil Cytoskeleton 2003, 55:77-96; J Neuropathol Exp Neurol 2006, 65:455-467). Here we determined the expression, distribution and interaction of betaIII-tubulin and gamma-tubulin in diffuse-type astrocytic gliomas (grades II-IV) (n = 17) and the human glioblastoma cell line T98G. By immunohistochemistry and immunofluorescence microscopy, betaIII-tubulin and gamma-tubulin were co-distributed in anaplastic astrocytomas and glioblastomas and to a lesser extent, in low-grade diffuse astrocytomas (P < 0.05). In T98G glioblastoma cells betaIII-tubulin was associated with microtubules whereas gamma-tubulin exhibited striking diffuse cytoplasmic staining in addition to its expectant centrosome-associated pericentriolar distribution. Treatment with different anti-microtubule drugs revealed that betaIII-tubulin was not associated with insoluble gamma-tubulin aggregates. On the other hand, immunoprecipitation experiments unveiled that both tubulins formed complexes in soluble cytoplasmic pools, where substantial amounts of these proteins were located. We suggest that aberrant expression and interactions of betaIII-tubulin and gamma-tubulin may be linked to malignant changes in glial cells.

Class III beta-tubulin isotype: a key cytoskeletal protein at the crossroads of developmental neurobiology and tumor neuropathology

The expression of the cytoskeletal protein class III beta-tubulin isotype is reviewed in the context of human central nervous system development and neoplasia. Compared to systemic organs and tissues, class III beta-tubulin is abundant in the brain, where it is prominently expressed during fetal and postnatal development. As exemplified in cerebellar neurogenesis, the distribution of class III beta-tubulin is neuron associated, exhibiting different temporospatial gradients in the neuronal progeny of the external granule layer versus the neuroepithelial germinal matrix of the velum medullare. However, transient expression of this protein is also present in the telencephalic subventricular zones comprising putative neuronal and/or glial precursor cells. This temporospatially restricted, potentially non-neuronal expression of class III beta-tubulin may have implications in the accurate identification of presumptive neurons derived from transplanted embryonic stem cells. In the adult central nervous system, the distribution of class III beta-tubulin is almost exclusively neuron specific. Altered patterns of expression are noted in brain tumors. In "embryonal"-type neuronal/neuroblastic tumors of the central nervous system, such as the medulloblastomas, class III beta-tubulin expression is associated with neuronal differentiation and decreased cell proliferation. In contrast, the expression of class III beta-tubulin in gliomas is associated with an ascending grade of histologic malignancy and with correspondingly high proliferative indices. Thus, class III beta-tubulin expression in neuronal or neuroblastic tumors is differentiation dependent, whereas in glial tumors, it is aberrant and/or represents "dedifferentiation" associated with the acquisition of glial progenitor-like phenotype(s). From a diagnostic perspective, the detection of class III beta-tubulin immunostaining in neoplastic cells should not be construed as categorical evidence of divergent neuronal differentiation in tumors, which are otherwise phenotypically glial. Because class III beta-tubulin is present in neoplastic but not in normal differentiated glial cells, the elucidation of molecular mechanisms responsible for the altered expression of this isotype may provide critical insights into the dynamics of the microtubule cytoskeleton in the growth and progression of gliomas.

Class III beta-tubulin in human development and cancer

The differential cellular expression of class III beta-tubulin isotype (betaIII) is reviewed in the context of human embryological development and neoplasia. As compared to somatic organs and tissues, betaIII is abundant in the central and peripheral nervous systems (CNS and PNS) where it is prominently expressed during fetal and postnatal development. As exemplified in cerebellar and sympathoadrenal neurogenesis, the distribution of betaIII is neuron-associated, exhibiting distinct temporospatial gradients according to the regional neuroepithelia of origin. However, transient expression of this protein is also present in the subventricular zones of the CNS comprising putative neuronal- and/or glial precursor cells, as well as in Kulchitsky neuroendocrine cells of the fetal respiratory epithelium. This temporally restricted, potentially non-neuronal expression may have implications in the identification of presumptive neurons derived from embryonic stem cells. In adult tissues, the distribution of betaIII is almost exclusively neuron-specific. Altered patterns of expression are noted in cancer. In "embryonal"- and "adult-type" neuronal tumors of the CNS and PNS, betaIII is associated with neuronal differentiation and decreased cell proliferation. In contrast, the presence of betaIII in gliomas and lung cancer is associated with an ascending histological grade of malignancy. Thus, betaIII expression in neuronal tumors is differentiation-dependent, while in non-neuronal tumors it is aberrant and/or represents "dedifferentiation" associated with the acquisition of progenitor-like phenotypic properties. Increased expression in various epithelial cancer cell lines is associated with chemoresistance to taxanes. Because betaIII is present in subpopulations of neoplastic, but not in normal differentiated glial or somatic epithelial cells, the elucidation of mechanisms responsible for the altered expression of this isotype may provide insights into the role of the microtubule cytoskeleton in tumorigenesis and tumor progression.

Aberrant localization of the neuronal class III beta-tubulin in astrocytomas

BACKGROUND

The class III beta-tubulin isotype (betaIII) is widely regarded as a neuronal marker in development and neoplasia. In previous work, we have shown that the expression of betaIII in neuronal/neuroblastic tumors is differentiation dependent. In contrast, the aberrant localization of this isotype in certain nonneuronal neoplasms, such as epithelial neuroendocrine lung tumors, is associated with anaplastic potential.

OBJECTIVE

To test the generality of this observation, we investigated the immunoreactivity profile of betaIII in astrocytomas.

DESIGN

Sixty archival, surgically excised astrocytomas (8 pilocytic astrocytomas, WHO grade 1; 18 diffuse fibrillary astrocytomas, WHO grade 2; 4 anaplastic astrocytomas, WHO grade 3; and 30 glioblastomas, WHO grade 4), were studied by immunohistochemistry using anti-betaIII monoclonal (TuJ1) and polyclonal antibodies. A monoclonal antibody to Ki-67 nuclear antigen (NC-MM1) was used as a marker for cell proliferation. Antibodies to glial fibrillary acidic protein (GFAP) and BM89 synaptic vesicle antigen/synaptophysin were used as glial and neuronal markers, respectively.

RESULTS

The betaIII immunoreactivity was significantly greater in high-grade astrocytomas (anaplastic astrocytomas and glioblastomas; median labeling index [MLI], 35%; interquartile range [IQR], 20%-47%) as compared with diffuse fibrillary astrocytomas (MLI, 4%; IQR, 0.2%-21%) (P <.0001) and was rarely detectable in pilocytic astrocytomas (MLI, 0%; IQR, 0%-0.5%) (P <.0001 vs high-grade astrocytomas; P <.01 vs diffuse fibrillary astrocytomas). A highly significant, grade-dependent relationship was observed between betaIII and Ki-67 labeling and malignancy, but this association was stronger for Ki-67 than for betaIII (betaIII, P <.006; Ki-67, P <.0001). There was co-localization of betaIII and GFAP in neoplastic astrocytes, but no BM89 synaptic vesicle antigen/synaptophysin staining was detected.

CONCLUSIONS

In the context of astrocytic gliomas, betaIII immunoreactivity is associated with an ascending gradient of malignancy and thus may be a useful ancillary diagnostic marker. However, the significance of betaIII-positive phenotypes in diffuse fibrillary astrocytomas with respect to prognostic and predictive value requires further evaluation. Under certain neoplastic conditions, betaIII expression is not neuron specific, calling for a cautious interpretation of betaIII-positive phenotypes in brain tumors.

Effects of inducible glial fibrillary acidic protein on glioma cell motility and proliferation

We have studied the effect of induced glial fibrillary acidic protein (GFAP) on motility, cell morphology, and proliferation of two originally GFAP-negative human glioma cell lines. Glioma cell lines U-1242 MG and U-251 MG sp subclone 3A were transfected with a vector system that allows for an inducible GFAP expression. This experimental system creates an "on/off" situation in which GFAP expression is suppressed by tetracycline. Inducible expression of GFAP in the absence of tetracycline was confirmed by immunofluorescence staining and Northern and Western blotting. The study showed that forced GFAP expression resulted in an inhibition of cell motility measured as the phagokinetic track area of individual cells seeded sparsely on a surface covered with gold particles. It also resulted in a change in cell morphology, with extended cell processes, and it was associated with a low fraction of cells in S-phase. We conclude that the down-regulation of GFAP expression that is often seen in gliomas in vivo may be an important parameter of tumor progression related mainly to the motile and thereby invasive properties of malignant glioma cells.

Class III beta-tubulin isotype (beta III) in the adrenal medulla: III. Differential expression of neuronal and glial antigens identifies two distinct populations of neuronal and glial-like (sustentacular) cells in the PC12 rat pheochromocytoma cell line maintained in a Gelfoam matrix system

BACKGROUND

The rat PC12 pheochromocytoma cell line provides an established system for the study of neuronal differentiation. To our knowledge, glial differentiation has not been reported in this cell line.

METHODS

We have studied, by immunohistochemistry and immunoblotting, the presence of neuronal cytoskeletal antigens [class III beta-tubulin isotype (beta III), microtubule associated proteins MAP2, MAP1B and tau, and different neurofilament (NF) protein components], and synaptophysin in comparison with the glial fibrillary acidic protein (GFAP) and S-100 protein in the PC12 cell line. In three different experiments, PC12 cells were maintained in a three-dimensional gelatin foam (Gelfoam) matrix system for up to 34 days with and without treatment with 1 mM dibutyryl cyclic (dc)AMP. Immunohistochemistry was performed on explants ranging from 2 to 32 days-in vitro, which were fixed in either Bouin's solution, 70% ethanol, or 10% neutral-buffered formalin and embedded in paraffin. Immunoblotting was performed on Gelfoam explants with a panel of antibodies against all aforementioned neuronal and glial markers. Additional immunoblot experiments using anti-GFAP and anti-beta III monoclonal antibodies in cell suspensions and homogenates from PC12 monolayer cultures were carried out to compare growth conditions in relation to the expression of these proteins.

RESULTS

Beta III and MAP2 were demonstrated by immunohistochemistry and immunoblotting of PC12 explants maintained for up to 32 days in Gelfoam matrices with and without treatment with dcAMP. Intense filamentous and granular beta III staining of PC12 cells was observed in dcAMP-treated cultures concomitant with neuronal morphologic alterations (neuritogenesis and ganglionic phenotype). In untreated cultures, beta III staining was present in less differentiated cells, as well in cells undergoing neuritic development. The neuronal phenotype of PC12 cells was confirmed by staining for MAP2, tau, and NF proteins, as well as for synaptophysin. The presence of beta III, MAP2, MAP1B, tau, and NF proteins was confirmed by immunoblotting. Clusters of GFAP-positive and S-100 protein-positive spindle cells, phenotypically distinct from the chromaffin-like or neuronal cells, were demonstrated in Gelfoam explants at 5-30 days in vitro. In 30-day-old cultures treated with dcAMP, there was strong filamentous GFAP and diffuse S-100 protein staining in an increased number of sustentacular-like PC12 cells. GFAP staining was corroborated by immunoblotting of explants maintained under identical conditions in vitro. In contrast, immunoblots performed on homogenates from PC12 suspension and monolayer cultures were GFAP-negative.

CONCLUSIONS

Neuronal and glial-like, presumed sustentacular, phenotypes were demonstrated in PC12 cells grown in Gelfoam matrices with and without treatment with dcAMP for up to 34 days. To our knowledge, the occurrence of glial differentiation in the PC12 line is a hitherto unreported finding. Adult rat medullary sustentacular cells are known to express S-100 and GFA proteins (Suzuki and Kachi, Kaibogaku Zasshi-Anat 70(2): 130-139, 1995), and the organ culture system employed in our study may well have favored this direction of differentiation.

Neuron-associated class III beta-tubulin, tau, and MAP2 in the D-283 Med cell line and in primary explants of human medulloblastoma

The D283 Med human medulloblastoma cell line and primary explants of five surgically excised medulloblastomas were cultured using a three-dimensional Gelfoam matrix system. The cultures were evaluated immunohistochemically for a series of antigenic determinants associated with neuronal or glial differentiation. Focal immunolocalization of class III beta-tubulin, microtubule-associated protein 2 (MAP2), and to a lesser degree tau, was demonstrated in all cultures. Class III beta-tubulin isotype, MAP2, and tau protein were also detected by immunoblot in Gelfoam matrix cultures, monolayer cultures, and suspension cultures of D283 Med cells. Staining for neurofilament protein epitopes was highly variable, even among different cultures derived from the same original tumour, but time-dependent changes in neurofilament protein, which may have reflected neuronal differentiation, were not consistently shown. Widespread gamma-enolase and focal synaptophysin reactivities were visualized in all cultures, but no S-antigen staining was detected. Leu 7 labelling was variably present in half of the cultures of D283 Med cells, but was more abundant in explants derived from four of the five original tumours. Vimentin was consistently found in D283 Med cultures at all time points. No immunoreactivity for glial fibrillary acidic protein was detected in the D283 Med cell line. Conversely, staining for this protein was demonstrated in scattered astrocytic cells in the surgical specimens of all five medulloblastomas. Concomitant with increased time in culture, three of the primary tumours displayed increased numbers of glial fibrillary acidic protein-positive cells when cultured in the Gelfoam system, but the other two tumours had a minimal astrocytic component.(ABSTRACT TRUNCATED AT 250 WORDS)

Absence of neuron-associated microtubule proteins in the rat C-6 glioma cell line. A comparative immunoblot and immunohistochemical study

Three neuron-associated microtubule proteins, Class III beta-tubulin isotype, MAP-2, and tau, were evaluated in a comparative immunoblot and immunohistochemical study of the rat C-6 glioma cell line maintained for up to 31 days in vitro. Western blots on whole SDS extracts of cells grown: (i) as monolayers on plastic dishes (for 13 and 16 days); (ii) as monolayers on poly-D-lysine coated glass coverslips (for 3, 7, and 11 days); and (iii) as explants on Gelfoam matrices (for 10, 30, and 31 days) were probed with monoclonal antibodies (MoAb) specific for the above-mentioned microtubule proteins. For these and all other markers employed, immunoperoxidase histochemistry was performed only on the matrix cultures. The immunoblot experiments demonstrated that the Class III beta-tubulin isotype, MAP2, and tau were not expressed by the C-6 cell line in any of the culture conditions, nor were they found by immunohistochemistry. In contrast, explants from all culture conditions were positive for glial fibrillary acidic (GFA) protein and for a universal anti-beta-tubulin isotype MoAb by immunoblotting, as well as by immunohistochemistry in Gelfoam matrix cultures maintained in an organ culture system. Both sets of experiments indicate that these markers are not altered under three different conditions of growth over a one-month period in vitro. The expression of GFA protein and the absence of detectable levels of Class III beta-tubulin, MAP2, and tau are in keeping with the astrocytic phenotype of the C-6 cell line.