Cell surface tubulin in leukemic cells: molecular structure, surface binding, turnover, cell cycle expression, and origin

Wild type, dEX3 and 2B survivin isoforms localize to the tumor cell plasma membrane, are secreted in exosomes, and interact with extracellular tubulin

The Inhibitor of Apoptosis Protein survivin (svn) is upregulated in nearly all types of cancer and represents a promising therapeutic target. Localization to specific subcellular compartments and interactions with various binding partners allow survivin to play diverse roles in apoptosis resistance and mitosis. Survivin has recently been found in two extracellular compartments: the outer plasma membrane and secreted exosomes. In addition to svn-wt, splice variants svn-dEX3 and svn-2B are also overexpressed in human tumors. Here we show that, similarly to svn-wt, svn-dEX3 and svn-2B can be displayed on the outer plasma membrane, and secreted in exosomes. Additionally, we have identified a novel interaction of all three forms of survivin with secreted tubulin.

Identification of Extracellular Segments by Mass Spectrometry Improves Topology Prediction of Transmembrane Proteins

Transmembrane proteins play crucial role in signaling, ion transport, nutrient uptake, as well as in maintaining the dynamic equilibrium between the internal and external environment of cells. Despite their important biological functions and abundance, less than 2% of all determined structures are transmembrane proteins. Given the persisting technical difficulties associated with high resolution structure determination of transmembrane proteins, additional methods, including computational and experimental techniques remain vital in promoting our understanding of their topologies, 3D structures, functions and interactions. Here we report a method for the high-throughput determination of extracellular segments of transmembrane proteins based on the identification of surface labeled and biotin captured peptide fragments by LC/MS/MS. We show that reliable identification of extracellular protein segments increases the accuracy and reliability of existing topology prediction algorithms. Using the experimental topology data as constraints, our improved prediction tool provides accurate and reliable topology models for hundreds of human transmembrane proteins.

An optimized isolation of biotinylated cell surface proteins reveals novel players in cancer metastasis

Details of metastasis, the deadliest aspect of cancer, are unclear. Cell surface proteins play central roles in adhesive contacts between the tumor cell and the stroma during metastasis. We optimized a fast, small-scale isolation of biotinylated cell surface proteins to reveal novel metastasis-associated players from an isogenic pair of human MDA-MB-435 cancer cells with opposite metastatic phenotypes. Isolated proteins were trypsin digested and analyzed using LC-MS/MS followed by quantitation with the Progenesis LC-MS software. Sixteen proteins displayed over twofold expression differences between the metastatic and non-metastatic cells. Interestingly, overexpression of most of them (14/16) in the metastatic cells indicates a gain of novel surface protein profile as compared to the non-metastatic ones. All five validated, differentially expressed proteins showed higher expression in the metastatic cells in culture, and four of these were further validated in vivo. Moreover, we analyzed expression of two of the identified proteins, CD109 and ITGA6 in 3-dimensional cultures of six melanoma cell lines. Both proteins marked the surface of cells derived from melanoma metastasis over cells derived from primary melanoma. The unbiased identification and validation of both known and novel metastasis-associated proteins indicate a reliable approach for the identification of differentially expressed surface proteins.

Plasma membrane tubulin

The association of tubulin with the plasma membrane comprises multiple levels of penetration into the bilayer: from integral membrane protein, to attachment via palmitoylation, to surface binding, and to microtubules attached by linker proteins to proteins in the membrane. Here we discuss the soundness and weaknesses of the chemical and biochemical evidence marshaled to support these associations, as well as the mechanisms by which tubulin or microtubules may regulate functions at the plasma membrane.

Vinblastine, a chemotherapeutic drug, inhibits palmitoylation of tubulin in human leukemic lymphocytes

BACKGROUND

We have previously shown that tubulin, the major protein of microtubules, is posttranslationally modified by palmitoylation. In addition, we demonstrated that palmitoylation of tubulin is inhibited in vitro by stoichiometric levels of the chemotherapeutic drug, vinblastine. Here, we sought to determine whether a clinically relevant dose of vinblastine inhibits palmitoylation of tubulin in vivo.

METHODS

Human CEM leukemic lymphocytes were incubated with [3H]palmitate in the presence and absence of a low, clinically relevant dose of vinblastine. [3H]palmitoylated tubulin was identified by two-dimensional PAGE and autoradiography.

RESULTS

We found, first, that tubulin was palmitoylated in CEM cells. Second, the clinically relevant dose of vinblastine inhibited palmitoylation of tubulin in vivo in CEM cells. In addition, microtubules were disassembled and cells became apoptotic.

CONCLUSION

This study identifies a previously unknown mechanism of action of vinblastine, the depalmitoylation of tubulin, and suggests that depalmitoylation of tubulin may be a target for new chemotherapeutic drugs.

Mitochondrial proteins at unexpected cellular locations: export of proteins from mitochondria from an evolutionary perspective

Researchers in a wide variety of unrelated areas studying functions of different proteins are unexpectedly finding that their proteins of interest are actually mitochondrial proteins, although functions would appear to be extramitochondrial. We review the leading current examples of mitochondrial macromolecules indicated to be also present outside of mitochondria that apparently exit from mitochondria to arrive at their destinations. Mitochondrial chaperones, which have been implicated in growth and development, autoimmune diseases, cell mortality, antigen presentation, apoptosis, and resistance to antimitotic drugs, provide some of the best studied examples pointing to roles for mitochondria and mitochondrial proteins in diverse cellular phenomena. To explain the observations, we propose that specific export mechanisms exist by which certain proteins exit mitochondria, allowing these proteins to have additional functions at specific extramitochondrial sites. Several possible mechanisms by which mitochondrial proteins could be exported are discussed. Gram-negative proteobacteria, from which mitochondria evolved, contain a number of different mechanisms for protein export. It is likely that mitochondria either retained or evolved export mechanisms for certain specific proteins.

Vimentin-crossreactive antibodies induce cell death in primary cultures of embryonic spinal cord

Sera from guinea pigs inoculated with spinal cord gray matter were cytotoxic for embryonic day 14 (ED14) primary spinal cord cell cultures. All cell types including astroglia and neurons were affected. Immunoglobulin (IgG) from inoculated animals and biologically active complement proteins were both identified as necessary components for the observed cytotoxicity. Adsorption with cytoskeletal constituents removed the cytotoxic effect. Adsorption with purified vimentin substantially reduced cytotoxicity, while adsorption with glial fibrillary acid protein (GFAP) had no cytoprotective effects. Antiserum from vimentin-immunized guinea pigs was also toxic to spinal cord cell cultures. Antibody and complement-mediated toxicity appeared to result from interaction with vimentin bound to the surface of embryonic cultured glia and neurons, although interaction with a cross-reacting epitope could not be definitely excluded. Cytotoxic vimentin-directed antibody was not noted in control sera, and the presence of this antibody in guinea pig and human sera did not correlate with the clinicopathological state.

Cell death induced by vincristine in the intestinal crypts of mice and in a human Burkitt's lymphoma cell line

Although vincristine is widely used clinically in the treatment of some human cancers, its mechanism of action has not been clearly established. In this study, the patterns of cell death induced by vincristine in the intestinal crypts of mice and in a human Burkitt's lymphoma cell line were investigated by light and electron microscopy. Vincristine was found to enhance apoptosis of interphase cells in both systems and also to cause the arrest of cells in mitosis, the latter effect being more pronounced in the intestinal crypts. Arrested mitotic cells went on to die by a process that had a number of features in common with apoptosis. These include compaction of chromatin (following coalescence of chromosomes), condensation of the cytoplasm, initial preservation of organelle integrity, and eventually the fragmentation of the cell into a number of membrane-enclosed bodies which are morphologically similar to conventional apoptotic bodies. The results suggest that the cytocidal effect of vincristine is not solely dependent on metaphase arrest but is a cumulative one, resulting both from apoptosis of interphase cells and the 'apoptotic-like' death of cells arrested in metaphase.

Colchicine blocks beta adrenoceptor and class I antigen-specific interactions

Previously we have demonstrated a molecular relationship between H-2 class I antigens and beta adrenoceptors from cardiac tissue. Here we show this type of interaction taking place with beta adrenoceptors from splenic cells and their purified membranes and the participation of cytoskeletal proteins in the phenomenon. Alloimmune, as well as anti-class I but not anti-class II, antibodies were able to inhibit in a competitive manner the binding of (-)-[3H]dihydroalprenolol to splenic lymphocytes and their purified membranes, and to increase cyclic AMP levels in intact cells as a consequence of beta adrenoceptor activation. Furthermore, colchicine (a microtubule disrupting drug), but not cytochalasin B (a microfilament disrupting drug), was able to abrogate alloimmune antibody inhibition over the beta radioligand binding to its receptor on both intact splenocytes and their membranes. Alloantibody actions were significantly diminished by peripheral protein solubilization in purified spleen cell membranes. These data pointed indirectly to the participation of a colchicine binding protein in class I antigen hormone-receptor associations.

A possible role for K+ channels in tumor cell injury. Membrane vesicle shedding and nuclear DNA fragmentation

Mastocytoma P815 tumor cells subjected to low temperature (O degrees C/l h) and shifted to 22 degrees or 37 degrees C undergo morphological, physiological and biochemical changes which are analogous to those induced by immune effector cells, i.e., changes in cell-surface morphology and membrane permeability, elevated O2 consumption rates and nuclear DNA fragmentation [18-21]. Utilizing this low-temperature shift method for the induction of cell injury, we investigated the possible role of K+ channels in this process. Results show that the two classical K+ channel blockers, tetraethylammonium (TEA) and 4-aminopyridine (4-AP), inhibited the low temperature-induced cell-surface membrane vesicle shedding as well as the nuclear DNA-fragmentation process. These results indicate that K+ channel function is required for tumor-cell injury as manifested by nuclear DNA fragmentation and cell-surface membrane vesicle (MV) shedding.

Axonal tubulin and microtubules: morphologic evidence for stable regions on axonal microtubules

Biochemical studies indicate that axonal tubulin is composed of at least two distinct pools that differ in cold solubility and biochemical composition [Brady et al: J. Cell Biol. 99:1716-1724]. To determine the morphologic correlate of cold-insoluble tubulin, segments of rat optic nerves were exposed to a series of in vitro experimental conditions that affect microtubules (MTs), including cold, podophyllotoxin (PT), triflupromazine (TFP), and taxol, and then examined by electron microscopy. Longitudinal sections of control axons showed MTs oriented parallel to the long axis of the axons. Axons exposed to cold, PT, and TFP showed short segments of MTs in association with cytoskeletal disarray. Morphometric studies were used to distinguish between a simple malorientation of MTs (undulation or zigzags in their course) and the loss of labile segments of MTs, leaving the stable portions behind. The lengths of MT segments were measured in longitudinal sections, and the numbers of MTs were determined in the cross sections. All MT segment-length histograms showed a unimodal distribution. Cold and PT produced a simple shift of the control histogram to the shorter length MTs. In cross sections the numbers of MTs in cold- and PT-exposed axons were significantly decreased, indicating that the presence of short segments of MTs in the longitudinal plane of sections was due to a loss of portions of MTs. Taxol, an agent that promotes MT assembly, reversed the cold effect partially and resulted in increases in both MT segment length and number. These studies indicate that stable MT segments are portions of longer MTs containing both stable and labile regions. Furthermore, these findings are consistent with the hypothesis that cold-insoluble tubulin functions as a transportable MT-organizing complex in the axon.

Inhibition of axonal transport 'in vivo' by a tubulin-specific antibody

We have used antibodies against the major proteins of the cytoskeleton-tubulin, the neurofilament triplet proteins and actin-as in vivo probes to determine the contribution of separate components of the cytoskeleton in axonal transport. The injection of either Fast Blue or wheat germ agglutinin conjugated horseradish peroxidase into the caudate nucleus of adult rats resulted in the retrograde transport of these tracers to the neuronal cell bodies in the substantia nigra pars compacta. In experimental animals these tracer injections were immediately preceded by injections of antiserum against tubulin, neurofilament triplet protein or actin, into multiple sites in the caudate. Preimmune serum injection preceded tracer injection as a control in the contralateral caudate of the same animal. One antiserum against electrophoretically purified pig brain tubulin (NS-20) produced a dramatic decrease in the normal retrograde and anterograde transport of both tracers to the SN. Other antisera against tubulin, as well as neurofilament and actin antisera, had no effect on the axonal transport of the tracers. Affinity purified antibodies prepared from the NS-20 antitubulin serum also blocked axonal transport of the tracers. These results provide further support for a critical role of microtubules in axonal transport in vivo. Moreover, an antigenic determinant on tubulin that is uniquely recognized by the NS-20 antibodies may provide us with a way to define the site of association of transfer vesicles with microtubules.

Epstein-Barr virus-related ultrastructural modifications of plasma membrane during B-cell transformation

Ultrastructural modifications are described in the plasma membrane of in vitro established human B cells. By the freeze-fracture technique, intramembrane particles (IMPs) are quantified in B lymphocytes following Epstein-Barr virus (EBV) transformation in vitro, and in B-lymphoma (Burkitt-type) cells, either positive or negative for EBV genome. Analysis shows an overall increase in IMP density as compared to normal controls. Differences are observed between the protoplasmic and exoplasmic faces of fractured membranes as well as among in vitro transformed and clearly neoplastic cells. Results indicate that conformational changes in IMP distribution parallel neoplastic evolution of transformed cells.

Interaction of tubulin with octyl glucoside and deoxycholate. 1. Binding and hydrodynamic studies

Tubulin purified from calf brain cytoplasm, normally a compact water-soluble dimer, is able to interact with the mild detergents octyl glucoside (a minimum of 60 detergent molecules) and deoxycholate (95 +/- 8 molecules). Binding is cooperative and approaches saturation below the critical micelle concentration of the amphiphiles. Binding is accompanied by a quenching of the intrinsic protein fluorescence, but no spectral shape changes indicating denaturation such as in the case of sodium dodecyl sulfate are observed. Glycerol, which is known to be preferentially excluded from the tubulin domain and to favor the folded and associated forms of this protein, inhibits the binding of the mild detergents. Octyl glucoside induces a rapidly equilibrating tubulin self-association reaction characterized by a bimodal sedimentation velocity profile with boundaries at approximately 5 and 12 S. Full dissociation of this detergent restores the normal sedimentation behavior to 90% of the protein. Binding of deoxycholate slows the sedimentation velocity of tubulin from s(0)20,w = 5.6 +/- 0.2 S to s(0)20,w = 4.8 +/- 0.3 S. Measurements of the molecular weight of the tubulin-deoxycholate complex indicate an increase from 100,000 to 143,000 +/- 5,000. The diffusion rate consistently decreases from (5.3 +/- 0.5) X 10(-7) to (3.8 +/- 0.2) X 10(-7) cm2 S-1. This is most simply interpreted as an expansion of the undissociated tubulin dimer upon detergent binding (a change in the frictional ratio, f/f min, from 1.35 to 1.86). It is concluded that tubulin shows a reversible transition between the water-soluble state and amphipathic detergent-bound forms which constitute a model system of tubulin-membrane interactions.