Interaction of Vinblastine with Calf Brain Microtubule protein

Aspartic protease-pepstatin A interactions: Structural insights on the thermal inactivation mechanism

Aspartic proteases are the targets for structure-based drug design for their role in physiological processes and pharmaceutical applications. Structural insights into the thermal inactivation mechanism of an aspartic protease in presence and absence of bound pepstatin A have been obtained by kinetics of thermal inactivation, CD, fluorescence spectroscopy and molecular dynamic simulations. The irreversible thermal inactivation of the aspartic protease comprised of loss of tertiary and secondary structures succeeded by the loss of activity, autolysis and aggregation The enthalpy and entropy of thermal inactivation of the enzyme in presence of pepstatin A increased from 81.2 to 148.5 kcal mol^-1, and from 179 to 359 kcal mol^-1 K^-1 respectively. Pepstatin A shifted the mid-point of thermal inactivation of the protease from 58 °C to 77 °C. The association constant (K) for pepstatin A with aspartic protease was 2.5 ± 0.3 × 10 ⁵ M^-1 and ΔG^o value was -8.3 kcal mol^-1. Molecular dynamic simulation studies were able to delineate the role of pepstatin A in stabilizing backbone conformation and side chain interactions. In the Cα-backbone, the short helical segments and the conserved glycines were part of the most unstable segments of the protein. Understanding the mechanism of thermal inactivation has the potential to develop re-engineered thermostable proteases.

The Biological Activity of the Novel Vinca Alkaloids 4-chlorochablastine and 4-chlorochacristine

BACKGROUND

Vinca alkaloids are important cancer drugs belonging to the class of antimitotic agents. The most commonly used substances are vinblastine and vincristine, other compounds are vinorelbine and vinflunine. All of them are very effective drugs but their use is limited by severe side-effects including neurotoxicity and bone marrow depression. Therefore, it is very important to develop novel vinca alkaloids with similar efficacy but lower toxicity.

METHODS

Here, we analyzed two new compounds, 4-chlorochablastine and 4-chlorochacristine, with regard to their biological activity. These novel compounds were applied to a leukemia cell line at clinically relevant concentrations. For comparison, the established vinca alkaloids vinblastine, vincristine, vinorelbine, and vinflunine were also tested.

RESULTS

Both novel substances decreased cellular proliferation. Apoptosis was found to be increased using two different methods reflecting early and late apoptosis. Cell cycle analysis revealed a clear decrease in G1-cells and an increase in G2/M-cells indicating an arrest in mitosis. In general, 4- chlorochablastine and 4-chlorochacristine caused these effects at concentrations higher than those needed for vinblastine, vincristine, and vinorelbine, but the potency was approximately in the range of vinflunine.

CONCLUSION

Taken together, the results show first indications that these novel vinca alkaloids might be effective and that they warrant further analysis.

Identification of a less toxic vinca alkaloid derivative for use as a chemotherapeutic agent, based on in silico structural insights and metabolic interactions with CYP3A4 and CYP3A5

Vinca alkaloids are chemotherapeutic agents used in the treatment of both pediatric and adult cancer patients. Cytochrome P450 3A5 (CYP3A5) is 9- to 14-fold more efficient at clearing vincristine than cytochrome P450 3A4 (CYP3A4) is. However, patients who express an inactive form of the polymorphic CYP3A5 enzyme suffer from severe neurotoxicity during vincristine treatment, resulting in chemotherapy failure. Previous studies have found that the addition of new features to the parent drug can enhance its binding affinity to tubulin manyfold and could therefore yield novel anticancer drugs. However, there is no report of any study of the metabolic activities of CYP3A4 and CYP3A5 with respect to vincristine and vinblastine, so we studied the interactions of these two drugs and 15 vinca derivatives with CYP3A4 and CYP3A5 by performing docking studies using GOLD. Six of the vinca derivatives in complexes with CYP3A4 and CYP3A5 were further investigated in 100-ns molecular dynamic simulations. Interaction energies, hydrogen bonds, and linear interaction energies were calculated and principal component analysis was carried out to visualize the binding interface in each complex. The results indicate that the addition of dimethylurea at the C20' position in vincristine may increase its binding affinity and lead to enhanced interactions with the less polymorphic CYP3A4 rather than CYP3A5. Thus, dimethylurea vincristine may be a useful drug in cancer chemotherapy treatment as it should be significantly less likely than vincristine to induce severe neurotoxicity in patients. Graphical Abstract Proposed modification of Vinca alkaloid derivatives to decrease the neurotoxicity level in cancer patients exhibiting CYP3A4 gene rather than polymorphic CYP3A5 gene.

Thalidomide (5HPP-33) suppresses microtubule dynamics and depolymerizes the microtubule network by binding at the vinblastine binding site on tubulin

Thalidomides were initially thought to be broad-range drugs specifically for curing insomnia and relieving morning sickness in pregnant women. However, its use was discontinued because of a major drawback of causing teratogenicity. In this study, we found that a thalidomide derivative, 5-hydroxy-2-(2,6-diisopropylphenyl)-1H-isoindole-1,3-dione (5HPP-33), inhibited the proliferation of MCF-7 with a half-maximal inhibitory concentration of 4.5 ± 0.4 μM. 5HPP-33 depolymerized microtubules and inhibited the reassembly of cold-depolymerized microtubules in MCF-7 cells. Using time-lapse imaging, the effect of 5HPP-33 on the dynamics of individual microtubules in live MCF-7 cells was analyzed. 5HPP-33 (5 μM) decreased the rates of growth and shortening excursions by 34 and 33%, respectively, and increased the time microtubules spent in the pause state by 92% as compared to that of the vehicle-treated MCF-7 cells. 5HPP-33 (5 μM) reduced the dynamicity of microtubules by 62% compared to the control. 5HPP-33 treatment reduced the distance between the two poles of a bipolar spindle, induced multipolarity in some of the treated cells, and blocked cells at mitosis. In vitro, 5HPP-33 bound to tubulin with a weak affinity. Vinblastine inhibited the binding of 5HPP-33 to tubulin, and 5HPP-33 inhibited the binding of BODIPY FL-vinblastine to tubulin. Further, a molecular docking analysis suggested that 5HPP-33 shares its binding site on tubulin with vinblastine. The results provided significant insight into the antimitotic mechanism of action of 5HPP-33 and also suggest a possible mechanism for the teratogenicity of thalidomides.

Discovery of small molecule inhibitors that interact with γ-tubulin

Recent studies have shown an overexpression of γ-tubulin in human glioblastomas and glioblastoma cell lines. As the 2-year survival rate for glioblastoma is very poor, potential benefit exists for discovering novel chemotherapeutic agents that can inhibit γ-tubulin, which is known to form a ring complex that acts as a microtubule nucleation center. We present experimental evidence that colchicine and combretastatin A-4 bind to γ-tubulin, which are to our knowledge the first drug-like compounds known to interact with γ-tubulin. Molecular dynamics simulations and docking studies were used to analyze the hypothesized γ-tubulin binding domain of these compounds. The suitability of the potential binding modes was evaluated and suggests the subsequent rational design of novel targeted inhibitors of γ-tubulin.

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.

Stathmin/Op18 is a novel mediator of vinblastine activity

Microtubule (MT) dynamic instability is tightly regulated by stabilizing and destabilizing proteins, the latter being exemplified by stathmin/Op18, a protein known to destabilize MTs. Studies in cells have indicated that the level of stathmin expression modifies the cytotoxicity of antimicrotubule drugs, such as vinblastine (VLB). Using isothermal titration calorimetry and analytical ultracentrifugation, we show that VLB increases the affinity of stathmin for tubulin 50-fold (and vice versa). These results are the first biochemical evidence of the direct relationship between stathmin and an antimitotic drug, and reveal a new mechanism of action for VLB.

Rotenone inhibits mammalian cell proliferation by inhibiting microtubule assembly through tubulin binding

Rotenone, a widely used insecticide, has been shown to inhibit mammalian cell proliferation and to depolymerize cellular microtubules. In the present study, the effects of rotenone on the assembly of microtubules in relation to its ability to inhibit cell proliferation and mitosis were analyzed. We found that rotenone inhibited the proliferation of HeLa and MCF-7 cells with half maximal inhibitory concentrations of 0.2 +/- 0.1 microm and 0.4 +/- 0.1 microm, respectively. At its effective inhibitory concentration range, rotenone depolymerized spindle microtubules of both cell types. However, it had a much stronger effect on the interphase microtubules of MCF-7 cells compared to that of the HeLa cells. Rotenone suppressed the reassembly of microtubules in living HeLa cells, suggesting that it can suppress microtubule growth rates. Furthermore, it reduced the intercentrosomal distance in HeLa cells at its lower effective concentration range and induced multipolar-spindle formation at a relatively higher concentration range. It also increased the level of checkpoint protein BubR1 at the kinetochore region. Rotenone inhibited both the assembly and the GTP hydrolysis rate of microtubules in vitro. It also inhibited the binding of colchicine to tubulin, perturbed the secondary structure of tubulin, and reduced the intrinsic tryptophan fluorescence of tubulin and the extrinsic fluorescence of tubulin-1-anilinonaphthalene-8-sulfonic acid complex, suggesting that it binds to tubulin. A dissociation constant of 3 +/- 0.6 microm was estimated for tubulin-rotenone complex. The data presented suggest that rotenone blocks mitosis and inhibits cell proliferation by perturbing microtubule assembly dynamics.

A spectroscopic study of the interaction of isoflavones with human serum albumin

Genistein and daidzein, the major isoflavones present in soybeans, possess a wide spectrum of physiological and pharmacological functions. The binding of genistein to human serum albumin (HSA) has been investigated by equilibrium dialysis, fluorescence measurements, CD and molecular visualization. One mole of genistein is bound per mole of HSA with a binding constant of 1.5 +/- 0.2 x 10(5) m(-1). Binding of genistein to HSA precludes the attachment of daidzein. The ability of HSA to bind genistein is found to be lost when the tryptophan residue of albumin is modified with N-bromosuccinimide. At 27 degrees C (pH 7.4), van't Hoff's enthalpy, entropy and free energy changes that accompany the binding are found to be -13.16 kcal x mol(-1), -21 cal x mol(-1) K(-1) and -6.86 kcal x mol(-1), respectively. Temperature and ionic strength dependence and competitive binding measurements of genistein with HSA in the presence of fatty acids and 8-anilino-1-naphthalene sulfonic acid have suggested the involvement of both hydrophobic and ionic interactions in the genistein-HSA binding. Binding measurements of genistein with BSA and HSA, and those in the presence of warfarin and 2,3,5-tri-iodobenzoic acid and Förster energy transfer measurements have been used for deducing the binding pocket on HSA. Fluorescence anisotropy measurements of daidzein bound and then displaced with warfarin, 2,3,5-tri-iodobenzoic acid or diazepam confirm the binding of daidzein and genistein to subdomain IIA of HSA. The ability of HSA to form ternery complexes with other neutral molecules such as warfarin, which also binds within the subdomain IIA pocket, increases our understanding of the binding dynamics of exogenous drugs to HSA.

Pseudolarix Acid B, a New Tubulin-Binding Agent, Inhibits Angiogenesis by Interacting with a Novel Binding Site on Tubulin

Tubulin-binding agents have received considerable interest as potential tumor-selective angiogenesis-targeting drugs. Herein, we report that pseudolarix acid B (PAB), isolated from the traditional Chinese medicinal plant Pseudolarix kaempferi Gordon, is a tubulin-binding agent. We further demonstrate that PAB significantly and dose-dependently inhibits proliferation, migration, and tube formation by human microvessel enthothelial cells. It is noteworthy that PAB eliminated newly formed endothelial tubes and microvessels both in vitro and in vivo. In addition, PAB dramatically arrested the cell cycle at G2/M phase. PAB also induced endothelial cell retraction, intercellular gap formation, and promoted actin stress fiber formation in conjunction with disruption of the tubulin and actin cytoskeletons. All of these effects occurred at noncytotoxic concentrations of PAB. We found that these effects of PAB are attributable to depolymerization of tubulin by direct interaction with a distinct binding site on tubulin compared with those of colchicine and vinblastine. Taken together, these findings show that PAB is a candidate antiangiogenic agent for use in cancer therapy, and they provide proof of principle for targeting this novel binding site on tubulin as a new strategy for treating cancer.

BisANS binding to tubulin: isothermal titration calorimetry and the site-specific proteolysis reveal the GTP-induced structural stability of tubulin

Interactions of bisANS and ANS to tubulin in the presence and absence of GTP were investigated, and the binding and thermodynamic parameters were determined using isothermal titration calorimetry. Like bisANS binding to tubulin, we observed a large number of lower affinity ANS binding sites (N1 = 1.3, K1 = 3.7 x 10(5) M(-1), N2 = 10.5, K2 = 7 x 10(4)/M(-1)) in addition to 1-2 higher affinity sites. Although the presence of GTP lowers the bisANS binding to both higher and lower affinity sites (N1 = 4.3, N2 = 11.7 in absence and N1 = 1.8, N2 = 3.6 in presence of GTP), the stoichiometries of both higher and lower affinity sites of ANS remain unaffected in the presence of GTP. BisANS-induced structural changes on tubulin were studied using site-specific proteolysis with trypsin and chymotrypsin. Digestion of both alpha and beta tubulin with trypsin and chymotrypsin, respectively, has been found to be very specific in presence of GTP. GTP has dramatic effects on lowering the extent of nonspecific digestion of beta tubulin with trypsin and stabilizing the intermediate bands produced from both alpha and beta. BisANS-treated tubulin is more susceptible to both trypsin and chymotrypsin digestion. At higher bisANS concentration (>20 microM) both alpha and beta tubulins are almost totally digested with enzymes, indicating bisANS-induced unfolding or destabilization of tubulin structure. Again, the addition of GTP has remarkable effect on lowering the bisANS-induced enhanced digestion of tubulin as well as stabilizing effect on intermediate bands. These results of isothermal titration calorimetry, proteolysis and the DTNB-kinetics data clearly established that the addition of GTP makes tubulin compact and rigid and hence the GTP-induced stabilization of tubulin structure. No such destabilization of tubulin structure has been noticed with ANS, although, like bisANS, ANS possesses a large number of lower affinity binding sites. On the basis of these results, we propose that the unique structure of bisANS, which in absence of GTP can bind tubulin as a bifunctional ligand (through its two ANS moieties), is responsible for the structural changes of tubulin.

Distinctive interactions in the holoenzyme formation for two isoforms of glutamate decarboxylase

The interactions between glutamate decarboxylase (GAD) and its cofactor pyridoxal phosphate (PLP) play a key role in the regulation of GAD activity. The enzyme has two isoforms, GAD65 and GAD67. A comparison of binding constants, rate constants, and kinetic profiles for the formation of holoenzyme (holoGAD65 and holoGAD67) revealed that the two isoforms interact distinctively with the cofactor. GAD67 exhibits a higher binding constant for PLP binding, making it more difficult to dissociate PLP from holoGAD67 than holoGAD65. Meanwhile, PLP binding occurs at a much slower rate for GAD67 than GAD65, as evidenced by lower rate constants and a slower initial rate of the holoenzyme formation. Job's plots revealed a stoichiometry of 1:1 for PLP binding to GAD65 before and after the saturation level of PLP, while 1:2 for PLP binding to GAD67 prior to the saturation of PLP and 1:1 at the saturation level of PLP. These results suggested that the two binding sites of GAD65 exhibit similar affinities for PLP. In contrast, one binding site of GAD67 exhibits a significantly higher affinity for PLP than the other binding site. Based on these findings, it was proposed that a slower PLP binding to GAD67 than GAD65 and a less ease to dissociate PLP from holoGAD67 than holoGAD65 are important underlying factors. This attributes to GAD67 being more highly saturated by PLP and GAD65 being less saturated by PLP. A larger conformation change constant for GAD67 than GAD65 supported a significant conformational change induced by the initial PLP binding to GAD67, which affects the other binding site affinity of GAD67. The present studies provided valuable insights into distinctive properties between the two isoforms of GAD.

Nucleotide-dependent bisANS binding to tubulin

Non-covalent hydrophobic probes such as 5, 5'-bis(8-anilino-1-naphthalenesulfonate) (bisANS) have become increasingly popular to gain information about protein structure and conformation. However, there are limitations as bisANS binds non-specifically at multiple sites of many proteins. Successful use of this probe depends upon the development of binding conditions where only specific dye-protein interaction will occur. In this report, we have shown that the binding of bisANS to tubulin occurs instantaneously, specifically at one high affinity site when 1 mM guanosine 5'-triphosphate (GTP) is included in the reaction medium. Substantial portions of protein secondary structure and colchicine binding activity of tubulin are lost upon bisANS binding in absence of GTP. BisANS binding increases with time and occurs at multiple sites in the absence of GTP. Like GTP, other analogs, guanosine 5'-diphosphate, guanosine 5'-monophosphate and adenosine 5'-triphosphate, also displace bisANS from the lower affinity sites of tubulin. We believe that these multiple binding sites are generated due to the bisANS-induced structural changes on tubulin and the presence of GTP and other nucleotides protect those structural changes.

Inhibition of lipoxygenase 1 by phosphatidylcholine micelles-bound curcumin

Curcumin (diferuloyl methane) from rhizomes of Curcuma longa L. binds to phosphatidylcholine (PC) micelles. The binding of curcumin with PC micelles was followed by fluorescence measurements. Curcumin emits at 490 nm with an excitation wavelength of 451 nm after binding to PC-mixed micelles stabilized with deoxycholate. Curcumin in aqueous solution does not inhibit dioxygenation of fatty acids by Lipoxygenase 1 (LOX1). But, when bound to PC micelles, it inhibits the oxidation of fatty acids. The present study has shown that 8.6 microM of curcumin bound to the PC micelles is required for 50% inhibition of linoleic acid peroxidation. Lineweaver-Burk plot analysis has indicated that curcumin is a competitive inhibitor of LOX1 with Ki of 1.7 microM for linoleic and 4.3 microM for arachidonic acids, respectively. Based on spectroscopic measurements, we conclude that the inhibition of LOX1 activity by curcumin can be due to binding to active center iron and curcumin after binding to the PC micelles acts as an inhibitor of LOX1.

Localization of critical histidyl residues required for vinblastine-induced tubulin polymerization and for microtubule assembly

Vinblastine-induced tubulin polymerization is electrostatically regulated and shows pH dependence with a pI approximately 7.0 suggesting the involvement of histidyl residues. Modification of histidyl residues of tubulin with diethylpyrocarbonate (DEPC) at a mole ratio of 0.74 (DEPC/total His residues) for 3 min at 25 degreesC completely inhibited vinblastine-induced polymerization with little effect on microtubule assembly. Under these conditions DEPC reacts only with histidyl residues. For complete inhibition two histidyl residues have to be modified. Demodification of the carboxyethyl histidyl derivatives by hydroxylamine led to nearly complete recovery of polymerization competence. Labeling with [14C]DEPC localized both of these histidyl residues on beta-tubulin at beta227 and beta264. Similarly, tubulin modification with DEPC for longer times (8 min) resulted in complete inhibition of microtubule assembly, at which time approximately 4 histidyl residues had been modified. This inhibition by DEPC was also reversed by hydroxylamine. The third histidyl residue was found on alpha-tubulin at alpha88. Thus, two charged histidyl residues are obligatorily involved in vinblastine-induced polymerization, whereas a different histidyl residue on a different tubulin monomer is involved in microtubule assembly.

The C terminus of beta-tubulin regulates vinblastine-induced tubulin polymerization

Oligoanions such as sodium triphosphate or GTP prevent and/or reverse vinblastine-induced polymerization of tubulin. We now show that the anions of glutamate-rich extreme C termini of tubulin are similarly involved in the regulation of the vinblastine effect. Cleavage of the C termini by limited proteolysis with subtilisin enhances vinblastine-induced tubulin polymerization and abolishes the anion effect. Only the beta-tubulin C terminus needs to be removed to achieve these changes and the later cleavage of the alpha-tubulin C terminus has little additional effect. In fact, vinblastine concentrations >20 microM block cleavage of the alpha-tubulin C terminus in the polymer, whereas cleavage of the beta-tubulin C terminus proceeds unimpeded over the time used. The vinblastine effect on tubulin polymerization is also highly pH-dependent between pH 6.5 and 7.5; this is less marked, but not absent, after subtilisin treatment. A working model is proposed wherein an anionic domain proximal to the extreme C terminus must interact with a cationic domain to permit vinblastine to promote polymerization. Both exogenous and extreme C-terminal anions compete for the cationic domain with the proximal anionic domain to prevent vinblastine-induced polymerization. We conclude that the electrostatic regulation of tubulin polymerization induced by vinblastine resides primarily in the beta-tubulin C terminus but that additional regulation proximal in the tubulin molecule also plays a role.

Stabilization of microtubule dynamics by estramustine by binding to a novel site in tubulin: a possible mechanistic basis for its antitumor action

The cellular targets for estramustine, an antitumor drug used in the treatment of hormone-refractory prostate cancer, are believed to be the spindle microtubules responsible for chromosome separation at mitosis. Estramustine only weakly inhibits polymerization of purified tubulin into microtubules by binding to tubulin (Kd, approximately 30 microM) at a site distinct from the colchicine or the vinblastine binding sites. However, by video microscopy, we find that estramustine strongly stabilizes growing and shortening dynamics at plus ends of bovine brain microtubules devoid of microtubule-associated proteins at concentrations substantially below those required to inhibit polymerization of the microtubules. Estramustine strongly reduced the rate and extent both of shortening and growing, increased the percentage of time the microtubules spent in an attenuated state, neither growing nor shortening detectably, and reduced the overall dynamicity of the microtubules. Significantly, the combined suppressive effects of vinblastine and estramustine on the rate and extent of shortening and dynamicity were additive. Thus, like the antimitotic mechanisms of action of the antitumor drugs vinblastine and taxol, the antimitotic mechanism of action of estramustine may be due to kinetic stabilization of spindle microtubule dynamics. The results may explain the mechanistic basis for the benefit derived from combined use of estramustine with vinblastine or taxol, two other drugs that target microtubules, in the treatment of hormone-refractory prostate cancer.

Inhibition of mitosis and microtubule function through direct tubulin binding by a novel antiproliferative naphthopyran LY290181

The mechanism of action of a novel antiproliferative compound LY290181 [2-amino-4-(3-pyridyl)-4H-naphtho(1,2-b)pyran-3-carbonitrile] was characterized. LY290181 is a potent inhibitor of cell proliferation, producing 50% inhibition of vascular smooth muscle, endothelial, Chinese hamster ovary, HeLa, and human erythroleukemia cells at concentrations of 8-40 nM. Cell cycle analysis showed that LY290181 caused accumulation of smooth muscle cells at the G2/M phase and induced mitotic arrest in Chinese hamster ovary cells and HeLa cells. At low concentrations (3-30 nM), LY290181 blocked transition of cells from metaphase to anaphase and disrupted mitotic spindle organization. At high concentrations (>/=100 nM), LY290181 produced a concentration-dependent loss of cytoplasmic and spindle microtubules. LY290181 inhibited the polymerization of purified bovine brain microtubule protein into microtubules, and it depolymerized preformed microtubules. Using tubulin-1-anilino-8-naphthalene sulfonate complex fluorescence, we have shown that LY290181 directly interacted with tubulin in a unique manner. These studies show that LY290181 induces cell growth arrest in prometaphase/metaphase, and tubulin appears to be its molecular target.

Interaction of a new bis-indol derivative, KAR-2 with tubulin and its antimitotic activity

1. KAR-2 (3"-(beta-chloroethyl)-2",4"-dioxo-3,5"-spiro-oxazolidino- 4-deacetoxy-vinblastine), is a bis-indol derivative; catharantine is coupled with the vindoline moiety which contains a substituted oxazolidino group. Our binding studies showed that KAR-2 exhibited high affinity for bovine purified brain tubulin (Kd-3 microM) and it inhibited microtubule assembly at a concentration of 10 nM. 2. Anti-microtubular activity of KAR-2 was highly dependent on the ultrastructure of microtubules: while the single tubules were sensitive, the tubules cross-linked by phosphofructokinase (ATP: D-fructose-6-phosphate-1-phosphotransferase, EC 2.7.1.11) exhibited significant resistance against KAR-2. 3. The cytoplasmic microtubules of Chinese hamster ovary mammalian and Sf9 insect cells were damaged by 1 microgram ml-1 KAR-2, as observed by indirect immunofluorescence and transmission electron microscopy. Scanning electron microscopy revealed intensive surface blebbing on both types of cells in the presence of KAR-2. 4. KAR-2 was effective in the mouse leukaemia P338 test in vivo without significant toxicity. Studies on a primary cerebro-cortical culture of rat brain and differentiated PC12 cells indicated that the toxicity of KAR-2 was significantly lower than that of vinblastine. The additional property of KAR-2 that distinguishes it from bis-indol derivatives is the lack of anti-calmodulin activity.

Suppression of microtubule dynamics by LY290181. A potential mechanism for its antiproliferative action

LY290181, 2-amino-4-(3-pyridyl)-4H-naphtho(1,2-b)pyran-3-carbonitrile, is a potent antiproliferative compound that blocks cells in the G2/M phase of the cell cycle by an apparent action on microtubules. In the present work we found that LY290181 bound to tubulin with high affinity (1 mol of LY290181 per mol of tubulin dimer; Ka, 3.8 x 10(5) M-1) and that it did not appear to bind at the colchicine or vinblastine-binding sites. LY290181 strongly stabilized microtubule dynamics as determined by video microscopy. It reduced the rate and extent of growing and shortening, reduced the catastrophe frequency and increased the rescue frequency, and increased the percentage of time the microtubules spent in an attenuated state. At the lowest effective LY290181 concentrations (0.1-0.3 microM), suppression of dynamics occurred with little reduction in polymer mass. However, at higher concentrations, LY290181 strongly reduced the polymer mass with half-maximal inhibition at a concentration of 0.75 microM. We suggest that LY290181 may exert its antiproliferative activity by stabilizing spindle microtubule dynamics by binding to tubulin at a novel site.

Immunohistochemical analysis of beta-tubulin isotypes in human prostate carcinoma and benign prostatic hypertrophy

BACKGROUND

beta-tubulin, the intracellular target of several antimicrotubule agents, is encoded by at least six genes and exists as multiple isotypes with tissue-specific expression. Previous in vitro studies indicated that tubulin isotype composition may affect polymerization properties, dynamics, and sensitivity to drugs.

METHODS

To investigate the isotype composition of beta-tubulin in human prostate, tissues were collected from 26 patients after radical prostatectomy and sections were stained with isotype-specific antibodies.

RESULTS

beta IV tubulin is the predominant isotype in benign prostatic hyperplasia (BPH) and adenocarcinoma, showing significantly stronger immunohistochemical expression than beta II and beta III, particularly in Gleason's grade 3 and 4 cancers. Staining for the beta II isotype was invariably weak and often absent in BPH and normal glands. There was a marked increase in beta II isotype stain from BPH to cancer in 77% of the patients, suggesting that the expression of this isotype is related to malignant status.

CONCLUSIONS

The beta II tubulin isotype is a potential marker for prostate adenocarcinoma. The possibility that tumor beta-tubulin isotype composition may effect the response to antimicrotubule drug therapy in prostate cancer and other tumors merit investigation.

Understanding and controlling the cell cycle with natural products

Small molecule natural products have aided in the discovery and characterization of many proteins critical to the progression and maintenance of the cell cycle. Identification of the direct target of a natural product gives scientists a tool to control a specific aspect of the cell cycle, thus facilitating the study of the cell-cycle machinery.

Energy transfer studies of the distances between the colchicine, ruthenium red, and bisANS binding sites on calf brain tubulin

Fluorescence energy transfer experiments were performed in order to measure the spatial separation between the colchine and Ruthenium Red binding sites, the high-affinity bisANS and Ruthenium Red sites, and the allocolchicine and high-affinity bisANS sites on calf brain tubulin. Energy transfer was observed between both colchicine and allocolchicine and Ruthenium Red, resulting in a distance of 40-45 A between these sites on the tubulin molecule. No detectable energy transfer could be observed when allocolchicine was used as fluorescence donor and bisANS as acceptor or when bisANS was used as donor and Ruthenium Red as acceptor. This indicates that the distance of separation between the allocolchicine and bisANS sites is greater than 50 A, while that between the bisANS and Ruthenium Red sites is greater than 72 A. On the basis of these and previous distance measurements (Ward & Timasheff, 1988), two triangles of binding sites have been defined (colchicine-bisANS-E-site and colchicine-bisANS-Ruthenium Red). Since the dihedral angle between them is not known, a schematic model has been drawn with all the sites located in a single plane.(ABSTRACT TRUNCATED AT 250 WORDS)

Cooperative multiple binding of bisANS and daunomycin to tubulin

The binding of daunomycin and bisANS to tubulin was studied by direct equilibrium techniques. Both ligands generated abnormal Scatchard plots. Their concave-downward nature indicated positive cooperativity. The data conform to tubulin possessing ca. 35 daunomycin binding sites with a binding constant of 570-1430 M-1. The binding of bisANS is characterized by 1 strong binding site (KA = 4.5 x 10(5) M-1) and 40-50 lower affinity sites. Hill plots of both showed low degrees of cooperativity (m = 1.8 for daunomycin and 2.3 for bisANS). A detailed analysis was carried out of the cooperativity of binding of daunomycin to tubulin. Concentration differences spectra and sedimentation velocity analysis of daunomycin showed that this molecule undergoes self-association in the drug concentration range used in the binding study. The low level of polymerization (approximately tetramer), however, indicated that this could not be the source of the observed cooperativity between 35 molecules. Both the shape and concentration dependence of the daunomycin concentration difference spectra were strikingly similar to those generated on the binding of daunomycin to tubulin, which indicates the stacking of daunomycin in both cases. The observed Scatchard plot of the binding was found to be consistent with a process that involves in part ligand-ligand interactions when complexed to tubulin. Examination of the binding of bisANS in the presence of daunomycin revealed a strong increase of bisANS binding to tubulin, which suggests a loosening of tubulin structure with the exposure of new sites as these ligands bind. The mutual interaction between the two ligands in dilute solution was demonstrated by difference spectroscopy.(ABSTRACT TRUNCATED AT 250 WORDS)

Chemically-induced aneuploidy in mammalian oocytes

The ability of certain chemicals to elevate the frequency of aneuploidy above spontaneous levels in mammalian experimental models prompts the concern that a similar situation might exist in humans. Validation of experimental models for aneuploidy studies is in progress since there is much to be learned about the causes and mechanisms of chemically-induced aneuploidy. Several biological variables have been shown to influence the results from aneuploidy assays. In this review, we examine these variables as they relate to female germ cell aneuploid assays. Also, we have found that the aneuploidy results obtained from different cell types, sexes, and experimental models cannot necessarily be expected to agree due to certain anatomic and physiologic differences and the end points measured.

Aging of tubulin at neutral pH: the destabilizing effect of vinca alkaloids

The effect of the vinca alkaloid drugs, vincristine, vinblastine, catharanthine, and vindoline, on the aging process of tubulin has been examined. It was found that addition of vincristine or vinblastine accelerated by a factor of 3-3.5 the transformation of tubulin from the 5.8 S alpha-beta-tubulin dimer to paucidisperse polymers, with an average sedimentation coefficient of 9 S, previously observed in the absence of drugs (V. Prakash and S. N. Timasheff, 1982, J. Mol. Biol. 160, 499-515). This transformation of tubulin from 5.8 S to "9 S" followed pseudo-first-order kinetics whether the starting protein was predominantly dimeric (i.e., at low drug concentration) or self-associated into the reversible linear polymers induced by the vinca alkaloid drugs at high drug concentration (G. C. Na and S. N. Timasheff, 1980, Biochemistry 19, 1355-1365; V. Prakash and S. N. Timasheff, 1985, Biochemistry 24, 5004-5010). Identical kinetics were found in a fluorescence examination of the loss by tubulin of its ability to bind colchicine specifically, indicating that the rate determining step is a protein conformational change that induces a major change in the far uv circular dichroism spectrum of tubulin. The found lack of an effect of dithiothreitol on the aging and aggregation processes is consistent with the irreversible aggregation being due to the intermolecular coalescence of nonpolar patches on the protein. The observations that vincristine binds to aged tubulin and that the aging of tubulin is accompanied by quenching of the tryptophan fluorescence similar to that which occurs on the binding of the vinca drugs has led to the proposal that the vinca alkaloids stabilize the aged conformation of the protein by interacting with nonpolar regions that may be related to the aggregation sites.

Interaction of Prodan with tubulin. A fluorescence spectroscopic study

The compound 6-propionyl-2-(N,N-dimethyl)-aminonaphthalene (Prodan), an efficient fluorescent probe for proteins, is shown to bind to tubulin. Detailed experiments on fluorescence enhancement, anisotropy and energy transfer were carried out to unravel the nature of Prodan-tubulin interaction and the Prodan-binding site on tubulin. It was found that Prodan binds to tubulin at a rigid site, with a stoichiometry of 1:1 and a dissociation constant of 20 microM. Competition experiments using 1,8-[3H]anilinonaphthalenesulfonate ([3H]ANS) indicated that the three aminonaphthalenes, ANS, bis(1,8-anilinonaphthalenesulfonate) and Prodan probably bind to a common region on the tubulin molecule.

Cellular uptake and tubulin binding properties of four Vinca alkaloids

The in vitro effects of four Vinca alkaloids, vinblastine (VLB), vincristine (VCR), vindesine (VDS) and vinepidine (VPD), on B16 melanoma proliferation, binding to bovine brain tubulin and B16 melanoma cell extracts, and uptake by the B16 cells were compared. The relative binding affinities to bovine brain tubulin were VPD greater than VCR congruent to VDS greater than VLB with the Ka for VPD being about 4-fold higher than that for VLB. On the other hand, the relative effects on B16 cell proliferation were exactly the opposite. Differences were found in the degree of concentration of the four alkaloids by the cells: 100-fold for VLB, 50-fold for VCR and VDS, and 20-fold for VPD. At the extracellular concentrations of drugs which inhibit proliferation by 50%, the intracellular concentration would still be far less than the tubulin concentration. Thus, it is likely that all of the Vinca alkaloids would be bound to tubulin and difference in uptake rather than Ka values is the major factor in determining the relative effectiveness of the drugs. L cells showed 50% the sensitivity of B16 melanoma cells toward VLB and 30% the sensitivity toward VPD. The L cells also concentrated these drugs to a lesser extent than did the B16 cells.

Physical and spectroscopic methods for the evaluation of the interactions of antimitotic agents with tubulin

The physico-chemical methods for the study of the binding of ligands to tubulin are examined in-depth, emphasizing the assumptions on which they are based and their limitations. The criteria of specificity and linkage to protein self-association are presented. It is shown that, of the direct equilibrium binding techniques, Hummel-Dreyer gel permeation chromatography and rapid ultracentrifugation are applicable only when binding is not linked to protein self-association. Disc filtration is valid only when the reverse unbinding reaction is very low. Binding linked to protein self-association can be measured by batch gel permeation or by dialysis equilibrium. The indirect techniques, such as fluorescence perturbation or difference absorption spectroscopy are discussed in terms of the assumptions on which they are based. They are shown to be used best only after characterization of the binding by direct techniques. Equilibrium binding parameters can also be deduced from careful kinetic experiments. Comparison of calorimetrically measured enthalpies of binding to van't Hoff enthalpies derived from equilibrium measurements indicates that the method of choice is calorimetric, while comparison with van't Hoff analysis can reveal the existence of reaction steps not detected by equilibrium measurements. Use of other indirect approaches, such as titration of an enzymic activity, can also lead to the detection of additional steps. The criteria are set up for the proper data analysis of ligand binding linked to protein self-association and the selection of the proper mode of linkage. It is shown how the thermodynamic contributions of various moieties of a ligand can be established by a rational structural modification of the ligand and the proper analysis of the binding measurements, in which all non-specific entropic contributions are taken into account. It is demonstrated also how a similar analysis of binding data can lead to conclusions about the reaction pathway from a comparison of equilibrium thermodynamic measurements on judiciously modified ligand molecules.

Effects of antimitotic agents on tubulin-nucleotide interactions

The interaction of antimitotic drugs with guanine nucleotides in the tubulin-microtubule system is reviewed. Antimitotic agent-tubulin interactions can be covalent, entropic, allosteric or coupled to other equilibria (such as divalent cation binding, alternate polymer formation, or the stabilization of native tubulin structure). Antimitotics bind to tubulin at a few common sites and alter the ability of tubulin to form microtubules. Colchicine and podophyllotoxin compete for a common overlapping binding site but only colchicine induces GTPase activity and large conformational changes in the tubulin heterodimer. The vinca alkaloids, vinblastine and vincristine, the macrocyclic ansa macrolides, maytansine and ansamitocin P-3, and the fungal antimitotic, rhizoxin, share and compete for a different binding site near the exchangeable nucleotide binding site. The macrocyclic heptapeptide, phomopsin A, and the depsipeptide, dolastatin 10, bind to a site adjacent to the vinca alkaloid and nucleotide sites. Colchicine, vinca alkaloids, dolastatin 10 and phomopsin A induce alternate polymer formation (sheets for colchicine, spirals for vinblastine and vincristine and rings for dolastatin 10 and phomopsin A). Maytansine, ansamitocin P-3 and rhizoxin inhibit vinblastine-induced spiral formation. Taxol stoichiometrically induces microtubule formation and, in the presence of GTP, assembly-associated GTP hydrolysis. Analogs of guanine nucleotides also alter polymer morphology. Thus, sites on tubulin for drugs and nucleotides communicate allosterically with the interfaces that form longitudinal and lateral contacts within a microtubule. Microtubule associated proteins (MAPs), divalent cations, and buffer components can alter the surface interactions of tubulin and thus modulate the interactions between antimitotic drugs and guanine nucleotides.

Interactions of the catharanthus (Vinca) alkaloids with tubulin and microtubules

The dimeric Vinca alkaloids represent a group of important anti-tumor compounds whose intracellular target is tubulin, the protein monomer of microtubules. In this review data on the binding of these drugs to tubulin and microtubules in vitro are examined. The binding to tubulin is linked to a protein self-association reaction described by Na and Timasheff (1986a) as a ligand-induced plus ligand-mediated isodesmic self-association reaction. The simplest model which fits the binding data is one in which there is one intrinsic site which is linked to the self-association process. Effects of solution variables on the binding and self-association explain the wide variation of reported apparent binding constants for Vinca alkaloids to tubulin. The Vinca drugs also bind to microtubules via a low number of sites at the ends of microtubules with apparent high affinity and which are involved in the inhibition of tubulin dimer addition to the microtubule ends, and to sites along the microtubule wall with apparent low affinity which are involved in the disruption of the microtubules into spiraled protofilaments. This review also compares available binding data for different natural and semi-synthetic Vinca alkaloids.

Specific alterations in the biological activities of C-20'-modified vinblastine congeners

Both the anti-tumor and toxic activities of the vinca alkaloid dimers, vinblastine (VBL) and vincristine (VCR), may reside at the level of their known cellular target, the microtubule system. The contributions made by each of the various actions of these alkaloids on this system are unknown. We have used new, complete synthetic methodologies to create a series of eight C-20' alkyl congeners of VBL and have examined these compounds for their abilities to (1) inhibit microtubule assembly, (2) disassemble preformed microtubules, and (3) induce spiral aggregate formation, using purified brain microtubule protein. By combining turbidimetric and electron microscopic techniques, we discovered that each of the various effects of VBL on the microtubule system in vitro was amenable to alteration by specific modification at this single molecular site. In addition, we report two new aberrations of VBL action--the induction of spirals by a concentration of congener below 1 microM and the formation of "opened" microtubules polymerized in the presence of congener. The relationship between anti-microtubule action in vitro and the cellular activities of growth inhibition and mitotic arrest by the congeners was examined in leukemic and colon cancer cell lines. In general, we found that both cellular perturbations were correlated to the ability of the congeners to inhibit microtubule polymerization rather than to the actions of spiral formation or microtubule disassembly. These results are a breakthrough in the structure/function relationship of the vinca alkaloid dimers and should provide the means to determine the role of specific anti-microtubule activities to the complex biological actions of these natural product drugs.

Interaction of normal and glycated human haemoglobin with erythrocyte membranes from normal and diabetic individuals

The interaction between normal or glycated haemoglobin and the cytoplasmic surface of human erythrocyte ghost membranes from normal and diabetic individuals was studied at low pH and low ionic strength. Haemoglobin binding to the membrane was monitored by quenching of a fluorescent probe, 12-(9-anthroyl) stearic acid, embedded in the membrane. The quenching occurs by energy transfer from the probe to the membrane-bound haemoglobin molecules. It was found that both glycated and non-glycated haemoglobin bind with higher affinity to membranes from diabetics than to control erythrocyte ghosts. The binding of glycated haemoglobin is significantly less than that of normal haemoglobin to red blood cell membranes from both normal and diabetic individuals.

Influence of the guanine nucleotide phosphorylation state and of Mg2+ ions on the interaction of vinzolidine/tubulin 6 S: a fluorescence quenching study

The binding of the new vincaalkaloid vinzolidine to tubulin 6 S was investigated by using fluorescence quenching methods. The value of the apparent equilibrium binding constant was found to depend on the phosphorylation state of the guanine nucleotide bound to the tubulin exchangeable site (E-site), with Ka values of 4.9 X 10(4) and 8.19 X 10(4) M-1 for GTP- and GDP-tubulin, respectively. The effect of Mg2+ ions on this binding was more important on GTP-tubulin than on GDP-tubulin, and might be related to the existence of Mg2+ site(s) independent of the nucleotide.

Effect of solution variables on the binding of vinblastine to tubulin

Vinblastine binding to tublin was measured in different buffers using tubulin prepared by two different methods and three different binding assay methods. In 100 mM 1,4-piperazinediethanesulfonic acid (Pipes) buffer containing 1 mM MgSO4 and 1 mM ethylene glycol bis (beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA), the data appeared to be consistent with one site with a Ka value of 3.4 X 10(6) M-1 and another site with a Ka value of 2.8 X 10(5) M-1. However, in buffers of lower ionic strengths and without Mg2+ the Ka values were lower. The lowest value (2 X 10(4) M-1) was obtained in 10 mM phosphate buffer, in which only one site was evident under the conditions used. Neither the binding assay used nor the method for tubulin preparation affected the Ka value. Using HPLC, aggregation induced by vinblastine was evident in buffers which gave the largest Ka values. Tubulin aggregation in the presence of vinblastine was also confirmed by analytical ultracentrifugation. The results support the proposal of Na and Timasheff [Biochemistry 25, 6214 (1986)] that the apparent Ka value is influenced by the degree of aggregation induced by vinblastine and that the intrinsic binding constant to the dimer is represented by the lowest value, about 2 X 10(4) M-1.

Interaction of tubulin with the macromolecular apolar probe, octyl sepharose

Binding of the microtubule protein, tubulin, to hydrophobic groups immobilized on octyl sepharose has been investigated. The results indicate that tubulin binds to octyl sepharose in a time-, temperature-, and concentration-dependent manner. Binding is multiphasic, with one fast phase and at least two slow phases, and is influenced by the presence of antimitotic drugs. Colchicine, vinblastine and podophyllotoxin enhance the fast binding of tubulin with very little effect on the slow binding. Pre-incubation of tubulin with the apolar probe, bis(1,8-anilinonaphthalenesulfonate) (BisANS) enhances both the rapid and slow phases of binding of tubulin to octyl sepharose. 1,8-Anilinonaphthalenesulfonate, the monomer of BisANS, has no effect. These results are consistent with a model for tubulin decay which involves the appearance of hydrophobic sites with time.