GTP influences the binding of vincristine in human tumor cytosols

Vinca alkaloids as a potential cancer therapeutics: recent update and future challenges

Vinca alkaloids including vincristine, vinblastine, vindesine, and vinflunine are chemotherapeutic compounds commonly used to treat various cancers. Vinca alkaloids are one of the first microtubule-targeting agents to be produced and certified for the treatment of hematological and lymphatic neoplasms. Microtubule targeting agents like vincristine and vinblastine work by disrupting microtubule dynamics, causing mitotic arrest and cell death. The key issues facing vinca alkaloids applications include establishing an environment-friendly production technique based on microorganisms, as well as increasing bioavailability without causing harm to patient's health. The low yield of these vinca alkaloids from the plant and the difficulty of meeting their huge colossal demand around the globe prompted researchers to create a variety of approaches. Endophytes could thus be selected to produce beneficial secondary metabolites required for the biosynthesis of vinca alkaloids. This review covers the significant aspects of these vital drugs, from their discovery to the present day, in a concise manner. In addition, we emphasize the major hurdles that must be overcome in the coming years to improve vinca alkaloid's effectiveness.

Chemomodulation of drugs involved in multidrug resistance in chronic lymphatic leukemia of the B-cell type

Reduced drug accumulation may be one reason for intrinsic drug resistance in chronic lymphatic leukemia of the B-cell type (B-CLL). Immunophenotyped leukemic human B-cells from 38 cases of B-CLL were characterized for P-glycoprotein (PGP) content. In all, 30 cases of B-CLL were additionally analyzed for further parameters: accumulation of daunorubicin (DNR, n = 20) and rhodamine 123 (Rh123, n = 30) in both the presence and the absence of verapamil (VRP). Also, 16 cases of B-CLL were analyzed for vincristine (VCR) accumulation with or without VRP. Concerning the relative expression of PGP, these 16 cases of B-CLL were representative for the whole group of 30 cases. Spontaneous accumulation of Rh123 and VCR varied over a wide range: accumulation of Rh123, by a factor of 11.8; accumulation of VCR, by a factor of 9.7; and accumulation of DNR, by a factor of 3.6. VRP modulated the accumulation of RH123 in 16/30 cases (53%), that of VCR in 69% of cases, and that of DNR in 11% of cases. The maximal VRP-mediated increases in accumulation amounted to factors of 1.3 for DNR, 2.3 for Rh123, and 7.8 for VCR. Spontaneous drug accumulation did not correlate with the extent of chemomodulation. The amount of PGP in B-CLL cells (all cases studied were PGP-positive) did not correlate with drug accumulation or with the extent of VRP-mediated chemomodulation. Thus, high expression of PGP is only partially responsible for defective drug accumulation in B-CLL. Only the degree of chemomodulation by VRP is predictive for the extent of the PGP-related functional drug accumulation defect. Thus, in B-CLL, PGP-independent drug accumulation defects seem to be as important as those mediated by PGP. The extent of this drug accumulation defect varies for the different drugs in the following order VCR > Rh123 > DNR. The relevance of PGP-mediated and -independent drug accumulation defects in vivo may depend to a large extent on the drug being used and on the individual cell type. Both types of defect in drug accumulation are of high importance when regimens include VCR a drug commonly used in second-line chemotherapy of B-CLL. Both defects in drug accumulation may be responsible for intrinsic VCR resistance in B-CLL.

Induction of HSP70 is associated with vincristine resistance in heat-shocked 9L rat brain tumour cells

The most prominent cellular changes in heat-shock response are induction of HSPs synthesis and reorganisation of cytoskeleton. Vincristine was used as a tool to evaluate the integrity of microtubules in 9L rat brain tumour cells recovering from heat-shock treatment. Cells treated at 45 degrees C for 15 min and recovered under normal growing condition became resistant to vincristine-inflicted cytotoxicity and microtubule destruction. Among all HSPs, the level of HSP70 and the degree of vincristine resistance are best correlated. HSP70 and tubulin were found to be associated with each other as they were co-immunoprecipitated by either anti-HSP70 or anti-beta-tubulin monoclonal antibody. The current studies establish for the first time that HSP70 can complex with tubulin in cells and this association may stabilise the organisation of microtubules thus protect the heat-treated cells from vincristine damage. These findings are noteworthy in combining hyperthermia and chemotherapy in the management of malignant diseases.

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.

Influence of guanine nucleotides on vincristine binding in tumor cytosols and purified tubulin: evidence for an inhibitor of vincristine binding

In cytosols from human rhabdomyosarcoma xenografts, the formation of vincristine (VCR)-tubulin complex and its stability were increased by GTP (Bowman et al.: Biochem. Biophys. Res. Commun., 135:695-700, 1986). We have further examined this modulation to determine whether a) GTP was protecting the VCR binding site from denaturation, b) the enhancement of complex formation was guanosine specific, and c) whether this influence was a direct interaction between GTP, VCR, and tubulin, or was mediated through another factor. In GTP-depleted cytosols from tumor xenografts HxRH18 and HxRh12, VCR binding activity was stable for at least 2 hours at 37 degrees C, indicating that the enhancement of complex formation and stability was not due to protection of tubulin integrity as measured by VCR binding; 10 nM GTP increased complex formation slightly, with complex formation increasing as GTP concentrations were increased to 5 microM, where maximum effect was observed. GTP and GDP (0.1 mM) both increased complex formation three-fold, while GMP, GMP-PNP, and ITP increased formation 1.5-fold. IMP, CTP, and ATP had no significant effect. Therefore, the modulation of VCR binding was relatively specific for the guanine nucleotides GDP and GTP. Microtubule protein, purified from Rh18 and Rh12 tumors by cycles of polymerization-depolymerization, bound VCR rapidly and binding was not influenced by GTP. This suggested that GTP modulation of VCR binding in cytosols was through a soluble factor lost in tubulin purification. In experiments with cytosol fractionated by molecular weight, there was inhibition of VCR binding activity by fractions with an mw range 20-50 kD. This inhibition was decreased by 25% by the addition of GTP. These data suggest that in tumor cytosols there may be competition between VCR and a natural ligand that is modulated by GTP. Two potential models for VCR binding are proposed.

Altered molecular properties of tubulin in a multidrug-resistant variant of Chinese hamster cells selected for resistance to vinca alkaloids

The basis for the markedly altered intracellular binding of [3H]vincristine in a multidrug-resistant variant (DC-3F/VCRd-5L) of Chinese hamster lung cells (DC-3F) was investigated. Binding of [3H]vincristine by protein in cytosol derived from each cell type exhibited a differing requirement for GTP in MgCl2 containing buffer of low-ionic strength. Binding of [3H]vincristine occurred to cytosolic protein derived from both variant and parental DC-3F cells, but after removal of GTP, binding only occurred to cytosolic protein from parental cells regardless of the presence of added GTP. Although binding by cytosolic protein from parental DC-3F cells did not require GTP, the addition of 0.1 mM GTP increased by two-fold the rate and extent of binding. When cytosol from variant and parental DC-3F cells was incubated with low concentrations of [3H]vincristine in high-ionic strength buffer and analyzed by molecular-sieve HPLC, most of the protein binding [3H]vincristine in parentally derived cytosol eluted as Mr 110,000-115,000 daltons, corresponding to that for dimeric tubulin. The same binding species was not detected in cytosol derived from variant cells. However, these same fractions derived with both parental and variant cytosols contained tubulin as shown by SDS-PAGE and immunoblotting. A smaller peak of [3H]vincristine binding and an amount of tubulin equal to that found in later fractions were found in the void volume during the same HPLC elution runs with cytosol from both variant and parental DC-3F cells. Evidence was also obtained for differences between parental and variant DC-3F cells in beta-tubulin isoforms following isoelectric focusing and immunoblotting. Parental-cell cytosol contains a single isoform of beta-tubulin. However, in variant cell cytosol the same isoform and, in addition, three more basic isoforms were found. These alterations in [3H]vincristine binding and in isoform compositions of beta-tubulin in variant versus parental DC-3F cells may have importance in regard to vincristine resistance in DC-3F cells.

Comparative uptake, retention and action of vincristine, vinblastine and vindesine on murine leukaemic lymphoblasts sensitive and resistant to vincristine

1. The uptake and retention of vincristine (VCR), vinblastine (VBL) and vindesine (VDS) were evaluated comparatively with respect to their cytotoxic action on a murine lymphoblastic leukaemia (L5178Y). 2. The same parameters were measured on a derived subline of cells resistant to VCR (L5178Y/r) in order to determine whether the different degree of resistance to each alkaloid correlates with the amount of drug associated with the cells. 3. VCR was the most active on L5178Y cells (IC50 = 5.8 x 10(-9) M) while the activity of VBL and that of VDS were similar (IC50 4.4 x 10(-8) M and 3.5 x 10(-8) M, respectively). Nevertheless, a considerably larger amount of VBL was taken up by the cells compared to VDS, although there were no significant differences in their cytotoxic action. 4. The VCR resistant cell line also expressed resistance to VDS, whose IC50 was increased by a factor of 11.4, but not to VBL. However, the uptake and retention of the three alkaloids were similarly reduced in L5178Y/r cells regardless of the degree of resistance expressed. 5. Although a decreased drug uptake and/or retention by the cells provides an explanation for the resistance to vinca alkaloids, they do not seem to be the only factors accounting for the resistance shown by the cell line which we have isolated. 6. The results seem to indicate that part of the VBL taken up by the cells is not used to induce the cytotoxic effect, but is diverted to some cellular compartment(s) or rate controlling process(es) which are different from the target that mediates its cytotoxic action.

Formation and stability of vincristine-tubulin complex in kidney cytosols. Role of GTP and GTP hydrolysis

Vincristine-tubulin complex formed in the 100,000 g fraction of mouse kidney dissociated rapidly at 37 degrees in the absence of guanosine-5'-triphosphate (GTP). In the presence of 2 mM GTP, there was a substantial (2.8-fold) increase in complex stability; NaF (100 mM) but not beta-glycerophosphate (1 mM) also reduced the rate of dissociation. Further, complex was stabilized by other ribonucleoside-5'-triphosphates (but not their respective 5'-monophosphates), and a nonhydrolyzable analogue of GTP. Stability of the VCR-tubulin complex formed in cytosol from kidney and separated from unbound VCR and GTP by gel filtration was influenced by the concentration of GTP. These results appear not to be a consequence of denaturation of tubulin during incubation, as VCR binding activity remained constant under experimental conditions both in the presence and after the removal of GTP. Further, the rate of formation of the VCR-tubulin complex in kidney was also influenced by the concentration of GTP and was increased by the addition of NaF. In the absence of added GTP, virtually no complex was isolated. ATP, CTP, or ITP has little effect on complex formation, suggesting that the effect may be GTP specific. These data suggest that the destabilizing activity in cytosols prepared from mouse kidney, and the failure to form a stable VCR-tubulin complex in kidney, are in part the consequence of rapid hydrolysis of GTP by a pyrophosphohydrolase. Direct measurement of the hydrolysis of GTP showed that the activity in kidney (9.26 nmol/min/mg protein) was 9.3-fold greater than in tumor extracts.