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Staurosporine, a protein kinase inhibitor, induces neurite outgrowth in pheochromocytoma cells and, therefore, may serve as a potential prototype for neurotropic drugs. The principal aim of the present study was to characterize the cytoskeletal properties of neurites induced in pheochromocytoma cells by staurosporine, in comparison to those induced by nerve growth factor, with emphasis on tubulin and tau proteins. Two major findings are described: a) staurosporine rapidly induces outgrowth of neurites that are resistant to colchicine treatment; and b) staurosporine treatment causes a rapid increase in tau protein levels, with a time course similar to the initiation of its neurotropic effects. The following observations exclude tubulin as the cellular target for staurosporine action: a) the level, cellular distribution, and assembly properties of tubulin are not affected by staurosporine treatment; and b) colchicine uptake, its binding to tubulin, and its interference with tubulin polymerization are not changed by staurosporine. On the other hand, staurosporine treatment causes a transient, dose-dependent increase in tau protein levels. This increase, which is already evident after 1 hr, reaches a maximum of 2 to 3 fold after 5 hr of treatment and declines to basal level within the next 10 to 15 hr. The rapid, transient increase of tau protein levels induced by staurosporine is reminiscent of its neurotropic properties. Here we characterize and compare the cytoskeletal properties of neurites induced by treatment with staurosporine and with nerve growth factor, and we offer a mechanistic explanation for the rapid stabilization of staurosporine induced neurites.

Background: Taxol is a potent inhibitor of the replication of eukaryotic cells and has significant antitumor activity in human malignancies. The drug induces the formation of bundles of stable microtubules and blocks cells in the mitotic phase of the cell cycle. In vitro, taxol enhances the polymerization of tubulin to microtubules that are resistant to depolymerization. Although it is evident that taxol interacts with the tubulin-microtubule system, no information has been available on the binding site for the drug on the microtubule., Purpose: Our purpose was to determine if taxol binds to one or both of the tubulin subunits., Methods: In the absence of a photoaffinity-labeled analogue of taxol, [3H]taxol was used directly to photolabel tubulin. A complex of microtubule protein and [3H]taxol was irradiated by ultraviolet light and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis., Results and Conclusions: The radiolabeled drug preferentially binds covalently to the beta-subunit of tubulin, and the binding can be competed with unlabeled taxol., Implications: This observation is the first step in a study to determine the binding site for taxol on the microtubule.

Taxol stabilizes microtubules against the depolymerizing effects of cold temperature, drugs and Ca++. In this report, the effect of alkaline pH on microtubules polymerized in the presence of taxol has been studied. Although taxol-microtubules are more stable than microtubules assembled in the presence of GTP, taxol-microtubules can be partially disassembled when the pH becomes more alkaline. A portion of the recovered tubulin dimer is assembly competent upon pH adjustment to approximately 6.6 and the microtubules formed upon the induction of assembly by GTP are normal as judged by electron microscopy. The data indicate that alkaline pH can be used to recover assembly-competent tubulin from a taxol-microtubule complex. At pH 6.6, taxol-induced polymers consisted of two components. The majority were microtubules, but in addition hoops and ribbons were also present. At alkaline pH, the microtubules were more stable than the hoops and ribbons and at pH greater than 7.5 they were the only stable structures. Microtubules stabilized by taxol are protected against the depolymerizing action of podophyllotoxin even at alkaline pH, whereas the hoops and ribbons are depolymerized.

Taxol, a potent inhibitor of cell replication, enhances the assembly of tubulin into stable microtubules and promotes the formation of microtubule bundles in cells. In addition to its unique mechanism of action, taxol exhibits unusual promise as an antitumor agent, but its application in cancer chemotherapy is hampered by its limited availability. In order to better define the structure-activity profile of taxol for the design of more accessible drugs and to provide insight into the chemical features of the taxol-microtubule interaction, taxol analogues 3-8, with deleted A-ring side chain substituents and both R and S C-2' configurations, were synthesized from baccatin III through esterification at the hindered 13-hydroxyl. Employing an improved hydroxyl protection strategy, lactate analogues 3 and 4 were prepared with reasonable efficiency owing to their simple side-chain structures, while N-benzoylisoserine analogues 7 and 8 were synthesized through esterification reactions whose rates were enhanced greatly by the participation of the amide functionality. Although less biologically active than taxol, analogues 5-7 were found to promote the polymerization of tubulin and to be cytotoxic; 5 and 6 were considerably more effective than 7, whereas 3, 4, and 8 were least active. Interestingly, tubulin polymerization was sensitive to the C-2' configuration only when the amide substituent was present in the side chain. This observation suggests that the 3'-amide substituent plays an important role in preorganizing the taxol side chain to bind to microtubules.

RP 56976 (taxotere), a new semisynthetic analogue of taxol, is a potentially important chemotherapeutic agent for the treatment of cancer. We report here that this drug is a potent inhibitor of cell replication and, like taxol, promotes the in vitro assembly of stable microtubules in the absence of guanosine triphosphate and induces microtubule-bundle formation in cells. Compared with taxol, RP 56976 is slightly more active as a promoter of tubulin polymerization. As an inhibitor of cell replication, RP 56976 is 2.5-fold more potent than taxol in J774.2 and P388 cells and at least 5-fold more potent in taxol-resistant cells.

The hydrolysis products of taxol have been isolated by high-performance liquid chromatography and identified by nuclear magnetic resonance and mass spectroscopy. In contrast to taxol, the major hydrolysis product, baccatin III, has little cytotoxic activity and does not promote in vitro microtubule assembly. In cell culture medium, the concentration of taxol decreases with time and 7-epitaxol, which exhibits properties comparable to those of taxol both on cells and on in vitro microtubuli polymerization, is formed. Baccatin III is found in small quantities in the cell medium, although it is barely detectable within the cells. It is concluded that 7-epitaxol is the major derivative of taxol found in cells and that its presence does not alter, in a major way, the overall biological activity of taxol.