Transfer and amplification of a mutant beta-tubulin gene results in colcemid dependence: use of the transformant to demonstrate regulation of beta-tubulin subunit levels by protein degradation

Restoration of Mitochondrial Gene Expression Using a Cloned Human Gene in Chinese Hamster Lung Cell Mutant

BACKGROUND

Gal^-32 is a Chinese hamster lung cell nuclear mutant that is unable to grow in galactose due to a defect in mitochondrial protein synthesis. Since the product of the Gal^-32 gene was unknown, it was imperative to use phenotypic complementation to clone a human gene that corrected the Gal^-32 mutation.

RESULTS

Recessive Gal^-32 cells were co-transformed with pSV2-neo plasmid DNA and recombinant DNA from a human genomic library containing the dominant human Gal⁺ gene and a chloramphenicol-resistance (cam^r ) gene present in the pSV13 vector. Primary transformants were selected by growth in galactose and the neomycin analog G418. In order to rescue the human Gal⁺ gene, a genomic library was constructed with primary transformant DNA and the pCV108 cosmid vector. The cam^r gene was used to identify clones with the nearby human sequences. DNA from two cam^r , Alu-hybridizing clones was able to transform the recessive Gal^-32 cells to the Gal⁺ phenotype and to restore mitochondrial protein synthesis.

CONCLUSION

These data demonstrate the isolation of two pCV108-transformant recombinant clones containing a human gene that complements the Chinese hamster Gal^-32 mutation and restores galactose metabolism.

New insights into mechanisms of resistance to microtubule inhibitors

Mechanisms to explain tumor cell resistance to drugs that target the microtubule cytoskeleton have relied on the assumption that the drugs act either to suppress microtubule dynamics or to perturb the balance between assembled and nonassembled tubulin. Recently, however, it was found that these drugs also alter the stability of microtubule attachment to centrosomes, and do so at the same concentrations that are needed to inhibit cell division. Based on this new information, a new model is presented that explains resistance resulting from a variety of molecular changes that have been reported in the literature. The improved understanding of drug action and resistance has important implications for chemotherapy with these agents.

Paclitaxel-dependent cell lines reveal a novel drug activity

We previously described the isolation of Tax 18 and Tax 11-6, two paclitaxel-dependent cell lines that assemble low amounts of microtubule polymer and require the drug for cell division. In the present studies, fluorescence time-lapse microscopy was used to measure microtubule dynamic instability behavior in these cells. The mutations were found to cause small decreases in microtubule growth and shortening, but the changes seemed unable to explain the defects in microtubule polymer levels or cell division. Moreover, paclitaxel further suppressed microtubule dynamics at low drug concentrations that were insufficient to rescue the mutant phenotype. Wild-type (WT) cells treated with similar low drug concentrations also had highly suppressed microtubules, yet experienced no problems with cell division. Thus, the effects of paclitaxel on microtubule dynamics seemed to be unrelated to cell division in both WT and mutant cell lines. The higher drug concentrations needed to rescue the mutant phenotype instead inhibited the formation of unstable microtubule fragments that appeared at high frequency in the drug-dependent, but not WT, cell lines. Live cell imaging revealed that the fragments were generated by microtubule detachment from centrosomes, a process that was reversed by paclitaxel. We conclude that paclitaxel rescues mutant cell division by inhibiting the detachment of microtubule minus ends from centrosomes rather than by altering plus-end microtubule dynamics.

alpha-Tubulin limits its own synthesis: evidence for a mechanism involving translational repression

A Chinese hamster alpha-tubulin cDNA was modified to encode an 11-amino acid carboxyl-terminal extension containing the immunodominant epitope from influenza hemagglutinin antigen (to create HA alpha 1-tubulin) and was cloned into a vector for expression in mammalian cells. 12 stable CHO cell lines expressing this HA alpha 1-tubulin were isolated and characterized. HA alpha 1-tubulin incorporated into all classes of microtubules, assembled to the same extent as the endogenous tubulin, and did not perturb the growth of the cells in which it was expressed. However, overexpression of HA alpha 1-tubulin strongly repressed the synthesis of endogenous alpha-tubulin while having little or no effect on the synthesis of beta-tubulin. Treatment of transfected cells with sodium butyrate to induce even greater expression of HA alpha 1-tubulin led to a further decrease in synthesis of endogenous alpha-tubulin that was fully reversible upon removal of the inducer. Decreased synthesis of alpha-tubulin in transfected cells did not result from decreased levels of alpha-tubulin mRNA, as demonstrated by ribonuclease protection assays. On the other hand, colchicine, a drug previously shown to destabilize the tubulin message, caused a clear reduction in both protein synthesis and mRNA levels for transfected HA alpha 1-tubulin and endogenous alpha-tubulin, thus indicating that the decreased alpha-tubulin synthesis observed as a result of HA alpha 1-tubulin overexpression is distinct from the previously described autoregulation of tubulin. The results are consistent with a mechanism in which free alpha-tubulin inhibits the translation of its own message as a way of ensuring stoichiometric synthesis of alpha- and beta-tubulin.

Mutations affecting assembly and stability of tubulin: evidence for a nonessential beta-tubulin in CHO cells

Eight strains of Chinese hamster ovary (CHO) cells having an assembly-defective beta-tubulin were found among revertants of strain Cmd 4, a mutant with a conditional lethal mutation in a beta-tubulin gene (F. Cabral, M. E. Sobel, and M. M. Gottesman, Cell 20:29-36, 1980). The altered beta-tubulins in these strains have electrophoretically silent alterations or, in some cases, an increase or a decrease in apparent molecular weight based on their migration in two-dimensional gels. The identity of these variant proteins as beta-tubulin was confirmed by peptide mapping, which also revealed the loss of distinct methionine-containing peptides in the assembly-defective beta-tubulins of lower apparent molecular weight. The altered mobility of these beta-tubulin polypeptides was not the result of a posttranslational modification, since the altered species could be labeled in very short incubations with [35S]methionine and were found among in vitro-translated polypeptides by using purified mRNA. In at least one strain, an altered DNA restriction fragment could be demonstrated, suggesting that an alteration occurred in one of the structural genes for beta-tubulin. Assembly-defective beta-tubulin was unstable and turned over with a half-life of only 1 to 2 h in exponentially growing cells. This rapid degradation of a tubulin gene product resulted in approximately 30% lower steady-state levels of both alpha- and beta-tubulin yet did not affect the growth rate of the cells or the distribution of the microtubules as judged by immunofluorescence microscopy. These results argue that CHO cells possess a beta-tubulin gene product that is not essential for survival.

Mutations affecting assembly and stability of tubulin: evidence for a nonessential beta-tubulin in CHO cells

Eight strains of Chinese hamster ovary (CHO) cells having an assembly-defective beta-tubulin were found among revertants of strain Cmd 4, a mutant with a conditional lethal mutation in a beta-tubulin gene (F. Cabral, M. E. Sobel, and M. M. Gottesman, Cell 20:29-36, 1980). The altered beta-tubulins in these strains have electrophoretically silent alterations or, in some cases, an increase or a decrease in apparent molecular weight based on their migration in two-dimensional gels. The identity of these variant proteins as beta-tubulin was confirmed by peptide mapping, which also revealed the loss of distinct methionine-containing peptides in the assembly-defective beta-tubulins of lower apparent molecular weight. The altered mobility of these beta-tubulin polypeptides was not the result of a posttranslational modification, since the altered species could be labeled in very short incubations with [35S]methionine and were found among in vitro-translated polypeptides by using purified mRNA. In at least one strain, an altered DNA restriction fragment could be demonstrated, suggesting that an alteration occurred in one of the structural genes for beta-tubulin. Assembly-defective beta-tubulin was unstable and turned over with a half-life of only 1 to 2 h in exponentially growing cells. This rapid degradation of a tubulin gene product resulted in approximately 30% lower steady-state levels of both alpha- and beta-tubulin yet did not affect the growth rate of the cells or the distribution of the microtubules as judged by immunofluorescence microscopy. These results argue that CHO cells possess a beta-tubulin gene product that is not essential for survival.

2,4-Dichlorobenzyl thiocyanate, an antimitotic agent that alters microtubule morphology

A compound of simple structure, 2,4-dichlorobenzyl thiocyanate (DCBT), is an antimitotic agent with a number of unusual properties. The drug causes an extreme reorganization of microtubules in cells in culture. Most normal microtubules disappear, and remaining tubulin-containing structures appear to be bundled or aggregated. DCBT irreversibly inhibits in vitro polymerization of purified tubulin, but only after a prolonged preincubation of the protein with the drug. Binding of radiolabeled DCBT to tubulin similarly requires a long incubation time, with the reaction not being complete even after 6 hr at 37 degrees C. A specific interaction with tubulin is also shown by the crossresistance to DCBT of Colcemid-resistant cells with an altered beta-tubulin. A human KB carcinoma cell line and a Chinese hamster ovary cell line selected for crossresistance to multiple chemotherapeutic agents, including most antimitotic drugs, are sensitive to DCBT. Initial structure-function studies have demonstrated weak antimitotic and antitubulin activity with the parent compound benzyl thiocyanate. Chlorination at either position 2 or position 4 of the phenyl ring produces compounds of intermediate activity (4-chlorobenzyl thiocyanate is more active than 2-chlorobenzyl thiocyanate). The thiocyanate moiety appears to be essential for activity.