Maria Kavallaris

Microtubules and resistance to tubulin-binding agents

Microtubules are dynamic structures composed of α–β-tubulin heterodimers that are essential in cell division and are important targets for cancer drugs. Mutations in β-tubulin that affect microtubule polymer mass and/or drug binding are associated with resistance to tubulin-binding agents such as paclitaxel. The aberrant expression of specific β-tubulin isotypes, in particular βIII-tubulin, or of microtubule-regulating proteins is important clinically in tumour aggressiveness and resistance to chemotherapy. In addition, changes in actin regulation can also mediate resistance to tubulin-binding agents. Understanding the molecular mechanisms that mediate resistance to tubulin-binding agents will be vital to improve the efficacy of these agents.

Metronomic chemotherapy: new rationale for new directions

Tumor angiogenesis is recognized as a major therapeutic target in the fight against cancer. The key involvement of angiogenesis in tumor growth and metastasis has started to redefine chemotherapy and new protocols have emerged. Metronomic chemotherapy, which is intended to prevent tumor angiogenesis, is based on more frequent and low-dose drug administrations compared with conventional chemotherapy. The potential of metronomic chemotherapy was revealed in animal models a decade ago and the efficacy of this approach has been confirmed in the clinic. In the past 5 years, multiple clinical trials have investigated the safety and efficacy of metronomic chemotherapy in a variety of human cancers. While the results have been variable, clinical studies have shown that these new treatment protocols represent an interesting alternative for either primary systemic therapy or maintenance therapy. We review the latest clinical trials of metronomic chemotherapy in adult and pediatric cancer patients. Accumulating evidence suggests that the efficacy of such treatment may not only rely on anti-angiogenic activity. Potential new mechanisms of action, such as restoration of anticancer immune response and induction of tumor dormancy are discussed. Finally, we highlight the research efforts that need to be made to facilitate the optimal development of metronomic chemotherapy.

Bcl-2 inhibits Bax translocation from cytosol to mitochondria during drug-induced apoptosis of human tumor cells

The pro-apoptotic protein, Bax, has been reported to translocate from cytosol to mitochondria following exposure of cells to apoptotic stresses including cytokine withdrawal and treatment with glucocorticoids and cytotoxic drugs. These observations, coupled with reports showing that Bax causes the release of mitochondrial cytochrome c, implicate Bax as a central mediator of the apoptotic process. In this report we demonstrate by subcellular fractionation a significant shift in Bax localization from cytosol to cellular membranes in two human tumor cell lines exposed to staurosporine or etoposide. Immunofluorescence studies confirmed that Bax specifically relocalized to the mitochondria. This redistribution of Bax occurred in concert with, or just prior to, proteolytic processing of procaspase-3, activation of DEVD-specific cleavage activity and degradation of poly(ADP-ribose) polymerase. However, Bax membrane translocation was independent of caspase activity as determined using the broad-range caspase inhibitor z-VAD-fmk. High level overexpression of the anti-apoptotic protein Bcl-2 prevented Bax redistribution to the mitochondria, caspase activation and apoptosis following exposure to staurosporine or etoposide. These data confirm the role of Bax in mitochondrial cytochrome c release, and indicate that prevention of Bax translocation to the mitochondrial membrane represents a novel mechanism by which Bcl-2 inhibits drug-induced apoptosis.

Class III β-tubulin mediates sensitivity to chemotherapeutic drugs in non-small cell lung cancer

First line therapy for non–small cell lung carcinoma (NSCLC) commonly includes combination therapy with a tubulin-binding agent (TBA) and a DNA-damaging agent. TBAs suppress microtubule dynamics by binding to the β-tubulin subunit of α/β-tubulin, inducing mitotic arrest and apoptosis. Up-regulation of class III β-tubulin (βIII-tubulin) has been implicated in clinical resistance in NSCLC, ovarian and breast tumors treated in combination with a TBA and DNA-damaging agent. To investigate the functional significance of βIII-tubulin in resistance to both these classes of agents, small interfering RNA (siRNA) was used to silence the expression of this isotype in two NSCLC cell lines, NCI-H460 and Calu-6. Reverse transcription-PCR and immunoblotting showed that βIII-siRNA potently inhibited the expression of βIII-tubulin, without affecting the expression of other major β-tubulin isotypes. Clonogenic assays showed that βIII-siRNA cells were significantly more sensitive to TBAs, paclitaxel, vincristine, and vinorelbine, and for the first time, DNA-damaging agents, cisplatin, doxorubicin, and etoposide compared with controls. Cell cycle analysis of H460 βIII-siRNA cells showed reduced accumulation at the G2-M boundary and an increase in the sub-G1 population in response to TBA treatment compared with control cells. Importantly, βIII-siRNA cells displayed a significant dose-dependent increase in Annexin V staining when treated with either paclitaxel or cisplatin, compared with controls. These findings have revealed a novel role for βIII-tubulin in mediating response to both TBA and DNA-damaging agent therapy and may have important implications for improving the targeting and treatment of drug-refractory NSCLC. [Cancer Res 2007;67(19):9356–63]

Antisense oligonucleotides to class III β-tubulin sensitize drug-resistant cells to Taxol

SummaryA major impediment to the successful use of Taxol in the treatment of cancer is the development of drug resistance. The major cellular target of Taxol is the microtubule that is comprised of α- and β-tubulin heterodimers. Binding sites for Taxol have been delineated on the β-tubulin subunit that has six isotypes. We have recently described increased expression of the brain-specific human class III β-tubulin isotype, encoded by the Hβ4 gene, in both Taxol-resistant ovarian tumours and non-small-cell lung cancer cell lines. To evaluate directly the role of the class III β-tubulin isotype in mediating Taxol resistance, antisense phosphorothioate oligodeoxynucleotides (ODN) targeted against various regions of the Hβ4 gene have been designed and examined for their efficacy in reducing Hβ4 gene and protein expression. Taxol-resistant lung cancer cells, A549-T24, which are 17-fold resistant to Taxol and display a fourfold increase in Hβ4 expression compared to the parental A549 cells, were treated with 1 μM antisense ODNs. Two ODNs, AS1 and AS3, were found to reduce mRNA expression by 40–50%, as determined by reverse transcription polymerase chain reaction. A concentration-dependent reduction in Hβ4 mRNA expression was demonstrated with AS1 ODN. Immunofluorescence staining of cells treated with AS1 ODN revealed a decrease in class III protein expression which corresponded to a 39% increase in sensitivity to Taxol (P < 0.005). These findings support an important role for Hβ4 (class III) β-tubulin expression in Taxol resistance and have potential implications for the treatment of Taxol-resistant tumours.

Acid-Labile Core Cross-Linked Micelles for pH-Triggered Release of Antitumor Drugs

Micelles of a model amphiphilic block copolymer, poly(hydroxyethyl acrylate)-block-poly(n-butyl acrylate) (PHEA-b-PBA), synthesized via the RAFT polymerization were cross-linked by copolymerization of a degradable cross-linker from the living RAFT-end groups of PBA chains, yielding a cross-linked core without affecting significantly the original micelle size. The cross-linker incorporation into the micelles was evidenced via physicochemical analysis of the copolymer unimers formed upon acidic cleavage of the cross-linked micelles. High doxorubicin loading capacities (60 wt %) were obtained. Hydrolysis of less than half of the cross-links in the core was found to be sufficient to release doxorubicin faster at acidic pH compared to neutral pH. The system represents the first example of core-cross-linked micelles that can be destabilized (potentially both above and below CMC) by the pH-dependent cleavage of the cross-links and the subsequent polarity change in the core to enable the release of hydrophobic drugs entrapped inside the micelle.

Cytoskeleton and paclitaxel sensitivity in breast cancer : the role of β-tubulins

The antineoplastic effect of paclitaxel is mainly related to its ability to bind the β subunit of tubulin, thus preventing tubulin chain depolarization and inducing apoptosis. The relevance of the Class I β‐tubulin characteristics have also been confirmed in the clinical setting where mutations of paclitaxel‐binding site of β‐tubulin Class I have been related to paclitaxel resistance in non small cell lung and ovarian cancers. In the present study, we verified the hypothesis of a relationship between molecular alterations of β‐tubulin Class I and paclitaxel sensitivity in a panel of breast cell lines with different drug IC50. The Class I β‐tubulin gene cDNA has been sequenced detecting heterozygous missense mutations (exon 1 and 4) only in MCF‐7 and SK‐BR‐3 lines. Furthermore, the expression (at both mRNA and protein level) of the different isotypes have been analyzed demonstrating an association between low cell sensitivity to paclitaxel and Class III β‐tubulin expression increasing. Antisense oligonucleotide (ODN) experiments confirmed that the inhibition of Class III β‐tubulin could at least partially increase paclitaxel‐chemosensitivity. The hypothesis of a relationship between β‐tubulin tumor expression and paclitaxel clinical response has been finally verified in a series of 92 advanced breast cancer patients treated with a first line paclitaxel‐based chemotherapy. Thirty‐five percent (95% CI: 45–31) of patients with high Class III β‐tubulin expression showed a disease progression vs. only 7% of patients with low expression (35% vs. 7%, p < 0.002). Our study suggests that Class III β‐tubulin tumor expression could be considered a predictive biomarker of paclitaxel‐clinical resistance for breast cancer patients.

Microtubules: a dynamic target in cancer therapy.

The tubulin/microtubule system is an important target for anticancer therapy. Two of the most clinically valuable groups of these agents are the vinca alkaloids and taxanes. In recent years, new tubulin‐binding agents have been under preclinical or clinical development. One of these classes of agents, epothilones, has shown great promise in phase III clinical trials. What all these agents share in common, is that they bind to β‐tubulin and disrupt microtubule function during mitosis which in turn leads to mitotic arrest and cell death. In addition, these agents can inhibit angiogenesis. Not withstanding their effectiveness, drug resistance can pose a major clinical problem. This review provides an overview of the mechanisms mediating resistance to tubulin‐binding agents related to the cellular target and discusses strategies to overcome this important clinical problem.

Microtubule alterations and mutations induced by desoxyepothilone B: implications for drug-target interactions.

Epothilones, like paclitaxel, bind to beta-tubulin and stabilize microtubules. We selected a series of four leukemia sublines that display increasing levels of resistance to the epothilone analog desoxyepothilone B (dEpoB). The dEpoB cells selected in 30-140 nM were approximately 15-fold cross-resistant to paclitaxel, while 300 nM selected cells were 467-fold resistant to this agent. The dEpoB-selected cells are hypersensitive to microtubule destabilizing agents, and express increased levels of class III beta-tubulin and MAP4. A novel class I beta-tubulin mutation, A231T, that affects microtubule stability but does not alter paclitaxel binding, was identified. The 300 nM selected cells acquired a second mutation, Q292E, situated near the M loop of class I beta-tubulin. These cells fail to undergo drug-induced tubulin polymerization due to dramatically reduced drug binding. The dEpoB-resistant leukemia cells provide novel insights into microtubule dynamics and, in particular, drug-target interactions.

Tissue-specific tropomyosin isoform composition.

Four distinct genes encode tropomyosin (Tm) proteins, integral components of the actin microfilament system. In non-muscle cells, over 40 Tm isoforms are derived using alternative splicing. Distinct populations of actin filaments characterized by the composition of these Tm isoforms are found differentially sorted within cells (Gunning et al. 1998b). We hypothesized that these distinct intracellular compartments defined by the association of Tm isoforms may allow for independent regulation of microfilament function. Consequently, to understand the molecular mechanisms that give rise to these different microfilaments and their regulation, a cohort of fully characterized isoform-specific Tm antibodies was required. The characterization protocol initially involved testing the specificity of the antibodies on bacterially produced Tm proteins. We then confirmed that these Tm antibodies can be used to probe the expression and subcellular localization of different Tm isoforms by Western blot analysis, immunofluorescence staining of cells in culture, and immunohistochemistry of paraffin wax-embedded mouse tissues. These Tm antibodies, therefore, have the capacity to monitor specific actin filament populations in a range of experimental systems.