Characterization of a human βV-tubulin antibody and expression of this isotype in normal and malignant human tissue

Nuclear βII-Tubulin and its Possible Utility in Cancer Diagnosis, Prognosis and Treatment

Microtubules are organelles that usually occur only in the cytosol. Walss et al. (1999) discovered the βII isotype of tubulin, complexed with α, in the nuclei of certain cultured cells, in non-microtubule form. When fluorescently labeled tubulins were microinjected into the cells, only αβII appeared in the nucleus, and only after one cycle of nuclear disassembly and reassembly. It appeared as if αβII does not cross the nuclear envelope but is trapped in the nucleus by the re-forming nuclear envelope in whose reassembly βII may be involved. βII is present in the cytoplasm and nuclei of many tumor cells. With some exceptions, normal tissues that expressed βII rarely had βII in their nuclei. It is possible that βII is involved in nuclear reassembly and then disappears from the nucleus. Ruksha et al. (2019) observed that patients whose colon cancer cells in the invasive front showed no βII had a median survival of about 5.5 years, which was more than halved if they had cytosolic βII and further lessened if they had nuclear βII, suggesting that the presence and location of βII in biopsies could be a useful prognostic indicator and also that βII may be involved in cancer progression. Yeh and Ludueña. (2004) observed that many tumors were surrounded by non-cancerous cells exhibiting cytosolic and nuclear βII, suggesting a signaling pathway that causes βII to be synthesized in nearby cells and localized to their nuclei. βII could be useful in cancer diagnosis, since the presence of βII in non-cancerous cells could indicate a nearby tumor. Investigation of this pathway might reveal novel targets for chemotherapy. Another possibility would be to combine αβII with CRISPR-Cas9. This complex would likely enter the nucleus of a cancer cell and, if guided to the appropriate gene, might destroy the cancer cell or make it less aggressive; possible targets will be discussed here. The possibilities raised here about the utility of βII in cancer diagnosis, prognosis, biology and therapy may repay further investigation.

Intrinsic and Extrinsic Factors Affecting Microtubule Dynamics in Normal and Cancer Cells

Microtubules (MTs), highly dynamic structures composed of α- and β-tubulin heterodimers, are involved in cell movement and intracellular traffic and are essential for cell division. Within the cell, MTs are not uniform as they can be composed of different tubulin isotypes that are post-translationally modified and interact with different microtubule-associated proteins (MAPs). These diverse intrinsic factors influence the dynamics of MTs. Extrinsic factors such as microtubule-targeting agents (MTAs) can also affect MT dynamics. MTAs can be divided into two main categories: microtubule-stabilizing agents (MSAs) and microtubule-destabilizing agents (MDAs). Thus, the MT skeleton is an important target for anticancer therapy. This review discusses factors that determine the microtubule dynamics in normal and cancer cells and describes microtubule-MTA interactions, highlighting the importance of tubulin isoform diversity and post-translational modifications in MTA responses and the consequences of such a phenomenon, including drug resistance development.

Tubulin heterogeneity regulates functions and dynamics of microtubules and plays a role in the development of drug resistance in cancer

Microtubules, composed of αβ-tubulin heterodimers, exhibit diverse structural and functional properties in different cell types. The diversity in the microtubule structure originates from tubulin heterogeneities, namely tubulin isotypes and their post-translational modifications (PTMs). These heterogeneities confer differential stability to microtubules and provide spatial cues for the functioning of the cell. Furthermore, the altered expressions of tubulin isotypes and PTMs are prominent factors for the development of resistance against some cancer drugs. In this review, we summarize our current knowledge of the tubulin isotypes and PTMs and how, together, they control the cellular functions of the microtubules. We also describe how cancer cells use this tubulin heterogeneity to acquire resistance against clinical agents and discuss existing attempts to counter the developed resistance.

Expression of βV-Tubulin in Secretory Cells of the Fallopian Tube Epithelium Marks Cellular Atypia

Supplemental digital content is available in the text.

Objectives

Class V Beta tubulin isotype (βV-tubulin) was recently found to have tissue-specific expression patterns in epithelial tissues with secretory function and aberrant expression in tumors. The aims of this pilot study were (a) to examine expression of βV-tubulin in the fallopian tube epithelium (FTE) of patients who underwent salpingectomy, (b) to characterize FTE atypia in high-risk patients with BRCA mutations, and (c) to determine expression of βV-tubulin in serous ovarian neoplasms.

Methods

Immunohistochemistry, with a highly specific antibody developed in our laboratory against human βV-tubulin, was used to evaluate expression in paraffin-embedded sections of the fallopian tube (n = 82) and tumors (n = 13), from prospectively selected cases, categorized by reason for salpingectomy.

Results

βV-tubulin, when present, was expressed in secretory cells and essentially never in ciliated cells of the FTE. Histologically “normal” FTE had very rare, scattered βV-tubulin–positive cells; percentage positivity increased in cases of serous ovarian neoplasms. The highest expression was observed in FTE from patients with BRCA mutant breast cancer. Four distinct types of FTE atypia were delineated in patients with known BRCA mutations. In a few additional test cases of ovarian neoplasms, βV-tubulin was highly expressed, with the extent and intensity of staining elevated in high-grade serous carcinomas compared with serous borderline tumors.

Conclusions

In summary, βV-tubulin was localized to secretory cells of the distal FTE and its expression varied according to the clinical diagnosis. The frequency of these cells and thus expression of βV-tubulin were dramatically enriched in tissue obtained from BRCA mutant cases, which also exhibited pronounced histologic atypia indicative of early predysplastic aberrations. Furthermore, elevated expression of βV-tubulin correlated with poor differentiation status in serous ovarian neoplasms.

Taxol Analogues Exhibit Differential Effects on Photoaffinity Labeling of β-Tubulin and the Multidrug Resistance Associated P-Glycoprotein

Several next-generation taxanes have been reported to possess high potency against Taxol-resistant cancer cell lines overexpressing βIII-tubulin and/or P-glycoprotein (P-gp), both of which are involved in drug resistance. Using a photoaffinity Taxol analogue, 2-( m-azidobenzoyl)taxol, two potent next-generation taxanes, SB-T-1214 and SB-CST-10202, exhibited distinct inhibitory effects on photolabeling of β-tubulin from different eukaryotic sources that differ in β-tubulin isotype composition. They also specifically inhibited photolabeling of P-gp, and the inhibitory effect correlated well with the steady-state accumulation of [³H]vinblastine in a multidrug resistant (MDR) cell line, SKVLB1. Several microtubule-stabilizing agents (MSAs)-resistant cell lines from the human ovarian cancer cell line Hey were isolated, and their MDR1 and βIII-tubulin levels determined. Distinct potencies of the two taxanes against different MSA-resistant cells expressing unique levels of MDR1 and βIII-tubulin were found. Cytotoxicity assays, done in the presence of verapamil, indicated that SB-T-1214 is a substrate, although not as good as Taxol, for P-gp. The mechanisms involved in drug resistance are multifactorial, and the effectiveness of new Taxol analogues depends on the interaction between the drugs and all possible targets; in this case the two major cellular targets are β-tubulin and P-gp.

Enhanced stability of microtubules contributes in the development of colchicine resistance in MCF-7 cells

Understanding the mechanism of resistance to tubulin-targeted anticancer drugs is important for improved chemotherapy. In this work, a colchicine-resistant MCF-7 cell line (MCF-7_Col30) was generated by the gradual increment of colchicine treatment and the MCF-7_Col30 showed ∼8-fold resistance towards colchicine. MCF-7_Col30 cells showed ∼2.5-fold resistance against microtubule depolymerizing agents, vinblastine, and nocodazole. In contrast, it displayed more sensitivity towards paclitaxel, a microtubule-polymerizing agent. MCF-7 and MCF-7_Col30 cells showed similar sensitivity towards cisplatin. Further, the level of P-glycoprotein did not increase in MCF-7_Col30 cells. MCF-7_Col30 cells resisted the microtubule depolymerizing effects of colchicine. The time-lapse imaging of individual microtubules in live cells showed that the dynamics of microtubules in MCF-7_Col30 cells was suppressed as compared to the parent MCF-7 cells. The levels of tubulin acetylation and glutamylation increased in MCF-7_Col30 cells than the parent MCF-7 cells suggesting that microtubules are stabilized in MCF-7_Col30 cells. Interestingly, the level of βIII tubulin was increased by 2.3 folds whereas that of βII and βIV tubulin was decreased by 55 and 150%, respectively in MCF-7_Col30 cells. The results suggested that the changes in the level of β-tubulin isoforms and the post-translational modifications of microtubules altered the stability and dynamics of microtubules and contributed to the development of colchicine-resistance in MCF-7 cells.

An Essential Role of Maspin in Embryogenesis and Tumor Suppression

Maspin (SerpinB5) is an epithelial-specific tumor suppressor gene product that displays context-dependent cellular functions. Maspin-deficient mouse models created to date have not definitively established maspin functions critical for cancer suppression. In this study, we generated a mouse strain in which exon 4 of the Maspin gene was deleted, confirming its essential role in development but also enabling a breeding scheme to bypass embryonic lethality. Phenotypic characterization of this viable strain established that maspin deficiency was associated with a reduction in maximum body weight and a variety of context-dependent epithelial abnormalities. Specifically, maspin-deficient mice exhibited pulmonary adenocarcinoma, myoepithelial hyperplasia of the mammary gland, hyperplasia of luminal cells of dorsolateral and anterior prostate, and atrophy of luminal cells of ventral prostate and stratum spinosum of epidermis. These cancer phenotypes were accompanied by increased inflammatory stroma. These mice also displayed the autoimmune disorder alopecia aerate. Overall, our findings defined context-specific tumor suppressor roles for maspin in a clinically relevant model to study maspin functions in cancer and other pathologies. Cancer Res; 77(4); 886-96. ©2017 AACR.

2-(m-Azidobenzoyl)taxol binds differentially to distinct β-tubulin isotypes

There are seven β-tubulin isotypes present in distinct quantities in mammalian cells of different origin. Altered expression of β-tubulin isotypes has been reported in cancer cell lines resistant to microtubule stabilizing agents (MSAs) and in human tumors resistant to Taxol. To study the relative binding affinities of MSAs, tubulin from different sources, with distinct β-tubulin isotype content, were specifically photolabeled with a tritium-labeled Taxol analog, 2-(m-azidobenzoyl)taxol, alone or in the presence of MSAs. The inhibitory effects elicited by these MSAs on photolabeling were distinct for β-tubulin from different sources. To determine the exact amount of drug that binds to different β-tubulin isotypes, bovine brain tubulin was photolabeled and the isotypes resolved by high-resolution isoelectrofocusing. All bands were analyzed by mass spectrometry following cyanogen bromide digestion, and the identity and relative quantity of each β-tubulin isotype determined. It was found that compared with other β-tubulin isotypes, βIII-tubulin bound the least amount of 2-(m-azidobenzoyl)taxol. Analysis of the sequences of β-tubulin near the Taxol binding site indicated that, in addition to the M-loop that is known to be involved in drug binding, the leucine cluster region of βIII-tubulin contains a unique residue, alanine, at 218, compared with other isotypes that contain threonine. Molecular dynamic simulations indicated that the frequency of Taxol-accommodating conformations decreased dramatically in the T218A variant, compared with other β-tubulins. Our results indicate that the difference in residue 218 in βIII-tubulin may be responsible for inhibition of drug binding to this isotype, which could influence downstream cellular events.

Effect of CH-35, a novel anti-tumor colchicine analogue, on breast cancer cells overexpressing the βIII isotype of tubulin

The subunit protein of microtubules is tubulin, which has been the target for some of the most successful and widely used anti-tumor drugs. Most of the drugs that target tubulin bind to the β subunit. There are many isotypes of β-tubulin and their distributions differ among different tissues. The βIII isotype is over-expressed in many tumors, particularly those that are aggressive, metastatic, and drug resistant. We have previously reported the design and synthesis of a series of compounds to fit the colchicine site on βIII but not on the other isotypes. In the current study, we tested the toxicity and the anti-tumor activity of one of these compounds, CH-35, on the human breast tumor MDA-MB-231 over-expressing βIII in a xenogeneic mouse model. We found that CH-35 was as toxic as Taxol® in vivo. Although the βIII-over-expressing cells developed into very fast-growing tumors, CH-35 was more effective against this tumor than was Taxol. Our results suggest that CH-35 is a promising candidate for future drug development.

Proteomics of cancer cell lines resistant to microtubule-stabilizing agents

Despite the clinical success of microtubule-interacting agents (MIA), a significant challenge for oncologists is the inability to predict the response of individual patients with cancer to these drugs. In the present study, six cell lines were compared by 2D DIGE proteomics to investigate cellular resistance to the class of MIAs known as microtubule-stabilizing agents (MSA). The human lung cancer cell line A549 was compared with two drug-resistant daughter cell lines, a taxol-resistant cell line (AT12) and an epothilone B (EpoB)-resistant cell line (EpoB40). The ovarian cancer cell line Hey was compared with two drug-resistant daughter cell lines, an EpoB-resistant cell line (EpoB8) and an ixabepilone-resistant cell line (Ixab80). All 2D DIGE results were validated by Western blot analyses. A variety of cytoskeletal and cytoskeleton-associated proteins were differentially expressed in drug-resistant cells. Differential abundance of 14-3-3σ, galectin-1 and phosphorylation of stathmin are worthy of further studies as candidate predictive biomarkers for MSAs. This is especially true for galectin-1, a β-galactose-binding lectin that mediates tumor invasion and metastasis. Galectin-1 was greatly increased in EpoB- and ixabepilone-resistant cells and its suppression caused an increase in drug sensitivity in both drug-sensitive and -resistant Hey cells. Furthermore, the growth medium from resistant Hey cells contained higher levels of galectin-1, suggesting that galectin-1 could play a role in resistance to MSAs.