Abstract IA9-1: Is taxol resistance related to tubulin isotypes?

Abstract

Abstract Microtubules are an important component of the euraryotic cytoskeleton and are essential for normal cell division. They are also a validated target for antitumor drugs, such as taxol, that bind to the – tubulin subunit in the microtubule polymer. Endogenous resistance or the development of drug resistance during treatment with taxol is a serious clinical problem. Understanding the emergence of taxol resistance and being able to predict patient response would enhance the efficacy of the drug. A number of mechanisms, such as overexpression of P-glycoprotein that acts as an ATP-dependent drug-efflux pump, or mutations in the drug-binding site in tubulin, have been described. Of particular interest to our research, is the aberrant expression of β-tubulin isotypes in cancer. Tubulin is composed of α- and β-tubulin subunits that exist as heterodimers. In humans, there are seven β-tubulins and eight β-tubulins, each the product of a distinct gene located on different chromosomes. Each isotype can undergo extensive posttranslational modifications such as acetylation, phosphorylation, polyglutamylation, polyglycylation and reversible tyrosination, all of which add to the complexity of the tubulin/microtubule system. Most of the divergence between the isotypes is found in the last 15-20 amino acids at the carboxyl end of tubulin and in addition, most posttranslational modifications of tubulin occur in this region. For the most part, tubulin isotypes have been detected at the mRNA level or at the protein level with isotype-specific antibodies. Our laboratory has developed proteomic methods, including highresolution isoelectrofocusing, CNBr cleavage and mass spectrometry that allow us to determine the isotype content and posttranslational modifications of tubulin in cells and tissues in a single experiment. We have utilized taxol, which enhances tubulin polymerization, to isolate sufficient tubulin from cells and are developing methods to quantify tubulin isotype expression by mass spectrometry. The overexpression of βIII-tubulin has been associated with resistance to taxanes in a range of tumor types and in taxol-resistant cells generated in our laboratory. βIII-tubulin is normally expressed only in neuronal cells. However, it is aberrantly expressed in numerous malignant tissues and could be a meaningful prognostic factor for treatment outcome. As has been shown by others in different tumors, in our laboratory an immunohistochemical analysis of 47 tumor specimens from head and neck patients determined that overexpression of β-III tubulin was highly prognostic of survival. Citation Information: Clin Cancer Res 2010;16(14 Suppl):IA9-1.