Investigating the Specific Interactions Between Microtubule Stabilising Agents and β-Tubulin

Microtubule stabilising agents are a class of cytotoxic compounds that cause mitotic arrest through inhibition of microtubule function. They specifically target β-tubulin subunits promoting tubulin polymerisation, which eventually leads to cell death. Members of this drug class include the cancer chemotherapeutics paclitaxel and ixabepilone. However, like many cytotoxic agents, tumour cells often develop multi-drug resistance phenotypes limiting the effectiveness of such compounds. This results from the expulsion of these drugs from cells by efflux pumps, as well as mutation of their binding site. Much effort has been focused on improving the utility of this important drug class in the ongoing fight against cancer.  The microtubule stabilising agents peloruside A and laulimalide originate from marine sponge species native to the South Pacific. They have similar pharmacological profiles to paclitaxel and ixabepilone, however with several unique properties. They are poor substrates for efflux pumps and target a different region on β-tubulin subunits, giving them the potential for treatment of resistant tumours. This represents a novel mechanism of action that may be exploited for drug development, and further characterisation of the binding site is warranted.  The aim of this study is to investigate the contribution of two amino acids of human βItubulin to the interactions with peloruside A and laulimalide. Specifically, glu127 and lys124 have been predicted by computational modelling and analogue studies to form hydrogen bonds and other associations with the two compounds. These amino acids are located on β-tubulin subunits adjacent to the main binding pocket of peloruside A and laulimalide, and represent a potential inter-protofilament interaction that does not occur with other microtubule stabilising agents. This binding mechanism has not yet been shown by crystallography and is hence based solely on in silico work, requiring biological validation.  HEK293 cells were transfected with βI-tubulin with these amino acids mutated to alanines to prevent hydrogen bond formation. Cell proliferation assays, flow cytometry, and immunoblotting were used to study the effect loss of the inter-protofilament interaction has on the bioactivity of peloruside A and laulimalide. These mutations did not significantly alter the concentration-response of cells to either drug in the cell proliferation assay. However, accumulation of cells in the G2/M phase of the cell cycle and the proportion of transfected cells showing signs of mitotic arrest significantly decreased for E127A mutant cells compared to wild type βI-tubulin transfected control cells treated with peloruside A. Furthermore, a similar reduction in cell cycle block was also seen in E127A mutant cells treated with the negative control ixabepilone, which binds to a different site on β-tubulin.  No evidence seen in this study suggests that either amino acid plays a major role in peloruside A or laulimalide target binding. However, the amino acid E127 is important for inter-protofilament associations independent of drug treatment, as its mutation appeared to reduce global stability of microtubule structures. This information requires further validation, it may be useful in the design of future analogue syntheses as development of these promising drug candidates continues.