Delineating the interaction of combretastatin A-4 with αβ tubulin isotypes present in drug resistant human lung carcinoma using a molecular modeling approach.

Tubulin isotypes are known to regulate microtubule dynamic instability and contribute to the development of drug resistance in certain types of cancers. Combretastatin-A4 (CA-4) has a potent anti-mitotic, vascular disrupting and anti-angiogenic activity. It binds at the interface of αβ tubulin heterodimers and inhibits microtubules assembly. Interestingly, the CA-4 resistant human lung carcinoma shows alteration of βI and βIII isotype levels, a higher expression of βI tubulin isotype and a decreased expression of βIII tubulin isotypes has been reported in drug resistant cell lines. However, the origin of CA-4 resistance in lung carcinoma is not well understood. Here, we investigate the interaction and binding affinities of αβI, αβIIb, αβIII and αβIVa tubulin isotypes with CA-4, employing molecular modeling approaches. Sequence analysis shows that variations in residue composition at the CA-4 binding pocket of βI, βIII and βIVa tubulin isotypes when compared to template βIIb isotype. Molecular docking result shows that the CA-4 prefers 'cis' conformation in all αβ-tubulin isotypes. Molecular dynamics simulation reveal role of H7 helix, T7 loop and H8 helix of β-tubulin in lower binding affinity of αβI and αβIII isotypes for CA-4. The order of binding energy for CA-4 is αβIIb > αβIVa > αβI > αβIII. This suggest that drug resistance is induced in human lung carcinoma cells by altering the expression of β-tubulin isotypes namely βI and βIII which show lowest binding affinities. Our present study can help in designing potential CA-4 analogs against drug-resistant cancer cells showing altered expression of tubulin isotypes. Abbreviations: CA-4 combretastatin-A4 MD molecular dynamics RMSD root mean square deviation DSSP dictionary of secondary structure of proteins VMD visual molecular dynamics Communicated by Ramaswamy H. Sarma.