Regulation of β-tubulin isotypes by micro-RNA 100 in MCF7 breast cancer cells.

Antimitotic drugs are key components of combination chemotherapy protocols for hematological and solid tumors. The taxanes (e.g., paclitaxel) bind to the β subunit of the tubulin heterodimer and reduce microtubule dynamics, leading to cell cycle arrest in G2/M. The effectiveness of combination chemotherapy is limited by tumor resistance to drugs initially or as a cumulative effect after several cycles of treatment. Because changes in the drug receptor may be linked to drug resistance, we investigated changes in β-tubulin isotypes in response to paclitaxel treatment in MCF7 breast cancer cells. We found that paclitaxel induced a 2-3 fold increase in mRNA for β-tubulin IIA and III genes, TUBB2A, and TUBB3. β-Tubulin class III protein increased; however, β-tubulin class II protein was not detected in these cells. Paclitaxel treatment following pretreatment with actinomycin D showed that the change in β-tubulin class III was due to increased transcription and linked to G2/M arrest. The increase in β-tubulin IIA mRNA was due to both enhanced stability and increased transcription, unassociated with G2/M arrest. We used micro-RNA superarrays to look for changes in families of micro-RNAs that might be linked to drug-induced changes in β-tubulin isotype mRNA and/or protein. We found a significant decrease in the tumor suppressor, miR-100, in MCF7 cells in response to paclitaxel treatment. Transfection of MCF7 cells with miR-100 significantly reduced β-tubulin I, IIA, IIB and V mRNA and prevented paclitaxel-induced increases in β-tubulin isotypes. This is the first report of a micro-RNA that regulates these specific β-tubulin isotype mRNAs.