Evasion of apoptosis is a known feature of cancer cells. One mechanism of deregulating the apoptotic pathway is through overexpression of antiapoptotic BCL2 family members. ABT-263 (navitoclax) is a first-in-class BCL2 family inhibitor that restores the ability of cancer cells to undergo apoptosis. However, many cancer cells are resistant to ABT-263 due to high levels of a BCL2 family member, MCL1, which is not targeted by the drug. MCL1 expression is regulated transcriptionally, translationally, and through proteasome-mediated degradation. Recently, MCL1 expression was shown to be affected by microRNAs (miRNA). To identify miRNAs that modulate the sensitivity of cancer cells to ABT-263, we screened a library of 810 human miRNA mimics in HCT-116 cells in the presence of ABT-263. The screen revealed 19 miRNAs that sensitize HCT-116 cells to ABT-263. Fifteen of these miRNAs were also shown to sensitize CHL1 melanoma cells to the same agent. We further evaluated 12 of the strongest sensitizers in these cell lines. We found that these sensitizers induced apoptosis only in the presence of ABT-263. In addition, whereas all 12 of these miRNAs reduced MCL1 protein expression, only 10 of them targeted MCL1 through direct binding to the 3'-untranslated region of the gene, raising the possibility that other resistance regulators of MCL1 expression may be identified using our method. Finally, because sensitizing miRNA expression is lower in tumors compared with normal tissues, our data can facilitate the design of miRNA replacement therapies to increase sensitivity to BCL2 antagonists.
©2010 AACR.