MiR-20a and miR-20b negatively regulate autophagy by targeting RB1CC1/FIP200 in breast cancer cells.

Aims: RB1CC1/FIP200 was essential for autophagosome formation. Therefore, RB1CC1/FIP200 cellular levels are critical for the activation of the autophagy pathways. Following the screen of miRNAs affecting RB1CC1/FIP200 level and rapamycin-induced autophagy, we discovered miR-20a and miR-20b could regulate autophagy by targeting RB1CC1/FIP200.

Main Methods: Inhibitory effect of miR-20a and 20b on basal and rapamycin-stimulated autophagy was demonstrated using various autophagic tests including GFP-LC3 puncta analysis, LC3II/LC3I gel shift and TEM observation.

Key Findings: We discovered RB1CC1/FIP200 as cellular targets of miR-20a and miR-20b. Upon miR-20a and miR-20b overexpression, both mRNA and protein levels of RB1CC1/FIP200 decreased. miR-20a and miR-20b target sequences present in the 3' UTR of RB1CC1/FIP200 mRNAs and introduction of mutations abolished the miR-20a and miR-20b responsiveness. In MCF7 and MDA-MB-231 breast cancer cells, miR-20a and miR-20b over-expression attenuated basal and rapamycin-induced autophagy; while suppression of miR-20a or miR-20b by specific antagomir showed normal rapamycin-induced autophagic activity.

Significance: To our knowledge, this is the first study showing the significance of miR-20a and miR-20b regulating autophagy by targeting RB1CC1/FIP200.