Total RNA was extracted from HepG2.2.15 using TRIzol (15596018, Invitrogen), RNA purity and concentration were then examined using NanoDrop 2000, RNA integrity and quantity were measured using the Agilent 2100 system. Sequencing libraries were generated using NEBNext Ultra RNA Library Prep Kit for Illumina (NEB, USA) following manufacturer's recommendations and index codes were added to attribute sequences to each sample.