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MicroRNAs are small highly conserved noncoding RNAs that are widely expressed in multicellular organisms and participate in the regulation of various cellular processes including autophagy and viral replication. Evidently, microRNAs are able to modulate host gene expression and thereby inhibit or enhance hepatitis B virus (HBV) replication. The miR-99 family members are highly expressed in the liver. Interestingly, the plasma levels of miR-99 family in the peripheral blood correspond with HBV DNA loads. Thus, we asked whether the miR-99 family regulated HBV replication and analyzed the underlying molecular mechanism. Compared with primary hepatocytes, miR-99 family expression was downregulated in hepatoma cells. Transfection of miR-99a, miR-99b, and miR-100 markedly increased HBV replication, progeny secretion, and antigen expression in hepatoma cells. However, miR-99 family had no effect on HBV transcription and HBV promoter activities, suggesting that they regulate HBV replication at posttranscriptional steps. Consistent with bioinformatic analysis and recent reports, ectopic expression of miR-99 family attenuated IGF-1R/Akt/mTOR pathway signaling and repressed insulin-stimulated activation in hepatoma cells. Moreover, the experimental data demonstrated that the miR-99 family promoted autophagy through mTOR/ULK1 signaling and thereby enhanced HBV replication. In conclusion, the miR-99 family promotes HBV replication posttranscriptionally through IGF-1R/PI3K/Akt/mTOR/ULK1 signaling-induced autophagy.

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