Another miRNA significantly altered in response to 2/3 PH was miR-378 (Fig. 2A). In contrast to miR-21, miR-378 expression declined after 2/3 PH, suggesting that it might inhibit a proliferation-promoting gene (Fig. 4A). Loss of miR-378 expression in livers of Dgcr8del/fl, Alb-Cre+/− mice

lacking oval cells indicated that it functions predominantly in hepatocytes in the liver (Fig. 4B). Among 64 genes predicted as targets of miR-378 in both mice and humans, four genes were previously reported to have a positive effect on cell proliferation (Supporting Information Table 3). Intriguingly, one of these genes, Odc1, encodes a polyamine-synthesizing enzyme that is needed for efficient and timely DNA synthesis in liver regeneration.28 Furthermore, Odc1 had find more the highest score and free energy of the predicted conserved miR-378 target genes with established proliferation-promoting function (Supporting Information Fig. 4B, Supporting Information Table 3). Using the

HSP inhibitor experimental strategy described above, we found that miR-378 inhibits Odc1 by direct targeting of a complementary sequence in its 3′UTR (Fig. 4C). These results suggest that efficient liver regeneration may involve release of Odc1 from repression by miR-378. Although miRNAs are known to regulate cell proliferation, little information exists on the miRNAs and their target genes involved in regeneration of the liver or other organs. We investigated this question by generating mice with hepatocyte-specific inactivation of DGCR8, an essential component of the microprocessor complex. DGCR8 anchors the primary miRNA transcript for cleavage by Drosha. Thus, DGCR8 acts upstream of Dicer and its deficiency leads to disruption of processing of miRNAs, not other small RNAs.17 In line with recent findings of impaired proliferation in DGCR8-deficient

mouse embryonic stem cells,29 we found that initiation of DNA synthesis was delayed in DGCR8-deficient hepatocytes after 2/3 PH. This finding suggested that miRNAs regulate hepatocyte G1 to S phase progression during liver regeneration. To identify miRNAs regulating the regenerative capabilities of hepatocytes, we screened for miRNA expression changes in livers of selleck compound adult wildtype mice after 2/3 PH. Because the miRNAs reported as promoters of proliferation in mouse embryonic stem cells are not expressed in hepatocytes, it was not surprising that none of these miRNAs was induced by 2/3 PH. Instead, we specifically found increased levels of miR-21, an established promoter of proliferation that is expressed at high levels in many types of cancer.20 Prompted by stringent target gene prediction we found that Btg2 is a direct target of miR-21. Moreover, our data suggest that increased miR-21 expression serves to antagonize the cell cycle inhibitor Btg2 during liver regeneration.