Peroxisome Proliferator-Activated Receptor α Regulates a MicroRNA-Mediated Signaling Cascade Responsible for Hepatocellular Proliferation

ABSTRACT Activation of peroxisome proliferator-activated receptor α (PPARα) leads to hepatocellular proliferation and liver carcinomas. The early events mediating these effects are unknown. A novel mechanism by which PPARα regulates gene expression and hepatocellular proliferation was uncovered. MicroRNA (miRNA) expression profiling demonstrated that activated PPARα was a major regulator of hepatic miRNA expression. Of particular interest, let-7C, an miRNA important in cell growth, was inhibited following 4-h treatment and 2-week and 11-month sustained treatment with the potent PPARα agonist Wy-14,643 in wild-type mice. let-7C was shown to target c-myc via direct interaction with the 3′ untranslated region of c-myc. The PPARα-mediated induction of c-myc via let-7C subsequently increased expression of the oncogenic mir-17-92 cluster; these events did not occur in Pparα-null mice. Overexpression of let-7C decreased c-myc and mir-17 and suppressed the growth of Hepa-1 cells. Furthermore, using the human PPARα-expressing mouse model, which is responsive to Wy-14,643 effects on β-oxidation and serum triglycerides but resistant to hepatocellular proliferation and tumorigenesis, we demonstrated a critical role for let-7C in liver oncogenesis. Wy-14,643 treatment did not inhibit let-7C or induce c-myc and mir-17 expression. These observations reveal a let-7C signaling cascade critical for PPARα agonist-induced liver proliferation and tumorigenesis.