Deletion of organic cation transporter Oct3 promotes hepatic fibrosis via upregulation of TGFβ

Organic cation transporters (OCT) are responsible for the intracellular uptake and detoxification of a broad spectrum of endogenous and exogenous substrates. OCTs are downregulated in cholestasis, fibrosis, and hepatocellular carcinoma, but the underlying molecular mechanisms and downstream effects of OCT deletion are unknown. Oct3-knockout (Oct3^-/-; FVB.Slc22a3^tm10pb) and wild-type (WT; FVB) mice were subject to escalating doses of carbon tetrachloride (CCl₄) or thioacetamide (TAA) for 6 wk to induce advanced parenchymal liver fibrosis. Secondary biliary fibrosis was generated by bile duct ligation. Liver fibrosis was assessed by hydroxyproline determination, quantitative Sirius red morphometry, and quantitative real-time PCR for fibrosis and inflammation-related genes. Ductular reaction was assessed by bile duct count per field of view in hematoxylin and eosin staining. General gene expression analyses were performed in liver tissue from untreated Oct3^-/- and WT mice. Finally, primary murine hepatocytes were treated with the nonselective OCT inhibitor quinine, and transforming growth factor-β1 (Tgfβ1) protein expression was quantified by quantitative real-time PCR and Western blot. Oct3^-/- mice developed significantly more fibrosis after bile duct ligation and CCl₄ treatment compared with WT mice. Ductular reaction was enhanced in the long-term model. Concomitantly, Oct1 mRNA expression was downregulated during cholestatic and chemically (TAA and CCl₄) induced fibrogenesis. The downregulation of Oct1 mRNA in fibrotic liver tissue reversed within 4 wk after TAA cessation. Gene expression analysis by next-generation sequencing revealed an enrichment of Tgfβ1 target genes in Oct3^-/- mice. Tgfβ1 mRNA expression was significantly upregulated after chemically induced fibrosis (P < 0.001) in Oct3^-/- compared with WT mice. Accordingly, in primary murine hepatocytes functional inhibition of OCT led to an upregulation of Tgfβ1 mRNA expression. Loss of Oct3 promotes fibrogenesis by affecting Tgfβ-mediated homeostasis in mice with chronic biliary and parenchymal liver damage and fibrosis.NEW & NOTEWORTHY We show for the first time that organic cation transporter 3 (Oct3) is not only downregulated in fibrosis but loss of Oct3 also leads to an upregulation of transforming growth factor-β contributing to fibrosis progression.