In this study, we investigated the role of leptin and its receptors (Ob-R) in profibrogenic responses in the liver using Zucker (fa/fa) rats, a natural occurring Ob-R-deficient animal.

Male Zucker (fa/fa) rats and their lean (+/?) littermates were given intraperitoneal injections of thioacetamide (TAA) (200 mg/kg body wt, 3 times/wk) for 4–8 weeks, and progression of hepatic fibrosis was evaluated. In vitro transactivation of hepatic stellate cells (HSCs) isolated from Zucker rats was evaluated by Western blotting and immunocytochemistry for α-smooth muscle actin and type I collagen. Further, a long-form Ob-R (Ob-Rb) in sinusoidal endothelial cells (SECs) and Kupffer cells was identified by reverse-transcription polymerase chain reaction. Moreover, transforming growth factor (TGF)-β1. messenger RNA in LSE cells, a human SEC-derived cell line, was measured by Northern blotting.

Although the normal liver does not produce leptin, activated HSCs produced leptin in vivo during fibrogenesis caused by TAA. In Zucker rats, TAA-induced hepatic fibrosis was prevented almost completely, whereas induction of TGF-β1 and activation of HSCs were abolished. It is less likely, however, that leptin plays an essential role in the activation of HSCs as a strong autocrine regulator, because HSCs isolated from Zucker rats undergo normal transactivation process in vitro. In contrast, SECs and Kupffer cells contain Ob-Rb, through which leptin up-regulates the expression of matrix remodeling genes including TGF-β1.

Collectively, these findings indicated that leptin and its functional receptors (Ob-Rb) play a pivotal role in profibrogenic responses in the liver.