Upregulation of hepatic melanocortin 4 receptor during rat liver regeneration

Abstract

Background: Melanocortin 4 receptor (MC4R) is predominantly recognized to mediate energy metabolism and anti-inflammation through the central nervous system. However, the expression of MC4R has recently been identified in rat liver and was shown to be upregulated during acute phase response. This study aims to investigate potential roles of MC4R in liver regeneration.

Materials and methods: Rat partial hepatectomy (PH) was performed, and MC4R expression was analyzed at different time points after resection. Sham-operated animals (SH) served as controls. In vitro primary hepatocytes (HCs) were isolated from normal rat liver and stimulated with α-melanocyte-stimulating hormone (MC4R agonist). Real-time polymerase chain reaction, Western blot, and immunofluorescence staining were applied to detect gene expression.

Results: Up to 8 h after PH, hepatic messenger RNA of proinflammatory cytokines interleukin 6 and tumor necrosis factor α reached peak values. Between 8 and 72 h after PH, rat liver regeneration was extremely active as assessed by the regeneration indices labeled by Ki-67. Immunofluorescence staining indicated that MC4R was mostly expressed in hepatocyte nuclear factor 4(+) cells (HCs) and upregulated during rat liver regeneration. Concurrently, the expression of hepatic MC4R protein was significantly higher in PH than in SH animals, and phosphorylated extracellular signal-regulated kinase 1/2 was remarkably increased in PH compared with SH animals (P < 0.05, respectively). In vitro experiments showed that the expression of proliferating cell nuclear antigen was significantly higher in HCs treated with α-melanocyte-stimulating hormone than in control HCs, which was correlated to the increase of phosphorylated extracellular signal-regulated kinase 1/2 and reduction of phosphorylated signal transducer and activator of transcription 3 (P < 0.05, respectively).

Conclusions: MC4R is predominantly expressed in HCs and upregulated during rat liver regeneration. In vitro stimulation of HC MC4R is associated with a modulation of extracellular signal-regulated kinase and signal transducer and activator of transcription 3 pathways regulating liver regeneration.

Keywords: Liver regeneration; MC4R; pERK; pSTAT3.