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The hepatic interleukin-6 receptor. Down-regulation of the interleukin-6 binding subunit (gp80) by its ligand
D Zohlnhöfer1, L Graeve, S Rose-John
1Institut für Biochemie RWTH Aachen, Germany.
Insights
The interleukin-6 (IL6) binding protein, gp80, is crucial for IL6 receptor internalization and degradation in HepG2 cells. This process limits IL6 signaling and suggests gp80 is not recycled, protecting against overstimulation.
Area of Science:
- Cell Biology
- Immunology
- Molecular Biology
Background:
- Interleukin-6 (IL6) mediates its effects through a receptor complex involving an IL6-binding protein (gp80) and a signal-transducing polypeptide (gp130).
- Understanding the IL6 receptor's (IL6R) dynamics is key to comprehending IL6 signaling regulation.
Purpose of the Study:
- To investigate the role of gp80 in the binding, internalization, and down-regulation of the hepatic IL6 receptor in human hepatoma cells (HepG2).
- To elucidate the contribution of gp80 to IL6 receptor trafficking and ligand-induced regulation.
Main Methods:
- Utilized transfected HepG2 cells overexpressing gp80 and parental cells for comparative analysis.
- Quantified IL6 binding affinity (Kd) and assessed IL6 internalization and degradation rates.
- Investigated the reappearance of IL6-binding sites and the effect of cycloheximide on gp80 recycling.
Main Results:
- gp80 mediates low-affinity binding of IL6 (Kd = 500 pM) and is rate-limiting for IL6 internalization and degradation.
- Internalization of IL6 led to rapid down-regulation of cell surface IL6-binding sites (t1/2 ≈ 15-30 min).
- Over 80% of internalized IL6 was degraded, and gp80 reappearance required >8 hours and was cycloheximide-sensitive, indicating no gp80 recycling.
Conclusions:
- gp80 is essential for IL6 binding, internalization, and degradation, controlling IL6 receptor dynamics.
- The lack of gp80 recycling suggests a mechanism for sustained regulation of IL6 signaling.
- Ligand-induced down-regulation of the hepatic IL6 receptor via gp80 may serve as a protective mechanism against IL6 overstimulation.
Abstract:
Interleukin-6 (IL6) exerts its action via a cell surface receptor composed of an 80 kDa IL6-binding protein (gp80) and a 130 kDa polypeptide involved in signal transduction (gp130). We studied the role of gp80 in binding, internalization and down-regulation of the hepatic IL6-receptor (IL6R) by its ligand in human hepatoma cells (HepG2). Comparison of transfected HepG2 cells overexpressing gp80 with parental cells indicate that gp80 is responsible for low affinity binding (Kd = 500 pM) of IL6. Furthermore, gp80 is rate-limiting in internalization and degradation of IL6. Internalization resulted in a rapid down-regulation (t1/2 approximately 15-30 min) of IL6-binding sites at the cell surface. More than 80% of the internalized [125I]rhIL6 was degraded. The reappearance of IL6-binding sites at the cell surface required greater than 8 h and was sensitive to cycloheximide, suggesting that gp80 is not recycled after internalization. The down-regulation of the hepatic IL6R by its ligand might play an important role as a protection against overstimulation.