Does hepcidin limit intestinal iron absorption and iron release from macrophages?

Hepcidin is established as the master regulator of systemic iron homeostasis. Its primary role is to serve as the negative feedback hormone that controls iron entry into the plasma, thereby maintaining iron concentrations within a narrow physiological range.

Mechanistic basis of iron regulation

Hepcidin controls iron flux by targeting ferroportin, the only known cellular iron exporter in mammals. This mechanism involves a precise molecular interaction:

• Molecular degradation: Hepcidin binds directly to ferroportin on the cell surface, inducing its internalization and subsequent lysosomal degradation.
• Intestinal absorption: In the duodenum, hepcidin degrades ferroportin on the basolateral membrane of enterocytes. This prevents dietary iron from being absorbed into the bloodstream, effectively trapping it within the intestinal cells until they are sloughed off.
• Macrophage recycling: Most daily iron requirements are met by macrophages recycling iron from senescent red blood cells. Hepcidin inhibits this release by degrading macrophage ferroportin, sequestering iron within the reticuloendothelial system.
• Signaling pathways: Hepcidin expression is regulated by several inputs. It is upregulated by iron through the BMP6-SMAD pathway and by inflammation via IL-6/STAT3 signaling. Conversely, it is suppressed by the hormone erythroferrone during periods of increased red blood cell production to maximize iron availability.

Consequences of inadequate hepcidin activity

When hepcidin production is inappropriately low or its activity is impaired, the body loses its ability to "turn off" iron entry into the plasma.

• Hyperferremia: Low hepcidin levels allow ferroportin to remain stable on cell membranes. This results in continuous, unregulated intestinal iron absorption and excessive iron release from macrophages, leading to elevated serum iron levels.
• Transferrin saturation (TSAT): As serum iron rises, it occupies more binding sites on transferrin, the primary iron transport protein. In cases of hepcidin deficiency (such as hereditary hemochromatosis), TSAT typically exceeds 45% and can often reach 80–100%.
• Clinical evidence: Research in hereditary hemochromatosis patients confirms that even small reductions in hepcidin relative to iron stores lead to progressive iron loading. Clinical trials of hepcidin mimetics have demonstrated that restoring hepcidin activity successfully reduces both serum iron and TSAT by rapidly internalizing ferroportin.

Bottom line

Hepcidin is the definitive hormone controlling iron entry into the blood. Inadequate hepcidin activity causes unregulated iron absorption and macrophage release via ferroportin, leading directly to elevated serum iron and high transferrin saturation.