Iron is an essential
trace mineral that plays a number of important physiological roles in humans, including
oxygen transport, energy metabolism, and
neurotransmitter synthesis.
Iron absorption by the proximal small bowel is a critical checkpoint in the maintenance of whole-body
iron levels since, unlike most other essential nutrients, no regulated excretory systems exist for
iron in humans. Maintaining proper
iron levels is critical to avoid the adverse physiological consequences of either low or high tissue
iron concentrations, as commonly occurs in
iron-deficiency anemia and hereditary
hemochromatosis, respectively. Exquisite regulatory mechanisms have thus evolved to modulate how much
iron is acquired from the diet. Systemic sensing of
iron levels is accomplished by a network of molecules that regulate transcription of the HAMP gene in hepatocytes, thus modulating levels of the serum-borne,
iron-regulatory
hormone hepcidin.
Hepcidin decreases intestinal
iron absorption by binding to the
iron exporter
ferroportin 1 on the basolateral surface of duodenal enterocytes, causing its internalization and degradation. Mucosal regulation of
iron transport also occurs during low-
iron states, via transcriptional (by
hypoxia-inducible factor 2α) and posttranscriptional (by the
iron-sensing
iron-regulatory protein/
iron-responsive
element system) mechanisms. Recent studies demonstrated that these regulatory loops function in tandem to control expression or activity of key modulators of
iron homeostasis. In health, body
iron levels are maintained at appropriate levels; however, in several inherited disorders and in other pathophysiological states,
iron sensing is perturbed and intestinal
iron absorption is dysregulated. The
iron-related phenotypes of these diseases exemplify the necessity of precisely regulating
iron absorption to meet body demands.