Fluctuations of Intracellular Iron Modulate Elastin Production

Abstract

Production of insoluble elastin, the major component of elastic fibers, can be modulated by numerous intrinsic and exogenous factors. Because patients with hemolytic disorders characterized with fluctuations in iron concentration demonstrate defective elastic fibers, we speculated that iron might also modulate elastogenesis. In the present report we demonstrate that treatment of cultured human skin fibroblasts with low concentration of iron 2-20 M (ferric ammonium citrate) induced a significant increase in the synthesis of tropoelastin and deposition of insoluble elastin. Northern blot and realtime reverse transcription-PCR analysis revealed that treatment with 20 M iron led to an increase of ϳ3-fold in elastin mRNA levels. Because treatment with an intracellular iron chelator, desferrioxamine, caused a significant decrease in elastin mRNA level and consequent inhibition of elastin deposition, we conclude that iron facilitates elastin gene expression. Our experimental evidence also demonstrates the existence of an opposite effect, in which higher, but not cytotoxic concentrations of iron (100 -400 M) induced the production of intracellular reactive oxygen species that coincided with a significant decrease in elastin message stability and the disappearance of iron-dependent stimulatory effect on elastogenesis. This stimulatory elastogenic effect was reversed, however, in cultures simultaneously treated with high iron concentration (200 M) and the intracellular hydroxyl radical scavenger, dimethylthiourea. Thus, presented data, for the first time, demonstrate the existence of two opposite iron-dependent mechanisms that may affect the steady state of elastin message. We speculate that extreme fluctuations in intracellular iron levels result in impaired elastic fiber production as observed in hemolytic diseases.Mature elastic fibers and laminae provide extensibility and elastic recoil to vascular walls and ligaments and form a connective tissue framework of lungs, elastic cartilage, and skin (1, 2). They are complex structures made of polymeric (insoluble) elastin and 12-nm microfibrils that consist of several glycoproteins, e.g. fibrillins, fibulins, and microfibril-associated glycoproteins (3-6). Elastic fiber formation (elastogenesis) is a complex process involving several intracellular and extracellular events. Cells (fibroblasts, endothelial cells, chondroblasts, or vascular smooth muscle cells) must first synthesize and secrete numerous glycoproteins to form a microfibrillar scaffold upon which tropoelastin, the precursor peptides, are properly assembled and covalently cross-linked by lysyl oxidase into a resilient polymer, insoluble elastin (7-10). Production of elastin reaches its highest levels in the third trimester of the fetal life and steadily decreases during early postnatal development (11,12). In undisturbed tissues, elastic fibers may last over the entire human lifespan (13,14).The net deposition of elastin appears to be controlled on both the transcriptional level (tropoelastin mRNA message express...

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