Identification of a Mechanism by Which Lens Epithelial Cells Limit Accumulation of Overexpressed Ferritin H-chain

Abstract

The primary cultures of canine lens epithelial cells were transiently transfected with cDNAs for dog ferritin H-or L-chains in order to study differential expression of these chains. By using chain-specific antibodies, we determined that at 48 h after transfection overexpression of L-chain was much higher (9-fold over control) than that of H-chain (1.7-fold). We discovered that differentially transfected cells secrete overexpressed chains as homopolymeric ferritin into the media. Fortyeight hours after transfection accumulation of H-ferritin in the media was much higher (3-fold) than that of L-ferritin. This resulted in lowering of the concentration of H-chain in the cytosol. Co-transfection of cells with both H-and L-chain cDNAs increased the intracellular levels of H-chain and eliminated secretion of Hferritin to the media. We concluded that lens epithelial cells differentially regulate concentration of both ferritin chains in the cytosol. The overexpressed L-chain accumulated in the cytosol as predominantly homopolymeric L-ferritin. This is in contrast to H-chain, which is removed to the media unless there is an L-chain available to form heteropolymeric ferritin. These data indicate that the inability of cells to more strictly control cytosolic levels of L-chain may augment its accumulation in lenses of humans with hereditary hyperferritinemia cataract syndrome, which is caused by overexpression of L-chain due to mutation in the regulatory element in the untranslated region of the mRNA of the chain.Ferritins are the highly conserved iron storage proteins found in species from bacteria to mammals. Mammalian ferritins, located predominantly in cytoplasm, are heteropolymers of 24 subunits of two types, heavy (H 1 ; 21 kDa) and light (L; 19 kDa). Despite high (ϳ50%) sequence identity and similar three-dimensional structures, the subunits are genetically different and each has a distinct and complementary role in storing iron. The H subunit has a ferroxidase center responsible for uptake and oxidation of Fe 2ϩ into ferric ions. Translocation of these ions into the core of the ferritin shell and their mineralization for long term storage is facilitated by the L subunit. The intracellular level of ferritin correlates with the size of the pool of intracellular iron, which plays the main role in the regulation of ferritin expression. The iron-controlled expression of both chains is primarily translational and involves binding of iron-regulatory proteins IRP1 and IRP2 (iron sensors) to iron-response element (IRE), a stem-loop structure present in the 5Ј-untranslated region of the mRNA (1). The expression of H-chain gene is also regulated transcriptionally by cytokines (2), hormones, oncogenes (3), and inducers of cell differentiation (4).The H and L subunit ratio of mammalian ferritins varies and is highly tissue-specific; however, mechanisms by which cells maintain the chain-specific ratio are not fully understood. Although both chains complement each other in the process of sequestering and storing iron, H-chain has ...

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