Intracellular iron metabolism

Secreted ferritin subunits are of two kinds in insects molecular cloning of cDNAs encoding two major subunits of secreted ferritin from Calpodes ethlius

In insects, holoferritin is easily visible in the vacuolar system of tissues that filter the hemolymph and, at least in Lepidoptera, is abundant in the hemolymph. Sequences reported for insect secreted ferritins from Lepidoptera and Diptera have high sequence diversity. We examined the nature of this diversity for the first time by analyzing sequences of cDNAs encoding two ferritin subunits from one species, Calpodes ethlius (Lepidoptera, Hesperiidae). We found that insect secreted ferritin subunits are of two types with little resemblance to each other. Ferritin was isolated from iron loaded hemolymph of C. ethlius fifth instar larvae by differential centrifugation. The N-terminal amino acid sequences for the nonglycosylated subunit with Mr 24,000 (S) and the largest glycosylated subunit with Mr 31,000 (G) were determined. The N-termini of the two subunits were different and were used to construct degenerate PCR primers. The same cDNA products were amplified from cDNA libraries from the midgut which secretes holoferritin and from the fat body which secretes iron-poor apoferritin. The G subunit most closely resembles the glycosylated ferritin subunit from Manduca sexta and the S subunit resembles the Drosophila small subunit. The S and G subunits from Calpodes were dissimilar and distinct from the cytosolic ferritins of vertebrates and invertebrates. Additional sequences were obtained by 5' and 3' RACE from separate fat body and midgut RACE libraries. cDNAs encoding both subunits had a consensus iron responsive element (IRE) in a conserved cap-distal location of their 5' UTR. An integrin-binding RGD motif found in the G subunit and conserved in Manduca may facilitate iron uptake through a calreticulin (mobilferrin)/integrin pathway. Calpodes and other insect ferritins have conserved cysteine residues to which fatty acids can be linked. Dynamic acylation of ferritin may slow but not prevent its passage out of the ER.

Unidirectional upregulation of the synthesis of the major iron proteins, transferrin-receptor and ferritin, in HepG2 cells by the acute-phase protein alpha1-antitrypsin

BACKGROUND/AIMS

We have previously shown that the hepatic acute-phase protein alpha1-antitrypsin (alpha1-AT) is an important mediator of changes in iron metabolism in the course of anaemia of chronic disease. Alpha1-AT profoundly reduces growth of erythroid cells by interfering with transferrin-mediated iron uptake. In the present work we investigate the effects of alpha1-AT on hepatic iron metabolism, as the liver plays a central role in body iron metabolism and in metabolic changes during acute-phase response.

METHODS

The human hepatoma cell line Hep G2 was cultured in RPMI 1640+10% FCS. The effect of alpha1-AT on transferrin-receptor binding was investigated in equilibrium binding assays with 125I-transferrin. Expression of transferrin receptor was determined by saturation experiments and intracellular ferritin was measured in cell lysates after incubating cells either alone or with alpha1-AT. To determine iron regulatory protein binding activity to iron responsive elements we used gel retardation assays and Northern blot analysis was carried out to investigate transferrin receptor and ferritin mRNA expression.

RESULTS

Alpha1-AT completely prevented transferrin from binding to its receptor and internalization of the transferrin-transferrin receptor complex on HepG2. In addition, alpha1-AT caused a distinct increase in iron regulatory protein binding activity to iron responsive elements, which is characteristic of iron deprivation. Normally, the synthesis of transferrin receptor and ferritin is regulated bidirectionally, but alpha1-AT promoted a unidirectional regulation. Alpha1-AT increased the synthesis of both transferrin receptor and ferritin and, moreover, increased cellular amounts of transferrin receptor mRNA and ferritin H-chain mRNA.

CONCLUSIONS

The effect of alpha1-AT on transferrin receptor synthesis appears to be mediated via activation of iron responsive element binding affinity of iron regulatory protein leading to an increased stability of transferrin receptor mRNA. Changes in ferritin, however, may be related to a transcriptionally mediated, iron-independent pathway which overrides the influence of activated iron regulatory protein. These specific changes in iron metabolism are the very ones seen in the course of anaemia of chronic disease. Our results emphasize the central role of alpha1-AT as a mediator of altered iron metabolism, characteristic of anaemia of chronic disease, not only with respect to erythroid cells but also with respect to liver cells.