Regulated secretion of glycosylated human ferritin from hepatocytes

Abstract

Serum ferritin has been used widely in clinical medicine chiefly as an indicator of iron stores and inflammation. Circulating ferritin also can have paracrine effects. Despite the clinical significance of serum ferritin, its secretion remains an enigma. The consensus view is that serum ferritin arises from tissue ferritinsprincipally ferritin light-which can be glycosylated. Ferritin heavy and light chains are cytosolic proteins that form cages of 24 subunits to store intracellular iron. We show that ferritin light is secreted when its expression is increased in stable, transfected HepG2 cells or adenovirus-infected HepG2 cells. Export occurs through the classical secretory pathway and some chains are N-glycosylated. Ferritins do not need to form cages prior to secretion. Secretion is blocked specifically, effectively, and rapidly by a factor in serum. The timing of this inhibition of ferritin secretion suggests that normally cytosolic ferritin L is targeted to the secretory pathway during translation despite the absence of a conventional signal sequence. Thus, secretion of glycosylated and unglycosylated ferritin is a regulated and not a stochastic process. IntroductionSerum ferritin has wide clinical utility primarily as an indicator of intracellular iron stores 1-3 and as an acute-phase reactant useful for the diagnosis and follow-up of adult Still disease 4 and systemic lupus erythematosis. 5,6 Elevated serum ferritin also has been used as a harbinger of tissue masses including ovarian carcinoma, 7 Hodgkin disease, 8 renal cell carcinoma, 9 and melanoma, 10 and preterm deliveries. 11,12 Ferritin has been linked to insulin resistance [13][14][15] and, controversially, to atherosclerosis. 16 Thus, serum ferritin has been used for the diagnosis and follow-up of numerous disorders, although it is most strongly linked to intracellular iron status and adult Still disease.Despite the frequent use in clinical medicine of measurements of serum ferritin, most aspects of its secretion remain unknown. An understanding of the mechanism and regulation of the secretion of ferritin would inform the use of this commonly used clinical test. Moreover, the trafficking of normally cytosolic ferritins is relevant to the physiology of iron homeostasis and the pathophysiology of many disorders in which intracellular ferritin could play a role. [17][18][19][20] There are 2 forms of ferritin that function together to store iron in cells. 2 In humans, ferritin light (L) has a molecular mass of 19 kDa and consists of 175 amino acids. Human ferritin L is 53% identical to ferritin heavy (H), which contains 183 amino acids and is 21 kDa. 21 Unlike ferritin L, ferritin H contains a ferroxidase site. Cages that store iron atoms are formed from 24 ferritin monomers in varying ratios of H/L. [21][22][23] Both ferritin H and L subunits are necessary for efficient iron storage; although ferritin L lacks a ferroxidase site, the protein nucleates iron mineralization. 21,24 Ferritin is secreted from liver 25,26 and lymphoid cells. 27 Some studies...

Serum ferritin is derived primarily from macrophages through a nonclassical secretory pathway

“…35 Further evidence for a lysosomal pathway of ferritin secretion comes from our finding that ferritin is clearly visible in BMDM lysosomes and autophagolysosomes ( Figure 6B-C). Indications that secretion of ferritin may be regulated are further supported by the finding that an unidentified serum factor inhibits ferritin secretion, 12 but further investigation is required to elucidate this regulatory mechanism. The inhibition of lysosomal processing of ferritin by Chloroquine, which impairs its secretion ( Figure 6A), further supports the lysosomal secretory pathway suggested for serum ferritin.…”

“…Further evidence for the ER-Golgi route for ferritin secretion was reported from hepatocyte cell-lines. 11,12 However, hepatocytes may not be the main source of serum ferritin (see the next section).…”

“…10 Indeed, it was shown in hepatocyte cell-models that inhibition of ER-Golgi trafficking abolished secretion of ferritin. 11,12 However, it was never shown that serum ferritin was N-glycosylated, and ferritin did not enter the ER even when translated on rough ER. 13,14 Thus, the mechanism of ferritin secretion remains unclear.…”

Ferroportin1 deficiency in mouse macrophages impairs iron homeostasis and inflammatory responses

“…There are also some data to suggest that efflux of the iron storage protein ferritin is a potential pathway for the export of macrophage iron. [34][35][36] Although we did not directly examine ferritin release in this study, the finding that ferritin levels did not decline over time in BMDMs from Fpn1 LysM/LysM mice that had been preloaded with iron ( Figure 3C) suggests that ferritin release was not playing a significant role. In vivo, Fpn1 LysM/LysM mice had significantly higher serum ferritin levels than that of control mice, suggesting a more complex situation that probably reflects ferritin release from a range of cell types.…”

EVALUATION OF SERUM FERRITIN AND SERUM IRON IN FREE-RANGING BLACK RHINOCEROS (DICEROS BICORNIS) AS A TOOL TO UNDERSTAND FACTORS AFFECTING IRON-OVERLOAD DISORDER

“…In hepatocytes there is direct evidence of regulated secretion of glycosylated ferritin (97). Secreted ferritin may originate from phagocytic cells that degrade haemoglobin (Figure 1).…”