Dysregulation of the sensory and regulatory pathways controlling cellular iron metabolism in unilateral obstructive nephropathy

Chronic kidney disease involves disturbances in iron metabolism including anemia caused by insufficient erythropoietin (EPO) production. However, underlying mechanisms responsible for the dysregulation of cellular iron metabolism are incompletely defined. Using the unilateral ureteral obstruction (UUO) model in Irp1+/+ and Irp1-/- mice, we asked if iron regulatory proteins (IRPs), the central regulators of cellular iron metabolism and suppressors of EPO production, contribute to the etiology of anemia in kidney failure. We identified a significant reduction in IRP protein level and RNA binding activity that associates with a loss of the iron uptake protein transferrin receptor 1 (TfR1), increased expression of the iron storage protein subunits H- and L-ferritin, and a low but overall variable level of stainable iron in the obstructed kidney. This reduction in IRP RNA binding activity and ferritin RNA levels suggests the concomitant rise in ferritin expression and iron content in kidney failure is IRP dependent. In contrast, the reduction in the Epo mRNA level in the obstructed kidney was not rescued by genetic ablation of IRP1, suggesting disruption of normal hypoxia-inducible factor (HIF)-2α regulation. Furthermore, reduced expression of some HIF-α target genes in UUO occurred in the face of increased expression of HIF-α proteins and prolyl hydroxylases 2 and 1, the latter of which is not known to be HIF-α mediated. Our results suggest that the IRP system drives changes in cellular iron metabolism that are associated with kidney failure in UUO but that the impact of IRPs on EPO production is overridden by disrupted hypoxia signaling.NEW & NOTEWORTHY This study demonstrates that iron metabolism and hypoxia signaling are dysregulated in unilateral obstructive nephropathy. Expression of iron regulatory proteins (IRPs), central regulators of cellular iron metabolism, and the iron uptake (transferrin receptor 1) and storage (ferritins) proteins they target is strongly altered. This suggests a role of IRPs in previously observed changes in iron metabolism in progressive renal disease. Hypoxia signaling is disrupted and appeared to dominate the action of IRP1 in controlling erythropoietin expression.

A synergistic role of IRP1 and FBXL5 proteins in coordinating iron metabolism during cell proliferation

Iron-regulatory protein 1 (IRP1) belongs to a family of RNA-binding proteins that modulate metazoan iron metabolism. Multiple mechanisms are employed to control the action of IRP1 in dictating changes in the uptake and metabolic fate of iron. Inactivation of IRP1 RNA binding by iron primarily involves insertion of a [4Fe-4S] cluster by the cytosolic iron-sulfur cluster assembly (CIA) system, converting it into cytosolic aconitase (c-acon), but can also involve iron-mediated degradation of IRP1 by the E3 ligase FBXL5 that also targets IRP2. How CIA and FBXL5 collaborate to maintain cellular iron homeostasis through IRP1 and other pathways is poorly understood. Because impaired Fe-S cluster biogenesis associates with human disease, we determined the importance of FBXL5 for regulating IRP1 when CIA is impaired. Suppression of FBXL5 expression coupled with induction of an IRP1 mutant (IRP1^3C>3S) that cannot insert the Fe-S cluster, or along with knockdown of the CIA factors NUBP2 or FAM96A, reduced cell viability. Iron supplementation reversed this growth defect and was associated with FBXL5-dependent polyubiquitination of IRP1. Phosphorylation of IRP1 at Ser-138 increased when CIA was inhibited and was required for iron rescue. Impaired CIA activity, as noted by reduced c-acon activity, was associated with enhanced FBXL5 expression and a concomitant reduction in IRP1 and IRP2 protein level and RNA-binding activity. Conversely, expression of either IRP induced FBXL5 protein level, demonstrating a negative feedback loop limiting excessive accumulation of iron-response element RNA-binding activity, whose disruption reduces cell growth. We conclude that a regulatory circuit involving FBXL5 and CIA acts through both IRPs to control iron metabolism and promote optimal cell growth.

Selective Inhibition of Collagen Prolyl 4-Hydroxylase in Human Cells

Collagen is the most abundant protein in animals. Its overproduction is associated with fibrosis and cancer metastasis. The stability of collagen relies on post-translational modifications, the most prevalent being the hydroxylation of collagen strands by collagen prolyl 4-hydroxylases (CP4Hs). Catalysis by CP4Hs enlists an iron cofactor to convert proline residues to 4-hydroxyproline residues, which are essential for the conformational stability of mature collagen. Ethyl 3,4-dihydroxybenzoate (EDHB) is commonly used as a "P4H" inhibitor in cells, but suffers from low potency, poor selectivity, and off-target effects that cause iron deficiency. Dicarboxylates of 2,2'-bipyridine are among the most potent known CP4H inhibitors but suffer from a high affinity for free iron. A screen of biheteroaryl compounds revealed that replacing one pyridyl group with a thiazole moiety retains potency and enhances selectivity. A diester of 2-(5-carboxythiazol-2-yl)pyridine-5-carboxylic acid is bioavailable to human cells and inhibits collagen biosynthesis at concentrations that neither cause general toxicity nor disrupt iron homeostasis. These data anoint a potent and selective probe for CP4H and a potential lead for the development of a new class of antifibrotic and antimetastatic agents.

The mTORC1/4E-BP pathway coordinates hemoglobin production with L-leucine availability

In multicellular organisms, the mechanisms by which diverse cell types acquire distinct amino acids and how cellular function adapts to their availability are fundamental questions in biology. We found that increased neutral essential amino acid (NEAA) uptake was a critical component of erythropoiesis. As red blood cells matured, expression of the amino acid transporter gene Lat3 increased, which increased NEAA import. Inadequate NEAA uptake by pharmacologic inhibition or RNAi-mediated knockdown of LAT3 triggered a specific reduction in hemoglobin production in zebrafish embryos and murine erythroid cells through the mTORC1 (mammalian target of rapamycin complex 1)/4E-BP (eukaryotic translation initiation factor 4E-binding protein) pathway. CRISPR-mediated deletion of members of the 4E-BP family in murine erythroid cells rendered them resistant to mTORC1 and LAT3 inhibition and restored hemoglobin production. These results identify a developmental role for LAT3 in red blood cells and demonstrate that mTORC1 serves as a homeostatic sensor that couples hemoglobin production at the translational level to sufficient uptake of NEAAs, particularly L-leucine.

Low intracellular iron increases the stability of matriptase-2

Matriptase-2 (MT2) is a type II transmembrane serine protease that is predominantly expressed in hepatocytes. It suppresses the expression of hepatic hepcidin, an iron regulatory hormone, by cleaving membrane hemojuvelin into an inactive form. Hemojuvelin is a bone morphogenetic protein (BMP) co-receptor. Here, we report that MT2 is up-regulated under iron deprivation. In HepG2 cells stably expressing the coding sequence of the MT2 gene, TMPRSS6, incubation with apo-transferrin or the membrane-impermeable iron chelator, deferoxamine mesylate salt, was able to increase MT2 levels. This increase did not result from the inhibition of MT2 shedding from the cells. Rather, studies using a membrane-permeable iron chelator, salicylaldehyde isonicotinoyl hydrazone, revealed that depletion of cellular iron was able to decrease the degradation of MT2 independently of internalization. We found that lack of the putative endocytosis motif in its cytoplasmic domain largely abolished the sensitivity of MT2 to iron depletion. Neither acute nor chronic iron deficiency was able to alter the association of Tmprss6 mRNA with polyribosomes in the liver of rats indicating a lack of translational regulation by low iron levels. Studies in mice showed that Tmprss6 mRNA was not regulated by iron nor the BMP-mediated signaling with no evident correlation with either Bmp6 mRNA or Id1 mRNA, a target of BMP signaling. These results suggest that regulation of MT2 occurs at the level of protein degradation rather than by changes in the rate of internalization and translational or transcriptional mechanisms and that the cytoplasmic domain of MT2 is necessary for its regulation.

Multiple determinants within iron-responsive elements dictate iron regulatory protein binding and regulatory hierarchy

Iron regulatory proteins (IRPs) are iron-regulated RNA binding proteins that, along with iron-responsive elements (IREs), control the translation of a diverse set of mRNA with 5' IRE. Dysregulation of IRP action causes disease with etiology that may reflect differential control of IRE-containing mRNA. IREs are defined by a conserved stem-loop structure including a midstem bulge at C8 and a terminal CAGUGH sequence that forms an AGU pseudo-triloop and N19 bulge. C8 and the pseudo-triloop nucleotides make the majority of the 22 identified bonds with IRP1. We show that IRP1 binds 5' IREs in a hierarchy extending over a ninefold range of affinities that encompasses changes in IRE binding affinity observed with human L-ferritin IRE mutants. The limits of this IRE binding hierarchy are predicted to arise due to small differences in binding energy (e.g., equivalent to one H-bond). We demonstrate that multiple regions of the IRE stem not predicted to contact IRP1 help establish the binding hierarchy with the sequence and structure of the C8 region displaying a major role. In contrast, base-pairing and stacking in the upper stem region proximal to the terminal loop had a minor role. Unexpectedly, an N20 bulge compensated for the lack of an N19 bulge, suggesting the existence of novel IREs. Taken together, we suggest that a regulatory binding hierarchy is established through the impact of the IRE stem on the strength, not the number, of bonds between C8 or pseudo-triloop nucleotides and IRP1 or through their impact on an induced fit mechanism of binding.

Fatty-acyl esters of retinol (vitamin A) in the liver of the harp seal (Phoca groenlandica), hooded seal (Cystophora cristata), and California sea lion (Zalophus californianus)

The fatty-acid composition of retinyl esters in the livers of two species of phocid seal, the harp seal (Phoca groenlandica, n = 20) and the hooded seal (Cystophora cristata, n = 15), and one species of otariid seal, the California sea lion (Zalophus californianus, n = 6), was determined. Vitamin A ranged in concentration from 4 to 1024 nmol retinol/g liver for the phocids and from 381 to 979 nmol/g liver for the otariids. In most of the livers, retinyl palmitate was not the principal ester, and the palmitate + stearate + oleate trio of retinyl esters represented less than 50% of the total. In all samples, the retinyl esters contained 20:1, 20:4, 20:5, and 22:6 in unusually large amounts. Retinyl esters tended to be richer than whole-liver lipids in 20:5 + 22:6, whereas whole-liver lipids were richer in 18:0 and 18:2. Therefore, the pool of acyl donors used for the esterification of retinol may be distinct from that used for other lipids. Birth-to-weaning changes were seen only in the harp seals. In the pups, the hepatic vitamin-A concentration increased 454%, while the proportion of 18:0 and 20:1 in the retinyl esters rose and that of 14:0 + 16:1 and 20:4 fell. Concomitantly, in their mothers, the proportion of 20:4 increased but that of 16:0 and 18:0 decreased.