Katalin Sipos

Biogenesis of cytosolic ribosomes requires the essential iron-sulphur protein Rli1p and mitochondria.

Mitochondria perform a central function in the biogenesis of cellular iron–sulphur (Fe/S) proteins. It is unknown to date why this biosynthetic pathway is indispensable for life, the more so as no essential mitochondrial Fe/S proteins are known. Here, we show that the soluble ATP‐binding cassette (ABC) protein Rli1p carries N‐terminal Fe/S clusters that require the mitochondrial and cytosolic Fe/S protein biogenesis machineries for assembly. Mutations in critical cysteine residues of Rli1p abolish association with Fe/S clusters and lead to loss of cell viability. Hence, the essential character of Fe/S clusters in Rli1p explains the indispensable character of mitochondria in eukaryotes. We further report that Rli1p is associated with ribosomes and with Hcr1p, a protein involved in rRNA processing and translation initiation. Depletion of Rli1p causes a nuclear export defect of the small and large ribosomal subunits and subsequently a translational arrest. Thus, ribosome biogenesis and function are intimately linked to the crucial role of mitochondria in the maturation of the essential Fe/S protein Rli1p.

Hexose phosphorylation and the putative calcium channel component Mid1p are required for the hexose-induced transient elevation of cytosolic calcium response in Saccharomyces cerevisiae

Saccharomyces cerevisiae responds to environ‐mental stimuli such as an exposure to pheromone or to hexoses after carbon source limitation with a transient elevation of cytosolic calcium (TECC) response. In this study, we examined whether hexose transport and phosphorylation are necessary for the TECC response. We found that a mutant strain lacking most of the known hexose transporters was unable to carry out the TECC response when exposed to glucose. A mutant strain that lacked the ability to phosphorylate glucose was unable to respond to glucose addition, but displayed a normal TECC response after the addition of galactose. These results indicate that hexose uptake and phosphorylation are required to trigger the hexose‐induced TECC response. We also found that the TECC response was significantly smaller than normal when the level of environmental calcium was reduced, and was abolished in a mid1 mutant that lacked a subunit of the high‐affinity calcium channel of the yeast plasma membrane. These results indicate that most or all of the TECC response is mediated by an influx of calcium from the extracellular space. Our results indicate that this transient increase in plasma membrane calcium permeability may be linked to the accumulation of Glc‐1‐P (or a related glucose metabolite) in yeast.

NUCLEOLIN PROMOTES SECONDARY STRUCTURE IN RIBOSOMAL RNA

The effect of nucleolin on the secondary structure of RNA was studied using circular dichroism (CD). Nucleolin caused decreases in the main positive bands and shifts to higher wavelengths in the CD spectra of synthetic polynucleotides such as poly(G) and poly(A) indicating helix destabilizing activity. In contrast, nucleolin effected increases in signal and shifts to lower wavelengths of the peaks of CD spectra of ribosomal RNA, suggesting enhancement of secondary structure. Another major nucleolar RNA binding protein, B23, had helix destabilizing activity but did not enhance RNA secondary structure. It is proposed that nucleolin promotes formation of secondary structure in preribosomal RNA during the early stages of ribosome biogenesis.

Question: Why are mitochondria essential for life?

Is There an Answer? is intended to serve as a forum in which readers to IUBMB Life may pose questions of the type that intrigue biochemists but for which there may be no obvious answer or one may be available but not widely known or easily accessible. Readers are invited to e-mail f.vella@sasktel.net if they have questions to contribute or if they can provide answers to questions that are provided here from time to time. In the latter case, instructions will be sent to interested readers. Answers should be, whenever possible, evidencebased and provide relevant references. – Frank Vella

Serum prohepcidin levels in chronic inflammatory bowel diseases

BACKGROUND AND AIMS One of the major symptoms of chronic inflammatory bowel diseases is anemia. The two most common diseases are Crohns disease and ulcerative colitis. Anemia may develop due to intestinal bleeding, iron absorption disturbances, or high levels of inflammatory cytokines. It is not clear whether or not hepcidin, the only known hormone regulating cellular iron uptake in mammals is involved. The transcription of hepcidin is controlled by the iron status of the body, hypoxia, and/or inflammation. This study was meant to find relationship between serum prohepcidin levels and clinical parameters of iron homeostasis or inflammatory state in patients suffering from Crohns disease or ulcerative colitis. METHODS Serum prohepcidin levels were measured with ELISA in 72 patients diagnosed with ulcerative colitis and 30 patients suffering from Crohns disease. RESULTS In both groups serum iron levels were lower, while levels of C-reactive protein were higher than in the healthy controls. Serum prohepcidin levels showed no significant differences compared to those in the control group. In the affected patients only weak correlations were observed between prohepcidin levels and diagnostic parameters: in Crohns disease prohepcidin levels correlated positively with transferrin levels, total iron-binding capacity, transferrin saturation, activity index, and serum albumin levels, while in ulcerative coltitis prohepcidin levels were related to transferrin levels and transferrin saturation. CONCLUSION It seems obvious that serum prohepcidin level determination in itself is not a satisfactory diagnostic or prognostic measure in anemia of chronic inflammatory bowel diseases.

Simultaneous Determination of 4-Substituted Cathinones (4-MMC, 4-MEC and 4-FMC) in Human Urine by HPLC–DAD

This study developed a selective and rapid high-performance liquid chromatography-diode array detection method for the confirmation of different cathinone derivates in human urine. Samples were prepared by solid-phase extraction (SPE) using procaine hydrochloride as the internal standard. The chromatographic separation was performed on a Kinetex PFP column using isocratic elution. The mobile phase was composed of a mixture of acetonitrile (33%, v/v) and 0.005M ammonium trifluoroacetate buffer (67%, v/v; pH 4.93 ± 0.03) with a flow rate of 0.350 mL/min. The diode array detection was performed at 262 nm. The method was linear over the concentration range of 25-2,400 ng/mL. Intra-day and inter-day precision values for cathinones were less than 1.26% (relative standard deviation). The limit of detection for any compounds extracted from human urine by the optimized SPE method was 40 ng/mL and the limit of quantification was 100 ng/mL in the urine. The recovery rate of SPE was between 71 and 82% with a lower relative standard deviation than 2.35%.

Prohepcidin binds to the HAMP promoter and autoregulates its own expression.

Hepcidin is the major regulatory peptide hormone of iron metabolism, encoded by the HAMP (hepcidin antimicrobial peptide) gene. Hepcidin is expressed mainly in hepatocytes, but is also found in the blood in both a mature and prohormone form. Although, the function of mature hepcidin and the regulation of the HAMP gene have been extensively studied, the intracellular localization and the fate of prohepcidin remains controversial. In the present study, we propose a novel role for prohepcidin in the regulation of its own transcription. Using indirect immunofluorescence and mCherry tagging, a portion of prohepcidin was detected in the nucleus of hepatocytes. Prohepcidin was found to specifically bind to the STAT3 (signal transducer and activator of transcription 3) site in the promoter of HAMP. Overexpression of prohepcidin in WRL68 cells decreased HAMP promoter activity, whereas decreasing the amount of prohepcidin caused increased promoter activity measured by a luciferase reporter-gene assay. Moreover, overexpression of the known prohepcidin-binding partner, α-1 antitrypsin caused increased HAMP promoter activity, suggesting that only the non-α-1 antitrypsin-bound prohepcidin affects the expression of its own gene. The results of the present study indicate that prohepcidin can bind to and transcriptionally regulate the expression of HAMP, suggesting a novel autoregulatory pathway of hepcidin gene expression in hepatocytes.

α‐1 Antitrypsin binds preprohepcidin intracellularly and prohepcidin in the serum

Recent discoveries have indicated that the hormone hepcidin plays a major role in the control of iron homeostasis. Hepcidin regulates the iron level in the blood through the interaction with ferroportin, an iron exporter molecule, causing its internalization and degradation. As a result, hepcidin increases cellular iron sequestration, and decreases the iron concentration in the plasma. Only mature hepcidin (result of the cleavage of prohepcidin by furin proteases) has biological activity; however, prohepcidin, the prohormone form, is also present in the plasma. In this study, we aimed to identify new protein–protein interactions of preprohepcidin, prohepcidin and hepcidin using the BacterioMatch two‐hybrid system. Screening assays were carried out on a human liver cDNA library. Preprohepcidin screening gave the following results: α‐1 antitrypsin, transthyretin and α‐1‐acid glycoprotein showed strong interactions with preprohepcidin. We further confirmed and examined the α‐1 antitrypsin binding in vitro (glutathione S‐transferase, pull down, coimmunoprecipitation, MALDI‐TOF) and in vivo (ELISA, cross‐linking assay). Our results demonstrated that the serine protease inhibitor α‐1 antitrypsin binds preprohepcidin within the cell during maturation. Furthermore, α‐1 antitrypsin binds prohepcidin significantly in the plasma. This observation may explain the presence of prohormone in the circulation, as well as the post‐translational regulation of the mature hormone level in the blood. In addition, the lack of cleavage protection in patients with α‐1 antitrypsin deficiency may be the reason for the disturbance in their iron homeostasis.

PCR-based identification of drowning: four case reports

Proper diagnosis in drowning victims is often difficult due to the lack of signs specific to drowning. The diatom test is a widely used procedure for the diagnosis. Some types of water contain only minimal amounts of diatom cells which may provide false-negative results, while a negative diatom test result does not exclude drowning. In proving drowning, we used a polymerase chain reaction (PCR)-based biological method in addition to the conventional methods. DNA was extracted from postmortem spleen tissues and water of the drowning site. Samples were tested with algae (diatoms and small green algae)- and cyanobacteria (blue-green algae)-specific primers. We present here multiple drowning cases in which diatom tests of the postmortem tissue samples and the water were negative. In each case, the presence of phytoplanktonic DNA strengthened the autopsy diagnosis of drowning even in the absence of visible diatoms. In the future, the PCR method may be of consideration as a possible supplement of the diatom test in the examination of presumed drowning cases.

Hepcidin and its potential clinical utility

A number of pathophysiological conditions are related to iron metabolism disturbances. Some of them are well known, others are newly discovered or special. Hepcidin is a newly identified iron metabolism regulating hormone, which could be a promising biomarker for many disorders. In this review, we provide background information about mammalian iron metabolism, cellular iron trafficking, and the regulation of expression of hepcidin. Beside these molecular biological processes, we summarize the methods that have been used to determine blood and urine hepcidin levels and present those pathological conditions (cancer, inflammation, neurological disorders) when hepcidin measurement may have clinical relevance.