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Erythropoietin: induction of synthesis to signal transduction

D Roberts and D J Smith

INTRODUCTION

The hormone erythropoietin (EPO), released under hypoxic conditions, acts primarily to stimulate erythrocyte (RBC) production. The association between lack of oxygen, a blood-borne compound, and RBC formation was suggested as early as at the turn of the century (Carnot & DeFlandre, 1906), but it was not until 1957 that the kidneys were identified as the major site of EPO production in the adult mammal (Jacobson et al. 1957). The first purification of EPO was from plasma of anaemic sheep in 1971 (Goldwasser & Kung, 1971). Despite an extensive procedure, the yield and specific activity of the product were extremely low (0·4% and 8250 U/mg protein respectively). In 1977, the same procedure was used with 2550 litres of urine from anaemic humans, generating a product with a specific activity of 74 000 U/mg protein (Miyake et al. 1977). Characterization of the hormone was hampered by the difficulty of obtaining a

DOI:
https://doi.org/10.1677/jme.0.0120131
Volume 12: Issue 2,
Pages:
131–148 (18 total)
Restricted access

Up-regulation of insulin-like growth factor-I (IGF-I) receptor gene expression in patients with reduced serum IGF-I levels

R Eshet, H Werner, B Klinger, A Silbergeld, Z Laron, D LeRoith, and C T Roberts Jr

ABSTRACT

We have analysed the expression of the IGF-I receptor gene in lymphocytes of patients with low levels of circulating IGF-I (four patients with isolated GH deficiency (IGHD) and one Laron-type dwarf (LTD)) in comparison with a control group exhibiting normal serum IGF-I levels and endocrine profiles. 125I-Labelled IGF-I binding assays were performed on erythrocytes to determine the number of IGF-I binding sites per cell and their dissociation constants. Erythrocytes from patients with IGHD or LTD contained significantly (P=0·002) more receptors per cell (10·9±3·1 binding sites/cell), with a reduced affinity (K d = 0·49±0·05 nm), than erythrocytes from controls (2·0±0·4 sites/cell; K d = 0·14 nm). The levels of IGF-I receptor mRNA in circulating lymphocytes were determined by an RNA template-specific reverse transcription/polymerase chain reaction method. There was a statistically significant increase in IGF-I receptor mRNA levels in lymphocytes from patients with LTD or IGHD when compared with controls (3108·1±775·9 vs 576·0±465·7 arbitrary units, P=0·006). The increased level of IGF-I binding due to increased IGF-I receptor gene expression may represent a compensatory up-regulation process activated in response to the low levels of IGF-I in the circulation of patients with LTD or IGHD.

DOI:
https://doi.org/10.1677/jme.0.0100115
Volume 10: Issue 2,
Pages:
115–120 (6 total)
Restricted access

Differential accumulation of insulin-like growth factor-I in kidneys of pre- and postpubertal streptozotocin-diabetic rats

M Phillip, H Werner, T Palese, A A Kowarski, B Stannard, L A Bach, D LeRoith, and C T Roberts Jr

ABSTRACT

Nephropathy, one of the major complications of diabetes mellitus, is characterized by an early increase in kidney size. In experimental models of diabetes, this event is preceded by a rapid and transient rise in kidney IGF-I levels, at least in adult animals. Since diabetes-associated renal changes are uncommon in young patients, we investigated the early changes in the components of the IGF system following induction of diabetes in prepubertal and postpubertal rats. The rationale for this study was the evaluation of potential differences which could lead to kidney complications only at adult stages.

Unlike the situation in the postpubertal kidney, in which there was a transient accumulation of extractable IGF-I 24–48 h after streptozotocin (STZ) administration, there was a decrease of ∼12-fold in the level of IGF-I in the prepubertal kidney over the same period of time. Paradoxically, kidney IGF-I mRNA levels were reduced by ∼50% in the postpubertal rat 24 h after STZ treatment, whereas in the prepubertal kidney IGF-I mRNA levels were unaltered. Furthermore, the levels of IGF-I receptor mRNA and 125I-labelled IGF-I binding to kidney membranes of postpubertal diabetic rats were similar to the levels in control kidneys. On the other hand, both the levels of IGF-I receptor mRNA and 125I-labelled IGF-I binding were increased (∼2·5-fold (after 24 h) and ∼ 3-fold (after 48 h) respectively) in prepubertal animals. In addition, increased expression of IGF-binding protein (IGFBP)-1 mRNA was seen early in diabetes in both pre- and postpubertal rats.

The results of this study suggest that the transient accumulation of IGF-I in the kidney of the postpubertal diabetic rat may not be due to an increase in the local synthesis of IGF-I, but rather to an increase in IGF-I uptake from the circulation due to non-membrane-associated IGFBP-1. The lack of accumulation of IGF-I in the prepubertal kidney probably reflects the ∼ 10-fold lower levels of circulating IGF-I in young as compared with adult diabetic rats.

DOI:
https://doi.org/10.1677/jme.0.0120215
Volume 12: Issue 2,
Pages:
215–224 (10 total)
Free access

Distinct expression and activity profiles of largemouth bass (Micropterus salmoides) estrogen receptors in response to estradiol and nonylphenol

Tara Sabo-Attwood, Jason L Blum, Kevin J Kroll, Vishal Patel, Detlef Birkholz, Nancy J Szabo, Suzanne Z Fisher, Robert McKenna, Martha Campbell-Thompson, and Nancy D Denslow

Abstract

The estrogen receptor (ER) signaling cascade is a vulnerable target of exposure to environmental xenoestrogens, like nonylphenol (NP), which are causally associated with impaired health status. However, the impact of xenoestrogens on the individual receptor isotypes (α, βa, and βb) is not well understood. The goal of these studies was to determine the impact of NP on largemouth bass (Micropterus salmoides) ER isotype expression and activity. Here, we show that hepatic expression levels of three receptors are not equivalent in male largemouth bass exposed to NP by injection. Transcript levels of the ERα subtype were predominantly induced in concert with vitellogenin similarly to fish exposed to 17β-estradiol (E2) as measured by quantitative real-time PCR. NP also induced circulating plasma levels of estrogen, which may contribute to overall activation of the ERs. To measure the activation of each receptor isotype by E2 and NP, we employed reporter assays using an estrogen response element (ERE)–luciferase construct. Results from these studies show that ERα had the greatest activity following exposure to E2 and NP. This activity was inhibited by the antagonists ICI 182 780 and ZM 189 154. Furthermore, both βb and βa subtypes depressed ERα activation, suggesting that the cellular composition of receptor isotypes may contribute to the overall actions of estrogen and estrogenic contaminants via the receptors. Results from these studies collectively reveal the differential response of fish ER isotypes in response to xenoestrogens.

DOI:
https://doi.org/10.1677/JME-07-0038
Volume 39: Issue 4,
Pages:
223–237 (15 total)
Free access

Novel mitochondrial complex I inhibitors restore glucose-handling abilities of high-fat fed mice

Darren S D Martin, Siobhán Leonard, Robert Devine, Clara Redondo, Gemma K Kinsella, Conor J Breen, Victoria McEneaney, Mary F Rooney, Tim S Munsey, Richard K Porter, Asipu Sivaprasadarao, John C Stephens, and John B C Findlay

Metformin is the main drug of choice for treating type 2 diabetes, yet the therapeutic regimens and side effects of the compound are all undesirable and can lead to reduced compliance. The aim of this study was to elucidate the mechanism of action of two novel compounds which improved glucose handling and weight gain in mice on a high-fat diet. Wildtype C57Bl/6 male mice were fed on a high-fat diet and treated with novel, anti-diabetic compounds. Both compounds restored the glucose handling ability of these mice. At a cellular level, these compounds achieve this by inhibiting complex I activity in mitochondria, leading to AMP-activated protein kinase activation and subsequent increased glucose uptake by the cells, as measured in the mouse C2C12 muscle cell line. Based on the inhibition of NADH dehydrogenase (IC50 27µmolL−1), one of these compounds is close to a thousand fold more potent than metformin. There are no indications of off target effects. The compounds have the potential to have a greater anti-diabetic effect at a lower dose than metformin and may represent a new anti-diabetic compound class. The mechanism of action appears not to be as an insulin sensitizer but rather as an insulin substitute.

DOI:
https://doi.org/10.1530/JME-15-0225
Volume 56: Issue 3,
Pages:
261–271 (11 total)