Elsevier

Journal of Hepatology

Volume 47, Issue 2, August 2007, Pages 262-269
Journal of Hepatology

Succinate is a paracrine signal for liver damage

https://doi.org/10.1016/j.jhep.2007.03.016Get rights and content

Background/Aims

A G-protein-coupled succinate receptor has recently been identified in several tissues, including the liver. The objectives of this work were to determine the hepatic cell types that express this receptor and to determine its physiological role.

Methods

Expression and distribution of the succinate receptor was determined by RT-PCR and confocal immunofluorescence. Biochemical assays were used to measure succinate and cAMP. Cytosolic Ca2+ was monitored in single cells by time-lapse imaging. Western blot was used to study the effect of succinate on activation of hepatic stellate cells.

Results

The succinate receptor was expressed in quiescent hepatic stellate cells, and expression decreased with activation. Ischemia induced release of succinate in isolated perfused livers. In contrast to what is observed in cell expression systems, succinate did not inhibit cAMP production or increase cytosolic Ca2+ in primary hepatic stellate cells. However, succinate accelerated stellate cell activation.

Conclusions

Hepatic stellate cells express the succinate receptor. Succinate may behave as a paracrine signal by which ischemic hepatocytes trigger stellate cell activation.

Introduction

Succinate is an important metabolic molecule that constitutes one of the intermediates of the citric acid cycle. Recently, a succinate receptor was identified that suggests succinate may also act as a signaling molecule [1]. This newly described receptor is the former orphan G-protein-coupled receptor GPR91. This receptor is expressed in kidney, liver, spleen and to a lesser extent small intestine [1]. The distribution and function of the succinate receptor within the kidney has been examined, but nothing is known about the receptor in the liver. The principal signaling actions of succinate that are mediated by the receptor GPR91 in kidney are to increase Ca2+ via Gq and to decrease cAMP via Gi. The principal downstream effect that has been identified is an increase in renin release, which produces a rise in arterial blood pressure [1]. Given the important role of succinate as an intermediary of the central cross-road of metabolism – the citric acid cycle – and its novel signaling properties, an attractive hypothesis is that GPR91 might serve as a sensor of metabolic activity in organs or tissues. Of special interest is the fact that succinate levels rise in response to ischemia [2], [3], because GPR91 could be responsible for mediating the adaptation of organs and tissues to hypoxic conditions, including regulating secretion or repair and proliferation. Because the liver is the central metabolic organ and is particularly susceptible to ischemia, we examined the intrahepatic distribution and functional role of the succinate receptor.

Section snippets

Animals and materials

Male Sprague–Dawley rats (200–250 g, Charles River Laboratories, Boston, MA) were used for all studies. Animals were maintained on a standard diet and housed under a 12-h light–dark cycle. Ultra-pure succinic acid and ATP were purchased from Sigma Chemical Co. (St. Louis, MO). All other chemicals were of the highest quality commercially available.

Isolation of hepatic cell types

Hepatic stellate cells (HSC) were isolated from male Sprague–Dawley rats by Nycodenz gradient centrifugation, as described previously [4]. Primary

Expression and distribution of the succinate receptor in liver

Expression of the succinate receptor GPR91 was detected in rat liver using RT-PCR of mRNA extracted from whole liver samples (Fig. 1). The tissue distribution of this receptor was characterized by isolating hepatocytes, cholangiocytes, Kupffer cells, sinusoidal endothelial cells, hepatic stellate cells and portal fibroblasts and the mRNA specific for each of these cell types was extracted. The succinate receptor GPR91 was detected only in quiescent hepatic stellate cells (HSC), as well as in

Discussion

Succinate is an intermediary of the citric acid cycle, a central metabolic pathway which promotes the oxidative decarboxylation of acetyl-CoA and produces reducing equivalents, NADH and FADH2, used in the respiratory chain [2]. Succinate is produced by the oxidation of succinyl-CoA by the enzyme succinyl-CoA hydrolase and is further oxidized to fumarate by succinate dehydrogenase. The activity of the citric acid cycle is regulated to match metabolic demands [2], but pathological situations such

Acknowledgments

This work was supported by NIH Grants DK45710, DK34989, and TW01451.

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    The authors who have taken part in this study declared that they have no relationship with the manufacturers of the drugs involved either in the past or present and did not receive funding from the manufacturers to carry out their research. They received funding from NIH which enabled them to carry out their study.

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