We attempted to determine natural agonists of GPR39 in fetal bovine serum (FBS). FBS was conditioned to extract peptides and fractionated by two types of HPLC. The activity of each fraction was monitored by intracellular calcium mobilization. Then the purified active ingredient was analyzed by inductively coupled plasma mass spectrometry. In this fashion, Zn2+ ion was identified as an agonist of GPR39, though no peptidergic molecules were found. The calcium-mobilizing activity of Zn2+ was not abolished by pertussis toxin but was by a phospholipase C (PLC) inhibitor, U73122, indicating that the activity of GPR39 is mediated through the Gqalpha -PLC pathway. In addition, Zn2+ also activated mouse and rat GPR39, showing that the function of GPR39 as a Zn2+ receptor is conserved across species. This study is the first exploration of GPR39 agonists in FBS and indicates that GPR39 functions as a Gq-coupled Zn2+-sensing receptor.

Type 2 diabetes (T2D) occurs when there is insufficient insulin release to control blood glucose, due to insulin resistance and impaired β-cell function. The GPR39 receptor is expressed in metabolic tissues including pancreatic β-cells and has been proposed as a T2D target. Specifically, GPR39 agonists might improve β-cell function leading to more adequate and sustained insulin release and glucose control. The present study aimed to test the hypothesis that GPR39 agonism would improve glucose stimulated insulin secretion in vivo. A high throughput screen, followed by a medicinal chemistry program, identified three novel potent Zn2+ modulated GPR39 agonists. These agonists were evaluated in acute rodent glucose tolerance tests. The results showed a lack of glucose lowering and insulinotropic effects not only in lean mice, but also in diet-induced obese (DIO) mice and Zucker fatty rats. It is concluded that Zn2+ modulated GPR39 agonists do not acutely stimulate insulin release in rodents.

Iron overload in the liver causes oxidative stress and inflammation, which result in organ dysfunction, making it a risk factor for non-alcoholic steatohepatitis (NASH) and hepatocellular carcinoma. We aimed to evaluate the effect of dietary iron restriction on disease progression in rats fed a choline-deficient L-amino acid-defined (CDAA) diet. Male F344 rats were fed a choline-sufficient amino acid-defined (control) diet, a CDAA diet or an iron-restricted CDAA diet for 4, 8 and 12 weeks. At each time point, hepatic iron levels, oxidative stress, inflammation and fibrosis were evaluated by immunohistochemistry. The iron-restricted CDAA diet significantly decreased serum iron levels for 12 weeks compared with the CDAA diet. Histological analysis confirmed that feeding with the CDAA diet induced hepatic iron overload and that this was associated with oxidative stress (number of 8-hydroxydeoxyguanosine-positive cells), inflammation (CD68 positive area) and fibrosis (Sirius Red positive area). Iron restriction with the CDAA diet significantly led to a reduction in the hepatic iron levels, oxidative stress, inflammation and fibrosis. Therefore, dietary iron restriction could be a useful therapeutic approach for NASH patients with hepatic iron overload.

No abstract

G-protein-coupled receptor 39 (GPR39), a member of the ghrelin receptor family, has a full-length isoform GPR39-1a and a truncated isoform GPR39-1b. While GPR39-1a was clarified as a receptor of Zn(2+), the characteristic property of GPR39-1b remains unknown. Therefore, in this study, the molecular functions of GPR39-1b were explored in cell culture. In contrast to GPR39-1a, GPR39-1b showed no response to Zn(2+) stimulation in calcium mobilization assays, suggesting that GPR39-1b is not a functional receptor of Zn(2+). To understand the signaling interaction of GPR39-1b, we investigated the dimerization between the isoforms, and conducted bioluminescence resonance energy transfer (BRET(2)) assays. The results indicated that GPR39-1b homodimerized, but did not heterodimerize with GPR39-1a. We subsequently attempted to search the heterodimeric counterparts of GPR39-1b. Neurotensin receptor 1 (NTSR1) was also targeted as a GPR39-1b interacting partner because of its highly conserved amino acid sequence and mRNA localization, which was similar to GPR39-1b. BRET(2) assays demonstrated that GPR39-1b heterodimerized with NTSR1. To examine the effect of GPR39-1b on NTRS1-mediated cAMP/PKA signaling, we used the cAMP responsive element-luciferase assays and observed that GPR39-1b attenuated neurotensin-induced NTSR1 signaling. Taken together, our results provided a novel regulatory mechanism for GPR39-1b in NTRS1 signaling.