Goldfish adiponectin : novel function as a satiety factor and autocrine/paracrine regulation at the hepatic level

Abstract

Adiponectin (AdipoQ), an adipokine with a high level in circulation, is known to have pleiotropic functions mediated by its receptor AdipoR1/R2. Although AdipoQ is involved in energy homeostasis and glucose/lipid metabolism, its functional role in appetite control is controversial and the corresponding information in lower vertebrates is still limited. To shed light on feeding regulation by AdipoQ in fish models, goldfish AdipoQ was cloned and shown to be structurally and evolutionarily related to AdipoQ in other vertebrates. Based on the cDNA obtained, recombinant goldfish AdipoQ was produced and confirmed to be bioactive by activating AdipoR1/R2 expressed in HepG2 cells. In goldfish, food intake increased plasma AdipoQ and AdipoQ mRNA levels in the liver and brain areas involved in feeding control including the telencephalon, optic tectum and hypothalamus. IP/ICV injection with recombinant AdipoQ, however, could reduce foraging activity and food intake in goldfish and the feeding inhibition observed was mediated by the central action of AdipoQ, which differentially modified orexigenic (NPY, AgRP, orexin, and apelin)/anorexigenic siganls (POMC, CART, CCK, and MCH) and their receptors (receptors for leptin, NPY, and ghrelin) expressed in the telencephalon, optic tectum and/or hypothalamus. Our findings provide evidence that AdipoQ not only can act as a feeding inhibitor but also a satiety factor in goldfish.

Spexin (SPX), a neuropeptide highly expressed within the brain, is another satiety factor identified in goldfish based on our previous study and its expression in the liver after a meal can contribute to the satiation response in fish models. In goldfish, food consumption could elevate plasma glucose, insulin, AdipoQ and SPX with parallel rises of AdipoQ and SPX gene expression in the liver. In goldfish hepatocytes, insulin also induced transcript expression of AdipoQ and SPX. Meanwhile, co-expression of SPX with its receptor GalR2/AdipoQ with its receptors AdipoR1/R2 was noted in individual hepatocytes, suggesting the possible local actions of the two factors at the hepatic level. This idea is supported by our findings that (i) SPX and AdipoQ mRNA in goldfish liver were elevated by IP injection of SPX, while a similar treatment with AdipoQ reduced hepatic expression of SPX and GalR2 transcripts with a parallel rise in AdipoQ gene expression; (ii) In goldfish hepatocytes, SPX produced locally could induce SPX and AdipoQ transcript expression via GalR2 coupled to PLC/IP3/PKC, Ca2+/CaM/CaMK-II and MEK1/2 /ERK1/2 cascades; (iii) At hepatocyte level, AdipoQ produced locally could inhibit SPX and GalR2 gene expression via AdipoR1/R2 coupled to AMPK/PPARα/γ, PI3K/Akt/mTOR and MKK3/6/p38 MAPK pathways and AdipoQ co-treatment was effective in blocking the SPX response induced by insulin and SPX, respectively, and (iv) AdipoQ induced AdipoQ transcript expression in goldfish hepatocytes was mediated by AMPK inhibition of STAT5 signaling. Our results suggest that the postprandial rise of insulin caused by glucose uptake can induce two satiety signals in goldfish liver, namely SPX and AdipoQ, and these two factors through local induction/autoregulation and feedback regulation can also fine-tune their output at hepatic level during the satiation response.