Functional Interaction of Spexin and Adiponectin Forming a Local Feedbackin Goldfish Hepatocytes for Feeding Regulation

Spexin (SPX), a neuropeptide with pleiotropic functions, has been confirmed to be a novel satiety factor in fish models. Adiponectin (AdipoQ), the most abundant adipokine in circulation, is involved in lipid and glucose metabolism and its insulin-sensitizing, cardioprotective, and anti-inflammatory actions are also well-documented. However, the interaction between SPX and AdipoQ has not be reported and very little is known regarding the functions of AdipoQ in non-mammalian species. In this study, AdipoQ was cloned in goldfish and found to be widely expressed at tissue level including the liver. Sequence alignment and in silico protein modelling revealed that its a.a. sequence and 3D protein structure were highly comparable to the mammalian counterparts. Recombinant protein of goldfish AdipoQ was expressed in E. coli and IP injection of the protein purified could suppress foraging activity and food intake in goldfish. Food intake in goldfish, interestingly, could elevate plasma levels of SPX and AdipoQ with parallel rises of their transcript expression in the liver. In primary culture of goldfish hepatocytes, SPX treatment was shown to induce protein phosphorylation of MEK(1/2) and ERK(1/2) with a parallel rise in AdipoQ mRNA level. SPX-induced AdipoQ mRNA expression, however, was sensitive to the blockade of PLC/PKC, Ca(2+)/CaMK-II and MEK(1/2)/ERK(1/2) but not cAMP/PKA cascades. In reciprocal experiments, AdipoQ treatment could induce protein phosphorylation of AMPK, Akt and P(38) MAPK in goldfish hepatocytes. Meanwhile, AdipoQ was also effective in reducing SPX mRNA level and this inhibitory effect was negated by blocking the AMPK/PPAR, PI3K/Akt and P(38) MAPK but not the MEK(1/2)/ERK(1/2) or PLC/PKC pathways. Apparently, the PI3K/Akt and P(38) MAPK cascades were functionally coupled with AMPK activation. These results imply that (i) AdipoQ, similar to SPX, can be induced by food intake and serve as a satiety signal in goldfish, (ii) AdipoQ expression in goldfish liver can be up-regulated by SPX via the PLC/PKC, Ca(2+)/CaMK-II and MEK(1/2)/ERK(1/2) pathways, which may enhance the satiation response caused by SPX after food intake, and (iii) AdipoQ can inhibit SPX expression at hepatic level via the AMPK/PPAR, PI3K/Akt and P(38) MAPK cascades, which may lead to signal termination of SPX. These findings, as a whole, suggest that AdipoQ production in goldfish liver not only can form a signal amplification step for the satiation response of SPX but also constitute a local feedback to turn off SPX signal at the hepatic level.