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The link between energy metabolism and reproduction is well known in vertebrates. Irisin, the product of fibronectin type III domain-containing protein 5 (FNDC5) gene, plays an important role in energy homeostasis. However, biological actions of irisin on reproduction remain elusive. To address this gap, we examined the direct effects of irisin on luteinizing hormone β (LHβ) and follicle-stimulating hormone β (FSHβ) gene expression in tilapia pituitary cells. As a first step, the transcripts of FNDC5 were detected in the proximal pars distalis (PPD), but not in the rostral pars distalis (RPD) and neurointermediate lobe (NIL) of the tilapia pituitary by RT-PCR. In the tilapia pituitary, irisin immunoreactive signals were also detected in PPD region. In primary cultures of tilapia pituitary cells, irisin was effective in stimulating both LHβ and FSHβ mRNA levels in vivo and in vitro. In cultured pituitary cells of tilapia, removal of endogenous irisin by immunoneutralization using irisin antiserum inhibited LHβ and FSHβ gene expression. Salmon gonadotrophin releasing hormone (sGnRH) increased LHβ and FSHβ mRNA levels in tilapia pituitary but these stimulatory actions were not either enhanced by treatment with irisin or blocked by irisin antiserum. Furthermore, the stimulation on LHβ and FSHβ mRNA expression was coincident with the enhancement of LHβ and FSHβ mRNA stability after irisin treatment. These results provide evidence that irisin may serve as a novel intrapituitary factor maintaining gonadotropins gene expression in tilapia pituitary.

Irisin, the product of fibronectin type III domain-containing protein 5 (FNDC5) gene, is well-documented to be a regulator of energy metabolism. At present, not much is known about its biological function in non-mammalian species. In this study, a full-length tilapia FDNC5 was cloned and its tissue expression pattern has been confirmed. Based on the sequence obtained, we produced and purified recombinant irisin which could induce uncoupling protein 1 (UCP1) gene expression in tilapia hepatocytes. Further, the rabbit polyclonal irisin antiserum was produced and its specificity was confirmed by antiserum preabsorption. In tilapia pituitary cells, irisin inhibited growth hormone (GH) gene expression and secretion and triggered rapid phosphorylation of Akt, Erk1/2, and p38 MAPK. Furthermore, irisin-inhibited GH mRNA expression could be prevented by inhibiting PI3K/Akt, MEK1/2, and p38 MAPK, respectively. Apparently, fish irisin can act directly at the pituitary level to inhibit GH transcript expression via multiple signaling pathways.

Dopamine (DA) is an important neurotransmitter in the central nervous system of vertebrates and possesses key hypophysiotropic functions. Early studies have shown that DA has a potent inhibitory effect on somatolactin (SL) release in fish. However, the mechanisms responsible for DA inhibition of SL gene expression are largely unknown. To this end, tilapia DA type-1 (D1) and type-2 (D2) receptor transcripts were examined in the neurointermediate lobe (NIL) of the tilapia pituitary by real-time PCR. In tilapia, DA not only was effective in inhibiting SL mRNA levels in vivo and in vitro, but also could abolish pituitary adenylate cyclase-activating polypeptide (PACAP)- and salmon gonadotropin-releasing hormone (sGnRH)-stimulated SL gene expression at the pituitary level. In parallel studies, the specific D2 receptor agonists quinpirole and bromocriptine could mimic the DA-inhibited SL gene expression. Furthermore, the D2 receptor antagonists domperidone and (-)-sulpiride could abolish the SL response to DA or the D2 agonist quinpirole, whereas D1 receptor antagonists SCH23390 and SKF83566 were not effective in this respect. In primary cultures of tilapia NIL cells, D2 agonist quinpirole-inhibited cAMP production could be blocked by co-treatment with the D2 antagonist domperidone and the ability of forskolin to increase cAMP production was also inhibited by quinpirole. Using a pharmacological approach, the AC/cAMP pathway was shown to be involved in quinpirole-inhibited SL mRNA expression. These results provide evidence that DA can directly inhibit SL gene expression at the tilapia pituitary level via D2 receptor through the AC/cAMP-dependent mechanism.

Somatolactin (SL), the member of the growth hormone (GH)/prolactin (PRL) family, is fish-specific pituitary hormone with diverse functions. However, little is known about its biological function in fish hepatocytes. Using tilapia as a model, SLtranscripts were shown to be widely expressed in various extrapituitary tissues with the relatively high expression level in liver. To explore the biological action of SL in hepatocytes, we produced and purified recombinant tilapia SL protein which could induce pigment aggregation in tilapia melanophores. Further, the antiserum for the SL was produced and its specificity was confirmed by antiserum preabsorption. During 4 week starvation, hepatic SL transcripts in starved fish were significantly higher than the control fish starting on the 1st week of starvation until 4th week. After re-feeding, the SL transcripts level returned to normal. Using primary cultures of tilapia hepatocytes, insulin-like growth factors (IGF1 and IGF2) gene expression were elevated by static incubation with recombinant tilapia SL. In contrast, removal endogenous SL by immunoneutralization using SL antiserum was shown to inhibit IGF1 and IGFgene expression. These findings, taken together, provide evidence for the first time that SL may serve as a novel regulator in fish stimulating IGF1 and IGF gene expression in hepatocytes.

The peptide hormone adropin plays a role in energy homeostasis. However, biological actions of adropin in non-mammalian species are still lacking. Using tilapia as a model, we examined the role of adropin in lipoprotein lipase (LPL) regulation in hepatocytes. To this end, the structural identity of tilapia adropin was established by 5′/3′-rapid amplification of cDNA ends (RACE). The transcripts of tilapia adropin were ubiquitously expressed in various tissues with the highest levels in the liver and hypothalamus. The prolonged fasting could elevate tilapia hepatic adropin gene expression, whereas no effect of fasting was observed on hypothalamic adropin gene levels. In primary cultures of tilapia hepatocytes, synthetic adropin was effective in stimulating LPL release, cellular LPL content, and total LPL production. The increase in LPL production also occurred with parallel rises in LPL gene levels. In parallel experiments, adropin could elevate cAMP production and up-regulate protein kinase A (PKA) and PKC activities. Using a pharmacological approach, cAMP/PKA and PLC/inositol trisphosphate (IP3)/PKC cascades were shown to be involved in adropin-stimulated LPL gene expression. Parallel inhibition of p38MAPK and Erk1/2, however, were not effective in these regards. Our findings provide, for the first time, evidence that adropin could stimulate LPL gene expression via direct actions in tilapia hepatocytes through the activation of multiple signaling mechanisms.