Possible involvement of PKC/MAPK pathway in the regulation of GnRH by dietary arachidonic acid in the brain of male tongue sole Cynoglossus semilaevis

Abstract

Our previous studies have indicated that dietary arachidonic acid (ARA) significantly affects the gonadal steroidogenesis in the marine teleost tongue sole Cynoglossus semilaevis, and this effect was more positive in male fish than in female fish (Aquaculture, 468,(378)(379)(380)(381)(382)(383)(384)(385). As a following up study, the present study was further aimed at investigating the possible mechanisms in the brain mediating the effects of dietary ARA on gonadal steroidogenesis. A 70-day feed trial was repeated with two-year-old tongue sole, using three experimental diets with graded levels of ARA, 0.34%, 2.53%, and 9.63% of total fatty acids. Each diet was fed to triplicate groups of 23 fish (15 males and 8 females). The results confirmed the positive effect of dietary ARA on testosterone production in male fish. The concentration of gonadotrophin-releasing hormone (GnRH) in serum responded to dietary ARA in a similar pattern with the testosterone concentration, but the concentration of gonadotrophin in serum was not affected by dietary ARA. The response of gene expression of PKCβ, ARRB1, ARRB2, ERK2 and ATF3 in the brain to dietary ARA was in good agreement with those of GnRH and testosterone, indicating the possible involvement of PKC-ARRB-ERK-ATF3 pathway in signalling transduction of GnRH. However, the PI3K/Akt and TLR/NF-κB pathways may not be directly involved in the regulation of GnRH metabolism by ARA. This is the first study reporting the possible involvement of PKC/MAPK pathways in regulation of reproductive endocrine processes by long-chain polyunsaturated fatty acid in brain of marine fish. K E Y W O R D Sarachidonic acid, brain, Cynoglossus semilaevis, PKC/MAPK, reproduction Abbreviations: 3β-HSD, 3β-hydroxysteroid dehydrogenase; Akt1, protein kinase B 1; ARA, arachidonic acid; ARRB, arrestin beta; ATF3, activating transcription factor 3; CREB, cAMP responsive element binding protein; DHA, docosahexaenoic acid; ELISA, enzyme-linked immunosorbent assay; EPA, eicosapentaenoic acid; ERK, extracellular-regulated protein kinase;

Arachidonic acid matters

“…138 Other studies revealed that, in tongue sole broodstock, dietary ARA supplementation significantly reduced the estradiol production in females, but stimulated the testosterone production in males. 125,150 The sex-dependent effects of ARA on gonadal steroidogenesis have also been observed in Senegalese sole, where steroid (11-ketotestosterone and testosterone) levels in male fish increased significantly with increasing dietary ARA (0.7%-6.0% TFA) in a dosedependent manner, whereas in females estradiol did not show any change related to dietary ARA content. 151 The distinct and more prominent roles of ARA in the reproductive physiology of male fish than in females were often also concurrent with substantially higher ARA accumulation in gonads of male fish.…”

“…In yellow catfish, it was observed that a medium ARA level (4.67% TFA) reduced the concentration of estradiol and 11‐ketotestosterone in the serum 138 . Other studies revealed that, in tongue sole broodstock, dietary ARA supplementation significantly reduced the estradiol production in females, but stimulated the testosterone production in males 125,150 . The sex‐dependent effects of ARA on gonadal steroidogenesis have also been observed in Senegalese sole, where steroid (11‐ketotestosterone and testosterone) levels in male fish increased significantly with increasing dietary ARA (0.7%–6.0% TFA) in a dose‐dependent manner, whereas in females estradiol did not show any change related to dietary ARA content 151 …”

“…A study on male tongue sole brains showed that the PKC-ARRB-ERK-ATF3 pathway was possibly involved in the stimulation of gonadotrophin-releasing hormone by dietary ARA (9.63% TFA), whereas the PI3K/Akt and TLR/NF-κB pathways was apparently not directly involved in this process. 150 T A B L E 3 Summary of ARA studies with fish broodstocks (2) Dietary ARA appears more important for male fish than for female fish, and more important for immature females than for mature females. (2) However, the effect of dietary ARA on ch25h transcription in the liver showed an opposite gender-difference pattern to those in the gonads and brain.…”

Cloning and Characterization of Cholesterol 25-Hydroxylase (ch25h) From a Marine Teleost, Chinese Tongue Sole (Cynoglossus semilaevis), and Its Gene Expressions in Response to Dietary Arachidonic Acid

“…The differential regulation of gonadal steroidogenesis and brain GnRH synthesis by dietary ARA between male and female Chinese tongue sole has been shown in our previous studies (Xu et al, 2017a(Xu et al, , 2019. Those results showed that dietary ARA supplementation stimulated the testosterone and GnRH production in males, but reduced the estradiol production in the females.…”

“…Only in zebrafish, the interferon independent antiviral role of 25HC was validated (Pereiro et al, 2017). In Chinese tongue sole, a recent study of ours with feeding trial followed by transcriptomic analysis has shown that ch25h transcription in the brain was significantly affected by dietary arachidonic acid (ARA), which plays important roles in fish reproduction (Izquierdo et al, 2001;Norberg et al, 2017), and this effect was different between male and female fish (Xu et al, 2019). Our previous studies have also shown that dietary ARA differentially regulates the gonadal steroidogenesis in Chinese tongue sole depending on fish gender (Xu et al, 2017a).…”