Subcellular fractionation of rainbow trout gonads with emphasis on microsomal enzymes involved in steroid metabolism

Tissue-specific expression and activity of cytochrome P450 1A and 3A in rainbow trout (Oncorhynchus mykiss)

Piscine cytochrome P450 (CYP) enzymes play an important role in the metabolism of xenobiotics. Xenobiotics often act as inducers of CYP1A1 and CYP3A expression and activity in fish. We compared constitutive mRNA expression of CYP1A1, CYP3A27, and CYP3A45 and catalytic activity of CYP1A (7-ethoxyresorufin-O-deethylation, EROD) and CYP3A-like (benzyloxy-4-trifluoromethylcoumarin-O-debenzyloxylation, BFCOD) enzymes in the following six rainbow trout tissues: liver, gill, heart, brain, intestine, and gonad. mRNA expression and activity were present in all investigated tissues. The CYP1A1 mRNA expression was higher in the liver, gill, heart, and brain compared to gonad and intestine. The intestine was the main site of CYP3A27 and CYP3A45 expression. The highest EROD and BFCOD activity was observed in liver tissue followed in descending order by heart, brain, gill, intestine, and gonad. Such differences might be related to the role of CYP physiological functions in the specific tissue. Rainbow trout exposure to 50 mg/kg of β-naphthoflavone for 48 h resulted in a 7.5- and 5.9-fold increase in liver EROD and BFCOD activity, respectively. In vitro EROD activity inhibition with ellipticine showed tissue-specific inhibition, while ketoconazole decreased BFCOD activity by 50-98 % in all tissues. Further studies are needed to identify all CYP isoforms that are responsible for these activities and modes of regulation.

Enzyme activities of intestinal triacylglycerol and phosphatidylcholine biosynthesis in Atlantic salmon (Salmo salar L.)

The substitution of fish oil with plant-derived oil in diets for carnivorous fish, such as Atlantic salmon, has previously revealed the potentially deleterious supranuclear accumulation of lipid droplets in intestinal cells (enterocytes) which may compromise gut integrity, and consequently, fish health. This suggests that unfamiliar dietary lipid sources may have a significant impact on intestinal lipid metabolism, however, the mode of lipid resynthesis is largely unknown in teleost fish intestine. The present study aimed at characterising three key lipogenic enzymes involved in the biosynthesis of triacylglycerol (TAG) and phosphatidylcholine (PC) in Atlantic salmon enterocytes: monoacylglycerol acyltransferase (MGAT), diacylglycerol acyltransferase (DGAT), and diacylglycerol cholinephosphotransferase (CPT). Furthermore, to investigate the dietary effect of plant oils on these enzymes, two experimental groups of fish were fed a diet with either capelin (fish oil) or vegetable oil (rapeseed oil:palm oil:linseed oil, 55:30:15 w/w) as the lipid source. The monoacylglycerol (MAG) pathway was highly active in the intestinal mucosa of Atlantic salmon as demonstrated by MGAT activity (7 nmol [1-(14)C]palmitoyl-CoA incorporated min(-1) mg protein(-1)) and DGAT activity (4 nmol [1-(14)C]palmitoyl-CoA incorporated min(-1) mg protein(-1)), with MGAT appearing to also provide adequate production of sn-1,2-diacylglycerol for potential utilisation in PC synthesis via CPT activity (0.4 nmol CDP-[(14)C]choline incorporated min(-1) mg protein(-1)). Both DGAT and CPT specific activity values were comparable to reported mammalian equivalents, although MGAT activity was lower. Nevertheless, MGAT appeared not to be the rate-limiting step in salmon intestinal TAG synthesis. The homology between piscine and mammalian enzymes was established by similar stimulation and inhibition profiles by a variety of tested cofactors and isomeric substrates. The low dietary n-3/n-6 PUFA ratio presented in the vegetable oil diet did not significantly affect the activities of MGAT, DGAT, or CPT under optimised assay conditions, or in vivo intestinal mucosa lipid class composition, when compared to a standard fish oil diet.

Effects of progesterone and estradiol on the reproductive axis in immature diploid and triploid rainbow trout

In fish species, many studies demonstrated the crucial role of estradiol (E2) in the development of the reproductive axis, but progesterone (P) has been described mainly as a precursor steroid and no clear role by itself has been reported. Moreover, a cooperative effect of P (or another progestin) and E2 in fish has never been reported to our knowledge. In the present work, we investigated the effects of P, alone or in combination with E2, on the reproductive-axis of immature rainbow trout (Oncorhynchus mykiss). Liver vitellogenin and estradiol receptor (rtER) mRNA levels increased after E2 treatment, but were unchanged by P treatments as a reflection of peripheral action of steroids. In contrast, at the pituitary level, LH contents increased after E2 and/or P treatments. Focusing on the brain level, we confirmed a clear up regulation of rtER expression by E2 in sterile triploid females, and we also demonstrated a similar stimulating effect of P alone but no cooperative effect together with E2. In conclusion, our data demonstrate that in immature trout, prior to the beginning of the first reproductive cycle, unlike E2, P is able to stimulate the reproductive brain-pituitary axis without affecting vitellogenin synthesis in the liver.