The Research Of Effects And Mechanisms Of Dietary Genistein On The Growth Of Juvenile Nile Tilapia,(Oreochromis Niloticus)

With the development of aquaculture, the demand for aquatic feed increasedgreatly. Moreover, Because of the high price and limited supply of animal protein, thesoybean protein has gradually become the preferred alternative. Soybean isoflavone isrich in soybean protein powder, so it is very important to find out the influence ofsoybean isoflavones on the growth of fish. Genistein shares structural features with17β-estradiol(E2) and it is one of the most potent and higest levels phytoestrogens insoybean and soy products.Therefore, we chose "New GIFT Strain Nile tilapia" as the experimental animalin this paper, and fed tilapia diets for8weeks with dietary genistein concentration0,30g/g,300g/g and3000g/g, and determined the effect of genistein on the growthof tilapia, and from the two aspects of nutrition and endocrine discussed themechanism of genistein affecting the growth of Nile tilapia. Our study provided thetheoretical basis for the future scientific evaluation of genistein on fish and exploringnew type of fish feed which was taken soybean meal as protein source. The resultsshowed that:1、With dietary genistein concentration0~3000g/g feeding tilapia diets for8weeks,30and300μg/g group of fish final weight and specific growth rate were notsignificantly different from control, while3000μg/g group decreased significantlycompared with control. Among each treatment group, feed intake, feed efficiencyratio, hepatosomatic index, viscerosomatic index and the fish body composition didnot differ significantly. It showed that low concentration of genistein had no effect onthe growth of tilapia, and high concentration of genistein could inhibit the growth oftilapia.2、From the study on the main digestive enzymes of tilapia showed that theactivities of stomach protease and intestinal amylase of300μg/g group feeding tilapiaafter8weeks were significantly lower than those of the control. In the3000μg/g group, the activities of stomach protease, trypsin, liver amylase and intestinal amylasewere significantly lower than those of the control. There were no significantdifference in SOD activity and stomach amylase among each treatment. The resultssuggested high level of genistein could interfere in the proteometabolism andglycometabolism, and reduce the tilapia conversion and absorption of nutrients.3、The growth of fish was mainly regulated by GH/IGF-I axis, and GH synthesisand secretion were regulated by ghrelin, GnRHs, NPY and PACAP which weresecreted by hyothalamus. With dietary genistein concentration0~3000g/g feedingtilapia diets for8weeks, GH levels in plasma were examined by radioimmunoassay.GH、ghrelin、GnRHs、GnRHRs、NPY、NPYRs and PACAP mRNA expression levelswere examined by real-time PCR. It turned out, GH levels in plasma and theexpression of mRNA decreased gradually in tilapia, and3000g/g group wassignificantly lower than that of the control, the changing trend was consistent with thegrowth of tilapia, and it confirmed the important role of pituitary GH in regulating thegrowth of tilapia. The detection on the main factors of hyothalamus which regulatedthe pituitary GH showed, the expression of ghrelin mRNA in3000μg/g group wassignificantly lower than that of the control treatment, so its promoting effect onpituitary GH was restrained. In addition, NPY mRNA expression reached the highestlevel in3000μg/g group, however, the expression level of NPY R1mRNA decreasedsignificantly. Its promoting effect on pituitary GH was also restrained. With theconcentration of dietary genistein increasing, the expressions of GnRHs, GnRHRs,NPYR2and PACAPmRNA were not significantly changed. It showed that highconcentration of genistein ruduced the synthesis and secretion of GH by inhibiting theexpression of ghrelin and NPY R1which were located on the up-stream of GH/IGF–Iaxis.4、GH was synthesised in the pituitary, following the blood circulation to theliver, then combined with the GHR in liver cells and stimulated the liver cells toreleasing IGF-I which combined with the IGFBP3, through the endocrine function tothe target cells, and promoted cell proliferation and growth ultimately. With dietarygenistein concentration0~3000g/g feeding tilapia diets for8weeks, IGF-I levels in plasma were examined by radioimmunoassay. IGF-I、GHR1、GHR2、IGF-IR、IGFBP3mRNA expression levels in liver were examined by real-time PCR. IGF-I level inplasma and the expression of mRNA decreased gradually in tilapia, and3000g/ggroup was significantly lower than that of the control, which was consistent with theperformance of growth and GH. In addition, the expression level of GHR2andIGF-IR mRNA in3000μg/g group decreased significantly compared to the controltreatment, while IGFBP3mRNA increased significantly compared to the controltreatment. It suggested that genistein may reduce the synthesis of IGF-I by inhibitingthe expression of liver GHR2, and also reduced the levels of free IGF-I by promotingthe synthesis of IGFBP3, then resulted in the decrease of the expression of IGF-IR,thus inhibited the function of GH mediated by IGF–I.In conclusion, it had no significant effect on the growth of tilapia in the lowconcentration (0~300g/g) of genistein, while it had significant effect when the theconcentration was higher than3000g/g. For its machanism, on one hand, it couldreduce the protein metabolism and glycometabolism of the tilapia by inhibiting themain activities of digestive enzymes of tilapia including stomach protease, trypsin,liver amylase and intestinal amylase, thus, reduce the tilapia conversion andabsorption of nutrients. On the other hand, it also could affect the GH/IGF–I axis.The high concentration of genistein could inhibit the expression of ghrelin and NPYR1, and thus reduced the synthesis and secretion of GH; By inhibiting the expressionof liver GHR2, IGF-IR, and IGFBP3, it reduced the synthesis of IGF-I and free IGF-I,resulting in inhibiting the function of GH mediated by IGF–I.