Effect Of Heat Stress On Mammary Gland Development And Nutritional Regulation

The mammary gland is an exocrine gland that is unique to mammals,and its secretion of milk is essential for the growth and development of offspring.The good development of the mammary gland is a prerequisite for its full play of lactation function.Therefore,study and elucidation of the underlying mechanisms involved in mammary gland development and nutritional regulation have important theoretical and practical significance for increasing the animal lactation capacity and further improving the animal husbandry production level.The development of animal mammary glands consists of several distinct stages,including embryonic,puberty,pregnancy,lactation and involution stages.Among them,pregnancy,lactation and involution stages are cyclically changed with the reproductive cycle.The development of pubertal mammary gland is mainly manifested as the growth and branching of the mammary ducts,while the development of the later period is mainly the development and involution of the mammary acinus.The development of the mammary gland during puberty has an important influence on the following development of the mammary gland and the lactation function.Mammary gland development is affected by many factors such as hormones,environment,nutrition and management.Nowadays,there are many researches focusing on hormonal regulation of mammary gland development.In contrast,there are few reports on the effects of hot environment(or heat stress)on breast development and on the nutritional regulation.Therefore,the aim of the present study is to investigate the effects of heat stress on the development of mammary glands in porcine and mouse),and to explore the regulatory role of nutritional factors such as tauriursodeoxycholic acid(TUDCA)and lauric acid in this process.For this purpose,we first compared the composition of mammary gland(DNA,RNA and protein content,etc.),expression of proliferation and inflammation-related genes in the mammary gland,serum levels of hormones in the lactating sows during winter(December to January)or summer(June)of southern China.Secondly,we investigated the effects of heat stress on mammary gland development(the number of catheter branches and terminal end buds(TEB),etc.),expression of mammary gland proliferation-related gene,signaling pathway involved in mammary gland proliferation,and serum levels of estrogen(E2)and insulin-like growth factor-1(IGF-1)in pubertal mice.On these basis,we further explore the possible beneficial effects of TUDCA and lauric acid(LA)on development of the damaged mammary glands caused by heat stress.The results are as follows:1.In Guangdong,the DNA content of mammary glands of lactating sows in summer(June)was significantly lower than that of sows in winter(December-January).Compared with lactating sows in winter,mRNA levels of proliferation-associated genes such as PCNA,Cyclin B1,Cyclin D3,Cyclin A2,Cyclin E2,and protein levels of PCNA were significantly lower in sow mammary glands in summer.In contrast,the expression of inflammation-associated gene TLR4 in mammary glands of lactating sows in summer was significantly higher than that in winter sows.Meanwhile,the phosphorylation level of AKT was significantly lower in the sow mammary gland than in the sow in the summer,indicating that the proliferation-related pathway AKT was significantly inhibited in the mammary gland of sow in summer.However,the concentration of E2 in lactating sows in the summer was comparable with that of lactating sows in winter.2.Heat stress significantly reduced average weekly feed intake and body weight of mice,while significantly increased average water intake.Heat stress significantly inhibiting mammary gland development of pubertal mice by reducing mammary duct branching and TEB number in mammary glands.Heat stress significantly promoted the protein expression of TLR4,and significantly decreased the Cyclin D1 protein expression and the Cyclin D3 mRNA expression in the mammary gland.In addition,heat stress significantly reduced the serum levels of proliferation-related hormones such as IGF-1 and E2.These above results suggest that heat stress inhibits the development of mammary gland in pubertal mice by inhibiting the expression of proliferation-related genes,promoting the expression of inflammatory genes,and reducing the serum levels of hormones related to proliferation.3.Compared with the heat stress group,dietary supplementation of 0.25% TUDCA significantly increased the average weekly feed intake,while significantly decreased the average weekly drinking water,with no significant effect on body weight and body weight gain in mice.RIA and ELISA results showed that TUDCA had no significant effect on serum levels of IGF-1 and E2 in mice.Results of temperature test showed that TUDCA significantly increased mice rectal temperature,with no significant effect on body surface temperature of mice.The findings of whole mount staining showed that TUDCA significantly increased the numbers of TEBs and ductal branching of mouse mammary glands.In addition,the results of gene expression showed that TUDCA significantly increased the mRNA expression levels of Cyclin D1 and Cyclin D3 and the phosphorylation level of AKT in mouse mammary glands.At the same time,TUDCA significantly reduced the expression of inflammatory protein TLR4 in mouse mammary glands.The above results suggest that TUDCA can relieve the inhibitory effect of heat stress on mammary gland development by increasing the body core temperature of mice and the expression of mammary gland proliferation-related genes.4.Compared with the heat stress group,dietary supplementation of 1% lauric acid had no significant effect on the food intake and body weight of the heat-stressed mice,while significantly decreased the average weekly drinking water of the mice.Results of temperature testing showed that lauric acid had no significant effect on rectal temperature and body surface temperature in mice.The findings of whole mount staining showed that lauric acid had no significant effect on ductal branching and TEB number of the mammary gland in heat-stressed mouse.Meanwhile,RIA and ELISA results showed that lauric acid had no significant effect on serum levels of IGF-1 and E2.The results of gene expression showed that although lauric acid significantly increased the mRNA expression of proliferation-related genes PCNA and Cyclin D3 in the mammary glands of heat-stressed mouse,but also significantly increased the protein expression of inflammation-associated gene TLR4.The above results suggest that lauric acid can’t alleviate the heat stress-induced inhibition of mice mammary development.In summary,heat stress exerts inhibitory effects on the development of mammary glands in lactating sows and pubescent mice.The inhibitory effects of heat stress on the development of mammary gland in pubertal mice achieved by regulating the expression of proliferative and inflammation-related genes and the serum levels hormone related to mammary gland-proliferating.Dietary supplementation of TUDCA can alleviate the heat stress-induced inhibitory effects on mammary gland development by increasing the body core temperature and the expression of proliferation related genes in mice mammary gland,and reducing the expression of inflammatory genes.