热应激通过诱导奶牛乳腺细胞凋亡减少乳蛋白

摘要

本试验旨在研究热应激时乳蛋白含量和产量降低与泌乳相关激素、乳蛋白合成相关信号通路及乳腺细胞凋亡的关系，并揭示热应激引起乳蛋白含量和产量下降的原因。选取4头泌乳日龄、体重、产奶量相近的经产健康荷斯坦奶牛作为试验动物。采取2×2交叉试验设计，试验共2期，每期18 d（其中预试期和试验期各9 d），2期之间间隔30 d。预试期为热中性环境，自由采食。试验期奶牛随机分为2组（ n＝2），分别为热应激组和配对限饲组。结果表明：1）热应激显著降低了乳蛋白的含量和产量（ P＜0．05）。2）热应激对酪氨酸激酶（ JAK）－信号转导及转录激活因子（STAT）信号通路及κ－酪蛋白（CSN3）基因表达量没有显著影响（P＞0．05）。3）热应激显著增加了哺乳动物雷帕霉素靶蛋白（ mTOR ）信号通路中的 mTOR 的基因表达量（ P＜0．05），且有增加核糖体S6蛋白激酶（S6K1）基因表达量的趋势（0．05≤P＜0．10）。4）热应激增加了乳腺细胞中与细胞凋亡相关的半胱氨酸蛋白酶3（ CASP3）、环氧合酶－2（ COX2）基因的表达量（P＜0．05），且有增加B淋巴细胞瘤－2相关X蛋白（BAX）基因表达量的趋势（0．05≤P＜0．10）。综合可知，热应激可能并不是通过调控单个乳腺细胞合成乳蛋白的能力来影响乳蛋白的含量和产量，而是通过诱导乳腺细胞的凋亡，减少可用于乳蛋白合成的乳腺细胞的数量来影响的。%This experiment was conducted to investigate the relationship among the decrease of milk protein content and yield under heat stress, hormones related to lactation, pathways involved in milk protein synthesis, and apoptosis of mammary cells, and to find reasons for milk protein decline caused by heat stress. Four health⁃y multiparous lactating Holstein cows with similar days in milk, body weight and milk yield were used as ex⁃periment animals. Crossover design was applied, there were two experimental periods, and each period lasted for 18 days ( 9 d of pretrial period, and 9 d of trail period) with 30 d between the two periods. Cows in pretrial period were exposed to thermal neutral conditions and allowed to eat ad libitum. Cows in trial period were ran⁃domly divided into two groups ( n=2) , which were heat stress group and pair⁃restricted feeding group. The re⁃sults showed as follows: 1) heat stress significantly decreased milk protein yield and content (P<0.05). 2) Heat stress had no significant influence on Janus kinase ( JAK)⁃signal transducer and activator of transcription ( STAT) pathway andκ⁃casein ( CSN3) gene expression ( P>0.05) . 3) In mTOR pathway, heat stress signifi⁃cantly increased the expression of mammalian target of rapamycin ( mTOR) gene ( P<0.05) , and had the tend⁃ency to increase the expression of ribosomal protein S6 kinase (S6K1) gene (0.05≤P<0.10). 4) Heat stress significantly increased the expressions of Caspase⁃3 ( CASP3) and cyclooxygenase⁃2 ( COX2) genes involved in apoptosis (P<0.05), and had the tendency to increase the expression of B⁃cell lymphoma⁃2⁃associated X protein (BAX) (0.05≤P<0.10). It is concluded that heat stress may not be able to decline milk protein con⁃tent and yield through regulating the capacity of individual mammary cells, but thorough induction of apoptosis of the mammary cells and decrease of the quantity of mammary cells used for milk protein synthesis.