LAT1调控奶牛乳腺乳蛋白合成的机理研究

Mammary gland is a highly specialized organ that produced milk protein during lactation. A steady and sufficient amino acid supply is necessary to maintain a high degree of milk protein synthesis. LAT1 is one of the amino acid transporters responsible for transmembrane movement of several essential amino acids in animal cells. Preliminary study showed that LAT1 expression was positively correlated with milk protein content in lactating mammary tissues of dairy cows. In cultured mammary epithelial cells of dairy cows, LAT1 overexpression increased milk protein synthesis, whereas LAT1 knockdown had the opposite effect. Prolactin treatment also increased LAT1 expression and milk protein synthesis in cultured mammary epithelial cells of dairy cows. These results suggest that LAT1 is involved in the regulation of amino acid uptake and milk protein synthesis in mammary gland of dairy cow. However, the underlying molecular mechanism has remained largely unexplored. Therefore, the aim of this study is to investigate the precise role of prolactin in LAT1 transporting essential amino acid, and analyze the regulation of prolactin signal on LAT1 expression and function. Then, using cultured mammary epithelial cells as a model, we investigate how LAT1 stimulate mTOR activation and milk protein synthesis via binding lysosomal-associated protein. These results will help us understand the process of lactation and elucidate the molecular mechanisms underlying LAT1 modulation of amino acid transport and milk protein synthesis.

泌乳乳腺是乳蛋白合成极其旺盛的场所，能否获得充足的氨基酸供给是顺利合成乳蛋白的关键。LAT1是重要的氨基酸转运载体，介导细胞对必需氨基酸的摄取。本研究的前期结果发现奶牛乳腺中LAT1的表达与乳蛋白率之间呈正相关；乳腺上皮细胞中LAT1基因过表达和干扰后，乳蛋白合成相应升高和降低；细胞培养基中添加催乳素也可以促进LAT1表达上调，进而提高乳蛋白合成。这些结果预示LAT1在奶牛乳腺对氨基酸摄取过程中发挥重要调控作用，进而影响乳蛋白的合成。为阐明奶牛乳腺中LAT1调控乳蛋白合成的分子机制，我们首先研究催乳素对LAT1氨基酸转运能力的调控作用；随后分析催乳素信号分子调节LAT1基因表达的方式；最后研究LAT1作为转运载体同溶酶体相关蛋白相互作用，激活mTOR信号通路，调节乳蛋白合成的方式。本研究的成果将使我们更清楚地理解乳腺的泌乳过程，明确LAT1调控奶牛乳腺氨基酸供给及乳蛋白合成的分子机制。

为满足泌乳期奶牛乳腺中乳蛋白的合成，乳腺上皮细胞对氨基酸的需求显著增加。LAT1是负责转运必需氨基酸进入细胞的主要转运载体，其在奶牛乳腺中的表达与乳蛋白合成呈正相关。为深入探讨LAT1的功能与乳蛋白合成之间的关系，本项目研究了奶牛乳腺上皮细胞中催乳素对LAT1氨基酸转运能力的调控作用，并分析催乳素信号调节LAT1表达与活性的方式，最后研究了LAT1激活mTOR信号通路，调节乳蛋白合成的分子方式。研究结果显示奶牛乳腺上皮细胞中催乳素通过LAT1介导细胞对Thr、Val、Met、Ile、Leu、Tyr、Lys以及His的摄取。催乳素通过激活JAK-STAT5以及PI3K-AKT信号通路调节LAT1的转录活性以及LAT1在细胞内的转位。在乳蛋白合成过程中，LAT1的表达与mTOR信号通路激活相关。Met作用可以诱导LAT1转位到细胞溶酶体膜，通过与溶酶体膜蛋白LAPTM4b结合，激活mTOR信号通路，从而诱导乳蛋白的合成。本研究结果使我们更清楚地了解LAT1调控奶牛乳腺氨基酸供给模式及乳蛋白合成的分子机制，为通过营养调控方式提高泌乳奶牛的乳品质提供理论支持。