Cyclic Phosphatidylinositol Metabolism and Resynthesis

在激素刺激下，磷脂酰肌醇二磷酸解后，磷脂酰肌醇转化为磷脂酰肌醇二磷酸。研究表明，除了质膜外，亚细胞器中的脂质必须是可获得的，以解释经常参与循环反应的大量PtdIns。到目前为止，已经提出了两种机制来解释这一点：(1)PtdIns转移蛋白促进细胞内PtdIns向质膜的转移，(2)代谢的大部分PtdIns被直接水解，可能是由于细胞内游离钙的增加，并且不参与三磷酸肌醇第二信使的产生，也不与激素受体复合体直接相关。第三种可能性被提出：(3)内膜PtdIns是由于受体的隔离和通过细胞内部的循环而进入的。已经设计了一系列实验来测试第三种可能性。它们包括钾耗竭和受体突变对内化的抑制。作为这一假设的推论，PtdIns的代偿性再合成不需要在质膜上发生。PtdIns合成酶的亚细胞定位将通过免疫细胞化学来确定。在细胞对激素的反应中，一个重要的信号通路涉及到一种称为磷脂酰肌醇二磷酸的膜成分的分解，以产生进一步的信号分子。为了保持细胞对激素的反应能力，这种材料必须重新合成。这项研究的目标是确定细胞内的重新合成发生在哪里，以及如何在细胞的激素刺激下进行控制。这项研究的结果将为细胞如何对激素和其他外部信号做出反应提供重要的新信息。

Hormone-stimulated hydrolysis of phosphatidyl inositol bis- phosphate is followed by conversion of phosphatidyl inositol to phosphatidyl inositol bis-phosphate. Studies indicate that lipid from subcellular organelles in addition to the plasma membrane must be accessible in order to account for the large amount of PtdIns that is often involved in the cycling response. To date, two mechanisms have been proposed to explain this: (1) A PtdIns transfer protein facilitates the transfer of intracellular PtdIns to the plasma membrane, and (2) The bulk of the PtdIns metabolized is hydrolyzed directly, perhaps as a result of the rise in intracellular free calcium, and is not involved in the generation of inositol tris phosphate second messengers or directly associated with a hormone-receptor complex. A third possibility is suggested: (3) Internal membrane PtdIns is accessed as a result of receptor sequestration and recycling through the interior of the cell. A series of experiments have been devised to test this third possibility. They involve inhibition of internalization by potassium depletion as well as by receptor mutation. As a corollary to this hypothesis, compensatory resynthesis of PtdIns need not take place at the plasma membrane. The subcellular location of the PtdIns synthetase enzyme will be determined by immunocytochemistry. %%% One important signaling pathway in the response of cells to hormones involves the breakdown of a membrane component known as phosphatidylinositol bis-phosphate to produce further signal molecules. To maintain a cell's ability to respond to hormone, this material must be resynthesized. The goal of this research is to determine where within the cell resynthesis takes place and how it is controlled in coordination with hormonal stimulation of the cell. The results of this research will contribute important new information on how cells respond to hormones and other external signals.