Cyclic Phosphatidylinositol Metabolism and Resynthesis

在激素刺激下，二磷酸磷脂酰肌醇水解后，磷脂酰肌醇转化为二磷酸磷脂酰肌醇。研究表明，除了质膜外，来自亚细胞器的脂质必须是可接近的，以解释经常参与循环反应的大量PtdIns。迄今为止，已经提出了两种机制来解释这一点：(1) PtdIns转运蛋白促进细胞内PtdIns转运至质膜；(2)代谢的大部分PtdIns直接水解，可能是细胞内游离钙增加的结果，不参与肌醇磷酸三酯第二信使的生成，也不直接与激素受体复合物相关。第三种可能性是：(3)通过细胞内部的受体隔离和再循环进入细胞膜内PtdIns。人们设计了一系列实验来检验这第三种可能性。它们包括钾消耗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发生在质膜上。PtdIns合成酶的亚细胞位置将通过免疫细胞化学确定。在细胞对激素的反应中，一个重要的信号通路涉及到一种被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。为了维持细胞对激素的反应能力，这种物质必须重新合成。这项研究的目的是确定细胞内的何处发生再合成，以及如何与细胞的激素刺激协调控制。这项研究的结果将为细胞如何对激素和其他外部信号作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.