ROLE OF CELL SURFACE IN INITIATION OF CELL DIVISION

凝血酶启动心肌细胞分裂机制的研究 培养的成纤维细胞表明，细胞表面的作用是 足以激活细胞。这引发了对细胞表面的研究 用于刺激细胞分裂所必需的相互作用和事件。 这些研究导致鉴定出一种细胞分泌的蛋白酶Nexin。 介导凝血酶与细胞的大部分特异性结合的蛋白质 浮出水面。蛋白酶Nexin由细胞释放到培养基中，在 它与凝血酶形成共价键。凝血酶-蛋白水解酶连接素 然后，复合体特异性地与细胞结合，并内化并 降级了。我们的研究表明，凝血酶与蛋白水解酶的联系 Nexin使凝血酶失活。此外，蛋白水解酶Nexin抑制 凝血酶的刺激，因此它代表了一种细胞 调节它们对凝血酶的有丝分裂反应。我们还演示了一个 细胞表面未连接凝血酶的结合部位和已显示的结合 凝血酶不是刺激细胞所必需的 组织。过去的研究表明，蛋白水解物的活性 凝血酶是细胞激活所必需的。事实上，我们所有的结果 指出需要通过以下方式切割一个或多个细胞表面蛋白 用于刺激的凝血酶。在这一点上，我们已经展示了几个细胞 表面蛋白被凝血酶切割。其中一种已经被确认 就是纤维连接蛋白。此外，细胞表面蛋白约为140 千道尔顿和55千道尔顿是凝血酶敏感的，但他们不是 但已被确认身份。正在进行更好地解析膜的研究 蛋白质，所以我们将能够识别其他可能是 对凝血酶敏感。我们将研究这些乳沟中哪些是必要的 通过确定它们是否出现在一系列大的 对病毒有反应或无反应的克隆细胞群 凝血酶的促有丝分裂作用。(A)

Our studies on the mechanism by which thrombin initiates division of cultured fibroblasts have shown that action at the cell surface is sufficient for cell activation. This prompted studies on the cell surface for interactions and events necessary for the stimulation of cell division. These studies led to the identification of protease nexin, a cell-secreted protein which mediates much of the specific binding of thrombin to the cell surface. Protease nexin is released by cells into the culture medium where it forms a covalent linkage with thrombin. The thrombin-protease nexin complexes then specifically bind to cells and are internalized and degraded. Our studies have shown that linkage of thrombin to protease nexin inactivates the thrombin. Moreover, protease nexin inhibits the stimulation by thrombin, and thus it represents a mechanism by which cells modulate their mitogenic response to thrombin. We have also demonstrated a cell surface binding site for unlinked thrombin and have shown that binding of thrombin to this site is not necessary for the stimulation of cell division. Past studies have shown that the proteolytic activity of thrombin is necessary for cell activation. In fact, all of our results point to a requirement for cleavage of one or more cell surface proteins by thrombin for the stimulation. At\this point we have shown that several cell surface proteins are cleaved by thrombin. One of these has been identified as fibronectin. In addition, cell surface proteins of about 140 kilodaltons and 55 kilodaltons were thrombin-sensitive, but they have not yet been identified. Studies\are underway to better resolve membrane proteins so we will be able to identify additional ones that might be thrombin-sensitive. We will study which of these cleavages are necessary for cell activation by determining whether they occur in a large series of cloned cell populations that are either responsive or unresponsive to the mitogenic action of thrombin. (A)