Regulation of the function of EGF receptors via P_2 purinergic receptors and related cell signaling systems

我们研究了嘌呤能受体和其他受体（缓激肽受体）的刺激如何引发细胞信号传导并影响其他受体，如EGF受体。我们选择A-431人表皮样癌细胞作为实验对象，因为它除了表达P_2嘌呤能受体外，还表达高水平的EGF受体。P_2嘌呤能受体的刺激刺激PLC活性，随后PKC激活和[Ca^<2+>]_i升高。从配体特异性增加[Ca^<2+>]_i来看，引起这种细胞活性的嘌呤能受体可能是P_<2Y>或P_<2U>型。升高[Ca^<2+>]_i的核苷酸均能抑制EGF受体的高亲和力结合，表明EGF受体的功能受P_2嘌呤能受体通过细胞信号转导系统调控。在缓激肽的刺激下，EGF的高亲和力结合也受到抑制。然而，缓激素对EGF高亲和力结合的抑制是短暂的。研究表明，缓激肽可以刺激磷酸化蛋白磷酸酶和蛋白激酶c。这种缓激肽磷酸化和去磷酸化EGF受体的双相作用表明，在调节磷酸化水平的过程中，受体功能受到稳态控制。除了PLC激活外，EGF和ATP的刺激激活了DG激酶或PLD，后者反过来提供PA并激活PKC/ [Ca2+]i系统，所有这些都被发现是刺激PI合成所必需的。目前的研究结果表明，配体刺激在激活第二信使和消耗其前体的同时，也刺激了第二信使前体的合成和回收途径。

We studied how stimulation of the purinergic receptors and other receptors (bradykinin receptors) elicites the cell signalling and affects other rceptors, e.g.the EGF receptor. We used A-431 human epidermoid carcinoma cells for the present experiment since they express high level of EGF receptors in addition to P_2 purinergic receptors. The stimulation of P_2 purinergic receptors stimulated PLC activity, followed by the PKC activation and [Ca^<2+>]_i elevation. From the ligand specificity to increase [Ca^<2+>]_i, the purinergic receptors responsible for provoking such cellular activity are suggested to be the P_<2Y> or P_<2U>-type. All the nucleotides that increased [Ca^<2+>]_i also inhibited the EGF receptor high affinity bindnig indicating that the function of EGF receptors is regulated by P_2 purinergic receptors via the cellular signal transduction system. The EGF high affinity binding was also inhibited when the cells were stimulated by bradykinin. However, the inhibition by bradykinin of the EGF high affinity binding was transient. Bradykinin was shown to stimulate a phosphoprotein phosphatase (s) as well as protein kinase C.Such biphasic effects of bradykinin to phosphorylate and dephosphorylate EGF receptors imply a homeostatic control of the receptor function in regulating phopshorylation level. In addition to PLC activation, stimulation by EGF and ATP activated either DG kinase or PLD which in turn supplied PA and activated PKC/ [Ca2+]i system, all of which were found to be essential for the stimulation of PI synthesis. The present results imply the general prospect that ligand stimulation, which mobilizes second messengers and consumes their precursors, simultaneously provokes the pathway to synthesize and salvage the second messenger precursors.