Previously postulated "ligand-independent" signaling of GPR4 is mediated through proton-sensing mechanisms.
Previously postulated "ligand-independent" signaling of GPR4 is mediated through proton-sensing mechanisms.
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DOI：
10.1016/j.cellsig.2007.03.009
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发表时间：
2007-08
影响因子：
4.8
通讯作者：
M. Tobo;H. Tomura;C. Mogi;Ju-Qiang Wang;Jin-Peng Liu;Mayumi Komachi;A. Damirin;Takao Kimura;N. Murata;H. Kurose;Koichi Sato;F. Okajima
中科院分区：
文献类型：
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作者：
M. Tobo;H. Tomura;C. Mogi;Ju-Qiang Wang;Jin-Peng Liu;Mayumi Komachi;A. Damirin;Takao Kimura;N. Murata;H. Kurose;Koichi Sato;F. Okajima
GPR 4最初被鉴定为鞘氨醇磷酸胆碱和溶血磷脂酰胆碱的受体;然而，脂质作用并不总是得到证实。相反，有时在GPR 4和其他OGR 1家族受体表达细胞中观察到配体非依赖性作用。在这里，我们研究了细胞外质子的可能参与，这是最近提出的GPR 4的另一种配体。在pH 7.4时，与载体转染细胞相比，GPR 4转染的RH 7777细胞中表皮生长因子诱导的细胞外信号调节激酶活性较低，与cAMP积累增加相关。血清反应元件（SRE）驱动的转录活性也明显高于GPR 4表达的HEK 293细胞比载体转染的细胞在pH 7.4。这些明显的配体独立的行动是非常小的碱性7.8。在表达GPR 4或其他OGR 1受体家族成员的HEK 293细胞中，细胞外酸化以依赖于G13蛋白/Rho信号通路的方式进一步增加SRE活性。表达GPR 4的细胞还显示出在pH 7.4下活化T细胞的钙调神经磷酸酶依赖性核因子（NFAT）启动子活化，并且该活性通过pH低于7.2与磷酸肌醇产生相关而进一步增加。然而，在相反的cAMP和SRE的反应，碱化到pH 7.8几乎没有影响的高基础活动。最后，GPR 4的表达几乎不调制鞘氨醇磷酸胆碱或溶血磷脂酰胆碱诱导的行动。这些结果表明，细胞外质子通过GPR 4受体在一些先前假设的配体非依赖性作用中发挥配体的作用。此外，GPR 4可能是一种多功能受体，与Gs、G13和Gq/11蛋白偶联，以响应细胞外酸化。
GPR4 was initially identified as a receptor for sphingosylphosphorylcholine and lysophosphatidylcholine; however, lipid actions have not always been confirmed. Instead, ligand-independent actions have sometimes been observed in GPR4- and other OGR1 family receptor-expressing cells. Here, we examined the possible involvement of extracellular protons, which have recently been proposed as another ligand for GPR4. At pH 7.4, the epidermal growth factor-induced extracellular signal-regulated kinase activity was lower in GPR4-transfected RH7777 cells, in association with increased cAMP accumulation, than in vector-transfected cells. The serum response element (SRE)-driven transcriptional activity was also clearly higher in GPR4-expressing HEK293 cells than in vector-transfected cells at pH 7.4. These apparent ligand-independent actions were very small at alkalinic 7.8. The SRE activity was further increased by extracellular acidification in a manner dependent on the G13protein/Rho signaling pathway in HEK293 cells expressing GPR4 or other OGR1 receptor family members. GPR4-expressing cells also showed a calcineurin-dependent nuclear factor of activated T cell (NFAT) promoter activation at pH 7.4, and this activity was further increased by pH below 7.2 in association with inositol phosphate production. In contrast to the cAMP and SRE responses, however, alkalinization to pH 7.8 hardly affected the high basal activity. Finally, the expression of GPR4 hardly modulated the sphingosylphosphorylcholine- or lysophosphatidylcholine-induced action. These results suggest that an extracellular proton play a role as a ligand in some of previously postulated ligand-independent actions through GPR4 receptors. Moreover, GPR4 may be a multi-functional receptor coupling to Gs, G13, and Gq/11proteins in response to extracellular acidification.