Misra U K, Gawdi G, Pizzo S V
Department of Pathology, Duke University Medical Center, Durham, North Carolina 27710, USA.
J Cell Biochem. 1996 Apr;61(1):61-71. doi: 10.1002/(SICI)1097-4644(19960401)61:1%3C61::AID-JCB8%3E3.0.CO;2-0.
Binding of receptor-recognized forms of tetrameric human alpha 2-macroglobulin (alpha 2M*) to a macrophage signaling receptor induces cAMP synthesis, increases in inositol 1,4,5-triphosphate (IP3) synthesis, and a concomitant rise in cytosolic free calcium ([Ca2+]i). The alpha 2M* signaling receptor is coupled to a pertussis-toxin insensitive G protein. Binding of alpha 2M* also occurs to the low density lipoprotein receptor-related protein/alpha 2M receptor (LRP/alpha 2MR), but this binding does not induce signal transduction. Rat alpha 1-inhibitor-3 (alpha 1I3) is a monomeric member of the alpha-macroglobulin/complement superfamily. Like alpha 2M, it can react with proteinases or methylamine which induces a conformational change causing activated alpha 1I3 to bind to LRP/alpha 2MR. We now report that alpha 1I3-methylamine binds to the macrophage alpha 2M* signaling receptor inducing a rapid rise in the synthesis of IP3 with a subsequent 1.5- to 3-fold rise in [Ca2+]i. alpha 1I3-methylamine binding to macrophages also caused a statistically significant elevation in cAMP. Native alpha 1I3, like alpha 2M, was unable to induce signal transduction. alpha 1I3 forms a complex with alpha 1-microglobulin, which has a distinct conformation from alpha 1I3 and is recognized by LRP/alpha 2MR. This complex also induces an increase in [Ca2+]i comparable to the effect of alpha 1I3-methylamine on macrophages. It is concluded that activation of alpha 1I3 by methylamine or binding of alpha 1-microglobulin causes similar conformational changes in the inhibitor, exposing the receptor recognition site for the alpha 2M* signaling receptor, as well as for LRP/alpha 2MR.
受体识别形式的四聚体人α2-巨球蛋白（α2M*）与巨噬细胞信号受体结合可诱导环磷酸腺苷（cAMP）合成、肌醇1,4,5-三磷酸（IP3）合成增加以及胞质游离钙（[Ca2+]i）随之升高。α2M信号受体与百日咳毒素不敏感的G蛋白偶联。α2M也可与低密度脂蛋白受体相关蛋白/α2M受体（LRP/α2MR）结合，但这种结合不诱导信号转导。大鼠α1-抑制因子3（α1I3）是α-巨球蛋白/补体超家族的单体成员。与α2M一样，它可与蛋白酶或甲胺反应，诱导构象变化，使活化的α1I3与LRP/α2MR结合。我们现在报告，α1I3-甲胺与巨噬细胞α2M信号受体结合，诱导IP3合成迅速增加，随后[Ca2+]i升高1.5至3倍。α1I3-甲胺与巨噬细胞的结合还导致cAMP在统计学上显著升高。天然α1I3与α2M一样，无法诱导信号转导。α1I3与α1-微球蛋白形成复合物，其构象与α1I3不同，可被LRP/α2MR识别。该复合物也可诱导[Ca2+]i增加，其效果与α1I3-甲胺对巨噬细胞的作用相当。结论是，甲胺对α1I3的激活或α1-微球蛋白的结合会使抑制剂发生类似的构象变化，暴露出α2M信号受体以及LRP/α2MR的受体识别位点。
