Herman A, Helman D, Livnah O, Gertler A
Institute of Biochemistry, Food Science and Nutrition, Faculty of Agricultural, Food and Environmental Quality Sciences, Wolfson Centre for Applied Structural Biology, Faculty of Life Sciences, Hebrew University of Jerusalem, Rehovot 76100, Israel.
J Biol Chem. 1999 Mar 19;274(12):7631-9. doi: 10.1074/jbc.274.12.7631.
Growth hormone receptor (GHR)-mediated activity of ruminant placental lactogens (PLs) and ovine (o) GH was compared, using cells transfected with full size human (h), rabbit (rb), and oGHRs. All three PLs acted as agonists in heterologous bioassays, whereas in homologous bioassays in cells transfected with oGHRs they antagonized the oGH activity. Despite these differences, oGH and PLs bound with similar affinity to the oGHR extracellular domain (oGHR-ECD), indicating that the binding occurs through hormone site I. Gel filtration of complexes between oPL and oGHR-ECD showed a 1:1 stoichiometry, confirming this conclusion. The oPL T185D and bPL T188D, which exhibited weak biological activity mediated through GHRs, behaved as site I antagonists, whereas oPL G130R and bPL G133R formed a 1:1 complex with GHR-ECDs and bound to h/rb/oGHR-ECDs with affinity similar to that of wild-type oPL. They had no agonistic activity in all models transfected with h/rb and oGHRs, but were antagonistic to all of them. In conclusion, ruminant PLs antagonize the activity of oGH in homologous systems, because they cannot homodimerize oGHRs, whereas in heterologous systems they act as agonists. The structural analysis hints that minor differences in the sequence of the GHR-ECDs may account for this difference. Since the initial step in the activity transduced through cytokine/hemapoietic receptors family is receptor homodimerization or heterodimerization, we suggest that the question of homologous versus heterologous interactions should be reexamined.
使用转染了全长人（h）、兔（rb）和绵羊（o）生长激素受体（GHR）的细胞，比较了反刍动物胎盘催乳素（PLs）和绵羊（o）生长激素（GH）的GHR介导活性。所有三种PLs在异源生物测定中均表现为激动剂，而在转染了oGHRs的细胞中的同源生物测定中，它们拮抗oGH活性。尽管存在这些差异，oGH和PLs与oGHR细胞外结构域（oGHR-ECD）的结合亲和力相似，表明这种结合是通过激素位点I发生的。oPL与oGHR-ECD之间复合物的凝胶过滤显示化学计量比为1:1，证实了这一结论。表现出通过GHRs介导的弱生物活性的oPL T185D和bPL T188D表现为位点I拮抗剂，而oPL G130R和bPL G133R与GHR-ECDs形成1:1复合物，并以与野生型oPL相似的亲和力结合到h/rb/oGHR-ECDs上。它们在所有转染了h/rb和oGHRs的模型中均无激动活性，但对所有这些模型均具有拮抗作用。总之，反刍动物PLs在同源系统中拮抗oGH的活性，因为它们不能使oGHRs同源二聚化，而在异源系统中它们表现为激动剂。结构分析表明，GHR-ECDs序列中的微小差异可能是造成这种差异的原因。由于通过细胞因子/造血受体家族转导活性的初始步骤是受体同源二聚化或异源二聚化，我们建议应重新审视同源与异源相互作用的问题。
