Chen X, Levkowitz G, Tzahar E, Karunagaran D, Lavi S, Ben-Baruch N, Leitner O, Ratzkin B J, Bacus S S, Yarden Y
Department of Chemical Immunology, Weizmann Institute of Science, Rehovot, Israel.
J Biol Chem. 1996 Mar 29;271(13):7620-9.
The group of subtype I transmembrane tyrosine kinases includes the epidermal growth factor (EGF) receptor (ErbB-1), an orphan receptor (ErbB-2), and two receptors for the Neu differentiation factor (NDF/heregulin), namely: ErbB-3 and ErbB-4. Here we addressed the distinct functions of the two NDF receptors by using an immunological approach. Two sets of monoclonal antibodies (mAbs) to ErbB-3 and ErbB-4 were generated through immunization with recombinant ectodomains of the corresponding receptors that were fused to immunoglobulin. We found that the shared ligand binds to highly immunogenic, but immunologically distinct sites of ErbB-3 and ErbB-4. NDF receptors differed also in their kinase activities; whereas the catalytic activity of ErbB-4 was activable by mAbs, ErbB-3 underwent no activation by mAbs in living cells. Likewise, down-regulation of ErbB-4, but not ErbB-3, was induced by certain mAbs. By using the generated mAbs, we found that the major NDF receptor on mammary epithelial cells is a heterodimer of ErbB-3 with ErbB-2, whereas an ErbB-1/ErbB-2 heterodimer, or an ErbB-1 homodimer, is the predominant species that binds EGF. Consistent with ErbB-2 being a shared receptor subunit, its tyrosine phosphorylation was increased by both heterologous ligands and it mediated a trans-inhibitory effect of NDF on EGF binding. Last, we show that the effect of NDF on differentiation of breast tumor cells can be mimicked by anti-ErbB-4 antibodies, but not by mAbs to ErbB-3. Nevertheless, an ErbB-3-specific mAb partially inhibited the effect of NDF on cellular differentiation. These results suggest that homodimers of ErbB-4 are biologically active, but heterodimerization of the kinase-defective ErbB-3, probably with ErbB-2, is essential for transmission of NDF signals through ErbB-3.
I型跨膜酪氨酸激酶家族包括表皮生长因子（EGF）受体（ErbB-1）、一个孤儿受体（ErbB-2）以及神经分化因子（NDF/heregulin）的两个受体，即ErbB-3和ErbB-4。在此，我们采用免疫学方法探讨了这两个NDF受体的不同功能。通过用与免疫球蛋白融合的相应受体的重组胞外域进行免疫，产生了两组针对ErbB-3和ErbB-4的单克隆抗体（mAb）。我们发现，共同配体与ErbB-3和ErbB-4的高度免疫原性但免疫学上不同的位点结合。NDF受体的激酶活性也有所不同；虽然ErbB-4的催化活性可被mAb激活，但在活细胞中ErbB-3不会被mAb激活。同样，某些mAb可诱导ErbB-4的下调，但不会诱导ErbB-3的下调。通过使用所产生的mAb，我们发现乳腺上皮细胞上主要的NDF受体是ErbB-3与ErbB-2的异二聚体，而ErbB-1/ErbB-2异二聚体或ErbB-1同二聚体是结合EGF的主要形式。与ErbB-2作为共享受体亚基一致，其酪氨酸磷酸化可被两种异源配体增强，并且它介导了NDF对EGF结合的反式抑制作用。最后，我们表明抗ErbB-4抗体可模拟NDF对乳腺肿瘤细胞分化的作用，但抗ErbB-3的mAb则不能。然而，一种ErbB-3特异性mAb可部分抑制NDF对细胞分化的作用。这些结果表明，ErbB-4同二聚体具有生物学活性，但激酶缺陷型ErbB-3（可能与ErbB-2）的异二聚化对于通过ErbB-3传递NDF信号至关重要。
