Subramaniam P S, Khan S A, Pontzer C H, Johnson H M
Department of Microbiology and Cell Science, University of Florida, Gainesville 32611, USA.
Proc Natl Acad Sci U S A. 1995 Dec 19;92(26):12270-4. doi: 10.1073/pnas.92.26.12270.
Interferon tau (IFN tau), originally identified as a pregnancy recognition hormone, is a type I interferon that is related to the various IFN alpha species (IFN alpha s). Ovine IFN tau has antiviral activity similar to that of human IFN alpha A on the Madin-Darby bovine kidney (MDBK) cell line and is equally effective in inhibiting cell proliferation. In this study, IFN tau was found to differ from IFN alpha A in that is was > 30-fold less toxic to MDBK cells at high concentrations. Excess IFN tau did not block the cytotoxicity of IFN alpha A on MDBK cells, suggesting that these two type I IFNs recognize the type I IFN receptor differently on these cells. In direct binding studies, 125I-IFN tau had a Kd of 3.90 x 10(-10) M for receptor on MDBK cells, whereas that of 125I-IFN alpha A was 4.45 x 10(-11) M. Consistent with the higher binding affinity, IFN alpha A was severalfold more effective than IFN tau in competitive binding against 125I-IFN tau to receptor on MDBK cells. Paradoxically, the two IFNs had similar specific antiviral activities on MDBK cells. However, maximal IFN antiviral activity required only fractional occupancy of receptors, whereas toxicity was associated with maximal receptor occupancy. Hence, IFN alpha A, with the higher binding affinity, was more toxic than IFN tau. The IFNs were similar in inducing the specific phosphorylation of the type I receptor-associated tyrosine kinase Tyk2, and the transcription factors Stat1 alpha and Stat2, suggesting that phosphorylation of these signal transduction proteins is not involved in the cellular toxicity associated with type I IFNs. Experiments using synthetic peptides suggest that differences in the interaction at the N terminal of IFN tau and IFN alpha with the type I receptor complex contribute significantly to differences in high-affinity equilibrium binding of these molecules. It is postulated that such a differential recognition of the receptor is responsible for the similar antiviral but different cytotoxic effects of these IFNs. Moreover, these data imply that receptors are "spare'' with respect to certain biological properties, and we speculate that IFNs may induce a concentration-dependent selective association of receptor subunits.
干扰素τ（IFNτ）最初被鉴定为一种妊娠识别激素，是一种与多种α干扰素（IFNα）相关的I型干扰素。绵羊IFNτ在马-达二氏牛肾（MDBK）细胞系上具有与人IFNαA相似的抗病毒活性，并且在抑制细胞增殖方面同样有效。在本研究中，发现IFNτ与IFNαA的不同之处在于，在高浓度时它对MDBK细胞的毒性比IFNαA低30倍以上。过量的IFNτ不会阻断IFNαA对MDBK细胞的细胞毒性，这表明这两种I型干扰素在这些细胞上对I型干扰素受体的识别方式不同。在直接结合研究中，125I-IFNτ对MDBK细胞上受体的解离常数（Kd）为3.90×10-10M，而125I-IFNαA的解离常数为4.45×10-11M。与更高的结合亲和力一致，在与125I-IFNτ竞争结合MDBK细胞上的受体时，IFNαA比IFNτ有效几倍。矛盾的是，这两种干扰素在MDBK细胞上具有相似的特异性抗病毒活性。然而，干扰素的最大抗病毒活性仅需要受体的部分占据，而毒性与受体的最大占据相关。因此，具有更高结合亲和力的IFNαA比IFNτ毒性更大。这两种干扰素在诱导I型受体相关酪氨酸激酶Tyk2以及转录因子Stat1α和Stat2的特异性磷酸化方面相似，这表明这些信号转导蛋白的磷酸化与I型干扰素相关的细胞毒性无关。使用合成肽的实验表明，IFNτ和IFNα在N末端与I型受体复合物相互作用的差异显著导致了这些分子在高亲和力平衡结合方面的差异。据推测，受体的这种差异识别是这些干扰素具有相似抗病毒但不同细胞毒性作用的原因。此外，这些数据意味着受体在某些生物学特性方面是“备用的”，并且我们推测干扰素可能诱导受体亚基的浓度依赖性选择性结合。
