SOCS-3 Is Tyrosine Phosphorylated in Response to Interleukin-2 and Suppresses STAT5 Phosphorylation and Lymphocyte Proliferation

Members of the recently discovered SOCS/CIS/SSI family have been proposed as regulators of cytokine signaling, and while targets and mechanisms have been suggested for some family members, the precise role of these proteins remains to be defined. To date no SOCS proteins have been specifically implicated in interleukin-2 (IL-2) signaling in T cells. Here we report SOCS-3 expression in response to IL-2 in both T-cell lines and human peripheral blood lymphocytes. SOCS-3 protein was detectable as early as 30 min following IL-2 stimulation, while CIS was seen only at low levels after 2 h. Unlike CIS, SOCS-3 was rapidly tyrosine phosphorylated in response to IL-2. Tyrosine phosphorylation of SOCS-3 was observed upon coexpression with Jak1 and Jak2 but only weakly with Jak3. In these experiments, SOCS-3 associated with Jak1 and inhibited Jak1 phosphorylation, and this inhibition was markedly enhanced by the presence of IL-2 receptor beta chain (IL-2Rβ). Moreover, following IL-2 stimulation of T cells, SOCS-3 was able to interact with the IL-2 receptor complex, and in particular tyrosine phosphorylated Jak1 and IL-2Rβ. Additionally, in lymphocytes expressing SOCS-3 but not CIS, IL-2-induced tyrosine phosphorylation of STAT5b was markedly reduced, while there was only a weak effect on IL-3-mediated STAT5b tyrosine phosphorylation. Finally, proliferation induced by both IL-2- and IL-3 was significantly inhibited in the presence of SOCS-3. The findings suggest that when SOCS-3 is rapidly induced by IL-2 in T cells, it acts to inhibit IL-2 responses in a classical negative feedback loop.

新近发现的SOCS/CIS/SSI家族成员被认为是细胞因子信号转导的调控因子，尽管已有部分家族成员的作用靶点与调控机制被提出，但这类蛋白质的确切功能仍有待阐明。迄今为止，尚无SOCS蛋白被明确证实参与T细胞内的白细胞介素-2（interleukin-2, IL-2）信号转导过程。本研究报道了T细胞系及人外周血淋巴细胞中，IL-2可诱导SOCS-3的表达。IL-2刺激后30分钟即可检测到SOCS-3蛋白，而CIS仅在刺激2小时后呈现低水平表达。与CIS不同，SOCS-3可在IL-2刺激下快速发生酪氨酸磷酸化。当与贾纳斯激酶1（Janus kinase 1, Jak1）、贾纳斯激酶2（Janus kinase 2, Jak2）共表达时，可观察到SOCS-3的酪氨酸磷酸化；而与贾纳斯激酶3（Janus kinase 3, Jak3）共表达时，磷酸化水平仅微弱升高。在上述实验中，SOCS-3可与Jak1结合并抑制其磷酸化，且IL-2受体β链（IL-2Rβ）的存在可显著增强这一抑制效应。此外，在T细胞经IL-2刺激后，SOCS-3可与IL-2受体复合物结合，尤其是与酪氨酸磷酸化的Jak1及IL-2Rβ相互作用。进一步研究发现，在仅表达SOCS-3而非CIS的淋巴细胞中，IL-2诱导的信号转导与转录激活因子5b（signal transducer and activator of transcription 5b, STAT5b）酪氨酸磷酸化水平显著降低，而其对IL-3介导的STAT5b酪氨酸磷酸化仅存在微弱影响。最后，SOCS-3的存在可显著抑制IL-2及IL-3诱导的细胞增殖。本研究结果表明，当IL-2在T细胞中快速诱导SOCS-3表达时，SOCS-3可通过经典的负反馈循环抑制IL-2介导的细胞应答反应。