SOCS-3 is tryrosine 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 a s regulators of cytokine signaling, and while targets and mechanisms have b een suggested for some family members, the precise role of these proteins r emains to be defined. To date no SOCS proteins have been specifically impli cated in interleukin-2 (IL-2) signaling in T tells. Here we report SOCS-3 e xpression in response to IL-2 in both T-cell lines and human peripheral blo od 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 C IS, SOCS-3 was rapidly tyrosine phosphorylated in response to IL-2. Tyrosin e phosphorylation of SOCS-3 was observed upon coexpression with Jak1 and Ja k2 but only weakly with Jak3. In these experiments, SOCS-3 associated with Jak1 and inhibited Jak1 phosphorylation, and this inhibition was markedly e nhanced by the presence of IL-2 receptor beta chain (IL-2R beta). Moreover, following IL-2 stimulation of T cells, SOCS-3 was able to interact with th e IL-2 receptor complex, and in particular tyrosine phosphorylated Jak1 and IL-2R beta. Additionally, in lymphocytes expressing SOCS-3 but not CIS, IL -2-induced tyrosine phosphorylation of STAT5b was markedly reduced, while t here was only a weak effect on IL-3-mediated STAT5b tyrosine phosphorylatio n. 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.