Prolactin (PRL) has been shown to activate the cytoplasmic tyrosine kinase Janus kinase 2 (Jak2) and the subsequent recruitment of various signaling molecules including members of the signal transducer and activator of transcription family of transcription factors. Recently, an expanding family of cytokine-inducible inhibitors of signaling has been identified that initially included four members: suppressor of cytokine signaling (SOCS)-1, SOCS-2, SOCS-3, and cytokine-inducible src homology domain 2 (SH-2) proteins. The present study analyzes the role of these members in PRL signaling. Constitutive expression of SOCS-1 and SOCS-3 suppressed PRL-induced signal transducer and activator of transcription 5-dependent gene transcription, and Jak2 tyrosine kinase activity was greatly reduced in the presence of SOCS-1 or SOCS-3. SOCS-1 was shown to associate with Jak2, whereas SOCS-2 was associated with the prolactin receptor. Co-transfection studies were conducted to further analyze the interactions of SOCS proteins. SOCS-2 was shown to suppress the inhibitory effect of SOCS-1 by restoring Jak2 kinase activity but did not affect the inhibitory effect of SOCS-3 on PRL signaling. Northern blot analysis revealed that SOCS-3 and SOCS-1 genes were transiently expressed in response to PRL, both in vivo and in vitro, whereas the expression of SOCS-2 and CIS genes was still elevated 24 h after hormonal stimulation. We thus propose that the early expressed SOCS genes (SOCS-1 and SOCS-3) switch off PRL signaling and that the later expressed SOCS-2 gene can restore the sensitivity of cells to PRL, partly by suppressing the SOCS-1 inhibitory effect.Prolactin (PRL) 1 exerts its effects via the PRL receptor (PRLR) and the activation of intracellular signaling molecules through the Jak-STAT pathway (1). Ligand binding leads to dimerization of the receptor and activation of Jak2, which in turn phosphorylates the PRL receptor and the transcription factor STAT5. However, less is known regarding how PRL signal transduction is switched off. In addition to Jak2 and STAT5 induction, PRL receptor activation also results in the stimulation of the protein tyrosine phosphatase SHP-2; however, in the case of the PRL receptor, a positive role in signal transduction has been assigned (2). Recently, a novel family of proteins capable of suppressing cytokine signal transduction has been identified that function in a classic negative feedback loop to regulate cytokine signaling (3-11). Expression of suppressor of cytokine signaling (SOCS)-1 suppresses interleukin 6-induced macrophage differentiation of murine myeloid leukemia cell line M1 and interleukin 6-induced receptor phosphorylation as well as STAT activation. SOCS-1 interacts with the catalytic region of Jak kinases and suppresses their tyrosine kinase activity and, as a result, the activation of STATs. Studies of mutated or deleted SOCS-1 proteins reported that the N-terminal domain and the SH-2 domain were required to inhibit leukemia-inhibitory factor signal transduction (12).SOCS-1 and SOCS...

A family of suppressors of cytokine signaling (SOCS) has recently been identified of which two members have been shown to block growth hormone (GH) signaling. Dose-response experiments were conducted in 293 cells and SOCS-1 and SOCS-3 were shown to inhibit the transcriptional activation of a GH-responsive element and suppressed Jak2 tyrosine kinase activity. SOCS-2 had two opposite effects: at low concentrations it inhibited GH-induced STAT5-dependent gene transcription, but restoration of GH signaling was observed at higher concentrations. In cotransfection studies, SOCS-2 was able to block the inhibitory effect of SOCS-1 but not that of SOCS-3 on GH signaling. These findings suggest that a major function for SOCS-2 is to restore the sensitivity to GH by overcoming the initial inhibitory effects of other endogenous SOCS molecules.z 1999 Federation of European Biochemical Societies.