We report here the role of one of the less studied members of the family of suppressors of cytokine signaling (SOCS), namely SOCS-7, in cytokine signaling. We demonstrate that SOCS-7 inhibits prolactin (PRL), growth hormone (GH), or leptin (LEP) signaling mediated through STAT3 and STAT5 in a dose-dependent manner. SOCS-7 also attenuated STAT3 and STAT5 signaling induced by overexpression of JH1, the catalytic subdomain of JAK2. Since SOCS-7 interacted with phosphorylated STAT3 or STAT5, we assumed that SOCS-7 acts at the level of STAT proteins. Indeed, we showed that SOCS-7 inhibits PRL-and leptin-induced STAT5 and STAT3 phosphorylation and prevented the nuclear translocation of activated STAT3. Taken together, our results indicate that SOCS-7 is a physiological dysregulator of PRL, leptin, and probably also GH signaling and that its mode of action is a novel variation of SOCS protein inhibition of cytokine-inducible STAT-mediated signal transduction.One of the main pathways for the termination of cytokineactivated JAK 1 /STAT signaling is mediated by a family of proteins discovered nearly a decade ago, the suppressors of cytokine signaling (SOCS) (1-4). These proteins' modulatory activity is not restricted to the JAK/STAT pathway; it has also been reported for activities mediated by receptor tyrosine kinases, such as c-Kit and insulin receptor substrate (IRS)-1 to -4 (5-7). The current paradigm attributes the main function of SOCS to the targeting of the affected proteins for ubiquitination and subsequent degradation by the proteasome, although additional mechanisms are also involved (4 -6). The SOCS family consists of eight proteins, termed SOCS-1 through -7 and CIS (cytokineinducible SH2 protein). Whereas CIS and SOCS-1 through -3 have been studied extensively, others, in particular SOCS-7, have been given much less attention. The latter protein was first discovered by Matuoka et al. (8), who cloned its partial cDNA sequence and called it NAP-4 due to its ability to interact with the adaptor proteins Nck, Grb2 (Ash), and Phospholipase C-␥-1. More recently, full sequences of mouse, rat, and human SOCS-7 have been reported (GenBank TM NM_138657 for mouse; XP_213443 for rat; and see Ref. 9 for human). The NAP-4 sequence lacks the 127 N-terminal amino acids of SOCS-7 but has an additional segment of 34 amino acids (exon 4). SOCS-7 is constitutively expressed in several tissues, most strongly in the testis and brain (8, 10, 11), and its expression in leukocytes is stimulated by prolactin (PRL), growth hormone (GH), and interleukin-6 (10). Recently, SOCS-7 (and SOCS-6) has been shown to interact with IRS-2 and IRS-4 (11). However, to date, no report concerning the possible biological role of SOCS-7 has been published aside from our recent work showing that SOCS-7 interacts with the cytoskeleton proteins actin and vinexin and is present in cell membranes (9). This latter finding prompted us to study whether SOCS-7 affects biological activities mediated through the membrane-embedded receptors of three cytokines: GH ...

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Receptors for prolactin (PRL-R) are expressed in normal leukocytes from rat and man. PRL signals through PRL-R associated Janus tyrosine kinase (Jak)-2 and signal transducers and activators of transcription (Stat). In addition, in human leukocytes PRL also activates the p38 MAP kinase pathway. PRL, at physiological concentrations, stimulates the expression of the interferon regulatory factor (IRF)-1 gene in rat spleen and bone marrow cells. In man, genes induced by PRL include several members of the 'suppressors of cytokine signaling' (SOCS) family and inducible nitric oxide synthase (iNOS; in mononuclear cells and in granulocytes) and IRF-1 (in granulocytes). Thus, in normal leukocytes, PRL induces the expression of several genes relevant to innate and acquired immune responses. Sex hormones, such as estrogen and PRL, have been implicated in the pathogenesis of murine and human SLE. Also defective signaling in leukocytes is a feature of the disease. What the origin is of aberrant signaling processes in SLE lymphocytes and how they relate to tolerance breakdown and immunopathology is still unknown. It is not unlikely that PRL is a player at some level. The exact contribution of PRL to immune responses in normal subjects and in SLE patients is not known. Further work should also indicate whether PRL might contribute to the onset or progression of the disease and assess the possible benefits of manipulating PRL concentrations in patients.

To evaluate the possible role of prolactin (PRL) in T-lymphocytes, we monitored gene induction in one cytotoxic T-lymphocyte (CTL) clone derived from a patient with hemochromatosis and in several T-helper clones generated from a normal donor and a patient with multiple sclerosis. The CTL clone expressed conventional PRL receptor (PRLR), and PRL induced the expression of suppressor of cytokine signaling-3 (SOCS-3) and increased the expression of SOCS-2 and cytokine-inducible src homology-2 containing protein (CIS, another member of the SOCS family). As is the case in granulocytes, expression of a conventional receptor for PRL could not be shown by polymerase chain reaction analysis on three helper clones. In addition, as in granulocytes, PRL modulated the expression of genes such as the interferon-regulatory factor-1, inducible nitric oxide synthase, CIS, and SOCS-2. These effects were also elicited with ovine PRL and could be prevented by anti-PRL antibodies. Thus, the use of clones allowed the detection of direct effects of PRL on T-cells, even when these have few or no detectable PRLR, confirming that human T-lymphocytes are targets for PRL.

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