Preparation and expression of biologically active prolactin and growth hormone receptors and suppressor of cytokine signaling proteins 1, 2, 3, and 6 tagged with cyan and yellow fluorescent proteins

SOCS6 is a selective suppressor of receptor tyrosine kinase signaling

The suppressors of cytokine signaling (SOCS) are well-known negative regulators of cytokine receptor signaling. SOCS6 is one of eight members of the SOCS family of proteins. Similar to other SOCS proteins, SOCS6 consists of an uncharacterized extended N-terminal region followed by an SH2 domain and a SOCS box. Unlike other SOCS proteins, SOCS6 is mainly involved in negative regulation of receptor tyrosine kinase signaling. SOCS6 is widely expressed in many tissues and is found to be downregulated in many cancers including colorectal cancer, gastric cancer, lung cancer, ovarian cancer, stomach cancer, thyroid cancer, hepatocellular carcinoma, and pancreatic cancer. SOCS6 is involved in negative regulation of receptor signaling by increasing degradation mediated by ubiquitination of receptors or substrate proteins and induces apoptosis by targeting mitochondrial proteins. Therefore, SOCS6 turns out as an important regulator of survival signaling and its activity is required for controlling receptor tyrosine kinase signaling.

Quantitative FRET imaging of leptin receptor oligomerization kinetics in single cells

BACKGROUND INFORMATION

Leptin, an adipocyte-secreted hormone, signals through activation of its membrane-embedded receptor (LEPR). To study the leptin-induced events occurring in short (LEPRa) and long (LEPRb) LEPRs in the cell membrane, by FRET (fluorescence resonance energy transfer) methodology, the respective receptors, tagged at their C-terminal with CFP (cyan fluorescent protein) or YFP (yellow fluorescent protein), were prepared.

RESULTS

The constructs encoding mLEPRa (mouse LEPRa)-YFP and mLEPRa-CFP, mLEPRb-YFP and mLEPRb-CFP were tested for biological activity in transiently transfected CHO cells (Chinese-hamster ovary cells) and HEK-293T cells (human embryonic kidney 293 T cells) for activation of STAT3 (signal transduction and activators of transcription 3)-mediated LUC (luciferase) activity and binding of radiolabelled leptin. All four constructs were biologically active and were as potent as their untagged counterparts. The localization pattern of the fused protein appeared to be confined almost entirely to the cell membrane. The leptin-dependent interaction between various types of receptors in fixed cells were studied by measuring FRET, using fluorescence lifetime imaging microscopy and acceptor photobleaching methods.

CONCLUSIONS

Both methods yielded similar results, indicating that (1) leptin receptors expressed in the cell membrane exist mostly as preformed LEPRa/LEPRa or LEPRb/LEPRb homo-oligomers but not as LEPRb/LEPRa hetero-oligomers; (2) the appearance of transient leptin-induced FRET in cells transfected with LEPRb/LEPRb reflects both a conformational change that leads to closer interaction in the cytosolic part and a higher FRET signal, as well as de novo homo-oligomerization; (3) in LEPRa/LEPRa, exposure to leptin does not lead to any increase in FRET signalling as the proximity of CFP and YFP fluorophores in space already gives maximal FRET efficiency of the preoligomerized receptors.

Identification of a nuclear localization signal in suppressor of cytokine signaling 1

Suppressor of cytokine signaling (SOCS) proteins are inducible feedback inhibitors of janus kinase and signal transducer and activators of transcription signaling pathways. In addition, SOCS1 has been identified to regulate stability of nuclear NF-kappaB subunits. However, details about the regulation of the nuclear pool of SOCS1 are unknown. Using different experimental approaches, we observed that SOCS1 but no further SOCS family members localized to the nucleus when expressed in various cell lines. Nuclear transport was confirmed for endogenous SOCS1 in macrophages stimulated with IFN-gamma. Sequence analysis revealed a bipartite nuclear localization signal (NLS) located between the src-homology 2 (SH2) domain and the SOCS box of SOCS1. Deletion of this region, introduction of a series of R/A point mutations, or substitution of this sequence with the respective region of SOCS3 resulted in loss of nuclear localization. Fusion of the SOCS1-NLS to cytokine-inducible SH2 region containing protein (CIS) resulted in nuclear localization of this otherwise cytoplasmic protein. SOCS1 mutants with loss of nuclear localization were still effective in suppressing IFN-alpha-mediated STAT1 tyrosine phosphorylation. However, they showed decreased inhibition of IFN-gamma-mediated induction of CD54. The results identify a hitherto unknown transport of SOCS1 into the nucleus which extends the spectrum of SOCS1 inhibitory activity.

The nuclear localization of SOCS6 requires the N-terminal region and negatively regulates Stat3 protein levels

We determined that endogenous- and overexpressed- SOCS6 was localized in both the nucleus and cytoplasm. The localization of SOCS6 depended on amino acids 1-210 in the N-terminal region of the protein, which contains an unidentified domain. GFP-tagged SOCS6 or the N-terminal region, was exclusively localized and widely distributed throughout the entire nucleus, whereas the C-terminal region displayed a nuclear omission pattern. We also demonstrated that the SOCS6 protein could decrease the levels of the Stat3 protein in the nucleus, and that its negative regulation of the Stat3 protein level was dependent on its C-terminal region. These observations suggest that SOCS6 is composed of at least two functional domains required for its biological role in localizing and degrading Stat3 in the nucleus.

Spatio-temporal kinetics of growth hormone receptor signaling in single cells using FRET microscopy

The growth hormone (GH) receptor (R)-mediated JAK2 (Janus kinase-2)-STAT5 (signaling transducer and activator of transcription-5) pathway involves a cascade of protein-protein interactions and tyrosine phosphorylations that occur in a spatially and temporally sensitive manner in cells. To study GHR dimerization or GH-induced conformational change of predimerized GHRs and STAT5 activation kinetics in intact cells, fluorescence resonance energy transfer (FRET) and live-cell imaging methods were employed. FRET measurements at the membrane of HEK-293T cells co-expressing GHRs tagged at the C-terminus with cyan (C) and yellow (Y) fluorescent proteins (FPs) revealed transient GHR dimerization lasting 2-3 min, with a maximum at 3 min after GH stimulation, which was sufficient to induce STAT5 activation. The transient nature of the dimerization or GH-induced conformational change of predimerized GHRs kinetics was not a result of GHR internalization, as neither potassium- nor cholesterol-depletion treatments prolonged the FRET signal. YFP-tagged STAT5 recruitment to the membrane, binding to GHR-CFP, and phosphorylation, occurred within minutes of GH stimulation. Activated STAT5a-YFP did not show nuclear accumulation, despite nuclear pSTAT5 increase, suggesting high turnover of STAT5 nuclear shuttling. Although GHR dimerization and STAT5 activation have been reported previously, this is the first spatially resolved demonstration of GHR-signaling kinetics in intact cells.

Suppressors of cytokine signaling (SOCS) 1 and SOCS3 interact with and modulate fibroblast growth factor receptor signaling

Fibroblast growth factor receptor (FGFR) signaling is transduced by the mitogen-activated protein kinase (MAPK) cascade and the signal transducers and activators of transcription (STATs). Suppressors of cytokine signaling (SOCS) proteins are expressed in response to cytokine-inducible stimulation of STAT phosphorylation, acting in a negative-feedback mechanism to hinder the activities of these receptors. However, there are no data concerning the role of SOCS proteins in the regulation of fibroblast growth factor receptor (FGFR) signaling. In the present study, we show that activation of FGFR in chondrocytes induces the expression of SOCS1 and SOCS3 mRNA, and that these proteins are constitutively associated with FGFR3, as demonstrated by co-immunoprecipitation studies. Transfection of cells with FGFR3-GFP and SOCS1-CFP revealed their colocalization, clustered prominently in the perinuclear cytosolic part of the cell. The effect of the interaction between FGFR3 and SOCS1 on receptor activity was investigated in a chondrocytic cell line overexpressing SOCS1. In these cells, STAT1 phosphorylation is repressed, MAPK phosphorylation is elevated and prolonged, and FGFR3 downregulation is attenuated. Expression of osteopontin (OPN), which is directly upregulated by FGF in chondrocytes, was stimulated by lower levels of FGF in cells expressing SOCS1 compared with parental cells. Blocking of MAPK phosphorylation by PD98059 decreased OPN expression in both cell types, but this decrease was more marked in cells expressing SOCS1. The presented results suggest a novel interaction between the SOCS1 and SOCS3 proteins and the FGFR3 signaling pathway.

Suppressor of cytokine signaling 7 inhibits prolactin, growth hormone, and leptin signaling by interacting with STAT5 or STAT3 and attenuating their nuclear translocation

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.

SOCS-1 localizes to the microtubule organizing complex-associated 20S proteasome

The regulation of cytokine signaling is critical for controlling cellular proliferation and activation during an immune response. SOCS-1 is a potent inhibitor of Jak kinase activity and of signaling initiated by several cytokines. SOCS-1 protein levels are tightly regulated, and recent data suggest that SOCS-1 may regulate the protein levels of some signaling proteins by the ubiquitin proteasome pathway; however, the cellular mechanism by which SOCS-1 directs proteins for degradation is unknown. In this report, SOCS-1 is found to colocalize and biochemically copurify with the microtubule organizing complex (MTOC) and its associated 20S proteasome. The SOCS-1 SH2 domain is required for the localization of SOCS-1 to the MTOC. Overexpression of SOCS-1 targets Jak1 in an SH2-dependent manner to a perinuclear distribution resembling the MTOC-associated 20S proteasome. Analysis of MTOCs fractionated from SOCS-1-deficient cells demonstrates that SOCS-1 may function redundantly to regulate the localization of Jak1 to the MTOC. Nocodazole inhibits the protein turnover of SOCS-1, demonstrating that the minus-end transport of SOCS-1 to the MTOC-associated 20S proteasome is required to regulate SOCS-1 protein levels. These data link SOCS-1 directly with the proteasome pathway and suggest another function for the SH2 domain of SOCS-1 in the regulation of Jak/STAT signaling.

The suppressor of cytokine signaling (SOCS)-7 interacts with the actin cytoskeleton through vinexin

To understand the function of the suppressor of cytokine signaling (SOCS)-7, we have looked for proteins interacting with SOCS-7 in a stringent yeast two-hybrid screen of a human leukocyte cDNA-library. We identified the cytoskeletal molecule vinexin as a partner interacting with SOCS-7. Tests with deletion mutants of SOCS-7 demonstrated that a central region of the molecule containing several proline-rich regions, N-terminal to the SH2 domain, was responsible for the binding to vinexin. It is thus likely that one of the SH3 domains of vinexin interacts with a poly-proline region of SOCS-7. The interaction with vinexin was confirmed biochemically as vinexin-alpha was co-precipitated with SOCS-7. Confocal laser-scanning microscopy in HEK293T, MCF-7, and 3T3-L1 cells showed that part of the transfected SOCS-7-green fluorescent protein (GFP) molecules merged with vinexin and with actin. Taken together, our data indicate that SOCS-7 interacts with vinexin and the actin cytoskeleton.

Ovine placental lactogen-induced heterodimerization of ovine growth hormone and prolactin receptors in living cells is demonstrated by fluorescence resonance energy transfer microscopy and leads to prolonged phosphorylation of signal transducer and activator of transcription (STAT)1 and STAT3

HEK-293T cells transiently transfected with ovine (o) GH receptor (GHR) and prolactin receptor (PRLR) constructs respectively tagged downstream with cyan or yellow fluorescent proteins were used to study ovine placental lactogen (oPL)-stimulated heterodimerization by fluorescence resonance energy transfer (FRET) microscopy. The oPL-stimulated transient heterodimerization of GHR and PRLR had a peak occurring 2.5-3 min after oPL application, whereas oGH or oPRL had no effect at all. The results indicate none or only little dimerization occurring before the hormonal stimulation. The effect of heterodimerization was studied by comparing activation of Janus kinase 2, signal transducer and activator of transcription (STAT)1, STAT3, STAT5, and MAPK in Chinese hamster ovary cells stably transfected with chimeric genes encoding receptors consisting of cytosolic and transmembrane parts of oGHR and oPRLR, extracellular domains of human granulocyte and macrophage colony-stimulating factor (hGM-CSF) receptor alpha or beta, and cells transfected with the two forms (alpha or beta) of PRLR and GHR. Functionality of those proteins was verified by hGM-CSF-induced phosphorylation of both intracellular PRLR and GHR domains and hGM-CSF-induced heterodimerization was documented by chimeric receptor coimmunoprecipitation. Homodimerization or heterodimerization of PRLRs and GHRs had no differential effect on activation of STAT5 and MAPK. However, heterodimerization resulted in a prolonged phosphorylation of STAT1 and in particular STAT3, suggesting that the heterodimerization of alpha-oGHR and beta-oPRLR is able to transduce a signal, which is distinct from that occurring on homodimeric associations.