Oncostatin M - Signal Transduction Through OSM Receptors

Signal Transduction Through OSM Receptors

Signalling by type I and type II OSM receptors have now been shown to be qualitatively distinct. These differences in signaling character, in addition to the tissue distribution profiles of OSMRb and LIFRb, offer another variable in the distinction between the common and specific cellular effects of OSM with respect to LIF. All IL-6 cytokines whether they homo- or heterodimerise gp130 seem to activate JAK1, JAK2 and to a lesser degree Tyk2. It should be noted however that JAK1, JAK2, and tyk2 are not interchangeable in the gp130 system, this has been demonstrated with the use of JAK1, Jak2 or Tyk2 deficient cell lines obtained from mutant mice. Cells from JAK1 deficient mice show reduced STAT activation and generation of biological responses in response to IL-6 and LIF. In contrast, fibroblasts derived from JAK2 null mice can respond to IL-6, with demonstratable tyrosine phosphorylation of gp130, JAK1 and TYK2. Thus it seems JAK1 is the critical JAK required for gp130 signalling. Activation of the same Jaks by all three receptor combinations (gp130/gp130, gp130/LIFR, gp130/OSMR) raises the question of how IL6, LIF and OSM can activate distinct intracellular signaling pathways. Selection of particular substrates, i.e. STAT isoform, depended not on which Jak is activated, but instead are determined by specific motifs, especially tyrosine-based motifs, within each receptor intracellular domain.

Aligning the intracellular domains of gp130, LIFR and hOSMR results in some interesting observations. Sequence identity is generally quite low across the group averaging at 4.6%. However as with many Class I Haematopoeitin receptors, two short membrane proximal motifs, termed box 1 and box 2 are present. In addition these receptors also contain a serine rich region and a third more poorly conserved motif termed box 3. Box 1 is present in all signalling cytokine receptors. It is characteristically rich in proline residues and is essential for the association and activation of JAKs. Box 2 is also important for association with JAKs. Gp130 contains box1 and box2 sequences within the membrane-proximal part of the cytoplasmic region, lying within the minimum 61 amino acids required for receptor activation. Mutations within the box1 region reduce the ability of gp130 to associate with Jaks and abolish ligand-induced activation of Jak1 and Jak2. Box 2 also contributes to activation and binding of JAKs. Studies with various gp130 truncation mutants show a reduction of Jak2 binding and abrogation of certain biological effects upon deletion of box2. However, Jaks are able to associate with gp130 devoid of box2 when overexpressed.

LIFR and OSMR also contain the membrane-proximal box1/box2-like regions. The first 65 amino acid residues in the cytoplasmic domain of LIFR, in combination with full length gp130, can generate signalling on treatment with LIF. Coprecipitation of Jak1, Jak2 and Tyk2 with receptors containing cytoplasmic parts of the LIFR or OSMR. All beta receptor subunits of the gp130 system also possess a box 3 region. This region corresponds to the C-terminal amino acids of the OSMR and LIFR receptors respectively. Box 3 is necessary for the action of OSMR; however Box3 is dispensable for the action of LIFR. In the case of gp130 box 3 is dispensable for activity, however the presence of an intact box 3 sequence is required for certain aspects of gp130 signalling, i.e. stimulation of transcription through the STAT-3 response element. In addition to the poor sequence conservation amongst the intracellular domains of gp130 receptors, the number and position of conserved tyrosine residues are also poorly conserved. For example LIFR and OSMR share three homologous tyrosines. In contrast none of the tyrosine residues present in the intracellular domain of gp130 share equivalents with LIFR or OSMR, even though the intracellular regions of LIFR and gp130 share more sequence identity than LIFR and OSMR.

Of the proteins recruited to type I cytokine receptors STAT proteins remain the best studied. Homodimerisation of gp130 has been shown to phosphorylate and activate both STAT1 and STAT3. gp130 preferentially activates STAT3 which it can do through four STAT3 activation consensus sequences YXXQ: (YRHQ), (YFKQ), Y905 (YLPQ), Y915 (YMPQ). The lower propensity for STAT1 activation may be a reflection of the lower number of STAT1 activation sequences, YZPQ (where X is any residue and Z is any uncharged residue), namely Y905 and Y915. Cytokines that signal via homodimeric complexes of LIFR or OSMR (i.e. devoid of gp130) are currently unknown in nature. However, various investigators have attempted artificial homodimerisation of LIFR and OSMR intracellular domains, with conflicting results, by constructing receptor chimeras that fuse the extracellular domain of one cytokine receptor with the intracellular domain of LIFR or OSMR.

Signalling by LIFR intracellular domain homodimerisation has been demonstrated in hepatoma and neuroblastoma cells, embryonic stem cells and COS-1 cells by utilizing chimeric receptors that homodimerise upon stimulation with their cognate cytokines (i.e. GCSF, neurotrophin-3, EGF). However a GCSFR/LIFR chimera was not capable of signaling in M1 or Baf cells.