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We describe the molecular cloning of a cDNA for the alpha chain of the human IL-11 receptor (IL-11R alpha) and demonstrate the requirement of either the human or mouse gp130 molecule for signalling. cDNA clones encoding IL-11R alpha were isolated from a bone marrow cDNA library using a fragment from the murine IL-11R alpha as a probe. The human receptor was predicted to consist of 422 amino acids and was found to share 84% identity with the murine protein. In the extra-cellular region it exhibited a single hemopoietin domain with conserved cysteine residues and WSTWS motif. The transmembrane region was followed by a short cytoplasmic tail which did not contain a tyrosine kinase domain. Interaction of the human IL-11R alpha with murine gp130 was demonstrated: expression of the human IL-11R alpha in murine M1 cells which constitutively express murine gp130 (and murine LIF receptor), resulted in the generation of specific high-affinity binding sites for IL-11 (Kd = 250 pM). In addition, expression of the human IL-11R alpha in these cells permitted the induction of macrophage differentiation in response to IL-11. These results suggested that the human IL-11R alpha chain was able to form a functional receptor complex in association with murine gp130. The requirement of gp130 for signalling was confirmed by expression of the human IL-11R alpha in Ba/F3 cells. BaF3 cells that expressed the human IL-11R alpha alone showed binding of radiolabelled IL-11 but no proliferative response. Introduction of human gp130 into these cells resulted in high-affinity IL-11 binding sites and IL-11 dependent cellular proliferation. Thus these results demonstrated the absolute requirement of gp130 for signalling.

SCL (TAL-1) is implicated in the generation of human T-cell acute lymphoblastic leukaemia. To directly examine the role of this putative oncogene, an SCL retrovirus was constructed and used to infect a v-ABL transformed T-lymphocyte cell line. Thirteen independent SCL-infected and four control cell lines were established and injected subcutaneously into syngeneic mice. Mice injected with SCL-infected clonal cell lines died significantly more rapidly than control animals. By day 200 46% (40/87) of animals injected with SCL-infected cell lines had died due to disseminated transplantable lymphoid tumours. In contrast only 22% of control mice were dead by day 200 (P0.0015). Of possible relevance to the enhanced tumourigenesis, some SCL-infected cell lines displayed increased clonogenicity in agar. Increased cell growth was even more striking when ex-vivo tumour-derived cell lines were studied. Thus, SCL can co-operate with v-ABL to hasten T-cell tumourigenesis. This is the first direct evidence demonstrating that SCL can behave as an oncogene.

The SCL gene encodes a putative transcription factor with a basic helix-loop-helix (B-HLH) motif and is known to be predominantly expressed in erythroid cells. Here we also demonstrate expression of SCL mRNA in normal mast cells, mast cell lines and megakaryocytic cell lines. SCL is therefore expressed in the same three lineages as GATA-1, a well-recognized hemopoietic transcription factor. SCL and GATA-1 mRNA were also co-expressed in interleukin 3-dependent primitive myeloid lines. In murine erythroleukemia (MEL) cells SCL and GATA-1 underwent coordinated biphasic modulation during hexamethylene bisacetamide (HMBA)-induced erythroid differentiation. The kinetics of SCL and GATA-1 mRNA expression was inversely correlated with changes in ID, a negative regulator of B-HLH proteins, and was distinct from changes in MYC, MYB and erythropoietin receptor transcripts. During myeloid differentiation of K562 cells, SCL and GATA-1 mRNA levels also underwent biphasic modulation. Thus SCL and GATA-1 are coordinately expressed in multiple hemopoietic lineages and coordinately regulated during induced erythroid and myeloid differentiation. In nonhemopoietic tissues SCL was only detected in adult and developing brain where GATA-1 is reportedly not expressed. In day 14.5 embryos analysed by in situ hybridization, SCL transcripts were detected in post-mitotic neurons in the metencephalon and roof of the mesencephalon. This suggests a previously unexpected role for SCL in neural differentiation.

The SCL gene encodes a member of the 'helix-loop-helix' family of DNA binding regulatory proteins. It is transcriptionally activated in some cases of T-cell acute lymphoblastic leukaemia by a reciprocal translocation involving the T cell receptor delta locus. In order to gain insight into the normal functions of SCL we have studied SCL mRNA levels in human and murine haemopoietic cell lines and normal tissues. We have observed high levels of SCL mRNA in all human and murine erythroid cell lines examined. Foetal and adult normal haemopoietic cell populations rich in erythroid precursors also expressed high levels of SCL mRNA. Our results suggest a previously unexpected role for SCL in the regulation of erythropoiesis.