Complex conserved organization of the mammalian leukemia inhibitory factorgene: Regulated expression of intracellular and extracellular cytokines

Leukemia inhibitory factor (LIF) is a member of the IL-6 family of pleiotro pic cytokines, which are extensively involved in modulating hematopoiesis a nd immunity. We have undertaken a detailed analysis of LIF genomic organiza tion and gene transcription and investigated the proteins expressed from al ternate transcripts. Previously unidentified LIF transcripts, containing al ternate first exons spliced onto common second and third exons, were cloned from murine embryonic stem cells, human embryonal carcinoma cells, and pri mary porcine fibroblasts, Based on sequence homology and position within th e genomic sequence, this confirmed the existence of the LIF-M transcript in species other than the mouse and identified a new class of transcript, des ignated LIF-T, Thus, a complex genomic organization of the LIF gene, conser ved among eutherian mammals, results in the expression of three LIF transcr ipts (LIF-D, LIF-M, and LIF-T) differentially expressed from alternate prom oters. The first exon of the LIF-T transcript contained no in-frame AUG, ca using translation to initiate downstream of the secretory signal sequence a t the first AUG in exon two, producing a truncated LIF protein that was loc alized within the cell. Enforced secretion of this protein demonstrated tha t it could act as a LIF receptor agonist. Regulated expression of biologica lly active intracellular and extracellular LIF cytokine could thus provide alternate mechanisms for the modulation of hematopoiesis and immune system function.