Human germ cell tumor cell lines express novel leukemia inhibitory factor transcripts encoding differentially localized proteins

Leukemia inhibitory factor, a double-edged sword with therapeutic implications in human diseases

Leukemia inhibitory factor (LIF) is a pleiotropic cytokine of the interleukin-6 (IL-6) superfamily. LIF was initially discovered as a factor to induce the differentiation of myeloid leukemia cells and thus inhibit their proliferation. Subsequent studies have highlighted the multi-functions of LIF under a wide variety of physiological and pathological conditions in a highly cell-, tissue-, and context-dependent manner. Emerging evidence has demonstrated that LIF plays an essential role in the stem cell niche, where it maintains the homeostasis and regeneration of multiple somatic tissues, including intestine, neuron, and muscle. Further, LIF exerts a crucial regulatory role in immunity and functions as a protective factor against many immunopathological diseases, such as infection, inflammatory bowel disease (IBD), and graft-verse-host disease (GVHD). It is worth noting that while LIF displays a tumor-suppressive function in leukemia, recent studies have highlighted the oncogenic role of LIF in many types of solid tumors, further demonstrating the complexities and context-dependent effects of LIF. In this review, we summarize the recent insights into the roles and mechanisms of LIF in stem cell homeostasis and regeneration, immunity, and cancer, and discuss the potential therapeutic options for human diseases by modulating LIF levels and functions.

Leukemia Inhibitory Factor: An Important Cytokine in Pathologies and Cancer

Leukemia Inhibitory Factor (LIF) is a member of the IL-6 cytokine family and is expressed in almost every tissue type within the body. Although LIF was named for its ability to induce differentiation of myeloid leukemia cells, studies of LIF in additional diseases and solid tumor types have shown that it has the potential to contribute to many other pathologies. Exploring the roles of LIF in normal physiology and non-cancer pathologies can give important insights into how it may be dysregulated within cancers, and the possible effects of this dysregulation. Within various cancer types, LIF expression has been linked to hallmarks of cancer, such as proliferation, metastasis, and chemoresistance, as well as overall patient survival. The mechanisms behind these effects of LIF are not well understood and can differ between different tissue types. In fact, research has shown that while LIF may promote malignancy progression in some solid tumors, it can have anti-neoplastic effects in others. This review will summarize current knowledge of how LIF expression impacts cellular function and dysfunction to help reveal new adjuvant treatment options for cancer patients, while also revealing potential adverse effects of treatments targeting LIF signaling.

Emerging Perspectives on Leukemia Inhibitory Factor and its Receptor in Cancer

Tumorigenesis and metastasis have deep connections to inflammation and inflammatory cytokines, but the mechanisms underlying these relationships are poorly understood. Leukemia Inhibitory Factor (LIF) and its receptor (LIFR), part of the interleukin-6 (IL-6) cytokine family, make up one such ill-defined piece of the puzzle connecting inflammation to cancer. Although other members of the IL-6 family have been shown to be involved in the metastasis of multiple types of cancer, the role of LIF and LIFR has been challenging to determine. Described by others in the past as enigmatic and paradoxical, LIF and LIFR are expressed in a diverse array of cells in the body, and the narrative surrounding them in cancer-related processes has been vague, and at times even contradictory. Despite this, recent insights into their functional roles in cancer have highlighted interesting patterns that may allude to a broader understanding of LIF and LIFR within tumor growth and metastasis. This review will discuss in depth the signaling pathways activated by LIF and LIFR specifically in the context of cancer-the purpose being to summarize recent literature concerning the downstream effects of LIF/LIFR signaling in a variety of cancer-related circumstances in an effort to begin teasing out the intricate web of contradictions that have made this pair so challenging to define.

A Zombie LIF Gene in Elephants Is Upregulated by TP53 to Induce Apoptosis in Response to DNA Damage

Large-bodied organisms have more cells that can potentially turn cancerous than small-bodied organisms, imposing an increased risk of developing cancer. This expectation predicts a positive correlation between body size and cancer risk; however, there is no correlation between body size and cancer risk across species ("Peto's paradox"). Here, we show that elephants and their extinct relatives (proboscideans) may have resolved Peto's paradox in part through refunctionalizing a leukemia inhibitory factor pseudogene (LIF6) with pro-apoptotic functions. LIF6 is transcriptionally upregulated by TP53 in response to DNA damage and translocates to the mitochondria where it induces apoptosis. Phylogenetic analyses of living and extinct proboscidean LIF6 genes indicates that its TP53 response element evolved coincident with the evolution of large body sizes in the proboscidean stem lineage. These results suggest that refunctionalizing of a pro-apoptotic LIF pseudogene may have been permissive (although not sufficient) for the evolution of large body sizes in proboscideans.

Alterations in the expression of leukemia inhibitory factor following exercise: comparisons between wild-type and mdx muscles

BACKGROUND

Leukemia inhibitory factor (LIF) is a pleiotropic cytokine, belonging to the interleukin-6 family of cytokines, that has been suggested to have positive effects on myogenesis following injury and to minimise dystrophic pathology in mdx mice. Previous reports have suggested that Lif mRNA is up-regulated in the limb and diaphragm muscles of mdx mice, in human cases of dystrophy and acutely following exercise. This study examined expression of Lif mRNA in the quadriceps muscles of mdx and wild-type mice that were either sedentary or allowed to exercise voluntarily for two weeks.

RESULTS

Exercise caused a decrease in Lif mRNA expression in wild-type muscle, but this was not the case in mdx muscle. Lif mRNA levels in sedentary mdx mice were similar to those in exercised wild type muscles, and in mdx mice there was no further decrease in levels following exercise. Similar down-regulation of Lif mRNA was observed in the tibialis anterior and diaphragm muscles of mdx mice at three and six weeks of age respectively, compared with wild-type controls. Transcripts for the LIF receptor (Lifr) were also down-regulated in these mdx muscles, suggesting LIF activity may be minimised in dystrophic muscle. However fluorescent immunohistochemical labeling of LIF did not correlate with transcript expression data, as LIF immunoreactivity could not be detected in wild-type muscle, where mRNA expression was high, but was present in dystrophic muscle where mRNA expression was low. This study also described the translocation of membrane proteins, including LIFR, to the nuclei of syncytial muscle cells during differentiation and fusion. In addition this study demonstrates that survival of donor myoblasts injected into dystrophic muscle was enhanced by co-administration of recombinant LIF.

CONCLUSIONS

This study provides new evidence to support a role for LIF in normal muscle biology in response to exercise. Although expression levels of Lif transcript in mdx muscles were not consistent with previous studies, the detection of LIF protein in mdx muscle but not wild-type muscle supports a role for LIF in dystrophy. This study also provides evidence of the differential localisation of the LIFR, and the potential for anti-inflammatory actions of LIF that promote survival of transplanted myoblasts in dystrophic muscle.*corresponding author: Jason White, Muscular Dystrophy Research Group, Murdoch Childrens Research Institute; email: jasondw@unimelb.edu.au.

Ontogenetic features of the expression of mRNA isoforms for leukemia-inhibitory factor in human fetal tissues and mononuclear cells

The expression of leukemia-inhibitory factor mRNA in human fetal tissues and mononuclear cells was studied during ontogeny. The expression of mRNA isoforms for leukemia-inhibitory factor was tissue-specific at the stage of prenatal development. The transition from antenatal and neonatal development to the postnatal period was accompanied by a decrease in the expression of mRNA isoforms for leukemia-inhibitory factor in mononuclear cells.

Polarized secretion of leukemia inhibitory factor

Background

The direction of cytokine secretion from polarized cells determines the cytokine's cellular targets. Leukemia inhibitory factor (LIF) belongs to the interleukin-6 (IL-6) family of cytokines and signals through LIFR/gp130. Three factors which may regulate the direction of LIF secretion were studied: the site of stimulation, signal peptides, and expression levels. Stimulation with IL-1β is known to promote IL-6 secretion from the stimulated membrane (apical or basolateral) in the human intestinal epithelial cell line Caco-2. Since LIF is related to IL-6, LIF secretion was also tested in Caco-2 following IL-1β stimulation. Signal peptides may influence the trafficking of LIF. Two isoforms of murine LIF, LIF-M and LIF-D, encode different signal peptides which have been associated with different locations of the mature protein in fibroblasts. To determine the effect of the signal peptides on LIF secretion, secretion levels were compared in Madin-Darby canine kidney (MDCK) clones which expressed murine LIF-M or LIF-D or human LIF under the control of an inducible promoter. Low and high levels of LIF expression were also compared since saturation of the apical or basolateral route would reveal specific transporters for LIF.

Results

When Caco-2 was grown on permeable supports, LIF was secreted constitutively with around 40% secreted into the apical chamber. Stimulation with IL-1β increased LIF production. After treating the apical surface with IL-1β, the percentage secreted apically remained similar to the untreated, whereas, when the cells were stimulated at the basolateral surface only 20% was secreted apically. In MDCK cells, an endogenous LIF-like protein was detected entirely in the apical compartment. The two mLIF isoforms showed no difference in their secretion patterns in MDCK. Interestingly, about 70% of murine and human LIF was secreted apically from MDCK over a 400-fold range of expression levels within clones and a 200,000-fold range across clones.

Conclusion

The site of stimulation affected the polarity of LIF secretion, while, signal peptides and expression levels did not. Exogenous LIF is transported in MDCK without readily saturated steps.

Evidence for heterodimeric association of leukemia inhibitory factor (LIF) receptor and gp130 in the mouse uterus for LIF signaling during blastocyst implantation

Implantation failure in mice lacking leukemia inhibitory factor (LIF) establishes that this cytokine is crucial to this process. LIF transcripts are expressed in the uterus in a biphasic manner: LIF is expressed in the gland on the morning of day 4 and again in stromal cells surrounding the blastocyst with the onset of implantation in the evening of day 4 of pregnancy. However, it is not yet clear whether both phases of LIF expression are required for implantation, since the receptor usage by uterine LIF still remains elusive. Here we have provided evidence that major cell types expressing theLIF receptor (LIFR) and its signal transducing partner gp130 are mostly disparate in the mouse uterus during the glandular LIF expression in the morning of day 4. In contrast, LIFR and gp130 expressions overlap in the luminal epithelium at the time of blastocyst attachment on the evening of day 4 when the second phase of LIF expression occurs in stromal cells surrounding the blastocyst, suggesting that LIF participates in implantation in a paracrine manner. Similar expression patterns for LIFR and gp130 were observed when a delayed implantation model was used. For example, a transient overlapping expression of LIFR and gp130 was evident at 12 h after estrogen-induced termination of delayed implantation. Coimmunoprecipitation experiments showed that LIFR and gp130 form heterodimers and are available for LIF signaling at the time of blastocyst attachment. We have also shown that an intra-peritoneal administration of recombinant LIF in LIF-deficient pregnant mice on the evening of day 4, close to the time when the second phase of LIF expression is normally observed, is sufficient to rescue implantation failure, and that there is no evidence of antagonistic action by soluble forms of the receptors. Collectively, our results have provided evidence that LIFR and gp130 form a functional heterodimer in the uterus during the attachment reaction to direct LIF signaling.

Alternative splicing of mRNA of mouse interleukin-4 and interleukin-6

Interleukin-4 and interleukin-6 are multifunctional regulatory proteins, which participate both in haemopoiesis and in immunopoiesis. The alternative splicing of these interleukins in humans is known to proceed in a tissue-specific manner. Additionally, changes in splicing can also be dependent on tissue pathology. In this work, we report on the presence of alternatively spliced mRNA (IL-4delta2mRNA), lacking exon 2, in mouse bone marrow and spleen cells. We find that in unstimulated cells IL-4mRNA levels strongly dominate over IL-4delta2mRNA levels. Both increase in response to stimulation, with the concentration of the alternative variant rising earlier and faster than that of the full-length variant. In all other tissues studied dominance of IL-4delta2mRNA over the full-length variant was not observed. In addition, we find expression of three forms of IL-6 mRNA: the full-length IL-6 mRNA, IL-6Delta3 mRNA, and IL-6Delta5 mRNA in the second and third trimester placenta tissue and in the spleen of mice immunized with a high dose of sheep erythrocytes. It is anticipated that translation of these mRNA variants can generate proteins capable of binding to some subunits of the IL-6 receptor, thus possessing effector function. Alternative splicing is discussed as a source of cytokines with new regulatory properties.

Expression of leukemia inhibitory factor (LIF) and its receptor gp190 in human liver and in cultured human liver myofibroblasts. Cloning of new isoforms of LIF mRNA

BACKGROUND: The cytokine leukemia inhibitory factor (LIF) mediates its biological effects through binding to its high affinity receptor made of the low-affinity LIF receptor subunit gp190 (LIF-R) and the gp130 subunit. LIF exerts several important effects in the liver, however, data on liver expression of LIF are scarce. The aim of this study was to examine the expression of LIF and LIF-R in human liver. RESULTS: LIF expression, analyzed by immunohistochemistry, was barely detectable in normal liver but was strong within cirrhotic fibrous septa and was found in spindle-shaped cells compatible with myofibroblasts. Accordingly, cultured human liver myofibroblasts expressed high levels of LIF as shown by ELISA and Northern blot. Biological assay demonstrated that myofibroblast-derived LIF was fully active. RT-PCR showed expression of the LIF-D and M isoforms, and also of low levels of new variants of LIF-D and LIF-M resulting from deletion of exon 2 through alternative splicing. LIF receptor expression was detected mainly as a continuous sinusoidal staining that was enhanced in cirrhotic liver, suggestive of endothelial cell and/or hepatocyte labeling. Immunohistochemistry, flow cytometry and STAT-3 phosphorylation assays did not provide evidence for LIF receptor expression by myofibroblasts themselves. LIF secretion by cultured myofibroblasts was down regulated by the addition of interleukin-4. CONCLUSIONS: We show for the first time the expression of LIF in human liver myofibroblasts, as well as of two new isoforms of LIF mRNA. Expression of LIF by myofibroblasts and of its receptor by adjacent cells suggests a potential LIF paracrine loop in human liver that may play a role in the regulation of intra-hepatic inflammation.

Leukemia Inhibitory Factor: An Important Regulator of Endometrial Function

PROBLEM

Leukemia inhibitory factor (LIF) is multifunctional cytokine that displays biological activities in different cells, including endometrial cells. The aim of this study is to describe implications of LIF on a physiological function of endometrium.

METHOD OF STUDY

The role of LIF in the endometrial function is reviewed and summarized from the available literature.

RESULTS

LIF plays an important role in a physiological function of endometrium. In human endometrial LIF expression depends on cellular localizations, steroid hormones, menstrual stages and a local cytokine network. Stronger LIF expression exists in an endometrial epithelium during a luteal phase of the menstrual cycle, which coincides with the time of an implantation. The impairments of the endometrial LIF expression may play a significant role in the pathological processes involving implantation and the infertility.

CONCLUSIONS

There is a substantial evidence that LIF is a potential regulator of the endometrial function and might be one of the factors that play a key role in human reproduction.

Biphasic effects of leukemia inhibitory factor on osteoblastic differentiation

Leukemia inhibitory factor (LIF) is a cytokine produced by multiple cell types including osteoblasts and which is active on bone metabolism. We have previously shown that in a bone nodule forming in vitro model of osteogenesis, the fetal rat calvaria (RC) cell model, LIF inhibits osteoblast differentiation, acting on late osteoprogenitors and/or early osteoblasts. These results are in contrast to in vivo experiments, in which LIF has been found to increase bone formation. To resolve this discrepancy, we have tested the effect of LIF on rat bone marrow (RBM) stromal cell cultures, an in vitro model encompassing earlier osteoprogenitor stages. LIF inhibited cell growth in early, proliferating RBM cultures, but increased the culture saturation density. The effect of LIF on bone nodule formation in this model was cell density dependent and biphasic. Continuous treatment with LIF reduced the number of bone nodules present in confluent, more mature cultures, and the inhibitory effect was strongest when cells were plated at higher cell density than lower. In contrast, during the early stages of RBM culture, nodule numbers were higher in LIF-treated dishes than in controls, and this effect was greater in lower density cultures. Acute LIF treatment restricted to early time points increased the final number of bone nodules formed in mature RBM cell cultures, but not in RC cell cultures. Our results indicate that LIF exerts complex, stage-specific effects on osteoprogenitor recruitment, differentiation, and bone formation, and that the effects are cell nonautonomous, in the rat bone marrow stromal cell model. J. Cell. Biochem. Suppl. 36: 63-70, 2001.

Leukemia-inhibitory factor-neuroimmune modulator of endocrine function

Leukemia-inhibitory factor (LIF) is a pleiotropic cytokine expressed by multiple tissue types. The LIF receptor shares a common gp130 receptor subunit with the IL-6 cytokine superfamily. LIF signaling is mediated mainly by JAK-STAT (janus-kinase-signal transducer and activator of transcription) pathways and is abrogated by the SOCS (suppressor-of cytokine signaling) and PIAS (protein inhibitors of activated STAT) proteins. In addition to classic hematopoietic and neuronal actions, LIF plays a critical role in several endocrine functions including the utero-placental unit, the hypothalamo-pituitary-adrenal axis, bone cell metabolism, energy homeostasis, and hormonally responsive tumors. This paper reviews recent advances in our understanding of molecular mechanisms regulating LIF expression and action and also provides a systemic overview of LIF-mediated endocrine regulation. Local and systemic LIF serve to integrate multiple developmental and functional cell signals, culminating in maintaining appropriate hormonal and metabolic homeostasis. LIF thus functions as a critical molecular interface between the neuroimmune and endocrine systems.

Intracellular and extracellular leukemia inhibitory factor proteins have different cellular activities that are mediated by distinct protein motifs

Although many growth factors and cytokines have been shown to be localized within the cell and nucleus, the mechanism by which these molecules elicit a biological response is not well understood. The cytokine leukemia inhibitory factor (LIF) provides a tractable experimental system to investigate this problem, because translation of alternatively spliced transcripts results in the production of differentially localized LIF proteins, one secreted from the cell and acting via cell surface receptors and the other localized within the cell. We have used overexpression analysis to demonstrate that extracellular and intracellular LIF proteins can have distinct cellular activities. Intracellular LIF protein is localized to both nucleus and cytoplasm and when overexpressed induces apoptosis that is inhibited by CrmA but not Bcl-2 expression. Mutational analysis revealed that the intracellular activity was independent of receptor interaction and activation and reliant on a conserved leucine-rich motif that was not required for activation of cell surface receptors by extracellular protein. This provides the first report of alternate intracellular and extracellular cytokine activities that result from differential cellular localization of the protein and are mediated by spatially distinct motifs.

Regulated expression of alternate transcripts from the mouse oncostatin M gene: implications for interleukin-6 family cytokines

Oncostatin M (OSM) is a member of the IL-6 family of polyfunctional cytokines. The characterized murine OSM transcript consists of three exons and encodes a secreted protein. Investigations of mOSM expression using the ribonuclease protection assay demonstrated novel sites of expression in undifferentiated but not differentiated pluripotent cells, and revealed the existence of alternatively spliced mOSM transcripts. cDNAs representing a novel mOSM transcript (mOSM 13) containing exon 1 spliced directly to exon 3 were isolated from bone marrow using Rapid Amplification of cDNA Ends (RACE) PCR and RT-PCR approaches. Expression of the mOSM 13 transcript was regulated in a tissue-specific manner and independently of mOSM transcript production, suggesting that its production is biologically significant. Splicing of exon 1 directly to exon 3 disrupts the OSM open reading frame of mOSM 13. Initiation of translation at sites within exon 3 of mOSM 13 would yield N-terminally truncated OSM proteins that are localized within the cell. The omission of exon 2 by alternate splicing and the production of intracellular proteins with alternate biological activities are conserved among several IL-6 family cytokines and are one manifestation of a more general phenomenon; the production of alternate cytokine transcripts encoding intracellular and extracellular proteins.

Complex Conserved Organization of the Mammalian Leukemia Inhibitory Factor Gene: Regulated Expression of Intracellular and Extracellular Cytokines

Leukemia inhibitory factor (LIF) is a member of the IL-6 family of pleiotropic cytokines, which are extensively involved in modulating hematopoiesis and immunity. We have undertaken a detailed analysis of LIF genomic organization and gene transcription and investigated the proteins expressed from alternate transcripts. Previously unidentified LIF transcripts, containing alternate first exons spliced onto common second and third exons, were cloned from murine embryonic stem cells, human embryonal carcinoma cells, and primary porcine fibroblasts. Based on sequence homology and position within the genomic sequence, this confirmed the existence of the LIF-M transcript in species other than the mouse and identified a new class of transcript, designated LIF-T. Thus, a complex genomic organization of the LIF gene, conserved among eutherian mammals, results in the expression of three LIF transcripts (LIF-D, LIF-M, and LIF-T) differentially expressed from alternate promoters. The first exon of the LIF-T transcript contained no in-frame AUG, causing translation to initiate downstream of the secretory signal sequence at the first AUG in exon two, producing a truncated LIF protein that was localized within the cell. Enforced secretion of this protein demonstrated that it could act as a LIF receptor agonist. Regulated expression of biologically active intracellular and extracellular LIF cytokine could thus provide alternate mechanisms for the modulation of hematopoiesis and immune system function.