Differentiation responses of embryonic endothelium to leukemia inhibitory factor

Immunolocalisation of leukaemia inhibitory factor in the cornea

AIM

Leukaemia inhibitory factor (LIF) is a pleotrophic cytokine expressed in a variety of cell types, and have shown to regulate stem cell proliferation, vascular genesis, inflammation, and immunity in various locations. Expression of LIF and its role in the cornea have not been studied previously. In this study, we examined the expression of LIF in the cornea.

MATERIALS AND METHOD

Immunohistochemistry was performed using polyclonal LIF antibodies, and Avidin-Biotin ABC complex on cultured human corneal epithelium corneal fibroblasts and wild-type murine corneal epithelium.

RESULTS

LIF was detected in the cytoplasm of murine corneal epithelium, cultured human corneal epithelium, and fibroblasts. The expression of LIF was mainly cytoplasmic.

CONCLUSION

LIF is expressed in the corneal epithelium and fibroblasts. It may have an important role in the maintenance of homeostasis of the corneal epithelium and cornea stroma. Further studies are necessary to elucidate the role of LIF in the cornea.

In vitro model for developmental progression from vasculogenesis to angiogenesis with a murine endothelial precursor cell line, MFLM-4

In the embryo, vascular networks are developed through both vasculogenesis, the assembly of vessels from endothelial progenitor cells or hemangioblasts, and angiogenesis, the sprouting of vessels from preexisting capillaries. Cell culture models using endothelial cells (EC) and various extracellular matrix components have been useful in understanding the cellular and molecular factors involved in angiogenesis. However, there are few models of vasculogenesis. Using a murine endothelial precursor cell line, MFLM-4, derived from e14.5 lung mesenchyme, we have developed a culture system that not only recapitulates many of the characteristics of vasculogenesis but also progresses into angiogenesis. By 8 h, MFLM-4 cultured on the basement membrane preparation Matrigel invade the matrix, coalesce, and assemble into large clusters of cells resembling blood islands. During vascular development, blood islands are the focal areas for coalescence of endothelial precursors. For MFLM-4, this phase of in vitro vasculogenic clustering does not require proliferation. If proliferation is not blocked, MFLM-4 progresses into an angiogenic phase with the clusters forming multicell angiogenic sprouts. Through 3 days of culture, lumens form within the clusters, adjacent clusters are connected with tube-like structures, and eventually an extensive network or plexus of clusters connected by capillary-like tubes is formed. MFLM-4 cultured on Matrigel provides an in vitro system for analysis of the multistage, concurrent processes of vasculogenesis and angiogenesis.

LIF transduces contradictory signals on capillary outgrowth through induction of stat3 and (P41/43)MAP kinase

The signaling pathways regulating blood vessel growth and development are not well understood. In the present report, an in vitro model was used to identify signaling pathways regulating capillary formation in embryonic endothelial cells. Basic fibroblast growth factor (bFGF) plus leukemia inhibitory factor (LIF) optimally stimulate the formation of capillary-like structures of the embryonic endothelial cell line IEM. LIF stimulation of IEM cells leads to activation of the Stat3 as well as the (P41/43)mitogen-activated protein kinase ((P41/43)MAPK) cascade, while bFGF does not activate Stat3 but does induce the (P41/43)MAPK cascade. Inhibition of Stat3 DNA-binding activity by expression of a dominant inhibitory Stat3 mutant increases the capillary outgrowth of the IEM cells induced by LIF. Increased Stat3 activity by overexpression of the wild-type Stat3 greatly reduced capillary outgrowth. In contrast, inhibition of the (P41/43)MAPK cascade using a MEK-1 inhibitor dramatically inhibits the LIF-induced capillary outgrowth. Moreover, the increased formation of capillary-like structures of the IEM cells mediated by Stat3 inhibition does not overcome the requirement for activation of the (P41/43)MAPK pathway for capillary outgrowth. Stat3 activity correlates with the LIF-induced expression of the negative feedback regulators of the Janus (JAK) family of tyrosine kinases, SOCS-1 and SOCS-3. These results provide evidence that Stat3 acts as a negative regulator of capillary outgrowth, possibly by increasing SOCS-1 or SOCS-3 expression. The contradictory signals stimulated by LIF could be necessary to control the intensity of the response leading to capillary outgrowth in vivo.

Saturable entry of leukemia inhibitory factor from blood to the central nervous system

Leukemia inhibitory factor (LIF) is a neurotrophic cytokine now under clinical investigation for its effects on the CNS. We studied its passage across the blood-brain barrier (BBB) from blood to brain and spinal cord. Although a large amount of LIF was reversibly associated with the cerebral vasculature, intact LIF did reach brain parenchyma. Multiple-time regression analysis showed ready access of LIF to the CNS at a rate much faster than that of the vascular marker albumin. Excess LIF inhibited the entry of 125I-LIF after administration i.v. or by in-situ perfusion in blood-free buffer. Efflux of LIF from brain to blood was slower than reabsorption by CSF bulk flow, indicating that LIF tended to be retained in the brain. Although ciliary neurotrophic factor (CNTF) and LIF bind to the same receptor complex, CNTF did not cross-inhibit the entry of LIF into the CNS. A monoclonal antibody to LIF, however, abolished the entry of LIF. Our results show that peripherally administered LIF readily enters the brain and spinal cord by a saturable transport system across the BBB that may have biological implications.

Tubedown-1, a novel acetyltransferase associated with blood vessel development

We have used an embryonic endothelial cell line (IEM cells) as an experimental system for identifying and characterizing new molecules which are regulated during blood vessel development. A novel gene isolated from IEM cells, tubedown-1 (tbdn-1), is expressed at high levels in unstimulated IEM cells and is downregulated during formation of capillary tube structures by the IEM cells induced by basic fibroblast growth factor (bFGF) and leukemia inhibitory factor (LIF) in vitro. Tbdn-1 is also downregulated in M1 myeloid leukemia cells after differentiation in response to LIF in vitro. Tbdn-1 is homologous to the yeast NAT-1 N-terminal acetyltransferases and encodes a novel protein of approximately 69 kDa associated with an acetyltransferase activity. Levels and distribution of tbdn-1 expression are regulated in both endothelial and hematopoietic cells during development in tissues such as the yolk sac blood islands, heart, and liver blood vessels. In the adult, tbdn-1 expression is low or undetected in most organs examined with the exception of the atrial endocardium, the endothelial and myeloid compartments of bone marrow, and the remodeling vascular bed of atretic ovarian follicles. The distribution and regulation of expression of tbdn-1 suggest that this novel acetyltransferase may be involved in regulating vascular and hematopoietic development and physiologic angiogenesis.

Leukemia inhibitory factor modulates cardiogenesis in embryoid bodies in opposite fashions

Cardiogenesis is a multistep process regulated by a hierarchy of factors defining each developmental stage of the heart. One of these factors, leukemia inhibitory factor (LIF), a member of the interleukin-6 family of cytokines, is expressed in embryonic and neonatal cardiomyocytes and induces cardiomyocyte hypertrophy. Many aspects of embryogenesis are faithfully recapitulated during in vitro differentiation of embryonic stem cells in embryoid bodies. We exploited this model to study effects of growth factors on commitment and differentiation of cardiomyocytes and on maintenance of their phenotype. We identified LIF as a factor affecting commitment and differentiation of cardiomyocytes in an opposite manner. Diffusible LIF inhibited mesoderm formation and hampered commitment of cardiomyocytes. Lack of both the diffusible and matrix-bound isoforms of LIF in lif-/- embryoid bodies did not interfere with commitment, but it severely suppressed early differentiation of cardiomyocytes. Onset of differentiation was rescued by very low concentrations of diffusible LIF; however, consecutive differentiation was attenuated in a concentration-dependent manner by diffusible LIF both in wild-type and lif-/- cardiomyocytes. Differentiation of cardiomyocytes was severely hampered but not completely blocked in lifr-/- embryoid bodies, suggesting additional, LIF-receptor ligand independent pathways for commitment and differentiation of cardiomyocytes. At the fully differentiated state, both paracrine and autocrine LIF promoted proliferation and increased longevity of cardiomyocytes. These findings suggest that both paracrine and autocrine and both diffusible and matrix-bound isoforms of LIF contribute to the modulation of cardiogenesis in a subtle, opposite, and developmental stage-dependent manner and control proliferation and maintenance of the differentiated state of cardiomyocytes.