HMEC-1: Establishment of an Immortalized Human Microvascular Endothelial Cell Line

Binding of tissue plasminogen activator to endothelial cells. The effect on functional properties. Localization of a ligand in the B-chain of tPA

The binding of 125I-labelled tissue plasminogen activator (tPA), the tPA A- or B-chain to endothelial cells (EC) were studied in suspensions of cultured human umbilical vein EC (HUVEC) or immortalized microvascular EC (HMEC). By determinations of the concentration-dependent binding it was shown that both the A-chain and the B-chain, which were isolated after partial reduction of two-chain tPA, contain ligands for binding to EC. The affinity for the B-chain was much higher than for the A-chain according to Scatchard analysis (Kd 24 and 515 nM, respectively), whereas the number of binding sites was higher for the A-chain than for the B-chain (Bmax 8 x 10(5) and 1.2 x 10(5), respectively). There were no cross interactions between the A- and B-chains and their binding sites. The binding of tPA to EC induced an almost 100-fold increase of the activation rate when compared to the same amount of enzyme in free solution, which in contrast to the fibrin-induced stimulation was not inhibited by antibodies against fibrin. The enzymatic activity of the B-chain was much less affected by the association to the cells. Both tPA and the tPA B-chain were largely protected against inhibition by an excess plasminogen activator type-1 (PAI-1) when bound to EC, whereas the same amount of free tPA was totally inactivated. The competition studies strongly indicated that an N-terminal segment in the B-chain, AKHRRSPGER, may be the ligand part of the B-chain. It is interesting to note that this polypeptide segment also participates in a binding site for PAI-1, necessary for effective inhibition. This implies a possible competition between PAI-1 and a tPA-receptor for binding of tPA. High molecular weight urokinase had no quenching effect on the binding of the B-chain to EC.

Human dermal endothelial cells express membrane-associated mast cell growth factor

Mast cell growth factor (MGF), a molecule that serves as a ligand for the receptor tyrosine kinase c-kit, is important in mast cell differentiation, migration, and activation. Previous studies of paraffin-embedded human skin using antibody to murine MGF and reverse transcription-polymerase chain reaction have demonstrated MGF protein and mRNA expression in keratinocytes and isolated dermal cells. We utilized a monoclonal antibody to human MGF to further define patterns of immunoreactivity in frozen specimens of neonatal and adult skin from normal individuals and from patients with urticaria pigmentosa. In addition to keratinocytes and isolated dermal cells in normal and urticaria pigmentosa skin, MGF was detected in cells lining superficial and mid-dermal vessels. Co-expression of MGF and the vascular antigen CD31, and immunoelectron microscopy, identified MGF-positive cells as endothelial cells. Patterns of endothelial MGF expression were not influenced by mast cell degranulation and endothelial E-selectin induction in vitro. By ultrastructure, unfixed specimens demonstrated MGF expression both within the endothelial cytoplasm and in association with lumenal, but not ablumenal, surfaces. Specimens fixed with Nakane's solution had diminished endothelial cytoplasmic MGF reactivity, but lumenal expression was maintained, suggesting persistence of a membrane-associated reactivity. MGF mRNA was also detected in cultured dermal microvascular endothelial cells using reverse transcription-polymerase chain reaction. These data establish human dermal endothelial cells as sites of MGF production and expression in human skin. Mast cell precursors must home to skin via vascular channels and differentiate in the immediate perivascular space. Thus, endothelial MGF may be an important determinant of adhesion and differentiation of mast cell progenitors expressing receptors for MGF.

Pharmacological modulation of endothelial cell-associated adhesion molecule expression: implications for future treatment of dermatological diseases

Skin diseases with an inflammatory component, regardless of their etiology, are characterized at some point by the extravasation and subsequent infiltration of leukocytes into the dermal and/or epidermal compartments. This trafficking pattern is determined by a complex series of events whereby the leukocytes interact with cell adhesion molecules (CAM), particularly those induced on endothelial cells following activation with various inflammatory mediators. Vascular CAMs belonging to the selectin family (i.e., P-selectin and E-selectin) are thought to mediate early and reversible events involving leukocyte rolling and margination along the lumenal surface of microvascular cells (post-capillary venules). Certain members of the immunoglobulin supergene family (i.e., VCAM-1 and ICAM-1) regulate later and irreversible steps which lead to firm attachment and subsequent diapedesis of leukocytes. Accumulating evidence suggests that if one blocks the ligand-binding sites between leukocytes and endothelial cells, or inhibits vascular CAM expression, hematopoietic cell extravasation and progressive inflammatory events can be greatly diminished. To identify such inhibitors we developed a cell-based Elisa using the human microvascular cell line HMEC-1. As reported in the present paper, this approach yielded a naturally-occurring, low molecular weight compound which potently inhibits cytokine-induced adhesion molecule expression on cultured endothelial cells, without modulating "house-keeping" proteins.

Development of an ELISA for quantification of human protein S in cell culture fluids using commercial polyclonal antisera

An enzyme-linked immunosorbent assay (ELISA) was developed to measure protein S antigen released into cell culture fluids. We used readily available commercial polyclonal antisera to develop the assay. This assay was sensitive with a detection limit of about 0.086 ng/ml. Between-assay precision (coefficient of variation) at levels of 0.2, 1.1, and 13.9 ng/ml was 14%, 15%, and 11% respectively. Specificity and accuracy were demonstrated from the use of: 1) culture fluids from 3-primary endothelial cell cultures and 7-cell lines known to constitutively produce protein S; 2) 2-cell lines not synthesizing protein S; and 3) from selected samples of normal and protein S deficient plasma. The ELISA described here was about 12-fold more sensitive and 40-fold more cost effective when compared to a commercial ELISA kit. Thus the assay provided a sensitive, specific, precise and economical method useful for the measurement of the nanogram amounts of protein S commonly encountered in cell culture fluids.

Characterization of expression and modulation of cell adhesion molecules on an immortalized human dermal microvascular endothelial cell line (HMEC-1)

We have recently reported the creation of the first immortalized cell line derived from human dermal microvascular endothelial cells (HMEC-1). In preliminary studies this line was found to closely resemble microvascular endothelial cells in regard to many phenotypic characteristics. Because two key functional features of endothelial cells are their ability to bind to peripheral blood leukocytes and extracellular matrix proteins via cell adhesion molecules, we have now characterized HMEC-1 in terms of expression and regulation of cell adhesion molecules of the integrin, immunoglobulin gene superfamily, and selectin families. HMEC-1 can either constitutively express or can be induced to express key integrins, including alpha-1, -2, -3, -4, -5, -6, and -V, as well as beta-1, -3, -4, and -5. They also express or are capable of expressing immunoglobulin gene superfamily molecules, such as intercellular adhesion molecule-1 and vascular cell adhesion molecule-1, and a member of the selectin family, E-selectin. A number of important cell adhesion molecules that are either constitutively expressed or that must be induced are regulated in a time- and dose-dependent fashion by selected cytokines. Experiments comparing the phenotypic characteristics of HMEC-1 with human dermal microvascular endothelial cells or human umbilical vein endothelial cells reveal HMEC-1 to have features of both small- and large-vessel endothelial cells.

Expression of SV40 tumour antigens enables human endothelial cells to grow independently from foetal calf serum and exogenous growth factors

Human endothelial cells were transfected with a plasmid containing the coding sequence of the large T protein of simian virus 40. Transfected cells were selected for their ability to grow in defined medium (DM). Several cell lines were derived and characterized in their response to endothelial cell growth supplement (ECGS), epidermal growth factor (EGF) and insulin (INS). In addition to cell lines that were dependent on these additives, others growing without any exogenous growth factor could be selected. No evidence of autocrine growth stimulation was found. For growth studies, a simple assay was used based on the acid phosphatase activity as a parameter for the cell number. Cell lines in defined medium showed less chromosome aberrations than those grown in serum-containing medium. Because of their long in vitro life span of about 100 generation doublings and defined medium requirements these cells represent valuable test material for all kinds of investigations on the vascular endothelium.

Inhibition of induced angiogenesis in a human microvascular endothelial cell line by ET-18-OCH3

Alkyl-lysophospholipids are a group of anti-cancer compounds that have previously been shown to have the unique feature of being selectively toxic to neoplastic tissues. One of these compounds, ET-18-OCH3, has been used for purging bone marrow of cancer cells in phase I clinical trials. Tumor-induced angiogenesis has been directly correlated with tumor growth and metastasis. In this study, we examined the effect ET-18-OCH3 has on a human microvascular endothelial cell line (HMEC-1), including the following functions: angiogenesis, cell-adhesion molecule expression, and cell-junction integrity. We found that ET-18-OCH3 (in vitro) reversibly inhibited induced angiogenesis at levels that did not affect viability. At lower concentrations, ET-18-OCH3 down-regulated the expression of cell-adhesion molecules and affected the integrity of cell-to-cell junctions. This observation demonstrates this versatile family of compounds to have additional targets of action.

Protein S, an antithrombotic factor, is synthesized and released by neural tumor cells

Protein S, an anticoagulant factor in the protein C antithrombotic pathway, was found to be synthesized and released by six tumor cell lines of neural origin by western blotting and ELISA. The rate of synthesis ranged from three- to 11-fold higher than that of a microvascular endothelial cell line and 36-144% that of a hepatoma cell line. The secreted protein S displayed specific anticoagulant activity similar to that of purified plasma protein S, implying that it was fully gamma-carboxylated. Ten primary brain tumor tissues also expressed protein S antigen, as shown by western blot analysis. Expression of anticoagulantly active protein S by neural cells raises important questions concerning possible physiologic roles for this multidomain protein beyond its function in control of thrombosis.

Role of microvascular endothelial cells in inflammation

Endothelial cells are critical elements in the evolution of all types of cutaneous inflammation. They participate through the synthesis and secretion of pro-inflammatory cytokines, including interleukin 1 (IL-1), IL-6, and IL-8, as well as M-CSF, G-CSF, GM-CSF, gro alpha, and MCP. They also express a series of cell-surface proteins and glycoproteins known as cell adhesion molecules that allow circulating leukocytes to bind to endothelial cells and allow endothelial cells to bind to matrix proteins. The regulated expression of these molecules, including those in the integrin, immunoglobulin gene, and selection families, allows for the precise trafficking of circulating leukocytes to sites of inflammation, injury, or immunologic stimulation in the skin. Furthermore, emerging evidence clearly indicates that selected differences exist between endothelial cells of the microvasculature and those that line large blood vessels. These include differences in secreted products, differences in the expression of cell adhesion molecules, and differences in cytokine-induced regulation of commonly expressed cell adhesion molecules, among others. Thus, a precise delineation of the biology of cutaneous microvascular endothelial cells is important to our understanding of cutaneous inflammation.

Viral susceptibility of an immortalized human microvascular endothelial cell line

CDC/EU.HMEC-1 is the first immortalized human microvascular endothelial cell line that retains morphologic, phenotypic, and functional characteristics of a normal human microvascular endothelial cell. This study evaluates a variety of viruses and their effects on this human endothelial cell line. The data indicate that adenoviruses, some herpesviruses, reoviruses and most picornaviruses grow well in HMEC-1, with distinctive cytopathic effects. The paramyxoviruses, however, do not appear to propagate, nor does HIV. The findings indicate that microvascular endothelial cells may act as a reservoir of these viruses; it also suggests the possibility that microvascular endothelium could be involved in the processing and presentation of antigen to immune cells.