Helen A. Bull

Silica‐associated systemic sclerosis is clinically, serologically and immunologically indistinguishable from idiopathic systemic sclerosis

To determine whether the clinical, immunological and serological features of patients with silica‐associated systemic sclerosis are different from patients with the‘idiopathic’ form of systemic sclerosis (SS) we studied 22 underground coal miners who were exposed to silica dust (SD), 30 mine workers who later developed silicosis (S) and 17 mine workers exposed to silica dust who subsequently developed a systemic sclerosis‐like disease (SA‐SS).

Pro-inflammatory mediators induce sustained release of prostaglandin E2 from human dermal microvascular endothelial cells

The vasodilator prostaglandin E2 has been proposed as a mediator of erythema in a variety of cutaneous inflammatory reactions and prostacyclin levels have been found to be elevated in ultraviolet induced erythema. Human recombinant interleukin iα and lipopolysaccharide induced a concentration‐ and time‐dependent release of prostaglandin E2, but not prostacyclin, from cultured neonatal and adult human dermal microvascular endothelial cells. Prostaglandin E2 was measurable at 2 h after stimulation with i U/ml interleukin Iα, levels increased rapidly up to 6 h and more slowly up to 24 h. Lipopolysaccharide (20 μg/ml) induced measurable release of prostaglandin E2 between 2 and 4 h after stimulation and release continued up to 24 h when incubation was terminated. With both agonists, release of prostaglandin E2 was inhibited by indomethacin and significantly reduced by cycloheximide. The sensitivity and magnitude of responses of the cutaneous endothelial cells to these pro‐inflammatory stimuli appeared to be dependent on their derivation.

Characterization of histamine receptor sub-types regulating prostacyclin release from human endothelial cells.

1 The histamine receptor sub‐types that are involved in the initiation and maintenance of prostacyclin (PGI2) release from human endothelial cells have been investigated. 2 Endothelial cells cultured from umbilical vein (HUVEC) were incubated with either histamine, the selective H1‐receptor agonists, 2‐methyl histamine (2‐MeHA) or thiazolylethylamine (ThEA), the H1‐agonist/H3‐antagonist, β‐histine (β‐His), the selective H2‐agonist, dimaprit, the H2‐agonist/H3‐antagonist, impromidine, the selective H3‐agonist, (R)α‐methylhistamine ((R)α‐MeHA) and the H3‐antagonist, thioperamide. 3 The H1‐agonists and the H3‐agonist (R)α‐MeHA induced a concentration (100 nm‐1 mm) and time‐dependent release of PGI2 as determined by radioimmunoassay for 6‐keto‐PGF1α, but were less potent than histamine itself. The rank order of potency was the same following 30 min and 24 h incubation, i.e. histamine > ThEA > 2‐MeHA ≫ β‐His > (R)α‐MeHA. 4 Histamine and 2‐MeHA (1 μm‐1 mm), ThHEA (10 μm‐1mm) and (R)α‐MeHA (1 mm), but not β‐His, induced a significantly greater increase in PGI2 release after 24 h incubation than after 30 min incubation (P < 0.05). 5 Neither the selective H2‐agonist, dimaprit, nor the H2‐agonist/H3‐antagonist, impromidine alone induced release of PGI2. 6 The H1‐antagonist, mepyramine (10 μm), abolished release of PGI2 induced by histamine, the H1‐agonists and (R)α‐MeHA but the H2‐antagonist cimetidine (10 μm) and the H2/H3‐antagonist, burimamide (10 μm) did not significantly modulate PGI2 release. 7 Although the H3‐agonist (R)α‐MeHA induced release of PGI2, it failed to modulate PGI2 release in the presence of histamine. 8 Low concentrations of the H3‐antagonist, thioperamide (100 nm) did not modulate histamine release of PGI2 at all but after 24 h incubation, thioperamide (10−4 m) partially reduced PGI2 release in the presence of histamine. 9 These results indicate that PGI2 from HUVEC is initiated and maintained via histamine H1‐receptor occupancy. There appears to be no involvement of either H2‐ or H3‐receptors in this particular endothelial cell histaminergic response.

Effect of human recombinant interleukin‐Iα on release of prostacyclin from human endothelial cells

Incubation of human recombinant IL‐1α (hrIL‐1α) with cultured human endothelial cells induced a dose‐and time‐dependent increase in the release of prostacyclin (PGI2). Above a dose of hrIL‐1α 0.05 units/ml and following a variable lag phase of between 2 and 4 h, PGI2 release (measured as the stable hydrolysis product 6‐keto‐prostaglandin F1α) was detected in the culture supernatant and levels continued to rise throughout a 48‐h incubation. The release of PGI2 required the continued presence of hrIL‐1α, did not demonstrate tachyphylaxis and was not reduced by pre‐incubation with the protein synthesis inhibitors cycloheximide, tunicamycin and actinomycin or by the calmodulin antagonist trifluoroperazine. The relationship of these results to ultraviolet radiation induced erythema is discussed.

Interleukin-1 potentiates histamine-induced release of prostacyclin from human endothelial cells

1 In human cultured umbilical vein endothelial cells, interleukin‐1 potentiated histamine‐induced release of prostacyclin in a time‐ and concentration‐dependent manner. 2 In cells incubated with interleukin‐1 for 24 h, maximal potentiation was observed when cells were pre‐incubated with 0.5 u ml−1 interleukin‐1 before stimulation with histamine (1 μm‐1 mm). 3 In cells incubated with 0.5 u ml−1 interleukin‐1, 20 min pre‐incubation was sufficient to induce a statistically significant potentiation of prostacyclin release induced by 1 μm histamine (P > 0.05). 4 Nifedipine but not cycloheximide, significantly (P > 0.05) inhibited histamine‐induced release of prostacyclin and interleukin‐1 potentiation of histamine‐induced release of prostacyclin (P > 0.05). 5 Incubation with 1 u ml−1 interleukin‐1 induced a two fold increase in cellular prostaglandin synthetase activity within 30 min. The enzyme activity increased up to 6 h and was maintained up to 24 h. In cells co‐incubated with cycloheximide and 1 u ml−1 interleukin 1, prostaglandin synthetase activity at 24 h was the same as that in unstimulated cells. Prostacyclin release was not significantly inhibited in cells co‐incubated with cycloheximide and interleukin 1. 6 These results suggest that interleukin‐1 potentiates histamine‐induced release of prostacyclin by rapid up‐regulation of prostaglandin synthetase activity as well as by inducing synthesis of enzyme protein. These mechanisms may act to potentiate/regulate vascular endothelial responses in inflammatory reactions.

Release of prostacyclin from cultured human endothelial cells following prolonged exposure to histamine

Histamine is an important pro‐inflammatory molecule mediating leukocyte margination, plasma extravasation and vasodilation, but its precise mode of action on vascular endothelium is unclear. We report that histamine is able to induce prolonged release of prostacyclin (PGI2) from human endothelial cells via occupancy of the H1 receptor, without an absolute requirement for the presence of histamine or synthesis of new enzyme protein to facilitate continued release of PGI2.

The haemostatic function of the vascular endothelial cell.

Since the mid 1970s, which saw the advent of techniques for the in vitro culturing of vascular endothelial cells, there has been an exponential increase in our understanding of the structure and function of this cell. One of its major roles is the prevention of thrombin generation and platelet adhesion to its luminal surface. Until recently, it was thought that platelet adhesion and fibrin clot formation only occurred at areas in the blood vessel where the endothelial cell monolayer had been removed. It is becoming increasingly apparent that this is not so. Under ideal conditions the endothelial cell will not facilitate platelet adhesion or activation of the coagulation cascade. However the endothelial cell is constantly being challenged and in order to maintain an anticoagulant thromboresistant luminal surface, it is involved in three pathways responsible for down-regulation of the coagulation cascade (Table 1). In addition thromboresistance is mediated by lipoxygenase and cyclo-oxygenase metabolites of unsaturated fatty acids synthesised by the endothelial cell. If perturbation of the endothelial cell is excessive or prolonged then the anticoagulant, antithrombotic mechanisms are over-ruled and the procoagulant pathways predominate with resultant generation of thrombin, fibrin clot formation and subsequent platelet adhesion and aggregation. There is down-regulation of fibrinolysis and the activated protein C pathway as well as synthesis of specific procoagulant and prothrombotic mediators (Table 2).