Laurence A. Harker

Homocystine-induced arteriosclerosis. The role of endothelial cell injury and platelet response in its genesis.

The atherogenic mechanism of homocystinemia has been defined by measuring endothelial cell loss and regeneration, platelet consumption, and intimal lesion formation in a primate model. Three groups of baboons were studied: (a) 8 control animals; (b) 15 animals after 3 mo of continuous homocystinemia; and (c) 11 animals after 3 mo of combined homocystinemia and oral treatment with dipyridamole. Experimental homocystinemia caused patchy endothelial desquamation comprising about 10% of the aortic surface despite a 25-fold increase in endothelial cell regeneration. Neither endothelial cell loss nor regeneration was changed significantly by dipyridamole. Homocystine-induced vascular deendothelialization produced a threefold increase in platelet consumption that was interrupted by dipyridamole inhibition of platelet function. All homocystinemic animals developed typical arteriosclerotic or preatherosclerotic intimal lesions composed of proliferating smooth muscle cells averaging 10-15 cell layers surrounded by large amounts of collagen, elastic fibers, glycosaminoglycans, and sometimes lipid. Intimal lesion formation was prevented by dipyridamole therapy. We conclude that homocystine-induced endothelial cell injury resulted in arteriosclerosis through platelet-mediated intimal proliferation of smooth muscle cells that can be prevented by drug-induced platelet dysfunction.

Thrombokinetics in man

Platelet production, distribution, and destruction have been quantitated in normal man and in selected patients with platelet disorders. In most instances, total production as calculated from the megakaryocyte mass agreed with production estimated from platelet turnover. In patients with megaloblastosis, a discrepancy between these two measurements indicated the presence of ineffective thrombopoiesis. Thrombopoiesis was regulated by (a) alterations in megakaryocyte number, and (b) changes in megakaryocyte volume (produced by changes in endomitosis). The volume-endomitosis changes were closely related to the peripheral platelet count and were a useful indicator of thrombopoietic stimulus. Thrombocytopenic disorders have been classified on the basis of the disturbed physiology into disorders of (a) production (hypoproliferative or ineffective), (b) distribution (splenic pooling), or (c) destruction (immune or consumptive). Less than a twofold increase in platelet production in the presence of significant thrombocytopenia was taken to represent impaired proliferation. Thrombocytosis was classified as reactive or autonomous. Reactive thrombocytosis was consistently associated with a mean megakaryocyte volume and endomitosis less than normal but appropriate for the elevated circulating platelet count. In contrast, the average megakaryocyte volume and nuclear number were always greater than normal in thrombocythemia findings indicating autonomy.

Homocysteine-induced endothelial cell injury in vitro: A model for the study of vascular injury

Direct chemical injury to vascular endothelium was determined in vitro by measuring independently cell detachment and release of 51Cr from labeled human endothelial cell monolayers. Homocysteine, a sulfhydryl amino acid, induced specific 51Cr release from endothelial cells in direct proportion to its concentration between 0.1 and 10 mM. The proportion of detached cells during exposure to homocysteine was also directly related to the concentration of homocysteine. In vitro preincubation of homocysteine resulted in a progressive loss of cytotoxicity with no activity persisting after 24 hours. Mercaptoethanol, a sulfhydryl agent similar to homocysteine, also induced endothelial injury in a concentration dependent manner. Penicillamine prevented the homocysteine mediated 51Cr-release at equimolar concentrations (p< 0.001). Catalase also inhibited the sulfydryl injury (90% reduction in 51Cr release at 0.1 mM homocysteine), but superoxide dismutase had no effect, thereby suggesting a role for hydrogen peroxide in mediating injury. Neither homocystine nor methionine, produced cell injury. Human arterial smooth muscle cells were insensitive to homocysteine levels less than 25 mM. These data demonstrate homocysteine-induced endothelial injury in vitro, and suggest that this process may in part be sulphydryl-mediated.

The bleeding time as a screening test for evaluation of platelet function.

Abstract The value of the standardized template bleeding time was studied in 100 normal subjects and 136 patients with various disorders. With normal platelets the bleeding time in this test is 4.5...

Platelet and Fibrinogen Consumption in Man

Abstract Survival and turnover measurements of platelets and fibrinogen in 35 normal subjects and 104 selected patients defined three types of consumptive processes involving the hemostatic apparatus. The first, characterized by combined platelet and fibrinogen consumption, represents an exaggeration of the physiologic hemostatic response. It occurs in patients with venous thrombosis, tissue trauma, widespread cancer, obstetric complications, and bacteremia. The result of activation of the coagulation system, this process can be modified by heparin. The second, characterized by selective platelet destruction, appears to reflect platelet thrombus formation on abnormal surfaces in the arterial system, including prosthetic devices and arterial thrombosis, thrombotic thrombocytopenic purpura, hemolytic-uremic syndrome, and vasculitis syndromes. This process is reversed by certain inhibitors of platelet function or adrenocortical steroid suppression of vascular inflammation. The third involves selective destru...

Neutrophil-mediated endothelial injury in vitro mechanisms of cell detachment.

Neutrophil-mediated endothelial injury was assessed in vitro using assays of cell lysis and cell detachment. Activation of human peripheral blood neutrophils adherent to human umbilical vein endothelial cell monolayers by serum-treated zymosan produced dose-dependent endothelial cell detachment without concomitant cell lysis. This injury was inhibited by neutral protease inhibitors, but not by catalase or superoxide dismutase. Neutrophils from a patient with chronic granulomatous disease also produced endothelial cell detachment when activated by serum-treated zymosan similar to normal neutrophils. Endothelial detachment was also produced by cell-free postsecretory media from activated neutrophils or by partially purified human neutrophil granule fraction and was inhibitable by tryptic, elastase, and serine protease inhibitors, but not by an acid protease inhibitor. Analysis of iodinated endothelial cell surface proteins that had been exposed to partially purified neutrophil granule fraction showed complete loss of proteins migrating in the region of fibronectin by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. This result was prevented in the presence of neutral protease inhibitors. We conclude that neutrophil-derived neutral proteases mediate endothelial cell detachment in vitro through digestion of endothelial cell surface proteins including fibronectin.

Glutathione redox cycle protects cultured endothelial cells against lysis by extracellularly generated hydrogen peroxide.

We have examined the role of the glutathione redox cycle as an antioxidant defense mechanism in cultured bovine and human endothelial cells by disrupting the glutathione redox cycle at several points. Endothelial glutathione reductase was selectively inhibited with 1,3-bis(chloroethyl)-1-nitrosourea (BCNU). Cellular stores of reduced glutathione were depleted by reaction with diethylmaleate (DEM) or 1-chloro-2,4-dinitrobenzene (CDNB) or by inhibition of glutathione synthesis with buthionine sulfoximine (BSO). Whereas several strains of untreated bovine and human endothelial cells were resistant to lysis by enzymatically generated hydrogen peroxide, BCNU-treated cells were readily lysed in a time- and dose-dependent manner. Glucose-glucose oxidase-mediated lysis of BCNU-treated bovine endothelial cells was catalase-inhibitable and directly related to BCNU concentration and endogenous glutathione reductase activity. Pretreatment of bovine endothelial cells with BCNU did not potentiate lysis by distilled water, calcium ionophore, lipopolysaccharide, or hypochlorous acid. Depletion of cellular reduced glutathione by reaction with DEM or CDNB or by inhibition of glutathione synthesis by BSO also potentiated endothelial lysis by enzymatically generated hydrogen peroxide. Inhibition of endothelial glutathione reductase by BCNU or depletion of reduced glutathione by BSO increased endothelial susceptibility to lysis by hydrogen peroxide generated by phorbol myristate acetate-activated neutrophils. We conclude that the glutathione redox cycle plays an important role as an endogenous antioxidant defense mechanism in cultured endothelial cells.

Studies of Platelet and Fibrinogen Kinetics in Patients with Prosthetic Heart Valves

Abstract Kinetic studies of platelet and fibrinogen in 18 patients with artificial heart valves indicate that platelets are selectively consumed by interaction with the prosthetic valves in an amount directly related to the surface area of the valve. Dipyridamole effectively prevents the valve-related consumption. Although acetylsalicylic acid itself has little capacity to correct the consumption of platelets in this setting, it has a potentiating effect on dipyridamole.

Thrombin stimulates tissue plasminogen activator release from cultured human endothelial cells.

The effect of thrombin on the release of tissue plasminogen activator from endothelial cells was studied in primary cultures of human umbilical vein endothelial cells. Tissue plasminogen activator concentration in conditioned medium was measured by a two-site radioimmunometric assay. The addition of increasing concentrations (0.01 to 10 U/ml) of thrombin to confluent cultures produced a saturable, dose-dependent increase in the rate of release of tissue plasminogen activator. A sixfold increase in tissue plasminogen activator concentration (from 2 to 12 ng/ml) occurred after the addition of 1 U/ml thrombin (8 X 10(-9) M) to cultures containing 5 X 10(4) cells/cm2. Enhanced release was not observed until 6 h after thrombin addition, reached a maximum rate of 1.3 ng/ml per h between 8 and 16 h, and then declined to 0.52 ng/ml per h after 16 h. The 6-h lag period before increased tPA release was reproducible and independent of thrombin concentration. Thrombin inactivated with diisopropylfluorophosphate or hirudin did not induce an increase in tissue plasminogen activator levels. A 50-fold excess of diisopropylfluorophosphate-treated thrombin, which inhibits binding of active thrombin to endothelial cell high affinity binding sites, did not inhibit the thrombin-induced increase. It is concluded that proteolitically active thrombin causes an increase in the rate of release of tissue plasminogen activator from cultured human endothelial cells. The 6-h interval between thrombin treatment and enhanced tissue plasminogen activator release may reflect a delaying mechanism that transiently protects hemostatic plugs from the sudden increase in the local concentration of this fibrinolytic enzyme.

Effects of polyethylene glycol-conjugated recombinant human megakaryocyte growth and development factor on platelet counts after chemotherapy for lung cancer.

BACKGROUND Polyethylene glycol (PEG)-conjugated recombinant human megakaryocyte growth and development factor (MGDF, also known as PEG-rHuMGDF), a recombinant molecule related to thrombopoietin, specifically stimulates megakaryopoiesis and platelet production and reduces the severity of thrombocytopenia in animals receiving myelosuppressive chemotherapy. METHODS We conducted a randomized, double-blind, placebo-controlled dose-escalation study of MGDF in 53 patients with lung cancer who were treated with carboplatin and paclitaxel. The patients were randomly assigned in blocks of 4 in a 1:3 ratio to receive either placebo or MGDF (0.03, 0.1, 0.3, 1.0, 3.0, or 5.0 microg per kilogram of body weight per day), injected subcutaneously. No other marrow-active cytokines were given. RESULTS In the 38 patients who received MGDF after chemotherapy, the median nadir platelet count was 188,000 per cubic millimeter (range, 68,000 to 373,000), as compared with 111,000 per cubic millimeter (range, 21,000 to 307,000) in 12 patients receiving placebo (P = 0.013). The platelet count recovered to base-line levels in 14 days in the treated patients as compared with more than 21 days in those receiving placebo (P<0.001). Among all 40 patients treated with MGDF, 1 had deep venous thrombosis and pulmonary embolism, and another had superficial thrombophlebitis. CONCLUSIONS MGDF has potent stimulatory effects on platelet production in patients with chemotherapy-induced thrombocytopenia.