Abstract. To investigate the greater enzymatic activity of the alternative pathway convertase (and the subsequent greater fixation of C3b) on paroxysmal nocturnal hemoglobinuria (PNH) erythrocytes, we have examined the topography of binding of C3b to PNH and normal erythrocytes. Using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography, the a-chain of C3b was found to bind via predominantly ester bonds to free hydroxyl groups on glycophorin-a, the major erythrocyte sialoglycoprotein. The pattern of binding of nascent C3b was the same for normal and PNH erythrocytes. Thus, although C3b binding to a different membrane constituent did not appear to account for the greater enzymatic activity of the alternative pathway convertase when affixed to PNH erythrocytes, it seemed possible that the glycoproteins to which C3b bound might be qualitatively abnormal on the PNH cells, and that structural differences in these molecules might impose modifications in the enzyme-substrate interactions of the alternative pathway convertase. Using methods for radiolabeling both protein and carbohydrate residues, we therefore compared the electrophoretic pattern of the cellsurface glycoproteins on PNH and normal erythrocytes. The glycophorin-a dimer was found to be qualitatively abnormal on the PNH cells as evidenced by its greater susceptibility to trypsin-mediated proteolysis. In addition, the abnormal erythrocytes from patients with PNH had Parts of this work were presented to the

Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired disorder associated with absence of expression of phosphatidylinositol (PI)- linked membrane proteins from circulating hematopoietic cells of multiple lineages. Recent work demonstrated that decay accelerating factor, one such PI-linked protein, bears the Cromer-related blood group antigens. This study demonstrated that other high incidence antigens, including Cartwright (Yta/Ytb), Holley-Gregory (Hy/Gya), John Milton Hagen (JMH), and Dombrock (Doa/Dob), are absent from the complement-sensitive (PNH III) erythrocytes of patients with PNH. The relatively normal, complement-insensitive erythrocytes from the same patients express these antigens normally. Therefore, these antigens most likely reside on PI-linked proteins absent from PNH III, but not PNH I, erythrocytes.

An arginine-glycine-aspartic acid sequence (RGD in the single letter code for amino acids) is present in the cell attachment site of both vitronectin and fibronectin. Inasmuch as fibronectin and synthetic peptides containing RGD enhance ingestion of opsonized particles by monocytes, we investigated the effects of vitronectin on phagocytosis by monocytes of sheep erythrocytes bearing IgG (EA) or complement C3b (EC3b). Peripheral blood monocytes were isolated by countercurrent elutriation and allowed to adhere to slides that had been coated with either vitronectin or fibronectin. Next, EA or EC3b were incubated with the adherent monocytes, and phagocytosis was subsequently quantified. Vitronectin caused the same dose dependent increase in phagocytosis as fibronectin. The augmentation of phagocytosis of EA induced by vitronectin could be inhibited by the F(ab')2 fragments of anti- vitronectin IgG but not by preimmune F(ab')2. The maximum phagocytosis of EA induced by vitronectin could not be enhanced by the addition of fibronectin, suggesting that vitronectin and fibronectin act on the same population of monocytes and that the two proteins stimulate the same mechanism through which the enhanced phagocytosis is mediated. Fibronectin and vitronectin caused a tenfold increase in the attachment of EC3b to monocytes, but phagocytosis was augmented minimally. These studies demonstrate that vitronectin modulates interactions between monocytes and opsonized particles.

To investigate the mechanism by which treatment of normal human erythrocytes with the sulfhydryl reagent 2-aminoethylisothiouronium bromide (AET) induces susceptibility to complement mediated lysis, the effects of AET on the structural and functional integrity of decay accelerating factor (DAF), membrane inhibitor of reactive lysis (MIRL), and complement receptor type 1 (CR1) were examined. Following treatment with AET, erythrocyte MIRL and CR1 were no longer recognized in situ by antibodies, and antibody binding to DAF was diminished by approximately 50%. These studies indicated that the structural integrity of the three complement regulatory proteins was either partially (DAF) or completely (MIRL and CR1) disrupted by AET. Subsequent experiments showed that functional inactivation paralleled the structural disruption. Treatment of normal erythrocytes with AET induced susceptibility to cobra venom factor-initiated hemolysis, indicating that the functional activity of MIRL had been destroyed. The capacity of erythrocyte CR1 to serve as a cofactor for factor I-mediated cleavage of iC3b to C3c and C3dg was lost following treatment with AET. C3 convertase activity increase markedly following treatment of erythrocytes with AET, but convertase activity on AET cells was approximately 50% less than that observed when DAF function on normal cells was completely inhibited by antibody. Susceptibility of AET cells to acidified serum lysis was shown to be due primarily to inactivation of MIRL. Unexpectedly, in acidified serum the activity of the amplification C3 convertase of the APC was found to be controlled by MIRL as well as by DAF. These studies show that AET induces susceptibility to complement-mediated lysis by disrupting the structural and functional integrity of membrane constituents that regulate the activity of both the C3 convertases and the membrane attack complex of complement.

Background. Oral contraceptives (OC) contain an orally active estrogen in combination with an orally active synthetic progestin derived from 19‐nortestosterone. OC have had an enormous positive impact on public health for the past three decades, and in the main, there has been a remarkably low incidence of troublesome side effects. Although estrogens are implicated in an increased incidence of breast and endometrial cancer, epidemiologic studies have not provided convincing evidence to support a direct correlation between OC use and an increase in breast cancer incidence. By contrast, OC do cause a decrease in the incidence of endometrial and ovarian carcinoma. During the past decade, several isolated reports have linked an increased incidence of breast cancer with the use of synthetic progestins. No mechanism for the proliferative potential of progestins has been offered. Therefore, the authors investigated this problem to formulate a hypothesis, based on laboratory data, that might be evaluated in populations at risk. Methods. The synthetic progestins (19‐nortestosterone derivatives) chosen for the study were norethynodrel, norethindrone, norgestrel (levonorgestrel), and gestodene. These were compared with the actions of medroxyprogesterone acetate (MPA). To determine whether the progestins produced their effects via the ER, the cells were transfected with a chloramphenicol acetyl transferase (CAT) reporter gene containing an estrogen response element only activated by ER. Results. The 19‐nortestosterone derivatives all stimulated the growth of estrogen receptor (ER)‐positive but not ER‐negative breast cancer cells in culture. Antiestrogens, but not the antiprogestin mifepristone (also known as RU 486), inhibited progestin‐stimulated cell proliferation. MPA did not stimulate cell proliferation. All the synthetic progestins that increased replication also activated CAT. Activation was blocked by antiestrogens but not by mifepristone; the synthetic progestin MPA was inactive. Conclusions. These studies provided direct evidence that some synthetic progestins exert estrogenic effects through the ER. The results demonstrated that progestins can have a dual effect on estrogen target tissues either to stimulate or differentiate cells. The results suggest that some beneficial estrogen‐like effects could be produced by synthetic progestins (e.g., bone preservation), but epidemiologic studies of OC use should focus of the “total estrogen” content to establish whether some formulations place some groups of women at greater risk of having breast cancer.

The erythrocyte membrane inhibitor of reactive lysis (MIRL) is an 18-Kd protein that controls complement-mediated hemolysis by restricting the activity of the membrane attack complex. MIRL expression on the erythrocytes of paroxysmal nocturnal hemoglobinuria (PNH) is abnormally low, and the greater susceptibility of PNH erythrocytes to complement is causally related to this deficiency. Inasmuch as other proteins that are deficient in PNH are anchored to the membrane through a glycosyl phosphatidylinositol moiety, studies were undertaken to determine if MIRL shares this structural feature. Normal human erythrocytes that had been radiolabeled with 125I were incubated with phosphatidylinositol- specific phospholipase C (PIPLC), and the supernate and the solubilized membrane proteins were immunoprecipitated using anti-MIRL antiserum. The MIRL that was specifically released into the supernate had an Mr of 19 Kd, while the MIRL that remained bound to the membrane had an Mr of 18 Kd. A quantitative assay showed that approximately 10% of erythrocyte MIRL was susceptible to PIPLC; however, treatment with PIPLC had no effect on either the electrophoretic mobility or the functional activity of purified MIRL. These studies show that the effects of PIPLC on MIRL are similar to those observed for other human erythrocyte membrane proteins that are anchored by a glycosyl phosphatidylinositol moiety.

When incubated in acidified serum, the erythrocytes of paroxysmal nocturnal hemoglobinuria (PNH) are hemolyzed through activation of the alternative pathway of complement (APC), but normal erythrocytes are resistant to this process. PNH cells are deficient in decay- accelerating factor (DAF), a complement regulatory protein that inhibits the activity of both the classical and the alternative pathways. However, deficiency of DAF alone does not account entirely for the aberrant effects of acidified serum on PNH cells. Recently, we have shown that PNH erythrocytes are also deficient in another complement control protein called membrane inhibitor of reactive lysis (MIRL) that restricts complement-mediated lysis by blocking formation of the membrane attack complex (MAC). To determine the effects of the DAF and MIRL on susceptibility to acidified serum lysis, PNH cells were repleted with the purified proteins. DAF partially inhibited acidified serum lysis by blocking the activity of the amplification C3 convertase. MIRL inhibited acidified serum lysis both by blocking the activity of the MAC and by inhibiting the activity the C3 convertase. When DAF function was blocked with antibody, normal erythrocytes became partially susceptible to acidified serum lysis. By blocking MIRL, cells were made completely susceptible to lysis, and control of C3 convertase activity was partially lost. When both DAF and MIRL were blocked, the capacity of normal erythrocytes to control the activity of the APC and the MAC was destroyed, and the cells hemolyzed even in unacidified serum. These studies demonstrate that DAF and MIRL act in concert to control susceptibility to acidified serum lysis; of the two proteins, MIRL is the more important. In addition to its regulatory effects on the MAC, MIRL also influences the activity of the C3 convertase of the APC. Further, in the absence of DAF and MIRL, the plasma regulators (factor H and factor I) lack the capacity to control membrane- associated activation of the APC.

Molecules that induce tissue factor expression by responsive cells such as endothelial cells and monocytes may be important in the regulation of hemostasis and, perhaps, in mediating certain hemostatic disorders. A constituent of normal human plasma capable of inducing tissue factor activity in human endothelial cells and monocytes has been isolated and identified as a derivative of, or modification associated with albumin. Procoagulant albumin caused a concentration-dependent induction of tissue factor expression by human endothelial cells, but bovine endothelial cells were unresponsive. The dose-response curve developed a plateau phase, indicating that the capacity of endothelial cells to respond to the stimulus was finite. The maximum response induced by the procoagulant albumin was similar to that observed for maximally effective concentrations of endotoxin, interleukin-1, and tumor necrosis factor. Time-course studies showed that procoagulant albumin produced peak activity in 4 to 6 hours. Identification of a procoagulant form of albumin in normal human plasma suggests a potential role for this constituent in regulation of hemostasis.