Expression of functional cell surface C1-inactivator by U937 cells.
We have previously shown that the human monocyte-like cell line U937 synthesizes C1-INA and expresses cell surface C1-INA. In this report we provide evidence that this surface-expressed C1-INA is functionally active. Intact U937 cells demonstrated functional C1-INA activity in a hemolytic assay. This activity was blocked when the cells were incubated with monospecific antibody to C1-INA, and was not detectable in cell-free supernatants of U937 cells. SDS-PAGE analysis of radiolabeled U937 cell surface proteins purified by anti-C1-INA affinity chromatography revealed two distinct bands. One protein had a Mr of 105 kDa identical to plasma C1-INA, and the second had a Mr of 200 kDa. We were unable to determine the identity of the 200 kDa protein by Western blotting with anti-C1-INA. However, the possibility exists that this 200 kDa molecule may represent a C1-INA receptor, a dimeric form of C1-INA, or an unrelated cell surface protein with affinity for C1-INA. Furthermore, we show that treatment of U937 cells with phorbol ester resulted in an increase in the percentage of cells expressing surface C1-INA. These results suggest that U937 cells express functional cell surface C1-INA, which could function in vivo to protect these human tumor cells from lysis by host complement.
Randazzo
B P
BP
Department of Medicine, State University of New York, Stony Brook 11794.
Fleit
H B
HB
Kaplan
A P
AP
Ghebrehiwet
B
B
D015153:Blotting, Western; D002460:Cell Line; D002474:Cell-Free System; D003174:Complement C1 Inactivator Proteins / Q000276:immunology / Q000302:isolation & purification* / Q000502:physiology; D003181:Complement C4 / Q000032:analysis; D003902:Detergents; D004591:Electrophoresis, Polyacrylamide Gel; D006461:Hemolysis; D006801:Humans; D007074:Immunoglobulin G / Q000502:physiology; D008565:Membrane Proteins / Q000276:immunology / Q000302:isolation & purification* / Q000502:physiology; D009000:Monocytes / Q000032:analysis*; D009500:Neutralization Tests; D013755:Tetradecanoylphorbol Acetate
D003174:Complement C1 Inactivator Proteins; D003181:Complement C4; D003902:Detergents; D007074:Immunoglobulin G; D008565:Membrane Proteins; D013755:Tetradecanoylphorbol Acetate