G. Bergamaschi et al., SAPORIN, A RIBOSOME-INACTIVATING PROTEIN USED TO PREPARE IMMUNOTOXINS, INDUCES CELL-DEATH VIA APOPTOSIS, British Journal of Haematology, 93(4), 1996, pp. 789-794
The plant toxin saporin is a ribosome-inactivating protein which inhib
its protein synthesis and growth of both normal and tumour cells. Its
cytotoxic activity can be increased by coupling with antibodies recogn
izing cell surface antigens. In this work we performed experiments to
test the hypothesis that saporin induces cell death via apoptosis. Exp
osure to saporin induced apoptosis in different cellular models, such
as human peripheral blood B lymphocytes and neutrophils, in the Daudi
B-cell line, and in the haemopoietic cell lines HL-60 and TF-1. This w
as indicated by: (a) the appearance of typical morphological features
such as chromatin condensation, nuclear fragmentation and blebbing of
plasma membranes; (b) DNA degradation into oligonucleosomal fragments;
(c) the appearance of apoptotic cells on DNA flow cytometry as a cell
population with reduced DNA content (A(O) region). The fraction of ce
lls showing features of apoptosis ranged from 19 +/- 5% for TF-1 cells
to 35 +/- 8% for neutrophils. In experiments with normal peripheral b
lood B lymphocytes or with Daudi cells, we compared the activity of na
tive saporin with that of an immunotoxin hybrid molecule obtained by b
inding the toxin to two bispecific antibodies recognizing both saporin
and the B lymphocyte-specific antigen CD22 (Sap/BsAb complexes). Sapo
rin bound to the antibodies was 2-3 logs more effective than native sa
porin in inducing apoptosis, with maximal inhibitions being observed a
t concentrations of 10(-6) M for native saporin and 10(-9)-10(-8) M fo
r the hybrid molecules. These findings indicate that treatment with sa
porin results in apoptosis of target cells and suggest that this may b
e relevant to the therapeutic use of saporin-containing immunotoxins.
In fact, if used in vivo as an immunotoxin, its cytotoxic activity cou
ld be devoid of more extensive and non-specific tissue damaging effect
s as would be the case if saporin induced necrosis of target cells.