Hydrogen peroxide produced during gamma-glutamyl transpeptidase activity is involved in prevention of apoptosis and maintenance of proliferation in U937 cells

It has been reported in several cell lines that exposure to low levels of r eactive oxygen species can exert a stimulatory effect on their proliferatio n. We have previously shown that mild oxidative conditions can also counter act apoptotic stimuli. A constitutive cellular production of low levels of superoxide and hydrogen peroxide originates from various sources; among the se, gamma-glutamyl transpeptidase (GGT), the plasma membrane-bound activity in charge of metabolizing extracellular reduced glutathione, has recently been included. Since the inhibition of GGT is a sufficient stimulus for the induction of apoptosis in selected cell lines, we investigated whether thi s effect might result from the suppression of the mentioned GGT-dependent p rooxidant reactions, on the theory that the latter may represent a basal an tiapoptotic and proliferative signal for the cell. Experiments showed that: 1) GGT activity in U937 monoblastoid cells is associated with the]producti on of low levels of hydrogen peroxide, and two independent GGT inhibitors c ause a dose-dependent decrease of such GGT-dependent production of H2O2; 2) GGT inhibition with acivicin results in cell growth arrest, and induces ce ll. death and DNA fragmentation with the ladder appearance of apoptosis; 3) treatment of cells with catalase-and even more with Trolox C-is able to de crease their proliferative rate; 4) GGT inhibition (with suppression of H2O 2 production) results in a down-regulation of poly(ADP-ribose) polimerase ( PARP) activity, which precedes the proteolytic cleavage of PARP molecule, s uch as that typically induced by caspases. The reported data suggest that t he low H2O2 levels originating as a by-product during GGT activity are able to act as sort of a 'life signal' in U937 cells, insofar as they can maint ain cell, proliferation and protect against apoptosis, possibly through an up-regulation of PARP activity.