del Bello B, Paolicchi A, Comporti M, Pompella A, Maellaro E
Institute of General Pathology, University of Siena Medical School, I-53100 Siena, Italy.
FASEB J. 1999 Jan;13(1):69-79. doi: 10.1096/fasebj.13.1.69.
It has been reported in several cell lines that exposure to low levels of reactive oxygen species can exert a stimulatory effect on their proliferation. We have previously shown that mild oxidative conditions can also counteract apoptotic stimuli. A constitutive cellular production of low levels of superoxide and hydrogen peroxide originates from various sources; among these, 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 this effect might result from the suppression of the mentioned GGT-dependent prooxidant reactions, on the theory that the latter may represent a basal antiapoptotic and proliferative signal for the cell. Experiments showed that: 1) GGT activity in U937 monoblastoid cells is associated with the production of low levels of hydrogen peroxide, and two independent GGT inhibitors cause a dose-dependent decrease of such GGT-dependent production of H2O2; 2) GGT inhibition with acivicin results in cell growth arrest, and induces cell 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 decrease their proliferative rate; 4) GGT inhibition (with suppression of H2O2 production) results in a down-regulation of poly(ADP-ribose) polimerase (PARP) activity, which precedes the proteolytic cleavage of PARP molecule, such as that typically induced by caspases. The reported data suggest that the 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 maintain cell proliferation and protect against apoptosis, possibly through an up-regulation of PARP activity.
据报道，在多种细胞系中，暴露于低水平的活性氧可对其增殖产生刺激作用。我们之前已经表明，轻度氧化条件也可抵消凋亡刺激。细胞持续产生低水平的超氧化物和过氧化氢源于多种来源；其中，γ-谷氨酰转肽酶（GGT），即负责代谢细胞外还原型谷胱甘肽的质膜结合活性，最近也被纳入其中。由于抑制GGT是在选定细胞系中诱导凋亡的充分刺激因素，我们研究了这种效应是否可能源于上述GGT依赖性促氧化反应的抑制，基于这样的理论，即后者可能代表细胞的一种基础抗凋亡和增殖信号。实验表明：1）U937单核细胞样细胞中的GGT活性与低水平过氧化氢的产生相关，两种独立的GGT抑制剂可导致这种GGT依赖性H2O2产生呈剂量依赖性降低；2）用阿西维辛抑制GGT会导致细胞生长停滞，并诱导细胞死亡和DNA片段化，呈现凋亡的梯状外观；3）用过氧化氢酶处理细胞——甚至用生育三烯酚C处理效果更明显——能够降低其增殖速率；4）GGT抑制（伴随H2O2产生的抑制）导致聚（ADP-核糖）聚合酶（PARP）活性下调，这先于PARP分子的蛋白水解切割，如通常由半胱天冬酶诱导的那样。所报道的数据表明，在GGT活性过程中作为副产物产生的低水平H2O2能够在U937细胞中充当一种“生命信号”，因为它们可以维持细胞增殖并防止凋亡，可能是通过上调PARP活性实现的。
