Enzymes of the gamma-glutamyl cycle in 'aging' WI-38 fibroblasts and in HeLa S3 cells

Senescent human fibroblasts show increased glycolysis and redox homeostasis with extracellular metabolomes that overlap with those of irreparable DNA damage, aging, and disease

Cellular senescence can modulate various pathologies and is associated with irreparable DNA double-strand breaks (IrrDSBs). Extracellular senescence metabolomes (ESMs) were generated from fibroblasts rendered senescent by proliferative exhaustion (PEsen) or 20 Gy of γ rays (IrrDSBsen) and compared with those of young proliferating cells, confluent cells, quiescent cells, and cells exposed to repairable levels of DNA damage to identify novel noninvasive markers of senescent cells. ESMs of PEsen and IrrDSBsen overlapped and showed increased levels of citrate, molecules involved in oxidative stress, a sterol, monohydroxylipids, tryptophan metabolism, phospholipid, and nucleotide catabolism, as well as reduced levels of dipeptides containing branched chain amino acids. The ESM overlaps with the aging and disease body fluid metabolomes, supporting their utility in the noninvasive detection of human senescent cells in vivo and by implication the detection of a variety of human pathologies. Intracellular metabolites of senescent cells showed a relative increase in glycolysis, gluconeogenesis, the pentose-phosphate pathway, and, consistent with this, pyruvate dehydrogenase kinase transcripts. In contrast, tricarboxylic acid cycle enzyme transcript levels were unchanged and their metabolites were depleted. These results are surprising because glycolysis antagonizes senescence entry but are consistent with established senescent cells entering a state of low oxidative stress.

Ammonia production by IMR-90 fibroblast cultures: effects of ammonia on glutathione, gamma-glutamyl transpeptidase, lysosomal enzymes, and cell division

gamma-Glutamyl transpeptidase has multi-catalytic activities. It degrades glutathione and can produce ammonia from glutamine. The present study was designed to examine whether the decreased cell proliferation, cellular glutathione content and concurrent increase in ammonia production in senescent cells in culture are the result of increased gamma-glutamyl transpeptidase activity. We used IMR-90 fibroblast and 3T3 LI preadipocyte cultures. The cellular glutathione content depended upon cell proliferation and cell density. The glutathione content was higher in cells at logarithmic growth, and lower at stationary growth or post confluency; dead cells had no detectable glutathione by the method currently used. The glutathione content was minimal in "old" IMR-90 cells, regardless of cell density. On the other hand, an increase occurred in the unit number of molecules of bound 5-iodoacetoamidofluorescein, an active-site directed stoichiometric inhibitor of transpeptidase. That result corresponded favorably with the increased enzyme activity, suggesting that the number of enzyme molecules per cell was increased. The inhibition of ammonia production of the cultures by inhibition of gamma-glutamyl transpeptidase by 5-iodoacetoamidofluorescein and reversible inhibition of ammonia production by a serine-borate mixture were consistent with our postulate. Addition of NH4Cl (0.1 mM) to IMR-90 cultures caused increased activities of transpeptidase and some of the lysosomal enzymes; concurrently, the amount of cellular glutathione and the number of cell divisions decreased. This suggests that the increased ammonia production presumably resulting from glutaminase activity of the observed increase of transpeptidase may profoundly affect certain cellular functions.

Theophylline and melanocyte-stimulating hormone effects on gamma-glutamyl transpeptidase and DOPA reactions in cultured melanoma cells

Gamma-glutamyl transpeptidase (GGT), an enzyme of the gamma-glutamyl cycle, was demonstrated in 3 of 6 cell lines derived from a single B16 murine melanoma. Its activity in these cells varied a great deal, appeared to be correlated with the developmental cycle of the cells, and was greatest in young, actively melanogenic cells. Generally, the activity seemed parallel to that of tyrosinase, an enzyme specific for melanin synthesis. The levels of both enzymes tended to decline with prolonged in vitro cultivation, but could be readily renewed after one animal passage. The 3 cell lines that were GGT-negative were nonpigmented and DOPA-negative; so was a nonmelanogenic and nonmelanocytic rhesus cell line. Melanocyte-stimulating hormone (MSH) and theophylline both enhanced pigmentation in murine melanoma cells. The mechanisms of their action apparently differed. We found that theophylline increased both DOPA- and GGT- reactive cells, whereas MSH only increased DOPA-reactive cells. All 3 GGT-positive lines were tumorigenic, and 2 GGT-negative line were not tumorigenic. Our observations suggest that GGT plays a role in the melanin biosynthetic pathway and the its activity is greater in melanoma cells that are tumorigenic.

Glucocorticoid receptors in WI-38 fibroblasts: characterization and changes with population doubling in culture

A high-affinity dexamethasone binding macromolecule was identified in WI-38 human fetal lung fibroblasts. High specificity of binding for glucocorticoids was shown by competition studies in which binding of dexamethasone was inhibited by cortisol and corticosterone but not by testosterone or 17 beta-estradiol. WI-38 cells exposed to [3H]dexamethasone at 30 degrees C were able to transfer the 3H-labeled steroid-receptor complex to the nuclear materal. A reduction of 30--50% was observed in the number of [3H]dexamethasone-receptor binding sites per cell as well as in the nuclear fraction of the cells as a function of age (passage levels 27 and 54). However, in the same cells no significant changes in affinity of receptor for [3H]dexamethasone as a function of the two passage levels were detected.