GAMMA-GUTAMYL TRANSPEPTIDASE MEDIATION OF TUMOR GLUTATHIONE UTILIZATION IN-VIVO

Background: Glutathione is a tripeptide used by cells to protect again st oxidative and free radical damage. It may also be involved in bioch emical mechanisms that cause some tumors to become resistant to antica ncer drugs, gamma-Glutamyl transpeptidase (GGTP) is a membrane-bound e nzyme that cleaves extracellular glutathione, providing cells with ami no acids necessary for intracellular synthesis of this compound. Incre ased expression of GGTP has been found in a number of human tumors; ho wever, few studies have examined the contribution of GGTP to tumor glu tathione metabolism in vivo. Purpose: Our goals were to study the util ization of host glutathione by 3-methylcholanthrene (MCA)-induced sarc omas grown in rats and to evaluate the involvement of tumor GGTP in th is process. Methods: The left ovaries of 21 female Fischer 344 rats we re isolated by laparotomy and placed in subcutaneous positions through stab wounds in the abdominal wall. A 3-mm cube of MCA sarcoma was the n sutured to each of the isolated ovaries. The MCA implants obliterate d the ovarian tissue, yielding isolated tumors with one arterial suppl y (the ovarian artery) and one draining vein (the ovarian vein, referr ed to as the tumor vein). After 2 weeks of tumor growth, blood was dra wn from the tumor vein, the inferior vena cava (IVC), and the aorta of 16 animals. Glutathione and cysteine concentrations in plasma samples from this blood were determined by high-performance liquid chromatogr aphy and used to calculate glutathione and cysteine utilization ratios for the tumor and the systemic circulations ([{concentration(aorta) - concentration(tumor) vein}/concentration(aorta)] x 100 and [{concentr ation(aorta) - concentration(Ivc)}/concentration(aorta)] x 100, respec tively). The utilization ratios from these control animals were compar ed with those from acivicin (AT-125; an irreversible CGTP inhibitor)-t reated rats (the remaining five animals). Data are presented as mean /- standard deviation; reported P values are from two-tailed tests of statistical significance. Results: In the control animals, glutathione and cysteine concentrations were significantly lower in the tumor vei n (3.55 +/- 1.9 and 5.69 +/- 2.8 mu M, respectively) and in the IVC (5 .65 +/- 2.3 and 7.17 +/- 2.4 mu M, respectively) than in the artery (1 2.48 +/- 5.7 and 12.33 +/- 5.9 mu M, respectively; all P values <.05). In addition, the glutathione utilization ratio was significantly high er for the tumor circulation than for the systemic circulation (69% +/ - 14% versus 52% +/- 14%; P < .003). The combined glutathione and cyst eine utilization ratio was also significantly higher for the tumor cir culation than for the systemic circulation (116% +/- 35% versus 88% +/ - 28%; P < .02). Treatment with AT-125 lowered the tumor glutathione u tilization ratio significantly (45% +/- 12% for treated animals versus 69% +/- 14% for control animals; P < .005). Conclusions: Our results show that glutathione and cysteine in the host circulation are used by MCA sarcomas, The significant reduction in tumor utilization of serum glutathione after treatment with AT-125, a GGTP inhibitor, indicates that GGTP is important in tumor glutathione metabolism.