Metabolism of cisplatin to a Nephrotoxin [Toxicology, 257(3), 174–175, doi 10.1016/j.tox.2008.12.014]

Employing pancreatic tumor γ-glutamyltransferase for therapeutic delivery

γ-Glutamyltransferase (γGT) is a cell surface enzyme that catalyzes hydrolysis of the bond linking the glutamate and cysteine residues of glutathione and glutathione-S-conjugates. We have observed that human pancreatic tumor cells and tumor-associated stellate cells express high levels of this enzyme when compared to normal pancreatic epithelial and stellate cells. Detection of the protein in tumor sections correlated with γGT activity on the surface of the cultured tumor and stellate cells. We tested whether the tumor γGT could be employed to deliver a therapeutic to the tumor endothelial cells. GSAO is a glutathione-S-conjugate of a trivalent arsenical that is activated to enter endothelial cells by γGT cleavage of the γ-glutamyl residue. The arsenical moiety triggers proliferation arrest and death of the endothelial cells by targeting the mitochondria. Human pancreatic tumor and stellate cell γGT activated GSAO in culture and γGT activity positively correlated with GSAO-mediated proliferation arrest and death of endothelial cells in Transwell and coculture systems. A soluble form of γGT is found in blood, and we measured the rate of activation of GSAO by this enzyme. We calculated that systemically administered GSAO would circulate through the pancreatic blood supply several times before appreciable activation by normal blood levels of γGT. In support of this finding, tumor γGT activity positively correlated with GSAO-mediated inhibition of pancreatic tumor angiogenesis and tumor growth in mice. Our findings indicate that pancreatic tumor γGT can be used to deliver a therapeutic to the tumor.

Possible role of cysteine-S-conjugate β-lyase in species differences in cisplatin nephrotoxicity

To better understand species differences in cisplatin nephrotoxicity, we focused on renal cysteine-S-conjugate β-lyase (C-S lyase), which may play a crucial role in the metabolism of platinum (Pt)-cysteine conjugates. Aminooxyacetic acid hemihydrochloride (AOAA), an inhibitor of C-S lyase, reduced renal injuries due to cisplatin in rats, suggesting involvement of C-S lyase. On day 5 following a bolus cisplatin injection, three species showed in vivo nephrotoxic potentials in the order of rats>mice=rabbits (the highest to lowest), based on body surface. The levels of renal Pt residue at the nephrotoxic dose were in order of rabbits>rats>mice. Meanwhile, the activity of endogenous (basal) mitochondrial aspartate aminotransferase (AST), one of the C-S lyases, in the renal cortex of naive animals was rats>mice=rabbits. In a qualitative Western blot analysis, expression of mitochondrial C-S lyase in the kidney was observed at approximately 37kDa in all five species used. In in vitro studies, the cytotoxicity of cisplatin was dependent on the expression level of C-S lyase mRNA in the respective renal cells. These results demonstrate that species differences in cisplatin nephrotoxicity are attributable to an interaction of renal Pt transition with C-S lyase activity.