Stable transfection of LLC-PK1 cells with human microsomal glutathione S-transferase gene increases haloalkene glutathione S-conjugate formation and cytotoxicity

Lactococcus lactis is an Efficient Expression System for Mammalian Membrane Proteins Involved in Liver Detoxification, CYP3A4, and MGST1

Despite the great importance of human membrane proteins involved in detoxification mechanisms, their wide use for biochemical approaches is still hampered by several technical difficulties considering eukaryotic protein expression in order to obtain the large amounts of protein required for functional and/or structural studies. Lactococcus lactis has emerged recently as an alternative heterologous expression system to Escherichia coli for proteins that are difficult to express. The aim of this work was to check its ability to express mammalian membrane proteins involved in liver detoxification, i.e., CYP3A4 and two isoforms of MGST1 (rat and human). Genes were cloned using two different strategies, i.e., classical or Gateway-compatible cloning, and we checked the possible influence of two affinity tags (6×-His-tag and Strep-tag II). Interestingly, all proteins could be successfully expressed in L. lactis at higher yields than those previously obtained for these proteins with classical expression systems (E. coli, Saccharomyces cerevisiae) or those of other eukaryotic membrane proteins expressed in L. lactis. In addition, rMGST1 was fairly active after expression in L. lactis. This study highlights L. lactis as an attractive system for efficient expression of mammalian detoxification membrane proteins at levels compatible with further functional and structural studies.

Role of microsomal glutathione transferase 1 in the mechanism-based biotransformation of glyceryl trinitrate in LLC-PK1 cells

Although glyceryl trinitrate (GTN) has been used in the treatment of angina for many years, details of its conversion to the proximal activator (presumed to be NO or an NO congener) of soluble guanylyl cyclase (sGC) are still unclear. We reported previously that purified microsomal glutathione transferase 1 (MGST1) mediates the denitration of GTN. In the current study, we investigated in intact cells whether this enzyme also converts GTN to species that activate sGC (mechanism-based biotransformation). We utilized LLC-PK1 cells, a cell line with an intact NO/sGC/cGMP system, and generated a stable cell line that overexpressed MGST1. MGST1 in the stably transfected cells was localized to the endoplasmic reticulum, and microsomes from these cells exhibited markedly increased GST activity. Although incubation of these cells with GTN resulted in a 3-4-fold increase in GTN biotransformation, attributed primarily to an increase in formation of the 1,3-glyceryl dinitrate metabolite, GTN-induced cGMP accumulation in cells overexpressing MGST1 was not different than that observed in wild type cells or in cells stably transfected with empty vector. To determine whether overexpression of NADPH cytochrome P450 reductase might act in concert with MGST1 to generate activators of sGC, we assessed GTN-induced cGMP accumulation in MGST1-overexpressing cells that had been transiently transfected with CPR. In this case, GTN-induced cGMP accumulation was also not different than that observed in wild type cells. We conclude that although MGST1 mediates the biotransformation of GTN in intact cells, this biotransformation does not contribute to the formation of activators of sGC.

Metabolism of Cisplatin to a nephrotoxin in proximal tubule cells

Cisplatin, a commonly used chemotherapeutic agent, is nephrotoxic. The mechanism by which cisplatin selectively kills the proximal tubule cells was heretofore unknown. Recent studies in mice and rats have shown that the nephrotoxicity of cisplatin can be blocked by acivicin or (aminooxy)acetic acid, the same enzyme inhibitors that block the metabolic activation of a series of nephrotoxic halogenated alkenes. In this study, it was hypothesized that cisplatin is activated in the kidney to a toxic metabolite through the same pathway that has been shown to activate the halogenated alkenes. This activation begins with the formation of a glutathione-conjugate that is metabolized to a cysteinyl-glycine-conjugate, to a cysteine-conjugate, and finally to a reactive thiol. In this study, a protocol was developed in which confluent monolayers of LLC-PK(1) cells were exposed to clinically relevant concentrations of cisplatin or cisplatin-conjugate for 3 h. Cell viability was assayed at 72 h. The role of gamma-glutamyl transpeptidase (GGT) and cysteine-S-conjugate beta-lyase in the metabolism of each of the cisplatin-conjugates was investigated. Pre-incubation of cisplatin with glutathione, cysteinyl-glycine, or N-acetyl-cysteine to allow for the spontaneous formation of cisplatin-conjugates increased the toxicity of cisplatin toward LLC-PK(1) cells. Inhibition of GGT activity showed that GGT was necessary only for the toxicity of the cisplatin-glutathione-conjugate. Inhibition of cysteine-S-conjugate beta-lyase reduced the toxicity of each of the cisplatin-conjugates. These data demonstrate that metabolism of cisplatin in proximal tubule cells is required for its nephrotoxicity. The elucidation of this pathway provides new targets for the inhibition of cisplatin nephrotoxicity.