Tumorigenicity assessments of Per.C6 cells and of an Ad5-vectored HIV-1 vaccine produced on this continuous cell line

PER.C6, a cell line derived from human embryonic retinal cells transformed with the Adenovirus Type 5 (Ad5) E1A and E1B genes, is used to produce E1-deleted Ad5 vectors such as the MRKAd5 HIV-1 gag vaccine. While whole, live PER.C6 cells are capable of growing as tumours when transplanted subcutaneously into immunodeficient nude mice at a high dosage, the process for vaccine production includes filtration steps and other methods which effectively preclude contamination by intact viable substrate cells. However, because of the neoplastic nature of this cell line, we carried out a series of investigations to assess the tumorigenic risk posed by residuals from the cell substrate in a vaccine. To address concerns about transmission of oncogenic DNA, we demonstrated that purified PER.C6 cellular DNA does not induce tumours in newborn hamsters or nude mice. To address concerns about other potential residuals, including hypothetical adventitious tumour viruses, we demonstrated that a PER.C6 cell lysate and a MRKAd5 HIV-1 gag vaccine produced on PER.C6 cells do not induce tumours in newborn hamsters or newborn rats. These results, in conjunction with the wide panel of viral safety tests performed on these cells, support the safety of the PER.C6 as a cell substrate for vaccine production.

Tobacco budworm P-glycoprotein: biochemical characterization and its involvement in pesticide resistance

Since pesticides have been shown to interact with P-glycoprotein (P-gp), the purpose of this study was to examine the possible role of P-gp in pesticide resistance in the tobacco budworm (Heliothis virescens). Using three P-gp antibodies, P-gp expression in various resistant populations of tobacco budworms was found to be 2-6-times that of the susceptible larvae. Tobacco budworm P-gp was glycosylated and localized primarily in the cuticle and fat body with little expression in the mid gut. To determine the role of P-gp in pesticide resistance, resistant tobacco budworm larvae were treated with a P-gp inhibitor, quinidine, and challenged with various doses of thiodicarb. Inhibition of P-gp decreased the LD50 for thiodicarb by a factor of 12.5. Quinidine treatment did not result in a significant inhibition of the P-450 system nor did it alter the feeding of the larvae, suggesting the potential involvement of P-gp in pesticide resistance. An age-dependent increase in P-gp expression was detected in resistant larvae as compared to control, susceptible larvae. This correlates with the reported age-dependent increase in resistance and is further evidence supporting the role of P-gp in the development of pesticide resistance.

P-glycoprotein involvement in cuticular penetration of [14C]thiodicarb in resistant tobacco budworms

Pesticides have been shown to interact with the multidrug resistance protein associated with cancer chemotherapy, P-glycoprotein (P-gp). P-gp, therefore, has also been implicated in the development of pesticide resistance. The purpose of this study was to characterize the effect P-gp has on the accumulation of the carbamate pesticide, thiodicarb. For these studies, resistant tobacco budworm larvae, expressing four times the P-gp as susceptible larvae, were pretreated with the P-gp inhibitor, quinidine, and challenged topically with thiodicarb. Quinidine enhanced thiodicarb toxicity in a dose-dependent manner, with mortality in the presence of P-gp inhibition increased up to 33%. Quinidine treatment increased [14C]thiodicarb accumulation 2- to 3-fold as compared to thiodicarb treatment alone. This study suggests that P-gp contributes to quinidine synergism of thiodicarb toxicity and suggests that P-gp may be involved in cuticular resistance to pesticides.

Chlorpyrifos oxon interacts with the mammalian multidrug resistance protein, P-glycoprotein

Multidrug resistance (MDR) to chemically unrelated therapeutic anticancer agents in mammalian cells is mediated by the overexpression of an ATP-dependent 150- to 180-kD membrane glycoprotein P-glycoprotein (P-gp). Although the complete physiological role of P-gp is unknown, it is proposed to function in cellular detoxification of xenobiotics. In this study, we investigated whether the organophosphorus insecticide chlorpyrifos (O,O-diethyl O-3,5,6-trichloro-2-pyridinyl phosphorothioate) or its metabolites interact with P-gp. Immunohistochemical analysis of tissues from male Fischer 344 rats administered chlorpyrifos (7.6 mg/kg gavage) showed increased P-gp expression in the kidney, adrenal, liver, jejunum, and stomach (tissues associated with elimination of xenobiotics), compared to control tissues. The most prominent increase was detected in the large bile ducts of the liver and the proximal tubule region of the kidney. P-gp expression was increased throughout the adrenal medulla and cortex, while a moderate increase was detected in the epithelial layers of the stomach and jejunum. To examine further the interaction between chlorpyrifos and P-gp, we evaluated whether chlorpyrifos or its active metabolite, chlorpyrifos oxon, could inhibit [3H]azidopine labeling of P-gp in MDR1 baculovirus-infected insect Sf9 cells. A concentration-dependent inhibition of [3H]azidopine labeling of P-gp was detected with chlorpyrifos oxon, while significant inhibition was not detected with chlorpyrifos. To correlate the binding of chlorpyrifos oxon to P-gp with a biochemical effect, we examined its ability to stimulate P-gp-mediated ATPase activity in these Sf9 cells. Chlorpyrifos oxon stimulated P-gp ATPase activity 1.75 times that of the positive control (10 microM verapamil). Taken together, these results suggest that chlorpyrifos oxon interacts with P-gp, and support the hypothesis that P-gp may play a role in the cellular detoxification of insecticides in mammalian tissues. To our knowledge this is the first report of an organophosphorus insecticide interacting with and increasing the expression of P-gp.

In vitro binding of [14C]2,5-hexanedione to rat neuronal cytoskeletal proteins

2,5-Hexanedione (2,5-HD) induces central-peripheral axonpathy characterized by the accumulation of 10-nm neurofilaments proximal to the nodes of Ranvier and a Wallerian-type degeneration. It has been postulated that neurofilament crosslinking may be involved in the production of this axonopathy. A potential initiating event in this neurotoxic process may be the direct binding of 2,5-HD to neurofilament and microtubule proteins. In this study, the in vitro binding of [14C]2,5-HD to neurofilament and microtubule proteins was examined. Neurofilament proteins isolated from rat spinal cord or microtubule proteins isolated from rat brain were incubated in the presence of 2,5-HD at concentrations ranging from 25 to 500 mM. Quantitative analysis of sodium dodecyl sulfate (SDS) polyacrylamide gels revealed a dose- and time-dependent binding of 2,5-HD to both neurofilament proteins and microtubule proteins. Expressed as pmol 2,5-HD bound per microgram protein, the observed relative binding was MAP2 > NF160 > NF200 > > NF68 > tubulin. These data demonstrate the direct binding of 2,5-HD to cytoskeletal proteins including both neurofilaments and microtubules.