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Characterization of biotransformation enzyme activities in primary rat proximal tubular cells
G J Schaaf1, E M de Groene, R F Maas
1Department of Veterinary Pharmacology, Pharmacy and Toxicology (VFFT), Faculty of Veterinary Medicine, Utrecht University, PO Box 80152 NL 3508 TD, Utrecht, The Netherlands. g.schaaf@vfft.vet.uu.nl
Abstract:
The proximal tubule is a frequent target for nephrotoxic compounds due to it's ability to transport and accumulate xenobiotics and their metabolites, as well as by the presence of an organ-selective set of biotransformation enzymes. The aim of the present study was to characterize the activities of different biotransformation enzymes during primary culturing of rat proximal tubular cells (PT cells). Specific marker substrates for determining cytochrome P450 (CYP450) activity of primary cultured PT cells include 7-ethoxyresorufin (CYP1A1), caffeine (CYP1A), testosterone (CY2B/C, CYP3A), tolbutamide (CYP2C) and dextromethorphan (CYP2D1). Activities of the CYP450 isoenzymes decreased considerably during culture with the greatest loss in activity within 24 h of culture. In addition, expression of CYP450 apoprotein, including CYP1A, CYP2C, CYP2D, CYP2E and CYP4A, was detected in microsomes from freshly isolated PT cells by immunoblotting using specific antibodies. CYP2B and CYP3A apoprotein could not be detected. Activity of the phase II biotransformation enzymes GST, GGT, beta-lyase and UGT was determined with 1-chloro-2,4-dinitrobenzene, L-glutamic acid gamma-(7-amido-4-methyl-coumarin), S-(1,1,2,2-tetrafluoroethyl)-L-cysteine and 1-naphthol, respectively, as marker substrates. Activity of the phase II enzymes remained more stable and, in contrast to CYP450 activity, significant activity was still expressed after 1 week of PT cell culture. Thus, despite the obvious advantages of PT cells as an in-vitro model for studies of biotransformation mediated toxicity, the strong time dependency of especially phase I and, to a lesser extent, phase II biotransformation activities confers limitations to their application.
Insights
Primary cultured rat proximal tubular cells show rapid loss of Phase I (cytochrome P450) enzyme activity within 24 hours, while Phase II enzymes remain stable for a week, limiting their in vitro toxicity study applications.
Area of Science:
- Toxicology
- Cell Biology
- Biochemistry
Background:
- The proximal tubule is a key site for xenobiotic metabolism and toxicity.
- Primary cultured proximal tubular (PT) cells are valuable in vitro models for studying drug metabolism and toxicity.
- Understanding the stability of biotransformation enzymes in cultured PT cells is crucial for interpreting experimental results.
Purpose of the Study:
- To characterize the activity and expression of Phase I (cytochrome P450) and Phase II biotransformation enzymes in primary cultured rat PT cells over time.
- To assess the stability of these enzyme activities during the initial culture period.
Main Methods:
- Primary rat PT cells were cultured.
- Cytochrome P450 (CYP450) enzyme activities were measured using specific marker substrates (e.g., 7-ethoxyresorufin, caffeine, testosterone).
- Expression of CYP450 apoproteins was analyzed by immunoblotting.
- Phase II enzyme activities (GST, GGT, beta-lyase, UGT) were determined using specific substrates.
Main Results:
- CYP450 enzyme activities significantly decreased within 24 hours of culture, with substantial loss observed.
- Expression of several CYP450 apoproteins (CYP1A, CYP2C, CYP2D, CYP2E, CYP4A) was detected in freshly isolated cells but not consistently throughout culture.
- Phase II enzyme activities remained relatively stable throughout one week of culture, showing greater resilience than Phase I enzymes.
Conclusions:
- Primary cultured PT cells exhibit a time-dependent decline in Phase I biotransformation enzyme activity, particularly within the first 24 hours.
- Phase II enzyme activities are more stable in cultured PT cells over a one-week period.
- The significant loss of Phase I activity over time presents a limitation for using these cells in long-term in vitro studies of biotransformation-mediated toxicity.