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Inhibition of stage-specific DNA synthesis in rat spermatogenic cells by polycyclic aromatic hydrocarbons
A Georgellis1, M Parvinen, J Rydström
1Department of Biochemistry, Arrhenius Laboratory, University of Stockholm, Sweden.
Abstract:
Changes in the rate of DNA synthesis in spermatogenic cells after treatment of segments of rat seminiferous tubule at defined stages of epithelial cycle with benzo[a]pyrene (BP) or 7,12-methylbenz[a]anthracene (DMBA) were studied. The incorporation of labeled thymidine into DNA was used as a measure of the rate of DNA synthesis. Very little or no inhibition of DNA synthesis at stages V and VIII of the cycle was observed at BP and DMBA concentrations lower than 100 microM. In contrast, in the presence of added mitochondria and/or microsomes from whole rat testis, 20 microM BP or DMBA inhibited DNA synthesis 5% and 80%, respectively. This inhibition of DNA synthesis was prevented by inhibitors of the cytochrome P-450 system and by free radical scavengers. These results suggest that polycyclic aromatic hydrocarbons (PAH) require metabolic activation in order to inhibit DNA replication in seminiferous tubules. The first step of this biotransformation is cytochrome P-450-dependent and occurs in Leydig cells. However, the metabolites produced in this step may be further metabolized to reactive metabolites by peroxidative pathways in the seminiferous tubules; these latter products may affect DNA replication.
Insights
Polycyclic aromatic hydrocarbons (PAHs) like benzo[a]pyrene and methylbenz[a]anthracene inhibit DNA synthesis in rat testes. This effect requires metabolic activation, involving cytochrome P-450 and potentially peroxidative pathways.
Area of Science:
- Toxicology
- Reproductive Biology
- Molecular Biology
Background:
- Polycyclic aromatic hydrocarbons (PAHs) are environmental contaminants.
- PAHs can affect reproductive health.
- Understanding PAH mechanisms in testes is crucial.
Purpose of the Study:
- To investigate the impact of benzo[a]pyrene (BP) and 7,12-methylbenz[a]anthracene (DMBA) on DNA synthesis in rat seminiferous tubules.
- To elucidate the metabolic activation pathways involved in PAH-induced inhibition of DNA replication.
Main Methods:
- Treatment of rat seminiferous tubules with BP or DMBA at specific epithelial cycle stages.
- Measurement of DNA synthesis rate via labeled thymidine incorporation.
- Inclusion of rat testis mitochondria and microsomes to assess metabolic activation.
Main Results:
- Low concentrations of BP and DMBA showed minimal inhibition of DNA synthesis.
- In the presence of mitochondria/microsomes, 20 microM BP and DMBA inhibited DNA synthesis by 5% and 80%, respectively.
- Inhibition was reversed by cytochrome P-450 inhibitors and free radical scavengers.
Conclusions:
- PAHs require metabolic activation to inhibit DNA replication in seminiferous tubules.
- Initial activation is cytochrome P-450-dependent in Leydig cells.
- Further metabolism via peroxidative pathways in tubules may generate reactive metabolites affecting DNA replication.