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Cell cycle-specific inhibition by retinoic acid of xenotropic murine retrovirus expression
Abstract:
Several retinoids were examined for their capacity to block chemically induced expression of endogenous xenotropic retrovirus from Kirsten sarcoma virus-transformed BALB/c mouse cells. Retinoic acid (RA) was found to inhibit induction of virus by 5-iododeoxyuridine, cycloheximide, and histidinol; inhibition was concentration (10(-4) to 10(-6) M) and time dependent (1 to 7 hr) and not a consequence of cytotoxicity. Following a 6-hr treatment with 10(-4) M RA, [3H]thymidine and [3H]uridine incorporation into total cellular DNA and RNA was reduced 37 and 63%, respectively. Heteronuclear RNA synthesis was reduced 36 and 7% within 4 hr by 10(-4) and 10(-5) M RA, respectively, indicating that inhibition was not the result of a general transcriptional block. Using synchronized cells, it was found that 5 X 10(-5) M RA added in G1 phase and followed by cycloheximide or 5-iododeoxyuridine induction inhibited virus expression 60 and 84%, respectively. Little or no inhibition was observed when RA was added during S phase with the inducers or during G2 phase followed by inducers. Cells synchronized by mitotic arrest showed a RA-mediated restriction point in early-to-mid-G1 phase as indicated by a delay in the onset of DNA synthesis and an inhibition of virus induction during S phase. The results show the presence in Kirsten sarcoma virus-transformed BALB/c cells of a RA-sensitive G1 restriction point for cell progression and suggest that inhibition of retrovirus activation may be related to an extended G1 phase.
Insights
Retinoic acid (RA) effectively inhibits the expression of xenotropic retrovirus in mouse cells. This inhibition occurs during the G1 phase of the cell cycle, suggesting a link between cell cycle progression and retrovirus activation.
Area of Science:
- Molecular Biology
- Virology
- Cell Biology
Background:
- Endogenous retroviruses can be expressed from transformed cells.
- Retinoids are known to modulate gene expression.
- Kirsten sarcoma virus-transformed BALB/c mouse cells express xenotropic retrovirus.
Purpose of the Study:
- To investigate the capacity of retinoids to block chemically induced xenotropic retrovirus expression.
- To determine the mechanism by which retinoic acid (RA) inhibits retrovirus induction.
Main Methods:
- Treatment of Kirsten sarcoma virus-transformed BALB/c mouse cells with various chemical inducers (5-iododeoxyuridine, cycloheximide, histidinol).
- Exposure to different concentrations and durations of retinoic acid (RA).
- Analysis of viral expression, cytotoxicity, DNA/RNA synthesis, and cell cycle progression using synchronized cells.
Main Results:
- Retinoic acid (RA) significantly inhibited virus induction in a concentration- and time-dependent manner without cytotoxicity.
- RA treatment reduced cellular DNA and RNA synthesis and heteronuclear RNA synthesis, but not as a general transcriptional block.
- RA added during the G1 phase, but not S or G2, effectively inhibited virus induction, indicating a RA-sensitive G1 restriction point.
Conclusions:
- Retinoic acid (RA) possesses potent inhibitory effects on xenotropic retrovirus expression in transformed mouse cells.
- The inhibition is mediated by a specific restriction point within the G1 phase of the cell cycle.
- RA-induced extension of the G1 phase may be the mechanism underlying the suppression of retrovirus activation.