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Reduced binding of epidermal growth factor by avian sarcoma virus-transformed rat cells
Abstract:
Rat cells transformed by Rous sarcoma virus and Fujinami sarcoma virus bound 5-10% of the amount of epidermal growth factor (EGF) bound by normal cells. Scatchard plot analysis indicated that the reduction in binding by transformed cells was due to a decreased number of receptors rather than to altered binding affinity. In experiments with temperature sensitive mutants of Rous sarcoma virus and Fujinami sarcoma virus significant loss of EGF binding occurred within one hour of shift from non-permissive to permissive temperature. Conditioned media from various normal and transformed cell lines were examined for the ability to inhibit EGF binding to normal cells or to cause "down regulation" of EGF receptors. No activity of either type was found. EGF-dependent phosphorylation in isolated membrane preparations was also examined. Membranes from normal cells displayed EGF-dependent phosphorylation of a Mr 180,000 protein presumed to be the EGF receptor. This activity was absent in membranes from transformed cells. The data suggest a close correlation between activation of avian sarcoma virus transforming gene products and modulation of the EGF growth regulatory system.
Insights
Avian sarcoma virus transformation reduces epidermal growth factor (EGF) receptors on rat cells. This modulation is linked to viral gene products, affecting cell growth regulation.
Area of Science:
- Molecular biology
- Cell signaling
- Virology
Background:
- Epidermal growth factor (EGF) is crucial for normal cell growth and regulation.
- Viral transformation can alter cellular processes, including growth factor signaling.
Purpose of the Study:
- To investigate the effect of avian sarcoma virus (ASV) transformation on EGF receptor expression and function in rat cells.
- To determine if viral transforming gene products modulate the EGF signaling pathway.
Main Methods:
- Quantifying EGF binding to normal and ASV-transformed rat cells using radioligand assays.
- Analyzing receptor binding affinity and number via Scatchard plot analysis.
- Assessing EGF-dependent receptor phosphorylation in isolated cell membranes.
Main Results:
- Transformed cells exhibited a 90-95% reduction in EGF binding compared to normal cells.
- This reduction was attributed to a decreased number of EGF receptors, not altered affinity.
- EGF-dependent phosphorylation of the EGF receptor (Mr 180,000) was observed in normal cell membranes but absent in transformed cells.
- Temperature-sensitive ASV mutants showed rapid loss of EGF binding upon temperature shift.
Conclusions:
- Activation of ASV transforming gene products is closely correlated with the downregulation of EGF receptors.
- Viral transformation significantly disrupts the EGF growth regulatory system.
- The findings suggest a direct link between viral oncogenes and the modulation of growth factor signaling pathways.