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A 34-kd protein with strong homology to ras-like proteins inhibits epidermal growth factor activity
1Department of Pathology and Cell Biology, Thomas Jefferson Medical College, Philadelphia, PA 19107.
Abstract:
Epidermal growth factor (EGF) and its analog, transforming growth factor-alpha, are felt to be important in oncogenesis. When malignant rabbit fibroma virus infects RK-13 rabbit kidney cells, a 34-kd protein that inhibits the effects of EGF on certain target cell lines is produced. We have purified this protein using high-pressure liquid chromatography and gel electrophoresis. This purified protein abolishes EGF-induced cellular proliferation. It also causes the EGF receptor-bearing A431 carcinoma cell line to stop proliferating in vitro. This purified 34-kd EGF inhibitor (EGFI) redirects cellular protein phosphorylation in the presence or absence of EGF. Whereas EGF increases phosphorylation of cellular proteins in normal rat kidney cells, clone 49F, and A431 EGFI generally decreases it. Both EGF and EGFI cause increased protein production in A431 and normal rat kidney cells. The major species of protein synthesized by cells seem invariant to EGFI, with or without EGF. The partial protein sequence of two fragments of EGFI shows striking similarity to two ras like proteins. Possible means by which such a ras-like protein might inhibit EGF-induced cellular proliferation are discussed. Therefore, a purified 34-kd ras-like protein inhibits EGF-induced cellular proliferation and changes the targets for cellular protein phosphorylation. Studies are in progress to characterize this protein further, both structurally and functionally.
Insights
A novel 34-kd protein, identified as an epidermal growth factor inhibitor (EGFI), halts cancer cell proliferation. This ras-like protein also alters cellular protein phosphorylation, offering new insights into oncogenesis.
Area of Science:
- Oncology
- Molecular Biology
- Virology
Background:
- Epidermal growth factor (EGF) and transforming growth factor-alpha are implicated in oncogenesis.
- Malignant rabbit fibroma virus infection of RK-13 cells produces a protein that inhibits EGF's effects.
Purpose of the Study:
- To purify and characterize a novel 34-kd protein that inhibits EGF-induced cellular proliferation.
- To investigate the mechanism by which this inhibitor affects cellular processes, including protein phosphorylation.
Main Methods:
- Purification of the 34-kd protein using high-pressure liquid chromatography and gel electrophoresis.
- Assays to measure EGF-induced cellular proliferation in target cell lines (A431 carcinoma cells, normal rat kidney cells).
- Analysis of cellular protein phosphorylation patterns in the presence and absence of EGF and the purified inhibitor.
Main Results:
- The purified 34-kd protein, termed EGFI, effectively abolished EGF-induced cellular proliferation in vitro.
- EGFI altered cellular protein phosphorylation, generally decreasing it, contrasting with EGF's effect.
- Partial protein sequencing revealed similarities between EGFI and ras-like proteins.
Conclusions:
- A purified 34-kd ras-like protein (EGFI) inhibits EGF-induced cellular proliferation.
- EGFI modifies cellular protein phosphorylation targets, suggesting a novel mechanism in cancer cell growth regulation.
- Further structural and functional characterization of EGFI is ongoing.