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Growth factor balance and tumor progression
M G Brattain1, G Howell, L Z Sun
1Medical College of Ohio, Toledo.
Abstract:
The role of balance of negative and positive autocrine growth factors in malignant progression is reviewed with an emphasis on transforming growth factor alpha (TGF-alpha) as a stimulating factor and transforming growth factor beta (TGF-beta) as an inhibiting factor. Evidence suggests that in normal cells TGF-alpha is down regulated in non-dividing or quiescent states in vitro. Tumor cells which have early stage characteristics as represented by poor clonal growth and poor tumorigenicity in athymic mice also show repression of TGF-alpha in non-dividing states. Progression of this phenotype is induced by uncontrolled low level expression of TGF-alpha by transfection with a constitutive expression vector for the polypeptide. Transfection of the unprogressed phenotype with a constitutive anti-sense vector for TGF-beta, also leads to tumor progression by repressing the autocrine negative TGF-beta activity normally expressed by these cells. Both the upregulation of TGF-alpha and the repression of TGF-beta generated in vivo progression without changing growth rates in vitro. Instead, clonality and ability to reenter the cell cycle from quiescence were increased. Thus, it is concluded that an autocrine balance of positive and negative factors is important for maintaining controlled re-entry into dividing states from non-dividing states and that disruption of this balance leads to malignant progression characterized by greater independence of the malignant cells from the control of exogenous growth factors.
Insights
The balance of growth factors like transforming growth factor alpha (TGF-alpha) and TGF-beta is crucial for cell cycle control. Disrupting this balance promotes malignant progression and cell independence.
Area of Science:
- Oncology
- Cell Biology
- Molecular Biology
Background:
- Autocrine growth factors regulate cell proliferation and quiescence.
- Transforming growth factor alpha (TGF-alpha) acts as a positive regulator, while TGF-beta acts as a negative regulator.
- Dysregulation of these factors is implicated in cancer development.
Purpose of the Study:
- To review the role of the balance between positive (TGF-alpha) and negative (TGF-beta) autocrine growth factors in malignant progression.
- To investigate how altering the expression of TGF-alpha and TGF-beta affects tumor cell behavior.
Main Methods:
- Review of existing evidence on TGF-alpha and TGF-beta in normal and tumor cells.
- In vitro studies involving transfection of tumor cells with expression vectors for TGF-alpha and anti-sense vectors for TGF-beta.
- Assessment of cellular phenotypes including clonal growth, tumorigenicity, and cell cycle re-entry.
Main Results:
- Upregulation of TGF-alpha or repression of TGF-beta in tumor cells promoted malignant progression in vivo.
- These changes increased clonality and the ability to re-enter the cell cycle from quiescence.
- In vitro growth rates remained unchanged, suggesting progression is linked to cell cycle control rather than proliferation rate.
Conclusions:
- An autocrine balance of positive and negative growth factors is essential for controlled cell cycle re-entry.
- Disruption of this balance leads to malignant progression and increased independence from external growth signals.
- This highlights the importance of autocrine signaling in maintaining cellular homeostasis and preventing cancer development.