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Transforming growth factor beta 1 inhibits epidermal growth factor receptor endocytosis and down-regulation in
G Baskin1, S Schenker, T Frosto
1Department of Medicine, University of Texas Health Science Center, San Antonio 78284-7878.
Abstract:
Incubation of fetal rat hepatocytes (FRH) with transforming growth factor beta 1 (TGF-beta 1) resulted in growth arrest and a biphasic effect on epidermal growth factor (EGF) receptor. After 2 h of exposure, EGF receptor (EGFR) was reduced by 43%. From 6 to 24 h, TGF-beta 1 exposure resulted in progressive increase in EGFR up to 74% over control. The increased binding was due to increase in high affinity EGF binding sites. FRH grown in medium containing EGF exhibited down-regulated EGFR with loss of high affinity EGF binding sites. With TGF-beta 1 exposure, high affinity EGFR was not down-regulated by EGF. Since down-regulation of EGFR involves internalization, the kinetics of EGF receptor-mediated endocytosis were examined. In TGF-beta 1-exposed FRH, EGF endocytosis was inhibited, with a reduction in the first order rate constant for the process from 0.078 to 0.043 min-1. Despite inhibition of growth, receptor down-regulation, and EGF endocytosis after TGF-beta 1 exposure, EGF-induced receptor autophosphorylation was preserved as demonstrated by [32P]phosphate-labeling of immunoprecipitated EGFR. These observations provide direct evidence that TGF-beta 1 regulates growth of fetal cells. Further, they suggest that TGF-beta 1 regulates endocytosis of EGF and possibly of other ligands.
Insights
Transforming growth factor beta 1 (TGF-β1) alters epidermal growth factor (EGF) receptor dynamics in fetal rat hepatocytes, inhibiting EGF endocytosis and preserving EGF-induced signaling, thereby regulating cell growth.
Area of Science:
- Cell Biology
- Molecular Biology
- Endocrinology
Background:
- Transforming growth factor beta 1 (TGF-β1) is a key regulator of cell growth and differentiation.
- Epidermal growth factor (EGF) receptor (EGFR) signaling is crucial for cell proliferation.
- The interplay between TGF-β1 and EGFR in fetal cells is not fully understood.
Purpose of the Study:
- To investigate the effect of TGF-β1 on EGFR expression, binding affinity, and endocytosis in fetal rat hepatocytes (FRH).
- To determine if TGF-β1 influences EGF-induced EGFR autophosphorylation.
- To elucidate the role of TGF-β1 in regulating FRH growth and EGFR-mediated endocytosis.
Main Methods:
- Incubation of FRH with TGF-β1 and EGF.
- Quantification of EGFR levels and EGF binding using radioligand assays.
- Analysis of EGFR endocytosis kinetics via measurement of the rate constant.
- Assessment of EGFR autophosphorylation using [32P]phosphate labeling.
Main Results:
- TGF-β1 induced biphasic changes in EGFR: an initial decrease followed by a significant increase in high-affinity binding sites.
- EGF alone down-regulated EGFR, but TGF-β1 exposure prevented this down-regulation.
- TGF-β1 inhibited EGF receptor-mediated endocytosis, reducing the rate constant from 0.078 to 0.043 min-1.
- EGF-induced EGFR autophosphorylation remained intact despite TGF-β1 treatment.
Conclusions:
- TGF-β1 regulates fetal rat hepatocyte growth by modulating EGFR expression and function.
- TGF-β1 inhibits EGFR endocytosis, suggesting a novel mechanism for regulating growth factor signaling.
- These findings highlight a complex cross-talk between TGF-β1 and EGFR pathways in controlling cell proliferation and ligand uptake.