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Down-regulation by growth factors of vascular smooth muscle angiotensin receptor gene expression
1Department of Pharmacology, Emory University School of Medicine, Atlanta, Georgia 30322.
Abstract:
The effects of epidermal growth factor, basic fibroblast growth factor, and platelet-derived growth factor-BB on angiotensin type 1 (AT1) receptor gene expression were examined in rat thoracic aorta vascular smooth muscle cells (VSMC) in culture. Incubation of serum-deprived VSMC with 20 ng/ml epidermal growth factor, 20 ng/ml basic fibroblast growth factor, or 50 ng/ml platelet-derived growth factor-BB reduces AT1 receptor mRNA levels, as assessed by Northern hybridization analysis, to approximately 30% of control levels. This effect is maximal 4 hr after addition of each growth factor to the culture medium and is sustained for up to 24 hr of incubation after a single dose. There is a correlative loss of membrane-associated AT1 receptors and angiotensin II-stimulated inositol phosphate production after 24 hr of growth factor treatment. The half-life of AT1 receptor mRNA is reduced significantly by growth factors, compared with that for cells treated with actinomycin D alone to block transcription. This suggests that growth factors activate a mechanism that involves post-transcriptional destabilization of AT1 receptor mRNA. This effect can be blocked by prior treatment of VSMC with actinomycin D or cycloheximide, suggesting that the effect of the growth factors on AT1 receptor gene expression is mediated through induction of an unknown gene or genes that function to destabilize AT1 receptor mRNA and that mRNA translation is essential for the destabilizing effect. Nuclear run-on assays reveal that the growth factors also significantly reduce the rate of de novo AT1 receptor gene transcription. Thus, down-regulation of AT1 receptor gene expression by growth factors also appears to involve mechanisms that decrease the rate of AT1 receptor gene transcription. These data reveal marked down-regulation of AT1 receptor gene expression in VSMC by growth factor receptor activation, through mechanisms that involve both attenuation of transcription and post-transcriptional mRNA destabilization.
Insights
Growth factors like EGF, FGF, and PDGF-BB significantly reduce angiotensin II type 1 receptor gene expression in vascular smooth muscle cells. This involves both decreased transcription and destabilized mRNA, impacting AT1 receptor levels.
Area of Science:
- Cardiovascular Biology
- Molecular Biology
- Cell Signaling
Background:
- Angiotensin II plays a key role in regulating vascular tone and blood pressure.
- The angiotensin type 1 (AT1) receptor mediates the effects of angiotensin II in vascular smooth muscle cells (VSMC).
- Growth factors are known to influence cellular processes, including gene expression.
Purpose of the Study:
- To investigate the impact of specific growth factors on AT1 receptor gene expression in rat thoracic aorta VSMC.
- To elucidate the molecular mechanisms underlying growth factor-mediated regulation of AT1 receptor expression.
Main Methods:
- Primary rat thoracic aorta VSMC were cultured and treated with epidermal growth factor (EGF), basic fibroblast growth factor (bFGF), or platelet-derived growth factor-BB (PDGF-BB).
- AT1 receptor mRNA levels were assessed using Northern hybridization.
- Changes in AT1 receptor protein levels and angiotensin II-stimulated signaling were measured.
- mRNA half-life was determined using actinomycin D.
- Gene transcription rates were analyzed via nuclear run-on assays.
Main Results:
- EGF, bFGF, and PDGF-BB treatment significantly reduced AT1 receptor mRNA levels to approximately 30% of control.
- This down-regulation was observed within 4 hours and sustained for 24 hours.
- A corresponding decrease in membrane-associated AT1 receptors and angiotensin II-stimulated inositol phosphate production was noted.
- Growth factors reduced the half-life of AT1 receptor mRNA and decreased the rate of de novo gene transcription.
- The effects were blocked by actinomycin D or cycloheximide, indicating a requirement for gene induction and mRNA translation.
Conclusions:
- Growth factor receptor activation leads to marked down-regulation of AT1 receptor gene expression in VSMC.
- This down-regulation occurs through a dual mechanism involving attenuation of AT1 receptor gene transcription and post-transcriptional mRNA destabilization.
- These findings highlight a novel pathway for regulating vascular AT1 receptor expression by growth factors.