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Cyclic AMP, early response gene expression, and DNA synthesis in rat smooth muscle cells
A Hultgãrdh-Nilsson1, V Querol-Ferrer, B Jonzon
1Department of Cell and Molecular Biology, Medical Nobel Institute, Karolinska Institutet, Stockholm, Sweden.
Abstract:
Smooth muscle cells (SMC) isolated from neonatal and adult rats differ markedly in their growth characteristics. The growth of neonatal cells is mainly due to autocrine stimulation, whereas the growth of adult SMC is dependent upon addition of exogenous mitogens. Increasing intracellular cyclic AMP (cAMP) levels effectively inhibits DNA synthesis in adult cells, but is essentially without effect on the rate of DNA synthesis in neonatal cells. In the present study we investigated whether this difference in cAMP sensitivity is due to an effect of cAMP on early response genes. The results show that increasing intracellular levels of cAMP by exposing the cells to the synthetic adenosine analogue N-ethyl-carboxamido adenosine (NECA) results in an accumulation of c-jun and c-fos mRNA in both cell types. NECA also lowered c-myc mRNA levels in neonatal cells, whereas it marginally increased the presence of c-myc mRNA in adult cells. Exposure to NECA also resulted in a limited increase in alpha-actin mRNA levels. NECA did not inhibit DNA synthesis or growth of adult SMC actively proliferating in the presence of 10% serum, suggesting that cAMP interferes with processes taking place during the early G1 phase or in the entry of growth-arrested cells into the G1 phase of the SMC cell cycle. It is concluded that the growth-inhibitory effect of NECA is unlikely to be due to actions of cAMP on early response genes. However, it cannot be completely excluded that an increased synthesis of jun/fos transcription factors may induce the transcription of other, growth-suppressing genes in the cells.
Insights
Neonatal and adult rat smooth muscle cells (SMC) show different growth responses to cyclic adenosine monophosphate (cAMP). cAMP inhibits adult SMC DNA synthesis but not neonatal, suggesting cAMP does not directly impact early response genes in this growth difference.
Area of Science:
- Cell Biology
- Molecular Biology
- Physiology
Background:
- Smooth muscle cells (SMC) from neonatal and adult rats exhibit distinct growth patterns.
- Neonatal SMC growth relies on autocrine stimulation, while adult SMC require external mitogens.
- Elevated cyclic adenosine monophosphate (cAMP) inhibits adult SMC DNA synthesis but not neonatal SMC DNA synthesis.
Purpose of the Study:
- To investigate if differential sensitivity to cAMP in rat SMC growth is linked to early response genes.
- To explore the impact of cAMP on gene expression in neonatal versus adult SMC.
Main Methods:
- Isolated neonatal and adult rat SMC were treated with N-ethyl-carboxamido adenosine (NECA) to increase intracellular cAMP levels.
- Quantitative analysis of c-jun, c-fos, c-myc, and alpha-actin mRNA expression.
- Assessment of DNA synthesis and cell proliferation in adult SMC under NECA treatment and serum stimulation.
Main Results:
- NECA-induced cAMP increase led to c-jun and c-fos mRNA accumulation in both neonatal and adult SMC.
- NECA decreased c-myc mRNA in neonatal SMC but slightly increased it in adult SMC.
- NECA did not inhibit DNA synthesis in actively proliferating adult SMC, suggesting cAMP's effect occurs in early G1 phase entry.
Conclusions:
- The growth-inhibitory effect of NECA is unlikely to be mediated by direct actions of cAMP on early response genes.
- While jun/fos transcription factors increase, their role in growth suppression via other genes remains a possibility.