Induction of cyclic AMP synthesis by forskolin is followed by a reduction in the expression of c‐myc messenger RNA and inhibition of ³H‐thymidine incorporation in human fibroblasts

Abstract

We have studied the effect of increased intracellular levels of cyclic AMP on the growth response to platelet-derived growth factor (PDGF) of human foreskin fibroblasts in culture. It was found that forskolin, a potent stimulator of adenylate cyclase activity, inhibits the stimulatory effect of PDGF on 3H-thymidine incorporation with a dose dependence similar to that observed with regard to cyclic AMP formation. A time-course study indicated that forskolin has no effect on ongoing DNA synthesis but affects events in the prereplicative phase. The cell-cycle block induced by forskolin was found to be reversible; after removal of the drug, DNA synthesis was initiated after a lag period, similar to that of the prereplicative phase of control cells. Forskolin had no effect on PDGF binding, receptor autophosphorylation, or c-fos mRNA expression. However, a reduction in PDGF-induced c-myc mRNA expression was observed in cultures given forskolin. Forskolin was also found to have a marked stimulatory effect on the expression of interferon-beta 2 mRNA expression. However, we were unable to demonstrate that the growth-inhibitory effect of forskolin is mediated by interferon-beta. In conclusion, an increase in cAMP levels leads to a reversible inhibition of PDGF-induced DNA synthesis in human fibroblasts, which may be related to an inhibition of c-myc mRNA expression.

Cyclic AMP-mediated Inhibition of Angiotensin II-induced Protein Synthesis Is Associated with Suppression of Tyrosine Phosphorylation Signaling in Vascular Smooth Muscle Cells

“…It is interesting to note that in contrast to the results presented here, increased cAMP has been shown to down-regulate c-myc mRNA expression in several cell types, including normal and neoplastic B cells (19), leukemic cells (20), fibroblasts (21), and a macrophage cell line (22). The cAMP-mediated reduction in c-myc RNA levels results from a decrease in c-myc transcription (20,22).…”

Beta-actin mRNA localization is regulated by signal transduction mechanisms.

“…In this study we found that activation of both kinases A and C (by forskolin and TPA respectively) inhibited the relocalization of /3-actin mRNA in serum-stimulated cells and caused delocalization of mRNA in cells grown in normal growth conditions. The inhibitory effect of these kinases on other PDGFinduced events is downstream of the receptor tyrosine kinase (Heldin et al, 1989;Graves et al, 1993), however beyond this the targets are not known. Activation of ras by PDGF is effected through several intermediate proteins (Lowenstein et al, 1992;Li et al, 1994;Chardin et al, 1993).…”

Cyclic AMP Inhibits the Proliferation of Thyroid Carcinoma Cell Lines through Regulation of CDK4 Phosphorylation

“…. ; Heldin et al, 1989;Mechta et al, 1989;Cowlen and Eling, 1992), repression of D-type cyclins (cyclin D1, cyclin D3; Cocks et al, 1992;Sewing et al, 1993;Ward et al, 1996;L'Allemain et al, 1997), and accumulation of the p27 kip1 CDK inhibitor Ward et al, 1996;van Oirschot et al, 2001;Kuiperij et al, 2005), which inhibits both CDK4 and CDK2 Kuiperij et al, 2005) and thus inactivating phosphorylations of pRb. Inhibition of cyclin D1-CDK4 activity by cAMP-dependent accumulation of p27 has been ascribed to p27 impairing the activating T172-phosphorylation of CDK4 by the constitutively active CDK-activating kinase (CAK, cyclin H-CDK7-Mat1; Kato et al, 1994).…”