Expression of angiotensin type II receptor downregulates Cdk4 synthesis and inhibits cell-cycle progression.

Accumulating evidence suggests that angiotensin II type II (AT(2)) receptor subtype negatively regulates cell proliferation in pathophysiological conditions associated with tissue remodeling. However, the mechanisms through which AT(2) receptor achieves this effect remain poorly understood. In this study, we demonstrate that expression of AT(2) receptor inhibits the proliferation of rat fibroblasts in a ligand-independent manner. The antiproliferative action of AT(2) is dependent on the density of surface receptors. We show that AT(2) receptor expression negatively regulates G1 phase progression in both cycling cells and G0-arrested cells stimulated to re-enter the cell cycle, but has no detectable effect on apoptosis. The delay in cell-cycle progression of AT(2)-expressing cells is associated with downregulation of cyclin E expression, decreased assembly of cyclin E-Cdk2 complexes, and the resulting attenuation of Cdk2 activation. The induction of Cdk4 expression and activity is also markedly attenuated, which likely contributes to the inhibition of cyclin E expression. Ectopic expression of Cdk4 alleviates the proliferation defect of AT(2)-expressing cells. These findings suggest that the growth-inhibitory effects of the AT(2) receptor are attributable in part to its spontaneous inhibitory action on the cell cycle machinery.

The Rb-family protein p107 inhibits translation by a PDK1-dependent mechanism.

The Rb family of proteins, which consists of Rb, p107 and p130, are critical regulators of cell proliferation. In addition to their inhibitory effects on cell cycle progression, Rb-family proteins repress transcription by RNA polymerases I and III, and may therefore restrain cell growth. However, it is not known if Rb, p107 or p130 have direct effects on protein synthesis. Here we report that ectopic expression of p107 in rat fibroblasts markedly attenuates the stimulation of mRNA translation and global protein synthesis by serum growth factors. This effect is associated with a reduction in the phosphorylation and activation of the serine-threonine kinases Akt1 and p70 S6 kinase (S6K1), two downstream targets of phosphoinositide-dependent kinase 1 (PDK1). We show that overexpression of p107 interferes with the recruitment of PDK1 to the plasma membrane in response to growth factors. Overexpression of PDK1 restores the defect in translation elicited by p107. These results suggest that p107 restricts cell growth by interfering with the phosphoinositide 3-kinase (PI3K) signaling pathway.

Role of p44/p42 MAP kinase in the age-dependent increase in vascular smooth muscle cell proliferation and neointimal formation.

OBJECTIVE: Age-dependent increase in vascular smooth muscle cell (VSMC) proliferation is thought to contribute to the pathology of atherosclerotic diseases. In this study, we investigated the role of mitogen-activated protein kinases (MAPKs) on VSMC proliferation and neointimal formation in the context of aging. METHODS AND RESULTS: VSMCs were isolated from the aorta of young and old rabbits. The proliferative index after serum stimulation was significantly increased in old versus young VSMCs. This was associated with a significant and specific age-dependent increase in p44/p42 MAPK activation. Treatment with MEK inhibitor PD98059 successfully inhibited p44/p42 MAPK activities and VSMC proliferation. These results were confirmed in vivo using a model of balloon injury in rabbit iliac arteries. p44/p42 MAPK activities were rapidly induced by angioplasty in young and old animals. However, the levels of p44/p42 MAPK activities achieved in arteries of old rabbits were significantly higher than those of young rabbits. This was associated with a higher cellular proliferative index and a significant increase in neointimal formation in old animals. Local delivery of PD98059 in old rabbits successfully inhibited p44/p42 MAPK activities after angioplasty, which led to a significant reduction in cellular proliferation and neointimal formation in treated animals. CONCLUSIONS: Our study suggests for the first time that increased p44/p42 MAPK activation contributes to augmented VSMC proliferation and neointimal formation with aging. p44/p42 MAPK inhibition could represent a novel therapeutic avenue against atherosclerotic diseases.

EGF receptor transactivation is obligatory for protein synthesis stimulation by G protein-coupled receptors.

The epidermal growth factor receptor (EGFR) was recently identified as a signal transducer of G protein-coupled receptors (GPCRs). In this study, we have examined the contribution of EGFR transactivation to the growth-promoting effect of GPCRs on vascular smooth muscle cells. Activation of the G(q)-coupled ANG II receptor or G(i)-coupled lysophosphatidic acid receptor resulted in increased tyrosine phosphorylation and activation of EGFR. Specific inhibition of EGFR kinase activity by tyrphostin AG-1478 or expression of a dominant-negative EGFR mutant abolished this response. Importantly, inhibition of EGFR function strongly attenuated the global stimulation of protein synthesis by GPCR agonists in vitro in cultured aortic smooth muscle cells and in vivo in the rat aorta and in small resistance arteries. The growth inhibition was associated with a marked reduction of extracellular signal-regulated kinase and phosphoinositide 3-kinase pathway activity and the resulting suppression of eukaryotic translation initiation factor 4E and 4E binding protein 1 phosphorylation. Our results demonstrate that EGFR transactivation is a physiologically relevant action of GPCRs linked to translational control and protein synthesis.