Cell Cycle Protein Expression in Vascular Smooth Muscle Cells In Vitro and In Vivo Is Regulated Through Phosphatidylinositol 3-Kinase and Mammalian Target of Rapamycin

Abstract

Abstract-Cell cycle progression represents a key event in vascular proliferative diseases, one that depends on an increased rate of protein synthesis. An increase in phosphatidylinositol 3-kinase (PI 3-kinase) activity is associated with vascular smooth muscle cell proliferation, and rapamycin, which blocks the activity of the mammalian target of rapamycin, inhibits this proliferation in vitro and in vivo. We hypothesized that these 2 molecules converge on a critical pathway of translational regulation that is essential for successful upregulation of cell cycle-regulatory proteins in activated smooth muscle cells. p70 S6 kinase, a target of PI 3-kinase and the mammalian target of rapamycin, was rapidly activated on growth factor stimulation of quiescent coronary artery smooth muscle cells and after balloon injury of rat carotid arteries. The translational repressor protein 4E-binding protein 1 was similarly hyperphosphorylated under these conditions. These events were associated with increases in the protein levels of cyclin B1, cyclin D1, cyclin E, cyclin-dependent kinase 1, cyclin-dependent kinase 2, proliferating cell nuclear antigen, and p21Cip1 in vivo and in vitro, whereas inhibition of the PI 3-kinase signaling pathway with either rapamycin or wortmannin blocked the upregulation of these cell cycle proteins, but not mRNA, and arrested the cells in vitro before S phase. In contrast to findings in other cell types, growth factor-or balloon injury-induced downregulation of the cell cycle inhibitor p27

Target of rapamycin (TOR): an integrator of nutrient and growth factor signals and coordinator of cell growth and cell cycle progression

“…phosphorylation), in parallel to its role in control of cell growth. Cell growth and cell cycle progression would therefore be coordinated by virtue of sharing a common upstream regulator, mTOR rapamycin inhibiting the expression of various cell cycle control proteins (Braun-Dullaeus et al, 2001), it not clear whether the observed effect is a direct effect of mTOR inhibition or whether it is an indirect secondary consequence of a delay in cell cycle progression. Certain cyclins may also be able to function as cell growth drivers, as it is reported that in Drosophila, cyclin D/ cdk4 complexes promote cell growth in addition to their role in driving cell cycle progression Meyer et al, 2000).…”

“…Rapamycin inhibits the increased proliferation and migration of vascular smooth muscle cells that occurs after injury (Poon et al, 1996;Gallo et al, 1999;Roque et al, 2000) and also induces their differentiation back toward a more mature phenotype (Martin et al, 2003). The injury-induced expression of several cell cycle regulatory proteins is inhibited by the in vivo administration of rapamycin to rats, suggesting a mechanism for the antirestenotic effect of rapamycin on proliferation (Braun-Dullaeus et al, 2001). After successful clinical trials, rapamycin (sirolimus) is now FDA approved for use on drug-eluting stents to inhibit restenosis after coronary artery intervention (Morice et al, 2002;Moses et al, 2003).…”

The cell cycle: A critical therapeutic target to prevent vascular proliferative disease

“…At the plasma membrane, Akt is activated, in part through phosphorylation by phosphoinositide-dependent kinase-1 and PIP3. Akt is then capable of phosphorylating a number of downstream targets, such as mammalian target of rapamycin (mTOR) (33)(34)(35). Mitogen-activated protein kinase pathway The mitogen-activated protein kinase (MAPK) pathway ( Figure 3) is a second intracellular signalling pathway that may be initiated by TKR activation; Ras is a membrane G-protein that transmits the signal received by the TKR at the cell surface to activate the intracellular MAPK pathway.…”

SHIP-2 and PTEN Are Expressed and Active in Vascular Smooth Muscle Cell Nuclei, but Only SHIP-2 Is Associated with Nuclear Speckles

“…We showed previously that phosphoinositide 3-kinase (PI3K) is essential for the VSMC cycle (3), and this enzyme was implicated recently in restenosis (4). PI3Ks make up a large family of lipid kinases, but class I PI3Ks are the most well known (5-7).…”