Unique, Highly Proliferative Growth Phenotype Expressed by Embryonic and Neointimal Smooth Muscle Cells Is Driven by Constitutive Akt, mTOR, and p70S6K Signaling and Is Actively Repressed by PTEN

Abstract

Background-At distinct times during embryonic development and after vascular injury, smooth muscle cells (SMCs) exhibit a highly proliferative, serum-independent growth phenotype. The aim of the present study was to evaluate the functional role of S6 ribosomal protein (S6RP) and upstream positive and negative regulators in the control of SMC serum-independent growth. Methods and Results-We previously reported increased expression of S6RP mRNA was associated with this unique growth phenotype. Using immunohistochemistry and Western blot analysis, we report high levels of total and phospho-S6RP and increased levels of Akt and p70S6K phosphorylation, upstream positive regulators of S6RP, in rat embryonic aortas and adult balloon-injured carotid arteries compared with quiescent adult aortas and uninjured carotid arteries. Western blot analysis demonstrated that cultured embryonic and neointimal SMCs that exhibited serumindependent growth capabilities expressed high levels of S6RP and constitutively active Akt, mTOR, and p70S6K. Pharmacological and molecular inhibition of phosphatidylinositol 3-kinase (PI3K) signaling pathways, using PI3K inhibitors, rapamycin, or dominant-negative Akt adenovirus, suppressed embryonic and neointimal SMC serumindependent growth. Finally, decreased activity of PTEN, an endogenous negative regulator of PI3K signaling, was associated with high in vivo SMC growth rates, and morpholino-mediated loss of endogenous PTEN induced a serum-independent growth phenotype in cultured serum-dependent SMCs. Conclusions-The possibility exists that cells that exhibit a distinct embryonic-like growth phenotype different from traditional SMCs are major contributors to intimal thickening. Growth of SMCs that exhibit this phenotype is dependent on constitutive Akt and mTOR/p70S6K signaling and is actively inhibited through the timed acquisition of the endogenously produced growth suppressor PTEN. Key Words: cells Ⅲ restenosis Ⅲ muscle, smooth Ⅲ aorta Ⅲ carotid arteries V ascular smooth muscle cell (SMC) proliferation plays a prominent role in normal vessel development and in many vascular pathologies, including restenosis after angioplasty and stent placement and atherosclerosis. The mechanisms that regulate SMC proliferation during vascular development and in response to vascular injury in the adult blood vessel, however, have yet to be fully elucidated. During development, aortic SMCs undergo a phase of rapid proliferation, during which the vessel wall acquires its complement of SMCs. 1 Replication rates decrease as the animal matures, with SMCs eventually reaching a quiescent state in the adult animal. 1,2 However, after injury to an adult artery, transient increases in SMC replication to levels similar to those exhibited during embryonic life are observed in the neointima. 2,3 Corresponding to high in vivo growth rates, SMCs cultured from embryonic aortas exhibit a distinct growth phenotype characterized by rapid serum-stimulated growth and the ability to replicate in a mitogen-or serum-independent man...

The survival pathways phosphatidylinositol‐3 kinase (PI3‐K)/phosphoinositide‐dependent protein kinase 1 (PDK1)/Akt modulate liver regeneration through hepatocyte size rather than proliferation†

“…Akt, mTOR, p70 S6K , and glycogen synthase kinase-3␤ are all good candidates for determining the original (inborn) cell size in various cells or organs. [19][20][21]23 This is consistent with our proposal that PDK1/Akt and associ- ated signals contribute to liver regeneration mainly by inducing cell growth (cell size), but not by inducing cell proliferation.…”

“…[14][15][16][17][18] Recent reports have shown that PI3-K/PDK1/Akt pathway and its associated molecules are responsible for determining cell size and functions. 6,[19][20][21][22][23][24] Many of these studies used animals with targeted gene disruption to demonstrate the crucial roles of these molecules (mTOR, p70 S6K , and glycogen synthase kinase-3) in determining the "inherent size of cells" in the specific organ. In contrast, liver-specific knockout of phosphatase tensin homolog deleted on chromosome 10, a negative regulator of PI3-K/Akt pathway, induced enlargement of the organ.…”

XBP 1-Deficiency Abrogates Neointimal Lesion of Injured Vessels Via Cross Talk With the PDGF Signaling

“…The Akt phosphorylation is derived from the increased PI3K activation or decreased phosphatase and tensin homolog dephosphorylation. 52,53 Suppression of PI3K reduces neointima formation, 54 whereas suppression of phosphatase and tensin homolog increases SMC hyperplasia. 55 In this study, we found that XBP1 splicing contributed to neointima formation via regulating SMC migration and proliferation.…”

“…The Akt phosphorylation is derived from the increased PI3K activation or decreased phosphatase and tensin homolog dephosphorylation. 52,53 Downloaded from http://ahajournals.org by on March 22, 2019 …”

ANG II-induced neointimal growth is mediated via cPLA2- and PLD2-activated Akt in balloon-injured rat carotid artery

“…Akt has been implicated in neointimal growth and VSMC proliferation after balloon injury (42). High levels of total phospho-Akt and phospho-ribosomal S6 kinase (p70S6K) have been shown in rat embryonic aorta and adult balloon-injured carotid arteries compared with quiescent adult rat aorta and uninjured carotid arteries (24). Akt is phosphorylated at 30 and 60 min after injury and to a lesser degree at 6 h after injury (39).…”