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.
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.


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 serum-independent 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 serum-independent 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.
Mourani
Peter M
PM
Department of Pediatrics, University of Colorado Health Sciences Center, Denver, Colo 80262, USA.
Garl
Pamela J
PJ
Wenzlau
Janet M
JM
Carpenter
Todd C
TC
Stenmark
Kurt R
KR
Weiser-Evans
Mary C M
MC
D000730:Androstadienes / Q000494:pharmacology; D000818:Animals; D001011:Aorta / Q000196:embryology / Q000254:growth & development; D020212:Carotid Artery Injuries / Q000378:metabolism; D002404:Catheterization / Q000009:adverse effects; D002455:Cell Division; D002478:Cells, Cultured / Q000378:metabolism; D002867:Chromones / Q000494:pharmacology; D016895:Culture Media, Serum-Free / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D009025:Morpholines / Q000494:pharmacology; D009131:Muscle, Smooth, Vascular / Q000166:cytology*; D032389:Myocytes, Smooth Muscle / Q000166:cytology* / Q000187:drug effects / Q000378:metabolism; D016376:Oligonucleotides, Antisense / Q000494:pharmacology; D051059:PTEN Phosphohydrolase; D010641:Phenotype; D019869:Phosphatidylinositol 3-Kinases / Q000502:physiology; D000081082:Phosphoinositide-3 Kinase Inhibitors; D010766:Phosphorylation; D047428:Protein Kinase Inhibitors; D011494:Protein Kinases / Q000502:physiology*; D011499:Protein Processing, Post-Translational; D017346:Protein Serine-Threonine Kinases; D017027:Protein Tyrosine Phosphatases / Q000502:physiology*; D011518:Proto-Oncogene Proteins / Q000502:physiology*; D051057:Proto-Oncogene Proteins c-akt; D051381:Rats; D017207:Rats, Sprague-Dawley; D038601:Ribosomal Protein S6 / Q000502:physiology*; D038762:Ribosomal Protein S6 Kinases, 70-kDa / Q000502:physiology*; D015398:Signal Transduction / Q000502:physiology*; D020123:Sirolimus / Q000494:pharmacology; D058570:TOR Serine-Threonine Kinases; D017539:Tunica Intima / Q000378:metabolism* / Q000473:pathology; D000077191:Wortmannin
D000730:Androstadienes; D002867:Chromones; D016895:Culture Media, Serum-Free; D004791:Enzyme Inhibitors; D009025:Morpholines; D016376:Oligonucleotides, Antisense; D051059:PTEN Phosphohydrolase; D019869:Phosphatidylinositol 3-Kinases; D000081082:Phosphoinositide-3 Kinase Inhibitors; D047428:Protein Kinase Inhibitors; D011494:Protein Kinases; D017346:Protein Serine-Threonine Kinases; D017027:Protein Tyrosine Phosphatases; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; D038601:Ribosomal Protein S6; D038762:Ribosomal Protein S6 Kinases, 70-kDa; D020123:Sirolimus; D058570:TOR Serine-Threonine Kinases; D000077191:Wortmannin; C085911:2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one; C546845:mTOR protein, rat; C494920:Akt1 protein, rat; C487216:Pten protein, rat