Signaling through PI3K/Akt mediates stretch and PDGF-BB-dependent DNA synthesis in bladder smooth muscle cells.
OBJECTIVE
Smooth muscle cells (SMC) of the bladder undergo hypertrophy and hyperplasia following exposure to sustained mechanical overload. Although superficial similarities in the response of the heart and bladder to hypertrophic stimuli suggest that similar molecular mechanisms may be involved, this remains to be demonstrated. In this study we compared signal transduction pathway activation in primary culture bladder SMC and cardiac myofibroblasts in response to cyclic stretch. The effects of growth factor stimulation on pathway activation in bladder SMC were also investigated.


METHODS
Primary culture rodent bladder SMC or cardiac myofibroblasts were subjected to cyclic stretch-relaxation in the absence or presence of pharmacologic inhibitors of the phosphoinositide-3-kinase, (PI3K)/Akt, extracellular signal-regulated kinase-mitogen activated protein kinase (Erk-MAPK) or the p38 stress-activated protein kinase-2 (SAPK2) pathways. In parallel experiments human bladder SMC were treated with platelet-derived growth factor-BB (PDGF-BB), heparin-binding EGF-like growth factor (HB-EGF) or fibroblast growth factor-2 (FGF-2). In each case the extent of DNA synthesis was determined by uptake of tritiated thymidine, and activation of specific signaling intermediates was determined by immunoblot analysis using antibodies to the non-phosphorylated and phosphorylated (activated) forms of Akt, p38 and Erk1/2.


RESULTS
Akt and p38 were rapidly phosphorylated in stretched bladder SMC and cardiac myofibroblasts, and stretch-induced DNA synthesis in these cells was ablated with inhibitors of PI3K or p38 but not Erk-MAPK. Similarly, PDGF-BB up-regulated DNA synthesis in bladder SMC in a p38 and Akt-dependent manner.


CONCLUSIONS
We conclude that distinct stimuli, such as mechanical stretch and PDGF-BB, promote DNA synthesis in bladder SMC through shared downstream signaling pathways. Furthermore, phenotypically similar cells from the bladder and heart show comparable pathway activation in response to stretch. These findings suggest that similar molecular mechanisms underlie the altered growth responses of the bladder and heart to mechanical overload. This study also provides the first report of Akt activation in bladder SMC and suggests that Akt, consistent with its pivotal role in cardiac hypertrophy, may also be a key regulator of remodeling in the SMC compartment of the bladder exposed to hypertrophic/hyperplastic stimuli in vivo.
Adam
Rosalyn M
RM
The Urologic Laboratory, Department of Urology, Children's Hospital Boston and Harvard Medical School, Boston, Massachusetts, USA.
Roth
Jonathan A
JA
Cheng
Hong-Lin
HL
Rice
Dana C
DC
Khoury
Joseph
J
Bauer
Stuart B
SB
Peters
Craig A
CA
Freeman
Michael R
MR
D000818:Animals; D000077214:Becaplermin; D006332:Cardiomegaly / Q000503:physiopathology; D002478:Cells, Cultured; D004247:DNA / Q000096:biosynthesis*; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004815:Epidermal Growth Factor / Q000494:pharmacology; D016222:Fibroblast Growth Factor 2 / Q000494:pharmacology; D066257:Heparin-binding EGF-like Growth Factor; D006984:Hypertrophy / Q000503:physiopathology; D036341:Intercellular Signaling Peptides and Proteins; D020928:Mitogen-Activated Protein Kinases / Q000037:antagonists & inhibitors / Q000502:physiology; D009130:Muscle, Smooth / Q000166:cytology / Q000378:metabolism / Q000502:physiology*; D009206:Myocardium / Q000166:cytology; D019869:Phosphatidylinositol 3-Kinases / Q000502:physiology*; D000081082:Phosphoinositide-3 Kinase Inhibitors; D010766:Phosphorylation; D010982:Platelet-Derived Growth Factor / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D017346:Protein Serine-Threonine Kinases / Q000502:physiology; D011518:Proto-Oncogene Proteins / Q000037:antagonists & inhibitors / Q000502:physiology*; D051057:Proto-Oncogene Proteins c-akt; D020574:Proto-Oncogene Proteins c-sis; D051381:Rats; D015398:Signal Transduction; D013314:Stress, Mechanical; D015854:Up-Regulation; D001743:Urinary Bladder / Q000166:cytology / Q000378:metabolism / Q000502:physiology*; D048051:p38 Mitogen-Activated Protein Kinases
D000077214:Becaplermin; D004247:DNA; D004791:Enzyme Inhibitors; D004815:Epidermal Growth Factor; D016222:Fibroblast Growth Factor 2; D066257:Heparin-binding EGF-like Growth Factor; D036341:Intercellular Signaling Peptides and Proteins; D020928:Mitogen-Activated Protein Kinases; D019869:Phosphatidylinositol 3-Kinases; D000081082:Phosphoinositide-3 Kinase Inhibitors; D010982:Platelet-Derived Growth Factor; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; D020574:Proto-Oncogene Proteins c-sis; D048051:p38 Mitogen-Activated Protein Kinases; C587883:HBEGF protein, human; C587885:Hbegf protein, rat; C494920:Akt1 protein, rat