Cyclic AMP activates B-Raf and ERK in cyst epithelial cells from autosomal-dominant polycystic kidneys.
BACKGROUND
The proliferation of mural epithelial cells is a major cause of progressive cyst enlargement in autosomal-dominant polycystic kidney disease (ADPKD). Adenosine 3', 5' cyclic monophosphate (cAMP) stimulates the proliferation of cells from ADPKD cysts, but not cells from normal human kidney cortex (HKC), through the activation of protein kinase A (PKA), mitogen-activated protein kinase kinase (MEK), and extracellular signal-regulated kinase (ERK/MAPK). In the current study, we examined the signaling pathway between PKA and MEK in ADPKD and HKC cells.


METHODS
Primary cultures of human ADPKD and HKC cells were prepared from nephrectomy specimens. We determined the effects of cAMP and epidermal growth factor (EGF) on the activation of ERK, B-Raf and Raf-1 in ADPKD and HKC cells by immune kinase assay and Western blot.


RESULTS
8-Br-cAMP increased phosphorylated ERK (2.7- +/- 0.6-fold, N = 7), and B-Raf kinase activity (3.6- +/- 1.1-fold, N = 5) in cells from ADPKD kidneys; levels of phosphorylated Raf-1 were not changed. Inhibition of PKA by H89 strikingly decreased cAMP-stimulated phosphorylation of ERK and B-Raf, and MAPK inhibition by PD98059 blocked the effect of the nucleotide to activate ERK. By contrast, in HKC cells 8-Br-cAMP did not activate B-Raf and ERK. EGF stimulated the phosphorylation of ERK and Raf-1 in both ADPKD and HKC cells, but had no effect on B-Raf. 8-Br-cAMP and EGF conjointly increased ERK activation above that of either agonist alone in ADPKD cells, and this combined effect was abolished by PD98059, indicating that ERK was activated by EGF- and cAMP-responsive cascades that converge at MAPK.


CONCLUSIONS
cAMP activates ERK and increases proliferation of ADPKD epithelial cells, but not cells from normal human kidney cortex, through the sequential phosphorylation of PKA, B-Raf and MAPK in a pathway separate from, but complementary to, the classical receptor tyrosine kinase cascade. Consequently, cAMP and EGF have great potential to accelerate the progressive enlargement of renal cysts.
Yamaguchi
Tamio
T
Kidney Institute, Kansas University Medical Center, Kansas City, Kansas 66160-7382, USA.
Nagao
Shizuko
S
Wallace
Darren P
DP
Belibi
Franck A
FA
Cowley
Benjamin D
BD
Pelling
Jill C
JC
Grantham
Jared J
JJ
D015124:8-Bromo Cyclic Adenosine Monophosphate / Q000494:pharmacology; D002478:Cells, Cultured; D000242:Cyclic AMP / Q000378:metabolism*; D004347:Drug Interactions; D004815:Epidermal Growth Factor / Q000494:pharmacology; D004847:Epithelial Cells / Q000201:enzymology; D005260:Female; D006801:Humans; D007672:Kidney Cortex / Q000166:cytology; D020935:MAP Kinase Signaling System / Q000187:drug effects / Q000502:physiology; D008297:Male; D008875:Middle Aged; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism*; D016891:Polycystic Kidney, Autosomal Dominant / Q000378:metabolism*; D048493:Proto-Oncogene Proteins B-raf; D019908:Proto-Oncogene Proteins c-raf / Q000378:metabolism*
D015124:8-Bromo Cyclic Adenosine Monophosphate; D004815:Epidermal Growth Factor; D000242:Cyclic AMP; D048493:Proto-Oncogene Proteins B-raf; D019908:Proto-Oncogene Proteins c-raf; D020928:Mitogen-Activated Protein Kinases