Activation of specific MEK-ERK cascade is necessary for TGFbeta signaling and crosstalk with PKA and PKC pathways in cultured rat articular chondrocytes.
OBJECTIVE
TGFbeta is a potent stimulator of cell growth in cultured rat articular chondrocytes (CRAC). The stimulatory effect is mediated through the immediate induction of c-fos gene by activating ERK of MAPK. The present study was undertaken to investigate the upstream regulators involved in TGFbeta-induced ERK activation in CRAC and to compare the results with the events in HepG2 cells.


RESULTS
In vitro kinase and trans-reporting assays showed that TGFbeta preferentially activated ERK and JNK pathways in CRAC and HepG2, respectively. ERK activation in CRAC was selectively inhibited by PD98059, a MEK inhibitor. Overexpression of wild or active forms of MEKK1, the upstream activator of ERK and JNK, decreased the TGFbeta-induced 3TP-luciferase activity in CRAC. In contrast, in HepG2 dominant negative form of MEKK1 or SEK1 ligand-dependent reporter activity was diminished. Transfection of TAK1, another MAPKKK, also positively and negatively regulated 3TP transcriptional activity of HepG2 and CRAC, respectively. Activation of PKA by 8-bromo-cyclic AMP or forskolin, and inhibition of PKC by calphostin C, resulted in a significant decrease in 3TP activity as well as in vitro ERK kinase activity in CRAC.


CONCLUSIONS
The results indicate that TGFbeta transduces a predominant signal pathway through MEK-ERK-Elk1, independent of MEKK1 or TAK1 pathway in CRAC. However, in HepG2, activation of MEKK1 and TAK1 is essential for TGFbeta-induced signal transmission. The results also demonstrated that in CRAC, MEK-ERK pathway activated by TGFbeta is negatively regulated by PKA cascade but transactivated by PKC.
Hirota
Y
Y
Department of Nature Medicine, Atomic Bomb Disease Institute, Nagasaki, Japan.
Tsukazaki
T
T
Yonekura
A
A
Miyazaki
Y
Y
Osaki
M
M
Shindo
H
H
Yamashita
S
S
D015124:8-Bromo Cyclic Adenosine Monophosphate / Q000494:pharmacology; D000818:Animals; D002358:Cartilage, Articular / Q000166:cytology; D002450:Cell Communication / Q000502:physiology*; D002478:Cells, Cultured / Q000166:cytology; D019902:Chondrocytes / Q000502:physiology*; D005576:Colforsin / Q000494:pharmacology; D017868:Cyclic AMP-Dependent Protein Kinases / Q000187:drug effects / Q000502:physiology*; D008297:Male; D020929:Mitogen-Activated Protein Kinase Kinases / Q000502:physiology*; D011493:Protein Kinase C / Q000187:drug effects / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D016212:Transforming Growth Factor beta / Q000502:physiology*; D014407:Tumor Cells, Cultured / Q000166:cytology; D015854:Up-Regulation
D016212:Transforming Growth Factor beta; D005576:Colforsin; D015124:8-Bromo Cyclic Adenosine Monophosphate; D017868:Cyclic AMP-Dependent Protein Kinases; D011493:Protein Kinase C; D020929:Mitogen-Activated Protein Kinase Kinases