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J. Cell Biol.,
Volume 145, Number 4, May 17, 1999 727-740
Department of Neurochemistry and Neuropharmacology, Biomedical Research Center, Osaka University Medical School, 2-2 Yamadaoka, Suita, Osaka 565-0871, Japan
The molecular mechanisms behind phenotypic modulation of smooth muscle cells (SMCs) remain unclear. In our recent paper, we reported the
establishment of novel culture system of gizzard SMCs (Hayashi, K., H. Saga, Y. Chimori, K. Kimura,
Y. Yamanaka, and K. Sobue. 1998. J. Biol. Chem. 273:
28860-28867), in which insulin-like growth factor-I
(IGF-I) was the most potent for maintaining the differentiated SMC phenotype, and IGF-I triggered the
phosphoinositide 3-kinase (PI3-K) and protein kinase
B (PKB(Akt)) pathway. Here, we investigated the signaling pathways involved in de-differentiation of gizzard SMCs induced by PDGF-BB, bFGF, and EGF. In
contrast to the IGF-I-triggered pathway, PDGF-BB,
bFGF, and EGF coordinately activated ERK and
p38MAPK pathways. Further, the forced expression of
active forms of MEK1 and MKK6, which are the upstream kinases of ERK and p38MAPK, respectively,
induced de-differentiation even when SMCs were
stimulated with IGF-I. Among three growth factors, PDGF-BB only triggered the PI3-K/PKB(Akt) pathway in addition to the ERK and p38MAPK pathways.
When the ERK and p38MAPK pathways were simultaneously blocked by their specific inhibitors or an active
form of either PI3-K or PKB(Akt) was transfected,
PDGF-BB in turn initiated to maintain the differentiated SMC phenotype. We applied these findings to
vascular SMCs, and demonstrated the possibility that
the same signaling pathways might be involved in
regulating the vascular SMC phenotype. These results
suggest that changes in the balance between the
PI3-K/PKB(Akt) pathway and the ERK and
p38MAPK pathways would determine phenotypes of visceral and vascular SMCs. We further reported that
SMCs cotransfected with active forms of MEK1 and
MKK6 secreted a nondialyzable, heat-labile protein
factor(s) which induced de-differentiation of surrounding normal SMCs.
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