| Research Abstract |
In previous study, we found that IGF-I triggered the phosphoinositide 3-kinase (PI3-K) and protein kinase B (PKB(Akt)) in cultured gizzard SMCs and this signaling pathway played a vital role in maintaining a differentiated phenotype of SMCs. Here, we investigated the signaling pathways involved in dedifferentiation of gizzard SMCs induced by PDGF-BB, bFGF, and EGF using the primary culture system described above. 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 dedifferentiation 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 tr
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ansfected, 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.
Further, we used mRNA subtraction method to reveal the molecular mechanisms underlying the phenotypic modulation of SMCs. With this approach, we found that a 10 kb mRNA encoding a homotypic cell adhesion molecule, cadherin 6B, was strongly expressed in differentiated vascular and visceral SMCs, but not in the dedifferentiated SMCs derived from them. In vivo, cadherin 6B was expressed in vascular and visceral SMCs, in addition to brain, spinal cord, retina, and kidney, at a late stage of chicken embryonic development. These results suggest that cadherin 6B is a novel molecular marker for SMC phenotype and is involved in the late differentiation of SMCs. Less
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