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Pulmonary, Allergy, and Critical Care Division, Department of Medicine, University of Pennsylvania Medical Center, Philadelphia, Pennsylvania 19104
The precise mechanisms that regulate
increases in airway smooth muscle (ASM) mass in asthma are unknown.
This study determined whether class IA phosphatidylinositol 3-kinase
(PI3K) is sufficient to stimulate DNA synthesis and characterized the
PI3K isoforms expressed in human ASM cells. ASM cells express class IA,
II, and III PI3K but not class IB. Because thrombin induces ASM cell proliferation, we investigated whether thrombin can stimulate class IA
PI3K. Transient transfection of ASM cells with hemagglutinin-tagged p85
PI3K followed by immunostaining revealed that in quiescent cells, p85
was expressed diffusely in the cytoplasm and after stimulation with
thrombin p85 translocated to the cell membrane. Microinjection of ASM
cells with a dominant negative class IA PI3K inhibited thrombin-induced
DNA synthesis by 30% and epidermal growth factor (EGF)- or
serum-induced DNA synthesis by 13 and 28%, respectively
(P < 0.05 by
2 analysis). In parallel
experiments, transfection or microinjection of cells with
constitutively active PI3K markedly increased DNA synthesis in
transfected cells 10.5-fold and in microinjected cells 12.7-fold
(P < 0.05 by
2 analysis) compared with
cells transfected or microinjected with control plasmid. Interestingly,
constitutively active PI3K augmented EGF-induced DNA synthesis but had
little effect on that induced by serum or thrombin in ASM cells.
Collectively, these data suggest that class IA PI3K is activated by
thrombin and is sufficient to induce ASM cell growth.
phosphatidylinositol 3-kinase; airway remodeling; asthma; signaling
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