Аннотация
Lysophosphatidic acid (LPA) utilizes a G-protein-coupled receptor
to activate the small GTP-binding protein Rho and to induce rapid
remodeling of the actin cytoskeleton. We studied the signal transduction
from LPA receptors to Rho activation. Analysis of the G-protein-coupling
pattern of LPA receptors by labeling activated G-proteins with alpha-32PGTP
azidoanilide revealed interaction with proteins of the Gq, Gi, and
G12 subfamilies. We could show that in COS-7 cells, expression of
GTPase-deficient mutants of Galpha12 and Galpha13 triggered Rho activation
as measured by increased Rho-GTP levels. In Swiss 3T3 cells, incubation
with LPA or microinjection of constitutively active mutants of Galpha12
and Galpha13 induced formation of actin stress fibers and assembly
of focal adhesions in a Rho-dependent manner. Interestingly, the
LPA-dependent cytoskeletal reorganization was suppressed by microinjected
antibodies directed against Galpha13, whereas Galpha12-specific antibodies
showed no inhibition. The tyrosine kinase inhibitor tyrphostin A
25 and the epidermal growth factor (EGF) receptor-specific tyrphostin
AG 1478 completely blocked actin stress fiber formation caused by
LPA or activated Galpha13 but not the effects of activated Galpha12.
Also, expression of the dominant negative EGF receptor mutant EGFR-CD533
markedly prevented the LPA- and Galpha13-induced actin polymerization.
Coexpression of EGFR-CD533 and activated Galpha13 in COS-7 cells
resulted in decreased Rho-GTP levels compared with expression of
activated Galpha13 alone. These data indicate that in Swiss 3T3 cells,
G13 but not G12 is involved in the LPA-induced activation of Rho.
Moreover, our results suggest an involvement of the EGF receptor
in this pathway.
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