Da. Troyer et al., TYROSINE PHOSPHORYLATION OF FOCAL ADHESION KINASE (P125(FAK)) - REGULATION BY CAMP AND THROMBIN IN MESANGIAL CELLS, Journal of the American Society of Nephrology, 7(3), 1996, pp. 415-423
Stress fibers, composed of actin filaments, converge upon and associat
e with a number of proteins, including focal adhesion kinase (p125(FAK
)), and integrin receptors to form areas of close contact between cell
s and the extracellular matrix referred to as focal adhesions. Treatme
nt of mesangial cells with cAMP-elevating agents causes a loss of foca
l adhesions, fragmentation of stress fibers, and decreased tyrosine ph
osphorylation of p125(FAK). Thrombin reverses these effects of cAMP, a
nd this model can be used to address some of the cellular mechanisms i
nvolved in regulating the loss and formation of focal adhesions. This
study reports the effects of cAMP and thrombin on mesangial cell shape
, distribution of actin, formation of stress fibers, and tyrosine phos
phorylation of p125(FAK). cAMP-treated cells display a condensed cell
body with slender processes that traverse the area formerly covered by
the cell. Addition of thrombin to these cells restores actin filament
s (stress fibers) and increases tyrosine phosphorylation of p125(FAK),
and the cells resume a flattened morphology, even In the continued pr
esence of cAMP-elevating agents. Peptides that mimic the tethered liga
nd portion of the thrombin receptor have the same effects on cell morp
hology and stress fiber formation as thrombin. In selected experiments
, agents that disrupt either stress fibers (cytochalasin D) or microtu
bules (nocodazole; Sigma Chemical, St. Louis, MO) were used to examine
the role of these cytoskeletal elements in thrombin-induced restorati
on of focal adhesions. Cytochalasin D blocked the ability of thrombin
to restore focal adhesions and phosphorylate p125(FAK). The effects of
nocodazole, an agent that destabilizes microtubules (but which has no
known receptor), are very similar to those of thrombin. The findings
discussed in this study indicate that thrombin can modulate the format
ion of focal adhesions. The organization of stress fibers and microtub
ules is apparently intimately related to the phosphorylation of p125(F
AK) and can be modulated by soluble receptor agonists such as thrombin
or via altered polymerization of microtubules.