Homeostatic intracellular-free Ca2+ is permissive for Rap1-mediated constitutive activation of alpha4 integrins on eosinophils.
Although much progress has been made in understanding the molecular mechanisms underlying agonist-induced "inside-out" activation of integrins, little is known about how basal levels of integrin function are maintained. This is particularly important for nonactivated eosinophils, where intermediate activation of alpha(4)beta(1) integrin supports recruitment to endothelial cells under flow conditions. Depletion of intracellular Ca(2+) and pharmacological inhibition of phospholipase C (but not other intracellular signaling molecules, including PI3K, ERK1/2, p38 MAPK, and tyrosine kinase activity) abrogated basal alpha(4) integrin activity in nonactivated eosinophils. Basal alpha(4) integrin activation was associated with activation of the small GTPase Rap1, a known regulator of agonist-induced integrin function. Basal Rap activation was dependent upon phospholipase C, but not intracellular Ca(2+). However, depletion of intracellular Ca(2+) in CD34(+) hematopoietic progenitor cells abolished RapV12-mediated induction of alpha(4) integrin activity. Thus, residual Rap activity or constitutively active Rap activity in Ca(2+)-depleted cells is not sufficient to induce alpha(4) integrin activation. These data suggest that activation of functional alpha(4) integrin activity in resting eosinophils is mediated by Rap1 provided that the intracellular-free Ca(2+) is at a normal homeostatic concentration.
Ulfman
Laurien H
LH
Department of Respiratory Medicine, University Medical Center Utrecht, Utrecht, The Netherlands. L.Ulfman@umcutrecht.nl
Kamp
Vera M
VM
van Aalst
Corneli W
CW
Verhagen
Liesbeth P
LP
Sanders
Marjolein E
ME
Reedquist
Kris A
KA
Buitenhuis
Miranda
M
Koenderman
Leo
L
D002118:Calcium / Q000378:metabolism*; D004804:Eosinophils / Q000276:immunology / Q000378:metabolism*; D006706:Homeostasis; D006801:Humans; D039441:Integrin alpha4 / Q000276:immunology / Q000378:metabolism*; D010738:Type C Phospholipases / Q000378:metabolism; D019010:Vascular Cell Adhesion Molecule-1 / Q000378:metabolism; D020827:rap1 GTP-Binding Proteins / Q000378:metabolism*
D019010:Vascular Cell Adhesion Molecule-1; D039441:Integrin alpha4; D010738:Type C Phospholipases; D020827:rap1 GTP-Binding Proteins; D002118:Calcium