Homeostatic Intracellular-Free Ca super(2+) Is Permissive for Rap1-Mediated Constitutive Activation of alpha sub(4) Integrins on Eosinophils

• Published in: The Journal of immunology (1950) Vol. 180; no. 8; pp. 5512 - 5519
• Main Authors: Ulfman, Laurien H, Kamp, Vera M, van Aalst, Corneli W, Verhagen, Liesbeth P, Sanders, Marjolein E, Reedquist, Kris A, Buitenhuis, Miranda, Koenderman, Leo
• Format: Journal Article
• Language: English
• Published: 01-04-2008
• ISSN: 0022-1767; 1550-6606

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 sub(4) beta sub(1) integrin supports recruitment to endothelial cells under flow conditions. Depletion of intracellular Ca super(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 sub(4) integrin activity in nonactivated eosinophils. Basal alpha sub(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 super(2+). However, depletion of intracellular Ca super(2+) in CD34 super(+) hematopoietic progenitor cells abolished RapV12-mediated induction of alpha sub(4) integrin activity. Thus, residual Rap activity or constitutively active Rap activity in Ca super(2+)-depleted cells is not sufficient to induce alpha sub(4) integrin activation. These data suggest that activation of functional alpha sub(4) integrin activity in resting eosinophils is mediated by Rap1 provided that the intracellular-free Ca super(2+) is at a normal homeostatic concentration.