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 Blood, 15 April 2005, Vol. 105, No. 8, pp. 3322-3329.
Prepublished online as a Blood First Edition Paper on December 21, 2004; DOI 10.1182/blood-2004-07-2881.

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RED CELLS

Role of Rap1 in promoting sickle red blood cell adhesion to laminin via BCAM/LU

Meghan M. Murphy, Mohamed A. Zayed, Allyson Evans, Carol E. Parker, Kenneth I. Ataga, Marilyn J. Telen, and Leslie V. Parise

From the Departments of Pharmacology, Biochemistry, and Medicine, University of North Carolina-Chapel Hill, Chapel Hill; and Department of Medicine, Duke University, Durham, NC.

Vaso-occlusion is a hallmark of sickle cell disease. Agonist-induced activation of sickle red blood cells (SS RBCs) promotes their adhesion to vascular proteins, potentially contributing to vasoocclusion. Previously, we described a cyclic adenosine monophosphate (cAMP)-dependent increase in SS RBC adhesion to laminin. Here, we investigated whether Rap1, a small guanosine triphosphatase (GTPase) known to promote integrin-mediated adhesion in other cells, was involved in this signaling pathway. We found that agonists known to induce cAMP signaling promoted the GTP-bound, active state of Rap1 in SS RBCs. The cAMP-dependent exchange factor Epac (exchange protein directly activated by cAMP) is a likely upstream activator of Rap1, since Epac is present in these cells and the Epac-specific cAMP analog 8CPT-2-Me (8-(4-cholorophenylthio)-2'-O-methyl-cAMP) activated Rap1 and promoted SS RBC adhesion to laminin. This 8CPT-2-Me-stimulated adhesion was integrin independent, since it was insensitive to RGD peptide or antibodies against the only known integrin on SS RBCs, {alpha}4{beta}1. However, this adhesion was completely inhibited by either a soluble version of basal cell adhesion molecule/Lutheran (BCAM/LU) or a BCAM/LU adhesion-blocking anti-body. Surprisingly, 8CPT-2-Me-activated Rap1 did not promote SS RBC adhesion to a known {alpha}4{beta}1 ligand, vascular cell adhesion molecule 1 (VCAM-1). These results demonstrate that Epac-induced Rap1 activation in SS RBCs promotes BCAM/LU-mediated adhesion to laminin. Thus, Epac-mediated Rap1 activation may represent an important signaling pathway for promoting SS RBC adhesion. (Blood. 2005;105:3322-3329)


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