SDF-1α induces differential trafficking of CXCR4-CXCR7 involving cyclophilin A, CXCR7 ubiquitination and promotes platelet survival

Platelet-derived SDF-1 alpha (CXCL12) mediates inflammatory and regenerative mechanisms. The present study characterizes the effect of SDF-1 alpha ligation in platelets. SDF-1 alpha (0-100 mu M) dose and time dependently caused internalization of its receptor CXCR4 (28.9 +/- 1.6 vs. 16.1 +/- 1.9 in SDF-1 alpha -treated platelets), coupled to the surface externalization of CXCR7 (65.5 +/- 8 vs. 162.8 +/- 27.6 following SDF-1 alpha treatment), both in vitro and in vivo. This was inhibited in the presence of AMD3100 (100 mu M), CXCR4 blocking and vesicular transport inhibitors (brefeldin A, 10 mu M; rapamycin, 100 nM). SDF-1 alpha/CXCR-4-mediated CXCR7 translocation was significantly reduced by inhibitors of ERK1/2-(U0126-10 mu M) and cyclophilinA (CyPA)(NIM811-10 mu M) by 28 and 46%, respectively. Further, SDF-1 alpha-induced downstream phosphorylation of Erk1/2 led to CyPA-dependent ubiquitination of CXCR7, which is essential for its surface translocation. CyPA-PPIase-activity inhibitor NIM-811, Erk1/2, and E1-ligase inhibitor(PYR-41-25 mu M) significantly abolished SDF-1 alpha-driven CXCR7 ubiquitination and subsequent surface translocation. SDF-1 alpha induced CXCR7 ubiquitination, and its surface exposure was observed in wild-type murine platelets, but not in CyPA-deficient platelets. SDF-1 alpha/CXCR4-CyPA-dependent CXCR7 translocation and its subsequent ligation attenuated activation-induced apoptosis both in vitro and when administered in vivo. This antiapoptotic effect of SDF-1 alpha was abrogated by blocking CXCR7, also significantly affected in Cypa(-/-) platelets. Thus, we decipher a novel mechanism, whereby SDF-1 alpha regulates relative receptor availability in circulating platelets and exerts its prosurvival benefits.