Abstract
The chemokine stromal cells derived factor-1 (SDF-1)/CXCL12 stimulates lymphocytes, hematopoietic stem cells (HSC) and hematopoietic progenitor cells (HPC) through the corresponding chemokine receptor CXCR4, which modulates homing, retention, and engraftment into the bone marrow. However, regulatory factors for CXCR4 excluding cytokines are totally unknown. Besides, all-trans retinoic acid (ATRA), a drug inducing cell differentiation and apoptosis, increases motility with unknown mechanisms. In the present study, we investigated the intracellular regulatory mechanisms of SDF-1α-induced migration of ATRA-treated cells. We initially surveyed cells exhibiting strongly membrane-associated CXCR4 (M-CXCR4), and found 3 out of 10 different cell lines (MOLT3, U266B1, and Raji). Of those, MOLT3 was identified as the only one enabled to respond to SDF-1α chemoattraction, accompanied by an increase in actin polymerization. When M-CXCR4 in MOLT3 was neutralized or antagonized by an antibody or the antagonist AMD3100, the migration activity to SDF-1α, analyzed using the Boyden chamber method, was totally abolished. ATRA pretreatment for 3 days induced growth inhibition of MOLT3, but apoptosis and differentiation did not occur. ATRA treatment alone had no effects on either CXCR4 gene expression or its total protein expression, and cell motility also did not increase; however, ATRA treatment dose dependently increased M-CXCR4 expression. When SDF-1α was added to the lower chamber, migration activity of cells pretreated by different doses of ATRA increased dose dependently and reached up to 5 times greater compared with cells lacking ATRA pretreatment. The increased migration activity was blocked by wortmannin, which inhibits phosphatidylinositol 3 kinase (PI3K), AKT inhibitor, and cytochalasin D, which inhibits actin polymerization; U0126, an inhibitor of MEK1/2, did not block the effects of ATRA and SDF-1α. The results indicate that M-CXCR4 is quite important for cell migration, and the intracellular signaling for the M-CXCR4/SDF-1 axis primarily involves the PI3K/Akt pathway, which ATRA positively affected. This is the fist time to show that migration activity to SDF-1 was accelerated by ATRA. Clinical application of this phenomenon may help to augment HSC/HPC homing after transplantation.
Down-regulation of the Phosphatidylinositol 3-Kinase/Akt Pathway Is Involved in Retinoic Acid-induced Phosphorylation, Degradation, and Transcriptional Activity of Retinoic Acid Receptor γ2
