Protein Phosphatase2A Regulates Stromal Cell- Derived Factor-1 Mediated Responses of CD34+ Cord Blood Cells.

The chemokine stromal cell- derived factor-1(SDF-1) and its receptor, CXCR4, play a major role in migration, retention, and development of hematopoietic progenitors in the bone marrow. During steady-state hematopoiesis, CXCR4/SDF-1 interaction restricts hematopoietic stem progenitor cells in marrow and disruption of the SDF-1/CXCR4 axis leads to their egress into circulation. However, biological responses to SDF-1 are rapidly attenuated by overlapping mechanisms that include peptide degradation by cell surface proteases, receptor uncoupling from heterotrimeric G-proteins and receptor endocytosis. It is known that phosphorylation of G-protein coupled receptors (GPCRs) by several protein kinases including serine-threonine kinase, protein kinase C (PKC), can result in receptor desensitization. This prompted us to investigate whether the role of serine- threonine phosphatase in the regulation of SDF-1 induced responses of CD34+ cord blood (CB) cells. To investigate the role of serine- threonine phosphatase we evaluated the effect of okadaic acid, a specific serine-threonine phosphatase inhibitor, on SDF-1 induced chemotaxis and adhesion of CD34+ cells from cord blood. Pre-incubation of CD34+ CB cells with okadaic acid (100–1000nM) significantly reduced SDF-1 directed chemotaxis and adhesion of CD34+ CB cells. This correlated with an increase in PKC phosphorylation. Although primary SDF-1 induced calcium flux in CD34+CB cells was unaffected by okadaic acid, if the cells were pretreated with SDF-1 and then stimulated with SDF-1, calcium flux was significantly less in CD34+CB cells pretreated with okadaic acid compared to untreated cells. Further, confocal microscopy was used to demonstrate the co-localization of serine threonine phosphatase and PKC. Using antibodies specific to PP2A catalytic subunit (PP2Ac) and various classes of PKC super family, it was observed in unstimulated CD34+ CB cells PP2Ac was mainly localized in nucleus and some phosphor-PKC-delta was detected in the cytoplasm. However, upon SDF-1 stimulation, co-localization of PP2Ac and phospho-PKC-delta was observed on the membrane of CD34+CB cells. These results indicate that serine-threonine phosphatase regulate SDF-1 mediated responses in CD34+CB cells and may play an important role in the ability of hematopoietic stem progenitor cells to continually respond to SDF-1.