Expansion of normal and leukemic human hematopoietic stem/progenitor cells requires rac-mediated interaction with stromal cells.
OBJECTIVE
To determine the involvement of Rac signaling in self-renewal and expansion on bone marrow stroma of normal CD34+ cells vs leukemic CD34+ cells from acute myeloid leukemia (AML) patients.


METHODS
Rac signaling was modulated by retroviral introduction of Racl-N17, Racl-V12, or by using the Rac inhibitor NSC23766. In long-term MS5 cocultures (leukemic) expansion, migration, adhesion, and presence of stem/progenitor cells were monitored in both normal as well as leukemic CD34+ cells.


RESULTS
Inhibition of Rac signaling impaired migration and adhesion of cord blood (CB) CD34+ cells on MS5 stroma. Long-term inhibition of Rac during a 5-week coculture period on stroma prevented association of hematopoietic progenitors with the bone marrow stromal cells and resulted in a dramatic decrease in the primitive stem cell frequency (long-term culture-initiating cell) in a dose-dependent manner. Many of these phenotypes were reversed in the presence of activated Racl-V12, including improved migration toward, and association with, MS5 cells. CD34+ AML cells were characterized by elevated levels of Rac activity (five of seven patients) and enhanced migration and adhesion to MS5 bone marrow stroma as compared to CB CD34+ cells. A dramatic decrease was observed in the formation of leukemic cobblestone area-forming cells as well as strongly diminished clonal expansion in the presence of the Rac inhibitor NSC23766.


CONCLUSIONS
Our data indicate that Rac signal transduction is required for the maintenance and expansion of both normal as well as leukemic stem/progenitor cells by mediating their interaction with stromal cells.
Rozenveld-Geugien
Marjan
M
Department of Hematology, University Medical Center Groningen, Groningen, The Netherlands.
Baas
Inge O
IO
van Gosliga
Djoke
D
Vellenga
Edo
E
Schuringa
Jan Jacob
JJ
D000634:Aminoquinolines / Q000494:pharmacology; D018952:Antigens, CD34 / Q000276:immunology; D002448:Cell Adhesion / Q000187:drug effects / Q000502:physiology; D002450:Cell Communication / Q000502:physiology; D018929:Cell Culture Techniques / Q000379:methods; D002465:Cell Movement / Q000187:drug effects / Q000502:physiology; D049109:Cell Proliferation / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D006412:Hematopoietic Stem Cells / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D006801:Humans; D007951:Leukemia, Myeloid / Q000378:metabolism*; D014411:Neoplastic Stem Cells; D011743:Pyrimidines / Q000494:pharmacology; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology; D017154:Stromal Cells / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D020744:rac GTP-Binding Proteins / Q000037:antagonists & inhibitors / Q000502:physiology*
D000634:Aminoquinolines; D018952:Antigens, CD34; C487513:NSC 23766; D011743:Pyrimidines; D020744:rac GTP-Binding Proteins