DUAL-ACTION OF RETINOIC ACID ON HUMAN EMBRYONIC FETAL HEMATOPOIESIS -BLOCKADE OF PRIMITIVE PROGENITOR PROLIFERATION AND SHIFT FROM MULTIPOTENT ERYTHROID MONOCYTIC TO GRANULOCYTIC DIFFERENTIATION PROGRAM

In preliminary studies, we have analyzed the hematopoietic growth fact or (HGF) requirement of hematopoietic progenitor cells (HPCs) purified from embryonic-fetal liver (FL) and grown in fetal calf serum-supplem ented (FCS+) clonogenic culture. The key role of erythropoietin (Epo) for colony formation by early erythroid progenitors (burst-forming uni ts-erythroid [BFU-E]) has been confirmed. Furthermore, in the absence of exogenous HGFs, FL monocytic progenitors (colony-forming unit monoc yte [CFU-M]) generate large colonies exclusively composed of monocytes -macrophages; these colonies are absent in FCS- clonogenic culture. On this basis, we have investigated the role of all-trans retinoic acid (ATRA) and its isomer 9-cis RA in FL hematopoiesis. Both compounds mod ulate the growth of purified FL HPCs, which show a dose-dependent shif t from mixed/erythroid/monocytic to granulocytic colony formation. Stu dies on unicellular and paired daughter cell culture unequivocally ind icate that the shift is mediated by modulation of the HPC differentiat ion program to the granulopoietic pathway (rather than RA-induced down modulation of multipotent/erythroid/monocytic HPC growth coupled with recruitment of granulocytic HPCs). ATRA and 9-cis RA also exert their effect on the proliferation of primitive HPCs (high-proliferative pote ntial colony-forming cells [HPP-CFCs]) and putative hematopoietic stem cells (HSCs; assayed in Dexter-type long-term culture). High concentr ations of either compound (1) drastically reduced the number of primar y HPP-CFC colonies and totally abolished their recloning capacity and (2) inhibited HSC proliferation. It is crucial that these results mirr or recent observations indicating that murine adult HPCs transduced wi th dominant negative ATRA receptor (RAR) gene are immortalized and sho w a selective blockade of granulocytic differentiation. Altogether, th ese results suggest that ATRA/9-cis RA may play a key role in FL hemat opoiesis via a dual effect hypothetically mediated by interaction with the RAR/RXR heterodimer, ie, inhibition of HSC/primitive HPC prolifer ation and induction of CFU-GEMM/BFU-E/CFU-M shift from the multipotent /erythroid/monocytic to the granulocytic-neutrophilic differentiation program. (C) 1996 by The American Society of Hematology.