Identification ofapopulation ofbipotent stemcells intheHL60 humanpromyelocytic leukemia cell line

HL60,thehumanpromyelocytic leukemia cell line, consists ofcells that resemble promyelocytes butaretrans- formed into moremature myeloid forms (polymorphonuclear leu- kocytes) that express mostofthefunctional characteristics ofter- minally differentiated myeloid forms whenincubated inthe presence ofdimethyl formamide (HCONMe2). WhenHL60cells areexposed tothephorbol diester 12-0-tetradecanoylphorbol 13- acetate (phorbol 12-myristate 13-acetate (PMA)), they shift from suspension toadherent onthesurface ofthetissue culture vessel, assume theappearance ofmacrophages, andacquire macrophage- associated surface markers, theenzyme-nonspecific esterase, the isozyme acid phosphatase, andthemyeloid-specific esterase. Al- though data from other laboratories suggest that HL60consists of stem cells that arebipotent with respect tomyeloid ormacrophage differentiation, unequivocal proof ofbipotency orevidence that ruled outthepresence oftwodifferent types ofstemcells, each committed todifferent lines ofhematopoietic differentiation, has beenlacking. Toresolve these twoalternatives, wehavedevel- oped acloned population ofHL60cells andstudied theproperties ofthese cells whenexposed toinducers ofmyeloid (HCONMe2) ormacrophage (PMA) differentiation. After 120hrofincubation with HCONMe2, 95%ofthecells acquire myeloid markers and lack specific macrophage markers, whereas thereverse istrue in thepresence ofPMA,clearly establishing that HL60isable to commit itself tothedevelopment oftwodifferent programs ofhe- matopoietic differentiation. Moreover, thecommitment tomac- rophage differentiation isirreversible after 6hrofincubation in PMAwhereas development ofthemyeloid program requires con- tinuous exposure ofthecells toHCONMe2. Commitment ofHL60 tomacrophage differentiation byPMAisnotaffected bysubse- quent addition ofHCONMe2, whereas HCONMe2-induced mye- loid differentiation isoverridden byexposure ofthecells toPMA. These data suggest that theHL60cell mayprovide amodel system for studying theprocess that generates several classes ofprecursor cells, eachcommitted todifferent lines ofhematopoietic differ- entiation, aswell astheprocess that mediates thematuration of these committed precursor cells totheir terminally differentiated state. Themechanism bywhich hematopoietic pluripotent stemcells