2 DISTINCTLY REGULATED EVENTS, PRIMING AND TRIGGERING, DURING RETINOID-INDUCED MATURATION AND RESISTANCE OF NB4 PROMYELOCYTIC LEUKEMIA-CELLLINE

In t(15;17) acute promyelocytic leukemia, all-trans retinoic acid (RA) induces leukemic cell maturation in vitro and remission in acute prom yelocytic leukemia patients, but in vivo treatments invariably lead to relapse with resistance to RA. NB4, a maturation-inducible cell line, and NB4-RA(r) sublines (R1 and R2) displaying no maturation in the pr esence of RA have been isolated from a patient in relapse. We show tha t resistance to maturation is not a mere unresponsiveness to RA: rathe r, R1 ''resistant'' cells do respond to RA (1 mu M) by sustained growt h, become competent to undergo terminal maturation, and up-regulate CD 11c/CD18 integrins. Interestingly, maturation of ''resistant'' cells, rendered competent by RA, can be achieved by cAMP-elevating agents (pr ostaglandin E, isoproterenol, cholera toxin, or phosphodiesterase inhi bitor) or stable agonistic cAMP analogs such as (Sp)-8-chloroadenosine cyclic 3',5'-phosphorothioate. This shows that activation of cAMP-dep endent protein kinase (cA kinase) can override the RA resistance and s uggests interdependent RA and cAMP signaling pathways in acute promyel ocytic leukemia maturation. No such cooperation was observed in the R2 resistant cells, though their cA-kinase was functional. (Rp)-8-Chloro adenosine cyclic 3',5'-phosphorothioate, which by displacing endogenou s cAMP inhibits the basal cA-kinase activity, decreased the response o f sensitive cells to RA. This raises the possibility that cA-kinase pl ays a key role in the maturation also of RA-sensitive cells. Our resul ts define two discrete steps in the maturation process: an RA-dependen t priming step that maintains proliferation while cells become compete nt to undergo maturation in response to retinoids and a cAMP-dependent step that triggers RA-primed cells to undergo terminal maturation. Un coupling RA and cAMP action might cause the so-called ''resistance.''