Cell proliferation and CD11b expression are controlled independently during HL60 cell differentiation initiated by 1,25 alpha-dihydroxyvitamin D-3 orAll-trans-retinoic acid

When 1 alpha ,25-dihydroxyvitamin D-3 (D-3) induces HL60 cells to different iate to monocytes, a burst of approximately three shortened cell cycles ("m aturation divisions") precedes exit from cell cycle and completion of matur ation. Here we show that similar maturation divisions occur during neutroph il differentiation induced by all-trans-retinoic acid (ATRA), but without s hortening of the cell cycle. Both ATRA and D-3 initiate these maturation di visions as cells pass through a "window of sensitivity" during early G1. We also investigated whether the initiation of maturation divisions and of th e expression of CD11b, an early-expressed maturation marker, are linked. Ce lls treated with D-3 or ATRA start to express CD11b after 9-14 h, before co mpleting the first maturation division. Elutriation was used to isolate sma ll HL60 cells (almost all in G1) and larger cells (in G1 and S phases) from unsynchronized populations. When these were cultured with D-3 or ATRA, mos t reentered cycle synchronously, multiplied, and differentiated. Following D-3 treatment, the G1-enriched small cells expressed CD11b slightly faster than unsynchronized cultures or fractions dominated by late G1 cells and/or S phase cells. D-3-induced CD11b expression occurred at a similar rate eve n in G1 cells that were held at the G1/S boundary by thymidine. In conclusi on, changes in the control of the cell cycle that characterize the onset of monocytic and neutrophil differentiation are only triggered in early G1, b ut CD11b expression can be initiated from most points in the cell cycle. Di fferentiating agents must therefore regulate the proliferation and the matu ration of differentiating myeloid cells by mechanisms that are at least par tly independent. (C) 2001 Academic Press.