Administration of low-dose cytarabine results in immediate S-phase arrest and subsequent activation of cell cycling in murine stem cells

Objective. Hematopoietic stem cells (HSC) are considered to display a quiescent state with low turnover rate. We investigated the cell-cycle kinetics of HSC after a single dose of cytarabine (Ara-C).

Materials and Methods. We analyzed by flow cytometry the cell-cycle status of lin(low)sca-1(+) c-kit(+) (LSK) stem cells isolated from the bone marrow of C57131/6 mice sacrificed at 0, 2, 4, 6, 8, 12, 20, 48, 72, and 96 hours after intraperitoneal injection of Ara-C (100 mg/kg) using 7-aminoactinomycin-D (7-AAD) for DNA staining. In vivo bromodeoxyuridine (BrdU) incorporation and Ki-67 expression in HCS were also measured.

Results. Two hours after administration of Ara-C, LSK cells ceased to incorporate BrdU. At 4 hours, a decrease of S-phase cells from 10% at baseline to 4% was found (p <0.05), followed by a rapid increase of BrdU and 7-AAD incorporation reaching a maximum of 28% S-phase cells at 20 hours (p <0.001). Ki-67 expression suggested recruitment of 20% of cells from GO into cell cycle. The total number of LSK cells increased 2.5-fold within this short time interval. After 72 hours, a recovery of cell cycling to baseline levels was observed.

Conclusion. This data shows that a single injection of Ara-C first rapidly induced S-phase arrest in HSC for up to 4 hours. Subsequently, an unexpectedly rapid activation of HCS with recruitment of GO cells into cell cycle was observed. The mechanism of cell-cycle activation of LSK cells remains unknown, but reduction of the number of differentiated end cell did not appear to be the primary trigger. (C) 2005 International Society for Experimental Hematology. Published by Elsevier Inc.