TELOMERASE REGULATION, CELL-CYCLE, AND TELOMERE STABILITY IN PRIMITIVE HEMATOPOIETIC-CELLS

Low levels of telomerase activity have recently been detected in human primitive hematopoietic cells, however, blood cells exhibit telomere shortening on cell proliferation. This challenging observation led us to study telomerase regulation and telomere length in human hematopoie tic progenitor cells from fetal liver (FL), cord blood (CB), periphera l blood (PB), and bone marrow (BM). We found telomerase activity in CD 34(+)/CD38(+) cells exceeding levels in CD34(+)/CD38(-), CD34(-), and mononuclear cells (P <.05). Baseline telomerase activity was highest i n BM (n = 5) CD34(+) cells, followed by PB (n = 20), CB (n = 11), and FL (n = 1). Within 48 hours to 72 hours of in vitro culture of CD34(+) cells in the presence of cytokines (KL, interleukin-3 [IL-3], IL-6, e rythropoietin, granulocyte colony-stimulating factor), telomerase acti vity was upregulated, peaked after 1 week of culture, and decreased to baseline levels or below detection after 3 to 4 weeks. Stimulation of CD34(+) cells with single cytokines resulted in no or minor telomeras e upregulation, whereas cytokine combinations resulted in a significan t telomerase increase (P <.001). There was a correlation between telom erase activity, cell cycle status by BrdU incorporation, and induction of phosphorylated retinoblastoma protein, CDC2, CDK2, cyclin D1, and cyclin A, but not cyclin E and B1 after 72 hours with multiple (but no t single) cytokines. In nonexpanding CD34(+) cells, telomerase was low or undetectable. Secondary CD34(+) cells showed a reduced ability to upregulate telomerase activity. Antiproliferative cytokines such as tr ansforming growth factor-beta 1 and high concentrations of all-trans-r etinoic acid in cytokine-supported CD34(+) cultures downmodulated telo merase activity. Average telomere lengths were 10.4 kbp, 7.4 kbp, and 7.6 kbp in CB, PB, and BM CD34(+) cells, respectively. In ex vivo expa nsion cultures, an average telomeric DNA loss of 1 to 2 kbp over 4 wee ks was observed. However, the rate of base pair loss per population do ubling was significantly lower during the first 2 weeks, when telomera se was upregulated, than during weeks 3 and 4 of culture. In summary, telomerase is upregulated in response to cytokine-induced proliferatio n and cell cycle activation in primitive hematopoietic cells. Telomera se is downregulated between weeks 3 and 4 of ex vivo expansion culture linked with decreased proliferation and greater expansion of more mat ure cell subsets, Our data suggest that telomerase activity in hematop oietic cells reduces, but does not prevent, telomere shortening on pro liferation. (C) 1997 by The American Society of Hematology.