Induction of Granulocytic Maturation in HL-60 Human Leukemia Cells by Free Radicals: A Hypothesis of Cell Differentiation Involving Hydroxyl Radicals

Abstract

Tumor cells usually contain lower superoxide dismutase (SOD) activity than differentiating cells, suggesting the involvement of oxygen free radicals in cell maturation. The effects of a system known to produce the OH˙ radicals were tested on HL-60 cells cultured under optimum conditions for 96 hr. Hydroxyl radicals were generated by a Fenton reaction, involving an ADP-Fe²⁺ (or ATP-Fe²⁺) complex and H₂O₂. Changes induced by OH˙ were compared to the effects of DMSO-induced differentiation of HL-60 cells. Cell numbers, viability, thymidine incorporation, TPA-induced NBT reduction and propidium iodide staining in flow cytometry were determined. The OH generating system inhibited the growth and thymidine incorporation of leukemic cells in a manner dependent on the dose of added H₂O₂ (from 0.005 to 0.05 mM). In addition, an increasing proportion of the treated cells displayed signs of cell differentiation. In DMSO-treated cells, SOD and catalase activities increased after 6 days of culturing. The results show that a portion of the OH˙ free radicals derived from H₂O₂, produced by the action of SOD, may be a necessary prerequisite for differentiation, whereas an overproduction of OH causes cell lethality or aging. We suggest that OH˙ free radicals may have a more complex role in cell physiology than simply causing oxidative damage.

Key Words:

• OH˙ free radicals
• HL-60 leukemia cell line
• DMSO-induced differentiation of HL-60 cells
• SOD
• catalase
• cell differentiation