Docosahexaenoic acid modulates different genes in cell cycle and apoptosis to control growth of human leukemia HL-60 cells

Although previous studies have shown that docosahexaenoic acid (DHA; 22:6ω3) from fish oils inhibits growth of different cancers, safety issues have been raised repeatedly about contaminations of toxins in these oils. Cultured microalgae are suggested recently as an alternative cleaner and safer source of the fatty acid. We investigated in this study the function of DHA from the enriched microalga Crypthecodinium cohnii (ADHA) in cell-growth control and its mechanism in human leukemia HL-60 cells. ADHA retarded proliferation of the leukemia cells dose-dependently by 4-93% of the control level, after 72-h incubations with 10-160 µM of the fatty acid; and the 50% inhibitory concentration (IC50) was estimated as 74 µM. DNA-flow cytometry study showed that ADHA arrested G0/G1 cells by 12-22% and induced apoptotic cells by 569-906% of their controls, after incubation with the IC50 of ADHA for 24, 48 and 72 h. The modes of cell-cycle arrest and pro-apoptotic actions of ADHA were further elucidated. Gene-array analysis illustrated that ADHA modulated a number of cell-cycle and apoptosis genes to control the cell growth; in particular, the fatty acid up-regulated the transcriptional repressor E2F-6 and pro-apoptotic Bax by 1435 and 4172% respectively, after 24 h of incubation. Semi-quantitative RT-PCR study further showed that ADHA induced elevation of the Bax mRNA transcript time-dependently. In meanwhile, ADHA also induced phosphorylation and thus inactivation of Rb protein in the leukemia cells. All these results suggest that ADHA up-regulates Bax and inactivates Rb protein to induce the cell-growth control and apoptosis in human leukemia HL-60 cells.